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Review

Recent Progress and Future Perspectives of MNb2O6 Nanomaterials for Photocatalytic Water Splitting

by
Parnapalle Ravi
and
Jin-Seo Noh
*
Department of Semiconductor Physics, Gachon University, 1342 Seongnam-daero, Sujeong-gu, Seongnam-si 13120, Gyeonggi-do, Republic of Korea
*
Author to whom correspondence should be addressed.
Materials 2025, 18(15), 3516; https://doi.org/10.3390/ma18153516
Submission received: 30 June 2025 / Revised: 24 July 2025 / Accepted: 25 July 2025 / Published: 27 July 2025
(This article belongs to the Special Issue Advanced Metal Oxide and Its Composites for Electro/Photo-Catalytic Hydrogen Generation and Supercapacitors)
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Abstract

The transition to clean and renewable energy sources is critically dependent on efficient hydrogen production technologies. This review surveys recent advances in photocatalytic water splitting, focusing on MNb2O6 nanomaterials, which have emerged as promising photocatalysts due to their tunable band structures, chemical robustness, and tailored morphologies. The objectives of this work are to (i) encompass the current synthesis strategies for MNb2O6 compounds; (ii) assess their structural, electronic, and optical properties in relation to photocatalytic performance; and (iii) elucidate the mechanisms underpinning enhanced hydrogen evolution. Main data collection methods include a literature review of experimental studies reporting bandgap measurements, structural analyses, and hydrogen production metrics for various MNb2O6 compositions—especially those incorporating transition metals such as Mn, Cu, Ni, and Co. Novelty stems from systematically detailing the relationships between synthesis routes (hydrothermal, solvothermal, electrospinning, etc.), crystallographic features, conductivity type, and bandgap tuning in these materials, as well as by benchmarking their performance against more conventional photocatalyst systems. Key findings indicate that MnNb2O6, CuNb2O6, and certain engineered heterostructures (e.g., with g-C3N4 or TiO2) display significant visible-light-driven hydrogen evolution, achieving hydrogen production rates up to 146 mmol h−1 g−1 in composite systems. The review spotlights trends in heterojunction design, defect engineering, co-catalyst integration, and the extension of light absorption into the visible range, all contributing to improved charge separation and catalytic longevity. However, significant challenges remain in realizing the full potential of the broader MNb2O6 family, particularly regarding efficiency, scalability, and long-term stability. The insights synthesized here serve as a guide for future experimental investigations and materials design, advancing the deployment of MNb2O6-based photocatalysts for large-scale, sustainable hydrogen production.

1. Introduction

Ensuring a reliable and sustainable global energy supply constitutes one of the foremost scientific and technological challenges of the 21st century. As of 2008, global energy consumption reached approximately 15 terawatts (TW), and projections suggest that this demand could nearly double by 2050 due to continued population growth and industrial expansion [1]. A range of alternative energy sources—including solar, wind, hydropower, and geothermal—offer more environmentally benign and sustainable options compared to conventional fossil fuels. Nevertheless, the integration of these renewable resources into the energy infrastructure presents significant challenges owing to their intrinsic limitations. Geothermal energy, for instance, is constrained by high operational costs and finite site lifespans. In detail, geothermal energy is a clean, reliable baseload power source, but its wider adoption is limited by high capital and operational costs. Installation costs range from USD 2000 to 6000 per kW, with drilling alone making up over half of this. A single well can cost up to USD 10 million, and exploratory failures increase expenses. Annual operation and maintenance add USD 150–190 per kW, with reinjection accounting for 12–15% of costs in some plants. The levelized cost of electricity (LCOE) rose from about USD 0.049/kWh in 2010 to USD 0.071/kWh in 2020, exceeding USD 0.10/kWh in difficult locations, making it less competitive than solar or wind. Geothermal plants typically maintain efficiency for 25–30 years, with reservoir declines of 0.5–1% annually. For instance, Kenya’s Aluto–Langano plant’s output fell from 7.3 MW to under 1 MW over 20 years due to reservoir degradation [2,3,4,5,6]. These issues highlight the need for innovation and cost-cutting measures to ensure geothermal energy’s long-term viability. Hydropower is associated with considerable environmental impacts and the substantial capital investment required for dam construction. Wind energy, while abundant and clean, suffers from intermittency and the lack of cost-effective large-scale energy storage solutions [7].
Hydrogen (H2) is considered an environmentally benign energy carrier and has garnered significant attention in response to the depletion of conventional fossil fuels such as petroleum and coal [8,9]. Among the various methods for hydrogen production, photocatalytic water splitting stands out as an efficient and sustainable approach, as it harnesses solar energy—a freely available and abundant resource globally. Consequently, hydrogen holds considerable promise as a clean energy source for the future, given its non-toxic nature and its potential to be produced from readily available natural resources like water and sunlight. These sources are both renewable and environmentally sustainable [10]. A wide range of semiconductor materials has been investigated for photocatalytic H2 evolution [11], with titanium dioxide (TiO2) remaining one of the most extensively studied candidates due to its advantageous attributes—namely, non-toxicity, robust photochemical stability, and economic viability [10]. Despite these merits, TiO2 is intrinsically limited by its wide bandgap (~3.2 eV), which restricts photoresponse to the ultraviolet (UV) region, comprising less than 5% of the solar spectrum. Moreover, the material exhibits a high electron–hole recombination rate, which further constrains its photocatalytic efficiency. To overcome these deficiencies, considerable effort has been directed toward heterojunction engineering, wherein TiO2 is coupled with narrow bandgap semiconductors capable of visible-light absorption and improved charge carrier separation. This strategy enhances solar light utilization and suppresses recombination losses, thereby improving overall photocatalytic performance [12,13,14]. Among the emerging visible-light-active semiconductors, graphitic carbon nitride (g-C3N4) has attracted significant attention. This metal-free polymeric material features a moderate bandgap of approximately 2.7 eV, making it suitable for harnessing visible light. In addition to its favorable optoelectronic properties, g-C3N4 exhibits notable thermal and chemical stability and can be synthesized via straightforward thermal polycondensation processes using low-cost precursors such as urea or melamine [15,16,17]. Thermogravimetric analysis studies consistently demonstrate that g-C3N4 is thermally stable up to approximately 600 °C in air or nitrogen atmospheres, with significant decomposition occurring only above this temperature [18,19]. For example, g-C3N4 materials synthesized from dicyandiamide or melamine exhibit pronounced stability until around 600 °C, after which gradual thermal degradation takes place. Further studies confirm that the material maintains its structural integrity under ambient and moderate high-pressure conditions up to 600–700 °C, with decomposition to graphite and nitrogen observed only above these thresholds. This high decomposition temperature underscores the notable thermal stability of g-C3N4 and supports its advantageous application as a robust photocatalyst in various high-temperature processes. These characteristics render g-C3N4 a compelling component in TiO2-based heterojunction systems, offering synergistic effects that advance the design of next-generation photocatalysts for sustainable hydrogen production. This pivotal discovery has spurred the development of a wide range of novel inorganic semiconductor photocatalysts aimed at enhancing H2 production efficiency. These materials include strontium titanate (SrTiO3) [20], zinc sulfide (ZnS) [21], niobates [22,23], TiO2 [24,25,26], CdS [12], cuprous oxide (Cu2O) [27], zinc oxide (ZnO) [28], bismuth ferrite (BiFeO3) [29], lanthanum ferrite (LaFeO3) [30], perovskite-type metal oxides [13,31], as well as tin dioxide (SnO2) [32], tungsten trioxide (WO3) [33], hematite (Fe2O3) [34], niobium pentoxide (Nb2O5) [23], vanadium pentoxide (V2O5) [35], and tantalum pentoxide (Ta2O5) [26,36]. The majority of these materials have band gaps greater than 3.0 eV, meaning they can only respond to ultraviolet (UV) light. Given that UV light constitutes only a small portion of the solar spectrum, visible-light-active materials are required to produce a greater amount of hydrogen.
In recent years, several review articles have been published focusing on hydrogen production through photocatalytic water splitting. For instance, the review by Acar et al. provides a comprehensive overview of various photocatalytic water-splitting methods and discusses key parameters such as bandgap engineering, performance evaluation, and chemical recycling strategies [37]. Similarly, Corredor et al. examined the influence of different system components on photocatalytic performance, specifically analyzing the roles of sacrificial agents, catalysts, photosensitizers, and solvents in hydrogen evolution [38]. Another notable review addresses a broader range of renewable energy sources, with a dedicated section on photocatalytic hydrogen production [39]. It highlights aspects such as the structural and optical properties of photocatalysts, photostability, the role of sacrificial reagents, recent technological developments, and limitations associated with photodetection systems.
However, this review offers a detailed discussion on narrow band gap materials of the AB2O6 family. In particular, transition metal niobates with the general formula MNb2O6 (M = Cu, Ni, Mg, Zn, Co, Fe, Mn., etc.) represent a class of emerging photocatalysts due to their favorable electronic structure, chemical stability, and visible-light activity. These materials typically exhibit orthorhombic or monoclinic crystal structures, with tunable bandgaps ranging from ~2.0 to 3 eV depending on the transition metal cation. Their tunable electronic structures, relative chemical stability, and visible light absorption capabilities make them promising candidates to replace conventional wide bandgap materials.

2. Methodology

This review was conducted through an extensive survey of the peer-reviewed literature on MNb2O6 nanomaterials, focusing on their synthesis, structural and optical properties, and photocatalytic water-splitting performance. Relevant studies were collected from databases such as ScienceDirect, Web of Science, Scopus, SpringerLink, and Google Scholar using targeted keywords including “MNb2O6,” “photocatalysis,” “water splitting,” and “hydrogen evolution.” Selection criteria included experimental or theoretical studies published between 2000 and 2025 that reported on the synthesis, band structure, photocatalytic activity, and compositional modifications of MNb2O6 materials. Key parameters, such as synthesis methods, bandgap energies, photocatalytic H2 evolution rates, and material structures, were extracted, compared, and critically analyzed. The goal was to provide a clear summary of the current state of research, identify promising compounds, highlight underexplored materials, and propose future directions for improving MNb2O6-based photocatalysts for solar-driven hydrogen generation.

3. Synthetic Approaches for MNb2O6 Nanomaterials

The synthesis of MNb2O6 compounds has been achieved through a variety of methods, each offering specific advantages in terms of morphology control, crystallinity, and scalability. Key methods employed in the literature are summarized below.

3.1. Chemical Transport Method

Early structural investigations of MNb2O6 compounds, including NiNb2O6, were performed using chemical transport methods. This approach involves transporting precursor materials in the vapor phase using agents such as Cl2 within a two-zone or vertical furnace system. The method enabled the growth of high-quality single crystals, suitable for detailed crystallographic and optical studies [40].

3.2. Solid-State Reaction

The solid-state reaction remains a widely used conventional method for synthesizing polycrystalline MNb2O6 materials. Typically, metal oxides (e.g., NiO, CuO) and Nb2O5 are mixed in stoichiometric ratios and subjected to high-temperature calcination. This method is often used as a precursor step for single crystal growth techniques like the floating zone method, which provides large, defect-minimized crystals for fundamental studies [41].

3.3. Solvothermal and Hydrothermal Methods

Low-temperature solution-based techniques, such as solvothermal and hydrothermal synthesis, have gained attention for producing MNb2O6 nanostructures with controlled morphology and phase purity. In solvothermal routes, metal chlorides and NbCl5 are dissolved in organic solvents and reacted in sealed autoclaves at elevated temperatures. Hydrothermal methods, by contrast, utilize aqueous basic media under similar thermal conditions to generate nanocrystals or microstructured powders. Reaction parameters such as temperature, duration, pH, and precursor ratios play crucial roles in determining the final product’s particle size and surface area—factors critical for photocatalytic efficiency [42,43].

3.4. Electrospinning

Electrospinning, followed by thermal treatment, has been employed to fabricate one-dimensional nanofibers of MNb2O6 materials such as CoNb2O6. This technique combines a polymer-based spinning solution with applied voltage to generate continuous fibers, which are then annealed to obtain crystalline oxide nanofibers. This method offers the advantage of producing high-aspect-ratio nanostructures with interconnected porous frameworks, beneficial for charge transport in photocatalysis [44].

4. Structural Features of MNb2O6 Nanomaterials

MNb2O6 compounds commonly crystallize in distorted columbite-type structures (orthorhombic, space group Pbcn) or related monoclinic phases. The columbite-type structure remains stable for various divalent transition metal ions such as Co, Fe, and Cu in the MNb2O6 framework [42,45,46], allowing for the tuning of electronic conductivity by selecting specific M-site cations or their combinations. As illustrated in Figure 1, the columbite crystal structure [47,48] comprises layers of slightly distorted hexagonal close-packed oxygen octahedra oriented perpendicular to the a-axis. Within each b–c plane, cation-occupied octahedra form zigzag chains running along the c-axis, connected through shared edges. Along the a-axis, the octahedrally coordinated cations follow a periodic sequence of M–Nb–Nb–M–Nb–Nb–M, contributing to the anisotropic nature of the crystal structure and influencing both structural and electronic properties. As shown in Figure 1, the columbite structure includes three distinct types of oxygen coordination environments: O1, which is coordinated to two Nb atoms and one M atom (represented as gray spheres); O2, which is bonded to one Nb and two M atoms (white spheres); and O3, which is shared among three Nb-centered octahedra (black spheres). These variations in oxygen coordination play a crucial role in defining the local bonding environment and influence electronic and catalytic behavior.
The M-site cations occupy interstitial positions between the NbO6 frameworks and influence the overall distortion and connectivity of the octahedra. In orthorhombic columbite structures, the Nb atoms form chains of edge-sharing octahedra along one crystallographic axis. The degree of distortion in the NbO6 units varies depending on the size and electronic configuration of the M cation. This structural flexibility allows for tunable electronic and optical properties, making MNb2O6 materials suitable for photocatalytic applications. Additionally, the presence of M2+/M3+ cations can introduce localized electronic states, which affect band structure and charge separation dynamics. The distinguished crystallographic parameters for MNb2O6 (M = Fe, Cu, Mg, Zn, Co, Mn, and Ni) ceramics are summarized in Table 1. All samples are crystallized in a single-phase columbite structure with the same Pbcn space group [49].

5. Morphological Features of MNb2O6 Nanomaterials

The morphology of MNb2O6 nanomaterials plays a pivotal role in determining their photocatalytic efficiency, particularly for water-splitting applications. Key morphological attributes such as particle size, shape, aspect ratio, crystallinity, and surface area directly affect light absorption, charge carrier mobility, and surface reaction kinetics.
Various synthesis strategies have been employed to tailor these morphological features (Table 2). For instance, Lee et al. synthesized MnNb2O6, CuNb2O6, and ZnNb2O6 nanoparticles via a solvothermal method, producing uniformly distributed nanocrystals with high surface areas, which facilitated visible-light-driven photocatalytic activity [42]. Similarly, Chun et al. prepared ZnNb2O6 nanocrystals using a hydrothermal route, achieving rod-like morphologies that enhanced light scattering and improved surface reactivity [43]. One-dimensional (1D) morphologies have shown particular promise due to their enhanced charge transport properties. Park et al. synthesized CoNb2O6 nanofibers through electrospinning followed by thermal annealing, resulting in a fibrous network with improved conductivity and extended photogenerated charge carrier lifetimes [44].
Moreover, hollow or porous structures and hierarchical assemblies of MNb2O6 are being actively explored, as they provide high surface-to-volume ratios, short diffusion paths, and more exposed reactive facets. Such morphological engineering is vital for maximizing the active interface between the photocatalyst and the electrolyte, thereby enhancing water-splitting efficiency. Despite these advances, most studies have focused on only a few MNb2O6 compounds, such as MnNb2O6 and CoNb2O6. Systematic morphological studies on other members of this family (e.g., MgNb2O6, FeNb2O6, NiNb2O6) remain limited. Additionally, there is a need for correlative studies linking specific morphological traits to photocatalytic performance metrics to better inform rational design strategies.
In summary, tailoring the morphology of MNb2O6 nanomaterials is essential for optimizing their photocatalytic behavior. Future research should emphasize morphology–performance relationships across the broader MNb2O6 family, using advanced characterization techniques such as SEM, TEM, BET, and AFM, alongside performance testing under standard photocatalytic conditions.

6. Optical Properties of MNb2O6 Compounds

A critical factor influencing the photocatalytic performance of MNb2O6 materials is their band gap energy, which determines their light absorption capability. We have compiled and compared available experimental band gap values with theoretical calculations to provide a comprehensive overview.
Experimental band gaps of MNb2O6 compounds typically fall within the visible to near-UV range. For instance, MnNb2O6 exhibits band gaps between 2.3 and 2.7 eV, making it active under visible light [42]. CuNb2O6 shows somewhat narrower band gaps around 1.9–2.1 eV, which correlates with its strong photocatalytic and photoelectrochemical activities [51]. CoNb2O6′s band gap is around 2.4 eV, supporting its visible-light-driven hydrogen evolution capabilities [44]. NiNb2O6 presents band gaps from 2.2 to 2.9 eV, consistent with its reported photocatalytic hydrogen production under UV-visible irradiation [52].
In contrast, ZnNb2O6, MgNb2O6, and FeNb2O6 exhibit wider band gaps ranging from approximately 3.6 to 4.0 eV, restricting their absorption primarily to the UV region [43,50]. These values align with their observed photocatalytic activities, mainly for UV-assisted processes rather than visible-light water splitting.
Theoretical band gap calculations obtained via density functional theory (DFT) tend to slightly overestimate experimental values but remain in good agreement overall. For example, DFT predicts band gaps of ~2.5–2.8 eV for MnNb2O6 and ~2.0–2.2 eV for CuNb2O6, closely matching experimental measurements. Similarly, theoretical values for ZnNb2O6 and MgNb2O6 are about 3.7–4.2 eV, consistent with experimental data (Table 3).
This comparison not only validates theoretical models but also reinforces the suitability of particular MNb2O6 compounds (e.g., MnNb2O6, CuNb2O6, CoNb2O6, NiNb2O6) for visible-light-driven photocatalytic water-splitting applications. The wider band gap materials may require further modifications such as doping or heterojunction formation to enhance visible light absorption and overall photocatalytic efficiency.
The band structures of all the compounds studied by Naveed-Ul-Haq et.al, obtained through GGA + U calculations and plotted along high-symmetry paths (Figure 2a–d), reveal wide insulating band gaps ranging from 2.64 to 2.98 eV. In comparison, experimental band gap values have been reported as 2.21–2.48 eV for MnNb2O6 [56] and 2.42 eV for CoNb2O6 [57]. NiNb2O6 shows a notably lower band gap of 1.86 eV, which closely aligns with the 1.98 eV reported by Karmakar et al. [58]. Notably, the band gap remains consistent across both the majority and minority spin channels. The Fermi level is positioned at the top of the valence band, suggesting a p-type conductivity character for these materials.
Figure 3a–d present the total density of states (TDOS) along with the element-resolved partial density of states (PDOS) for the investigated compounds. In MnNb2O6, CoNb2O6, and NiNb2O6, the valence band (VB) edge is predominantly composed of O 2p orbitals, with smaller contributions from the d-orbitals of the respective transition metals (Mn, Co, and Ni). In contrast, for FeNb2O6, the upper VB edge is mainly dominated by Fe 3d states, with only minimal involvement from O 2p orbitals. On the conduction band (CB) side, the lower edge in both MnNb2O6 and FeNb2O6 is primarily shaped by Nb 4d states, accompanied by minor contributions from O 2p states. For CoNb2O6, the Co 3d orbitals contribute more significantly to the CB minimum compared to the Mn and Fe counterparts. Uniquely, in NiNb2O6, the lower VB edge is almost entirely formed by Ni 3d states, differing from the other compounds. All the materials exhibit an antiferromagnetic ground state, with no net magnetic moment, as the spins of the transition metal ions (Mn, Fe, Co, and Ni) are aligned in an antiparallel configuration [54,59].
Moreover, the oxygen vacancies are prevalent point defects in niobates and typically act as donor states, shifting the Fermi level closer to the conduction band and thereby favoring n-type conductivity. In contrast, copper-based niobates may exhibit p-type behavior due to the involvement of Cu 3d orbitals in hole generation; their defect chemistry and behavior in related systems suggest that Mg, Fe, Ni, Zn, and Mn variants are predominantly n-type (Table 4). Co and Cu substitutions, however, may introduce more complex or ambipolar conduction characteristics.

7. Photocatalytic Performance of MNb2O6 Compounds

The MNb2O6 family, where M represents divalent or transition metals such as Mn, Fe, Co, Ni, Mg, Cu, and Zn, exhibits a rich variety of structural and optical properties, positioning these materials as attractive candidates for photocatalytic water splitting. However, only a subset of these materials has demonstrated significant activity under experimental conditions, highlighting both the potential and the gaps in our current understanding.

7.1. Visible-Light-Active Niobates: MnNb2O6, CuNb2O6, and NiNb2O6

Among the MNb2O6 compounds, MnNb2O6, CuNb2O6, and NiNb2O6 are particularly promising due to their moderate band gaps (~2.2–2.9 eV), which enable visible-light absorption.
MnNb2O6-based Composites: Several studies have explored MnNb2O6 integrated with g-C3N4 or porous morphologies to enhance photocatalytic activity. For instance, a MnNb2O6/g-C3N4 binary system synthesized via solvothermal and sonochemical methods achieved degradation efficiencies over 94% for various antibiotics under visible light, indicating strong potential for water purification and solar energy applications. High-surface-area MnNb2O6 also demonstrated enhanced performance in photodegradation tasks.
CuNb2O6 Photocatalysis: CuNb2O6 has been used both as a standalone material and in composites. Li et al. fabricated p-type CuNb2O6 via spray pyrolysis for use as a photocathode in photoelectrochemical cells. CuNb2O6/g-C3N4 and CuNb2O6 polymorph studies revealed that monoclinic phases exhibit superior photocatalytic behavior. Further, CuNb2O6-based heterojunctions, such as TiO2/CuNb2O6, significantly enhanced hydrogen evolution rates, with performance reaching 146 mmol h−1 g−1 in optimized composites (Figure 4). In detail, the structural analysis by X-ray diffraction (XRD) confirms the successful formation of the TiO2/CuNb2O6 composite, exhibiting distinct peaks corresponding to both anatase TiO2 and orthorhombic CuNb2O6 phases with no impurity signals, which indicates good crystallinity and effective interfacial contact. Morphological studies using SEM and TEM reveal that CuNb2O6 nanoparticles are uniformly anchored on the TiO2 surface, providing a high density of well-connected interfacial sites that facilitate efficient charge transfer and limit electron–hole recombination. Optical analysis, based on diffuse reflectance UV-Vis spectroscopy, shows that the composite’s band gap is significantly narrowed (2.26–2.77 eV) compared to pure TiO2 (~3.2 eV), resulting in extended visible light absorption up to around 410 nm. This improved visible-light harvesting capability is directly linked to the composite’s enhanced photocatalytic activity [60].
NiNb2O6 Heterojunctions and Solid-State Systems: NiNb2O6 has been investigated in heterostructures with reduced graphene oxide (RGO) and exfoliated g-C3N4 (E-gC3N4), showing efficient degradation of pollutants and enhanced photocatalytic activity under visible and sunlight irradiation. S-scheme heterojunctions like TiO2/NiNb2O6 composites improved charge separation, achieving hydrogen production rates up to 114 mmol h−1 g−1. Solid-state synthesized NiNb2O6 also showed promising hydrogen evolution under visible light without co-catalysts, with a band gap around 2.9 eV (Figure 5). In detail, XRD analysis confirms the coexistence of both TiO2 and NiNb2O6 phases in the composite, indicating successful synthesis. SEM and TEM images show that NiNb2O6 is well-dispersed on the TiO2 surface, resulting in good interfacial contact. Optical measurements reveal a reduced band gap for the composite compared to pure TiO2, enabling enhanced absorption of visible light and improved photocatalytic activity [52].

7.2. CoNb2O6: Emerging Potential Through Heterostructure Engineering

CoNb2O6 has shown measurable hydrogen evolution activity when integrated into heterostructures, such as with TiO2 in S-scheme composites. These engineered systems benefit from improved charge separation and extended light absorption, leading to enhanced photocatalytic performance. Despite this progress, direct evidence of H2 production from pure CoNb2O6 without composite engineering remains scarce. Satiya et al. reported the design and fabrication of a novel 2D/2D CoNb2O6/g-C3N5 type-II heterojunction photocatalyst aimed at enhancing visible-light-driven degradation of cefuroxime, a commonly used antibiotic and emerging water pollutant. The composite structure was engineered to maximize interfacial contact between the layered CoNb2O6 and g-C3N5, promoting efficient charge separation and transfer across the heterojunction. This synergistic interaction significantly improved the photocatalytic performance under visible light irradiation [61]. In a recent study, Mohammad et al. investigated the electrochemical properties of trirutile-phase CoNb2O6 microspheres and reported their pseudocapacitive behavior governed by anion intercalation mechanisms. This form of charge storage, which involves reversible insertion/extraction of anions within the electrode structure, significantly enhances the material’s capacitive performance. Furthermore, the authors demonstrated that CoNb2O6 exhibits considerable electrocatalytic activity toward water-splitting reactions, indicating its dual functionality as both an energy storage medium and a catalyst for sustainable energy applications [62].

7.3. Limited Exploration: ZnNb2O6, MgNb2O6, and FeNb2O6

Experimental data on MgNb2O6 and FeNb2O6 remain scarce for photocatalytic water splitting, although their structural properties have been characterized.
ZnNb2O6 Systems: ZnNb2O6 has a wide band gap (~3.8 eV), restricting activity to UV light. However, composites like ZnNb2O6/g-C3N4 integrated with nitrogen-doped graphene quantum dots (NGQDs) have improved visible-light hydrogen evolution. Optimized heterojunctions showed rates of 340.9 µmol h−1 g−1 (Figure 6). Furthermore, noble metal-loaded ZnNb2O6 (e.g., with Pt or Ag) significantly enhanced hydrogen generation, achieving up to 3200 µmol h−1 g−1 in methanol–water systems (Figure 7).
MgNb2O6 Applications: While MgNb2O6 is not yet established for water splitting, it has shown promise in dye degradation under UV light. Recent work on Dy- and Er-doped MgNb2O6 materials has highlighted their wide bandgap (~4–5 eV) and moderate dye-removal efficiencies, confirming their suitability for pollutant degradation.
Finally, MnNb2O6, NiNb2O6, CoNb2O6, and CuNb2O6 have shown the most promising hydrogen evolution rates under visible light, attributed to their optimal band gaps (2.1–2.9 eV), well-ordered crystal structures, and favorable morphologies. On the other hand, MgNb2O6, FeNb2O6 and ZnNb2O6 lack detailed photocatalytic hydrogen production reports. Their structural and morphological features suggest potential, but the absence of systematic hydrogen evolution studies represents a clear research gap.

8. Influence of Specific Surface Area on the Photocatalytic Performance of MNb2O6 Nanomaterials

The specific surface area of MNb2O6 nanomaterials, as determined by BET (Brunauer–Emmett–Teller) analysis, plays a crucial role in influencing their photocatalytic water-splitting performance. A higher specific surface area provides a greater number of active sites for surface redox reactions, enhances the adsorption of water and sacrificial agents, and facilitates efficient charge separation by shortening the migration paths of photogenerated carriers. For example, in the TiO2/CuNb2O6 composite reported by Ravi et al., the BET surface area increased from ~91 m2/g for pure CuNb2O6 to ~132 m2/g for the composite. This enhancement corresponded with a significant improvement in hydrogen evolution rate from ~21 mmol g−1 h−1 to ~146 mmol g−1 h−1 under visible light. Similarly, the g-C3N4/ZnNb2O6 heterojunction developed by Yan et al. exhibited a surface area of ~56.8 m2/g compared to ~18.9 m2/g for pure ZnNb2O6, resulting in an increased H2 generation rate of ~340 µmol g−1 h−1. In both cases, the elevated surface area enabled better light absorption, enhanced reactant diffusion, and reduced electron–hole recombination. These results underscore that tailoring the surface area through heterostructure formation and nanostructuring is a viable strategy to improve the photocatalytic efficiency of MNb2O6 materials, making BET analysis a vital tool in photocatalyst optimization. The comparative Table 5 below summarizes the structural, morphological features, synthesis methods, surface areas, and photocatalytic hydrogen production performance of various MNb2O6 compounds.
Among the MNb2O6 family, Mn, Cu, Ni, and Co-based compounds have shown the most promise for visible-light photocatalytic hydrogen evolution. Their performance is largely dependent on material morphology, composite design, and the engineering of heterojunctions that facilitate charge separation. Conversely, Mg, Fe, and Zn-based niobates require further exploration to unlock their full potential. Future work should prioritize systematic comparisons, cocatalyst optimization, and exploration of doped and hybrid systems across the full MNb2O6 family to advance sustainable hydrogen generation technologies.

9. Research Gaps in Advancing MNb2O6-Based Photocatalysts

MnNb2O6 and CuNb2O6 have emerged as promising candidates for photocatalytic water splitting due to their suitable bandgap energies and responsiveness to visible light. CoNb2O6 has also demonstrated measurable hydrogen evolution under solar irradiation, particularly when incorporated into heterostructured composites that enhance charge separation and light-harvesting efficiency. However, in contrast to these encouraging findings, other members of the MNb2O6 family—such as MgNb2O6, FeNb2O6, NiNb2O6, and ZnNb2O6—remain largely unexplored in terms of their direct photocatalytic hydrogen production. This gap underscores the need for systematic experimental studies to assess and unlock their full potential.
Although several MNb2O6 compounds have shown promise for photocatalytic water splitting, significant knowledge gaps persist across this material family. Notably, NiNb2O6, MnNb2O6, and CuNb2O6 have demonstrated favorable photocatalytic hydrogen evolution performance, attributed to their appropriate band gap energies (~2.0–2.9 eV) and visible-light responsiveness. CoNb2O6 has also been reported to exhibit appreciable hydrogen production under solar irradiation, particularly when incorporated into heterostructured composites that enhance charge separation and light harvesting.
In contrast, limited experimental data are available for other MNb2O6 members such as MgNb2O6, FeNb2O6, and ZnNb2O6. These materials have primarily been explored for UV-assisted dye degradation or dielectric applications, with few or no reports evaluating their direct hydrogen evolution performance. Even in cases where structural and optical properties have been characterized, systematic assessment of their photocatalytic behavior—especially under visible light—remains absent.
Moreover, there is a lack of studies exploring band structure engineering, heterojunction formation, or surface modification strategies for the lesser-known MNb2O6 phases. Without such investigations, the full photocatalytic potential of these materials remains untapped. In addition, few reports provide detailed insight into charge carrier dynamics or employ advanced electronic characterization techniques such as Mott–Schottky analysis, Hall-effect measurements, or Seebeck coefficient evaluation to validate theoretical predictions and guide material optimization.
Furthermore, comparative studies across the MNb2O6 series are notably scarce. Most available reports investigate single compounds under varying experimental conditions, limiting the ability to derive meaningful structure–property relationships or performance trends.
In summary, although select MNb2O6 compounds exhibit promising photocatalytic activity, comprehensive and systematic studies across the broader material family are critically needed. Addressing these gaps through coordinated experimental efforts, advanced characterization, and rational design strategies will be essential for unlocking the full potential of MNb2O6-based nanomaterials in sustainable hydrogen production.

10. Conclusions

This review highlights the recent progress in MNb2O6-based nanomaterials for photocatalytic water splitting, emphasizing their structural, morphological, optical, and performance characteristics. Among the reported compounds, narrow-bandgap MNb2O6 compounds such as MnNb2O6, CuNb2O6, NiNb2O6, and CoNb2O6 offer significant visible-light absorption (band gaps, 2.0–2.9 eV), directly translating to improved solar-to-hydrogen efficiency. Notably, optimized TiO2/CuNb2O6 and TiO2/NiNb2O6 composites achieved H2 evolution rates of up to 146 mmol h−1 g−1 and 114 mmol h−1 g−1, respectively, while g-C3N4/ZnNb2O6 heterojunctions reached 340 μmol h−1 g−1 under visible light—outperforming many conventional oxide photocatalysts. Structurally, MNb2O6 compounds typically adopt a columbite-type framework, which offers flexibility for electronic and morphological modifications. Morphological analysis reveals a strong correlation between surface area and activity; for instance, the hydrogen evolution rate for TiO2/CuNb2O6 increased substantially—from 21 to 146 mmol h−1 g−1—as the BET surface area was enhanced from ~91 to 132 m2/g. Additionally, strong alignment between theoretical and experimental band gap values supports the rational design of MNb2O6-based photocatalysts for visible light applications. Despite notable progress in Mn-, Cu-, Ni-, and Co-based systems, limited studies are available on MgNb2O6, FeNb2O6, and ZnNb2O6 for overall water splitting. These materials are often restricted to UV-driven dye degradation, and their potential under visible light remains underexplored. Unlike broader reviews, this review presents the first focused comparison of the MNb2O6 family, compiling quantitative data on structural, optical, and photocatalytic performance metrics. It also emphasizes the role of heterojunction construction, defect engineering, and surface area optimization in enhancing photocatalytic outcomes. Looking ahead, research should focus on (i) the development of underexplored MNb2O6 phases and their heterostructures, (ii) detailed investigations linking defect structures, charge carrier dynamics, and morphology to activity, and (iii) the adoption of scalable synthesis methods suited for practical applications. Standardized benchmarking of photocatalytic metrics will also be essential for identifying structure–property–performance relationships across the MNb2O6 series. In conclusion, MNb2O6 compounds represent a versatile and promising class of visible-light-active photocatalysts. Bridging current knowledge gaps through integrated experimental and computational studies will be key to advancing their application in sustainable hydrogen production.

Author Contributions

P.R.: Conceptualization, methodology, validation, data curation, writing—original draft. J.-S.N.: Supervision, funding acquisition, editing and critical revision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Korea (NRF) grant number 2019R1A2C1008746 and the APC was funded through a special invitation.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C1008746).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The crystal structure of columbite-type MNb2O6, highlighting the distinct oxygen sites: O1 (gray spheres), O2 (black spheres), and O3 (white spheres). Copyright 2023, Elsevier.
Figure 1. The crystal structure of columbite-type MNb2O6, highlighting the distinct oxygen sites: O1 (gray spheres), O2 (black spheres), and O3 (white spheres). Copyright 2023, Elsevier.
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Figure 2. The electronic band structures of (a) MnNb2O6 (b) FeNb2O6, (c) CoNb2O6, and (d) NiNb2O6 nanomaterials. Copyright 2023, Elsevier.
Figure 2. The electronic band structures of (a) MnNb2O6 (b) FeNb2O6, (c) CoNb2O6, and (d) NiNb2O6 nanomaterials. Copyright 2023, Elsevier.
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Figure 3. Total and element-resolved density of states (DOS) plots for (a) MnNb2O6 (b) FeNb2O6, (c) CoNb2O6, and (d) NiNb2O6. Copyright 2023, Elsevier.
Figure 3. Total and element-resolved density of states (DOS) plots for (a) MnNb2O6 (b) FeNb2O6, (c) CoNb2O6, and (d) NiNb2O6. Copyright 2023, Elsevier.
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Figure 4. (a) Schematic mechanism of TiO2/CuNb2O6 system, (b) H2 evolution with time, and (c) average rate of H2 production. Reprinted with permission from ref. [50]. Copyright ©2023 American Chemical Society.
Figure 4. (a) Schematic mechanism of TiO2/CuNb2O6 system, (b) H2 evolution with time, and (c) average rate of H2 production. Reprinted with permission from ref. [50]. Copyright ©2023 American Chemical Society.
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Figure 5. (a) Schematic mechanism of TiO2/NiNb2O6 system, (b) H2 evolution with time, and (c) average rate of H2 production. Copyright 2023, Elsevier.
Figure 5. (a) Schematic mechanism of TiO2/NiNb2O6 system, (b) H2 evolution with time, and (c) average rate of H2 production. Copyright 2023, Elsevier.
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Figure 6. (a) Possible mechanism occurring in g-C3N4/ZnNb2O6. Photocatalytic hydrogen evolution profiles of various samples under visible light irradiation (λ > 420 nm) as a function of time (b,d). Comparative analysis of hydrogen evolution rates under visible light illumination for the different samples (c,e). Copyright 2017, Elsevier.
Figure 6. (a) Possible mechanism occurring in g-C3N4/ZnNb2O6. Photocatalytic hydrogen evolution profiles of various samples under visible light irradiation (λ > 420 nm) as a function of time (b,d). Comparative analysis of hydrogen evolution rates under visible light illumination for the different samples (c,e). Copyright 2017, Elsevier.
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Figure 7. Photocatalytic hydrogen evolution rates of ZnNb2O6 synthesized over 7 days (ZnNb2O6–7D) loaded with 1 wt% cocatalyst, measured under UV light irradiation in (a) pure water and (b) a 20 vol% methanol aqueous solution. (Open access).
Figure 7. Photocatalytic hydrogen evolution rates of ZnNb2O6 synthesized over 7 days (ZnNb2O6–7D) loaded with 1 wt% cocatalyst, measured under UV light irradiation in (a) pure water and (b) a 20 vol% methanol aqueous solution. (Open access).
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Table 1. Structural parameters of MNb2O6 transition metal niobates.
Table 1. Structural parameters of MNb2O6 transition metal niobates.
CompoundSpace Groupa (Å)b (Å)c (Å)Notes on Structure
MgNb2O6 (columbite)Pbcn14.18755.70015.0331NbO6 octahedra form edge-sharing 1D chains along the c-axis; MgO6 connects via corners (ui.adsabs.harvard.edu)
FeNb2O6 (columbite)Pbcn~14.2815.73665.1234Details from single-crystal studies; demonstrates full orthorhombic symmetry
NiNb2O6 (columbite)Pbcn14.0325.6875.033Prepared as single crystals; consistent with columbite-type structure
ZnNb2O6 (columbite)Pbcn14.2085.7265.040In line with earlier findings, edge-sharing NbO6 chains are evident
MnNb2O6Pbcn14.395.745.09Powder, orthorhombic columbite; a = 0.5766 nm, b = 1.4439 nm, c = 0.5085 nm (V = 0.4234 nm3) (akjournals.com)
CoNb2O6Pbcn~14.18~5.72~5.04Orthorhombic columbite phase confirmed
CuNb2O6Pbcn14.105.705.03Analogous orthorhombic parameters expected; comparable to other MNb2O6 columbites
Table 2. Morphological features of MNb2O6 nanomaterials and their photocatalytic implications.
Table 2. Morphological features of MNb2O6 nanomaterials and their photocatalytic implications.
CompoundMorphologySynthesis MethodKey Features & ObservationsReference
MnNb2O6NanoparticlesSolvothermalUniform dispersion, high surface area; enhances visible-light absorption and H2 evolution[42]
CuNb2O6NanoparticlesSolvothermalSmall particle size; suitable bandgap for photocatalysis[42]
ZnNb2O6NanorodsHydrothermalRod-like morphology improves charge transport and light harvesting[43]
CoNb2O6NanofibersElectrospinning + Annealing1D structure enhances electron mobility; hierarchical fibrous network improves separation[44]
FeNb2O6Micro-/nanoparticlesSolid-state/Sol–gelLimited reports on photocatalytic water splitting; morphology less controlled[46]
NiNb2O6Crystals, bulk powderSolid-state/Floating ZoneMostly studied for magnetic/dielectric properties; photocatalytic morphology underexplored[40]
MgNb2O6Irregular particlesSolid-stateRarely explored for photocatalysis; lack of controlled morphology studies[50]
Table 3. Comparison of experimental and theoretical band gap values for various MNb2O6 compounds, along with their synthesis methods and corresponding literature references. The synthesis approach influences the morphology and crystallinity of the materials, thereby affecting their optical and photocatalytic properties.
Table 3. Comparison of experimental and theoretical band gap values for various MNb2O6 compounds, along with their synthesis methods and corresponding literature references. The synthesis approach influences the morphology and crystallinity of the materials, thereby affecting their optical and photocatalytic properties.
CompoundExperimental Band Gap (eV)Theoretical Band Gap (eV)Synthesis MethodKey References
MnNb2O62.3–2.72.5–2.8Solid-state reaction[53]
CuNb2O61.9–2.12.0–2.2Sol–gel method[51]
CoNb2O6~2.42.3–2.5Hydrothermal method + annealing[44]
NiNb2O62.2–2.52.4–2.7Solid-state reaction[54]
ZnNb2O63.6–3.83.7–3.9Hydrothermal method[55]
MgNb2O6~3.94.0–4.2Sol–gel auto-combustion[50]
FeNb2O6~4.04.0–4.2Solid-state reaction[54]
Table 4. Bandgap nature of MNb2O6 transition metal niobates.
Table 4. Bandgap nature of MNb2O6 transition metal niobates.
CompoundConductivity TypeReasoning/Evidence
MgNb2O6Likely n-typeAnalogous dielectric niobates tend to be n-type due to oxygen vacancies (pmc.ncbi.nlm.nih.gov, reddit.com); Mg has no open d-orbitals.
FeNb2O6Likely n-typeTransition-metal niobates often show n-type behavior from oxygen-vacancy donors.
NiNb2O6Likely n-typeSimilar logic: oxygen deficiencies act as electron donors in Nb–O systems.
ZnNb2O6Predominantly n-typeWidely used as a dielectric; typical oxide defects (e.g., O-vacancies) lead to n-type conduction.
MnNb2O6Likely n-type, possibly p-typeNo direct measurement. Structurally similar oxides are n-type, but Mn d-states could allow minority hole conduction.
CoNb2O6Likely n-type, possible mixed behaviorNo direct data. Cobalt oxides can sometimes be p-type, but niobate structure suggests n-type dominance.
CuNb2O6Potentially p-type, possibly ambipolarCu2+ often introduces hole carriers; certain copper niobates show p-type or bipolar conductivity (needs experimental confirmation).
Table 5. Structural phases, morphologies, synthesis routes, surface areas, and hydrogen production performances of selected MNb2O6 nanomaterials. Note that data are based on available experimental reports and indicate that while MnNb2O6 and CuNb2O6 demonstrate promising photocatalytic activity, other members of the MNb2O6 family remain underexplored in the context of hydrogen evolution.
Table 5. Structural phases, morphologies, synthesis routes, surface areas, and hydrogen production performances of selected MNb2O6 nanomaterials. Note that data are based on available experimental reports and indicate that while MnNb2O6 and CuNb2O6 demonstrate promising photocatalytic activity, other members of the MNb2O6 family remain underexplored in the context of hydrogen evolution.
CompoundCrystal Structure (Space Group)MorphologySynthesis MethodSurface Area (m2/g)H2 Evolution Rate (μmol h−1 g−1)Reference
MnNb2O6Orthorhombic (Pbcn)Rod-like nanoparticlesHydrothermal~28.3Not reported[53]
CuNb2O6Orthorhombic (Pbcn)Plate-like microstructuresSol-gel~91~21,000[60,63]
CoNb2O6OrthorhombicNanofibers in compositesElectrospinning + Calcination~12.7~190 (in heterojunction)[62]
MgNb2O6OrthorhombicIrregular granulesSolid-state reactionNot reportedNot reported[50]
FeNb2O6MonoclinicAgglomerated particlesSolid-state reaction~5.1Not reported[54]
NiNb2O6MonoclinicDense particlesSolid-state reaction~9616,000[52]
ZnNb2O6OrthorhombicPorous, nanoscale aggregatesHydrothermal~18.923[43]
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Ravi, P.; Noh, J.-S. Recent Progress and Future Perspectives of MNb2O6 Nanomaterials for Photocatalytic Water Splitting. Materials 2025, 18, 3516. https://doi.org/10.3390/ma18153516

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Ravi P, Noh J-S. Recent Progress and Future Perspectives of MNb2O6 Nanomaterials for Photocatalytic Water Splitting. Materials. 2025; 18(15):3516. https://doi.org/10.3390/ma18153516

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Ravi, Parnapalle, and Jin-Seo Noh. 2025. "Recent Progress and Future Perspectives of MNb2O6 Nanomaterials for Photocatalytic Water Splitting" Materials 18, no. 15: 3516. https://doi.org/10.3390/ma18153516

APA Style

Ravi, P., & Noh, J.-S. (2025). Recent Progress and Future Perspectives of MNb2O6 Nanomaterials for Photocatalytic Water Splitting. Materials, 18(15), 3516. https://doi.org/10.3390/ma18153516

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