The Influence of Defect Engineering on the Electronic Structure of Active Centers on the Catalyst Surface

Abstract

Defect engineering has recently emerged as a cutting-edge discipline for precise modulation of electronic structures in nanomaterials, shifting the paradigm in nanoscience from passive ‘inherent defect tolerance’ to proactive ‘defect-controlled design’. The deliberate introduction of defect—including vacancies, dopants, and interfaces—breaks the rigid symmetry of crystalline lattices, enabling new pathways for optimizing catalysis performance. This review systematically summarizes the mechanisms underlying defect-mediated electronic structure at active sites regulation, including (1) reconstruction of the electronic density of states, (2) tuning of coordination microenvironments, (3) charge transfer and localization effects, (4) spin-state and magnetic coupling modulation, and (5) dynamic defect and interface engineering. These mechanisms elucidate how defect-induced electronic restructuring governs catalytic activity and selectivity. We further assess advanced characterization techniques and computational methodologies for probing defects-induced electronic states, offering deeper mechanistic insights at atomic scales. Finally, we highlight recent breakthroughs in defect-engineered nanomaterials for catalytic applications, including hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and beyond, while discussing existing challenges in scalability, defect stability, and structure–property causality. This review aims to provide actionable principles for the rational design of defects to tailor electronic structures toward next-generation energy technologies.

1. Introduction

With the rapid advancement of nanotechnology and characterization techniques, materials science is transitioning into an era of atomic-scale design. The “perfection” of traditional crystalline materials is being reexamined—the deliberate introduction of specific defects not only preserves material functionality but also unlocks novel electronic states [1]. Defect engineering, the methodology for purposefully introducing defects such as vacancies, dopant, and grain boundaries, is redefining the foundational paradigms in catalysis, optoelectronic devices, and energy storage [2,3,4,5,6]. As a core strategy for electronic structure modulation, its essence lies in leveraging defect-induced local atomic rearrangements and electronic state reconstructions to overcome the limitation of ideal crystalline lattices, thereby granting multidimensional degrees of freedom for the design of novel material systems [7,8,9]. This review systematically explores the mechanisms of defect-mediated electronic structure regulation, synthesizes recent insights into the defect-electronic structure–property triad, and establishes a conceptual framework for the rational design of defect at specific electronic architectures.

In recent years, defect-mediated electronic structure modulation has been increasingly recognized as a multiscale and synergistic phenomenon. At the atomic scale, vacancies or dopant atoms can directly influence the electron donor/acceptor characteristics of active sites by altering localized charge density and bonding strength. For example, Fe doping combined with Co vacancies in CoSe₂ nanobelts regulates the electronic structure of Co₂ sites, reconstructing the electronic density of states near the Fermi level [10]. At the mesoscopic scale, extended defects such as interfaces and dislocations influence macroscopic carrier transport pathways via mechanisms including space-charge regions and quantum confinement effects. An example is the WS₂–WO₃ heterostructure, where abundant heterogeneous interfaces generate a strong interfacial electric field. These fields elevate the d-band center of W atoms and enhance the adsorption of intermediates [11]. Dynamically, the evolution of defect structures during operation triggers real-time electronic state reconstruction, which critically determines catalyst activity and stability. In the case of self-supported coral-like Ce₂W₂O₉ arrays, the bond structure of Ce atoms was regulated by the W defects formed through in situ electrochemical reconstruction, and the adsorption of intermediates by Ce active sites was optimized [12].

This review elucidates the physicochemical mechanisms by which defect engineering modulates electronic structures, organized through a tripartite analytical framework:

• Multidimensional regulatory strategies: We systematically dissect how defect engineering modulate the electronic structures of active sites, including electronic density-of-states reconstruction, coordination microenvironment tuning, charge transfer dynamics, and localization effects. Based on this, we summarized the main defect types in nanocatalysts and their regulatory effects on the electronic structure, as shown in

  Table 1. The main types of defects and their effects on electronic structures.

  The Main Types	The Effects on Electronic Structures
  Vacancy defect	Introduces local electronic states, forms defect energy levels in the band gap, creates charge traps, changes the energy gap, and alters the electron spin
  Doping defect	Induces charge transfer and orbital hybridization, regulates the position of Fermi energy levels, introduces impurity energy levels, and changes the band gap
  Coordination defect	Induces the rearrangement of orbital electrons and regulates the position of the Fermi energy level
  Lattice distortion defect	Changes the bond length, adjusts the electronic state density, and increases the center of the D-band
  Interface defect	Forms the space charge region at the grain boundaries, generates an interfacial electric field that affects carrier migration, and alters the energy band structure

  .

• Multiscale characterization techniques: We critically examined advanced techniques used for probing defect-electronic structure–property relationships. These includes defect-specific spectroscopies, electronic structure analyses and computational modeling approaches.
• Performance benchmarks: We highlight representative catalytic systems where defect-mediated electronic tailoring leads to enhanced performance, with a focused on hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and other relevant reaction systems.

This review focuses on the regulation mechanism, characterization and effect of defect engineering strategies on the electronic structure on the surface of nanocatalysts. It provides a detailed analysis of how defect engineering regulates the electronic structure and comprehensively summarizes research strategies such as defect characterization, electronic structure characterization, theoretical calculation and machine learning. This review also evaluates the significant contributions of the defect engineering regulation electronic structure strategy in fields such as HER and OER. By integrating regulation strategies, characterization techniques, computational modeling and experimental outcomes, this paper establishes the general design principles for defect-driven electronic modulation, aiming to provide a unified perspective for the rational design of the next generation of functional materials.

2. Mechanisms Underlying Defect Engineering-Mediated Modulation of Electronic Structures at Active Sites

2.1. Electronic Density of States Reconstruction

2.1.1. Defect Energy Level Engineering

Defect energy level engineering enables precise modulation of electronic structure through mechanisms including bandgap tuning, carrier dynamics regulation, and interfacial electron transfer optimization. By tailoring band structures to enhance light absorption and charge separation kinetics, coupled with the modification of local electronic environments to generate highly active catalytic sites, this approach offers innovative strategies for designing high-performance photocatalysts. The deliberate introduction of defect energy levels has been successfully applied in a range of applications including photocatalysis, molecular oxygen activation, and pollutant degradation, firmly establishing defect engineering as a key methodology for achieving atomic-level control over electronic architectures.

The strategic tuning of oxygen vacancy (OV) concentrations has emerged as a critical factor in optimizing charge carrier dynamics and separation efficiency in S-scheme heterojunction photocatalysts. A notable example was reported by Xu et al., who constructed a WO_3−x/In₂S₃ heterostructure and elucidated the critical role of defect energy levels in prolonging carrier lifetimes [13]. Moderate OV concentrations introduced intermediate defect states within the WO_3−x bandgap, which acted as electron reservoirs. These states significantly extended carrier lifetimes by facilitating the rapid electron trapping and delayed release of photogenerated electrons. X-ray photoelectron spectroscopy (XPS) analysis revealed negative shifts in W 4f and O 1s binding energies, along with changes in the W⁵⁺/W⁶⁺ ratio with increasing calcination temperature, confirming OV-induced redistribution of electronic density. Density functional theory (DFT) calculations further demonstrated that progressive OV incorporation led to the formation of shallow-to-deep defect levels, changing the bandgap of WO₃. Density of states (DOS) analysis corroborated this effect, showing an increasing depth of defect levels with higher OV concentrations (Figure 1). Femtosecond transient absorption spectroscopy (fs-TAS) provided direct kinetic evidence that optimal OV concentrations prolonged carrier lifetimes by functioning as electron traps, whereas excessive deep defect states (e.g., in WO_3−x-500) that quenched the deeply trapped electrons and shortened lifetimes. In situ irradiation XPS (ISI-XPS) and fs-TAS measurements collectively confirmed that intermediate defect levels—introduced by controlled OVs—play a critical role in electron storage and delayed release, thereby enhancing charge carrier separation. Notably, the optimized defective S-scheme heterojunction exhibited exceptional CO₂ photoreduction performance, achieving nearly 100% CO selectivity. These findings underscore the efficacy of defect-level engineering in tailoring charge transfer kinetics and photocatalytic efficiency.

As another example of the energy level regulation electronic structure induced by oxygen vacancies, Yang et al. engineered a Ni₂P/Bi₃O₄Br-OVs heterojunction system, in which OVs introduced localized defect energy levels within the Bi₃O₄Br bandgap, substantially optimizing its electronic properties [14]. The introduced OVs generated a deep defect level associated with Bi 6p orbitals of Bi^(3−x)+ near the mid-gap region, facilitating excitation under low-energy photons and serving as electron-trapping centers that improved O₂ activation. Electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) analyses confirmed the presence of OVs and their modulatory effects on Bi³⁺ electronic states, as indicated by negative binding energy shifts in Bi 4f and Br 3d binding energies, indicating surface charge redistribution. Theoretical simulations further revealed that dual active sites (OVs and Ni₂P) synergistically enhanced oxygen activation and charge separation, generating holes (h⁺) while converting O₂ into superoxide radicals (·O₂⁻), which serve as pivotal reactive intermediates in the photocatalytic oxidation process.

Introducing sulfur vacancies (SVs) into two-dimensional nanostructures has emerged as an effective strategy to optimize piezocatalytic performance via band structure engineering and charge carrier regulation. Ma et al. synthesized ultrathin 2D sulfur vacancy-engineered (SV-engineered) Cu@SnS_2−x nanosheets through a heterovalent substitution strategy, wherein Sn⁴⁺ sites were partially replaced by Cu²⁺ [15]. The incorporation of Cu²⁺ preferentially induced controlled SVs within the SnS₂ lattice, which significantly modified its electronic structure and charge transfer kinetics. These SVs narrowed the bandgap of SnS₂ from 2.16 eV (pristine) to 1.62 eV (Cu@SnS_2−x), thereby facilitating electron-hole (e⁻-h⁺) separation and enhancing sonocatalytic efficiency. Combined experimental and theoretical analyses elucidated the role of SVs in electronic modulation. X-ray photoelectron spectroscopy (XPS) demonstrated negative shifts in binding energies for both S and Sn, indicative of localized electron density redistribution around Sn atoms. DFT calculations revealed an upward shift in the Fermi level of SnS₂ following Cu doping, suggesting the formation of defect states t the lower region of CB edge. These states promoted electron transfer, improving the separation of ultrasonically generated carriers (Figure 2a,b). Piezoresponse force microscopy (PFM) further confirmed the defect-mediated charge separation mechanism, as evidenced by a pronounced increase in the piezoelectric coefficient signal amplitude for Cu@SnS_2−x. Notably, the Cu@SnS_2−x nanosheets exhibited ultrasound-enhanced, tumor microenvironment (TME)-responsive Fenton-like catalytic activity and glutathione depletion capability, enabling efficient tumor cell ablation via biocompatible piezocatalytic therapy. This study highlights the biomedical promise of defect engineering in designing electronic structures for targeted therapeutic applications.

For metal-free nanomaterials, the introduction of midgap states through heteroatom doping can effectively inhibit charge recombination and improve the migration of photogenerated carriers. Among various doping strategies, nitrogen doping has been particularly effective in inducing defect energy levels, thereby facilitating the spatial separation of photogenerated electron-hole pairs. Xing et al. reported the synthesis of nitrogen-doped porous thin-walled graphitic carbon nitride (g-C₃N₄) nanotubes (denoted as CNx) [16]. The incorporation of nitrogen atoms introduced midgap energy levels within the electronic band structure, resulting in a reduction in the bandgap from 2.46 eV to 2.28 eV for the CN0.75 composition. These nitrogen-induced midgap states enhance visible light harvesting by enabling the absorption of lower-energy photons and act as effective charge separation centers rather than recombination centers. Consequently, CN0.75 exhibited remarkable photocatalytic performance for tetracycline degradation, achieving a degradation efficiency of 93.0% within 60 min. Moreover, the material demonstrated excellent stability, with only a 4.1% decrease in photocatalytic efficiency after five consecutive cycles. This study underscores the significance of defect-level engineering as a general and effective strategy for optimizing the performance of photocatalysts in applications related to energy conversion and environmental remediation.

2.1.2. Fermi Level Shift

Modulating the Fermi level represents one of the central mechanisms by which defect engineering regulates electronic structures. By introducing defect configurations such as dopants and vacancies, the Fermi level undergoes precise shifts, which in turn alters the electronic interactions between reactants and the catalyst surface. This approach enables systematic tuning of adsorption energetics and charge transfer dynamics at active sites, offering a strategic pathway to enhance catalytic selectivity and activity for targeted reactions.

Interface engineering has become a powerful strategy for tuning Fermi levels and promoting directional charge transfer across catalytic heterojunctions. In this context, Lin et al. constructed a multi-armed MoSe₂/CdS S-scheme heterojunction featuring strong Mo–S coupling and tunable selenium vacancy (V_se) and Mo⁵⁺ concentrations [17]. The synergistic interplay between V_se vacancies and Mo⁵⁺ ions served as a key mechanism for Fermi level adjustment. By modulating the synthesis temperature (160–220 °C), the concentration of donor and acceptor in MoSe₂ dynamically evolved. As temperature increased, V_se concentration decreased, causing a downward shift in MoSe₂’s E_F. However, the simultaneous reduction in Mo⁵⁺ concentration counteracted this trend, resulting in a nonlinear E_F response (Figure 3): E_F initially decreased (ΔE_F = −0.08 eV from 160 °C to 180 °C) and then increased (ΔE_F = +0.05 eV from 180 °C to 220 °C). This nonlinear relationship modulated the E_F offset between CdS and MoSe₂, creating a robust built-in electric field. The strengthened field drove spatial separation of high-reducibility photoelectrons in CdS’s conduction band (CB) and high-oxidability photo-holes in MoSe₂’s valence band (VB), achieving an outstanding hydrogen evolution activity of 52.62 mmol g⁻¹ h⁻¹ under visible light, corresponding to an apparent quantum efficiency of 34.8% at 400 nm. This work establishes defect-concentration-driven E_F modulation as a universal strategy for designing high-performance heterojunction photocatalysts.

The strategic engineering of interfacial kinetics through oxygen vacancies (OVs) has emerged as a powerful approach to control charge carrier dynamics in photocatalytic heterostructures. As demonstrated by Liu et al., the synthesis of a BiVO₄/BiOBr-Ov (BVB-Ov) heterostructure revealed that Ov introduction elevates the Fermi level of BiOBr-Ov above that of BiVO₄, thereby reconfiguring the interfacial electron transfer pathway [18]. Notably, the elevated Fermi level of BiOBr-Ov induced a directional migration of photogenerated electrons from BiOBr-Ov to BiVO₄ upon heterojunction formation. This process triggered downward band bending at the BiVO₄ surface and established a pronounced built-in electric field. The resultant band alignment optimization facilitated spatially directed charge transfer across the interface, leading to a remarkable enhancement in carrier migration efficiency. Crucially, the presence of oxygen vacancies mediated a transition from Type-II to Z-scheme charge transfer mechanism, subsequently promoting the generation of oxidative •O₂⁻ radicals. The composite containing 20% BVB-Ov exhibited optimal photocatalytic activity, achieving 91% degradation of oxytetracycline (OTC) within 60 min under visible light irradiation (Figure 4). This study highlights Fermi level modulation via oxygen vacancies as an effective strategy for heterojunction-type conversion, providing a viable route to enhance photocatalytic performance through controlled interfacial energetics.

2.2. Coordination Environment Engineering

2.2.1. Low-Coordination Environments Induce the Exposure of Catalytically Active Sites

In low-coordination defect engineering, catalytic activity can be enhanced by exposing highly active sites and reconstructing localized electronic states to optimize adsorption energetics of reaction intermediates. Yu et al. developed amorphous PdSe₂ nanoparticles (a-PdSe₂ NPs) with disordered atomic arrangements, which generated low-coordinated Pd sites (Figure 5), that break the symmetric coordination constraints inherent to crystalline PdSe₂ [19]. DFT calculations reveal that the Pd-d and Se-p orbital states in a-PdSe₂ shift closer to the Fermi level compared to crystalline PdSe₂, resulting in the formation of continuous conduction bands, metallic behavior, and pronounced electronic state occupancy near the Fermi level. This electronic reconfiguration optimizes the formation energy of the *OOH intermediate, effectively suppressing O–O bond cleavage and promoting selective H₂O₂ generation. Experimentally, a-PdSe₂/C maintains >90% H₂O₂ selectivity across a broad potential window (0.3–0.6 V vs. reversible hydrogen electrode, RHE) in 0.1 M KOH, surpassing the performance of crystalline Pd-based catalysts. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) analyses further confirmed localized charge redistribution at undercoordinated Pd sites. This work establishes a novel paradigm for tailoring electronic structures via amorphous defect engineering and elucidates the structure–performance relationship between dynamic coordination defects and catalytic activity. These findings provide critical insights for the precision design of catalysts with highly exposed, electronically optimized active sites.

The synergy between amorphization and oxygen doping presents a powerful strategy for modulating low-coordination environments and optimizing electronic states in electrocatalysts. Zhao et al. systematically demonstrated this strategy by transforming crystalline PtTe₂ nanosheets (c-PtTe₂ NSs) into oxygen-doped amorphous structures (a-PtTe₂ NSs) via an oxygen-assisted calcination method [20]. During the crystalline-to-amorphous transition, oxygen doping disrupts the long-range order of c-PtTe₂ NSs, inducing structural amorphization and exposing reconfigured interfacial Pt atoms. Partial density of states (PDOS) analysis reveals that the d-band center of a-PtTe₂ NSs shifts upward relative to c-PtTe₂ NSs, strengthening hydrogen intermediate (H*) adsorption and facilitating rapid proton transfer kinetics. Under an applied electric field, electron accumulation at the exposed Pt sites and their expanded orbital volume further enhanced H* conversion efficiency. Consequently, a-PtTe₂ NSs exhibit optimized Gibbs free energy for H* adsorption and exceptional HER performance, including an ultralow initiation potential (~0 mV vs. RHE), an overpotential of 14 mV at 10 mA cm⁻², and a Tafel slope of 24.5 mV dec⁻¹. This study demonstrates that controlled amorphization of 2D layered materials circumvents the inherent crystallographic constraints on active site exposure, offering a novel strategy to design catalysts with high-density, undercoordinated active centers.

Tailoring coordination environments through defect engineering stabilizes unconventional active site geometries, thereby enhancing catalytic performance in complex reactions such as nitrogen reduction. Wang et al. engineered a defect-rich photocatalyst composed of Fe single atoms anchored in hollow BiOBr microtubes, where synergistic oxygen (V_O) and bromine vacancies (V_Br) generate low-coordinated FeO₅ active centers, significantly boosting nitrogen reduction reaction (NRR) efficiency [21]. In the pristine Fe_SA-Bi/BiOBr-V_Br system, each Fe atom is coordinated by four oxygen atoms from [Bi₂O₂]²⁺ layers (Figure 6b). Introducing dual vacancies disrupts this symmetry, resulting in a pentacoordinated Fe center bound to four oxygen atoms from [Bi₂O₂]²⁺ and one oxygen atom located in the Br vacancy (denoted as Fe_SA-Bi/h-BiOBr-V_O,Br; Figure 6a). This low-coordination environment induces localized polar charge redistribution on Fe sites, accelerating electron transfer from Fe to N₂ and lowering the activation energy of the potential-determining step (N₂ → NH₃). Consequently, the optimized catalyst achieves enhanced NH₃ production with high intrinsic activity and selectivity. This work provides a universal framework for designing defect-driven catalysts via precise coordination environment modulation.

The integration of undercoordinated multimetallic sites within well-defined frameworks offers a promising strategy for steering complex reaction pathways such as CO₂ photoreduction. Wang et al. engineered a multi-atomic catalyst (MAC) with undercoordinated triangular Cu₃ sites (denoted as Cu₃ MAC) via a “pre-locking and nanoconfinement polymerization” strategy, enabling highly selective CO₂ photoreduction to ethanol [22]. EXAFS analysis confirmed the coordination environment: the Cu–N and Cu–Cu coordination numbers for triangular Cu₃ MAC were determined as 1.95 and 1.83, respectively, indicating the formation of a low-coordination triangular Cu₃ geometry. Under light irradiation, these Cu₃ motifs facilitate electron accumulation, generating a high proportion of Cu⁺ active species critical for asymmetric C–C coupling during CO₂ reduction. The Cu₃ MAC exhibits exceptional photocatalytic performance for CO₂-to-ethanol conversion, attributed to the synergistic interplay of high-density Cu₃ sites with mixed Cu⁺/Cu²⁺ oxidation states, which enhance CO₂ chemisorption, accelerate charge separation, and stabilize the *CHCHOH intermediate. This “geometric-electronic dual modulation” strategy establishes a versatile framework for designing selective multicarbon-product catalysts.

Inducing low-coordination sites by constructing lattice mismatch interfaces represents an effective strategy for optimizing the electronic structure and improving catalytic activity. Zhang et al. engineered a self-supporting NiO/Co₃O₄ hybrid catalyst with abundant heterointerfaces, where low-coordination atoms (LCAs) at lattice-mismatched interfaces optimize electronic configurations and expose active sites for the OER [23]. The LCAs mediate electronic modulation, shifting Co and Ni cations to the favorable oxidation species in the intermediate spin states. DFT calculations reveal that interfacial LCAs downshift the Co d-band center below the Fermi level, thereby weakening oxygen intermediate adsorption strength and enhancing intrinsic activity. The hybrid catalyst achieves an OER overpotential of 262 mV at 10 mA cm⁻², outperforming NiO (456 mV), Co₃O₄ (490 mV), Ni foam (680 mV), and commercial IrO₂ (361 mV), with a Tafel slope of 58 mV dec⁻¹. This study exemplifies interface defect engineering: precise control over LCA spatial distribution and electronic density of states circumvents adsorption–desorption limitations in conventional metal oxides, paving the way for efficient, stable OER electrocatalysts.

2.2.2. Lattice Distortion Engineering for Electronic Structure Modulation

Lattice distortion has emerged as a vital tool for inducing local strain fields and electronic configurations at catalytic sites. By altering bond lengths, and reducing coordination numbers of active sites, strain engineering effectively elevates d-band centers, and modulates the electronic density of states, thereby enhancing catalytic performance.

In a representative study, Liu et al. engineered a strained RuO_2−x system via molten salt-assisted quenching, generating localized tensile strain on RuO_2−x surfaces, and incorporating Sr/Ta dopants to optimize its electronic structure [24]. The tensile strain induces spatial elongation of Ru–O bonds and reduced covalency, thereby suppressing lattice oxygen participation and mitigating structural degradation (Figure 7). Concurrently, Sr–Ta co-doping synergistically modulates the electronic environment, optimizing deprotonation kinetics at oxygen sites and tuning intermediate adsorption energetics at Ru sites, which collectively lower reaction energy barriers. The strained catalyst achieves an oxygen evolution reaction (OER) overpotential of 166 mV at 10 mA cm⁻² in 0.5 M H₂SO₄, with stability metrics exceeding pristine RuO₂ by an order of magnitude. Long-term stability evaluations in single-cell and proton exchange membrane (PEM) electrolyzers further validate its exceptional durability and activity, surpassing most state-of-the-art acidic OER catalysts.

Strain engineering at the nanoscale offers a powerful strategy for reconfiguring surface atomic structures and tuning electronic states, particularly in transition metals with intrinsically flexible bonding networks. Li et al. synthesized lattice-distorted Cu containing abundant dislocation-nanotwinned structures (DNTs) via electrochemical reduction of Cu₂O, generating strained nanostructures enriched with dislocations, axial-shear strain, and reduced surface coordination numbers (CN) [25]. The elecated tensile strain and decreased CN reduce Cu–Cu electron sharing, shifting the d-band center upward and lowering the hydrogen adsorption ΔG_H*. At a coordination number of 5, ΔG_H* on Cu(111)/(110) surfaces approaches zero, indicating optimal hydrogen evolution reaction (HER) activity. Although dislocation-twinned Cu initially matches Pt/C in performance at low overpotentials due to limited active site accessibility, it surpasses Pt/C under high-current-density conditions, delivering an overpotential of 301 mV (vs. 425 mV for Pt/C) at 500 mA cm⁻². This strain engineering strategy establishes Cu as a scalable, cost-effective HER catalyst for industrial-scale applications.

Carbon-based nanomaterials with engineered strain and defect sites offer versatile platforms for modulating reactivity, especially in oxidation and halogenation reactions. Zhang et al. synthesized defect-rich, strain-engineered carbon nanotubes (CNTs) characterized by enlarged diameters, enhanced porosity, and expanded interlayer spacing, which destabilize sp²-hybridized C–C bonds within the CNT framework [26]. This structural modification introduces localized tensile strain and modulates sp²-hybridized C–C bonding, thereby promoting electron transitions from 1s to π* orbitals and enhancing surface reactivity. The elongation of sp² bonds at strain sites enhances electronic interactions with chlorine molecules via an initial Yeager-type adsorption mechanism. As a result, this strain-driven process elevates ClO• selectivity from 38.8% (conventional defective CNTs) to 87.5%, enabling deep mineralization of 2,4-dichlorophenol (2,4-DCP). The study underscores the pivotal role of strain fields in governing adsorption configurations and regulating chlorination pathways.

Tuning lattice distortion and cation site occupation in mixed-metal oxides can significantly influence defect formation and catalytic functionality. Zhang et al. synthesized an inverse spinel MnO_x/Fe_1.8O_2.4−x via a sol–gel method, wherein Mn³⁺ preferentially occupies tetrahedral sites, inducing lattice distortion and facilitating the generation of highly active oxygen vacancy (OV) sites near Fe centers [27]. These OVs enhance the adsorption capacity of Fe³⁺ for oxygen atoms, promoting the adsorption and activation of oxygen-containing molecules. The resulting catalyst demonstrates exceptional selectivity for hydrogenating unsaturated aldehydes to their corresponding unsaturated alcohols. For α,β-unsaturated fatty aldehydes, butenal is converted to butenol with a selectivity of 99.9%. Similarly, the hydrogenation of α,β-unsaturated aromatic aldehyde cinnamaldehyde achieves 98.6% selectivity. In bio-based furfural hydrogenation, the catalyst attains a conversion rate of 93.5% with 97% selectivity after 2 h. At 200 °C, the hydrogenation of the unsaturated carbonyl compound trans-benzyl acetone also achieves 99.9% selectivity.

2.3. Charge Transfer and Localization Effects

2.3.1. Defects as Charge Traps

Defect-induced charge trapping is a critical mechanism for achieving spatial charge separation and prolonging carrier lifetimes in photocatalytic systems. Engineered defects can drive spatial charge separation, and facilitate directional migration of photogenerated carriers by acting as electron or hole traps. By suppressing electron-hole recombination and prolonging charge carrier lifetimes, these defects further generate localized energy levels that alter the band structure, thereby significantly enhancing catalytic activity.

In a representative study, Ma et al. synthesized cyano-defect-enriched graphitic carbon nitride (g-C₃N₄) with high surface area (denoted as NCN) and coupled it with metallic 1T-WS₂ cocatalysts to construct a 1T-WS₂/NCN photocatalytic system [28]. The strategically introduced cyano defects in NCN act as electron-trapping centers, promoting spatial segregation of photogenerated charge carriers. This defect-mediated charge separation facilitates the directional migration of carriers, enabling efficient electron excitation from the valence band to unoccupied conduction band orbitals and generating unpaired electrons. Upon photoexcitation, the strongly bound excitons formed within 1T-WS₂ layers interact with these trapped electrons to form trions, which localize at catalytically active edge sites. This charge concentration mechanism culminates in the efficient reduction of H⁺ to H₂. The optimized 1T-WS₂/NCN system exhibits a remarkable hydrogen evolution rate of 5769.3 μmol·g⁻¹ under 180 min of visible-light irradiation, representing a 90.6-fold enhancement compared to pristine g-C₃N₄.

Single-atom catalysts (SACs) are profoundly enhanced by defect engineering strategies that create localized trap states, enabling precise regulation of carrier dynamics and modulation of the electronic band structure. Zhang et al. engineered a single indium-atom catalyst (In-Nv-CNF) by introducing N₂C vacancies into carbon nitride (CN) frameworks [29]. These vacancies induce intraband defect states that function as electron traps, effectively suppressing electron-hole recombination and extending hole lifetimes (Figure 8a). As a result, the In-Nv-CNF achieves a photogenerated hole concentration of 1.12 × 10¹⁵ spins·mm⁻³, surpassing pristine CN and indium nanoparticle-decorated CN (In NPs/CN) by 74.7- and 24.9-fold, respectively. The optimized In-Nv-CNF demonstrates a 50-fold enhancement in photo-oxidative degradation rates compared to pristine CN, achieving efficient decomposition of tetracycline and ciprofloxacin under visible-light irradiation.

Asymmetric active sites, coupled with defect-induced charge localization, can synergistically enhance multielectron catalytic pathways such as C–C coupling in CO₂ reduction. Su et al. engineered asymmetric Cu-Cu^δ+-W^δ+ triatomic sites on oxygen-vacancy-rich CuWO₄ (CWO-OVs) for photodriven CO₂ C–C coupling [30]. OVs functions as electron traps to prolong charge carrier lifetimes and enable a continuous proton-coupled electron transfer (PCET) process (Figure 8b). These synergistic effects drove propionic acid production at 86.46 μmol g⁻¹ h⁻¹ with 89.27% electron selectivity, surpassing pristine CWO. This work demonstrates how defect-mediated charge localization and site asymmetry synergize to optimize multicarbon synthesis.

Embedding catalytic components within defect-rich carbon matrices can significantly enhance interfacial charge transfer and reactive species generation through synergistic electronic modulation. Bai et al. synthesized Co-substituted Bi₃O₄Cl quantum dots (CBO) embedded in defect-rich biochar (NBC) to create NCBO. Carbon defects in NBC act as electron traps, forming localized energy levels that redistribute electron density and modify the band structure [31]. These defect-mediated changes with quantum confinement enhance interfacial charge transfer and regenerate Co/Bi active sites, enabling efficient •OH and ¹O₂ generation for pollutant mineralization. NCBO achieves complete detoxification of TeCG via a self-sustaining redox cycle, offering a novel strategy for advanced oxidation processes.

Cations and anions, functioning as positive and negative charge traps, respectively, can significantly enhance charge separation efficiency and catalytic performance. Zhang et al. engineered Cu-induced adaptive sulfur vacancies on ZnIn₂S₄ nanosheets to regulate charge separation kinetics and construct hydrogen-migration gradient channels [32]. In this system, Cu dopants act as hole traps, while S vacancies serve as electron traps, synergistically suppressing electron-hole recombination. The distinct sulfur atom sites establish a ΔG_H* gradient, enabling surface-confined hydrogen migration driven by photothermal effects without interlayer transport. The optimized catalyst, with 5 mol% Cu confined in ZnIn₂S₄, achieves a hydrogen evolution reaction (HER) rate of 9.8647 mmol g⁻¹ h⁻¹ (14.8-fold enhancement over pristine ZnIn₂S₄) and an apparent quantum efficiency (AQE) of 37.11% at 420 nm, highlighting the pivotal role of defect engineering in tailoring reaction dynamics and catalytic activity.

2.3.2. The Metal-Support Interaction

The metal-support interaction (EMSI) represents a crucial mechanism that can regulate the electronic structure of metal active sites, inducing electron redistribution between the metal sites and support atoms. This interaction effectively enhances the reactivity of the metal sites and modulates the adsorption energy of intermediate species, thereby tuning their adsorption, improving catalytic activity, and controlling reaction pathways.

Xie et al. utilized this principle to synthesize Mo-doped VO₂ nanobelts, where atomically dispersed Mo was embedded within the VO₂ matrix to construct a high-efficiency electrocatalyst for NRR [33]. The system utilizes VO₂ as a support to strongly adsorb H-adsorbed N₂, providing an N₂ source to suppress the competing HER. Critically, electronic metal-support interaction between the support and Mo facilitates electron transfer from Mo to the support, generating electron-deficient regions around Mo. These regions accommodate the lone electron pairs of N₂ molecules, effectively activating the N≡N bond and reducing the energy barrier for the initial hydrogenation step. The electron-deficient sites also inhibit H⁺ adsorption, thereby enhancing the Faradaic efficiency (F_E) of NRR while suppressing HER. The Mo/VO₂ catalyst achieves a remarkable NH₃ yield of 190.1 μg NH₃ mg cat.⁻¹ h⁻¹ and an F_E of 32.4% at −0.5 V vs. RHE, representing 10.8-fold and 2.8-fold improvements over pristine VO₂, respectively. This work provides a strategic approach for designing high-efficiency electrocatalytic NRR catalysts.

Plasma-assisted defect engineering can significantly enhance metal-support interactions by introducing surface vacancies and heteroatom dopants, which jointly modulate interfacial charge transfer and catalytic stability. Zhu et al. developed Ir@Sr-p-TiO₂ nanowires via plasma treatment, integrating Ir nanoparticles and Sr single atoms for bifunctional OER and HER [34]. The enhanced activity stems from plasma-induced oxygen vacancies (Vo•) and Sr adsorption, which strengthen the metal-support interaction between Ir NPs and TiO₂. Vo• and Sr promote charge transfer from TiO₂ to Ir, optimizing the adsorption of OER/HER intermediates on oxygen- and hydroxyl-covered Ir NPs. Strong MSI also enhances structural stability against chemical corrosion. It is worth noting that the OER activity of the Ir@Sr-p-TiO₂ NWs is evaluated by Sr adsorbed d-TiO₂(004) with presence of an Ir cluster of 12 atoms (Ir@Sr-d-TiO₂(004), showed in Figure 9a. Figure 9b shows the OER processes on Ir@Sr-d-TiO₂(004) including four steps. Surprisingly, the catalyst requires overpotentials of only 250 and 32 mV to drive 10 mA cm⁻² for OER and HER, respectively, surpassing commercial IrO₂ and Pt/C. This design underscores the importance of defect engineering and MSI in bifunctional catalysis.

Metal–organic frameworks offer tunable platforms for integrating single atoms with defect-rich supports, enabling covalently electronic metal–support interactions (EMSI) that accelerate interfacial charge transfer. Ren et al. engineered Ru single atoms on defect-rich UiO-66 (Zr) via photochemical strategies. Covalent-bond-mediated EMSI between Ru SAs and UiO-66 accelerates interfacial charge transfer, enhancing photocatalytic activity [35]. The introduction of defects increased the NH₃ yield from 4.57 to 16.28 μmol g⁻¹ h⁻¹, while Ru loading further boosted it to 53.28 μmol g⁻¹ h⁻¹. DFT studies reveal that EMSI-induced charge redistribution directs Ru d-orbital electrons into N₂ π*-antibonding orbitals, activating the N≡N bond. This work advances MOF-based photocatalysts and deepens mechanistic understanding of N₂ activation.

The nature of the EMSI profoundly influences the electronic configuration of single-atom catalysts (SACs), particularly through orbital-level metal-support interactions that dictate adsorption behavior and reaction pathways. Zhang et al. investigated Cu single-atom catalysts on metal oxides (Al₂O₃, CeO₂, TiO₂) to correlate EMSI with CO₂RR performance [36]. EMSI-induced charge transfer modulates Cu’s electron density, yielding distinct highest occupied orbitals: Al₂O₃-CuSAC: 3d_yz facilitates CO adsorption via 3d_yz-π* backdonation, weakening C-O bonds and lowering C–C coupling barriers for multi-carbon products. TiO₂-CuSAC: 3d_z² hybridizes with H₂O σ/σ* orbitals, over-activating H₂O dissociation to favor HER over CH₄. CeO₂-CuSAC: 3d_x²−y² aids CO₂ activation, while localized Cu states suppress C–C coupling. Moderate H₂O activation enables *CO hydrogenation with controlled HER. This study provides a comprehensive orbital-level framework for tailoring SAC-support combinations to achieve product-selective CO₂RR.

Singhvi et al. designed Ni-Cu-Zn/CeO₂ catalysts with strong SMSI between trimetallic nanoparticles and defective CeO₂ [37]. XANES and XAS analyses confirm the electron redistribution from Ni 4s/4p to CeO₂ which forms SMSI, while Cu and Zn develop partial positive charges via Ce interactions. Cu-Ni alloying boosts CO selectivity and stability, while Zn doping enhances CO adsorption and tunes Ni’s electron density. The catalyst achieves 49,279 mmol CO g⁻¹ h⁻¹ at 650 °C with 99% selectivity and 100 h stability, highlighting the synergy of electronic modulation and defect dynamics in CO₂ utilization.

2.4. Modulation of Spin States and Magnetic Coupling

2.4.1. Impact of Spin-State Transitions on Catalytic Reactions

Spin state modulation enables the regulation of electronic structures, modifies the hybridization of electron orbitals, and elevates the d-band center. By altering the spin configuration of transition metal centers, we can reorganize orbital occupancy and facilitate the adsorption and activation of intermediates, thereby enhancing catalytic efficiency.

In regard to this strategy, Wang et al. introduced multiple defects to induce coordination unsaturation at Fe sites in NiFe-layered double hydroxide, increasing d-orbital splitting energy and stabilizing low-spin Fe³⁺ (LS Fe³⁺, t₂g⁵eg⁰, Figure 10) [38]. XANES analysis revealed a disordered NiFe-LS structure due to high defect density, featuring electron transitions from 1s to unoccupied 3d orbitals. Spatially resolved XAS demonstrated spin-dependent electron reorganization between t₂g and eg orbitals, altering hybridization between metal 3d and O 2p states. Compared to high-spin Fe³⁺ (HS, t₂g³eg²), the LS configuration weakened O* intermediate adsorption at Fe³⁺ sites and established an optimal adsorption hierarchy (OH* > O* > OOH*), shifting the rate-determining step from O* → OOH* to more energetically favorable OH* → O*. The catalyst achieved an ultralow overpotential of 244 mV at 500 mA cm⁻² (110 mV lower than HS NiFe-LDH), outperforming most reported NiFe-based systems. This work establishes a mechanistic link between spin-state transitions and adsorption energetics, positioning spin modulation as a key axis in rational catalyst design.

The co-engineering of lattice-site defects and oxygen vacancies offers a synergistic pathway to modulate spin states and electronic configurations of catalysts. Pei et al. co-incorporated La-site defects and oxygen vacancies into MOF-derived La_0.8FeO_3−δ, activating peroxymonosulfate (PMS) for levofloxacin (LVX) degradation under visible light [39]. Dual defects elongated Fe–O bonds, triggering a spin-state transition from low-spin to medium-spin Fe, which upshifted the d-band center and reduced the Fe(II)-PMS* activation barrier. This promoted Fe(IV)=O formation, enhancing PMS adsorption and LVX degradation. The catalyst exhibited exceptional stability under high salinity and natural organic matter, which photoelectrons promote the reduction of Fe(III) to Fe(II), thereby achieving the complete cycle of Fe(II)-Fe(IV)-Fe (II). This study exemplifies how defect-driven spin-state modulation can strategically enhance Fenton-like processes, extending its applicability in advanced oxidation technologies.

Huang et al. demonstrated chemically controlled spin-state transitions in a {Fe-Pt} Hofmann clathrate [40]. High-spin (HS) Fe(II) only facilitated photocatalytic ORR, requiring sacrificial agents. Low-spin (LS) Fe(II) enabled simultaneous ORR and water oxidation (WOR), achieving a H₂O₂ production rate of 66,000 μM g⁻¹ h⁻¹ under visible light. In situ characterization and DFT revealed LS-induced charge transfer between Fe(II) and Pt(IV), lowering energy barriers for ORR/WOR. Beyond water splitting, the system was further applied in upcycling sodium alkenesulfonate to high-value bromopropane, showcasing spin-state gating of cascade reactions. This work pioneers the use of spin-crossover modulation for programmable photocatalysis, showcasing how electronic spin modulation can couple reaction selectivity with catalytic functionality.

2.4.2. Defect-Engineered Magnetic Synergy: Tailoring Cooperative Spin Interactions via Structural Imperfections

Defect engineering induces interatomic ferromagnetic coupling, which imparts a spin-polarized magnetic response to the material, thereby enhancing its catalytic performance under external magnetic fields. This spin polarization facilitates spin-selective charge transfer and intermediate adsorption, particularly beneficial for spin-sensitive catalytic processes such as oxygen evolution or peroxide formation. Sun et al. developed a platform for synthesizing single-atom spin catalysts with broadly tunable magnetic substituents (M1) in MoS₂ matrices [41]. Among all M1/MoS₂ systems, Ni₁/MoS₂ adopts a distorted tetrahedral configuration, enabling ferromagnetic coupling between Ni₁ sites and proximal S atoms. This induces global room-temperature ferromagnetism, which facilitates spin-selective charge transfer to generate triplet O₂ during the OER. In water-splitting cells, Ni₁/MoS₂ exhibits exceptional magneto-catalytic performance, delivering a current density ≈10-fold higher (1.5–1.8 V) under a 502 mT field compared to zero-field conditions, outperforming commercial IrO₂. The enhancement arises from field-induced spin alignment and optimized spin density distribution at S active sites, driven by field-modulated Ni–S orbital hybridization. This tailors adsorption/desorption energetics for radical intermediates (*OH/*O), lowering the O₂ formation barrier by 0.072 eV in ferromagnetic configurations versus antiferromagnetic counterparts. Ni₁/MoS₂ also demonstrates superior stability in pure and seawater electrolytes, highlighting its potential for practical brine electrolysis. These findings establish ferromagnetic single-atom catalysts as platforms for spin-polarized electrocatalysis.

Engineering cation vacancies offer a powerful strategy for modulating spin states and surface electronic structures, with profound implications for magnetic ordering and catalytic activity. Wang et al. synthesized β-Ni(OH)₂ nanosheets with tunable Ni²⁺ vacancy concentrations via hydrothermal growth followed by alkaline etching. The introduction of Ni²⁺ vacancies trigger a spin-state transition, inducing orbital charge redistribution and asymmetric surface charge density [42]. This vacancy-driven modulation converts Ni(OH)₂ from antiferromagnetic to ferromagnetic, while activating electronic states near the Fermi level to enhance intrinsic OER activity. Under magnetic fields, vacancy-rich Ni(OH)₂ exhibits pronounced spin-electron exchange with oxygen intermediates, reducing the OER overpotential by 20 mV at 20 mA cm⁻². In contrast, pristine β-Ni(OH)₂ shows negligible magnetic response. The synergy between defect engineering and spin manipulation offers a paradigm for designing magneto-responsive catalysts, with implications for spin-level mechanistic insights in electrocatalysis.

2.5. Dynamic Defect and Interface Engineering

2.5.1. Defect Reconfiguration Under Reaction Conditions

The generation of dynamic defect sites via electrochemical or allied reactions enables precise modulation of a catalyst’s electronic structure under working conditions, thereby enhancing its intrinsic activity and operational durability. Ren et al. synthesized a self-supported coral-like Ce₂W₂O₉ array on carbon cloth via a facile ion-exchange method [12], followed by in situ electrochemical surface reconstruction to induce W vacancies, which dynamically modifies the local bonding environment of Ce atoms. Experimental and theoretical analyses reveal that optimized Ce sites lower the energy barrier of the potential-determining step (*OOH formation), accelerating O₂ generation and enhancing intrinsic activity. In Ce₂W₂O₉, Ce injects electrons into W sites via Ce–O–W chains. W vacancy formation during reconstruction activates adjacent Ce centers, tuning the Ce–O bond strength and optimizing oxygen intermediate adsorption energetics. The reconstructed R- Ce₂W₂O₉ electrode achieves exceptional stability and durability, delivering 351 mV overpotential at 100 mA cm⁻² and maintaining performance for >1000 h at 20 mA cm⁻². This work provides mechanistic insights into rare-earth oxide electrocatalysts for industrial OER applications.

Controlling interfacial atomic mobility represents a promising strategy for enhancing catalyst stability and activity, particularly in alkaline HER systems. Li et al. engineered a nitrogen-doped carbon shell-encapsulated NiCoS heterostructure (CN@NiCoS) featuring dynamically regulated sulfur migration [43]. Sulfur vacancies at the Ni₃S₂–Co₉S₈ interface served as migration channels, and the migrating S atoms are captured via C–S bonds in the CN shell, inhibiting sulfide dissolution. The synergistic effect of sulfur-doped CN and S-vacancy pairs induces an upshift in the d-band center near the Fermi level, thereby enhancing HER kinetics and durability. The catalyst exhibits low overpotentials of 4.6 mV (freshwater) and 8 mV (seawater) at 10 mA cm⁻², alongside 1000 h stability. This strategy guides interfacial atomic mobility design for robust alkaline HER.

Wang et al. anchored RuO₂ nanoparticles on LiCoO₂ nanosheets (RuO₂/LiCoO₂) to construct a catalyst capable of dynamic self-optimizing during OER [44]. The electron-donating LiCoO₂ induces charge transfer from Co to Ru, resulting in stabilized Ru valence. Operando Li⁺ diffusion and extraction trigger interfacial restructuring, moderately weakening Ru–O covalency to suppress lattice oxygen participation while balancing activity and stability. This restructuring optimizes Ru sites for *OOH intermediate adsorption, thereby reducing the rate-determining step barrier. The catalyst achieves 10 mA cm⁻² at 150 ± 2 mV overpotential with >2300 h stability in 0.5 M H₂SO₄ and operates continuously for 2000 h in proton exchange membrane electrolyzers at 1 A cm⁻². This work resolves the stability-activity trade-off in acidic OER catalysts.

2.5.2. Heterojunction Interface Engineering

The construction of heterojunction interfaces induces an interfacial electric field, which significantly enhances charge transfer and redistribution at the interface. This built-in electric field facilitates the directional migration of photogenerated carriers and promotes the adsorption of reaction intermediates, thereby optimizing catalytic efficiency.

Engineering interfacial electric fields within heterostructures have proven highly effective for regulating electronic structure and accelerating multistep reaction kinetics. As an important heterogeneous component which can be applied to different properties [45,46], WO₃ can be combined with other materials to construct the interfacial electric field and thereby regulate the electronic state. Wang et al. constructed a robust interfacial electric field in WS₂–WO₃ heterostructures, significantly accelerating the electrochemical nitrogen reduction reaction kinetics [11]. Schematic diagram of WS₂–WO₃ synthesis and reaction on the surface is shown in Figure 11. The abundant heterogeneous interfaces in WS₂–WO₃ possess strong interfacial electric fields that upshift the d-band center of W atoms, enhancing the adsorption of intermediate species (*–NH₂ and *–NH). This interface enables lower energy barriers for *–NH₂ and *–NH adsorption compared to pristine components, thereby shifting the rate-determining step from *NNH formation to *N generation while reducing overall surface energy barriers. The optimized WS₂–WO₃ heterostructure demonstrates superior catalytic performance with an NH₃ production rate of 62.38 μg h⁻¹ mg cat⁻¹ and a Faradaic efficiency of 24.24%. This work elucidates how interfacial electric fields in WS₂–WO₃ regulate d-band centers and intermediate adsorption to enhance ENRR, providing critical insights for rational catalyst design.

Defect-engineered heterojunctions incorporating oxygen vacancies offer a powerful strategy to modulate interfacial charge distribution and promote lattice oxygen-mediated mechanisms in OER catalysis. Huang et al. developed a defect-engineered Fe₂O₃@CeO₂-OV heterojunction with abundant oxygen vacancies supported on nickel foam for efficient OER [47]. Oxygen vacancies predominantly localized on CeO₂ nanocluster surfaces intensify Fe₂O₃-CeO₂ interactions, inducing strong CeO₂ → Fe₂O₃ charge transfer and interfacial electron density redistribution. These OVs promote robust cluster-support interactions, downshifting the Ce-5d band center relative to Fe-3d orbitals, thereby reducing free energy barriers for *(O_L-O) formation and O₂ desorption steps in the lattice oxygen-mediated mechanism. The Fe₂O₃@CeO₂-OV electrode achieves ultralow overpotentials of 172 mV and 317 mV at 10 mA cm⁻² and 1000 mA cm⁻², respectively, with exceptional stability.

Coupling plasmonic materials with strain- and defect-engineered semiconductors provides a promising route for enhancing light harvesting and catalytic activation in solar-driven nitrogen fixation. Li et al. designed a CeO₂-AD/Au heterostructure by anchoring strain-engineered CeO₂ nanosheets with Vo defects onto Au hollow nano-mushrooms (HNMs) for N₂ photofixation [48]. Plasmonic Au HNMs broaden light absorption to the NIR-II region while generating hot electrons/holes. Atomically sharp CeO₂/Au interfaces facilitate rapid hot electron transfer, enhancing utilization efficiency. Concurrently, interface compressive strain induces abundant strain-OV defects that promote N₂ adsorption and in situ activation. This synergistic structural/defect engineering enables solar-driven nitrogen fixation efficiency comparable to natural photosynthesis, highlighting its potential for artificial photosynthetic systems.

Integrating oxygen vacancies into heterojunctions offers a dual advantage: modulating interfacial electronic configurations and enhancing carrier transport dynamics. Zu et al. synthesized WO₃-OV/In₂S₃ S-scheme heterojunctions through hydrogen-thermal treatment, where OVs alter interfacial electronic configurations [49]. Sulfur-anchored OVs increase electron density near W 5d band, forming intimate interfacial contact that enhances charge transfer. Femtosecond transient absorption spectroscopy (fs-TAS) reveals OV-induced electron trapping states enable additional charge transport pathways, improving charge separation efficiency by 34.5-fold and 2.9-fold compared to WO₃-OV and In₂S₃ alone, respectively. The optimized heterojunction outperforms non-defective WO₃/In₂S₃, demonstrating OVs’ dual role in carrier dynamics modulation and thermodynamic parameter optimization for photocatalysis.

S-scheme heterojunctions offer a strategic architecture for achieving both directional charge separation and selective retention of high-energy carriers, crucial for efficient photocatalysis. Deng et al. fabricated In₂O₃/Nb₂O₅ S-scheme heterojunctions, achieving sub-10 ps ultrafast interfacial electron transfer [50]. Fs-TAS confirms selective charge recombination between weak electrons (In₂O₃ CB) and holes (Nb₂O₅ VB), while preserving strong electrons (Nb₂O₅ CB) and holes (In₂O₃ VB) for photoreactions. CO₂-TPD and DFT calculations verify enhanced CO₂ chemisorption/activation at heterointerfaces. The optimized nanofibers achieve a CO production rate of 0.21 mmol g⁻¹ h⁻¹ without sacrificial agents, showcasing fs-TAS’s utility in probing S-scheme charge dynamics. This work establishes a framework for designing advanced heterostructures with dual-enhanced charge separation and reactant activation capabilities.

3. Multiscale Characterization of Defect–Electronic Structure–Performance Correlations

3.1. Defects Characterization

3.1.1. Electron Paramagnetic Resonance (EPR)

Electron paramagnetic resonance is a widely used technique for characterizing material defects. Its working principle relies on the Zeeman splitting of unpaired electron spin states under an external magnetic field, where specific microwave frequencies induce spin-state transitions. The resulting absorption signals provide critical insights into electronic spin configurations. EPR is particularly effective in probing transient radical intermediates in chemical reactions, determining oxidation states and coordination geometries of transition metal ions, and investigating semiconductor defects (e.g., dopant sites) and catalytic active centers. Through its ability to resolve spin-sensitive interactions at the atomic scale, EPR has become an indispensable tool for elucidating structure–activity relationships in catalytic and electronic materials.

In the study by Wu et al., defect-rich iron-nickel oxyhydroxide (d-(Fe, Ni)OOH) was synthesized for efficient OER [51]. The introduction of engineered oxygen vacancies not only enhanced active site exposure and maximized the catalyst’s effective surface area but also modulated the local coordination environments and chemisorption properties of Fe/Ni sites. EPR confirmed the oxygen vacancies, with d-(Fe, Ni)OOH exhibiting a stronger magnetic signal at g ≈ 2.004 compared to pristine (Fe, Ni)OOH, indicating a higher unpaired electron density induced by oxygen defects (Figure 12).

In MOFs-related materials, Ding et al. developed defective MOFs (denoted as NiFc*_′xFc_1–x) featuring unsaturated metal sites [52]. These defects facilitated in situ transformation into metal hydroxides during OER, yielding materials with abundant oxygen vacancies that promoted the adsorption of oxygen intermediates and significantly boosted OER activity. EPR analysis revealed an intensified and broadened characteristic signal at g = 2.13 for NiFc′Fc, attributed to increased unpaired electrons from Fc incorporation.

In recent studies about defects in transition metal oxides, Bi et al. demonstrated the synthesis of Ni-doped TiO₂ containing triply associated O-Ti-O vacancies (V_OV_TiV_O) [53]. Hydroxyl groups adsorbed on unsaturated cationic sites critically promoted V_OV_TiV_O formation by facilitating H₂O dissociation and OH* deprotonation, enabling defect regeneration. EPR spectra showed distinct signals at g = 2.002 (unpaired electrons at oxygen vacancies). Ni-TiO₂-N₂ exhibited the strongest signal, confirming abundant oxygen vacancies. Strikingly, Ni-TiO₂-Air displayed an additional signal at g = 1.99, attributed to Ti vacancies, and g = 1.97 (Ti³⁺ centers in TiO₂), highlighting annealing atmosphere dependent vacancy configurations.

3.1.2. Positron Annihilation Lifetime Spectroscopy (PALS)

Positron Annihilation Lifetime Spectroscopy is a widely used technique for investigating material defects. When high-energy positrons (β⁺) are implanted into a material, they annihilate with electrons, emitting gamma rays. By measuring annihilation lifetimes, the size and concentration of defects such as vacancies and voids can be deduced. PALS enables quantitative evaluation of microscopic defects in metals, ceramics, polymers, and other materials. In addition, PALS is also employed to track crystallization processes in amorphous materials, segmental motions in polymer chains, pore size distribution/connectivity in nanoporous materials (e.g., MOFs, zeolites), and radiation-induced defects in nuclear or semiconductor systems.

In-depth characterization of vacancy configurations in doped metal oxides can be achieved through a combination of PALS and DFT simulations. In the study by Bi et al., triple O-Ti-O vacancy associates (V_O-V_Ti-V_O) in Ni-doped TiO₂ nanoparticles were characterized using PALS combined with DFT calculations [53]. All samples exhibited three distinct lifetime components (τ₁, τ₂, and τ₃) in their PALS profiles: τ₁ (~250 ps): Related to positron annihilation in defect-free regions or small defects (e.g., single vacancies or vacancy clusters). τ₂ (350–550 ps): Corresponded to large vacancy clusters in crystalline and grain boundary regions. τ₃ (5–15 ns): Attributed to positronium annihilation at large voids or interfaces. DFT calculations revealed that the primary defect in Ni-TiO₂-air (τ₁ = 232.3 ± 4.1 ps) originated from O-Ti-O trimeric vacancy associates (V_O-V_Ti-V_O), while Ni-TiO₂ (197.8 ± 2.2 ps) and Ni-TiO₂-N₂ (197.7 ± 4.5 ps) contained isolated single vacancies (V_O, V_Ti) or O-O vacancy clusters (V_O-V_O/V_O-V_O-V_O), with predicted lifetimes of 189.8–199.0 ps. This combined experimental–computational framework enables atomic-level identification of vacancy types and configurations, offering a robust strategy for elucidating defect structures in functional metal oxides.

Recent studies about multicomponent nanozyme systems also showed the effectiveness of PALS. Zhang et al. developed a symbiotic nanozyme system by anchoring bacteroid-like CeO_x nanoclusters onto Mn₃O₄ nano-supports (CeO_x/Mn₃O₄) [54]. A “material exchange” process between Ce and Mn atoms optimized the ratios of Ce³⁺/Ce⁴⁺ and Mn³⁺/Mn²⁺ while enhancing vacancy concentrations via interfacial defect engineering. The resulting CeO_x/Mn₃O₄ exhibited robust catalase-like (CAT-like) and superoxide dismutase-like (SOD-like) activities. PALS analysis gradually increased τ₁ values (163.9 ± 7.3 ps for CeO₂, 167.1 ± 1.3 ps for Mn₃O₄, and 204.3 ± 2.6 ps for CeO_x/Mn₃O₄), indicating a rise in both the number and size of small vacancy-type defects within the annealed symbiotic system (Figure 13). The τ₁ ≈ 163 ps signal in CeO₂ was attributed to VO’’ and VO’’VO’’ vacancy configurations.

3.2. Electronic Structure Characterization

3.2.1. X-Ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS)

X-ray Photoelectron Spectroscopy and Ultraviolet Photoelectron Spectroscopy are among the most widely used techniques for analyzing the electronic structures of nanomaterials. XPS, a surface-sensitive analytical method based on the photoelectric effect, provides critical insights into elemental composition, chemical states, and electronic configurations at material surfaces. It enables the direct identification of surface elements, quantitative concentration analysis, differentiation of oxidation states, and even functional group characterization. In contrast, UPS employs ultraviolet photons to excite valence electrons, elucidating valence band energy-level distributions. This technique is particularly suited for studying organic compounds, semiconductors, and adsorbed species, as it measures work functions, determines band structures (e.g., direct/indirect bandgaps in semiconductors/insulators), and probes molecular orbitals such as the highest occupied molecular orbital (HOMO) and electron delocalization. Together, XPS and UPS offer complementary insights into surface chemistry and electronic structure, making them indispensable tools in the characterization of nanoscale materials and interfaces.

For intermetallic alloy catalysts, XPS successfully determines the surface characteristics including engineered porosity and defect architectures Wu et al. synthesized a highly porous intermetallic alloy (CoCuMoNi) with abundant defect sites [55]. The CoCuMoNi catalyst possesses the bifunctional water decomposition performance of both HER and OER, attributed to the ternary interaction of heterogeneous structure interfaces, abundant defects and internal electric fields. XPS analysis revealed the chemical states and electron transfer dynamics in the nanoporous CoCuMoNi electrode and control samples (Figure 14). Multiple oxidation states were identified: Ni⁰ and Ni²⁺ for Ni; Mo⁰, Mo⁴⁺, and Mo⁶⁺ for Mo; Cu⁰, Cu¹⁺, and Cu²⁺ for Cu; Co⁰, Co²⁺, and Co³⁺ for Co. The electronegativity differences among the elements and the heterojunction structure induced significant interfacial charge redistribution. Covalent bonding between Mo-Ni, Cu-Ni, and Co-Ni facilitated electron transfer from less electronegative Mo to more electronegative Ni, followed by further transfer to Cu and Co. Conversely, Co-Mo and Co-Cu bonding enabled electron transfer from Mo to Co and subsequently to Cu. Overall, Ni and Mo acted as electron donors, while Cu and Co served as acceptors, forming a robust, highly active catalytic architecture.

In terms of intra-band defect levels, Xu et al. developed a WO_3−x/In₂S₃ S-scheme heterojunction with tunable intra-band defect levels for enhanced CO₂ photoreduction [13]. The charge transfer pathway was investigated via radical trapping experiments and in situ irradiated XPS (ISI-XPS). Compared to pristine WO_3−x and In₂S₃, the composite showed significantly intensified DMPO-•OH and DMPO-•O₂⁻ signals, confirming efficient separation and preservation of high-energy electrons (In₂S₃ conduction band, CB) and holes (WO_3−x valence band, VB). Under illumination, W 4f and O 1s binding energies (BEs) in WO_3−x/In₂S₃ shifted positively, while In 3d and S 2p BEs exhibited negative shifts, directly evidencing electron transfer from WO_3−x to In₂S₃ and validating the S-scheme mechanism. Notably, WO_3−x/In₂S₃-550 displayed the most pronounced BE shifts, indicating that optimized intra-band defect levels enhanced charge carrier transfer efficiency.

Li et al. engineered a TiO₂ heterostructure, c-TiO₂@a-TiO_2−x(OH)_y, featuring surface-frustrated Lewis pairs of HO-Ti-[O]-Ti embedded in an amorphous shell surrounding a crystalline core [56]. These SFLPs can heterolytically dissociate H₂, forming protonated hydroxyls and hydrides, which achieved CO₂ reduction. UPS analysis mapped the band-edge positions: the valence band edges of c-TiO₂ and a-TiO_2−x(OH)_y (0.0015 < x < 0.0031) were −7.7 eV and −7.59 eV (vs. vacuum), respectively, with corresponding conduction band edges at −4.13 eV and −4.21 eV (Figure 15). The band structure information obtained from UPS provides a basis for analyzing the electron-hole transfer, separation and recombination mechanisms.

3.2.2. X-Ray Fine Structure Analysis (XAFS)

X-ray Absorption Fine Structure is an advanced characterization technique that analyzes subtle energy-dependent variations in X-ray absorption. It provides quantitative insights into the local atomic structure including coordination environment, bond lengths, and chemical states. It comprises two core components: X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). XANES focuses on the fine structure near the absorption edge, where edge shifts reflect oxidation states and pre-edge peak intensity/shape reveals orbital hybridization. EXAFS analyzes post-edge oscillations, using frequency to determine bond lengths, amplitude to quantify coordination numbers, and phase shifts to identify neighboring atoms.

Yang et al. engineered Fe–N₃ sites with varying Fe–N bond lengths via carbon defect introduction, leveraging XAFS for structural differentiation [57]. Fe K-edge XANES spectra exhibited similar pre-edge features and line shapes due to shared “T” geometry across Fe–N₃ configurations, yet the main peak shifted systematically (Figure 16). Compared to the reference (M0), M1 and M4 showed lower main peak energies, while M3, M5, and M6 displayed higher energies. According to Natoli’s rule, these shifts inversely correlate with the average Fe–N bond length squared. By modifying bonding distances via defects, the local coordination structure of Fe–N₃ sites was tailored, directly impacting catalytic thermochemical activity. Critically, this structural tuning was quantitatively linked to XANES features, establishing an experimentally accessible structure–activity descriptor.

X-ray analyses are also capable of determining mixed metal valence state, which is crucial for transition metals. Liu et al. designed a heteronuclear FeMn dual-atom catalyst anchored on nitrogen-doped carbon (FeMn_DSA/dNC) using a defect-trapping strategy [58]. This defect engineering optimized electronic structures toward the Sabatier optimum, significantly enhancing ORR performance. XAFS elucidated the electronic/coordination environments: Fe K-edge XANES positioned absorption edges between FeO and Fe₂O₃, indicating mixed Fe²⁺/Fe³⁺ states. The k³-weighted Fourier-transform EXAFS revealed a primary Fe–N shell at ~1.50 Å and a heterometallic Fe–Mn scattering path at ~2.45 Å. Mn K-edge XANES matched Mn phthalocyanine (Mn²⁺), with EXAFS confirming Mn–N bonds (~1.38 Å) and Fe–Mn distances (~2.45 Å). EXAFS fitting and wavelet transform analysis validated the heteronuclear Fe–Mn pairs: Fe coordinates with four N atoms and one Mn atom, while Mn adopts a similar N₄ configuration with Fe coupling. Distinct WT intensity maxima (Fe: 4.8 Å⁻¹; Mn: 5.1 Å⁻¹) diverged from metallic Fe/Mn standards (8.2 and 7.7 Å⁻¹), confirming atomic dispersion.

In addition, Zhou et al. developed a defect-driven activation strategy for Co-based MOFs (D/CoFc-MOF), tuning adsorption behavior via local coordination and electronic reconfiguration [59]. XAS analysis revealed that Co K-edge XANES showed edge shifts toward higher energy in D/CoFc-MOF vs. CoFc-MOF, suggesting increased Co oxidation states (+2.1 to +2.2). EXAFS identified the first Co–O coordination shell (~1.5 Å), with shortened bond lengths in D/CoFc-MOF indicating enhanced Co–O covalency (Figure 17). WT-EXAFS at ~3.8 Å⁻¹ confirmed Co–O paths without Co–Co contributions, proving atomic dispersion. Electronic structure analysis via orbital-projected partial density of states (PDOS) revealed t₂g (d_xy, d_yz, d_xz) and e_g (d_x², d_z²) orbital occupations, with defect engineering reducing d-electron counts (7.15 → 7.13 e⁻) at Co orbitals and increasing the oxidation state of Co.

3.2.3. Electron Energy Loss Spectroscopy (EELS)

Electron Energy Loss Spectroscopy is a characterization technique that analyzes energy losses of high-energy electrons interacting with materials to determine the composition, electronic structure, and chemical environments, often coupled with transmission electron microscopy (TEM). EELS enables elemental identification via ionization edges, resolves unoccupied electronic states through near-edge fine structures, distinguishes chemical states, and extracts coordination information via extended energy loss fine structure (EXELFS) oscillations. Additionally, EELS probes valence/conduction band structures and interfacial charge transfer dynamics.

Li et al. anchored strain-engineered CeO₂ nanosheets with abundant strain-vo defects onto Au hollow nano-mushrooms, constructing a semiconductor/plasmonic metal heterostructure [48]. Interfacial compressive strain induced strain-vo defects that enhanced nitrogen adsorption and in situ activation. Aberration-corrected HAADF-STEM and EELS mapped Ce³⁺ and Ce⁴⁺ distributions in CeO₂-AD/Au (Figure 18). EELS spectra of Ce M-edges revealed reduced peak positions in regions near Au HNMs (Region 1: M₅-edge by 1.8 eV, M₄-edge by 2.0 eV) compared to bulk CeO₂ (Region 2). The M₅/M₄ intensity ratio in Region 1 matched Ce³⁺, while Region 2 aligned with Ce⁴⁺. Oxygen K-edge EELS highlighted weakened peak a intensity (O–Ce⁴⁺ bonds) in Region 1, corroborating Ce³⁺ predominance at the CeO₂/Au interface. This Ce³⁺ enrichment, stabilized by strain-induced vo defects, enhanced catalytic stability. In contrast, CeO₂-LD/Au exhibited significantly reduced interfacial Ce³⁺ content, as quantified by EELS valence mapping.

In another study, Li et al. synthesized a TiO₂ heterostructure, c-TiO₂@a-TiO_2−x(OH)_y, featuring surface-frustrated Lewis pairs of HO-Ti-[O]-Ti in an amorphous shell encapsulating a crystalline core [56]. Ti L-edge EELS at interfacial regions revealed structural and electronic disparities: Core regions showed characteristic Ti(IV) L_2,3-edge splitting, consistent with crystalline TiO₂ symmetry. Surface regions exhibited suppressed splitting and a −2.2 eV energy shift, indicative of Ti³⁺-rich amorphous a-TiO_2−x(OH)_y with distorted Ti–O₆ octahedra and O vacancies. The reduced t_2g-e_2g splitting and symmetry breaking at the interface were attributed to charge redistribution and stabilization of low-valent Ti in oxygen-deficient amorphous domains.

3.3. Theoretical Calculations

3.3.1. Density Functional Theory (DFT) Calculations

Density Functional Theory is a quantum mechanics-based computational method that investigates the ground-state properties of many-body systems by describing electron density rather than wavefunctions. As one of the most widely used theoretical tools in computational chemistry and materials science, DFT enables predictions of energy, structure, electronic properties, and reaction mechanisms for molecular, crystalline, and surface systems. Its applications span from evaluating molecular stability assessments, calculating band structure and density of states, mapping charge distribution, and analyzing transition-state/reactivity.

In the study by Zhou et al., DFT calculation was applied to analyze the electron properties of MOFs. They developed a defect-driven activation strategy to control adsorption behavior in Co-based MOFs (denoted as D/CoFc-MOF) by modulating local coordination geometry and electronic configurations [59]. DFT calculations were performed to analyze DOS, crystal orbital Hamilton population (COHP) of Co–O bonds, d-band center shifts (E_d), and energy-level alignments. Structural models included a pristine CoFc-MOF and its defect-engineered variant (D/CoFc-MOF) with non-bridging Fc’ ligands, where Co sites were preferentially exposed as active centers for oxygen evolution reaction (OER). The defect-induced band engineering markedly changed the E_d level from −1.60 eV (CoFc-MOF) to −1.83 eV (D/CoFc-MOF), optimizing electron transfer efficiency and reshaping electron density distributions. This enabled balanced adsorption/desorption of oxygen intermediates (OH*, O*, OOH*) through enhanced Co–O covalency, as evidenced by: Stronger orbital overlap between Co-3d and O-2p states in D/CoFc-MOF. Integrated COHP (ICOHP) values confirm tighter Co–O bonding compared to CoFc-MOF.

In another study, Chen et al. utilized ultrasonic cavitation to introduce controlled defects into FeCoNi/FeAl₂O₄ hybrid coatings, optimizing OER activity [60]. DFT studies on the heterojunction’s electronic structure employed: Special quasi-random structure (SQS) models for FeCoNi (metallic phase) and FeAl₂O₄ (spinel phase), validated against HAADF-STEM data. Charge density difference maps showing electron accumulation at the heterointerface, with Bader charge analysis confirming electron depletion in FeCoNi and accumulation in FeAl₂O₄. Projected DOS (PDOS) revealing d-d orbital coupling between Co/Ni 3d states near E_F, critical for intermediate stabilization. Broad e_g-t_2g splitting in Fe 3d orbitals, indicative of rapid electron transfer, and Al 2p/O 2p hybridization enhancing metal–oxygen covalency. These computational insights rationalized the superior OER performance, with E_d shifts and interfacial charge redistribution serving as key descriptors for defect-mediated catalyst optimization.

3.3.2. Machine Learning(ML)-Driven Prediction

Machine Learning leverages data-driven pattern recognition to predict chemical compositions, properties, reaction pathways, and material performance, significantly accelerating chemical research and materials discovery. Lin et al. demonstrated a closed-loop workflow integrating ML-assisted prediction, multi-objective optimization, and experimental synthesis to coordinately optimize defect content and thermal stability in UiO-66(Ce), achieving enhanced catalytic hydrogenation of dicyclopentadiene [61]. The ML workflow comprised three key steps:

• Automated Data Extraction: A line-recognition algorithm and thermogravimetric analysis curve evaluation method were developed to automate data extraction and batch calculations, minimizing manual errors and improving efficiency.
• Pure-Phase Subspace Delineation: Given the sensitivity of UiO-66(Ce) synthesis to parameters, ML screened >200,000 experimental protocols to define a tailored synthesis subspace, preventing impurity phases like CSUST-1 or Ce(HCO₂)₃.
• Closed-Loop Optimization: A hybrid utility function combining probability of achievement and weighted-sum techniques balanced defect density and thermal stability.

This enabled Pareto-frontier optimization, achieving target MOFs (T_d > 300 °C, defect content > 40%) within two experimental iterations. The optimized UiO-66(Ce) exhibited approximately 10-fold higher activity in DCPD hydrogenation compared to conventional approaches. This work establishes a universal data-driven paradigm for intelligent materials synthesis, advancing beyond traditional trial-and-error methods reliant on prior knowledge.

4. Defect Engineering-Driven High-Efficiency Catalytic Applications

4.1. Hydrogen Evolution Reaction (HER)

Hydrogen Evolution Reaction refers to the electrochemical process of generating hydrogen gas at the electrode surface via reduction, a pivotal reaction in water-splitting technologies for hydrogen production. As a key half-reaction in water-splitting and electrolysis systems, HER plays a central role in sustainable hydrogen production and renewable energy conversion technologies. Defect engineering has emerged as a powerful strategy to enhance HER activity by increasing active sites, modulating electronic structures, promoting charge transfer, and enabling surface reconstruction and synergistic effects.

Heteroatom doping is an effective strategy to manipulate the electronic structure and defect states in photocatalysts for enhanced hydrogen evolution performance. Pei et al. synthesized a boron-doped CRP (B-CRP) photocatalyst via a mild boric acid-assisted hydrothermal strategy. Boron incorporation filled vanadium vacancies, suppressed deep trap states, and introduced shallow trap states in CRP’s band structure [62]. These STS prolonged charge carrier lifetimes by inhibiting deep trapping and recombination, while boron dopants acted as active sites to boost HER. The optimized B-CRP achieved a photocatalytic HER rate of 1392 μmol g⁻¹ h⁻¹, outperforming pristine CRP (4-fold) and ARP (10-fold). Stability tests confirmed structural integrity and sustained activity over multiple cycles.

Ligand-assisted defect and phase engineering provides a versatile route for tuning interlayer structure, orbital interactions, and enhancing HER catalytic activities. Liu et al. developed a Co-doped 1T-MoS₂ catalyst (Co-1T-MoS₂-bpe) via ligand modulation using a CoMo-MOF precursor [63]. The multidentate 1,2-bis(4-pyridyl)ethane (bpe) ligand expanded interlayer spacing, exposed active sites, and accelerated water dissociation. DFT calculations revealed that π-conjugated ligand coordination weakened Mo 3d/S 2p orbital hybridization, enhancing S-2p/H-1s orbital overlap for optimized H adsorption. Co-1T-MoS₂-bpe delivered an ultralow overpotential of 118 mV at 10 mA cm⁻² in alkaline HER (Tafel slope: 83 mV dec⁻¹) via the Volmer-Heyrovsky pathway, and achieved overpotential at 200 mA cm⁻² which is lower than Pt/C. Integrated into an anion-exchange membrane water electrolyzer, it maintained 100 mA cm⁻² at 2.0 V for 10 h, surpassing Pt/C performance.

In addition to metal doping, nonmetallic heteroatom doping also showed the capability for improving HER activities. Zhang et al. engineered N,B,P/S-codoped carbon catalysts (BPN-C, BSN-C) from ZIF-derived N-doped carbon [64]. Nonmetal dopants induced defects, redistributed electron density, and increased active sites. BSN-C exhibited superior HER activity in alkaline media: η₁₀ = 129.7 mV and a Tafel slope of 72.7 mV dec⁻¹, outperforming BPN-C, BN-C, and N-C. After 1000 cycles, BSN-C retained 90.68% of the onset overpotential, demonstrating exceptional stability.

4.2. Oxygen Evolution Reaction (OER)

The oxygen evolution reaction is an anodic process generating oxygen through oxidation, serving as a critical half-reaction in water splitting for oxygen production and a cornerstone of metal-air batteries and CO₂ reduction technologies. Defect engineering—introducing vacancies, dopants, and grain boundaries—enhances OER activity by tailoring electronic structures and surface microenvironments to reduce reaction energy barriers.

Dynamic reconstruction of MOFs into active transition metal (oxy)hydroxides has emerged as a promising approach to OER activity through in situ defect and composition modulation. Zhang et al. incorporated monocarboxylic acid-functionalized linkers (ferrocene carboxylic acid, FcCA) into NiBDC-FcCA MOF, creating unsaturated sites that dynamically reconstruct into Fe-doped Ni(OH)₂/NiOOH during OER [65]. Oxygen vacancies and Fe dopants persisted throughout reconstruction, optimizing electron transport and weakening oxygen intermediate adsorption on Ni sites to favor OOH* formation. NiBDC-FcCA exhibited a ninefold higher current density at 1.6 V vs. NiBDC. The optimized NiBDC-FcCA/Fe foam achieved an OER overpotential of 250 mV at 200 mA cm⁻², with negligible activity loss after 1200 h at 1 A cm⁻². When integrated into an anion-exchange membrane water electrolyzer, it delivered 1 A cm⁻² at 1.85 V, outperforming commercial Pt/C||RuO₂ (1.92 V).

Defect engineering in 2D MOFs offers a tunable platform for enhancing bifunctional electrocatalysis (OER/HER) by optimizing active site exposure and intermediate adsorption energetics. Zhao et al. employed targeted defect engineering to create high-density unsaturated Fe sites in 2D Fe-MOFs. These defects promoted H₂O adsorption and *OH intermediate formation, modulating OER rate-determining steps [66]. Oxygen vacancies further optimized O and H adsorption, enabling dual OER/HER functionality. The optimized Fe-MOF achieved overpotentials of 259 mV (OER) and 36 mV (HER) at 10 mA cm⁻² in 1.0 M KOH. Fe-MOF_0.3 demonstrated exceptional stability in AEMWE under cyclic potentials (1.6–1.9 V), maintaining performance for 125 h.

Anion defect engineering offers a promising route to modulate lattice structure, charge distribution, and catalytic pathways for OER enhancement. Gu et al. introduced anionic iodine into Cu lattices, inducing charge redistribution and lattice distortion to regulate surface reconstruction during OER [67]. Iodine defects weakened Cu–S bonds, facilitating Cu²⁺ → Cu³⁺ oxidation and generating oxygen vacancies (V_O). These V_O sites provided spatial regions for OH⁻ adsorption and shifted the OER pathway to an oxygen vacancy oxidation mechanism (OVOM). The optimized V_O-I_0.033-CuOS achieved a 189 mV overpotential at 10 mA cm⁻² and 1250 h stability. In an AEMWE electrolyzer, it delivered 1 A cm⁻² at 1.65 V with a hydrogen production cost of $1.70/kg, surpassing the U.S. DOE’s 2026 target.

Understanding the distinct roles of cationic and anionic vacancies is essential for rationally designing high-performance OER catalysts. Zhang et al. investigated the roles of O (V_O) and Co (V_Co) vacancies in defective Co₃O₄ in initial activity and reconstruction process [68]. Initial OER activity followed Co₃O₄-V_Co > Co₃O₄-V_O > pristine Co₃O₄, indicating the improvement from the introduction of Co or O vacancies. Specifically, V_O enhanced *OH adsorption but slowed deprotonation, driving irreversible reconstruction into amorphous [Co(OH)₆] intermediates, while Co vacancies accelerated deprotonation but reduced the reconstruction rate. Both O and Co vacancies yields reconstructed Co bridge sites with high OER activity, leading to the increase or decrease in the Co–Co bond length, respectively, which Co₃O₄-V_Co has the best activity (η₁₀ = 262 mV vs. 320/300 mV for pristine Co₃O₄/Co₃O₄-V_O).

4.3. Carbon Dioxide Reduction(CO₂RR)

The electrochemical and photochemical reduction of carbon dioxide offers a sustainable route to convert greenhouse gas into value-added fuels and chemicals. CO₂ reduction is a catalytic process driven by external electric fields or light to convert CO₂ into products including CO, CH₄, C₂H₄, and HCOOH. Defect engineering—introducing vacancies, dopants, or grain boundaries—enhances CO₂RR activity and selectivity by optimizing electronic structures, exposing active sites, promoting CO₂ adsorption/activation, modulating intermediate adsorption energies, and improving charge transfer kinetics.

Engineering heterojunctions and anion vacancies provide a synergistic approach to enhance light absorption, charge separation, and intermediate stabilization in photocatalytic CO₂ reduction. Lai et al. synthesized an In₂S₃/In₂O₃ spatial heterojunction (ISIO(V_S)) with sulfur vacancies via polyvinylpyrrolidone treatment [69]. The heterojunction and S vacancies enhance visible-light utilization and charge separation. S vacancies lower the work function of In₂S₃, intensifying built-in electric fields to improve charge separation. DFT calculations reveal that S vacancies induce charge accumulation on adjacent In atoms, facilitating CO₂ activation. In situ DRIFTS and ΔG analysis confirm that ISIO (VS) exhibits a lower energy barrier for *CHO formation than CO desorption, favoring CH₄ selectivity (95.93%, 16.52 μmol g⁻¹ h⁻¹). S vacancies also strengthen In–C interactions in *CHO intermediates, lowering ΔG_*CHO.

Gradient defect engineering enables spatial modulation of electronic structure and internal electric fields to enhance photocatalytic CO₂ reduction. Wang et al. introduced a 3–4 nm gradient W vacancy layer on Bi₂WO₆ nanosheets, creating an inner-to-outer tandem homojunction with a graded Fermi level [70]. This directional internal electric field drives photoelectron migration from bulk to surface, providing a “highway” for charge separation. W vacancies alter the coordination of O/W atoms, shifting CO₂ adsorption from weak/strong to moderate bonding, reducing the *COOH formation barrier. The defect-engineered Bi₂WO₆ achieves a CO production rate of 30.62 μmol g⁻¹ h⁻¹ with 99% selectivity, without requiring cocatalysts or sacrificial agents.

Defect engineering in layered double hydroxides (LDHs) offers an effective strategy to tune surface reactivity and adsorption properties for selective CO₂ photoreduction. Wang et al. engineered oxygen vacancies (V_O) on ZnAl LDH to regulate CO₂ adsorption and surface reactivity [71]. V_O alters the electronic structure of Zn/Al sites, enabling selective chemisorption of H₂O and CO₂. Under illumination, CO₂ adsorbs on Al sites to form *COOH, which is subsequently protonated to *CO. Simultaneously, H₂O dissociates at Zn sites to supply protons. Dynamic structural evolution (layer expansion and crystal contraction) enhances CO production, with Vo-ZnAl achieving 17.2 μmol g⁻¹ h⁻¹ significantly outperforming pristine ZnAl (6.3 μmol g⁻¹ h⁻¹).

Introducing linker vacancies in MOFs offers a strategic route to promote dynamic structural evolution during CO₂RR catalysis. A et al. developed a Cu-MOF (UC-Cu-BTEC) with linker vacancies for CO₂RR [72]. Vacancies induce distortion in Cu dimers within the framework, preserving structural integrity while creating undercoordinated Cu sites. During CO₂RR, defective Cu dimers reconstruction generate abundant uncoordinated Cu species, enhancing *CO coverage and stabilizing C₂ intermediates. This promotes C–C coupling, yielding a C₂₊ Faradaic efficiency (F_EC2+) of 77.2% and sustaining F_EC2+ > 70% across a 400 mV potential window. In flow cells, UC-Cu-BTEC achieves a C₂₊ partial current density of -153.5 mA cm⁻² under neutral conditions.

4.4. Ammonia Synthesis Catalysis

Ammonia synthesis catalysis refers to the reduction of nitrogen sources (e.g., N₂) into ammonia (NH₃) via electrochemical or catalytic processes. This reaction is central to fertilizer production, energy storage, and emerging green hydrogen carriers. Defect engineering optimizes catalytic activity by modulating lattice defects (e.g., vacancies, doping, dislocations) to tailor electronic structures, enhance surface reactivity, suppress side reactions, and expose active sites for nitrogen activation.

Single-atom catalysts with engineered coordination environments offer a promising platform for high-efficiency nitrate reduction to ammonia. Ding et al. developed a defect-enriched coordination polymer (d-CoCP) with well-defined, unsaturated single-atom Co sites [73]. Methanol etching of a cobalt-based coordination polymer (CoCP) introduced nitrogen vacancies, creating localized undercoordinated Co sites. These vacancies reduced the energy barrier of the rate-limiting step in nitrate reduction to ammonia (NRA), enhancing NO₃⁻ adsorption and conversion. The optimized d-CoCP achieved an NH₃ production rate of 0.78 mmol h⁻¹ cm⁻² at 173.3 mA cm⁻² with 97% Faradaic efficiency (F_E), surpassing pristine CoCP and other single-atom catalysts. Stability tests confirmed negligible activity loss over 100 h.

The coupling of intrinsic piezoelectric fields with defect engineering offers a unique strategy for enhancing nitrogen reduction reaction performance under piezo-photocatalytic conditions. Yuan et al. engineered BaTiO₃ (BTO) with oxygen vacancies to synergize defects and piezoelectric fields for nitrogen reduction (NRR) [74]. Ti³⁺ sites near OVs possess unpaired d-orbital electrons, facilitating N₂ activation. Piezoresponse force microscopy and Kelvin probe force microscopy revealed enhanced polarization and charge separation in Ti³⁺-OV-modified BTO. In situ characterization and DFT studies showed that piezoelectric fields shift the Ti³⁺ d-band center toward the Fermi level, lowering the energy barrier for the rate limiting step. The optimized BTO with moderate oxygen vacancies achieved an NH₃ yield of 106.7 μmol g⁻¹ h⁻¹ under piezo-photocatalysis.

The size and spatial distribution of surface defects significantly influence nitrogen reduction performance by modulating adsorption dynamics and competing reaction pathways. Sun et al. synthesized VO_x nanobelts with controlled defect sizes (0–1 nm, 1–2 nm, > 5 nm) [75]. Small defects limited spatial flexibility for N₂ hydrogenation, while large defects favored competing HER. Medium-sized defects (1–2 nm) balanced electronic and geometric effects, promoting optimal N₂ adsorption and activation. The intermediate defect size yielded an NH₃ rate of 81.94 ± 1.45 μg h⁻¹ mg⁻¹ and F_E of 31.97 ± 0.75% at −0.5 V vs. RHE, demonstrating the critical role of defect dimensions in eNRR efficiency.

The synergistic integration of surface-loaded noble metals and defect-rich transition metal frameworks provides a powerful strategy for enhancing nitrogen reduction electrocatalysis. Guo et al. fabricated multi-defect nanocrystalline Cu via thermal quenching, where stress-induced defects and nonequilibrium phase transitions enhanced intermediate adsorption and proton transport [76]. Surface-loaded Ir species provided active H*, synergizing with defect-rich Cu to modulate the d-band center and optimize intermediate adsorption. The CuIr electrode achieved 87% NH₃ selectivity and 6.1 mmol h⁻¹ cm⁻² yield at −0.22 V vs. RHE, outperforming annealed counterparts.

4.5. Volatile Organic Compounds (VOCs) Degradation

Volatile organic compounds are toxic or carcinogenic pollutants (e.g., benzene, formaldehyde, chlorofluorocarbons) that pose significant risks to environmental and human health. These compounds are commonly emitted from industrial emissions, vehicle exhaust, and household products. Their degradation—typically transforming VOCs into benign products (e.g., CO₂, H₂O)—is critical for air purification. Defect engineering enhances VOC degradation efficiency by tailoring atomic-scale defects (e.g., vacancies, doping, lattice distortion) to optimize surface oxygen activity, inhibit charge recombination, and promote free radical generation.

Embedding atomically dispersed active sites within nanostructured carbon frameworks presents a promising approach for advanced VOC degradation via reactive oxygen species generation. Wei et al. designed a ZnO-carbon nanoreactor with carbon vacancy-rich Fe-N₄ sites via a self-carbothermal reduction strategy [77]. Carbon vacancies near Fe-N₄ sites facilitated C–O bond formation, enabled O to obtain more electron transfered from C at C–O and reduced the rate-determining step’s energy barrier, enhancing the oxygenation and reduction activity. The optimized Fe-NCv-900 activated peroxymonosulfate to generate reactive oxygen species, achieving 97% phenol removal within 5 min (13.5-fold faster than origin Fe-NC-900). The catalyst also exhibited broad pH adaptability (3–9).

Engineering high-index facets and defect-rich metal oxide supports offers a robust strategy for boosting redox activity and pollutant degradation selectivity. Sun et al. synthesized Ru-loaded high-index anatase TiO₂ {201} (Ru/{201}-TiO₂) rich in oxygen vacancies (V_O) and Ti³⁺ defects [78]. The strong metal-support interaction and defect-mediated electron transfer enabled efficient O₂/H₂O activation and ROS generation. Ru/{201}-TiO₂ achieved 100% chlorobenzene conversion without polychlorinated byproducts, with excellent stability for olefinic and alkenes CVOCs.

Amorphous metal oxide catalysts with tailored electronic structures and abundant defect sites offer promising platforms for efficient VOC degradation. Zhu et al. developed amorphous CeMn_x catalysts, where CeO₂ dissolution into MnO₂ lattices created abundant defects [79]. Ce doping increased the conduction band’s density of states, narrowed the bandgap, and reduced the work function, enhancing electron transfer for chlorobenzene (CB) degradation. At 200 °C, CeMn_0.16 achieved higher CO₂ yield and Cl selectivity than MnO₂.

In another study, Zhong et al. introduced pentagon defects into Co-N₄-anchored carbon (C₅-Co-N₄) via selective pyridine N-etching [80]. The defects elevated Co’s electron density and d-band center, enabling efficient PMS activation. C₅-Co-N₄ exhibited a turnover frequency (TOF) of 9.37 min⁻¹ and degraded 4-chlorophenol (4-CP) with 0.38 wt.% Co loading, outperforming conventional catalysts.

4.6. Other Catalytic Reactions

Defect engineering has emerged as a pivotal strategy to enhance diverse catalytic reactions, including selective toluene oxidation and glycerol oxidation. Shi et al. developed Ni-doped monolayered Bi₂WO₆ nanosheets (Ni/BWO) with varying Ni mass fractions for selective photocatalytic toluene oxidation [81]. Ni dopants mediate cascade active units at the atomic scale, comprising unsaturated metal atoms and spatially separated Bi···O frustrated Lewis pairs (FLPs). These FLPs activate toluene C–H bonds via concerted Bi···C and O···H coordination, with O₂ adsorbed at unsaturated W sites. This configuration bridges charge transfer, accelerating toluene oxidation by photoinduced radicals. Additionally, FLPs and W sites establish a unique pathway for C–H dehydrogenation, O₂ activation, and oxygen transfer. At 1.8 wt.% Ni, Ni/BWO achieves a toluene conversion rate of 4560 μmol g⁻¹ h⁻¹ with over 90% benzaldehyde selectivity.

Wu et al. engineered a spatially asymmetric defect structure in CuCo₂O₄ by partially refilling sulfur into high-density oxygen vacancies (HV_O-S) [82]. This optimization lowers the glycerol adsorption barrier and thermodynamically favors C–C bond cleavage. The HV_O-S catalyst delivers a Faradaic efficiency (F_E) of 98.5% for formate at 1.36 V vs. RHE, outperforming low-vacancy catalysts (HV_O: 75.3%). DFT calculations reveal that asymmetric defects enhance local charge transfer, reduce glycerol activation barriers, and facilitate secondary C–C cleavage. The strategy also improves C–C scission in ethylene glycol and glucose electrooxidation, demonstrating universal applicability. In a paired electrolysis system coupling glycerol oxidation with nitrate reduction (NO₃-RR), HV_O-S (anode) and CuCo₃N (cathode) achieve a cell voltage of 1.27 V at 10 mA·cm⁻², with 94% FE for formate, 99% FE for NH₃, and 24% energy savings compared to RuO₂//Pt/C.

5. Conclusions and Perspectives

Defect engineering has emerged as a transformative strategy for precise modulation of material electronic structures, offering capabilities far beyond the intrinsic limitation of conventional perfect crystals, demonstrating immense potential in catalysis. By introducing vacancies, dopants, grain boundaries, and other defect configurations, it enables atomic-scale tailoring of band structures, charge transport, surface adsorption, and spin polarization. This review systematically elucidates the coupling mechanisms between defects and electronic properties, summarizes advanced characterization techniques and computational tools, and highlights recent breakthroughs in defect-engineered catalysis. Key mechanisms include defect-induced energy levels, Fermi level shifts, coordination environment tuning, lattice distortion, charge trapping, metal-support interactions, spin-state transitions, ferromagnetic coupling, defect dynamics, and interface engineering.

Despite significant progress, critical challenges remain:

• Precise Structure–Property Relationships: Quantitative models linking defect types (point/line/surface), spatial distribution (monodispersed/clustered), and concentrations to electronic responses are lacking. Breakthroughs require ultrafast spectroscopy (e.g., femtosecond X-ray absorption) combined with machine learning to establish a “defect configuration–electronic response” database.
• Multi-Defect Synergy: Single defects often suffer from limited functionality or instability. Cooperative defects (e.g., oxygen vacancies with metal dopants) can induce interfacial charge redistribution and strain–field coupling. Future work should focus on in situ nanofabrication for spatially ordered assembly of multi-defect systems.
• Dynamic Defect Quantum Control: Current studies emphasize static defects, yet dynamic evolution under operando conditions (e.g., electrochemical-bias-induced vacancy migration) may trigger electronic state reorganization or quantum phase transitions. Atomic-resolution spatiotemporal characterization (e.g., in situ TEM or time-resolved X-ray probes) is essential.
• Sustainable Application: Advancing green synthesis and scalable manufacturing will align defect engineering with global sustainability and carbon neutrality goals.
• AI-Driven Design: Graph neural networks could predict defect-induced electronic perturbations, while automated platforms like droplet microreactors enable tailored synthesis for energy catalysis or optoelectronics.

Defect engineering is fundamentally reshaping the paradigm of materials science, transitioning from “structure determines properties” to a forward-looking perspective of “defects create functionality”. With the integration of atomic-scale manufacturing and multiscale theory, the field will evolve from “observing defects” to “programming defects,” offering transformative solutions to global energy and environmental crises.