Nanostructured MoO₃ for Efficient Energy and Environmental Catalysis

Abstract

This paper mainly focuses on the application of nanostructured MoO₃ materials in both energy and environmental catalysis fields. MoO₃ has wide tunability in bandgap, a unique semiconducting structure, and multiple valence states. Due to the natural advantage, it can be used as a high-activity metal oxide catalyst, can serve as an excellent support material, and provide opportunities to replace noble metal catalysts, thus having broad application prospects in catalysis. Herein, we comprehensively summarize the crystal structure and properties of nanostructured MoO₃ and highlight the recent significant research advancements in energy and environmental catalysis. Several current challenges and perspective research directions based on nanostructured MoO₃ are also discussed.

1. Introduction

In the 21st century, the impending depletion of fossil fuels and urgent environmental concerns are among the most challenging issues. Exploration of renewable and sustainable energy and the development of new and improved materials are the solutions to keep environmental sustainability [1,2,3,4]. Catalysts are increasingly important in the development of innovative clean energy, environmental protection, and energy conversion [5,6,7]. Accordingly, the search for developing superior nanostructured catalysts is a sustainable alternative. Noble metal catalysts such as Pt, Pd, Rh, and Au exhibit high activity, to tackle the energy and environmental challenges. However, the noble metal catalysts are impacted by their scarcity, high cost, and relatively low stability, which impede their general use in large scale applications. As a result, catalysts with higher activity and lower cost are urgently required for large-scale practical applications [8]. Transitional metal oxides dominate widespread applications in energy and environmental catalysis on account of their relatively low cost, high activity, and stability [9,10,11,12].

Molybdenum oxide (MoO₃), a kind of transition metal oxide with a n-type semiconducting, nontoxic nature and high stability, has attracted a lot of attention. In particular, nanostructured MoO₃ has demonstrated superior properties to bulk MoO₃, which is successfully employed in rechargeable batteries [13], capacitors [14], photocatalysis [15], electrocatalysis [16], gas sensors [17], and other applications [6]. The extended tunnels between the MoO₆ octahedra in MoO₃ are suitable for insert/de-insert mobile ions, such as H⁺ and Li⁺, and multiple oxidation states can enable rich redox reactions. Moreover, the superiorities of low cost, chemical stability, high theoretical specific capacity (1117 mA·h/g), and the environmentally friendly nature make nanostructured MoO₃ exceptional electrode materials for rechargeable batteries capacitors [13,18]. MoO₃ has been investigated as the photocatalyst in terms of its anisotropic layered structure for absorbing UV, as well as visible light. Introducing a defect band by H⁺ intercalation or oxygen vacancies can create defect state and decrease MoO₃ bandgap, which effectively increase the photocatalytic activity [15]. Each oxygen atom bonds to only one molybdenum atom of MoO₆ octahedra, and oxygen vacancy generates Mo dangling bond. MoO₃ favors the adsorption of water molecules in oxygen vacancies, which act as electron acceptors and consequently reduce the energy barrier make it highly reactive for electrocatalysis [2,19]. As a good gasochromic material, MoO₃ has efficient positive-ion accommodation and good charge transfer, and it can be used as an optical-based gas sensor. Moreover, relying on the change in the conductance of the oxide-on-gas adsorption/reaction, MoO₃ has been used for NO, NO₂, CO, H₂, NH₃, and other gases [17]. The unique structure strongly affects the performances. Large efforts have been made to obtain nanostructured MoO₃ with appealing properties by engineering multiple synthetic strategies for the applications in various fields. A variety of methods have been developed, including hydrothermal method [20], vapor deposition method [21], sol-gel method [22], electrochemical deposition [23], exfoliation method [24], and so forth [25]. Furthermore, many strategies have been used to improve the performance of nanostructured MoO₃ by controlling compositions, crystal facets, structural blocks, and defects, as well as tailoring the interfacial and electronic structures through doping and hybridization.

Though many of the literature studies have reported on nanostructured MoO₃, a thorough overview of nanostructured MoO₃ for efficient energy and environmental catalysis is still lacking so far. Hence, in this paper, we present a comprehensive review of the important advances in nanostructured MoO₃ materials. We comprehensively introduce the crystalline structures and mainly focus on summarizing the exploitation of nanostructured MoO₃ in environmental catalysis and energy conversion. We also provide a brief perspective on the current challenges and opportunities for effectively utilizing nanostructured MoO₃ and taking full advantage of MoO₃ in constructing highly efficient materials.

2. Phase Structure and Morphology of Nanostructured MoO₃

2.1. The Phase Structure of MoO₃

MoO₃ involves four different polymorphs (Figure 1), namely the thermodynamically stable orthorhombic phase (α-MoO₃), the metastable monoclinic phase (β-MoO₃), the hexagonal phase (h-MoO₃), and the high-pressure monoclinic (MoO₃-II) [6,20,26,27]. Each polymorph displays very unique physical and chemical properties, including bandgap energies, refractive indices, and mechanical hardness [28].

Among them, α-MoO₃ has been extensively studied because of its outstanding electrochemical and catalytic activities. α-MoO₃ phase is constituted by corner-sharing [MoO₆] octahedra along [001] and [100] directions. Two sublayers are stacked together by sharing the edges of octahedral along the [001] direction, to form a layer with the orthorhombic symmetry (a = 3.963 Å, b = 13.86 Å, c = 3.696 Å). Furthermore, an anisotropic structure with layers along the [010] direction (b axis) with weak van der Waals forces leads to the formation of α-MoO₃ (see Figure 1a). The layered structure of α-MoO₃ has relatively higher tolerance to nonstoichiometry such as the unusual pentavalent ion Mo⁵⁺, which exhibits high affinity for oxygen, and the layer structure allows the small guest ions (such as Li⁺, H⁺) to be well accommodated, without structural change in principle [20,29,30]. Mai et al. [31] proved that the conductivity and electroactivity of lithiated MoO₃ nanobelt were highly improved compared to that of a non-lithiated MoO₃ nanobelt.

The monoclinic structure of β-MoO₃ is markedly different from the crystal structure of α-MoO₃, which possesses a ReO₃-related structure. The MoO₆ octahedral units share corner oxygen atoms in the direction of the c axis, and edge sharing occurs in the direction of the c axis (see Figure 1b). A transformation from the β to α phase took place spontaneously at the temperature ranging from 387 to 450 °C, according to the reported result. Moreover, β-MoO₃ exhibited higher catalytic properties than α-MoO₃ in some catalysis reactions [32,33,34].

Hexagonal h-MoO₃ is built up by zigzag chains of MoO₆ octahedra linked to each other by corner sharing along the c axis. A very salient crystalline structure for h-MoO₃ is the presence of a tunnel (~ 3.0 Å in diameter) running along the c direction. It can serve as a conduit and intercalation site for mobile ions (see Figure 1c). The h-MoO₃ phase can generally be formulated as (A₂O)_x·MoO₃·(H₂O)_y, where A represents an alkali-metal ion or ammonium ion, and the exact values of x and y depend on the details of the preparation and subsequent treatment [6,26,32,35]. The tunnel structure in h-MoO₃ exhibits considerably higher sensitivity, coloration efficiency, and faster response in comparison with the orthorhombic α-MoO₃. It is mainly attributed to the h-MoO₃ of tunnel structure with a higher structure openness degree, giving rise to an accelerated electron–hole separation in photochromism and a facile Li⁺ ion insertion/extraction in electrochromism [36].

High pressure modification of molybdenum trioxide, MoO₃-II (ε-MoO₃), the structure of MoO₃-II is monoclinic, P2_l/m, with unit cell parameters: a = 3.954(l) Å, b = 3.687(2) Å, c = 7.095(4) Å (see Figure 1d). Like the α-MoO₃ layered structure, the individual MoO_3/3O_2/2O_1/1 layers of MoO₃-II and α-MoO₃ are virtually identical. The stacking sequence of the layers of MoO₃-II (aaa) differs from that of α-MoO₃ (aba), which is equated with an improved packing efficiency for the layers of MoO₃-II versus those of α-MoO₃. The metastable MoO₃-II can convert rapidly to stable orthorhombic phase α-MoO₃ at temperatures above 200 °C, which is relatively more stable than h-MoO₃ [27,28].

The above four different crystal structures have their unique properties, which accordingly influence their applications. The crystal structures of α-MoO₃ and MoO₃-II have distinctive double layers. Some positive ions can easily inject into the layer structure, which can be applied in electrochromism, catalysis, and as an electrode material of lithium ion batteries. However, MoO₃-II is metastable, the crystal structure is easily converted to stable α-MoO₃, which limits its application. The monoclinic structure of β-MoO₃ is a monoclinic ReO₃-related structure, in which each MoO₆ octahedral shares all the corners with adjacent MoO₆ octahedral. Breaking the Mo–O bonds can produce more unsaturated Mo atoms on the surface compared with α-MoO₃, which would behave as active centers for oxidation of small organic molecules, such as partial oxidation of methanol [34]. Unlike α-MoO₃, metastable h-MoO₃ easily permits the ready intercalation and migration of some monovalent cations because of the open structure tunnels. The unique structure would play an important role in enhancing the charge transfer characteristics and displaying an ionic conductive nature. The h-MoO₃ has the potential to exhibit excellent photochromic, electrochromic, and electrochemical properties. Crystal structure is the intrinsic property, but all the stoichiometric MoO₃ phases have wide bandgaps, which seems to impact some applications. Creating oxygen vacancies, reducing crystal dimensions, introducing dopants, or transforming into quantum dots can be used to manipulate the band structure and improve the performance of MoO₃.

2.2. The Morphology of Nanostructured MoO₃

A large number of architectures with controllable sizes or morphologies have been reported to optimize the structure and composition, such as zero-dimensional (0D) quantum dots (QDs), and one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) architectures, which is significant for revealing the relationship between structure and performance [6].

QDs represent small-size nanocrystals with all three dimensions in several nanometers. The properties were changed significantly from those bulk semiconductors due to their small sizes and the quantum confinement effect [37]. Lu et al. [38] prepared MoO₃ QDs through combining intercalation and thermal exfoliation and studied the optical properties of MoO₃ QDs. The dispersion of MoO₃ QDs showed a tunable strong localized surface plasmon resonance (LSPR) after simulated solar light illumination and the plasmon peak red shifted with an extension of illumination time, which was different from the MoO₃ nanosheets. Lu and co-workers [39] also observed the morphology changes in combination with different photochromic phenomena through preparation of quantum dots from MoO₃ nanosheets by UV irradiation (see Figure 2a). It suggested that the morphology changes were mainly influenced by consumption of photoexcited holes.

One-dimensional structure is mainly used to describe structures of which the growth is along one direction and the one dimension is less than 100 nm. One-dimensional architectures can be divided into nanowires, nanorods, nanotubes, nanobelts, and so on [26,40,41,42,43,44,45], which are widely used in many fields due to their unique structural traits. For instance, Meduri et al. [40] reported a MoO_3−x nanowire array (see Figure 2b). The 1D MoO₃ nanowires could provide good conduction pathways for electronic conductivity, along with a shorter path for lithium diffusion, which showed lower lithium intercalation voltages and flat voltage plateau. One dimensional α-MoO₃ nanorods were prepared with a size of about 50 nm in diameter and few microns long by solution combustion method; the TEM image of α-MoO₃ nanorods is shown in Figure 2c. By comparison and analysis, the thermodynamically stable α-MoO₃ single phase exhibited high specific capacitance and good cycle stability for supercapacitor applications [41]. Single-walled MoO₃ nanotubes were synthesized by Wang and co-workers, and the MoO₃ SWNTs (see Figure 2d) were synthesized by a thiol-assisted hydrothermal method through an interface controlled self-rolling mechanism [42]. Enyashin et al. [43] proposed the atomic models of MoO₃ nanotubes and studied the electronic structure and chemical-bonding indices. The results showed that the holey wall structures of MoO₃ nanotubes made it possible to dope the materials with various atoms. Moreover, α-MoO₃ nanobelts can display excellent performance for supercapacitors. It was reported that the α-MoO₃ nanobelts’ electrode exhibited a higher specific capacitance of 369 F/g and a good cycle stability, with more than 95% of the initial specific capacitance maintained after 500 cycles, which suggested the α-MoO₃ nanobelts could be a potential electrode material [44]. Zheng et al. [26] prepared h-MoO₃ nanobelts and studied the photochromic and electrochromic properties; the TEM image of as-prepared h-MoO₃ nanobelts is shown in Figure 2e. Higher structure openness degree in the tunnel structure of h-MoO₃ could promote electron–hole separation and allow cation insertion/extraction and diffusion which exhibited a better performance on photochromic and electrochromic response than that of α-MoO₃.

Two-dimensional nanostructured MoO₃ attracted extensive attention due to its tunable bandgap and high charge carrier mobility. Balendhran and co-workers presented a 2D MoO₃ based biosensing platform. The nanostructured film made of 2D α-MoO₃ nanoflakes was used as the conduction channel, which significantly reduced response time due to the high permittivity of the 2D α-MoO₃ nanoflakes [46]. In addition, Balendhran et al. [47] also demonstrated that the energy bandgap of 2D high-dielectric α-MoO₃. The α-MoO₃ forming MoO_3−x can reduce the bandgap and enhance charge carrier mobility. Compared with the bulk MoO₃, 2D-MoO₃ nanosheets had a better chemical sensor performance due to the 2D structure with a large surface area and more reactive sites [48] (see Figure 2f). Cheng et al. [49] reported that MoO_3−x nanosheets can display strong LSPR from the visible to the near-infrared region, owing to the layered crystal structure, which could be used as highly efficient catalysts for the hydrogen generation from ammonia borane with visible light irradiation.

Three-dimensional architectures can increase the surface area and provide more facets with active sites. Three-dimensional architectures mainly include nanoflowers, nanospheres, and mesoporous structures. Nanoflowers can be described as compositions of low-dimensional nano building blocks for certain structures, such as nanorods, nanosheets, and so forth. Sui et al. [50] reported a flower-like α-MoO₃ with hierarchical structure (see Figure 2g). The sensors based on α-MoO₃ flowers exhibited highly sensitive and distinctively selective to trimethylamine. The α-MoO₃ flowers may have less agglomerated configurations, more effective charge transportation, and expose more active sites, as well as specific facets. Another type of promising architecture, nanospheres were widely used in the fields of catalysis, sensing energy storage conversion, and so on [51]. Generally, hollow MoO₃ nanospheres were prepared by drawing support from templates. Triblock copolymer micelles have been used as templates to form hollow MoO₃ nanospheres. The size could be adjusted by choosing a suitable micellar core [52]. Du et al. [53] synthesized highly dispersed MoO₃ nanospheres by ultrasonic irradiation (see Figure 2h) and probed their growth mechanism. Mesoporous structure because of their intrinsic high surface areas and the nano-wall structure were beneficial to diffusing the ions and electrons, which can enhance the electrochemical or photocatalytic properties [6]. Brezesinski and co-workers reported the ordered mesoporous α-MoO₃ nanocrystalline walls for the application of thin-film pseudocapacitors, which exhibited the superior capacitive charge-storage properties compared with both the mesoporous amorphous and nonporous crystalline MoO₃ [54]. Luo et al. also prepared mesoporous MoO_3−x (see Figure 2i) with good activity and stability for HER under both acidic and alkaline conditions [55].

Based on the above overview, the performance of MoO₃ is generally influenced by the morphology, and the essence is that the structure of MoO₃ is different. The controllable preparation of diversified nanostructured MoO₃ is an efficient approach to achieve excellent performance. However, it should be noted that some nanostructured MoO₃ with well-controlled morphologies and sizes were not applied well. Hence, it is better to rationally design and synthesize with well-controlled nanostructured MoO₃ based on advances in nanomaterials science and engineering.

3. The Application of MoO₃ in Energy Related Catalysis

Due to its low-cost, nontoxicity, multiple oxidation states, and van der Waals gap along the [010] direction, molybdenum oxide (MoO₃) has attracted more and more attention in hydrogen evolution, oxygen evolution, and fuel cells in recent years. However, because of its poor inherent electrical conductivity and slow electrochemical kinetic process, the widespread application of MoO₃ is limited, and a great amount of effort has been devoted to enhancing the catalytic performance of MoO₃.

3.1. Application in Hydrogen Evolution

Owing to high calorific value and its renewable and clean properties, hydrogen (H₂) is regarded as one of the most promising energy carriers for the future. As a layered n-type semiconductor, MoO₃ has been studied as a catalyst for electrocatalytic hydrogen evolution, photocatalytic hydrogen evolution, and ammonia borane dehydrogenation.

3.1.1. Electrocatalytic Hydrogen Evolution Reactions

Electrocatalytic hydrogen evolution reaction (HER) is an efficient method. It is an environmentally friendly process that does not create any by-products. Although the layered structure of MoO₃ is suitable for insertion/removal of small ions such as H⁺, MoO₃ always shows poor catalytic performance for HER, due to the lack of active sites [55]. Therefore, MoO₃ always serves as core the substance, as the MoO₃/MoS₂ core–shell structure [56,57,58]. The MoO₃ core provides high specific surface area and facile charge transport, as depicted in Figure 3a. In order to eliminate the influence of the resistance of the solution and obtain the real kinetic activity of the MoO₃/MoS₂, an iR-corrected test was also performed, and the data is shown in Figure 3b. The HER activity of MoO₃/MoS₂ remained stable even if it was tested for 10,000 cycles (Figure 3c) [56]. Besides, Pt [59], Pd [60], RuO₂ [19] can also load on MoO₃, to get excellent electrocatalysts for electrocatalytic hydrogen evolution.

In order to extend application in electrochemical hydrogen evolution, researchers found that oxygen vacancies can dramatically improve catalytic performance of MoO₃. Figure 3d showed that MoO₃ with oxygen vacancies possessed a much narrower band gap compared to commercial MoO₃, and the color changed from greenish to blue (Figure 3d onset). Figure 3e demonstrates the formation of oxygen vacancies in MoO₃. As shown in Figure 3f, the oxygen vacancies which were close to Mo⁵⁺ can serve as active sites [55]. In this way, 2D α-MoO₃ nanosheets were fabricated and exhibited a considerable HER performance with low overpotential of 142 mV, to achieve 10 mA/cm² current density [61]. Zhang et al. also found that MoO_3−y (valence state of V and VI) can be used for efficient electrocatalytic hydrogen evolution [62]. Second, the morphology is another key factor. Mesoporous MoO₃ [55] and MoO₃ nanosheets [61] mentioned above have demonstrated the importance of morphology. Zhang et al. synthesized porous MoO₃ with large specific surface area of 113.8 m²/g, and it can work as a bifunctional electrocatalyst for (oxygen evolution reactions) OER and HER [63]. The large specific surface area of as-synthesized porous MoO₃ increased electrochemical active area (0.057 mF/cm²) which was four times bigger than commercial MoO₃ (0.012 mF/cm²). Third, hetero-atoms doping is another efficient strategy to improve electrocatalytic performance of MoO₃. Li et al. have confirmed that P doped MoO_3−x nanosheet (P–MoO_3−x) can show the low overpotential of 161 mV to reach 10 mA/cm² current density with low Tafel slope of 42 mV per decade [64]. They also found that P doping sites could attract protons to adjacent oxygen vacancies and then form H_ads on oxygen vacancies, which meant that P doping sites and oxygen vacancies promoted HER together. Haque et al. also demonstrated that NH₄⁺ doped MoO₃ (2D Crys–AMO) just need 138 mV to achieve 10 mA/cm² current density. After NH₄⁺ doping, orthorhombic MoO₃ was transformed into hexagonal phase, resulting in the formation of highly ordered intracrystalline pores (serve as active sites) [65]. When the hetero-atoms doping leads to the structure defect, such as oxygen vacancies or transformation of intracrystalline phase, the HER activity of MoO₃ may be promoted dramatically. In addition, when the atomic radius of hetero-atoms is similar to oxygen, it may be easier to be doped into the lattice of MoO₃. Stability is another crucial factor for catalysts. In general, MoO₃-based nanocatalysts, which served as substrate or active center, possessed excellent stability in both acid and alkaline media for HER [19,55]. The current density can remain stable at a specific voltage for 12 [55], 24 [19], and even 40 h [65].

3.1.2. Photocatalytic Hydrogen Evolution Reactions

Because solar energy is inexhaustible, photocatalytic hydrogen evolution reaction is an attractive strategy to produce hydrogen from water [66]. MoO₃ is a well-known direct-band-gap semiconductor with high work function and good hole conductivity. However, due to wide band gap energy (about 3.0 eV), the photo-induced electrons/holes (e⁻/h⁺) pairs are easy to recombine, resulting in low conversion efficiency of incident light [67]. To solve this issue, MoO₃-based nanocomposites were synthesized and studied. Ma et al. fabricated an MoO₃/polyimide composite, and the growth of MoO₃ increased the light absorption and suppressed the recombination of e⁻/h⁺ pairs [66]. MoO₃–TiO₂ nanotubes were studied, and MoO₃–TiO₂ annealed at 650 °C showed an almost-100-times-higher donor concentration than pure TiO₂ nanotubes. The MoO₃–TiO₂ nanotubes possessed lower charge transfer resistance and improved separation efficiency because of the appearance of MoO₃ [68]. Esparza et al. fabricated a Mo-coated Pt HER catalyst which was O₂-insensitive and stable in acidic media. The formed Mo membrane kept H₂ and O₂ far away from Pt, suppressing both oxygen reduction reactions (ORR) and hydrogen oxidation reactions (HOR). Therefore, the HER efficiency of the Mo-coated Pt HER catalyst was promoted [69]. Guo et al. synthesized MoS₂@MoO₃ core–shell nanowires with a high hydrogen evolution rate of 841.4 µmol/(h·g). The MoO₃ widened the range of light absorption and produced more photo-induced carrier, which accelerated HER rate greatly [70]. Direct bonds formed between CdS NWs and MoO_x clusters were achieved. The MoO_x clusters induced deep electron-trap states by generating long-lived electrons, to improve the activity. The MoO_x clusters could also effectually dissociate adsorbed water molecules, resulting in improved photocatalytic HER activity [71]. Under light irradiation, no obvious diminution in the photocatalytic activity of MoO₃-based materials for HER was observed for 15–30 h [66,67,68,69,70,71].

3.1.3. Ammonia Borane Dehydrogenation

Due to nontoxicity, room-temperature stability, and high hydrogen storage content (19.6 wt.%), ammonia borane (NH₃BH₃; AB) has been regarded as an attractive hydrogen storage material candidate [49,72]. With suitable catalysts, the hydrolysis of NH₃BH₃ can be obtained as illustrated in Equation (1):

NH₃BH₃ + 2H₂O → NH₄⁺ + BO₂⁻ + 3H₂

(1)

MoO₃ shows strong LSPR signal, a near-field enhancement phenomenon, in the visible light region. Yamashita group firstly reported visible-light-induced hydrogen evolution enhancement from NH₃BH₃ solution in plasmonic MoO_3−x [49], and the blue color of MoO_3−x (Figure 3g) was consistent with another report [55]. Figure 3h showed that the MoO_3−x is orthorhombic phase. The activity of MoO_3−x was obviously higher than commercial MoO₃ under visible light irradiation (Figure 3i,j). After that, they prepared Pd/MoO_3−x hybrid and it exhibited great plasmon-enhanced hydrolysis of NH₃BH₃ [72]. The MoO_3−x nanoparticles were fabricated, and the enhanced LSPR property generated by the introduction of oxygen vacancies [73]. Further, they explored the effect of reduction temperature to MoO₃, and MoO_3−x-200 °C (H₂ reduction temperature) showed a higher dehydrogenation activity [74]. The oxygen vacancies could narrow the band gap of MoO₃, then increased the absorption of visible light in a wider range. Lu et al. synthesized MoO₃-doped MnCo₂O₄ microspheres comprised of nanosheets. The Mo doping provided a small pore diameter leading to an enhanced specific surface area (BET area changed from 13.2 to 62.1 m²/g), which is contributed to enhance hydrolysis of NH₃BH₃ [75]. The stability of existing MoO₃-based catalyst is not desirable due to relatively short test time (60–70 min) [72,73,74,75].

In summary, the application of MoO₃ in hydrogen evolution is still in preliminary stage. And the further improvement in the activity and stability is still anticipated. Most of works reported that MoO₃ can serve as substance instead of active center in electrocatalytic and photocatalytic hydrogen evolution. As for NH₃BH₃ dehydrogenation, Yamashita group has been devoted to developing MoO₃ as a promising material. There still exists broad space for the researchers to develop nanostructured MoO₃ as an excellent catalyst for hydrogen evolution. The possible strategy including introduction of oxygen vacancies, hetero-atom doping and hybridization with other materials.

3.2. Application in Oxygen Evolution

3.2.1. Electrocatalytic Oxygen Evolution Reactions

Electrocatalytic OER is another half-reaction of water splitting. Owing to transfer of four electrons for the evolution of an O₂ molecule, OER is more sluggish compared to HER and becomes the rate-determining step of water splitting [76,77]. There are just a few of research studies about MoO₃ applied in OER, in which MoO₃ is regarded to be catalytically inert for OER [78]. Tariq et al. prepared IrO₂–MoO₃ composites and 30% mole fraction of iridium contents, which would be more favorable for OER. The mass specific OER activity of iridium active centers was greatly enhanced by seven-fold [76]. NiO@MoO₃/Vulcan carbon was also investigated by Illathvalappil. The MoO₃ would react with NiO to form a thin NiMoO₄ film which could stop MoO₃ from dissolving in the alkaline media, resulting in excellent stability at 1.51 V (vs. RHE) for 15 h in 1 M of KOH solution [77]. Guo et al. fabricated Co-doped MoO_x (CMO) core–shell structure. Figure 4a showed the CMO with uniformly spherical structure of 500 nm in diameter. Further, on a damaged area (Figure 4b), the core–shell structure can be observed, which means that the CMO possesses a MYLIKE chocolate-like structure. And Figure 4c further demonstrated the uniform distribution of Co, Mo, and O in the CMO. The CMO exhibits the overpotential 340 mV to reach 10 mA/cm² current density with a Tafel slope of 49 mV per decade (Figure 4d) [78]. The co-doping, bringing numerous active sites (about 6.550 × 10⁻³ mol/g) and amorphous structure of the CMO, was responsible for the improvement of OER activity. The durability of these catalysts was not so good, and the current density could remain stable at 10 mA/cm² for only 3–15 h in KOH aqueous solution [76,77,78].

3.2.2. Photoelectrochemical Oxygen Evolution Reactions

Photoelectrochemical (PEC) water splitting has been regarded as a potential avenue for sustainable energy supply. A main obstacle is the need of efficient and stable water oxidation photocatalysts [79,80]. As a high work function (>6.3 eV) and layered semiconductor, MoO₃ always hybridizes with other materials (such as BiVO₄) in order to match band potential and obtain an efficient catalyst. Lou et al. fabricated a Bi₂MoO₆/MoO₃ heterojunction by using anodic oxidation of a molybdenum foil and, subsequently, a hydrothermal method at 160 °C for 24 h. The Bi₂MoO₆/MoO₃ photoanode showed about 100% faradic photocurrent-to-O₂ conversion efficiency. They found that, when Bi³⁺ was introduced into the MoO₃ membrane, the valence band moved upward, which would lengthen the wavelength of light absorbed by Bi₂MoO₆/MoO₃. The photocurrent could remain stable for 8 h by using the Bi₂MoO₆/MoO₃ [79]. He et al. prepared the MoO₃/BiVO₄ heterojunction film by a drop-casting method, wherein the BiVO₄-based precursor was dropped on FTO glass and then dried and annealed. At 0.8 V (vs. SCE), the photocurrent of MoO₃/BiVO₄ was six times higher than bare BiVO₄ film. The improvement can be attributed to band potentials and conductivity differences between MoO₃ and BiVO₄ [80]. MoO₃/Ag/TiO₂ nanotube arrays were also successfully synthesized and investigated systemically. The enhanced photoelectrochemical performance is related to the tight contact at the interface. Specifically, the photoinduced electrons could move from valence band of TiO₂ to conduction band of MoO₃ through a Ti–O–Mo bond [81]. Chen et al. prepared Mo-doped BiVO₄/MoO_x electrode (1.2% Mo), and Figure 4e shows the formation of the Mo-doped BiVO₄/MoO_x heterojunction. With the increase of Mo doping amount, MoO_x would gradually accumulate and form a film on the surface of BiVO₄, and then continue to grow on the film. At 1.23 V (vs. RHE), the photocurrent density of the catalyst (2.67 mA/cm²) was five-fold higher than that of BiVO₄ electrode [82]. The valence and conduction band edges of BiVO₄ and MoO_x are suitable for forming the type II heterojunction at the interface and then accelerate the carrier transfer and separation.

In short summary, the research on MoO₃ for the application of OER and PEC water splitting is limited, but the published reports indicate that MoO₃ is a promising OER and PEC catalyst through morphology and structure design. MoO₃ has a wide band gap (about 3 eV) and easily forms heterojunctions with other materials, so as to obtain photoelectric OER catalysts with higher activity. More effective methods are needed to exploit the potential of MoO₃. Meanwhile, the reason for enhanced catalytic behavior also needs to be further investigated, to provide a principle for designing next-generation electrocatalysts.

3.3. Application in Fuel Cells

3.3.1. Direct Methanol Fuel Cells

With increasing demand of energy, the direct methanol fuel cells (DMFCs) have attracted more and more attention due to their high energy density (5 kW·h/L). However, the low activity of catalysts and the use of noble metals block its commercial application [83,84,85,86]. Nonstoichiometric MoO₃ (MoO_x) has a rutile-type structure and shows metallic conductivity, which is relatively stable and highly active [83]. Cabrera et al. first reported Pt/MoO_x/glassy carbon used for DMFCs by using an electrochemical deposition method. The lower valence of molybdenum and the proton spillover effect from hydrogen molybdenum bronze may account for the improved activity [83,84]. Justin et al. also prepared Pt–MoO₃/C composite. Insulating MoO₃ was electrochemically reduced to conductive hydrogen molybdenum bronze (HxMoO₃), which can keep the Pt surface clean in order to oxidize methanol. MoO₃ plays a crucial role in the transition process of adsorption intermediates to carbon dioxide [85]. Zhang et al. fabricated Pt/MoO₃ nanowires by using an impregnation–calcination method. After loading of Pt nanoparticles, the Pt/MoO₃ maintained its morphology of nanowires, as shown in Figure 5a. Figure 5b,c shows cyclic voltammetry, linear sweep curves of Pt/MoO₃ and Pt/C, respectively. The Pt/MoO₃ showed much higher electrocatalytic activity and stability for the methanol oxidation, compared to Pt/C (the same Pt loading) [86]. The promoted activity may be attributed to more active catalytic sites and three-dimensional structures generating reactant diffusion microchannels. The stability of MoO₃-based catalysts mentioned above is not satisfactory, and the time of stability test is less than 1 h.

3.3.2. Oxygen-Reduction Reactions

As a type of high efficiency, environmentally friendly energy conversion device, fuel cells have attractive much more attention in recent years [87]. Platinum (Pt) is a benchmark catalyst and always shows excellent performance for ORR. However, it is impossible to use Pt in practical applications that are large in scale because of its scarcity and high cost. Therefore, developing low-content Pt catalysts is a promising way and can be used for real application eventually. Recent researches indicated that MoO₃ can modify a Pt electrode to enhance its catalytic performance and stability [88,89]. After electrodeposition of MoO₃, the Pt/MoO_x/GCE showed a significant increase in the cathodic peak current. The improved performance could be ascribed to the combinations between Pt and MoO₃, which is a doped effect that always happens between transition elements and leads to the optimal mutual electronic density of states [90]. Karuppasamy et al. fabricated Au–MoO₃ and efficiently decreased the usage amount of Au. The Au/MoO₃ exhibited a one-dimensional morphology of nanorods, and there was no obvious aggregation of Au nanoparticles (Figure 5d). Further, according to particle size of statistics, the mean diameter of Au nanoparticles was 5.9 nm (Figure 5e). After 1000 cycles, no obvious change in the half-wave potential was observed, indicating excellent durability. The HRTEM was employed to study the specific structure of Au/MoO₃. The MoO₃ made the appearance of low-coordinated stepped Au atoms in the edge, such as (210), (310), and (410), indicating these were main planes of Au nanoparticles (Figure 5f). Then, Figure 5g,h confirmed that the majority of planes of Au nanoparticles was (111) plane. Due to the presence of MoO₃, the formation of poisonous intermediates, such as OH species, was suppressed on the high index plane of Au nanoparticles; therefore, oxygen was reduced more efficiently [91].

In summary, the glassy carbon electrodes with Pt-modified MoO₃ have been investigated widely for DMFCs and ORR. The MoO₃ can modify the electronic structure of Pt and suppress the formation of poisonous intermediates on high index plane of Au. However, MoO₃ directly used for DMFCs or ORR has not been reported. This phenomenon may be assigned to the relatively fewer research studies and the fact that MoO₃ always serves as a catalyst of partial oxide, suggesting it does not exist at the active sites for breaking O–O bonds. Besides these points, the oxidation mechanism of methanol and oxygen is complex, and there is no clear mechanism explanation about MoO₃-based materials for DMFCs and ORR. A lot of work still needs to be done for MoO₃-based materials applied in DMFCs and ORR.

4. MoO₃ Applied in Environmental Catalysis

4.1. Photodegradation of Organic Pollutants

Photocatalysis is the acceleration of a photoreaction rate in the presence of a catalyst, and it is a green and sustainable catalytic technology that has been widely studied for chemical synthesis, water treatment, environmental cleaning, and self-cleansing processes [92]. The photocatalytic reaction occurs at the interface, based on the absorption of photons with energy larger than the band gap of photocatalyst, with electrons excited from the valence band to the conduction band and producing electron–hole pairs. Photocatalytic efficiency mainly depends on the power and wavelength of the photon source; the properties of the catalyst include its electronic structure, defect density, surface area, and surface-to-volume ratio [93]. Therefore, improving the adsorption capacity of organic pollutants and enhancing the ability of light capture and photo-induced generation performance of electron–hole pairs are important. MoO₃ is one of the most important photocatalysts because of its interesting semiconducting layered structure, rich chemistry associated with multiple valence states and high thermal and chemical stability. Due to the abovementioned outstanding properties, nanostructured MoO₃ has been extensively explored for photocatalytic processes, which are summarized below.

Organic dyes are widely used in textile manufacturing, but they are an environmental threat; hence, the treatment of dyes in wastewater is highly necessary [94]. In general, the degradation rates of rhodamine B (RhB), methylene orange (MO), and methylene blue (MB) are used to evaluate the photocatalytic activity of catalysts. Owing to the distinctive layered structure, MoO₃ exhibited strong adsorption performance and good photocatalytic activity for photodegradation organic pollutants [95,96,97,98]. Inadequately, the large band gap (2.7–3.2 eV) of MoO₃ usually limits the photocatalytic performance of decomposing organic pollutants [99,100]. Based on MoO₃, various strategies are employed to develop the new efficient photocatalytic materials with the desired properties. These include the doping collaborated with other applicable semiconductors or elements and the fabrication of composites of heterostructures [15].

Hybrid nanomaterials can be designed to help enhance the degradation efficiency. Lots of efforts have been made in the design of MoO₃-based hybrids such as Eu(Gd)-doped MoO₃ [101,102], rGO/C-MoO₃ [15], and MoO₂/MoO₃ [100]. For instance, Phuruangrat et al. synthesized Gd-doped MoO₃. The Gd-doped MoO₃ nanobelts showed better degradation activity of MB than pure MoO₃, 99% of MB could be degraded by 3 mol% Gd-doped MoO₃ under visible-light irradiation in 60 min (see Figure 6a). MoO₂/MoO₃ hybrid nanostructures were prepared by Xi and co-workers [100]. The photocatalytic activities in RhB degradation for pure MoO₃ and MoO₂/MoO₃ nanocomposites with different contents of MoO₂ under visible-light irradiation are shown in Figure 6b. MoO₂/MoO₃ exhibited high-efficiency degradation for RhB with the assistance of H₂O₂. The ·OH was a dominant reactive species for the degradation of RhB under visible-light irradiation, and the synergistic effect between dyes and MoO₂/MoO₃ nanoparticles accelerated the production of hydroxyl radical (·OH) from H₂O₂ in this system.

Nanocomposites of heterostructures have also been explored to harvest visible-light nanocomposite, and exhibited extremely enhanced visible-light photocatalytic activity for decomposing organic pollutants. Lu et al. [103] fabricated p-MoO₃ nanostructures/n-TiO₂ nanofiber heterojunctions. The p-n nanoheterojunctions decreased photoluminescence intensity, suppressed the recombinations of photogenerated electrons and holes, and enhanced charge separation and photocatalytic efficiencies. Figure 6c–e showed the energy band alignment of p-MoO₃ nanostructures and n-TiO₂ nanofiber heterojunctions and the postulate mechanism for photodegradation of RhB under UV irradiation. Nanostructures/n-TiO₂ nanofiber heterojunctions exhibited a two-times-higher first-order rate constant for the degradation of RhB than that of TiO₂ nanofibers. Significant improvement of the visible-light-driven photodegradation has also been observed, using Bi₂Mo₃O₁₂/MoO₃ [104], MoS₂/MoO₃ [105,106], and TiO₂/MoO₃ heterostructures [107]. It was attributed to combining a feasible semiconducting material with MoO₃ to form heterostructural photocatalysts that can enhance the interfacial charge transfer and minimize the recombination of photogenerated electron–hole pairs, so that the photocatalytic efficiency is greatly improved.

Besides, to further shorten degradation period and improve degradation efficiency, the MoO₃-based direct solid-state Z-scheme system with a visible-light-driven semiconductor photocatalysts have been studied. He et al. [108] developed Z-scheme type MoO₃–g-C₃N₄ for enhanced photodegradation activity of methylene orange under visible light irradiation. The high photocatalytic activity of MoO₃–g-C₃N₄ is mainly attributed to the synergetic effect of MoO₃ and g-C₃N₄ in electron–hole pair separation via the charge migration between the two semiconductors (see Figure 7a). Figure 7b showed the PL spectra of MoO₃, g-C₃N₄ and 1.5 wt.% MoO₃–g-C₃N₄ samples. The 1.5 wt.% MoO₃–g-C₃N₄ sample had the strongest PL peak, the reason was the combination of high concentrations of the electron on the CB of g-C₃N₄ and holes on the VB of MoO₃, which generated higher concentration ·OH species. Feng et al. [99] reported AgBr/MoO₃ monolithic catalyst for degrading RhB under visible light irradiation. The formation of well-defined novel Z-scheme between AgBr and MoO₃ effectively accelerated dye-sensitization and charge transfer, resulting in high activity in degrading RhB solution. Figure 7c showed the schematic illustration of photosensitized degradation of the RhB dyes over the AgBr/MoO₃ composite before and after contacting for the AgBr/MoO₃ system. Further experiments suggested that ultrafast degradation of the RhB on the AgBr/MoO₃ nanocomposites was due to both the photocatalytic process and the dye sensitization. The charge-transfer mechanism was shown in Figure 7d. These results opened up a new avenue in surface- and interface-engineering techniques, enhancing the further utilization in the field of energy transformation and environmental improvement.

Apart from the use of decomposing dyes in wastewater, MoO₃ is also effective for photoreduction of aqueous Cr (VI). Very recently, Zhang et al. [109] prepared three-dimensional (3D) MoO₃@ZIF-8 core–shell nanorod composite photocatalysts and studied the Cr (VI) degradation mechanism, which were shown in Figure 8a,b. Compared with the pure ZIF-8 and MoO₃ nanowires, the MoO₃@ZIF-8 catalysts exhibited superior photocatalytic activity for Cr (VI) reduction under visible light (see Figure 8c). The reduction of Cr (VI) was up to 100% with 15% of ZIF-8 in 45 min. Moreover, the composite had good recyclability, and the photocatalytic activity remained almost constant after four cycles. Recently, Jing et al. [110] designed and synthesized Mo₂C/MoO₃ and employed it as catalyst for the photoreduction of Cr (VI) and photodegradation of MO under visible light. The photocatalytic reaction mechanism of photodegradation of MO and photoreduction of Cr (VI) for Mo₂C/MoO₃ under visible-light irradiation was illustrated (see Figure 8d). The Mo₂C/MoO₃ heterostructure showed the good photocatalytic activity for wastewater treatment.

In brief, MoO₃ has shown a good activity in degradation of the organic pollutants due to the design of morphology, hybrid structure, and heterostructure in catalysts. However, most photocatalytic processes were finished to decompose dyes under visible-light irradiation, so it is meaningful to synthesize nano-photocatalytic materials with a wide variety of wavelength bands. Additionally, based on the reported works, the study of stability and recyclability is poor; however, it is crucial for the practical applications of catalysts.

4.2. Selective Catalysis to Reduce Air Pollutants

4.2.1. Selective Catalytic Reduction of NO_x with NH₃

Nitrogen oxides (NO_x) cause many environmental pollution problems, such as acid rain, photochemical smog, and ozone depletion. Selective catalytic reduction (SCR) of NO_x with NH₃ has become the dominant technology for decreasing NO_x emission from stationary and mobile sources [111,112]. MoO₃ plays an important role in traditional catalysts, for example, with MoO₃-based catalysts for SCR exhibiting good catalytic NO_x removal performance. The redox behavior of V₂O₅–MoO₃/TiO₂ catalysts was investigated by Casagrande and co-workers, who pointed out that the ternary catalysts are more easily reduced and reoxidized than the corresponding binary samples (V₂O₅/TiO₂ and MoO₃/TiO₂ catalysts) [113]. Recently, based on CeO₂/MoO₃ binary or ternary catalysts that were favored by researchers and CeO₂ that possesses excellent redox property also with plenty of Lewis acid sites, the catalysts with MoO₃ loaded on CeO₂ have been well studied, with much more acid sites on surface. The synergistic effect between CeO₂ and MoO₃ contributed to the selective oxidation of NH₃ to N₂ [114,115]. For instance, Zhu et al. [116] studied the surface structure of M_xO_y/MoO₃/CeO₂ system (M = Ni, Cu, Fe) and its influence on SCR of NO by NH₃. The results revealed that the intensity of the interaction between MoO₃ and other metal oxides was different, which could be listed as follows: NiO/MoO₃/CeO₂ > CuO/MoO₃/CeO₂ > Fe₂O₃/MoO₃/CeO₂. The NO conversion for different catalysts in ‘‘NO + NH₃ + O_2′’ reaction is shown in Figure 9a. Figure 9b shows the schematic drawing of ammonia adsorption and decomposition on Brønsted and Lewis acid sites. The reactivity of “NO + NH₃ + O₂” reaction was strongly associated with acid properties of the catalysts. Additionally, some researchers studied the catalytic behaviors of MoO₃-based catalysts with respect to resistance to phosphorus, as shown in Figure 9c,d. The 1.3P/Ce–Mo(0.5)–O catalyst showed a higher NH₃ reaction rate than the other two catalysts below 350 °C, which suggested that the resistance of the CeO₂ catalyst to phosphate was improved with the addition of Mo, and phosphorus poisoning significantly affected catalyst activity [115]. Nevertheless, the NO_x conversion is still within a relatively narrow temperature window for MoO₃-based NH₃–SCR catalysts, and it is necessary to develop catalysts with superior catalytic performance at both high and low temperatures, with excellent resistance to coexisting poisoning pollutants such as SO₂ and phosphate.

4.2.2. Selective Catalytic Oxidation of Propene

Propene has an extensive production and application in numerous industries, and it is a primary contributor to photochemical smog, making it unfriendly to the environment [117]. There are lots of reports focusing on the catalytic oxidation of propene. MoO₃ is well-known for its structure sensitivity in selective oxidation of propene. The products of selective oxidation mainly include acrylic acid and acrolein. Schuh et al. [118] studied the influence of the morphology of α-MoO₃ in the selective oxidation of propylene. The results suggested that the morphologies of the samples have a significant effect not only on the selectivity to acrolein, but also on the propylene conversion (see Figure 10a,b). The rod-like structures with (100) facet seemed to be of decisive importance for the catalytic activity, and the same conclusion was drawn by Volta and coworkers [119,120]. Moreover, supported Mo oxides may exhibit different structural and catalytic properties for selective oxidation of propene. Ressler et al. [121] systematically studied the catalytic properties of MoO₃ supported on nanostructured SiO₂ and compared the conversion quantity of h-MoO₃/SiO₂ with that of α-MoO₃ by mass spectrometric analysis of the gas-phase composition during thermal treatment of in propene and oxygen (see Figure 10c). It exhibited stability and catalytic properties different from other binary bulk oxides and could directly oxidize propene to acrylic acid without additional metal sites. Besides this, MoO₃ was used for the epoxidation of propylene to propylene oxide (PO) by molecular oxygen [122]. The work of Jin et al. [123,124] on Ag–MoO₃ and Ag–MoO₃–ZrO₂ catalysts showed that MoO₃ resulted in a significant improvement in the efficiency of the catalyst. The addition of MoO₃ increased the selectivity to 34% at a propylene conversion of 4.6%, whereas the pure Ag catalyst had a PO selectivity of only 0.8% at a propylene conversion of 11%. The olefinic carbon of propylene was easily adsorbed by adding MoO₃ into the silver catalyst. MoO₃ was as an electron and structure-type bifunctional promoter in this system.

4.2.3. Selective Catalytic Oxidation of Methane

As one of the greenhouse gases, methane effects on global warming. Hence, its emissions have attracted more and more attention [125]. The conversion of CH₄ into chemicals is much more economical and energy-efficient compared to CO₂. Many publications in this field have been totally committed to the oxidation of methane at relatively high temperature, using transition metal oxides catalysts. The catalysts based on MoO₃ were the most widely studied because of their higher activity and selectivity for methane oxidation [126]. Liu et al. [127] prepared MoO₃/SiO₂ catalysts and studied the kinetics and mechanism of the partial oxidation of methane, using N₂O as the oxidant. These researchers also found that the selective oxidation reaction was initiated by the formation of O⁻ ions generated from the interaction of N₂O. The structural effects of MoO₃ on partial oxidation of methane to formaldehyde were investigated by Smith et al. The results indicated that Mo=O sites located on the side plane tend to form formaldehyde [128]. Moreover, Arena et al. [129] studied the working mechanism of MoO₃/SiO₂ catalysts in the partial oxidation of methane to Formaldehyde. The influence of the oxide loading on the surface structure and compared with the dispersity of MoO₃/SiO₂ and V₂O₅/SiO₂ catalysts. Taylor et al. [130] prepared a series of catalysts based on MoO₃ and WO₃, the MoO₃ based catalysts were more effective for the production of methanol. The Cu/MoO₃ and Ga₂O₃/MoO₃ catalysts showed selectivity and methanol yield advantage, respectively. MoO₃ demonstrated oxygen insertion ability due to its n-type semiconductivity. However, the mechanism of the co-operative effect in catalysis was not well studied. Most studies focused on the catalytic activity and selectivity, but the research of durability is rather poor.

4.2.4. Other Catalysis to Reduce Air Pollutants

There are only a few reports about MoO₃ catalysts applied for oxidation of CO and some volatile organic compounds (VOCs), such as methanol, (CH₃)₂S₂, benzene, and chlorobenzene [131,132,133]. Mohamed et al. [131] reported the oxidation of CO to CO₂ by MoO₃/CeO₂, and the results suggested that the dispersed MoO_x species and Ce³⁺/Ce⁴⁺ redox couples had high capacity toward oxygen, which was most likely to be the active species for CO oxidation. Wang et al. [132] studied the catalytic incineration of (CH₃)₂S₂ on CuO–MoO₃/γ-Al₂O₃ and the promoter effect. The results revealed that CuO–MoO₃/γ-Al₂O₃ has a good activity and durability, and Cr₂O₃ was the most effective promoter. Moreover, MoO₃ as a dopant in V₂O₅–MoO₃/TiO₂ catalysts improved redox properties and enhanced chlorobenzene oxidation catalytic activity at a low temperature, and this illuminated new applications for pollutants [133]. These researches will lay an early foundation for exploring MoO₃ catalytic properties in the catalysis of other VOCs.

5. Summary and Outlook

Nanostructured MoO₃ with unique structures and wide availabilities become one of the most promising materials employed for versatile applications. In this review, we have presented the basic structure and main applications of nanostructured MoO₃. The MoO₃ mainly consists of four phases: orthorhombic (α), monoclinic (β), hexagonal (h), and high pressure monoclinic (ε). All of these phases display unique physical and chemical properties for different performances.

We mainly outlined the applications of nanostructured MoO₃ in the field of energy and environmental catalysis, including water splitting, fuel cells, photocatalytic degradation, and selective thermocatalysis. The catalytic activities of nanostructured MoO₃ will be improved through the design of morphology, hybridization, and hybrid structure. The bandgap structure is optimized, and charge transfer is enhanced by constructing heterojunction nanocomposites. Although progress has been made in catalysis based on nanostructured MoO₃, the current research is at nascent stage. It is still required to develop new nanostructured MoO₃ materials to meet more demands in practical applications. For instance, selective catalysis to remove air pollutants requires very high temperatures; therefore, the exploitation of new and improved nanostructured MoO₃ applied to mild conditions is needed. The catalytic nature and mechanism in MoO₃-based catalysts are still challenging and need further studies. Limited research has been done to investigate the stability and recyclability in the selective thermocatalysis. There are numerous research studies about MoO₃ materials used for energy catalysis, especially in the field of hydrogen evolution. All kinds of strategies, including (1) morphology control, (2) phase transition, (3) introduction of oxygen vacancies, (4) hetero-atom doping, and (5) hybridization with other materials, have been employed to improve catalytic performance and durability of MoO₃-based materials. However, the activity or performance of MoO₃-based materials still do not completely reach the commercial standard, and the mechanism is not fully understood. The strategies mentioned above have been carried out, but there is no systematic and deep study on morphology, phase, oxygen vacancies, etc. Therefore, many problems, such as ‘how do oxygen vacancies influence catalytic performance?’ still need to be solved. It is more common to see that MoO₃ serves as a support material instead of active centers as a whole. Although some researchers have devoted themselves to addressing this [55,61,62,63,64,65], the activity enhancement is still limited. On the other hand, the durability of MoO₃-based materials still needs to be improved, and the stability test time should be prolonged. Thus, MoO₃ has huge treasure to be excavated for HER, OER, and fuel cells. In short summary, generally speaking, there are two aspects for further studying MoO₃ in the future: (i) the great improvement of catalytic performance and stability; and (ii) insight into structure-activity relationship and mechanism. Novel strategies and new methods must be proposed in order to achieve these long-term goals.