WO₃/MoO₃ Nanocomposite Thin Films and Heterostructures: Interfacial Synergy for Smart and Sustainable Technologies

Abstract

Tungsten trioxide (WO₃) and molybdenum trioxide (MoO₃) are redox-active Group VI transition-metal oxides widely used in functional thin-film technologies. Many studies have examined single-phase WO₃ and MoO₃ films. However, WO₃/MoO₃ nanocomposite thin films and heterostructures have not yet been comprehensively reviewed as interface-engineered platforms for smart and sustainable technologies. This review addresses this gap by analyzing WO₃/MoO₃ thin-film nanocomposites through the concept of interfacial synergy. Particular attention is paid to the structural complementarity of WO₃ and α-MoO₃, oxygen nonstoichiometry, mixed W⁶⁺/W⁵⁺/W⁴⁺ and Mo⁶⁺/Mo⁵⁺/Mo⁴⁺ valence states, crystallographic-shear suboxides, W–O–Mo interfaces, and fabrication routes for mixed, graded, and multilayer films. The functional advantages of these systems do not arise from simply combining the two oxides. Instead, they result from charge and oxygen-vacancy redistribution, shortened ion–electron transport pathways, phase stabilization, and the formation of new active sites at interface boundaries. The review also emphasizes the need to distinguish genuine interfacial synergy from apparent improvements caused by surface area, film thickness, porosity, hydration, or measurement conditions. Finally, the review links structural features, defect chemistry, and interface-controlled properties to device-level functionality. This framework highlights promising directions for WO₃/MoO₃ nanocomposite films in a wide range of smart and sustainable technologies.

1. Introduction

1.1. Structural Complementarity of WO₃ and α-MoO₃

Thin films of transition metal oxides (TMOs) occupy a central position in modern functional electronics [1], electrochemistry [2], photonics [3], sensing [4], and energy-saving technologies [5]. Their importance stems from high chemical and thermal stability, potential radiation resistance, compatibility with low-temperature deposition on transparent and flexible substrates, widely tunable optical properties, and the ability to reversibly change cation valence states [6,7]. Unlike many narrowly functional materials, oxide films can simultaneously serve as an active optical layer [8], an ion-electron conductor [9], a surface catalyst [10], a buffer interlayer, or an electrode [11]. Consequently, developing thin-film oxide systems increasingly involves designing composite and heterostructured materials with controlled synthesis, defect structures, interface boundaries, and multilevel architectures rather than optimizing an individual oxide compound [12,13].

Tungsten trioxide (WO₃) and molybdenum trioxide (MoO₃) are redox-active Group VI TMOs widely employed in functional thin-film technologies (Figure 1a). Their W⁶⁺ and Mo⁶⁺ cations have a d⁰ electronic configuration [5,14,15,16]. Both oxides can form oxygen-deficient phases, WO_3−x and MoO_3−x (Figure 1b), in which oxygen vacancies and the associated mixed-valence states of the cations largely govern their electronic, optical, electrochemical, and catalytic properties [17,18,19]. Despite their similar local coordination—both crystal structures are built from MO₆ octahedra—their crystallographic frameworks differ fundamentally (Figure 1).

Figure 1. Crystallographic shear (CS) mechanism and phase families in WO₃ and MoO₃ systems: (a) crystal structures of WO₃ and α-MoO₃; (b) schematic representation of CS plane formation and the evolution of Magnéli phases.

WO₃ adopts a three-dimensional framework structure derived from the ReO₃-type lattice, whereas α-MoO₃ possesses a layered orthorhombic structure (space group Pbnm/Pnma) with van der Waals gaps separating the bilayers [5,19,20,21]. Consequently, MoO₃ exhibits pronounced anisotropy in ion transport and surface reactivity. However, it also suffers from intrinsically low electronic conductivity and a tendency toward structural rearrangement during deep ion intercalation [22,23,24].

The structural differences between WO₃ and MoO₃ play a pivotal role in determining ion transport, mechanical stability, cycling stability, and the overall functional performance of thin-film devices. The formation of oxygen vacancies and localized W⁵⁺/W⁴⁺ and Mo⁵⁺/Mo⁴⁺ states is particularly important. Changes in the coordination and connectivity of MO₆ octahedra reinforce these effects. Together, these factors strongly influence electronic transport, optical absorption, surface chemical reactivity, and the intercalation of small cations such as H⁺, Li⁺, Na⁺, and K⁺ [24,25]. These structural and defect-related characteristics are particularly important for controlling broadband electrochromic modulation and for designing composite electrochromic films based on WO₃/MoO₃ heterostructures [19,22]. These combined structural and redox features make WO₃ and MoO₃ attractive for diverse functional applications, including electrochromic devices [22], gas sensors [26], lithium-ion batteries [27], adsorption technologies [28], photocatalysis [29], and other related technologies [20,30]. Consequently, WO₃ and MoO₃ provide complementary material platforms for the development of advanced ion-intercalation-based functional thin-film systems.

1.2. Beyond Single-Phase Thin Films: Interface-Engineered WO₃/MoO₃ Nanocomposites—Motivation and Scope of the Review

Despite these advantages, single-phase WO₃ and MoO₃ thin films still suffer from fundamental limitations [31]. For WO₃, typical challenges include the trade-off among high optical modulation, fast electrochromic switching, and cycling stability, along with the need to balance electronic conductivity and ion diffusion. For MoO₃, the main limitations are low electronic conductivity, anisotropic ion transport, sensitivity to hydration, and pronounced volume expansion during deep Li⁺ intercalation [32,33]. In photocatalysis and photoelectrochemistry, both oxides are limited by charge-carrier recombination and a narrow spectral response. In sensing applications, high sensitivity is often accompanied by insufficient selectivity, humidity interference, and slow recovery of the initial signal [19,20,21,22]. Thus, simply improving the morphology of a single-phase oxide does not necessarily produce a qualitative breakthrough in functional performance.

Consequently, increasing attention has shifted from single-phase WO₃ and MoO₃ thin films toward compositionally and interface-engineered WO₃/MoO₃ systems, including mixed oxides, nanocomposite coatings, compositionally graded films, heterostructures, and nanolaminated hybrid architectures [34]. Their superior performance arises not merely from the additive properties of the two constituent oxides but from interfacial synergy. This synergy involves charge redistribution across the W–O–Mo interface, modulation of oxygen-vacancy concentration, formation of mixed-valence W⁶⁺/W⁵⁺ and Mo⁶⁺/Mo⁵⁺ states, reorganization of H⁺/Li⁺ intercalation pathways, and modification of optical absorption [35]. Within these architectures, WO₃ can serve as a relatively stable electrochromic framework, whereas MoO₃ provides layered ion-transport channels, a high work function, surface redox activity, and additional adsorption sites [36,37]. However, improved functional performance requires precise control of the composition, phase state, and robust interfacial architecture of the WO₃/MoO₃ system [24,34,35].

However, WO₃/MoO₃ systems remain inherently complex to interpret because the nominal W:Mo ratio does not necessarily reflect the actual active phase composition. Hydration, annealing temperature, oxygen partial pressure, substrate characteristics, and film thickness can all shift the equilibrium among amorphous, crystalline, hydrated, substoichiometric, and mixed-phase structures. For WO₃, oxygen-deficient suboxides and Magnéli phases, including W₁₈O₄₉, W₂₀O₅₈, W₂₅O₇₃, and related CS phases of the WO_3−x system, are particularly important. Their CS planes generate new electronic states [38]. In the case of MoO₃, suboxides such as Mo₄O₁₁, Mo₈O₂₃, Mo₉O₂₆, and other CS phases also play significant roles. However, their thermodynamic stability and Li⁺ intercalation behavior differ substantially from those of the tungsten-based phases [31]. Therefore, conclusions regarding interfacial synergy should be based not only on the evaluation of functional performance but also on direct characterization of the phase composition, oxidation states, and interfacial structure.

This review is motivated by the recognition that further improvements in WO₃ and MoO₃ thin films can no longer be achieved merely by optimizing the individual single-phase oxides. For electrochromic devices, it is essential to simultaneously achieve high coloration efficiency (CE), fast switching kinetics, and excellent cycling stability. In gas sensors, the respective contributions of oxygen vacancies, humidity, catalytic active sites, and phase composition must be distinguished. For energy-storage applications, the low electronic conductivity and mechanical degradation of MoO₃ during deep Li⁺ intercalation must be overcome. In photocatalysis and photoelectrochemistry, efficient charge separation, controlled oxygen nonstoichiometry, and proper energy-band alignment are critical for maximizing performance [37]. These challenges require a transition from simply combining WO₃ and MoO₃ to rationally designing reproducible, interface-engineered WO₃/MoO₃ platforms. In such platforms, composition, oxygen-vacancy concentration, phase boundaries, and W–O–Mo interfacial contacts are interconnected parameters that govern functional performance.

Thus, the aim of this review is to establish a unified concept of interfacial synergy in WO₃/MoO₃ nanocomposite thin films and heterostructures as interface-engineered materials for smart windows, sensors, energy-storage devices, organic electronics, and photoactive systems. To achieve this aim, the article:

• Compares the structural and electronic features of WO₃ and α-MoO₃.
• Examines the role of oxygen vacancies, mixed-valence states, and CS-suboxides.
• Summarizes methods for fabricating mixed, compositionally graded, and multilayer WO₃/MoO₃ thin films.
• Analyzes the functional applications of WO₃/MoO₃ nanocomposite thin films.
• Identifies the limitations that hinder the transition from laboratory-scale demonstrations to reproducible devices.

2. Structural, Electronic, Defect, and Interfacial Fundamentals of WO₃/MoO₃ Nanocomposite Thin Films

This section discusses the physicochemical basis of interfacial synergy in WO₃/MoO₃ nanocomposite thin films. The main emphasis is placed on three interconnected levels: the structural complementarity of WO₃ and α-MoO₃; the defect chemistry of oxygen-deficient WO_3−x/MoO_3−x phases; and the role of W–O–Mo interfaces in coupled ion–electron transport. This approach allows WO₃/MoO₃ to be considered not as a mechanical mixture of two oxides but as a defect- and interface-engineered thin-film architecture.

2.1. Crystalline Architecture and Structural Complementarity of WO₃ and α-MoO₃

WO₃ and MoO₃ are closely related in terms of their chemical nature and redox activity, yet they represent distinct types of oxide frameworks [39]. WO₃ adopts a distorted ReO₃-type three-dimensional framework composed of corner-sharing WO₆ octahedra. This framework readily accommodates tilting, displacement, and local rearrangement of the octahedra during the intercalation of small cations. Consequently, the properties of WO₃ thin films are highly sensitive to crystallinity, the degree of hydration, oxygen nonstoichiometry, porosity, and annealing conditions. Each of these factors directly influences the optical response, electrochromic switching kinetics, and cycling stability [40].

In contrast to WO₃, α-MoO₃ possesses a layered orthorhombic crystal structure in which distorted MoO₆ octahedra are arranged into bilayers separated by van der Waals gaps. The layered nature of α-MoO₃ is clearly reflected in the morphology of its microcrystals and nanoplates. Figure 2 presents SEM images, reported by Martín-Ramos et al., of α-MoO₃ with a multilayered plate-like architecture [41]. These micrographs clearly illustrate why α-MoO₃ is widely regarded as a source of anisotropic ion-transport pathways and highly surface-active crystallographic planes.

Figure 2. (a–c) SEM micrographs of α-MoO₃ crystals exhibiting a multilayered stacked morphology acquired at magnifications of ×20,000, ×50,000, and ×120,000, respectively; (d) SEM image of MoO₃ crystals obtained by calcination of commercial molybdic acid (MoO₃·H₂O); (e) high-magnification SEM image showing the layered stacking of an α-MoO₃ crystal consisting of 44 individual layers. Reproduced from [41] (MDPI, CC BY 4.0).

This architecture increases the sensitivity of MoO₃ to hydration, reduction, phase transformations, and mechanical degradation during deep Li⁺ insertion. In this respect, α-MoO₃ can be compared with graphite, the classical layered anode material for lithium-ion batteries (LIBs). In graphite, lithiation mainly proceeds by reversible intercalation between graphene layers to form Li_x C₆, giving a theoretical intercalation capacity of approximately 372 mAh g⁻¹. MoO₃ must be treated differently because two electrochemical regimes are possible. Limited lithiation may involve comparatively reversible insertion and pseudocapacitive redox within the oxide framework, whereas deeper lithiation leads to conversion chemistry. The frequently quoted value of approximately 1117 mAh g⁻¹ corresponds to the complete six-electron reaction:

MoO₃ + 6Li⁺ + 6e⁻ → Mo⁰ + 3Li₂O (Mo⁶⁺ → Mo⁰)

(1)

It is therefore the theoretical limit of deep conversion, not a reversible electrochromic or intercalation capacity. Approaching this limit requires extensive Mo–O bond breaking, phase reconstruction, and large volume changes; therefore, the 1117 and 372 mAh g⁻¹ values should not be compared as equivalent reversible intercalation capacities. Consequently, nanostructuring, carbon matrices, and interfacial stabilization are important for retaining the lower-depth reversible storage contribution and limiting conversion-induced mechanical damage [39,41,42,43,44,45,46].

The structural differences between the two oxides provide a fundamental design advantage for WO₃/MoO₃ nanocomposites. Within these WO₃/MoO₃ architectures, WO₃ can serve as a structurally robust three-dimensional electrochromic or pseudocapacitive framework. MoO₃, meanwhile, contributes layered ion-transport channels, a high work function, coordinatively unsaturated surface Mo–O_x sites, and additional Mo⁶⁺/Mo⁵⁺ redox couples capable of participating in charge transfer, adsorption, and catalytic reactions. Consequently, the rational design of WO₃/MoO₃ nanocomposites requires consideration of the nominal W:Mo ratio and phase connectivity. It must also account for the spatial distribution of the MoO₃ phase, the continuity of coupled ion–electron transport pathways, and the structural stability of W–O–Mo heterointerfaces during prolonged electrochemical cycling. Figure 3 illustrates how the structural asymmetry of WO₃ and α-MoO₃ can be exploited to construct WO₃/MoO₃ heterointerfaces. The three-dimensional WO₃ framework provides structural robustness and distributed intercalation sites, whereas the layered α-MoO₃ matrix creates anisotropic ion-transport pathways together with a highly surface-active architecture.

Figure 3. Structural complementarity of WO₃ and α-MoO₃ as a basis for WO₃/MoO₃ heterointerface formation and interfacial synergy.

Microscopic analysis of thin films confirms that the structural architecture of an oxide is directly related to its functional properties. Deposition and annealing conditions affect grain structure, thickness, crack formation, porosity, and the accessibility of active surface sites [42,43]. The relationship between fabrication method, composition, and morphology is illustrated in Figure 4 using data reported by Lábadi et al. [14].

Figure 4. Photograph and SEM micrograph of a compositionally graded WO₃/MoO₃ thin-film structure: (a) photograph of a 30 × 30 cm² substrate coated with compositionally graded WO₃/MoO₃ layers; the interference fringes reflect spatial variations in film thickness and composition; (b) SEM micrograph of a 4 × 4 μm² area recorded from the central region of a 10 cm long Si substrate. Reproduced from [14] (MDPI, CC BY 4.0).

The compositionally graded WO₃/MoO₃ film deposited on a large-area substrate exhibits pronounced interference fringes (Figure 4a), reflecting spatial variations in coating thickness and composition. The corresponding SEM surface image shows a dense and relatively uniform thin-film morphology (Figure 4b), which is characteristic of coatings formed by reactive magnetron sputtering (RMS).

For WO₃, the relationship among annealing temperature, film morphology, and electrochromic switching speed is fundamentally important. The switching process is governed by the intercalation and deintercalation of small cations and electrons. Figure 5 shows FE-SEM images of the surfaces and cross-sections of electrodeposited WO₃ films after annealing at 50, 250, and 450 °C. According to the micrographs, the transition from 50 to 250 °C is accompanied by grain coarsening and densification of the coating, whereas annealing at 450 °C leads to the formation of cracks and agglomerated regions. Such morphological changes may deteriorate the uniformity of Li⁺/H⁺ insertion, increase local mechanical stresses, and reduce the cycling stability of the electrochromic response [43].

Figure 5. FE-SEM surface and cross-sectional images of electrodeposited WO₃ thin films annealed at different temperatures: (a1) surface morphology after annealing at 50 °C; (a2) corresponding cross-sectional morphology after annealing at 50 °C; (b1) surface morphology after annealing at 250 °C; (b2) corresponding cross-sectional morphology after annealing at 250 °C; (c1) surface morphology after annealing at 450 °C; (c2) corresponding cross-sectional morphology after annealing at 450 °C. The images illustrate the temperature-dependent evolution of grain structure, film densification, thickness, and crack formation. Reproduced from [43] (MDPI, CC BY 4.0).

Table 1 summarizes the key structural features of WO₃ and α-MoO₃ and illustrates how the differences between the three-dimensional framework-type and layered oxide matrices can be exploited in the design of composite thin films. Comparison of these parameters shows that the functional value of WO₃/MoO₃ systems is not determined by the properties of either oxide alone. Instead, it depends on their controlled integration within a stable architecture that jointly defines phase connectivity, interfacial W–O–Mo contacts, and coupled ion–electron transport pathways.

Table 1. Structural features of WO₃ and α-MoO₃ governing the behavior of WO₃/MoO₃.

No	Parameter	WO3	α-MoO3	Design Implication for WO3/MoO3	Ref.
1	Oxide matrix	Distorted ReO3-type three-dimensional network of corner-sharing WO6 octahedra; a framework for reversible H+/Li+ insertion	Layered MoO6 bilayers separated by van der Waals gaps; anisotropic ion transport	Combining the three-dimensional stability of WO3 with the layered transport channels of MoO3 increases functional flexibility	[39,40,41]
2	Morphology	Grain size, porosity, thickness, and crack formation depend strongly on annealing and deposition conditions	Plate-like and multilayered morphology increases the surface area and accessibility of interlayer regions	Morphology should enhance transport while avoiding uncontrolled degradation, cracking, and mechanical instability	[41,43]
3	Defect sensitivity	Oxygen vacancies and W5+/W4+ states modulate electronic conductivity and the electrochromic response	Oxygen vacancies and Mo5+/Mo4+ states strongly affect conductivity, work function, and structural stability	Defect density should be controlled rather than maximized, because excessive nonstoichiometry may reduce stability	[39,47,48]
4	Intercalation behavior	More distributed three-dimensional intercalation of small cations within the WO3 framework	Directional Li+ intercalation along the layers; risk of lattice expansion during deep lithiation	MoO3 is preferably introduced as a thin layer, nanodomain, or interfacial modifier rather than as a thick unstable phase	[45]
5	Electronic profile	WO3 generally exhibits n-type semiconducting behavior under oxygen-deficient conditions	The high work function of MoO3 is associated with its layered dipolar structure and surface electronic states	The W–O–Mo interface can tune charge transfer, contact selectivity, and coupled ion–electron transport	[44,47]

2.2. Oxygen Vacancies, Mixed-Valence States, and Crystallographic-Shear Suboxides

Oxygen vacancies are one of the key tools for tuning the properties of WO₃, MoO₃, and their nanocomposite heterostructures. Their formation disrupts the local coordination of MO₆ octahedra, alters the electron density at the cations, and generates the mixed-valence states W⁶⁺/W⁵⁺/W⁴⁺ or Mo⁶⁺/Mo⁵⁺/Mo⁴⁺. In WO₃, vacancies can act as donor defects, enhance n-type conductivity, and influence the electrochromic response [47]. In MoO₃, oxygen vacancies can increase conductivity and surface activity, but in excess they initiate hydration, phase drift, and reduction to lower oxide states [48].

XPS provides one of the most direct methods for investigating valence states and oxygen nonstoichiometry. For WO₃, the W 4f and O 1s regions allow the surface chemical state to be evaluated by tracking the relative contributions of stoichiometric W⁶⁺ centers, reduced W⁵⁺/W⁴⁺ states, and oxygen-containing surface components. In particular, deconvolution of the W 4f spectrum reveals the degree of tungsten reduction and the possible formation of oxygen vacancies. The O 1s region distinguishes lattice oxygen, hydroxyl groups, and adsorbed oxygen-containing species. Figure 6 presents a representative XPS analysis of a WO₃ film, including the survey spectrum as well as detailed spectra of the W 4f and O 1s regions [43].

Figure 6. XPS spectra of the electrodeposited WO₃ thin film: (a) survey spectrum; (b) high-resolution W 4f core-level spectrum showing the W 4f₇/₂ (35.16 eV) and W 4f₅/₂ (37.41 eV) components; (c) high-resolution O 1s core-level spectrum containing the lattice-oxygen component at 530.68 eV and the higher-binding-energy OH⁻ component at 532.35 eV. In the high-resolution spectra, the experimental signal is shown together with the overall fitted envelope and the individual deconvoluted peak components used to identify the corresponding chemical states. Reproduced from [43] (MDPI, CC BY 4.0).

Figure 7 compares the Mo 3d and O 1s XPS spectra of untreated and thermally annealed sputtered MoO₃ films. The Mo 3d envelope is dominated by the Mo⁶⁺ spin–orbit doublet, confirming MoO₃ as the principal surface phase. A minor Mo⁵⁺ contribution is present on the lower-binding-energy side and becomes slightly more pronounced after annealing at 300 °C, indicating increased oxygen deficiency. The narrowing of the Mo⁶⁺ peaks with increasing annealing temperature reflects a more uniform local chemical environment around molybdenum. In the O 1s region, the main component at approximately 530.4–530.6 eV is assigned to lattice oxygen, whereas the higher-binding-energy contribution is associated mainly with surface hydroxyl and contamination-related oxygen species.

Figure 7. High-resolution XPS spectra of sputtered MoO₃ thin films: (A) Mo 3d and (B) O 1s regions for the untreated film (a) and films annealed at 200 °C (b) and 300 °C (c). Reproduced from [49] (MDPI, CC BY 4.0).

Upon deeper reduction in WO_3−x and MoO_3−x, the defect structure evolves from dilute, statistically distributed oxygen vacancies toward more ordered CS suboxide phases. In such structures, oxygen deficiency is accommodated not merely as isolated point defects but through shear-plane formation. These planes alter the local connectivity of MO₆ octahedra, creating edge-sharing regions in addition to corner-sharing linkages. Figure 8 integrates the oxygen-deficiency-driven phase evolution and defect-engineering pathways in WO_3−x and MoO_3−x. In Figure 8, the arrows indicate the progression from stoichiometric oxides toward increasingly oxygen-deficient and reduced states. The color coding distinguishes the WO_3−x and MoO_3−x structural-evolution pathways and the principal defect states, while the vacancy, electron/polaron, and mixed-valence symbols represent oxygen vacancies, localized charge carriers, and reduced W/Mo centers, respectively. With increasing reduction, WO_3−x commonly forms relatively ordered crystallographic-shear suboxides, including W₁₈O₄₉, W₂₀O₅₈, W₂₅O₇₃, and W₅O₁₄, which can enhance electronic conductivity, near-infrared absorption, and electrochemical activity. In contrast, MoO_3−x exhibits a more complex phase landscape: Mo₈O₂₃ and Mo₉O₂₆ are associated with shear structures, whereas Mo₄O₁₁ and related suboxides contain mixed Mo coordination environments and are not strict CS phases. Thus, the common MO_3−x notation does not imply identical defect topologies or reduction mechanisms in the W–O and Mo–O systems. Oxygen vacancies, mixed-valence states, and small polarons may improve optical modulation, ion transport, surface reactivity, and charge conduction; however, excessive reduction can promote hydration, phase drift, irreversible ion trapping, and structural degradation.

Figure 8. Oxygen-deficiency-driven structural evolution and defect-engineering pathways in WO_3−x and MoO_3−x systems, illustrating the formation of mixed-valence states, small polarons, crystallographic-shear or mixed-coordination suboxides, their functional contributions, and the associated stability risks.

The different phase-evolution pathways of oxygen-deficient WO_3−x and MoO_3−x systems have important consequences for the long-term stability of WO₃/MoO₃-based devices operating under repeated ion insertion/extraction or prolonged illumination conditions. Because tungsten- and molybdenum-based suboxides accommodate oxygen deficiency through different structural mechanisms, their responses to continuous redox cycling are not equivalent. WO_x systems generally evolve through relatively ordered crystallographic-shear structures, which can preserve extended lattice connectivity and provide enhanced electronic transport. In contrast, MoO_x exhibits a broader range of reduced phases, including mixed-coordination suboxides, which may induce stronger local structural rearrangement, lattice distortion, and phase instability during prolonged operation.

Therefore, the stability of WO₃/MoO₃ heterostructures is architecture-dependent and determined by the balance between defect activation and structural preservation. Under repeated H⁺/Li⁺ intercalation or prolonged photoexcitation, differences in oxygen-vacancy mobility, reduction pathways, and local lattice response can lead to asymmetric degradation mechanisms, including chemical strain accumulation, interlayer sliding, trapped charge formation, and interfacial stress development. Recent studies have demonstrated that coherent interfacial constraint can substantially suppress structural deformation. For example, coherent pinning reduced the b-axis expansion of α-MoO₃ from approximately 16% to approximately 2%. This result highlights the importance of mechanical coupling at heterointerfaces [45]. Conversely, uncontrolled reduction and vacancy redistribution may accelerate phase drift and loss of functional reversibility [50].

Consequently, the long-term reliability of WO₃/MoO₃ devices should not be evaluated solely from the initial electrochemical, optical, or photocatalytic response. A comprehensive stability assessment requires monitoring phase retention, oxidation-state distribution, oxygen-vacancy evolution, impedance characteristics, and optical/electrochemical recovery after prolonged operation. Since WO_3−x and MoO_3−x follow inherently different phase-evolution pathways, degradation processes in WO₃/MoO₃ heterostructures are expected to be spatially and chemically asymmetric. Thus, stable device architectures should be designed not only to maximize interfacial activity but also to maintain phase compatibility, controlled defect mobility, and reversible W–O–Mo interfacial coupling during long-term operation.

For oxygen-deficient MoO_3−δ thin films, δ denotes a slight deficiency of lattice oxygen and the associated oxygen-vacancy concentration. Variations in oxygen nonstoichiometry, molybdenum oxidation state, and the fraction of suboxide components directly affect electrode behavior in lithium microbatteries [51,52]. In oxygen-deficient MoO_3−x, oxygen vacancies can enhance the pseudocapacitive contribution to charge storage by increasing the density of redox-active sites and facilitating coupled ion–electron transport [53]. Therefore, increased conductivity, changes in optical coloration, or improved capacity in WO₃/MoO₃ thin films should not automatically be interpreted as direct evidence of interfacial synergy. Such effects may originate not only from the formation of W–O–Mo interfaces, but also from changes in porosity, film thickness, hydration degree, oxygen-vacancy concentration, or the formation of suboxide phases.

To correctly distinguish these contributions, studies of WO₃/MoO₃ heterostructures should rely on comprehensive phase, valence-state, morphological, and electrochemical characterization, including XRD, Raman spectroscopy, XPS, XANES/EXAFS, EPR, TEM/EELS, and electrochemical impedance spectroscopy. Figure 8 summarizes the sequential evolution of oxide defect structures from stoichiometric MO₃ to oxygen vacancies, mixed-valence states, small polarons, and CS suboxides. The lower part of the scheme highlights the dual role of defects. Moderate defect engineering can improve optical modulation, ion insertion, coupled ion–electron transport, and surface reactivity. Excessive reduction, however, increases the risk of irreversible phase degradation, mechanical instability, loss of optical transparency, and deterioration of long-term cycling durability. Thus, defect engineering in WO₃/MoO₃ nanocomposite thin films should be regarded as a controlled balance between functional activation and structural destabilization, rather than as a simple route toward maximizing oxygen-vacancy concentration.

Table 2 summarizes the key defect states governing the behavior of WO₃- and MoO₃-based oxide systems, including oxygen vacancies (V_O), crystallographic-shear suboxides, and W–O–Mo interfacial contacts.

Table 2. Defect states in WO₃- and MoO₃-based oxide systems.

No	Defect State	Typical Manifestation	Functional Contribution	Associated Risks	Characterization
1	Oxygen vacancies (VO)	Reduced W5+/W4+ or Mo5+/Mo4+ centers; local lattice relaxation	Higher electronic conductivity; stronger visible/NIR absorption; enhanced pseudocapacitive response	Excessive reduction; sensitivity to hydration; drift of optical and electrochemical properties	XPS; Raman spectroscopy; EPR; UV–Vis–NIR spectroscopy; EIS
2	Shear/suboxide phases	WO3−x and MoO3−x suboxides; CS structures; Magnéli-type phases	Enhanced conductivity; modified redox behavior; altered electrochemical response	Irreversible phase transitions; poor reproducibility; reduced optical transparency	XRD; Raman spectroscopy; HRTEM; XPS; XANES/EXAFS
3	W–O–Mo interfacial contacts	Heterointerfaces in mixed, compositionally graded, and multilayer films; charge and vacancy redistribution	Interfacial charge transfer; coupled ion–electron transport; new active sites; possible interfacial synergy	Difficult attribution of effects; metastable interfaces; dependence on composition and phase connectivity	STEM-EDS/EELS; TEM; XPS depth profiling; EIS; electrochemical testing

Table 2 highlights the dual role of defect states in WO₃/MoO₃ systems. Oxygen vacancies, mixed-valence centers, and crystallographic-shear suboxides can enhance electronic conductivity, optical absorption, ion insertion, and pseudocapacitive charge storage. However, excessive defect formation may also cause phase instability, loss of transparency, poor cycling durability, and reduced reproducibility. W–O–Mo interfaces are therefore especially important because they can promote charge redistribution, oxygen-vacancy redistribution, and coupled ion–electron transport. Nevertheless, only complementary structural, valence-state, morphological, and electrochemical characterization can support an interpretation of these effects as true interfacial synergy. Relevant methods include XRD, Raman spectroscopy, XPS, XANES/EXAFS, EPR, TEM/EELS, and EIS [14,47,48,49,51,52,53].

2.3. W–O–Mo Interfaces: Composition, Charge Transport, and Interfacial Synergy

In WO₃/MoO₃ heterostructures, the heterointerface acts as an active functional region rather than a passive physical contact [14,54,55]. The intimate coupling between the two oxides promotes equilibration of their electrochemical potentials, which can induce interfacial charge redistribution, local band bending, modulation of oxygen-vacancy concentration, and the emergence of an internal electric field. In photocatalysis, such interfacial effects are crucial for promoting electron–hole separation. In electrochromic devices, they can govern the kinetics of coupled electron–cation injection. In gas sensors, they can modulate the depletion-layer width and the density of chemisorbed oxygen species.

Compositionally graded WO₃/MoO₃ thin films are of particular interest for analyzing W–O–Mo interfaces, because they allow the W/Mo ratio, coating thickness, and degree of interfacial interaction to be systematically varied within a single substrate. This architecture makes it possible to directly correlate the local film composition with its optical or electrochemical response and, therefore, to distinguish composition-driven effects from those associated with thickness, morphology, phase connectivity, and defect distribution. Figure 9a compares the Mo content determined by two independent methods: spectroscopic ellipsometry (SE) and Rutherford backscattering spectrometry (RBS). Such cross-validation is essential for reliable analysis of charge transport, interfacial synergy, and functional response. Conclusions should be based on the measured spatial distribution of Mo within the film rather than on the nominal composition of the target or precursor mixture [14].

Figure 9. Compositionally graded WO₃/MoO₃ thin films: validation of Mo-content distribution and its effect on coloration efficiency: (a) verification of the Mo content in a compositionally graded WO₃/MoO₃ thin film using spectroscopic ellipsometry (SE) and Rutherford backscattering spectrometry (RBS); (b) coloration efficiency (CE) as a function of MoO₃ atomic fraction in mixed WO₃/MoO₃ thin films with different thicknesses. Reproduced from [14] (MDPI, CC BY 4.0).

In this sense, compositionally graded WO₃/MoO₃ films provide a powerful model platform for separating genuine interfacial effects from apparent performance changes caused by variations in film thickness, morphology, or phase composition.

The coloration efficiency of WO₃/MoO₃ thin films further demonstrates that the maximum functional response is not necessarily achieved at the limiting compositions corresponding to pure WO₃ or pure MoO₃. Figure 9b shows the dependence of coloration efficiency on the MoO₃ content in the film at different wavelengths in the 400–800 nm range. The most pronounced maximum is observed at an intermediate composition, close to approximately 60 at.% MoO₃, indicating the existence of an optimal W/Mo ratio for the electrochromic response. This maximum cannot be explained by a simple additive combination of the individual properties of the two oxides. It most likely reflects the combined effects of local composition, film thickness, phase state, oxygen-defect concentration, and W–O–Mo interface formation. Therefore, composition is more than a chemical parameter in the analysis of WO₃/MoO₃ systems. It is also a key factor governing interfacial connectivity, coupled ion–electron transport, and the efficiency of optical modulation [14].

In WO₃/MoO₃ heterostructures, the interfacial region is important not only as a zone of charge redistribution, but also as a domain where coupled ion–electron transport pathways can be formed. For electrochromic and pseudocapacitive thin films, the functional response depends on electronic-level alignment and the W/Mo oxidation states. It also depends on the accessibility of reversible insertion and extraction channels for small cations such as H⁺, Li⁺, Na⁺, and K⁺. In this regard, WO₃ can be regarded as a relatively stable three-dimensional framework for reversible ion insertion, whereas MoO₃ is more effectively introduced as a thin layer, nanodomain, compositionally graded region, or interfacial modifier. This design strategy exploits the layered nature of MoO₃ and additional Mo⁶⁺/Mo⁵⁺ redox centers while limiting deep irreversible intercalation and the associated structural deformation. A similar principle is realized in WO₃/MoO₃ nanohybrid electrodes, where short diffusion lengths, a large interfacial area, and shortened charge-transfer pathways contribute to an improved pseudocapacitive response [56]. The functional advantage of WO₃/MoO₃ architectures therefore does not arise from combining two redox-active oxides alone. It depends on controlling the spatial distribution of MoO₃ to enhance interfacial charge transfer and reversible ion transport while avoiding deep structural degradation.

However, the interpretation of transport effects in WO₃/MoO₃ composites requires caution, because pronounced changes in the electrochemical response may also arise in other WO₃-based heterostructures. Figure 10 shows scan-rate-dependent cyclic voltammograms of WTi-1, WTi-5, and WTi-10 WO₃/TiO₂ heterostructured thin films [57]. The increase in current density with increasing scan rate indicates active ion–electron transport and redox participation within the films. Among the presented samples, WTi-5 exhibits the strongest current response and the largest enclosed CV area, suggesting a more favorable balance between electroactive WO₃ domains, TiO₂ modification, morphology, and charge-transport pathways. In contrast, the lower response of WTi-10 indicates that excessive modification may reduce electrochemical accessibility or hinder charge transport. These results illustrate that variations in CV shape, current density, and apparent capacitance can originate from film architecture, grain structure, porosity, thickness, phase composition, and defect state, rather than from interfacial synergy alone.

Figure 10. Scan-rate-dependent cyclic voltammograms of WO₃/TiO₂ heterostructured thin films: (a) WTi-1, (b) WTi-5, and (c) WTi-10 recorded at scan rates from 10 to 100 mV s⁻¹ in the potential range from −1.0 to +1.0 V vs. Ag/AgCl. Reproduced from [57] (MDPI, CC BY 4.0).

When analyzing WO₃/MoO₃ systems, an increase in current or capacitance should be considered evidence of improved coupled ion–electron transport only after electrochemical data have been correlated with morphological, phase, and valence-state characterization. Enhanced electrochemical response in WO₃/MoO₃ architectures should not be assigned to interfacial synergy solely from CV behavior. Independent structural and chemical evidence must confirm the formation and functional role of W–O–Mo interfaces [57].

Figure 11 presents a qualitative scheme of band alignment and transport processes at the W–O–Mo interface in WO₃/MoO₃ heterostructures. The band alignment shown in Figure 11 should be regarded as preparation-specific rather than universal. In particular, the electronic levels of MoO₃ depend strongly on the crystallographic polymorph, crystallographic orientation, oxygen stoichiometry, hydration state, and interfacial termination. Combined VB-XPS and DFT analyses showed that the valence-band edge of α-MoO₃ is approximately 0.9 and 1.0 eV higher, respectively, than that of β-MoO₃, whereas the conduction-band edges of these two phases remain close to −2.5 eV [58]. Phase-dependent band structures and work functions have also been demonstrated for polymorphic molybdenum oxides [59]. Metastable h-MoO₃ possesses a structurally distinct framework, and its band-edge positions therefore cannot be assigned from those of α- or β-MoO₃ without direct characterization [60]. Moreover, oxygen vacancies generate Mo 4d-derived polaronic states and progressively transform the electronic and structural properties of MoO_3−x [61,62]. For one specifically prepared monoclinic WO₃/defect-rich MoO_3−x heterostructure, Mott–Schottky and optical analyses yielded CBE/VBE values of 0.43/3.08 V versus RHE for WO₃ and 0.15/3.05 V versus RHE for MoO_3−x. These values correspond to approximate CBO and VBO magnitudes of 0.28 and 0.03 eV, respectively, and support type-II alignment only for that particular architecture [63]. Accordingly, reliable band-offset assignment requires phase- and stoichiometry-resolved UPS/XPS measurements or geometry-matched first-principles interface calculations.

Figure 11. Preparation-dependent electronic structure and coupled ion–electron transport in WO₃/MoO₃ heterostructures: (a) qualitative band-alignment schemes for WO₃ interfaced with α-MoO₃, β-MoO₃, h-MoO₃, and oxygen-deficient MoO_3−x, illustrating the dependence of the conduction- and valence-band offsets on MoO₃ polymorph and oxygen stoichiometry; the experimentally derived CBO ≈ 0.28 eV and VBO ≈ 0.03 eV for monoclinic WO₃/defect-rich MoO_3−x are shown only as a preparation-specific example [63]; (b) schematic representation of coupled ion transport and oxygen-vacancy/charge redistribution across WO₃-rich domains, the W–O–Mo interfacial region, and MoO₃-rich domains.

When favorable offsets are realized, interfacial charge redistribution and local band bending can assist carrier separation. At the same time, the W–O–Mo interfacial region may undergo oxygen-vacancy redistribution, thereby modifying the local defect structure and facilitating coupled ion–electron transport pathways. In this architecture, WO₃-rich domains provide predominantly isotropic pathways for the insertion of small cations. By contrast, anisotropic ion transport is most characteristic of α-MoO₃-rich layered domains, whereas transport in β-MoO₃, h-MoO₃, or MoO_3−x—containing regions may differ because of their distinct structures and defect states. Thus, the functional behavior of WO₃/MoO₃ heterostructures does not arise from simple mixing of the two oxides. Instead, it requires coordinated control of band alignment, composition, oxygen-defect chemistry, phase connectivity, and the spatial organization of the W–O–Mo interface.

Thus, W–O–Mo interfaces in WO₃/MoO₃ heterostructures should be regarded as active regions in which composition, defect chemistry, oxidation states, and phase connectivity jointly determine the electronic structure and coupled ion–electron transport. However, band alignment in these systems cannot be reduced to a single universal type-I or type-II scheme. Instead, it depends strongly on oxygen nonstoichiometry, Mo content, hydration degree, phase state, layer thickness, substrate effects, and thermal-treatment conditions. Therefore, the specific interfacial scenario must be experimentally verified using XPS/UPS, Mott–Schottky analysis, photocurrent measurements, electrochemical impedance spectroscopy, and valence-state characterization [64].

Interfacial synergy in WO₃/MoO₃ systems is not an automatic consequence of mixing two oxides. Enhanced electrochromic efficiency, photocatalytic activity, or pseudocapacitive response may originate not only from W–O–Mo interfaces, but also from changes in porosity, thickness, phase composition, hydration, or oxygen-vacancy concentration. Therefore, true synergy can only be demonstrated by correlating composition, structure, W/Mo oxidation states, the presence of interfacial W–O–Mo contacts, and transport–functional response within the same series of samples. Consequently, high-performance WO₃/MoO₃ composite films should be designed as interface-engineered systems. Optimization should address the W/Mo ratio, phase connectivity, defect density, interfacial stability, and accessible ion–electron transport pathways. This conclusion leads directly to the next section. The synthesis method determines film thickness, morphology, porosity, phase composition, defect structure, and the quality of W–O–Mo interfaces.

2.4. Operational Criterion and Minimum Evidence for Interfacial Synergy

The rule-of-mixtures approach is commonly used to assess hybrid effects in composite materials [65,66,67,68]. On this basis, we introduce a first-order additive null model for evaluating the functional response of WO₃/MoO₃ nanocomposite thin films. For a composite containing a WO₃ fraction x, the expected additive response is defined as

$${\mathrm{P}}_{\mathrm{add}}{=\mathrm{xP}}_{{\mathrm{WO}}_3}+\left(1-\mathrm{x}\right){\mathrm{P}}_{{\mathrm{MoO}}_3}$$

(2)

where P_WO3 and P_MoO3 are the normalized responses of matched single-phase reference films. The non-additive contribution is then operationally defined as

$${\Delta \mathrm{P}}_{\mathrm{syn}}{=\mathrm{P}}_{\mathrm{obs}}-{\mathrm{P}}_{\mathrm{add}}$$

(3)

where P_obs is the experimentally observed response of the WO₃/MoO₃ composite or heterostructure. Equations (2) and (3) are used here as a first-order screening framework rather than as a universal mixing law. A positive ΔP_syn value exceeding the combined experimental uncertainty indicates positive non-additivity. However, its attribution to W–O–Mo interfacial synergy additionally requires correlation with an interface-sensitive parameter and the exclusion of differences in thickness, loading, porosity, roughness, hydration, phase composition, and oxygen nonstoichiometry.

A positive ΔP_syn value alone does not establish W–O–Mo interfacial synergy because an apparently non-additive response may also arise from differences in thickness, morphology, porosity, phase composition, hydration, or oxygen-vacancy concentration. Accordingly, four minimum conditions should be met before interfacial synergy is assigned. First, pure-WO₃, pure-MoO₃, physical-mixture, and heterostructure controls should be prepared using identical substrates, post-heat treatment parameters, and active-material loadings. Second, the functional response should be normalized by active mass, area, or film thickness, as appropriate. Third, phase composition, roughness, porosity, hydration, and oxygen nonstoichiometry should be quantitatively constrained. Finally, at least one interface-sensitive structural, chemical, electronic, or transport parameter should be measured, or the interfacial density should be systematically varied at fixed composition. Operando characterization substantially strengthens the mechanistic assignment but is not required to satisfy this minimum evidentiary standard. The complete evidence-based validation workflow is summarized in Figure 12.

Figure 12. Minimum evidence-based workflow for assigning interfacial synergy in WO₃/MoO₃ nanocomposite thin films and heterostructures. The arrows indicate the sequential validation pathway from matched reference samples and normalized functional measurements to structural/chemical interface verification and, finally, evidence-supported assignment of genuine W–O–Mo interfacial synergy.

3. Synthesis Strategies for WO₃/MoO₃ Nanocomposite Thin Films and Heterostructures

The preceding analysis of structural, defect-related, and interfacial factors shows that the synthesis method determines both the coating process and the final thin-film architecture. Depending on the fabrication route, a WO₃/MoO₃ film may exist as an atomically mixed oxide, a compositionally graded layer, a nanolaminate, an island-like heterostructure, a porous electrode, or a hybrid film incorporating a conductive matrix. These architectures differ in thickness, phase state, oxygen nonstoichiometry, W/Mo distribution, defect density, and the stability of W–O–Mo interfacial contacts. Therefore, in this section, fabrication methods are considered not merely as technological procedures, but as tools for controlling interfacial architecture, defect chemistry, and coupled ion–electron transport. This perspective makes it possible to distinguish genuine W–O–Mo interfacial synergy from performance improvements caused only by variations in thickness, porosity, secondary phases, or the degree of oxide reduction [14,15].

3.1. Fabrication Strategy as a Tool for Engineering the Interfacial Architecture

Tungsten and molybdenum oxides have been studied individually for several decades. Their nanocomposite combinations, however, have attracted particular attention only recently. The growing interest in these systems arises from the possibility of integrating the three-dimensional WO₃ framework with the layered α-MoO₃ matrix, thereby enabling simultaneous control over oxygen vacancies, coupled ion–electron transport, and photoinduced conductivity modulation within a single heterostructure. This combination of mechanisms has opened a new research direction involving MoO₃/WO₃ heterojunctions for ultralow-power optoelectronic synaptic elements and neuromorphic devices. Although this field should still be regarded as an emerging application area, recent studies on artificial synapses based on MoO₃/WO₃ heterostructures already demonstrate its feasibility [55]. For this review, these results show that the W–O–Mo interface extends beyond electrochromic, sensing, and photocatalytic applications. It can also enable devices whose functional response depends on controlled and reversible conductivity modulation.

For WO₃/MoO₃ systems, the choice of fabrication method determines not only the morphology, but also the actual nature of the active phase. The same nominal W:Mo composition may correspond to a dense amorphous mixed oxide, a phase-separated coating, a nanolaminate with sharp interfaces, a compositionally graded layer, or a porous electrode material. Therefore, each fabrication route should be evaluated against four key criteria. These criteria include precise control of the W:Mo ratio and oxygen stoichiometry, reproducibility of W–O–Mo interfaces, and compatibility with the target substrate. Relevant substrates include ITO/FTO, glass, silicon (Si), metals, and flexible polymer supports [15,40,43,64,69,70,71].

MoO₃ incorporation into WO₃ is meaningful only when it generates a measurable interfacial effect, including faster coupled ion–electron transport, higher CE, enhanced pseudocapacitive response, improved charge separation, or the emergence of selective adsorption sites. When performance enhancement is governed mainly by porosity, thickness, or surface-area effects, the system should be considered morphologically optimized, rather than genuinely synergistic. Accordingly, Table 3 treats WO₃/MoO₃ thin-film fabrication methods as strategies for engineering interfaces, phase composition, and defect chemistry rather than merely as separate deposition techniques.

Table 3. Systematization of fabrication methods for WO₃/MoO₃ composite thin films and their role in interfacial design.

Method	Typical Architecture	Parameters	Advantages	Limitations/Risks	Target Applications	Ref.
Sol–gel, spin coating/dip coating	Mixed oxides, Mo-doped WO3, porous coatings	Precursor composition, pH, viscosity, deposition speed, annealing	Low cost, large-area processing, compatibility with glass and ITO/FTO	Hydration, shrinkage, cracking, residual organic phases	Electrochromic coatings, photocatalysis, gas sensors	[16,35,72,73]
Spray coating/spray pyrolysis	Mesoporous coatings, graded layers, oxide mixtures	Substrate temperature, aerosol flow rate, droplet size	Scalability and simple thickness tuning	Inhomogeneity, roughness, complicated phase control	Sensing, photocatalysis, protective coatings	[74]
Hydrothermal/solvothermal synthesis	Nanosheets, nanorods, MoO3 ribbons, 3D structures	Temperature, pressure, mineralizer, growth time	Control over crystallinity and high surface area	Difficult transfer to uniform thin films	Photocatalysis, gas sensors, pseudocapacitive electrodes	[64,70]
Electrodeposition/anodization	Hydrated WO3/MoO3−x layers, Mo-modified WO3 photoanodes	Potential, current, pH, peroxo-complex concentration, deposition charge	Low-temperature processing, direct growth on conductive substrates	Limited to conductive substrates; sensitivity to pH and electrolyte composition	Electrochromics, flexible electrodes, supercapacitors	[57,64,65]
Reactive magnetron sputtering	Atomically mixed oxides, W:Mo gradients, multilayer coatings	Power applied to W/Mo targets, O2/Ar ratio, pressure, substrate position	Reproducibility, clean films, composition-property mapping	Vacuum processing, cost, risk of hidden phase inhomogeneity	Smart windows, organic electronics, sensors	[14,15,66]
ALD/CVD	Nanolaminates, ultrathin interfaces, conformal layers	Number of cycles, precursors, temperature, oxidant/plasma	Sub-nanometer control and high conformality	Slow growth, expensive precursors, narrow temperature window	Microelectronics, supercapacitors, buffer layers	[56,75,76,77]
PLD, e-beam evaporation, and thermal evaporation	Dense layers, model heterostructures, nonequilibrium phases	Beam/laser energy, O2 pressure, substrate temperature	High purity and possibility of stabilizing nonstoichiometry	Limited area, particulates/droplets in PLD, difficult scaling	Fundamental studies, model interfaces	[71]

Overall, no single method is universal. The optimal route must be selected according to the target architecture and application. To complement the method-by-method comparison in Table 3, Figure 13 presents a qualitative decision matrix for selecting fabrication routes for WO₃/MoO₃ thin films. The methods are compared using six practically relevant criteria: thickness control, large-area compatibility, processing cost, compatibility with low-temperature substrates, porosity and texture tunability, and interface cleanliness. The matrix highlights the trade-off between scalable, low-cost processing and precise interfacial control, supporting the development of hybrid fabrication strategies. The scores are comparative and should not be interpreted as universal quantitative rankings.

Figure 13. Qualitative decision matrix for selecting fabrication routes for WO₃/MoO₃ thin films. Fabrication-route decision matrix for WO₃/MoO₃ films (scores: 1–5; 5 = favorable).

A technically feasible approach is a two-stage, interface-selective process. First, the bulk WO₃/MoO₃ layer is deposited by scalable reactive co-sputtering, spray/slot-die coating, or electrodeposition. A limited number of spatial ALD/PEALD cycles then form conformal W–O–Mo junctions, followed by mild oxidative annealing. This confines the costly self-limiting process to the nanoscale interfacial region while preserving high throughput. Spray coating has enabled ultrathin W–Mo oxide films from aqueous nanoparticle inks [66], whereas ALD has produced wafer-scale WO₃–MoO₃ heterostructures below 10 nm [57]. Scale-up should be benchmarked using equal-loading controls, in-line thickness, composition, and sheet-resistance monitoring, process yield, and cost per unit area. Most importantly, improved performance should be attributed to true W–O–Mo interfacial synergy only when synthesis conditions, W/Mo distribution, oxygen stoichiometry, phase connectivity, defect structure, and functional response are correlated within the same sample series.

3.2. Vacuum Deposition Methods

3.2.1. Reactive Magnetron Sputtering

Reactive magnetron sputtering (RMS) is one of the most versatile and reproducible methods for establishing composition–structure–property relationships in the WO₃/MoO₃ system. When separate W and Mo targets are used together with a controlled Ar/O₂ plasma, a continuous composition gradient can be generated within a single substrate. This combinatorial strategy greatly accelerates the search for the optimal W:Mo ratio and enables direct correlation of local film thickness, composition, and optical response without the need to fabricate a large series of individual samples [14,15]. In RMS, the W:Mo ratio can be finely tuned by adjusting the power applied to each target. The oxygen partial pressure controls the oxidation state and oxygen stoichiometry of the deposited oxide, from substoichiometric WO_x/MoO_x phases to nearly stoichiometric WO₃/MoO₃. Compared with sol–gel processing or conventional CVD routes, this method provides higher reproducibility of composition and cleaner control over film architecture. This combinatorial approach is particularly important for WO₃/MoO₃ thin films. Their maximum functional response is often observed at intermediate compositions rather than at the limiting states corresponding to pure WO₃ or pure MoO₃. Nevertheless, the deposited-film composition must be independently verified, for example by SE/RBS analysis. Nominal target parameters do not always reproduce the actual spatial distribution of W and Mo within the coating.

Figure 14 presents a cross-sectional schematic of the RMS chamber used for WO₃/MoO₃ thin-film deposition. Two magnetron targets, W and Mo, are positioned inside the chamber and directed toward a glass substrate, with a target-to-target separation of 700 mm. The simultaneous supply of Ar and O₂, combined with vacuum pumping, creates a controlled reactive plasma environment for oxide-film growth. By independently adjusting the power applied to each target, this configuration allows the fabrication of mixed, compositionally graded, or multilayer WO₃/MoO₃ coatings with controlled W/Mo distribution and tunable interfacial architecture.

Figure 14. Cross-sectional schematic view of the sputtering chamber and W/Mo target arrangement used for combinatorial RMS of WO₃–MoO₃ thin films. The W and Mo targets are separated by 700 mm and positioned above a glass substrate, while Ar and O₂ gas inlets and vacuum pumping provide a controlled reactive atmosphere for oxide-film deposition. Reproduced from [15] (MDPI, CC BY 4.0).

3.2.2. Pulsed Laser Deposition (PLD), Electron-Beam Evaporation, and Thermal Evaporation

PLD, electron-beam evaporation, and thermal evaporation are primarily suitable for fabricating dense layers, model heterostructures, and nanolaminated architectures. Their main advantages are high deposition purity and the ability to stabilize nonequilibrium states. However, oxygen pressure and subsequent phase diagnostics are critical for WO₃/MoO₃ systems. Insufficient oxygen activity may lead to the formation of WO_3−x/MoO_3−x suboxides, whereas high-temperature annealing can promote crystallization, mechanical stress accumulation, and phase segregation [53,70,75,78]. Therefore, these methods are particularly useful when fundamental interfacial effects need to be studied in a well-controlled architecture. However, their application to scalable electrochromic coatings and sensing films is limited by deposition area, equipment cost, and the need for strict control of oxygen activity.

3.2.3. Atomic Layer Deposition (ALD) and Chemical Vapor Deposition (CVD)

ALD and CVD are particularly important when interface precision is more critical than deposition rate. ALD is based on self-limiting surface reactions and is therefore well suited for fabricating ultrathin WO₃/MoO₃ nanolaminates, coating porous frameworks, and forming conformal oxide layers on three-dimensional substrates. In contrast to atomically mixed oxides, where the functional response is governed by the volume distribution of W–O–Mo bonds, nanolaminate architectures are controlled by the density, sharpness, and stability of interfacial boundaries. Decreasing the nanolaminate period increases the number of W–O–Mo interfaces, but also imposes stricter requirements on adhesion, mechanical compatibility, and diffusion stability during subsequent thermal treatment. For MoO₃-based layers, precursor chemistry, deposition temperature, and plasma activation are especially important, as they determine whether amorphous MoO_x, oriented α-MoO₃, or partially reduced mixed phases are formed [54,75,76,79].

For WO₃/MoO₃ nanolaminates, precursor selection must be evaluated at the supercycle level rather than only for the individual oxides. A suitable precursor must be sufficiently volatile and thermally stable, exhibit self-limiting surface chemistry, and produce volatile reaction by-products; additionally, the Mo and W subcycles must share compatible substrate-temperature ranges, oxidants, plasma conditions, and purge requirements. Otherwise, precursor carry-over, unequal growth per cycle, ligand-derived impurities, interfacial etching, or phase-selective crystallization may convert an intended abrupt W–O–Mo boundary into a compositionally graded or chemically contaminated region. Table 4 summarizes representative precursor chemistries and the minimum controls required to assess their compatibility in WO₃/MoO₃ supercycles.

Table 4. Representative Mo- and W-oxide precursor chemistries and their implications for WO₃/MoO₃ nanolaminate supercycles.

Oxide/Precursor–Reactant Pair	Verified Deposition Behavior	Main Risk in a WO3/MoO3 Supercycle	Required Process and Interface Control	Ref.
MoO3: Mo(CO)6 + O3/H2O	Self-limiting MoO3 growth was reported at 152–172 °C. Above this narrow interval, thermal decomposition of the carbonyl precursor becomes increasingly important	The verified Mo window does not overlap the classical W(CO)6/O3 window. Carbonyl decomposition, delivery instability and ligand-derived carbon can broaden or chemically grade the nominal interface	Independent saturation and decomposition tests for each half-cycle; QCM or in situ ellipsometry; carbon-sensitive XPS; precursor-line temperature and dose-repeatability monitoring	[80]
MoO3/MoOx: MoO2(acac)2, Mo(OiPr)4 or bis(ethylbenzene)Mo + H2O (comparative screening)	Comparative QCM screening found useful linear growth only for Mo(OiPr)4 and bis(ethylbenzene)Mo; the latter showed a 135–150 °C window and ~0.08 Å cycle−1. MoO2(acac)2 did not yield a practical ALD process under the tested conditions	Low volatility and weak surface reactivity can require long dosing, produce very low growth per cycle, increase precursor carry-over and make the Mo subcycle highly sensitive to surface termination. Calcination may also alter a previously deposited WO3 layer	Report bubbler and delivery-line temperatures, dose and purge lengths, QCM saturation curves and post-calcination composition. Verify carbon and depth-dependent Mo loading rather than assuming β-diketonate transferability	[81]
MoO3/MoOx: MoO2(thd)2 + O3	Uniform and conformal growth was demonstrated at 200–275 °C. The as-deposited films can contain α-MoO3, β-MoO3 and substoichiometric MoOx in condition-dependent proportions	Although the temperature range is compatible with some W processes, phase mixing, surface roughness and partial precursor decomposition can obscure the chemical abruptness of ultrathin nanolaminates	Pulse/purge saturation, GPC and roughness mapping; XRD/Raman phase analysis; XPS or ERDA impurity quantification; cross-sectional STEM-EELS for intermixing	[82]
MoOx: (NtBu)2(NMe2)2Mo + O2 plasma	PEALD was demonstrated over 50–350 °C, generally yielding slightly oxygen-deficient MoOx. Precursor delivery and thermal history strongly affect nucleation, density and crystallization; boosted delivery can favor oriented α-MoO3 at high temperature	Plasma exposure, unequal radical recombination and precursor-delivery transients can alter oxygen deficiency and phase selection from one nanolaminate period to the next. Nitrogen/carbon residues and plasma damage must be excluded	Match plasma power, exposure geometry and purge time between Mo and W subcycles; map GPC, density and O/Mo ratio; analyze C/N by XPS or ERDA and phase by XRD/GIXRD	[83,84]
H:MoOx: MoCl5 + H2O	Low-temperature ALD below 150 °C has been used to deposit hydrogenated MoOx, with deposition temperature controlling hydrogenation, work function and electrical behavior	The halide chemistry introduces possible Cl retention and corrosive HCl-containing by-products, while incorporated H changes the electronic state. These effects may chemically modify the underlying WO3 and shift band alignment	Quantify Cl and H, work function and Mo valence; verify reactor and substrate compatibility; use extended purging and examine interfacial corrosion or etching by depth-resolved XPS/TEM	[85]
WO3: W(CO)6 + O3	A narrow WO3 ALD window of 195–205 °C with a saturated growth rate of approximately 0.21 Å cycle−1 was reported	The window lies above the Mo(CO)6 window; a nominal carbonyl–carbonyl supercycle therefore lacks a demonstrated common temperature window. Different decomposition and crystallization behavior can generate roughness and composition gradients	Demonstrate a genuine common window experimentally or deposit the oxides in spatially separated zones. Use QCM/FTIR saturation tests, carbon XPS and equal-thickness controls	[86]
WO3: (tBuN)2(Me2N)2W + O2 plasma	PEALD WO3 growth was demonstrated over 100–400 °C. The broad range overlaps the Mo imido-amido/O2-plasma process and is therefore attractive for paired supercycles	A nominally overlapping temperature range does not ensure matched surface chemistry: the Mo and W precursors can have different saturation doses, ligand removal, oxygen deficiency and crystallization thresholds	Calibrate both half-cycles on the same starting surfaces; match radical dose and substrate temperature; compare GPC and incubation; quantify W/Mo, O/M and C/N depth profiles	[87]
Matched Mo/W bis(tert-butylimido)-bis(N,N′-diisopropylacetamidinato) precursors + O3	Both oxides were deposited near 300 °C using chemically analogous precursors. W growth was saturative (~0.45 Å cycle−1), whereas Mo showed high growth (up to ~2 Å cycle−1) but incompletely saturative surface reactions and stronger phase sensitivity	Even chemically matched precursor families can generate unequal GPC, non-equivalent ligand removal and dissimilar crystallization, producing composition and roughness gradients across short-period nanolaminates	Determine saturation separately for each oxide and for cross-nucleation surfaces; normalize subcycle thickness, not merely cycle count; verify interface sharpness by XPS depth profiling, XRR and STEM-EDS/EELS	[88]

The comparison shows that no precursor pair can be assumed a priori to yield an abrupt and compositionally uniform WO₃/MoO₃ nanolaminate. The classical carbonyl processes have narrow and non-overlapping verified windows: 152–172 °C for Mo(CO)₆ and 195–205 °C for W(CO)₆. By contrast, β-diketonate and imido-amidinato routes may require heated delivery and can exhibit phase mixing or incomplete saturation. Imido-amido/O₂-plasma chemistries offer a broader nominal temperature overlap, but the Mo and W subcycles still require independent calibration because their ligand removal, oxygen deficiency, incubation, and crystallization behavior are not equivalent. Halide chemistry enables low-temperature H:MoO_x deposition but introduces additional concerns associated with chlorine-containing by-products, hydrogen incorporation, and possible interface corrosion. Consequently, reliable nanolaminate fabrication requires saturation testing on cross-nucleation surfaces, thickness-normalized rather than cycle-count-normalized subcycles, and direct verification of composition and interface abruptness by QCM or in situ ellipsometry, XRR, depth-resolved XPS, and cross-sectional STEM-EDS/EELS.

Hybrid architectures further expand the functionality of WO₃/MoO₃ systems beyond binary oxide films. The incorporation of reduced graphene oxide (rGO), carbon nanotubes (CNTs), or conductive nanoparticles can compensate for the intrinsically low electronic conductivity of MoO₃ and accelerate pseudocapacitive processes. In gasochromic, photocatalytic, and transparent-electrode systems, additional components such as Pd, Pt, Ag, TiO₂, ZnO, or carbon phases may also be introduced. However, their contribution must be clearly distinguished from genuine W–O–Mo interfacial synergy [89,90,91].

3.3. Solution-Based Fabrication Methods

Table 5 systematizes the main solution-based and electrochemical approaches used for the fabrication of WO₃/MoO₃ composite thin films. For each method, the key processing parameters, typical resulting architectures, and characteristic deposition-related challenges are identified. This comparison provides a practical basis for selecting an appropriate fabrication route depending on the required morphology, scalability, phase/interface control, and substrate-related limitations. In solution-based methods, parameters such as precursor chemistry, solvent composition, pH, viscosity, drying regime, and annealing temperature largely determine film uniformity, porosity, hydration degree, cracking tendency, and residual organic content. In electrochemical approaches, the applied potential or current density, electrolyte composition, pH, peroxo-complex stability, deposition charge, and substrate conductivity directly affect film thickness, oxygen deficiency, hydration state, and accessibility of ion-transport pathways. Accordingly, these methods are not merely low-cost alternatives to vacuum deposition. They are also flexible tools for controlling porosity, surface area, defect concentration, and electrochemically active interfaces in WO₃/MoO₃ thin-film architectures. The data summarized in Table 5 show that each fabrication method has a specific set of controllable parameters and associated risks. For example, sol–gel processing and spin coating enable molecular-level mixing of W and Mo components, but are highly sensitive to shrinkage, cracking, hydration, and residual organic species. Hydrothermal synthesis provides highly developed nanostructured surfaces but usually requires an additional transfer or fixation step, during which adhesion and film uniformity become critical. Electrochemical deposition allows film thickness to be precisely controlled through the deposition charge, but is limited to conductive substrates and is sensitive to the electrolyte composition, pH, and molybdenum oxidation state. Therefore, reproducible WO₃/MoO₃ composite films require careful reporting and control of all key processing parameters. This requirement includes parameters often omitted from brief experimental descriptions.

Table 5. Critical processing parameters of solution-based and electrochemical methods for WO₃/MoO₃ composite thin films.

Method	Formed Structure	Controlled Parameters	Main Risks	Parameters That Must Be Reported
Sol–gel + spin coating	Molecularly mixed W/Mo oxides; thin porous layers	Precursor concentration; pH; viscosity; spin speed (rpm); number of coating cycles; annealing regime	Shrinkage; cracking; residual water/organic species; hydration	Precursors; solvent; sol aging time; spin speed; thickness per cycle; annealing regime
Dip coating	Uniform coatings over large areas	Withdrawal speed; viscosity; substrate wettability; humidity	Runoff/streaks; thickness gradients; edge effects	Withdrawal speed; atmosphere; humidity; substrate preparation
Spray coating/spray pyrolysis	Mesoporous layers; aerosol-derived mixed oxides; compositionally graded coatings	Substrate temperature; aerosol flow rate; droplet size; nozzle–substrate distance	Surface roughness; compositional inhomogeneity; difficult phase control	Flow rate; nozzle–substrate distance; solvent; substrate temperature; number of passes
Hydrothermal/solvothermal synthesis + transfer	Nanorods; nanosheets; MoO3 ribbons; hierarchical structures	Temperature; pressure; mineralizer; growth time; pH	Poor adhesion; layer inhomogeneity; difficult integration into uniform thin films	Anchoring/transfer method; washing/annealing regime; precursor loading; layer thickness
Electrodeposition/anodization	Hydrated WO3/MoO3−x layers; porous electrochemically active films	Potential/current; pH; electrolyte composition; deposition charge; electrode geometry	Valence instability of Mo; limitation to conductive substrates; high sensitivity to electrolyte composition	Deposition mode; electrolyte formulation; electrode area; passed charge; pH; temperature; post-treatment

3.3.1. Sol–Gel Processing and Related Solution-Based Deposition Methods

Sol–gel processing (Figure 15) and related solution-based methods remain among the most accessible routes for fabricating WO₃/MoO₃ thin films. These approaches enable W and Mo precursors to be mixed at the molecular level and deposited by spin coating, dip coating, or spray coating. For WO₃, commonly used precursors include peroxotungstic, tungstate, and alkoxide-based systems, whereas MoO₃ films can be prepared from molybdates, peroxomolybdate complexes, or organosols. After deposition, mild thermal treatment is critical. It removes solvent and organic residues and also determines the hydration state, crystallinity, porosity, and possible formation of substoichiometric oxide regions [35,70,72,73,90].

Figure 15. Schematic representation of the spin-coating route for WO₃/MoO₃ nanocomposite thin-film fabrication from a mixed W/Mo precursor solution.

The chief advantage of solution-based routes is their flexibility. Their key limitation is poor reproducibility. Small variations in pH, humidity, sol aging time, viscosity, or drying rate can lead to different degrees of hydration, shrinkage, cracking, and phase evolution. For solution-derived WO₃/MoO₃ films, performance is governed by the complete chemical and processing history rather than by the nominal deposition method alone. Key variables include precursor composition, solvent system, aging conditions, thickness per coating cycle, annealing temperature, and annealing duration.

Figure 15 schematically illustrates the spin-coating route for fabricating WO₃/MoO₃ nanocomposite thin films from a mixed W/Mo precursor solution. The process includes precursor dispensing onto the substrate, high-speed rotation driven by centrifugal forces, solvent evaporation, and thermal treatment, resulting in a mixed or interfacially coupled WO₃/MoO₃ oxide architecture. This scheme emphasizes that the final film uniformity, porosity, phase distribution, and degree of W–O–Mo interfacial contact are governed by precursor chemistry, viscosity, rotation speed, solvent evaporation kinetics, wetting behavior, and annealing conditions. The blue-to-yellow gradient schematically represents the spatial variation in the relative W/Mo precursor composition across the coating and is qualitative rather than a calibrated concentration scale; blue arrows indicate centrifugal spreading during spinning, whereas gray dashed arrows indicate solvent evaporation.

Although sol–gel processing is an economically attractive and versatile route, it often suffers from limited thickness control, film nonuniformity, shrinkage-induced defects, and difficulties in ensuring reproducible coating quality.

3.3.2. Hydrothermal Methods

Hydrothermal and solvothermal methods typically produce nanostructured active surfaces rather than dense, smooth coatings. These surfaces may comprise nanoribbons, nanorods, nanosheets, or hierarchical assemblies. This approach is particularly effective for α-MoO₃, whose layered crystal structure provides enhanced accessibility to interlayer spaces and increases the density of surface-active sites. However, when hydrothermally synthesized materials are transferred onto a substrate, adhesion, layer uniformity, and mechanical stability must be carefully controlled [35,64,70].

Figure 16 presents SEM images of MoO₃ samples obtained by hydrothermal synthesis using different additives. At the same scale of 500 nm, the micrographs reveal pronounced morphology changes depending on the type and concentration of the additive, ranging from loosely aggregated particles to more ordered nanoribbon- or rod-like structures. This comparison demonstrates that hydrothermal synthesis conditions strongly influence crystallite size, shape, aggregation degree, and surface accessibility, which are critical parameters for optimizing the active surface of MoO₃-based and WO₃/MoO₃ nanocomposite materials.

Figure 16. SEM images of MoO₃ samples obtained by hydrothermal synthesis using different additives. The micrographs illustrate the effect of additive type and concentration on the morphology, size, and aggregation behavior of MoO₃ nanostructures. Reproduced from [71] under the terms of the Creative Commons Attribution license (CC BY 4.0).

3.3.3. Electrochemical Methods

Electrochemical deposition, particularly cathodic deposition, and anodization should be considered separately because they provide low-temperature routes for forming hydrated WO₃ and Mo-containing oxide layers directly on conductive substrates. Unlike in spin coating, where solution viscosity, rotation speed, and drying conditions mainly govern film thickness, the thickness deposited electrochemically is directly related to the charge passed. The potential, current density, pH, electrolyte composition, and electrode geometry control the morphology. These approaches are especially promising for electrochromic coatings, flexible electrodes, photoanodes, and pseudocapacitive systems, where short diffusion pathways and good electrical contact with the substrate are essential.

Figure 17 shows FE-SEM images of WO₃ and WO₃–MoO₃ nanostructures obtained by electrochemical anodization with a molybdenum-containing additive [69,73,91]. The micrographs reveal a highly developed porous and granular surface morphology rather than a dense compact film. At lower magnification, the coatings exhibit continuous surface coverage with a rough, interconnected nanostructure. At higher magnification, they show fine grains, open pores, and locally aggregated nanoclusters. Such morphology is important because it increases the electrochemically active surface area and facilitates electrolyte penetration, thereby shortening ion-diffusion pathways and improving access to redox-active W/Mo centers.

Figure 17. FE-SEM images of anodically formed oxide nanostructures: (a) WO₃, (b) WO₃ prepared with a 0.01 M molybdate-containing additive, (c) TiO₂, and (d) TiO₂ prepared with 0.01 M Zn(NO₃)₂. The comparison illustrates the effect of Mo- and Zn-containing additives on the surface morphology of anodically synthesized oxide nanostructures. Reproduced from Ref. [72] under the terms of the Creative Commons Attribution license (CC BY 4.0).

However, comparison of the different panels indicates that the addition of Mo species and variation in the electrochemical regime can substantially change surface porosity, grain size, particle packing, and the degree of structural uniformity. These changes may strongly influence the photocurrent, capacitance, or electrochromic response even without true W–O–Mo interfacial synergy. Therefore, increases in current, capacitance, or photocurrent in such systems should not automatically be attributed solely to W–O–Mo interfacial effects. They must be interpreted by correlating electrochemical data with SEM morphology, XRD/Raman phase analysis, and XPS-based evaluation of W/Mo oxidation states and oxygen-defect concentration. This is especially important for anodized WO₃–MoO₃ films, where the functional response can arise from a combined contribution of porosity, hydration, oxygen vacancies, Mo-induced surface modification, and interfacial charge-transfer pathways.

3.4. Post-Treatment

Post-treatment should be regarded as a critical and independent stage in the fabrication of WO₃/MoO₃ nanocomposite thin films. Annealing, mild reduction, plasma treatment, hydration, or re-oxidation can substantially modify the phase composition, oxygen nonstoichiometry, oxidation states of W and Mo, defect density, and stability of W–O–Mo interfaces. These changes directly affect the functional behavior of the films, including optical modulation, ion insertion, charge transport, sensing response, photocatalytic activity, and pseudocapacitive charge storage [35,40,43,73,89].

The key point is that the optimal post-treatment regime does not necessarily correspond to the highest crystallinity or the maximum concentration of oxygen vacancies. Excessive crystallization may reduce ion-accessible pathways, whereas over-reduction can generate unstable suboxide phases, charge traps, or irreversible optical and electrochemical degradation. Therefore, post-treatment must be optimized according to the intended application rather than treated as a universal final annealing step.

Table 6 summarizes how the choice of fabrication method and post-treatment strategy should be aligned with the target function of the WO₃/MoO₃ film. For electrochromic devices, the priorities are high optical modulation and cycling stability. For sensors, surface accessibility and humidity stability are critical. Energy-storage systems require control of ion transport, defect stability, and resistance growth. For photocatalytic or photoelectrochemical systems, phase composition, defect-mediated recombination, and band alignment are decisive. Thus, post-treatment serves as the final tool for balancing crystallinity, defect chemistry, interfacial stability, and functional performance in WO₃/MoO₃ thin-film architectures.

Table 6. Selection of synthesis methods for WO₃/MoO₃ composite thin films according to target function.

No	Target Function	Preferred Architecture	Optimal Methods	Parameters	Critical Risk
1	Electrochromic smart windows	Amorphous or weakly crystalline mixed WO3/MoO3 films; compositionally graded layers	Reactive sputtering; sol–gel/spin coating; electrodeposition	ΔT, coloration efficiency, cycling stability	Ion trapping, residual coloration, loss of transparency
2	Gas sensors	Porous WO3/MoO3 surfaces and W–O–Mo heterointerfaces	Spray coating; hydrothermal growth; PVD followed by annealing	Sensitivity, selectivity, response/recovery time, humidity stability	MoO3 hydration, signal drift, humidity interference
3	Supercapacitors/LIBs	Nanolaminates, porous electrodes, WO3/MoO3/rGO hybrids	ALD; electrodeposition; hydrothermal synthesis combined with a conductive matrix	Capacitance/capacity, retention, EIS, rate capability	MoO3 volume expansion, increase in resistance
4	Photocatalysis/PEC	Defective WO3/MoO3−x heterostructures with controlled band alignment	Anodization; hydrothermal growth; spray coating; CVD	Photocurrent, IPCE, EIS, pollutant degradation	Defect-mediated recombination, photocorrosion
5	OLEDs/transparent electrodes	Thin MoO3-rich/WO3 interlayers; oxide/metal/oxide stacks	PVD; e-beam evaporation; ALD; multilayer deposition	Transparency, sheet resistance	Work-function shift, metal diffusion, contact instability

Analysis of current fabrication strategies shows that there is no universal method for producing WO₃/MoO₃ composite thin films. When reproducible composition mapping and identification of the optimum electrochromic performance are required, RMS is the most rational approach. When porosity, low cost, and scalability are prioritized, solution-based methods are preferable. When short diffusion pathways and strong adhesion to conductive substrates are essential, electrodeposition and anodization are particularly suitable. If the key requirement is the density, sharpness, and stability of interfaces, ALD/CVD and nanolaminate architectures offer clear advantages.

For further discussion of WO₃/MoO₃-based devices, it is important to distinguish four major types of systems: atomically mixed oxides, compositionally graded coatings, nanolaminates/heterostructures, and hybrid composites incorporating conductive matrices. This classification makes it possible to link the fabrication route with the dominant functional mechanism and to distinguish genuine W–O–Mo interfacial synergy from apparent performance improvements caused by film thickness, porosity, secondary phases, or different degrees of oxide reduction.

4. Functional Devices and Properties of WO₃/MoO₃ Nanocomposite Films and Heterostructures

Building on the fabrication strategies discussed in Section 3, Section 4 moves beyond synthesis and addresses how WO₃/MoO₃ nanocomposite thin films and heterostructures translate structural complexity into device-relevant functionality. This section does not treat performance as a set of isolated peak values, such as optical modulation, sensitivity, capacitance, photocurrent, or charge-storage capacity. Instead, it emphasizes the mechanistic links among processing history, chemical composition, phase constitution, oxygen nonstoichiometry, defect chemistry, W–O–Mo interfacial coupling, and long-term operational durability. Within this framework, WO₃/MoO₃-based architectures are examined as multifunctional, defect- and interface-engineered platforms. Nanoscale heterogeneity is treated as a functional design principle for electrochromic devices, gas sensors, supercapacitors, lithium-ion energy-storage systems, photoelectrochemical cells, transparent electrodes, and organic electronic devices.

4.1. Technological Applications of WO₃/MoO₃ Nanocomposite Thin Films and Heterostructures

The principal application areas of WO₃/MoO₃-based nanocomposite thin films and heterostructures, summarized in Figure 18, demonstrate the broad multifunctionality of these architectures, spanning electrochromic devices, gas sensors, photocatalysts, supercapacitors, lithium-ion energy-storage systems, transparent electrodes, and organic optoelectronic devices.

Figure 18. Multifunctional application landscape of WO₃/MoO₃ nanocomposite thin films and heterostructures.

WO₃ and MoO₃ thin films exhibit a broad range of chromogenic effects. Electrochromism involves reversible color and optical modulation under an applied potential. Other responses include UV-induced photochromism, gasochromism in reducing atmospheres such as H₂ or H₂S, and thermochromism driven by temperature changes [5,7,8,19,20,40,92]. The chromogenic response of WO₃ and MoO₃ is largely governed by the reversible intercalation and deintercalation of small cations coupled with electron injection. Under electrochemical polarization, for instance, H⁺ or Li⁺ ions enter the oxide lattice together with charge-compensating electrons. This process forms bronze-like phases such as H_xWO₃, Li_xWO₃, H_xMoO₃, or Li_xMoO₃. This coupled ion–electron insertion partially reduces W⁶⁺/Mo⁶⁺ to W⁵⁺/W⁴⁺ or Mo⁵⁺/Mo⁴⁺ states. The resulting electronic transitions produce intense blue or dark-blue coloration and pronounced changes in optical absorption [5,7,8,19,20,30,40,77,78,92,93,94,95,96,97]. In a generalized form, the electrochromic switching mechanism can be represented by the following reaction:

MO₃ + x(A⁺ + e⁻) ⇌ AxMO₃, where M = W or Mo; A⁺ = H⁺, Li⁺, Na⁺, or K⁺

(4)

Here, x denotes the degree of intercalation, i.e., the number of inserted cations and charge-compensating electrons per oxide formula unit. In an idealized case involving monovalent cations, x may vary over a relatively broad range. However, in real WO₃ and MoO₃ electrochromic thin films, the reversible intercalation window is typically much narrower and is limited by the onset of ion trapping, phase transformations, and structural degradation. Therefore, the kinetics and reversibility of reaction (4) critically depend on the crystallinity of the film, the density of oxygen vacancies, and the state of interfacial boundaries. These factors become particularly important when moving from single-phase oxides to nanocomposite and heterostructured architectures [5,14,78].

In this sense, nanocomposite and heterostructured WO₃/MoO₃ architectures should not be viewed merely as mixed-oxide systems, but as interfacially engineered materials in which crystallinity, oxygen-defect chemistry, and W–O–Mo boundary states collectively define the electrochromic response.

4.2. From Interface-Engineered WO₃/MoO₃ Materials to Functional Device Architectures

The transition from material to device requires identifying the true active region of a WO₃/MoO₃ architecture. Depending on the application, this region may correspond to the bulk of the mixed-oxide film, the surface accessible for adsorption and ion insertion, the W–O–Mo interface, oxygen-vacancy-rich domains, or the electrode/electrolyte contact. In electrochromic devices, both the bulk and surface participate in coupled cation–electron transport. In gas sensors, the response is governed mainly by the surface and depletion layer. In supercapacitors, accessible redox centers and ion-transport channels dominate. In photoelectrochemical systems, the active regions generate and separate charge, whereas energetically selective contacts govern optoelectronic devices.

Accordingly, device optimization cannot be reduced to selecting a nominal W:Mo ratio. It requires matching the active-layer geometry, phase constitution, oxygen-vacancy concentration, mixed-valence W⁵⁺/W⁴⁺ and Mo⁵⁺/Mo⁴⁺ states, and the quality of W–O–Mo interfacial contacts to the operating mechanism of each device.

Before discussing specific device classes, it is important to distinguish intrinsic WO₃/MoO₃ and Mo–W–O architectures from the broader family of WO₃- or MoO₃-based composites containing secondary oxides, sulfides, polymers, or carbonaceous phases. Mixed WO₃–MoO₃ electrochromic films have been studied since the late 1980s [95]. The modern field, however, has shifted from simple W–Mo oxide blending toward deliberate design of WO₃/MoO₃ interfaces, gradient coatings, nanolaminates, and defect-engineered Mo–W–O heterostructures. Table 7 summarizes representative architectures in which the functional response is directly associated with the coexistence of W- and Mo-oxide components or with the formation of a mixed Mo–W–O oxide matrix.

Table 7. Main WO₃/MoO₃ and Mo–W–O architectures, fabrication methods, and device-oriented applications.

No	Architecture	Fabrication Method	Key Mechanism	Application	Ref.
1	Amorphous WO3/MoO3 films	Thin-film deposition of an amorphous mixed oxide film	Intervalence charge transfer between W and Mo centers	Electrochromic absorption	[97]
2	Mo-doped WO3/W1−y Mo_yO3	Magnetron sputtering	Mo tunes defect density, conductivity, and spectral response	Electrochromic layers for smart windows	[77,98]
3	Sol–gel MoO3–WO3 films	Dip-coating from a P123-assisted sol–gel system; annealing	Porous matrix accelerates ion intercalation	Electrochromic coatings on ITO	[16]
4	WO3–MoO3 nanocomposite films	Cathodic electrodeposition	Coupled H+/Li+ intercalation; increased active surface area	Electrochromic films	[75]
5	MoO3/WO3 nanocomposite films	Solution processing/dip-coating	MoO3 optimizes surface roughness and ion transport	Electrochromic modulation	[99]
6	Mixed MoO3–WO3 films	Atmospheric chemical vapor deposition	W–Mo network enhances electron transfer between redox centers	Electrochromic and optoelectronic coatings	[92]
7	WO3/MoO3 gradient films	Reactive magnetron co-sputtering	Properties are governed by the W:Mo ratio and film thickness	Optimization of coloration efficiency	[14]
8	WO3/MoOx hybrid films	Chemical/solution-based formation	MoOx modifies charge balance and switching reversibility	Complementary electrochromic devices	[35]
9	MoO3–WO3/Ag/MoO3–WO3	Multilayer oxide/metal/oxide architecture	Ag improves conductivity, while W–Mo oxides preserve transparency and electrochromic response	ITO-free transparent electrodes	[90]
10	MoWOx/Mo_yW1−y O3−δ	Solvothermal synthesis followed by film formation	Mixed oxide matrix enables VIS/NIR plasmonic control	Dual-band smart windows	[100]
11	Ultrathin W–Mo oxide films	Spray-coating from aqueous nanoparticle ink	Porous W–Mo network provides high contrast and fast charge transfer	High-contrast electrochromic coatings	[78]
12	Mo-doped WO3 nanowires	Hydrothermal growth followed by Mo modification	Increased number of active sites and redox-storage capability	Electrochromic/energy-storage devices	[30]
13	Porous W–Mo oxide electrodes	Solution-based formation of porous films	Porous channels facilitate ion and electron transport	Energy-storing smart windows	[101]
14	MoOx/W–Mo oxide nanowires	Solution-based nanohybridization	MoOx enhances switching reversibility	Reversible energy-storing smart windows	[102]
15	WO3/MoO3 nanoparticulate composite films	Electrodeposition	Increased optical modulation and areal capacitance	Electrochromic energy storage	[94]
16	MoO3/WO3 heterojunction	Electron-beam evaporation	Photo- and electrically tunable conductance plasticity	Optoelectronic synapses	[56]
17	WO3–MoO3 nanohybrid	Atomic layer deposition	High density of W–O–Mo interfaces and short diffusion pathways	Thin-film supercapacitors	[64]
18	MoO3/WO3/rGO	Solution/chemical synthesis with rGO	rGO increases conductivity and buffers deformation	Supercapacitors; electrocatalysis	[103]
19	WO3/WO3–MoO3 photoanodes	Electrochemical formation of WO3 followed by Mo modification	Mo lowers charge-transfer resistance	Photoelectrocatalysis	[72]
20	WO3/MoO3−x heterostructure	Hydrothermal/solvothermal growth of MoO3−x on WO3	Oxygen vacancies and heterojunction improve e−/h+ separation	RhB degradation; H2 evolution	[104]
21	WO3−x/MoO3−x heterostructure	In situ solvothermal reduction	Defective region and vacancy bridges enhance charge separation	CO2 photoreduction	[105]
22	WO3/MoO3−x from W–Mo alloy scrap	Oxidative chemical recycling of W–Mo alloy scrap	Mixed W/Mo surface combines adsorption and photocatalysis	Removal of dyes and pollutants	[106]
23	Combinatorial Mo–W–O libraries	Thermal co-evaporation; combinatorial deposition	Composition-dependent oxide chemistry and surface acidity	Antibacterial films; screening	[107]
24	Plasmonic Mo–W hybrid oxide	Solvothermal synthesis	LSPR and activation of visible-light charge transfer	Visible-light-enhanced catalysis	[108]
25	Molybdenum–tungsten oxides	Solvothermal synthesis	The Mo/W ratio determines band structure and charge transfer	Photoelectrochemical water splitting	[109]

Electrochromic coatings and smart windows remain the most established and intensively studied application field for WO₃/MoO₃-based architectures. Their performance is primarily governed by the controlled design of W–O–Mo interfaces, porosity, and oxygen-defect chemistry, which together facilitate coupled ion–electron transport and improve switching kinetics. Beyond electrochromics, recent studies demonstrate a clear expansion of WO₃/MoO₃ systems toward energy storage, photocatalysis, and neuromorphic optoelectronics, indicating a gradual transition from conventional chromogenic coatings to multifunctional device platforms. Consequently, this functional diversification calls for application-specific design strategies that link interfacial architecture and defect-mediated transport to the dominant electrochemical, catalytic, or optoelectronic process.

4.3. Electrochromic Devices and Smart Windows

Electrochromic devices represent the most established application of WO₃/MoO₃-based composites, since both oxides participate in reversible coupled ion–electron processes. WO₃ provides pronounced cathodic coloration through the reduction of W⁶⁺ to W⁵⁺ and/or W⁴⁺ during the intercalation of small cations, whereas MoO₃ can modulate the work function, defect chemistry, charge transport, and chemical stability of the interface. For WO₃- and WO₃/MoO₃-based electrochromic films, the key performance parameters are optical modulation, coloration efficiency, switching kinetics, and cycling stability [14,16,22,35,74,77,78,93,94,96,97,98,99,100,110].

The WO₃/MoO₃ composition should not be optimized by simply maximizing the content of either component. Combinatorial WO₃/MoO₃ thin films show that the highest coloration efficiency is achieved at an intermediate MoO₃ content, where interfacial synergy reflects a balance among optical absorption, film thickness, electrical conductivity, and ion-insertion kinetics [14].

A representative example of an interface-oriented strategy is the work of Dong et al. [90]. In that study, an electrochromic device was fabricated without a conventional ITO-based transparent conducting electrode. In the multilayer MoO₃–WO₃/Ag/MoO₃–WO₃ architecture, the thin Ag interlayer provides electrical conductivity, whereas the MoO₃–WO₃ oxide layers contribute to the electrochromic response, optical transparency, and interfacial stability. This design demonstrates that WO₃/MoO₃ composite films can serve not only as active chromogenic materials but also as interface-engineered functional layers, enabling transparency, conductivity, and reversible optical switching to be integrated within a single ITO-free electrochromic device.

Historically, electrochromic smart windows have been the most developed application area for WO₃, because dynamic modulation of transmittance in the visible and near-infrared regions enables effective control of light and heat fluxes through facade glazing [5,7,111,112,113]. Depending on climate, facade orientation, and control strategy, such systems can provide a substantial reduction in building energy consumption. For example, in an office building located in a hot and humid climate, electrochromic glazing was calculated to reduce energy use by 17–23% compared with conventional low-E glazing [112]. For more complex self-regulating electrochromic systems with integrated energy-storage functionality, building-scale calculations for the U.S. office stock indicate considerable potential for electricity savings and CO₂-emission reduction. However, such quantitative estimates should be regarded as scenario-dependent values governed by device architecture, climatic zone, operating regime, and the assumed energy-generation model [114].

WO₃ is regarded as one of the benchmark cathodic electrochromic materials owing to reversible cation intercalation, stable W⁶⁺/W⁵⁺ redox switching, and pronounced modulation of optical absorption [19]. These features allow WO₃ to dynamically tune visible and near-infrared transmittance, which is important for energy-saving glazing, adaptive optical components, optical phase modulators, and optical-memory devices [5,7,8,19,40]. MoO₃ also exhibits electrochromic and photochromic behavior. However, its practical use in the pristine state is often limited by low intrinsic electronic conductivity, sluggish charge transport, and increased sensitivity to structural changes during deep ion intercalation [23,24,32,39,44,115]. At the same time, MoO₃ provides additional functionality. Its layered structure creates anisotropic ion-transport pathways, and its high work function makes it a valuable charge-selective buffer layer. The surface redox activity of the Mo⁶⁺/Mo⁵⁺ couple also opens opportunities for gas-sensitive coatings, photoactive heterojunctions, supercapacitors, and ion batteries [23,30,78]. Therefore, combining WO₃ and MoO₃ in nanocomposite films and heterostructures is particularly attractive. This approach integrates the strong electrochromic response of WO₃ with the anisotropic ion transport, high work function, and surface redox activity of MoO₃.

In recent years, electrochromic materials have increasingly been considered as both optical modulators and functional components of energy-storage devices. For WO₃/MoO₃-based systems, this concept is particularly natural, since the same H⁺/Li⁺ intercalation process that induces optical coloration can also contribute to pseudocapacitive charge storage. In hybrid smart-window devices, therefore, not only the transmittance modulation ΔT but also the ability of the active layer to store charge without losing optical reversibility becomes technologically important [96,101,102].

However, coupling electrochromism with energy storage imposes more stringent requirements on structural and cycling stability. When the intercalation depth becomes excessive, especially in MoO₃-rich regions, the risk of volumetric stress, phase drift, and irreversible ion trapping increases. No universal MoO₃ fraction or layer thickness can be prescribed. For dual-function smart windows, the optimum should be reported as a capacity–optical-contrast–cycle-life Pareto front within a reversible electrochromic window. A composition maximizing CE in one film library must not be interpreted as a universal stability limit. Thin MoO₃-rich domains, a continuous WO₃-rich framework, controlled moderate oxygen deficiency, and charge-limited cycling provide a more transferable design rule than a single atomic-percent threshold.

For this reason, such devices are better served by architectures based on thin MoO₃ domains, nanolaminates, gradient coatings, or conductive carbon matrices, which shorten diffusion pathways while simultaneously buffering mechanical deformation. Thus, the optimal WO₃/MoO₃ architecture for electrochromic energy-storage windows should not maximize charge insertion, but rather balance optical contrast, reversible charge capacity, ion-transport kinetics, and long-term mechanical integrity.

4.4. Gas Sensors

In parallel with electrochromic technologies, gas-sensing applications of WO₃-, MoO₃-, and WO₃/MoO₃-based nanostructures and thin films are actively being developed. High specific surface area, defect-rich structures, oxygen vacancies, and heterojunction formation can enhance the chemiresistive response and sensitivity. These features may also improve selectivity toward NO₂, H₂S, CO, NH₃, and volatile organic compounds [4,26,36,39,69,71,116]. For selected sensor architectures, ppb-level detection has been reported, which is particularly important for environmental monitoring, industrial safety, and air-quality diagnostics.

In gas sensing, WO₃ is widely used to detect NO₂, H₂, H₂S, CO, and NH₃. MoO₃ and MoO_3−x are attractive because of their high surface acidity, tunable oxygen-vacancy concentration, and ability to interact selectively with water, oxygen, and reducing gas molecules [39,49,69,71,116,117]. In WO₃/MoO₃-based gas sensors, target molecules interact with surface oxygen species, oxygen vacancies, and W–O–Mo interfacial sites, leading to changes in the charge-carrier concentration and electrical resistance. Figure 19 illustrates this concept for a chemiresistive WO₃/MoO₃ nanocomposite sensing channel exposed to oxidizing and reducing gas molecules. In WO₃/MoO₃ nanocomposites, these effects can be further amplified through interfacial charge redistribution, modulation of the depletion-layer width, and the formation of additional adsorption-active sites at W–O–Mo boundaries. Thus, the gas-sensing performance of WO₃/MoO₃ heterostructures should be interpreted not only in terms of surface area or grain size, but also as the result of coupled surface chemistry, oxygen-defect engineering, and W–O–Mo interfacial electronic effects.

Figure 19. Schematic illustration of a WO₃/MoO₃ nanocomposite chemiresistive gas sensor. Dashed arrows indicate the approach and adsorption of gas molecules onto the WO₃/MoO₃ sensing surface; the dashed circular callout highlights a representative W–O–Mo interfacial site; the source–drain circuit illustrates the chemiresistive electrical readout.

In WO₃/MoO₃ heterostructures, the gas-sensing response can be enhanced by modulation of the potential-barrier height at W–O–Mo interfaces and by redistribution of adsorbed oxygen species. However, an increased response does not by itself prove interfacial synergy. Similar improvements may arise from reduced grain size, increased porosity, a higher concentration of oxygen vacancies, or changes in humidity. Figure 20 compares the H₂-sensing responses of different MoO₃ phases. It demonstrates that the phase state and morphology of MoO₃ can strongly affect the sensor signal even without a WO₃ component. For WO₃/MoO₃ composites, this highlights the need to verify the phase state of the MoO₃ component before attributing enhanced sensing performance to W–O–Mo interfacial synergy.

Figure 20. Gas-sensing response of different MoO₃ polymorphs toward H₂: (a) MoO₃-10, monoclinic phase (CrCl₃, 240 °C, 6 h); (b) MoO₃-8, orthorhombic phase (CTAB, 240 °C, 6 h); (c) MoO₃-1, hexagonal phase (90 °C, 3 h); and (d) sensitivity of the MoO₃ polymorphs. Reproduced from [71] under the terms of the Creative Commons Attribution license (CC BY 4.0).

For practical gas-sensing devices, a high response alone is insufficient. At present, stable operation under these conditions cannot be claimed for an unencapsulated, calibration-free chemiresistor. Without hydrophobic protection or active thermal stabilization, practical use would require a continuous WO₃-rich conduction network, spatially confined MoO₃ domains, independent humidity/temperature referencing, and multivariate drift compensation. Validation should include humidity cycling, mixed-gas interference, temperature variations, and long-term baseline stability. Although α-MoO₃@WO₃ nanosheets demonstrated enhanced triethylamine sensing at an optimum temperature of 200 °C, robust room-temperature operation in complex atmospheres was not established [56]. Operating temperature, baseline drift, selectivity in complex gas mixtures, and resistance to humidity must also be considered. WO₃/MoO₃ composites are particularly attractive because their sensing behavior can be tuned through the balance between MoO₃-derived acidic surface sites and n-type electronic transport in WO₃. Nevertheless, this interpretation requires mechanistic validation by DRIFTS, operando XPS, or at least systematic measurements under controlled humidity and mixed-gas atmospheres, since otherwise enhanced response may be incorrectly attributed to W–O–Mo interfacial synergy.

4.5. Supercapacitors and Lithium-Ion Batteries

In energy-storage applications, WO₃ and MoO₃ can participate in reversible pseudocapacitive and limited ion-insertion reactions, but deep lithiation of MoO₃ may proceed by conversion rather than intercalation. WO₃ is valuable as a comparatively stable pseudocapacitive framework. The layered structure of α-MoO₃ provides accessible Li⁺-transport pathways, but the commonly cited theoretical value of approximately 1117 mAh g⁻¹ is the six-electron deep-conversion limit for Equation (1). It is not a reversible electrochromic or intercalation capacity [22,39,45]. Therefore, high reversible capacity, specific capacitance, and optical charge storage must be discussed separately from the theoretical conversion limit. Developing WO₃/MoO₃ nanocomposites and hybridizing them with highly conductive carbon matrices such as rGO are effective strategies for increasing active-site accessibility, accelerating coupled electron–ion transport, buffering conversion-induced stress, and improving cycling stability. For example, a MoO₃/WO₃/rGO nanocomposite was reported to deliver a specific capacitance of 583 F g⁻¹ while retaining 92.6% of its capacitance after 2000 cycles [103]. Such ternary WO₃/MoO₃/carbon architectures demonstrate that high-performance electrodes require electronically conductive and mechanically resilient frameworks capable of stabilizing repeated ion insertion/extraction and limiting irreversible conversion.

Performance analysis in supercapacitors and LIBs should therefore distinguish three contributions: surface-controlled pseudocapacitance, comparatively reversible limited intercalation, and deep conversion that is often only partially reversible. WO₃ provides a relatively stable three-dimensional framework for reversible ion insertion, whereas α-MoO₃ offers layered transport pathways but becomes a major source of mechanical degradation when lithiation progresses toward Mo⁰/Li₂O formation. Capacities approaching the 1117 mAh g⁻¹ theoretical limit indicate increasing participation of the Mo⁶⁺ → Mo⁰ conversion pathway, not a reversible electrochromic intercalation process. The design objective should therefore not be to maximize MoO₃ content or lithiation depth. Instead, WO₃/MoO₃ architectures should combine mechanical stability, electronic connectivity, and ionic accessibility to sustain the genuinely reversible fraction of charge storage during long-term cycling [39,45,46].

Several architecture-level strategies are particularly promising for WO₃/MoO₃-based energy-storage electrodes. WO₃/MoO₃ nanolaminates can limit the thickness of MoO₃-rich layers and thereby suppress deep intercalation and mechanical degradation. WO₃/MoO₃/rGO and related carbon-hybrid architectures improve electronic conductivity while buffering intercalation-induced stress. ALD-derived nanohybrids, in turn, provide precise control over oxide-layer thickness, a high density of W–O–Mo interfaces, and short ion-diffusion pathways [64,79,103,118,119].

For such devices, performance evaluation should not be limited to specific capacitance alone. A comprehensive assessment must include capacitance retention after cycling, charge-transfer resistance, post-cycling morphological changes, and preservation of the phase composition. When the increase in capacitance originates from uncontrolled reduction of MoO₃ or the formation of suboxide phases, the initial improvement in electrochemical performance may be followed by accelerated degradation of the system. Therefore, composition alone does not govern the efficiency and durability of WO₃/MoO₃-based electrochemical devices. Electrode architecture, interfacial stability, phase integrity, and the ability of the active layer to maintain coupled ion–electron transport during long-term cycling are also critical.

Table 8 summarizes the key architectural requirements for WO₃/MoO₃-based electrodes, including film thickness, electrical conductivity, defect chemistry, porosity, and interfacial state, and relates them to their functional roles, potential degradation risks, and appropriate diagnostic methods. Such a framework provides a basis for moving beyond isolated performance metrics toward the rational design of mechanically stable, electronically connected, and ionically accessible electrode architectures.

Table 8. Architectural requirements for WO₃/MoO₃ electrodes for energy storage.

Parameter	Desired State	Functional Rationale	Potential Risk	Diagnostics
MoO3-domain thickness	Thin layer or nanodomain	Shortens Li+-diffusion pathways and reduces intercalation-induced stress	Deep intercalation and loss of contact between active domains	SEM/TEM before and after cycling
Electrical conductivity	Continuous electronic network	Reduces internal resistance and improves rate capability	Insulating MoO3 domains limit rate performance	EIS; four-point probe
Defect chemistry	Moderate concentration of oxygen vacancies	Enhances redox activity and facilitates ion–electron transport	Phase drift and formation of MoO2/suboxide phases	XPS, XRD, Raman spectroscopy
Porosity	Accessible but mechanically stable porous structure	Increases the accessible area of active redox sites	Cracking and loss of adhesion	SEM; profilometry
Interface	Stable W–O–Mo contacts	Facilitates charge transfer and interfacial ion–electron transport	Phase segregation and degradation of interfacial connectivity	STEM-EDS/EELS; XPS depth profiling

The optimization of each parameter involves a trade-off between performance enhancement and unavoidable degradation risks. For example, increasing the active-layer thickness can improve capacitance, but it also lengthens ion-diffusion pathways and may promote mechanical failure. Similarly, excessive defect density may enhance redox activity but can also induce phase drift and structural instability. Here, interfacial synergy requires stable W–O–Mo interfaces. These interfaces facilitate charge transfer but may be susceptible to phase segregation. The proposed set of diagnostic methods, including SEM, EIS, XPS, and STEM-EDS, enables a comprehensive assessment of whether the electrode meets the required architectural criteria and allows degradation processes to be identified at an early stage.

4.6. Photoelectrochemical and Photocatalytic Devices

In photocatalytic and photoelectrochemical systems, WO₃/MoO₃ heterostructures are attractive because they provide a platform for tuning photogenerated charge separation, oxygen-defect chemistry, and interfacial band alignment. WO₃ acts as a relatively stable n-type photoactive oxide, while oxygen-deficient MoO_3−x can reshape the interfacial energy landscape and introduce defect-mediated charge-transfer pathways. Nevertheless, defect engineering must remain controlled, since excessive oxygen vacancies or deeply reduced Mo species may become recombination centers and compromise long-term photoactivity.

Figure 21 presents a conceptual framework for designing WO₃/MoO₃-based photocatalytic and photoelectrochemical architectures.

Figure 21. Photocatalytic and photoelectrochemical design based on band alignment and interfacial effects.

The scheme emphasizes that device performance is governed not only by the energetic alignment of the constituent oxides but also by interfacial charge separation, surface-reaction kinetics, controlled defect chemistry, and resistance to photocorrosion. In this context, WO₃ acts as the photoactive and chemically stable framework, whereas MoO₃/MoO_x is most effective as a confined interfacial modifier coupled with an additional oxide or co-catalyst. The diagram is qualitative and does not imply a universal band-offset configuration for all WO₃/MoO₃ heterostructures.

Oxygen-deficient WO₃/MoO_3−x heterostructures can reduce electron–hole recombination, promote molecular oxygen activation, and enhance the photocatalytic oxidation of organic pollutants. For the optimized WO₃/MoO_3−x composition designated WOM83%, RhB degradation reached 98% within 40 min. The same composition exhibited a photocatalytic H₂ evolution rate of up to 4214.2 μmol h⁻¹·g⁻¹ [104]. Thus, interfacial synergy in WO₃/MoO_3−x heterostructures reflects the combined influence of oxygen-vacancy engineering, interfacial charge separation, band-structure modulation, and preservation of phase stability under illumination. Electrochemical impedance spectroscopy provides a useful tool for evaluating whether Mo modification reduces charge-transfer resistance. Figure 22 presents the EIS spectra of WO₃ and WO₃–MoO₃ photoanodes. A decrease in the Nyquist-arc radius for the optimized composition indicates facilitated interfacial charge transfer; however, this interpretation should be supported by complementary analysis of morphology and phase composition.

Figure 22. Electrochemical impedance spectroscopy (EIS) Nyquist plots of nanostructured photoanodes: (a) WO₃ and WO₃–MoO₃ prepared with different molybdate concentrations; (b) TiO₂ and TiO₂–ZnO prepared with different Zn(NO₃)₂ concentrations. Reproduced from [72] under the terms of the Creative Commons Attribution license (CC BY 4.0).

Photocurrent provides a direct measure of photogenerated charge-carrier generation, separation, and collection efficiency. Figure 23 presents the photoelectrochemical response of WO₃–MoO₃ nanostructures under illumination at different applied potentials. These results indicate that Mo-containing modification can promote charge separation and interfacial transfer; however, the optimum performance is governed by the molybdate-additive concentration, film thickness, surface morphology, and defect state.

Figure 23. Water-splitting photocurrent response curves recorded under chopped illumination for nanostructured photoanodes: (a) WO₃ and WO₃–MoO₃ prepared with different molybdate concentrations; (b) TiO₂ and TiO₂–ZnO prepared with different Zn(NO₃)₂ concentrations. Reproduced from [72] under the terms of the Creative Commons Attribution license (CC BY 4.0).

In photocatalytic systems, WO₃/MoO_3−x heterostructures can enhance the degradation of organic pollutants through more efficient e⁻/h⁺ separation and activation of molecular oxygen. Nevertheless, dye-degradation kinetics alone are insufficient to confirm a heterojunction-driven mechanism. PL quenching, EPR detection of reactive oxygen species, Mott–Schottky analysis, photocurrent measurements, and carefully designed control samples with comparable morphology should support this interpretation. These methods distinguish the effects of band alignment and interfacial charge transfer from simple surface-area or defect-concentration effects [37,104,105,106,108,109,120].

4.7. Optoelectronics

The high work function of MoO₃ makes it one of the most important hole-injection and charge-selective oxide interlayers in organic electronics. In such devices, MoO₃ does not necessarily function as a bulk active layer; its key role is to provide energetic alignment at the electrode/organic-semiconductor interface. In this application, WO₃–MoO₃ interfacial synergy can improve chemical stability, enable optical tuning, and support the formation of multilayer oxide stacks [11,23,121].

For OLEDs and organic optoelectronic devices, the main performance criteria include low turn-on voltage, high luminance, external quantum efficiency, operational stability, and resistance to contact degradation. In this context, excessive oxygen nonstoichiometry can be detrimental because it modifies the work function and introduces trap states. Therefore, for MoO₃/WO₃ interlayers, UPS/KPFM analysis and nanometer-scale thickness control are particularly important.

4.8. Biomedical and Antibacterial Applications of Mo–W–O Systems

Biomedical and antibacterial applications provide another context in which Mo–W–O interfacial synergy may be explored. The antibacterial activity of these materials against Escherichia coli has been associated with local acidification, surface redox processes, and reactive oxygen species. Visible-light pathways involving oxygen-deficient W₁₈O₄₉ may also contribute [107,122]. Although this research direction remains less mature than electrochromic, sensing, and energy-storage applications, it demonstrates the expanding functional scope of WO₃/MoO₃-based composites beyond conventional thin-film device technologies [122].

4.9. Reliability, Scalability, and Criteria for Device Translation

For all WO₃/MoO₃-based device platforms, the key challenge is the long-term preservation of functional performance, structural integrity, and interfacial synergy under realistic operating conditions. Degradation manifests differently depending on the application. In electrochromic devices, it appears as loss of optical modulation and residual coloration. In gas sensors, it leads to baseline drift and humidity sensitivity. In energy-storage electrodes, it causes capacitance fading and resistance growth. In photoelectrochemical systems, it results in photocorrosion or enhanced charge recombination. These failure modes demonstrate that high initial performance is not sufficient for device relevance unless the W–O–Mo interfacial architecture remains chemically and mechanically stable during cycling.

Device translation therefore requires moving beyond laboratory-scale optimization toward reproducible control of composition, thickness, morphology, defect chemistry, and W–O–Mo interfacial connectivity over large areas. Particularly promising approaches include combinatorial sputtering, ALD-derived nanolaminates, spray-coating with optical monitoring, and electrodeposition combined with in situ diagnostics. In this context, the ability to fabricate larger films is not the only measure of scalability. Spatial uniformity, adhesion, optical/electrical response, and cycling durability must also be preserved across the entire active area. Regardless of the fabrication route, it is essential to verify that W–O–Mo interfacial contacts and the associated functional response are retained after cycling. Therefore, reliability testing, post-mortem structural analysis, and operando characterization should become mandatory criteria for translating WO₃/MoO₃ architectures from model materials into practical functional devices.

Table 9 summarizes the main degradation scenarios for electrochromic, sensing, energy-storage, photoelectrochemical, and optoelectronic WO₃/MoO₃-based devices, together with mitigation strategies and post-test diagnostic methods. This framework provides practical criteria for assessing device reliability and for selecting architectures suitable for scalable implementation.

Table 9. Main degradation scenarios and mitigation strategies in WO₃/MoO₃-based devices.

Device	Degradation Scenario	Main Cause	Mitigation Strategy	Post-Test Diagnostics/Control
Electrochromic window	Loss of optical contrast; residual coloration	Ion trapping; excessive oxygen vacancies; cracking	Optimized porosity; thin MoO3 interfacial layer; mild annealing	UV–Vis cycling; XPS; SEM
Gas sensor	Signal drift; humidity interference	MoO3 hydration; changes in surface adsorbates	Hydrophobic protection; temperature modulation; humidity calibration	Repeated response cycles; DRIFTS/XPS
Supercapacitor	Capacitance fading	Volume expansion of MoO3; loss of electrical/mechanical contact	MoO3 nanodomains; conductive carbon matrix; ALD-limited oxide thickness	EIS; SEM/TEM; cyclic GCD testing
PEC/photocatalysis	Photocurrent decrease; loss of catalytic activity	Defect-mediated recombination; photocorrosion	Controlled oxygen vacancies; protective layers; pH optimization	Photocurrent; PL; post-test XRD/XPS
OLED/electrode interlayer	Increase in VO; luminance decay	Work-function shift; metal diffusion	Nanometer-scale control of MoO3/WO3; barrier layer	J–V–L; UPS/KPFM; aging tests

Overall, the most promising WO₃/MoO₃ systems are not bulk mixed oxides, but thin-film architectures with controlled phase connectivity, such as gradient layers, nanolaminates, MoO₃-rich interfaces, WO₃ frameworks with Mo-modified surfaces, and hybrids with conductive matrices. These architectures make it possible to combine the strong electrochromic response of WO₃, the layered ion-transport pathways of MoO₃, and tunable W–O–Mo interfacial coupling within a single multifunctional platform. Their technological value is therefore defined by more than the intrinsic properties of the individual oxides. It also arises from interfacial charge redistribution, oxygen-vacancy regulation, modified ion-transport pathways, and the formation of new active sites. Consequently, the transition from single-phase oxide films to interface-engineered WO₃/MoO₃ nanocomposites represents a key strategy for developing smart windows, gas sensors, supercapacitors, lithium-ion storage systems, photoactive coatings, organic electronics, and other sustainable device technologies.

5. Outlook for WO₃/MoO₃ Nanocomposite Thin Films: From Interface Engineering to Sustainable Device Technologies

The future development of WO₃/MoO₃ nanocomposite thin films will be defined not by the simple mixing of two oxides, but by the controlled engineering of phase connectivity, defect chemistry, and W–O–Mo interfacial states. This shift is important because the same nominal W:Mo ratio can lead to fundamentally different architectures, including amorphous mixed oxides, phase-segregated composites, nanolaminates, gradient coatings, and oxygen-deficient suboxide systems. Therefore, a key scientific challenge is to establish reproducible links between synthesis conditions, interfacial structure, oxygen-vacancy concentration, valence-state distribution, and device-level functionality.

First-principles modeling provides an important foundation for understanding WO₃/MoO₃ systems. Representative calculations have addressed several aspects of the individual oxides. These include WO₃ polymorph stability [123], oxygen-vacancy energetics [47], and polaron formation and transport [124,125]. For MoO₃, calculations cover vacancy formation [126], defect-mediated adsorption [127], and Li adsorption, diffusion, and lithiation-induced deformation [128,129]. Related computational–experimental work on coherent α-MoO₃/η-Mo₄O₁₁ lattice pinning [45] and experimental WO₃/MoO_3−x heterojunctions [104] further motivates explicit interface modeling. However, geometry-matched calculations for realistic WO₃/MoO₃ interfaces remain insufficient to establish universal W–O–Mo bonding configurations, vacancy-segregation energies, or valence- and conduction-band offsets. Future studies should therefore combine experimentally constrained interface models with calculations of defect energetics, band alignment, ion migration, and mechanically induced interfacial strain.

Among the different application areas, electrochromic coatings and smart-window devices remain the closest to practical implementation. WO₃ provides strong cathodic coloration and reversible ion insertion, whereas MoO₃ can modify charge transport, defect chemistry, optical response, and interfacial stability [130]. Future progress should therefore be assessed by more than record optical modulation or coloration efficiency. It also requires the combined optimization of optical contrast, switching kinetics, residual coloration, cycling durability, and large-area uniformity.

A particularly important direction is the integration of electrochromic and energy-storage functions within a single thin-film platform. Since optical switching and pseudocapacitive charge storage both rely on coupled ion–electron processes, WO₃/MoO₃ architectures can serve as multifunctional active layers for smart windows with energy-storage capability. Nanolaminates, porous but mechanically stable coatings, and hybrids with conductive carbon matrices are especially promising for this purpose. These architectures can shorten ion-diffusion pathways, improve electronic conductivity, and mitigate mechanical degradation during repeated insertion/extraction cycles.

Gas sensing, photocatalysis, and photoelectrochemical applications represent more complex but highly promising directions. In gas sensors, WO₃/MoO₃ composites can combine n-type transport, surface acidity, oxygen-vacancy chemistry, and W–O–Mo interfacial barriers. However, future studies must clearly distinguish true interfacial effects from changes caused by surface area, porosity, humidity, grain size, or phase composition. In photocatalysis and photoelectrochemistry, WO₃/MoO_3−x systems should be considered defect-engineered heterojunctions in which oxygen vacancies and band alignment govern charge separation, O₂ activation, and surface reactivity. The central task is to enhance charge separation while avoiding excessive defect-mediated recombination and photocorrosion.

Organic electronics, transparent electrodes, and charge-selective interlayers represent another important application field. In these systems, WO₃ and MoO₃ are used not as bulk active materials, but as ultrathin interface-modifying layers. The decisive parameters are work-function tuning, energetic alignment, nanometer-scale thickness control, and long-term contact stability.

From a technological perspective, the transition from laboratory samples to functional devices requires reproducibility over large areas. Scalable processing must simultaneously control composition, thickness, morphology, valence state, oxygen-vacancy concentration, and functional response. Combinatorial sputtering, ALD/MLD nanolaminates, spray-coating with in situ optical monitoring, and electrodeposition with charge/current diagnostics are particularly promising in this regard. Future progress requires controlled synthesis, evidence-based interfacial diagnostics, standardized testing protocols, and long-term reliability assessment. Meeting these requirements could transform WO₃/MoO₃ nanocomposite thin films from model materials into practical platforms for smart, energy-efficient, and sustainable devices.

6. Conclusions

This review demonstrates that WO₃/MoO₃ nanocomposite thin films and heterostructures should not be viewed as a simple combination of two related Group VI oxides. Instead, they represent a distinct class of interface-engineered functional materials. Their properties are governed by the coupled effects of WO₃ structural connectivity, the layered nature of α-MoO₃, oxygen nonstoichiometry, mixed W⁶⁺/W⁵⁺ and Mo⁶⁺/Mo⁵⁺ redox states, and the quality of W–O–Mo interfacial contacts. In these systems, composition, phase state, defect chemistry, morphology, and interface structure are not secondary factors. They are key design parameters that determine ion–electron transport, optical absorption, surface reactivity, charge separation, and long-term functional stability.

• Improved performance in WO₃/MoO₃ films should not automatically be attributed to W–O–Mo interfacial synergy, as similar effects may arise from changes in thickness, porosity, grain size, hydration, oxygen-vacancy concentration, or suboxide formation. Therefore, reliable interpretation of WO₃/MoO₃ nanocomposites requires direct correlation between functional testing and structural, morphological, valence-state, and interfacial diagnostics. These measurements should be performed on the same sample series. Methods such as XRD, Raman spectroscopy, XPS/XANES, SEM/TEM, EIS, PL spectroscopy, Mott–Schottky analysis, UPS/KPFM, and operando or post-mortem characterization are essential for distinguishing true interfacial effects from morphology- or defect-driven enhancement.
• The synthesis route plays a decisive role in defining the active architecture of WO₃/MoO₃ systems. Magnetron sputtering and combinatorial co-sputtering are particularly suitable for constructing composition–structure–property maps. Sol–gel processing, spin-coating, dip-coating, and spray-coating are attractive for scalable coatings. Electrodeposition and anodization enable porous, hydrated, and substrate-adherent layers. ALD/CVD provide nanometer-scale control of interfaces and nanolaminate architectures, whereas PLD and evaporation are valuable for preparing dense model heterostructures. Thus, the fabrication method should be considered a functional design tool. It determines film thickness, W/Mo distribution, phase connectivity, oxygen-defect density, and the stability of W–O–Mo contacts.
• WO₃/MoO₃-based thin films offer a broad application landscape, including electrochromic smart windows, electrochromic energy-storage devices, gas sensors, supercapacitors, lithium-ion storage systems, photocatalytic and photoelectrochemical coatings, transparent electrodes, OLEDs, organic optoelectronics, and antibacterial surfaces. Across these applications, their main advantage lies in the possibility of simultaneously tuning optical modulation, ion insertion, electron transport, surface redox activity, charge selectivity, and interfacial stability. However, practical implementation is still limited by insufficient reproducibility, phase instability, MoO₃ hydration, uncontrolled oxygen-defect formation, cycling-induced degradation, and the absence of standardized protocols for comparing device performance.
• The most promising future direction is the transition from uncontrolled mixed oxides to deliberately designed thin-film architectures. These include gradient coatings, nanolaminates, MoO₃-rich interfaces, WO₃ frameworks with Mo-modified surfaces, oxygen-deficient WO₃/MoO_3−x heterostructures, and hybrids with conductive matrices. Such architectures can integrate several key functions within a single multifunctional platform. These include the strong electrochromic response of WO₃, the layered ion-transport pathways of MoO₃, and tunable W–O–Mo interfacial coupling. If future studies combine controlled synthesis, evidence-based interfacial diagnostics, long-term reliability testing, and scalable processing, WO₃/MoO₃ nanocomposite thin films can progress from promising laboratory materials toward practical smart, energy-efficient, and sustainable device technologies.