Urbanization and industrialization form the foundation of modern civilization, driving substantial economic growth, technological advancements, and enhanced living standards. These processes have contributed to the rising universal energy demand through sustainable and environmentally friendly solutions which is a key challenge of the 21st century. The primary energy source has been covered by fossil fuels for industry and transportation which is being severely depleted at a rapid rate. Fossil fuels combustion significantly contributes to environmental issues through the generation and release of harmful by-product gases such as NO
X, SO
2, and CO
2. Among these, CO
2 is a major pollutant, not only from fossil fuel combustion but also from various other human activities. Its accumulation intensifies the greenhouse effect, ultimately driving global warming [
1,
2]. Therefore, addressing the reduction of these toxic gases is crucial. Clean energy systems are essential for minimizing reliance on fossil fuels and reducing environmental harm.
In recent times, researchers have become increasingly attentive to achieving environmental sustainability and tackling the growing crisis of environmental pollution. Hydrogen (H
2) stands out in the energy market for the upcoming decades with numerous benefits when considered with other traditional fuels such as petroleum and coal [
3,
4,
5]. Its ability to be stored for extended periods and transported across locations makes it a promising solution for upcoming needs for energy. In the long run, hydrogen fuel will replace hydrocarbon fuels because of its benefits and adaptability. Even though hydrogen is considered to be a potential candidate, its generation from hydrocarbons, is required for a low greenhouse gas (GHG) scenario. Solar, a zero carbon but renewable source of energy can fulfil the energy requirement of the world by its proper implementation. Hydrogen production photo catalytically is attracting significant attention as a promising self-determining method for efficient energy generation as it relies on water and sunlight—resources that are abundant and virtually limitless [
6,
7,
8,
9]. This approach is effective as it addresses energy demands as well as reduces the reliance towards fossil fuels. The idea of Honda and Fujishima of photo assisted hydrogen and oxygen generation by water splitting in 1972 paved the way for various approaches and photocatalysts for solar light driven catalytic H
2 production [
10]. Despite its potential, the progress of hydrogen production via photocatalysis has been slower as it faces a myriad of challenges. The primary challenge lies in its low efficiency, catalyst long term activity, band gap engineering, and reactor design that are largely influenced by the quantum efficiency [
11,
12]. These challenges demand the need for innovative and strategic solutions to make hydrogen as a clean energy source solutions to make it realistic for the future generation.
Over four decades of research, some hundreds of photocatalysts have been explored in the area of oxides [
13,
14], alloys [
15,
16], metal free catalysts [
7,
17], Metal Organic Frameworks (MOF) [
18,
19], organic polymers and complexes. TiO
2 has been the most widely considered and established since the early 20th century, owing to its non-toxicity, wide availability, stability, and affordability [
10,
20,
21]. The efficiency of TiO
2 remains significantly below the desired level due to several limitations, including its inability to utilize visible light effectively, a wide band gap (3.2 eV) resulting in low efficiency in the visible spectrum, slow mobility and kinetics of photogenerated charges, and rapid combination of generated electrons and holes. An ideal photocatalyst should have a band gap smaller than 3.0 eV to effectively capture and utilize visible light. To address these challenges, various innovative approaches, such as morphology engineering [
22,
23,
24], elemental doping [
25,
26,
27,
28], heterojunction formation [
29,
30,
31]
etc., have been investigated to enrich TiO
2 performance in various photocatalytic applications. Still tremendous investigations are going on band gap modification, stability and other properties of semiconductor based photocatalysts. Some of these semiconductors include g-C
3N
4 [
32,
33], KTaO
3 [
34,
35], ZrO
2 [
36,
37], SrTiO
3 [
38,
39] TiO
2 [
20,
40] and BiVO
4 [
41,
42]
etc. and the further modifications on these include the heterojunction formation, composites, elemental doping for enhanced photocatalytic HER.
WO
3 stands out as the most significant photocatalyst among metal oxides for the visible light region, owing to its remarkable chemical and physical properties. WO
3 is a promising semiconductor with comparatively lesser band gap energy of 2.8 eV along with the absorption capability in the visible spectrum [
43,
44,
45]. Moreover, WO
3 is cost-effective, exhibits low toxicity, and undergoes structural rearrangements that result in various polymorphs (e.g., triclinic, monoclinic, orthorhombic, tetragonal, cubic, and hexagonal). It also possesses a high oxidizing ability of the valence band holes, stable physicochemical properties, excellent solar light responsiveness, and highly tuneable structures. These properties make WO
3 an outstanding catalyst for photocatalytic HER [
46,
47].
WO
3 is a capable photocatalyst for HER because of its capability to absorb visible light and its high stability harsh reaction conditions. The VB edge of WO
3 is around 2.8 eV (
vs. NHE), making it suitable for water oxidation. However, the CB is not shifted to negative edge to ease water reduction. When WO
3 (p-type) is considered as photoanode, water reduction potential tends to be in the positive potential of CB. Consequently, in photoelectrochemical devices utilizing Pt is used as cathode and WO
3 as anode. This promotes WO
3 capability for OER in a photocatalytic system. One such example is where WO
3 with Pt was used for O
2 evolution and TaON with Pt acted as photocathode for HER in the photoelectrochemical system. The authors used IO
3−/I
− shuttle redox system to enhance oxygen generation [
48].
Numerous studies have highlighted photocatalytic WO
3 and the optimization of their physicochemical properties for photocatalytic applications. Considering the plethora of studies conducted on WO
3 towards photocatalysis, this review work aims to highlight the properties, latest approachs and modifications in WO
3 based photocatalysts for HER. It also discusses the fundamental aspects of photocatalytic HER and evaluates the performance of photocatalysts in HER. Furthermore, this review provides a detailed overview of the latest synthesis methods and strategies designed to improve the photocatalytic efficiency of WO
3 for HER applications.
Succinctly, photocatalytic HER is typically an uphill reaction that involves illuminating a semiconductor catalytic material with light, causing electrons in the VB to absorb energy and transition to the CB, creating holes in CB. This is the preliminary step, which is devoted to semiconductor’s photoexcited state. The first prerequisite is the realisation of high solar to hydrogen ratio (STH) under solar irradiation and a photocatalyst with an appropriate band gap [3,49,50]. The excitation step is primarily determined by the electronic band structure of the catalyst and levels of the conduction and valence bands. A photocatalyst with a narrow bandgap, the smaller the light energy and longer the corresponding wavelength of light can produce a greater number of excited electrons–hole pairs under such identical illumination conditions. Additionally, the lowest energy level of the CB must be more negative than the redox potential of H+/H2 (0 V vs. NHE), whereas the highest energy level of the VB must be more positive than the redox potential of O2/H2O (1.23 V). In the subsequent phase, photogenerated carriers travel to the semiconductor’s surface. Crystal structure, crystallinity and particle size have significant impact on this stage leading to more effective hydrogen production [51,52]. Additionally, particle size reduction shortens the migration distance for photogenerated electrons and holes to reach surface reaction sites, which in turn reduces the likelihood of the rate of recombination [53]. The final phase involves the catalytic reaction on the surface, wherein the electrons and holes that have migrated to the catalyst’s surface react with the adsorbed substrate. The photogenerated electrons decrease H+ to produce H2, whereas the photogenerated holes oxidize H2O to make oxygen, as illustrated in .
. Schematic representation of photocatalytic water splitting.
Surface area and active sites are the main criteria of this step [8,49,54,55,56].
The general mechanism of photocatalytic HER is shown in equation below:
All the abovementioned steps mark H2 generation from the semiconductor photocatalysts. Once electron/hole pairs are made, charge separation and recombination are two competitive steps that happen inside the semiconductor photocatalyst. These play a part in how well photocatalytic water splitting works to make hydrogen. Charge recombination is a thermodynamically favourable process that happens almost instantaneously either on the surface or in bulk. This phenomenon is regarded as a deactivation process and renders water splitting ineffective [11,56]. Another critical challenge to address is the surface back reaction, where H2O is formed on the surface of the photocatalyst from the generated H2 and O2 [57,58]. In recent years, significant advancements have been achieved to overcome these obstacles, focusing on catalyst design, reactor engineering, and the use of sacrificial agents, co-catalysts, and other innovative strategies.
2.1. Assessment of Photocatalytic Water Splitting
A collection of crucial metrics that are often used to quantitatively describe the substrate conversion efficiency is used to assess the rate of H
2 evolution during photocatalytic HER. This efficiency is described by the concept of overall quantum yield [
59,
60]. Under the given irradiation circumstances, the rate of gas evolution is typically measured as mol/h per catalyst quantity [g]. The overall quantum yield for H
2 generation is calculated using the following equation:
Quantum yield is precise in homogeneous processes. In heterogeneous systems, it provides an estimate of the amount of incident photons on the surface of the catalyst, which represents the maximum number of photons that could be absorbed. As a result, the idea of photonic yield has been introduced. It represents the photoreaction rate observed over a specific time period to the rate of photons incident within a particular wavelength range passing through the irradiation window of the reactor [61,62].
The second parameter for the assessment of H
2 evolution photo catalytically is Turnover number (TON). TON is often defined as the ratio of reacting molecules to active sites. TON is given by the equation:
The determination of photocatalyst active sites is indigenous. Thus, TON can be expressed as the proportion of reacting electrons to atoms in a photocatalyst.
Even though all these parameters are considered, it is necessary to keep in mind some crucial points while evaluating the quantum yield.
- (i)
-
Optimize the quantity of photocatalyst for each experimental setup, ensuring it does not alter the reaction rate. In addition, the catalyst should be dispersed evenly, which can be accomplished through effective stirring.
- (ii)
-
Consider initial reactant consumption and product production rates to avoid interference with measurements and reduce catalyst deactivation. Particularly, the quantum yield for substrate utilization and product production frequently fluctuates with irradiation time, especially during prolonged exposure.
- (iii)
-
Before light irradiation, reactants adsorbed on catalysts should reach a stable state, and the reaction rate should be proportional to the irradiation.
2.2. Factors Affecting Efficiency of Photocatalysts
2.2.1. Band Gap
One important thing that affects how well HER works is the band gap of a photocatalyst. Light harvesting, the efficient separation, transfer of photogenerated charge carriers, and surface reactions are the only things that make photocatalytic HER work. The semiconductor’s band gap must cover the reduction and oxidation potentials of water, which are +0 V and +1.23 V
vs. NHE, respectively, when the source solution has a pH of 0. According to theory, a semiconductor’s VB should be less than the redox potential of O
2/H
2O and its CB should be more than the redox potential of H
+/H
2. Thermodynamically, the initial step is the hydrogenation of intermediates; the active site absorbs H
+ to reduce to H
2 by use of photoelectrons. Even though there are numerous stable photocatalysts developed, the main constrain that prevails is the wide band gap, which hinders overall photocatalytic efficiency. Many researchers have concentrated on creating visible-light-responsive photocatalysts that are stable and have adequate band gap. Even the most effective and widely used photocatalyst TiO
2 faces limitations in meeting the requisite property of a photocatalyst, which has a band gap of 3.2 eV [
63]. In such scenario, creating bandgap tuneable semiconductors is especially useful, which is considered to be a more promising approach to solar H
2 production than the single photocatalyst-based water splitting system. Heteroatom doping and the utilization of material junctions are two well-known techniques for changing photocatalyst bandgaps and band locations. One of the best ways to make photocatalysts work better was to coat them with noble metal. This is because the noble metal particles on the surface of the TiO
2 can act as electron sinks, storing and moving photogenerated electrons. This helps separate charge carriers and lowers the rate at which they recombine. Some of the noble metals adopted include Pt [
64,
65], Ru [
66,
67] and Pd [
68,
69],
etc. Owing to the expensive part of noble metals, the other strategies adopted for the tunning of the band gap are the generation of composite photocatalysts of non-noble metals over singular ones [
70,
71], doping with metals and non-metals [
72], creating anion [
73,
74], and cation [
75] vacancies. These variations have a substantial impact on the band structure of semiconductors by changing how materials react to visible light, creating new energy levels across the band structures of the host semiconductor, increasing light absorption, allowing low-energy excitations, charge separation, and speeding up the separation of charges and catalytic activity for photocatalytic HER.
2.2.2. pH and Temperature
The pH of a solution is a crucial component in photocatalytic processes, as it significantly influences the process by affecting the adsorption of substrates onto the photocatalyst surface. It is very important that the pH of the photocatalytic system changes because it affects where the VB and CB edges are located. This, in turn, affects the semiconductor’s ability to oxidize or reduce, as shown in the equation below:
where $$E_{v b}^{0}$$ and $$E_{c b}^{0}$$ are the VB and CB potentials at zero pH. Estahbanati et al. looked at how pH affects the rates of photocatalytic HER in great detail [76]. They looked at different alcohols, such as ethanol, glycerol, and methanol, at pH levels ranging from 2 to 12. Using Pt/TiO2 as the photocatalyst, they saw that the production of hydrogen peaked at a pH of about 8 for all substrates. This was because the catalyst clumped together and the TiOH groups broke apart, as seen in . Another study examined the impact of pH variations on the photocatalytic HER mechanism, focusing on the role of dissolved metal ions as electron scavengers (metal ions) and hole scavenger (methanol). This study found significant changes in VB and CB edges by pH variations throughout photocatalytic water splitting over WO3, TiO2 (rutile), and NiO employing a 355 nm laser source [77].
. Kinetics on photocatalytic HER on Pt/TiO<sub>2</sub> on different alcohols. The rate of hydrogen production based on the present experimental results for glycerol (<b>a</b>), ethanol (<b>b</b>), and methanol (<b>c</b>) [
76]. Copyright 2019, Elsevier.
The production of electron-hole pairs is not directly influenced by temperature, which is why it has an insignificant impact on photocatalytic activity. However, temperature shows a substantial influence on the desorption of reaction products from the catalyst surface, accelerating the overall reaction rate. The temperature effect on photocatalysis differs depending on the photocatalyst. At higher temperatures, the possibility of transfer of electrons from the VB to higher energy states increases, boosting the production of electron-hole pairs and aiding in the first oxidation and reduction processes. Lower temperatures, on the other hand, have a detrimental effect on the reaction rate because they decrease product desorption, limit reactant adsorption and reduce HER efficiency. Huaxu et al. analysed photocatalyst Pt/TiO
2 where it generated 4.71 mmol g
−1 of H
2 in 4 h in 45 °C [
78]. The hydrogen production increased to 15.18 mmol g
−1 at 55 °C.
2.2.3. Light Intensity
Light intensity has the capability to enhance the photocatalytic water splitting that enhances energies above the activation threshold. Typically, a light source has the capacity to accelerate the formation of electron-hole pairs through chemical reactions rather than recombination reactions. Baniasadi et al. (2013) showed that using ZnS for photocatalytic hydrogen generation led to a 20% rise in photoactivity when light strength was raised from 900 to 1000 W m
−2 [
79].
2.2.4. Sacrificial Agents
Sacrifice agents are catalysts that play a prominent role in photocatalytic H
2 production by diminishing the recombination of e
−-h
+ recombination and holes to enhance photocatalytic hydrogen generation. Electron donors are used as sacrificial agents and scavenge the holes reducing charge carrier recombination. Further, the back reaction of the formation of water is suppressed, increasing the H
2 yield. The water-splitting reaction with the presence of sacrificial agents is a less uphill reaction than without sacrificial agents. The most common sacrificial agents include triethanolamine, sodium sulfide/sodium sulfite and methanol [
80]. The selection and concentration of sacrificial agents play a drastic role in photocatalytic hydrogen generation. For example, Na
2S/Na
2SO
3 mixture, methanol and triethanolamine (TEOA) are the most suitable sacrificial agents for sulphides, oxides and carbon based photocatalysts, respectively. TEOA can, for example, take in photogenerated holes and strengthen the bond between carbon-based g-C
3N
4 and H
2O, which speeds up the splitting and movement of photogenerated carriers on the surface [
81]. Among alcohols, methanol is considered to be the ideal sacrificial agent with one hydroxyl group and was the ideal feedstock for photo reforming reactions. This is attributed to the fast h
+ transfer process. The addition of methanol donates electrons that react irreversibly with photogenerated VB and enhances e
−-h
+ efficiency resulting in higher quantum efficiency. Bowker et al. demonstrated that applying primary alcohols over Au/TiO
2 catalysts resulted in higher rates of H
2 generation, whereas tertiary alcohols exhibited only minimal activity [
82]. Na
2S/Na
2SO
3 mixture acts as sacrificial inorganic since it is a particularly efficient hole acceptor, allowing for successful charge carrier separation. Na
2S/Na
2SO
3 mixture has been mostly used in sulphide-based catalysts such as CdS due to its more oxidisable nature compared to alcohol and reducing the undesired photocorrosion. Also, if there is sulfide in the solution around the Cd
2+, it can mix with S
2– to make CdS again [
83].
The crystal phases of WO
3 semiconductor (n-type) change over the temperature range of −180 °C to 900 °C. At temperatures ranging from 17 to 330 °C, the monoclinic I phase (γ-WO
3) develops, whereas the monoclinic II phase (ε-WO
3) is seen above − 43 °C. Between − 43 and 17 °C, the triclinic phase (δ-WO
3) takes place, and between 330 and 740 °C, the orthorhombic beta phase (β-WO
3) appears. At temperatures exceeding 740 °C, the tetragonal phase (α-WO
3) is formed as shown in . The most stable of these phases is γ-WO
3, which has a monoclinic crystal structure and a band gap energy between 2.4 and 2.8 eV. It is also the form of WO
3 that has been studied the most [
84,
85,
86]. Monoclinic WO
3 features a perovskite-like structure with a space group of P21/n, comprising a network of WO
6 octahedra. Each WO
6 unit is composed of eight tungsten atoms and 24 oxygen atoms, with edges shared by 8 oxygen atoms. These octahedra form a slightly distorted cubic arrangement within the structure. This well-defined atomic arrangement and high structural controllability create favourable conditions for achieving efficient photocatalytic activity. Also, the holes on the VB of WO
3 have a high oxidation potential (>2.5 eV), which means they can change OH
− into a hydroxyl radical. This makes it suitable for photocatalytic oxidation reactions, especially in the removal of environmental pollutants. Further, WO
3 is widely used in areas including photocatalytic disinfection, gas sensing, hydrogen evolution reactions, organic contaminant degradation in water by photocatalysis and photoelectrocatalysis and energy storage.
The deep valence band position of WO
3 of about 3.4 V makes it an efficient catalyst for visible light induced O
2 evolution. Considering its efficiency in photocatalytic OER, various strategies can be adopted to enhance its photocatalytic HER. This includes template-based synthesis, nano structuring, metal and non-metal doping, phase engineering, heterojunction formation
etc. One strategy adopted is enhancing the oxygen vacancy of WO
3 resulting in increased HER activity relative to their pristine WO
3 [
87]. The others include enhancing the surface properties and band gap via doping, defect engineering, pairing WO
3 with hydrogen evolution photocatalysts in a Z-scheme system that can leverage its strong oxidation ability while enhancing overall charge carrier separation.
The modification in WO
3 which includes morphology, size, crystal defects and exposed faces via different synthesis techniques can tune its optical and electrical properties. Nevertheless, the quick recombination of electrons and holes in WO
3, similar to other metal oxides, impedes its photocatalytic activity, resulting in a further decline in efficiency. Various methods are adopted to overcome this limitation, which include morphology engineering, crystal facet optimization, engineering of defects based on vacant oxygen, metal doping, creation of composites with carbon, heterojunction construction using a different semiconductor, and so on.
. Phases of WO<sub>3</sub> (tungsten—grey balls and oxygen—red balls) [
84]. Copyright 2022, Elsevier.
Synthesis strategies play an important part in tunning and optimising the parameters of WO
3 based photocatalysts for better efficacy towards HER. Basically, the synthesis technique utilized to produce metal oxide, whether in film or powder form, should be simple, cost effective, easy to handle, and capable of mass production of the product. Different synthesis strategies can be adopted to modify the structure, shape, properties and size of WO
3 which to a large extent can affect the photocatalytic activity. Different synthetic strategies have been reported which are environmentally friendly and less expensive, but at the same time have amazing photocatalytic properties. The predominant synthesis methods employed include sol-gel, hydrothermal, co-precipitation, solution combustion,
etc.
3.1.1. Co-Precipitation Method (CPM)
The coprecipitation approach is quick and economical for synthesizing metal oxides, involving the simultaneous formation of oxide particles during the precipitation process. This method integrates nucleation, growth, coarsening, and aggregation, offering key advantages such as homogeneity, precise composition control, and low-temperature synthesis compared to traditional techniques [
88]. Additionally, it requires neither expensive equipment nor stringent reaction conditions, rendering it a cost-efficient and straightforward approach. This method is widely employed for the development of double hydroxides and can be optimized by adjusting the pH, which significantly influences the structure, size, and activity of the synthesized materials. Numerous researchers have utilized the coprecipitation method for synthesizing WO
3 and its composites to enhance photocatalytic performance. For instance, a study on WO
3/CoWO
4 nanocomposites with varying cobalt concentrations (5–20 wt%) demonstrated improved photocatalytic efficiency, with 20 wt% Co@WO
3 showing the best performance. The coprecipitation process produced catalysts with smaller crystalline sizes, lowered band gap of 2.51 eV and larger surface areas, which contributed to their effectiveness in degrading harmful contaminants in water. Degradation efficiency of 86.50% was exhibited by 20 wt% Co@WO
3 towards methylene blue [
89]. Research by Banic et al. on novel WO
3/Fe
3O
4 magnetic photocatalysts was synthesised by the co-precipitation method using sodium tungstate and Na
2SO
4 precursors. During the synthesis by co-precipitation at pH of 1.8 WO
3 exhibited hexagonal morphology, while at pH 0.4 monoclinic morphology was adopted. This can be ascribed to the rising WO
3 content, which enhances all textural parameters while maintaining its mesoporous characteristics, alongside the presence of hematite and magnetite Fe
3O
4 in the photocatalyst, as indicated by the XRD studies. The presence of WO
3 in magnetite enhanced the BET surface area. The researchers generated WO
3/Fe
3O
4 with varied amounts of WO
3, and 6.1 WO
3/Fe
3O
4/H
2O
2 exhibited a degradation efficiency for thiacloprid that was 2.2 times more than that of bare Fe
3O
4, as illustrated in [
90]. Another group attempted to explore the optical characteristics of Cu doped WO
3 nanoparticles using the straightforward CPM [
91]. The nucleation process initiates in the Na
2WO
4 precursor, succeeded by the polymerization, culminating in a unique structural molecule. The optimum temperature (100 °C) eliminates the undesirable solvents, yielding a pure chemical. The process for doping Cu was also the same. The CPM offers numerous advantages, including high yield, low temperature
etc. CPM yielded a monoclinic structure with space group P21/n with a morphology characterized by nano-plates and rods. The optical absorbance edge was detected between 350 and 500 nm, and the band gap energy values for bare and Cu doped WO
3 were 3.12 and 3.36 eV, respectively, using the Kubelka-Munk equation.
. (<b>a</b>) XRD pattern of synthesised catalysts (<b>b</b>) BET of synthesised samples −196 °C (<b>c</b>) Kinetics of TCL photodegradation in WO<sub>3</sub>/Fe<sub>3</sub>O<sub>4</sub> in the presence of SS radiation (<b>d</b>) Charge transfer process [
90]. Copyright 2019, Elsevier.
3.1.2. Hydrothermal Synthesis
The hydrothermal method is a useful solution-based approach for manufacturing metal oxides, conducted under conditions of elevated temperature and pressure. One of its key advantages is the ability to tailor the size, crystallinity, specific surface area and morphology by adjusting the reaction parameters, which significantly influences their photocatalytic activity. In hydrothermal synthesis, the compositions of oxides that are synthesized can be carefully controlled by using liquid phase or multiphase chemical processes. The hydrothermal method offers certain benefits including environmental friendliness, moderate conditions for operation, cheap prices, excellent dispersion in solution, production viability, less expensive instrumentation, and rapid synthesis. Hu et al. employed a straightforward hydrothermal technique to synthesize one-dimensional hexagonally organized WO
3 nanorods with varied configurations. They used SnCl
4·5H
2O as the capping agent to improve photocatalytic activity [
92]. Palharim et al. explored the variation of hydrothermal temperature in the shape of WO
3-AgCl composites. At a reaction temperature of 120 °C, WO
3 formed as agglomerated nanorods [
93]. The shape of the WO
3 changed to agglomerated rectangular prisms at 180 °C. These may have fully surrounded the AgCl particles. Longer time results in thinner WO
3 nanorods at 120 °C and thicker prisms in rectangular shape at 180 °C. Growth of AgCl was more noticeable at 120 °C, particularly during the initial 12 h. The morphology of the catalysts, which was substantially influenced by both the reaction temperature and the synthesis duration, had a significant impact on the photocatalytic efficiency of the materials. Hexagonal WO
3 nanorods photocatalysts have been prepared through one-pot hydrothermal method using sodium tungstate in DI water as precursor by Yao et al. Nanorods exhibited BET surface area of 59.7 m
2g
−1 and pore size distribution peak centres at about 6.12 nm. These nanorods demonstrated excellent photocatalytic activity for the breakdown of organic dyes under visible light irradiation [
94]. A hydrothermal/sonication route is adopted for synthesizing hexagonal WO
3 nanocrystals with various morphologies and the enhanced activities towards photo electrocatalytic HER by Mohamed et al. [
95]. Adopting different synthesis parameters and precursors generated nano rod, nanosphere and nanotube morphology. Among which nanotube exhibited the highest efficacy of HER in the presence of light. The exposure of particular plane (411) and (112), oxygen vacancies and nanotube morphology obtained by the hydrothermal method plays a noteworthy role in the catalytic activity. This implies that the hydrothermal approach plays an important role in photocatalysis since it is less energy demanding; regulated, and convenient to use, with the ability to customize the shape and size in the 10–100 nm range.
3.1.3. Sol-Gel Synthesis
The sol-gel method is a popular procedure for fabricating materials such as thin films, inorganic nanoparticles, metal oxides, and other functional materials. Graham Thomas devised this process in 1864 for the manufacture of silica gels [
96]. The field rapidly gained interest, with significant contributions from notable researchers.
A “sol” is a colloidal suspension of very minute solid particles (1–1000 nm) dispersed in an aqueous medium that are held together by short-range van der Waals forces. This process uses a sequence of chemical events to turn a sol into a gel-like network that includes both solid and liquid phases. This process is broadly categorized into two types: aqueous and non-aqueous. The sol-gel method is highly valued for its ability to create materials with large surface areas and stable surfaces. These characteristics are essential because the physical and chemical properties of the synthesized materials are considerably influenced by the experimental conditions that are employed during the process. The sol-gel synthesis generally includes the following sequential steps [
97,
98].
Hydrolysis and Polycondensation: Precursors such as metal alkoxides undergo hydrolysis in water or alcohol, followed by condensation reactions where water or alcohol is eliminated, forming metal oxide linkages. These reactions lead to the growth of polymeric networks and colloidal particles in the liquid state.
Gelation: As condensation progresses, the solution’s viscosity increases, and there is the generation of a porous gel structure in the liquid phase.
Aging: During this phase, polycondensation continues, along with localized precipitation within the gel network. This procedure decreases porosity and augments the thickness of the layer that separates colloidal particles.
Drying: This critical step involves removing water and organic components from the gel to form a solid material. Structural dispersion is minimized by different drying methods like supercritical, thermal and freeze drying.
The sol-gel process is particularly effective for synthesizing uniform, and homogeneous compositions [
99,
100,
101]. This method’s capacity to generate mixed matrices is a significant advantage by carefully combining multiple metal oxide precursors. The parameters in this process such as the precursor type and solution’s pH, have an important part in the size and cross-linking of the resulting colloidal particles. In their study, Nagarjuna et al. used the sol-gel method to make WO
3 for the first time. This opens the door to large-scale production of supported oxides. The authors investigated the chemical changes, calcination temperature, structural evolution, and surface characteristics of WO
3 powders produced using this method. The synthesized WO
3 materials were employed to assess the photoreduction of the model priority pollutant, Cr(VI), under visible light [
102].
3.1.4. Solution Combustion Method
Solution combustion is a promising technology that requires self-sustaining exothermic reactions in an aqueous or sol-gel environment. A wide range of nanomaterials, oxides, alloys, sulphides as well as inorganic ceramics and composites with tailored properties, can be synthesised with this method. These materials find applications across various fields, such as catalysis, photocatalysis, electrocatalysis, heavy metal removal, sensors, and electronics. The technique’s remarkable versatility and efficiency have led to the development of numerous variants, facilitating significant improvements in the quality and functionality of the synthesized materials [
103,
104]. Researchers explored photocatalytic properties of nanoscale WO
3 is synthesized utilizing an ultra-rapid solution combustion process with various fuels [
105]. The fuels are glycerine, urea, citric acid
etc. This synthesis process produced nanoparticles of various shapes, such as rods, spheres, needles
etc. depending upon the fuel and amount of fuel. The synthesized method exhibited a considerable rate of photocatalytic performance towards methylene blue degradation.
3.2. WO3 Based Photocatalysts for HER
3.2.1. Bare WO
3
WO
3 is a highly versatile material with several advantages, including its low fabrication cost and abundant availability. Its ability to be fabricated as thin films or coatings makes it particularly appealing for photocatalytic HER. In its bulk monoclinic form, WO
3 possesses an indirect band gap of 2.62 eV, harnessing around 12% of the solar spectrum’s energy. This property makes it ideal for applications, such as photoelectrochemical cells, CO
2 photoreduction, and photocatalytic pollutants degradation. Significant research has taken place throughout the last three decades that has been dedicated to nanoscale structural control of tungsten oxide to enhance its photocatalytic performance. The blue colour of substoichiometric tungsten oxides is caused by the existence of defect states below the CB minimum. The defect states enable lower-energy excitations to the conduction band, leading to significant light absorption in the near-infrared spectrum. This phenomenon is primarily attributed to the excitation of free electrons in the conduction band, which originate from oxygen vacancies [
106]. Paik et al. synthesized sub-stoichiometric tungsten oxide nanowires, typically ranging from 50 to 250 nm in length and possessing a diameter of less than 5 nm, exhibiting a dark navy-blue colour due to oxygen deprivation, by a high-temperature nonaqueous colloidal technique [
107]. The presence of oxygen vacancies in the as-synthesized WO
3 nanowires (NWs) causes wide absorption from visible to infrared wavelengths. WO
3 NWs have a higher optical band gap (2.69 eV) than stoichiometric bulk WO
3 (3.05 eV). Photocatalytic hydrogen synthesis by photo reforming involves distributing nano particles in water/alcohol combinations and exposing them to UV/vis light. 1 wt% Pt serves as a co-catalyst on the WO
X NWs to enhance the kinetics of H
2 molecule production and function as an electron collector. The catalyst under UV/vis light exhibited hydrogen evolution of 464 μmol h
−1 g
−1 while bulk powder only produced less than 20 μmol h
−1 g
−1. Researchers also examined ethanol, methanol, and glycerol as sacrificial agents. c shows that H
2 evolution happens at about comparable rates in the occurrence of methanol and ethanol, however it befalls at half the rate in the presence of glycerol. The diminished rate of H
2 production in the presence of glycerol can be attributed to the substantial size of this sacrificial reagent within the photo reforming reaction, as well as the existence of secondary hydroxyl groups in glycerol.
The concept of oxygen vacancies in WO
3 renders it a compelling candidate for water splitting, as proposed by Wang et al. [
108]. A straightforward room-temperature solution processing method has been disclosed, which has the potential to alter the surface morphology and electrical structure of WO
3, enabling an extraordinary ability to promote photocatalytic HER without relying on a co-catalyst. Surface disordered layer was created using a disordering agent Li-ethylenediamine (Li-EDA). A moderate gap band state was established, resulting in a blue shift of both the CBM and the E
f in comparison to the bare WO
3 (d). Li-EDA treated WO
3(LT-WO
3) with new band structures exhibited photocatalytic HER of about 94.2 μmol h
−1 g
−1 under natural light without the assistance of a co-catalyst and was stable for 25 h (e).
. (<b>a</b>) Schematic representation of band shift from bulk to nano (<b>b</b>) Hydrogen production from 1 wt% Pt-loaded WO<sub>x</sub> NWs under UV/vis light illumination in 1:1 vol. MeOH/H<sub>2</sub>O mixture (<b>c</b>) Hydrogen evolution using a Pt/WO<sub>x</sub> NW catalyst in sacrificial agents [
107]. Copyright 2018. American Chemical Society. (<b>d</b>) Energy levels diagram of WO<sub>3</sub> and LT-WO<sub>3</sub> nanoplates (<b>e</b>) H<sub>2</sub> production rate from LT-WO<sub>3</sub> of 5 cycles in 25 h each [
108]. Copyright 2019. Royal Society of Chemistry.
WO
3 presents limitations, such as rapid recombination of photogenerated charge carriers and a lower CB level that surpasses the reduction potential of O
2/O
2−, leading to diminished O
2 molecule reduction during pollutant degradation via photocatalysis. This setback has prompted the search for acceptable methods to overcome these challenges, such as changing and regulating the structure of the semiconductor and lowering the band gap for improved photocatalysis via its production pathway and alternative improvement strategies. One such strategy was phase engineering that can actively tune the activity of photocatalysts by altering their band structure and active site configuration. Zhang et al. adjusted the precursor ratio for the synthesis of WO
3 by controlling phase changes [
109]. Oxygen vacancies were persuaded in WO
3 at a relatively low temperature, resulting in the transition of crystal structure from monoclinic to orthorhombic or pseudo cubic phase. Orthorhombic and pseudo cubic WO
3 exhibited photocatalytic HER activities with 268 and 340 μmol g
–1 h
–1 H
2 generation rates respectively, while monoclinic didn’t show any activity (). Pseudo cubic WO
3 was stable for 24 h without any noticeable degradation in the catalytic activity. Reduced orthorhombic and pseudo cubic WO
3 emerged from vacancy generated WO
3 leading to WO
6 octahedra distortion. These alterations push the CBM over H
2 reduction potentials and enhancing the photocatalytic HER activity.
Another modification is substitutional doping of WO
3 with a suitable dopant material, which results in a favorable shift in band-edge location and narrowing of the band gap, hence boosting photocatalytic efficiency. Metal doping has been one of the prime strategies to overcome the demerits of WO
3 as a photocatalyst. Cu doped WO
3 was established to be an excellent photocatalysts for HER by Yin et al. [
110]. The authors synthesized WO
3@Cu core-shell nanoparticles via sol–gel method. Copper served as an electron donor, increasing electron transfer efficiency and separating photogenerated electron-hole pairs, resulting in an H
2 generation rate of 37.78 μmol h
−1 g
−1 in visible light illumination.
. (<b>a</b>) Pseudo cubic WO<sub>3</sub> and band diagram of WO<sub>3</sub> of monoclinic, orthorhombic and pseudo cubic (<b>b</b>) HER activity of pseudo cubic-WO<sub>3–x</sub> for 24 h reaction in four cycles. [
109]. Copyright 2019. American Chemical Society.
3.2.2. Composites of WO
3
With Metal Oxides and Metal Sulphides
The extensive band gap, coupled with the unfavourable positioning of the band edges in WO
3, constrains the absorption of incident light and diminishes its efficacy in H
2 production. The VB edge of WO
3 is positive but CB edge is not negatively positioned compared to the redox potential of H
2 generation. This reduces the capability of H
2 production in the presence of light. In this scenario, the main method to overcome this is structural modification.
The key modification in forming heterojunction was with the most prominent and established photocatalyst TiO
2. This combination is perfect because it has a smaller bandgap than TiO
2 and a more positive conduction band edge. So, it can act as an electron trap, making charge separation work better and slowing the recombination of photogenerated carriers. One such combination was done by Camacho et al., where the researchers adopted sol-gel method to produce WO
3 in different molar ratios with TiO
2 nanoparticles. Further Pd metal was included in the TiO
2/WO
3 system [
111]. Pd acted as an electron trapper promoting the photocatalytic activity. The motive of this study was the formation of TiO
2/WO
3 and the combination of Pd to enhance the light absorption to the visible region. It was observed that 0.001 wt% Pd TiO
2/WO
3 exhibited significant hydrogen generation of 7.7% quantum yield in water-methanol mixture. Another method adopted was composite formation of morphologically modified WO
3 with TiO
2. Quantum dots of WO
3 on TiO
2 were made using solvothermal and hydrogen-reduction techniques. These caused oxygen vacancy flaws to form in WO
3 [
112]. These flaws kept TiO
2 and WO
3–x’s strong reductive and oxidative abilities while changing the charge-transfer route from type II heterojunction to Z-scheme. (a,b). This enhanced the photocatalytic HER activity with the rate of 17.7 mmol h
−1 g
−1.
To enhance photocatalytic activity, researchers have successfully produced a nanocomposite of TiO
2 core/TiO
2-WO
3 with core shell structure using acid precipitation preceded by thermal breakdown. TiO
2-WO
3 weight ratio was optimized to enable effective absorption of visible light. The hydrophilic nature of the composite enhanced water-splitting application. The coupled heterojunction in the shell structure facilitated the separation of charge carriers through the interaction of the Fermi levels of the shell components. With diethylamine hydrochloride, this procedure allowed electrons to migrate to the surface of the catalyst to generate 19.8 mL of hydrogen in one hour. The same group explored a multi-component n-n heterojunction consisting of WO
3, TiO
2, and Fe
2O
3 [
113]. This approach leverages the synergistic interactions across the heterojunctions, significantly boosting photocatalytic performance. WO
3 nanoparticles particularly help to boost visible light absorption, hence increasing the activity. The aforementioned group of researchers integrated semiconductors to create n-n heterojunctions between Fe
2O
3 and TiO
2 on one side, whereas TiO
2 and WO
3 on the other side, resulting in n-n heterojunctions. The tri-component photocatalytic system with WO
3 nanoparticles at Fe
2O
3 and TiO
2 interfaces suppressed electron-hole recombination quite well (c). WO
3 nanoparticles on Fe
2O
3 and TiO
2 surfaces adjusted the visible light band gap to 2.10 eV with hydrogen generation of 10.2 mL/h. CdO also has been considered to have incredible effect on WO
3 by altering its band gap. The presence of CdO makes the WO
3 more effective by promoting the charge separation and dropping the over potential of hydrogen evolution. The presence of CdO tuned the crystallite size of WO
3 producing excellent photocatalytic hydrogen production [
114].
. Z-scheme structure of (<b>a</b>) WO<sub>3</sub>/TiO<sub>2</sub> and (<b>b</b>) WO<sub>3–x</sub>/TiO<sub>2</sub> [
112] Copyright 2017. Elsevier and (<b>c</b>) Scheme showing plausible mechanism describing the separation of charge carriers in the WO<sub>3</sub>/TiO<sub>2</sub>/Fe<sub>2</sub>O<sub>3</sub> nano photocatalyst [
113]. Copyright 2020, Elsevier.
Metal sulphides have also been explored for enhancing photocatalytic HER properties of WO
3. Some of the most prominent metal sulphides used along with WO
3 are CdS, Ag
2S, SnS
2, ZnIn
2S
4. A group of researchers explored the combination of WO
3 and CdS to achieve Z-scheme mechanism for photocatalytic HER under visible light irradiation [
115]. The group used lactic acid as an electron donor. This combination enhanced the overall photocatalytic activity 5 times higher than CdS with the HER rate of 369 μmol h
–1 g
–1 with 20 wt% of CdS to WO
3. The presence of CdS with WO
3 results in the transfer of electrons from CdS to WO
3. Then, the interface band bending is moulded which results in potential barrier. On exposure of catalyst to light, electrons from CB of CdS cross the potential barrier can travel to WO
3. The photogenerated electrons in WO
3 combine with CdS holes. This makes it much less likely that photogenerated charge recycling will happen. More photogenerated electrons in CdS are available to reduce H
+ to H
2, which makes the photocatalytic H
2 evolution activity very strong. The same combination of composite was modified by another group in 2019 incorporating oxygen vacancies along with MoS
2 as cocatalyst for photocatalytic HER. The group tuned the concentration of WO
3 and cocatalyst MoS
2 along with synthesis time for high efficacy creating oxygen vacancy [
116]. The existence of vacancies significantly enhanced light-capture capacity and promoted the transit of electrons and holes. H
2 generation was 2852.5 μmol g
−1 h
−1 for vacancy enriched CdS/WO
3 which was way better than bare CdS/WO
3 as mentioned in previous work. CdS-DETA has been synthesised by intercalating DETA organic molecules with CdS and incorporated to WO
3 as a direct Z scheme composite by solvothermal method. This improved the active sites and surface area for the favourable nanojunction structure, resulting in H
2 generation of 15,522 μmol/g h. Better H
2 evolution by photocatalysis is caused by the direct Z-scheme and the effective separation of electrons and holes. Separating electrons and holes in Z-scheme heterojunction composites stops holes from building up in the valence band of CdS due to WO
3. This stops CdS from corroding in sunlight and boosts photocatalytic activity [
117].
With Metal Free Carbon Materials
Carbon materials’ physio-chemical, electrical, and optical characteristics have been shown to vary depending on their many allotropic forms [
17,
118]. g-C
3N
4, an inorganic polymer has attained the greatest attention towards photocatalytic activity in the recent times. When g-C
3N
4 is combined with WO
3, a nanocomposite following Z scheme was made that works well as a photocatalyst when visible light hits it [
119]. The great efficiency of WO
3/g-C
3N
4 composites in photocatalytic processes for hydrogen generation has attracted a lot of interest. A lot of work has been conducted on composites of WO
3/g-C
3N
4 with varied loading concentrations of WO
3, which include 2D g-C
3N
4/WO
3 composites, composites of both 2D g-C
3N
4 and 2D WO
3 etc. One such work was pyrolysis method for the synthesis of WO
3/g-C
3N
4 composites by varying WO
3 contents [
120]. As WO
3 content was enhanced, the photocatalytic activity also increased 15 times than the pristine g-C
3N
4 with a rate of 400 μmol h
−1 g
−1c
at (b). Optimising WO
3 to g-C
3N
4 and decreasing photogenerated charge carrier recombination led to the Z scheme and increased hydrogen generation. Morphologically modified g-C
3N
4/WO
3-carbon microsphere also has prominent impact on photocatalytic HER. Because g-C
3N
4/WO
3 and carbon microspheres work together so well, the g-C
3N
4/WO
3-carbon microsphere showed great hydrogen generation photocatalytic activity [
121]. On the g-C
3N
4/WO
3-carbon microsphere hybrid photocatalyst, the g-C
3N
4 and WO
3 parts formed a Z-scheme heterojunction. This set-up shows that g-C
3N
4 and WO
3 can both make electrons and holes in the CB and VB, which results in a hydrogen generation rate of 1636.0 μmol h
−1 g
−1. Carbon microspheres also work as charge transfer tunnels, speeding up the movement of photogenerated electrons and making it harder for photogenerated electron-hole pairs to combine again. The addition of NiS represented an alternative approach to improving the photocatalytic hydrogen evolution reaction performance of g-C
3N
4/WO
3. [
122]. The growth of NiS-WO
3 in g-C
3N
4 changes its electronic structure and how its carriers behave. This NiS-assisted WO
3/g-C
3N
4 heterojunction system introduces additional active sites, enhancing the formation of heterojunctions and significantly improving charge separation and transfer efficiency enhancing the hydrogen production rate up to 2929.1 μmol/g h. The two main characteristics for this enhanced hydrogen production are that WO
3/g-C
3N
4 heterojunction suppressed the recombination rate and NiS provided more active sites as well as improved the electron mobility.
In 2018, a different group successfully made a heterojunction of WO
3/g-C
3N
4 as a 2D photocatalyst. The heterojunction has WO
3 nanoparticles that are 5 to 80 nm in size evenly placed on 2D g-C
3N
4 nanosheets using a hydrothermal method [
123]. The composite demonstrated superior H
2 efficacy compared to pure g-C
3N
4 and WO
3, achieving an H
2 generation rate of 1853 μmol h
−1 g
−1 with Pt as a co-catalyst. Because of the creation of 2D nano-architectures and the way that WO
3 and g-C
3N
4 work together, the 2D WO
3/g-C
3N
4 photocatalyst is better at making H
2. This is because there are more active sites on the photocatalyst surface. By making the Z-scheme, it was also possible to limit the mixing of photoexcited electrons and holes while increasing the range of visible light that could be absorbed.
. (<b>a</b>) Schematic diagram showing Z-scheme photocatalytic H<sub>2</sub>-evolution mechanism for the WO<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub> composite (<b>b</b>) Cycle study for photocatalytic HER [
123]. Copyright 2018, Elsevier.
Fu et al. designed a photocatalyst of WO
3 and g-C
3N
4 in 2D level [
124]. The authors made a step-like heterojunction photocatalyst out of 2D/2D WO
3 and g-C
3N
4 by letting the WO
3 and g-C
3N
4 nanosheets stick together on their own. The obtained 2D/2D WO
3/g-C
3N
4 photocatalysts with Pt as the cocatalyst exhibited better H
2-production activity with the rate of 982 μmol h
−1 g
−1 than g-C
3N
4 and WO
3. The researchers clarified the mechanism by which, following intimate contact between g-C
3N
4 and WO
3, electrons from g-C
3N
4 were transferred to WO
3 across their interface until equilibrium of their Fermi levels was attained, leading to a positive charge on g-C
3N
4. Concurrently, WO
3 attains a negative charge at the interface. Band edge of g-C
3N
4 has an upward curvature due to electron depletion, while WO
3 has a downward curvature. Light stimulated WO
3 and g-C
3N
4 electrons from VB to CB. The internal electric field, band edge bending, and coulombic interactions favor the recombination of some electrons (from WO
3’s CB) with holes (from g-C
3N
4 VB) and inhibit others. The heterojunction mechanism using the S scheme removed less efficient electrons from WO
3’s conduction band and holes from g-C
3N
4’s valence band. The enhanced charge carrier transfer method significantly increased the redox capacity of the 2D/2D WO
3/g-C
3N
4 composite heterojunction, establishing it as an effective catalyst for hydrogen generation ().
. (<b>a</b>) g-C<sub>3</sub>N<sub>4</sub> and WO<sub>3</sub> work function before contact. (<b>b</b>) Band edge bending and internal electric field at the interface of WO<sub>3</sub>/g-C<sub>3</sub>N<sub>4</sub> after contact. (<b>c</b>) The S-scheme charge transfer mechanism [
124]. Copyright 2019, Elsevier.
Graphene is a candid 2D photocatalysts with high chemical stability and large surface-area. The 2D structure and exceptional physical-chemical properties, make it a profound candidate in photocatalytic applications. Integrating WO
3 with graphene to form multicomponent heterojunctions can lead to the development of effective photocatalysts. This combination facilitates efficient charge separation and minimizes the rate of photo-induced charge recombination, attributed to graphene’s high specific surface area, rapid electron mobility, and excellent optical transparency. Tahir et al. successfully incorporated graphene into WO
3 through hydrothermal treatment, resulting in a composite with exceptional photocatalytic activity for H
2 generation [
125]. 7% graphene significantly reduced recombination of electron-hole and enhanced reduction reactions, achieving a hydrogen evolution rate of 288 µmol h
−1 g
−1. The better photocatalytic performance is due to the mutual interactions between graphene and WO
3. These interactions include WO
3 absorbing more visible light, having a larger surface area, and effectively separating charges.
Another group incorporated graphene nanoplates into TiO
2/WO
3 nanocomposite and investigated its photocatalytic properties towards H
2 generation [
126]. The incorporation of graphene enhanced the transport and lifetime of photo-generated excitons, explicitly showing H
2 generation of 15 μmol/g
cat with 15 mol% of WO
3. This enhanced H
2 generation was anticipated because of the formation of type II heterojunction as shown in . For type-II heterojunctions, the VB of WO
3 was less than that of TiO
2 where photogenerated holes in the VB of WO
3 can migrate to VB of TiO
2. Conversely, when CB of TiO
2 exceeds that of WO
3, electrons in the TiO
2 CB transfer to the WO
3 CB, leading to a spatial separation of electrons and holes. This may lead to a reduction in charge carrier recombination and an increase in charge carrier lifespan. A comprehensive list of composites of WO
3 along with their performances is provided in .
. (<b>a</b>) Type–II heterojunction (<b>b</b>) S–scheme of TiO<sub>2</sub>/WO<sub>3</sub> in UV light irradiation [
126]. Copyright 2023, Elsevier.
.
List of WO3 and its composites along with their photocatalytic HER performances.
| Sl No. |
Photocatalysts |
Light Source |
Solvent |
H2 Rate (μmol h−1 g−1) |
Ref. |
| 1 |
WOx NWs |
150 W Hg-Xenon lamp |
MeOH/H2O (1:1) |
464 |
[107] |
| 2 |
LT-WO3 |
150 W Xenon lamp |
Na2S/Na2SO3 |
94.2 |
[108] |
| 3 |
WO3–x |
150 W Xenon lamp |
Na2S/Na2SO3 |
340 |
[109] |
| 4 |
WO3/TiO2/Fe2O3 |
Solar Light |
H2O |
10.2 mL/h |
[113] |
| 5 |
WO3@Cu |
400 W Hg lamp |
MeOH/H2O |
37.78 |
[110] |
| 6 |
Pd/TiO2-WO3 |
Solar simulator |
MeOH/H2O |
5.3 × 10−5 mol/min gCat |
[111] |
| 7 |
WO3–xQDs/TiO2 |
Xenon lamp |
methanol/H2O |
17.7 mmol h−1 g−1 |
[112] |
| 8 |
Pd/WT |
UVP mercury lamp |
90:10 H2O/methanol |
5427.0 |
[127] |
| 9 |
ZnIn2S4/WO3 |
Xenon lamp—300 W |
10% methanol/H2O |
300 |
[128] |
| 10 |
CuCl/WO3 |
Xenon lamp—300 W |
10% methanol/H2O |
15.8 μmol h−1 |
[129] |
| 11 |
TiO2|Ti|WO3 |
Xenon lamp—300 W |
MeOH/H2O |
473.2 μmol |
[130] |
| 12 |
5 wt% WO3/ZnIn2S4 |
Xenon lamp—300 W |
Na2S + Na2SO3 |
1945.8 |
[131] |
| 13 |
2 mol% Ag/WO3 |
Xenon arc lamp |
H2O |
3 |
[132] |
| 14 |
WO3@MoS2/CdS |
Xenon lamp—300 W |
10 vol.% of lactic acid |
8.2 |
[133] |
| 15 |
CuO-WO3-CdS |
300 W Xenon lamp |
MB aqueous solution |
178 μmol/g |
[134] |
| 16 |
WO3-Cd0.5Zn0.5S |
300 W Xenon lamp |
Na2S + Na2SO3 |
42.26 mmol h−1 g−1 |
[135] |
| 17 |
CN/HWO-Pt |
300 W Xenon lamp |
10 vol% TEOA/H2O |
862 μmol h−1 |
[136] |
| 18 |
p-gC3N4/WO3 NTs |
300 W Xenon lamp |
Na2S + Na2SO3 |
547 |
[137] |
| 19 |
3% g-C3N4/WO3 |
Visible light |
H2O |
1425 |
[138] |
| 20 |
7% graphene/WO3 |
Metal-halide lamp |
Na2S + Na2SO3 |
288 |
[125] |
| 21 |
WO3@COF/rGO |
300 W Xenon lamp |
H2O |
26.7 |
[139] |
| 22 |
WO3(H2O)0.333/CdSe-DETA |
300 W Xenon lamp |
Na2S·9H2O + Na2SO3 |
2.4 |
[140] |
| 23 |
WO3-G |
500 W Xe |
10 vol% TEOA/H2O |
400 |
[141] |
| 24 |
WO3/Ni–ZnIn2S4 |
Xe light source |
Na2S + Na2SO3 |
9.29 mmol h−1 g−1 |
[142] |
| 25 |
g-CN/WO3/biochar/Cu2+-Carbon |
150 W Xe lamp |
20 mL TEOA/H2O |
1900 |
[143] |
| 26 |
g-C3N4/WO3 |
350 W Xe lamp |
H2O |
482 μmol h−1 |
[144] |
This paper is devoted to the memory of Professor David F. Ollis, who left us on 6 October 2023. Professor Ollis, who was internationally known to everyone in our fields of science and engineering, was a leader in numerous areas of chemical engineering and technology as he performed breakthrough research and pioneered many new approaches that shaped the way we practice engineering. His research contributions were characterized by thoroughness and great breadth. Professor Ollis played a pivotal role in introducing biochemical engineering to numerous chemical engineering departments across the United States and around the world. Beyond his research, he was an extraordinary and devoted educator who deeply cared about his students and their learning. Professor David F. Ollis was a good friend and mentor of mine (P.G. Smirniotis), and he will be missed very much, but his work will stay with us forever.
The authors acknowledge University of Cincinnati for providing research infrastructure.
P.C.M.: Original draft, Writing—review and editing. B.M.—Review and editing. P.G.S.: Supervision, review and editing.
Not applicable.
Not applicable.
The data used in the review can be accessed through the cited publication within the manuscript.
This research received no external funding.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
