Semiconductor Heterogeneous Photocatalysts

Semiconductor Heterogeneous Photocatalysts: History

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Subjects: Chemistry, Applied

Contributor:

Nahal Goodarzi

Zahra Ashrafi-Peyman

Elahe Khani

Alireza Z. Moshfegh

Semiconductor-based photocatalytic reactions are a practical class of advanced oxidation processes (AOPs) to address energy scarcity and environmental pollution. By utilizing solar energy as a clean, abundant, and renewable source, this process offers numerous advantages, including high efficiency, eco-friendliness, and low cost.

advanced oxidation process (AOPs)
semiconductor-based photocatalysts
H2 production
water splitting
CO2 reduction
dye and drug photodegradation

1. Introduction

With the continuous increase in global population and industrialization, the necessity for clean energy and a sustainable environment is increasing dramatically [1]. Based on U.S. Energy Information Administration (EIA) survey in 2019, energy consumption worldwide is expected to rise by approximately 50% between 2018 and 2050 [2]. About 83% of today’s energy supply comes from fossil fuels (e.g., gas, petroleum, and coal). Fossil fuel combustion is responsible for releasing large amounts of CO₂, accounting for around 76% of annual greenhouse gas emissions, which creates severe problems for the environment and public health, such as global warming and climate change [3,4]. Accordingly, eco-friendly and renewable energy resources, including solar, biomass, wind, geothermal, and hydropower, have been successfully exploited as practical alternatives for fossil fuels to address these problems [5,6]. Among several energy as mentioned above, solar energy, which is emitted from the sun at a rate of 3 × 10²³ kW and 1.8 × 10¹⁴ kW of which is intercepted by the earth, is the most affordable and applicable energy source [7]. Hence, designing an efficient technique to transform solar energy into hydrogen and clean fuels such as carbon-based fuels through water splitting and carbon dioxide (CO₂) reduction, respectively, shows great potential to address both energy crises and environmental problems [8,9]. As a non-carbon-based energy carrier, hydrogen energy has attracted considerable attention due to its numerous advantages and may become a chemical fuel in the future. It has been proven that hydrogen has the potential to be a preferred alternative to reduce human society’s reliance on non-renewable resources and decrease environmental concerns. Water, which covers 71% of the earth’s surface, has excellent potential as a source of hydrogen energy. Using water power, we can extract H₂ by using an appropriate photocatalyst. This process not only provides a means of generating energy, but also helps to reduce greenhouse gas emissions. While the high flammability of hydrogen is considered a disadvantage, it is also beneficial because it eliminates the need for additional energy to combust the fuel. Additionally, once released, hydrogen diffuses quickly and is non-toxic. These properties make hydrogen a potentially safe and efficient energy source for the future [10].

There are various approaches for solar hydrogen production, including photocatalytic (PC) [11], photothermal catalytic (PTC) [12], photovoltaic electrochemical (PV-EC) [13], photoelectrochemical (PEC) [14], and photobiological (PB) [15]. In addition, some of these methods, including PV-EC [16], PEC [17], and PC [18], are also utilized for CO₂ reduction. The core of all the mentioned methods is the semiconductor photocatalysis, which is known as one of the promising approaches in carbon dioxide reduction and hydrogen production [19]. Inspiring natural photosynthesis, Figure 1 illustrates the application of solar energy by using semiconductor photocatalysts toward hydrogen generation from water splitting (Figure 1a) and CO₂ reduction to valuable chemicals (Figure 1b).

Figure 1. A schematic diagram of (a) hydrogen production from water splitting and (b) reduction of CO₂ to useful chemicals using solar light.

The other most important aspect of environmental remediation in the modern world of today is water purification. Regarding the expansion of human activities, a significant number of organic contaminants, including industrial dyes, disinfectants, pesticides, agricultural fertilizers and pharmaceuticals compounds, as well as inorganic pollutants, including heavy metals, salts, solvents, and nutrients, are daily being released into the wastewater streams [21,22]. A schematic of various organic and inorganic water pollution sources is depicted in Figure 2.

Figure 2. Various organic and inorganic sources of water pollution in the environment.

Long-term persistence in the environment and the water solubility of a wide range of compounds mentioned above have a significant potential to cause severe problems to the environment and health globally [21]. To overcome these issues, various conventional purification methods, including physical techniques (e.g., adsorption, membrane separation, flocculation, and coagulation), chemical methods (e.g., advanced oxidation process (AOPs)), and several microorganism-assistant biological treatments have been extensively used [23]. Physical methods by using adsorption techniques can remove organic and inorganic pollutants under different operating conditions [24,25]. However, the incomplete removal of organic pollutants and the generation of waste products during treatment can lead to several challenges for the desired process [24,26]. On the other hand, despite the excellent selectivity, membrane separation technology often leads to high operating costs and increased energy consumption because of membrane fouling [24,27]. Biological treatments using microbiological organisms will play an essential role in removing emergent contaminants from wastewater in the future. However, the need for physicochemical pretreatment and generation of biological sludge are the significant disadvantages of these methods [28]. Chemical methods, significantly advanced oxidation processes (AOPs), can be considered as a beneficial alternative in water purification treatments [29]. AOPs, introduced in 1981 by Glaze et al. [30], are defined as water purification processes by generating a sufficient amount of free hydroxyl radicals (^•OH) as one of the most effective reactive oxygen species (ROS). In addition to hydroxyl radicals, other reactive species, including hydrogen peroxide (H₂O₂), anion radical (^•O₂⁻), and singlet oxygen (¹O₂), can be generated in AOPs [31]. Meanwhile, ¹O₂ is a highly selective oxidant that can quickly oxidize electron-rich moieties in organic pollutants owning to its electrophilic nature. This form of oxygen species possesses a prolonged lifetime (nearly 2–4 μs) and high concentration (e.g., 10⁻¹⁴–10⁻¹¹ M) in water and demonstrates excellent resilience to inorganic anions and natural organic matter in wastewater [32]. These properties make this species suitable for selectively oxidizing high-priority contaminants commonly found in wastewater, such as endocrine-disrupting chemicals (EDCs), antibiotics, and pharmaceuticals [33]. Moreover, ¹O₂ can be utilized for the inactivation of pathogenic microorganisms and the elimination of antibiotic-resistance genes in both natural and drinking water [34]. Several AOPs systems, such as catalytic ozonation, hydrogen peroxide (H₂O₂), persulfate (PS) activation processes, and photocatalysis, can be applied to generate ¹O₂ [32]. In some ozone-based systems, O₃ can be activated and produce free peroxide species (^•O₂) that can be directly converted to ¹O₂ through an electron transfer mechanism. H₂O₂, as a common oxidant, can generate a variety of ROS after catalytic activation, followed by ¹O₂ generation in the system. In persulfate (PS)-based AOPs, in the presence of carbon-based materials, the carbonyl (C=O) functional groups on the catalyst reacts with PS and generate ¹O₂. Among the aforementioned approaches, photocatalysis is the most effective method to produce ¹O₂ through either a directly energy transfer on the photocatalyst or indirectly from other ROS. Despite all the advantages of ¹O₂, this species has low redox potential (0.81 V/SHE). As a result, its reaction rate with most organic compounds is lower than that of observed with radicals [35]. Owing to the high oxidation ability of the ^•OH radical (E° (^•OH/H₂O) = 2.80 V/SHE), this species acts as a powerful agent to degrade a broad spectrum of toxic organic pollutants into harmless compounds and even mineralize them into CO₂ and H₂O [36,37]. There are six types of AOPs based on the source nature of the ^•OH radical generation, including (a) photocatalysis [38], (b) UV–H₂O₂ processes [39], (c) photo-Fenton [40], (d) ozonation [41], (e) electro-Fenton [42], and (f) Sonlysis [43]. Different types of AOPs processes are depicted in Figure 3a–f.

Figure 3. Different types of AOPs processes. (a) photocatalysis; (b) UV-H₂O₂ processes; (c) photo-Fenton; (d) ozonation; (e) electro-Fenton; (f) sonolysis.

Among various types of AOPs processes, the semiconductor-based photocatalytic process is highly expected to be the more desirable approach for wastewater treatment owing to its excellent properties, such as its environmentally friendly nature, simple manufacturing, complete mineralization capability, and reusability. Furthermore, the semiconductor photocatalyst can effectively decompose organic contaminants because of its high surface area using solar energy via a simple route and low operating cost without producing harmful byproducts [37,44,45].

2. Photoelectrochemical H₂ Production from Water Splitting

It is well known that the efficient conversion of solar energy to chemical energy for producing clean energy is based on utilizing an appropriate semiconductor photocatalyst. Thus, hydrogen can be produced via photoelectrochemical (PEC) water splitting on semiconductor photocatalysts with effective nanostructures with high surface-to-volume ratio and high light absorption ability. A typical PEC cell consists of two half-reactions (a) oxygen evolution reaction (OER) usually occurs on an n-type semiconductor as a photoanode, and (b) hydrogen evolution reaction (HER) occurs on a cathode as a counter electrode, as illustrated in Figure 4.

Figure 4. A schematic diagram of a typical photoelectrochemical (PEC) cell.

There are extensive efforts conducted by many researchers to maximize the efficiency of hydrogen production rate under solar irradiation by examining different nanostructured semiconductor photocatalysts. Table 1 presents some of the results obtained on different photoanode semiconductors.

Table 1. Some of the newest synthesized photocatalysts for H₂ production from water splitting.

Entry	Photocatalyst	Synthesis Method	Preparation Condition	Photocatalytic Test Condition	Photocurrent Density, Efficiency	Ref.
1	Sb-SnS	hydrothermal	180 ^◦C for 16 h.	Simulated solar, AM 1.5 G, 100 mW cm⁻², 0.5 M Na₂SO₄	3.3 mA cm⁻²	[198]
2	BiVO₄, Bi₂O₃, TiO₂	spin-coating and calcination	sol-gel-based spin coating method	Na₂SO₄ solution without hole scavengers	5.1, 3.4, and 2.1 mA cm⁻²	[199]
3	CN/Mn₂O₃	plasma-assisted liquid-based	melamine and C₃H₃N₃O₃	0.1 M Na₂SO₄, pH = 6.5	25 μA cm⁻²	[200]
4	TiB₂@AuNPs	spin-coating deposition	Ultrasonic assisted liquid-phase exfoliation	sunlight simulator (1 Sun), 0.1 M KOH	10 mA cm⁻²	[201]
5	NiO/BiVO₄	dip-coating	Ink: Ni nanocrystals	1 M KBi, hole scavenger: 0.2 M Na₂SO₃	4.41 mA cm⁻²	[202]
6	β-FeOOH/CN	particle-to-substrate contacts	precursor: DCDA and TCY	AM 1.5G illumination, 0.1 M NaOH	320 μA cm⁻²	[203]
7	Fe₂O₃ (Hematite)	CVD	precursor: ferrocene organometallic compound	Simulator solar AM 1.5 G filter, 1 M NaOH	2.5 mA cm⁻²	[204]
8	Ir SAs/NiO/Ni/ ZrO₂/n-Si	ALD process	precursor: TICP	simulated solar AM 1.5 G, 1 M NaOH	27.7 mA cm⁻²	[205]
9	Ni(OH)₂/Cl-BiVO₄	impregnation method	precursor: 0.5 M NH₄Cl	AM1.5 G filter, 100 mW cm^{− 2}, 0.5 M K₃BO₃, pH = 9.5	4.33 mA cm^{− 2}	[206]
10	Bi₂S₃-Cu₃BiS₃	in-situ decoration	precursor: Cu:Bi 3:1 ink	simulated solar light (AM 1.5G), K–Pi buffer	7.8 mA cm⁻²	[207]
11	BiVO₄/Vo_x	electrodeposition	precursor: BiOI	simulated solar light (AM 1.5G), 1 M borate buffer	6.29 mA cm⁻²	[208]

3. Photocatalytic CO₂ Reduction

The substantial fossil fuels consumption resulted in vast carbon emissions with an atmospheric concentration of over 400 ppm, as reported in 2019. The over usage of fossil fuels ((e.g., petroleum, gas, and coal)) consumption is responsible for discharging a large amount of CO₂ into the environment after their combustion. The release of CO₂ accounts for around 76% of annual greenhouse gas (GHG) emissions which creates severe problems for the environment and public health, such as global warming and climate changes, seawater acidification, and the upsurge of ocean levels. To solve these severe ecological issues, the conversion of CO₂ into valued small-molecule chemical products or energy fuels (such as CO, CH₄, HCOOH, and other chemicals) through photocatalytic technology, can solve the energy crisis as well as environmental issues. Extensive efforts have been conducted by many researchers to convert GHG, such as CO₂, into valuable products and reduce its catastrophic emission. As a result of this process, environmental problems such as climatic turbulences, which comprise ecological deterioration, seawater acidification, and the upsurge of ocean levels will be reduced [59,209,210,211,212,213,214]. Table 2 presents some results obtained on different photocatalyst semiconductors along with preparation methods, photocatalytic conditions, and product yield.

Table 2. A summary of the synthesized photocatalysts for CO₂ reaction.

Entry	photocatalyst	Synthesis Method	Preparation Condition	Photocatalytic Conditions	Photocatalytic Activity (Product Yield)	Ref.
1	MOF [Cu₃Th₆(µ₃-O)₄(µ₃-OH)₄(cpb•)₁₂] [Fe^III(CN)₆]₆	mixed, stirred and centrifuged	precursors: Th(NO₃)₄, formic acid,	300 W Xe lamp, 420 or 800 nm filter	CO production rate: 570.3 µmol g⁻¹h⁻¹	[215]
2	Porphyrin-based MOFs	solvothermal	precursor: TPP and MnCl₂	300 W Xe lamp, 1 sun (AM1.5 G)	CO production rate: 46.148 µmol g⁻¹h⁻¹	[214]
3	boron-doped g-C₃N₄/TiO_2−x	thermal reduction process	precursor: mixture of urea and NaBH₄	300 W Xe lamp	CO production rate: 265.2 µmol g⁻¹h⁻¹	[210]
4	Co-COF:	mix and sonication	precursor: TAPT, 120 °C, 96 h	300 W Xe lamp (Microsolar 300, cut-off 420 nm), (λ ≥ 420 nm)	CO production rate: 18,000 µmol g⁻¹h⁻¹ H₂ production rate: 800 µmol g⁻¹h⁻¹	[211]
5	GQDs/C-BN	hydrothermal	precursors: boric acid, melamine and glucose	300 W Xe lamp (λ = 200–420 nm or λ > 700 nm)	CO production rate: 33.47 μmol g⁻¹h⁻¹	[216]
6	Ni(OH)₂	hydrolyzation of CaO	stirring for 24 h at 298.15 K.	LED lamps (5 W, λ ≥ 420 nm)	CO production rate: 9.2 μmol g⁻¹h⁻¹	[217]
7	S-ZnS/ZnIn₂S₄	hydrothermal	precursor: Methylimidazole	300 W Xe lamp cut-off filter (λ > 420 nm)	CO production: 2075.7 ± 63.0 μmol g⁻¹h⁻¹ H₂ production: 2912.3 ± 185.9 μmol g⁻¹h⁻¹	[218]
8	BP-CN	mix and sonication	Precursor: [NH₃OH]⁺Cl⁻ and melamine	300 W Xe lamp	CO production rate: 44.6 μmol g⁻¹h⁻¹	[219]
9	P-doped PCN	thermal polymerization method	precursor: Melamine and NaH₂PO₂⋅H₂O	300 W Xe lamp, cut-off filter 420 nm	CH₄ production rate: 1.1 μmol g⁻¹ h⁻¹	[220]
10	Ni-BiOBr	hydrothermal	precursor: Bi(NO₃)₃⋅5H₂O	300 W xenon lamp cut-off 380 nm filter (λ ≥ 380 nm)	CO production rate: 378.7 μmol g⁻¹h⁻¹	[221]

4. Photocatalytic Dye/Drug Degradation

Based on a World Bank report, dyeing and textile industries have contributed to about 17–20% of water pollution [222]. According to a recently published article, the textiles, leathers, food, and paper industries are primarily in charge of producing dye wastewater, with an annual global production of 8 × 10⁵ tons of dyes, of which around 200,000 tones are textiles and dyes [223]. Numerous synthetic dyes used in textile, including both cationic dyes (e.g., safranin O (SO), rhodamine B(RhB), malachite green (MG), rhodamine 6G (Rh6G), methylene blue (MB), and crystal violet (CV)) and anionic dyes (e.g., eosin Y (EY), Eriochrome black T (EBT), phenol red (PR), methylene orange (MO), and Congo red (CR)) are toxic and harmful organic contaminants that will impede the photosynthesis process of aquatic plants and pose a threat to living creature health via the channels of food and drinking water supply [224,225,226].

Other organic wastes are produced by chemical and pharmaceutical industries that are harmful to society and the environment. During past decades, there has been a significant increase in the production and utilization of Pharmaceutical and Personal Care Products (PPCPs) to accommodate the demands of contemporary lifestyles and increased health care [227]. According to the European Union market, there are approximately 3000 frequently used drugs, and their usage is still on the rise worldwide [228]. Based on a recent report, the worldwide usage of antibiotics is believed to range from 100,000 to 200,000 metric tons. When excreted from the body, 70–90% of consumed antibiotics remain chemically unchanged or as active metabolites [229]. Moreover, the global production and consumption of PPCPs have significantly increased as a result of the COVID-19 pandemic in recent years. The findings of the National Health Commission of the People’s Republic of China indicate a dramatic increase in the consumption of antiviral and antibiotic drugs during the pandemic [230]. Pharmaceutical contaminants are found in water at concentrations as trace as ng·L⁻¹ to μg·L⁻¹. However, owning their chemical and physical properties, they can pose a serious threat to living organisms’ health even at these low concentration levels [231]. Given the severe problems resulting from water pollution caused by dye and pharmaceutical pollutants, as well as regarding the significant advantages of photocatalysis processes in removing harmful pollutants from water, numerous attempts have been undertaken to develop highly effective semiconductor-based photocatalysts to photodegrade of dye and pharmaceutical pollutants globally. A summary of various photocatalysts used for the degradation of dye and pharmaceutical contaminants in water, along with their synthesis method and reaction conditions, is presented in Table 3.

Table 3. A summary of various synthesized photocatalysts used for the degradation of dye and pharmaceutical contaminants in water.

Entry	Photocatalyst	Synthesis Method	Preparation Condition	Pollutant Type	Light Condition	Efficiency (%)	Time (min)	Ref.
1	CN/CQD/BiOCl_0.75Br_0.25	hydrothermal	pH = 5.8 [TC] = 100 mg·L⁻¹ Catalyst dose = 0.1 g·L⁻¹	Tetracycline (TC)	500 W Xe lamp (λ > 400 nm)	83.4	30	[232]
2	α-Fe₂O₃@TiO₂	sonication and wet impregnation	pH = 4.76 [CFX] = 20.5 mg·L⁻¹ Catalyst dose = 0.012 g·L⁻¹	Cefixime (CFX)	500 W halogen visible light (>400 nm)	98.8	103	[233]
3	DBS/CNNS	calcination	pH = 5.5 [MOX] = 50 mg·L⁻¹ Catalyst dose = 1 g·L⁻¹	moxifloxacin (MOX)	300 W xenon lamp (λ > 420 nm)	~100.0%	30	[234]
4	0.1 chl/0.1 SA-TiO₂	incipient wetness impregnation	pH = 6 [CPX] = 10 mg·L⁻¹ Catalyst dose = 0.75 g·L⁻¹	Ciprofloxacin (CPX)	Blue LED light (λ = 457 nm)	∼75%	120	[235]
5	ZnO/ZnIn₂S₄ (ZnO/ZIS)	hydrothermal	pH = 3 [CS] = 10 mg·L⁻¹ Catalyst dose = 0.40 g·L⁻¹	ceftriaxone sodium (CS)	500 W xenon lamp.	85.3%	150	[236]
6	In₂S₃/MQDs/SmFeO₃ (IMS)	sonication	[SMX] = 10 mg·L⁻¹, catalyst dose = 0.6 g·L⁻¹ pH = 5.3	sulfamethoxazole (SMX)	300 W Xe lamp	98.0% and 95.4% of SMX	120 and 90 min,	[237]
7	MgCr-LDH	formamide-assisted co-precipitation and mild hydrothermal	pH = 7 [MB] = 20 mg·L⁻¹ Catalyst dose = 30 mg	methylene blue (MB)	solar light	90.6	120	[238]
8	ZnO-CT	a green synthesis route using lemon leaf extract	pH = 7 [CR] = 20 mg·L⁻¹ Catalyst dose = 0.4 g·L⁻¹	Congo red (CR)	Natural sunlight, (λ = 408 nm)	97	90	[239]
9	Au/La₂Ti₂O₇/Ag₃PO₄	The in-situ precipitation	pH = 9.6 [RhB] = 10 mg·L⁻¹ Catalyst dose = 1 g·L⁻¹	Rhodamine (BRhB)	Natural sunlight	100	6	[240]
10	UiO-66-NH₂/PhC₂Cu	hydrothermal	pH = 9 [NOR] = 10 mg·L⁻¹ Catalyst dose = 0.2 g·L⁻¹	norfloxacin (NOR)	9 W LED lamp (455 nm)	97.9	60	[241]
11	CuPd/ZnO	hydrothermal and chemical reduction	pH = 2 [OM] = 40 mg·L⁻¹ Catalyst dose = 0.5 wt.%	methyl orange(MO)	solar simulator (λ = 440 nm)	95.3	45	[242]
12	mesoporous Fe/Al/La trimetallic nano-oxide(FAL)	chemical route	pH = 7 [dyes] = 10⁻⁵ M Catalyst dose = 0.30 g/100 mL	black 5 (RB5) methylene blue (MB) direct blue 71 (DB71) and	Sunlight	93.85 ± 2 90.51 ± 2 91.16 ± 2	90 45 60	[243]
13	Fe₂O₃/CNT/MIL	hydrothermal	pH = 7 [OFX] = 20 mg·L⁻¹, Catalyst dose = 100 mg·L⁻¹	ofloxacin (OFX)	300 W Xe lamp (λ > 420 nm)	99.3	60	[244]
14	α-NiMoO₄/ZnFe₂O₄/BC	Pyrolysis and hydrothermal	pH = 10 [KP] = 10 mg·L⁻¹, Catalyst dose = 100 mg·L⁻¹	ketoprofen (KP)	visible light (UV cutoff 150 W LS xenon arc lamp)	98.65	180	[245]
15	0D/2D AgI/CAU-17	deposition-precipitation	pH(for RhB degradation) = 3 [RhB] = 10 mg·L⁻¹, [KP] = 10 mg·L⁻¹, [MO] = 5 mg·L⁻¹, Catalyst dose = 0.25 mg·L⁻¹	Rhodamine B (RhB) Tetracycline (TC) methyl orange (MO)	500 W Xe lamp	96.7 81.3 50.3	90	[246]

5. Simultaneous Photocatalysis

In the last decade, heterogeneous photocatalytic reactions have experienced many efforts and devoted studies in developing advanced and innovative materials to confront both environmental and energy crises via the utilization of appropriate semiconducting photocatalysts in many essential chemical reactions, including wastewater treatment, H₂ generation, CO₂ reduction, organic transformations, N₂ photofixation, and biomass conversion to valuable products. It is well known in a conventional photocatalytic investigation, and these processes are occurred in a controlled condition and are discussed in the literature separately. However, a recent innovative approach is to engage two or more functions in one photocatalytic system simultaneously. The challenging point is that combining of two functions in one photocatalytic system requires a novel design, control, and engineering of an appropriate semiconductor photocatalyst with unique characteristics for each application in a particular environment. The concept of dual-purpose photocatalysis was discussed in the pioneering work of Kim et al. [247] on simultaneous hydrogen production and phenolic compounds’ degradation by using a TiO₂ surface decorated with platinum nanoparticles and fluorine atoms (F-TiO₂/Pt) as a photocatalyst. They obtained complete mineralization of organic compounds and found anoxic degradation of 4-chlorophenol accompanied by H₂ production. Because an appropriate photocatalyst characteristic is required for each process, thus the primary challenge is to conduct two or more kinds of applications simultaneously over one photocatalyst. Thus, it is an ideal approach to design and utilizes a special photocatalyst in at least two different concurrent applications. For example, 2D semiconductors (e.g., GO, rGO, and MXenes) and their hybrid combinations can be applied to produce hydrogen generation and pollutant degradation simultaneously. Nugmanova A. G. et al. [248] synthesized zinc porphyrin metal–organic frameworks non-covalently attached to graphene oxide (SURMOF/GO) in Pickering emulsions, and investigated its photocatalytic activity during photodegradation of rhodamine 6G (Rh6G) and 1,5-dihydroxynaphtalene (DHN). In another study, Nikoloudakis et al. [249] fabricated a covalently linked nickel (II) porphyrin–ruthenium(II) tris(bipyridyl) dyad for photocatalytic water oxidation reaction in dimethylformamide (DMF) using methyl viologen as a sacrificial electron acceptor.

In another study, Wu et al. [250] reported that the photodegradation yield of 4-chlorophenol was 99.9% after 60 min, and the H₂ production rate by oxygen vacancies-enriched titanium dioxide (O_v-TiO₂) photoanode was 198.2 μmol h⁻¹ cm⁻² that is significantly higher than that on the TiO₂ working electrode. For example, in the case of simultaneous organic pollutant oxidation and H₂ evolution, the chemical potential of pollutant oxidation can provide the chemical potential needed for hydrogen evolution (H⁺/H₂; 0 V vs. NHE) from water splitting [50]. In another study, dual functional photocatalysis was conducted to tackle both environmental and energy challenges.

Additional examples of various photocatalysts used for simultaneous pollutant photodegradation and H₂ evolution are presented in Table 4.

Table 4. A summary of various photocatalysts along with their reaction condition applied in simultaneous photodegradation of pollutant and H₂ evolution from water splitting.

Entry	Photocatalyst	Synthesis Method	Photocatalytic Test Condition	H₂ Activity (μmol h⁻¹ g⁻¹)	Pollutant: Photodegradation Efficiency (%)	Ref.
1	0.5 wt.% Pt/Zn-V-20	Calcination, hydrothermal	300 W Xe lamp AM 1.5 G filter 0.5 M Na₂SO₄, 15% CH₃OH	5230.4	2,2′,4,4′- tetrahydroxybenzophenone (BP-2): (99.6), methylene blue (MB): (99.4), acetaminophen (AAP): (92.0)	[253]
2	g-C₃N₄/BiOI/CdS	calcination, solvothermal, and solution chemical deposition	300-W Xe lamp with a λ > 420 nm cutoff filter	863.44	bisphenol A: (98.62)	[254]
3	Mo¹@CNNTs	template free polymerization	300 W xenon lamp coupled with λ = 420 nm cutoff filter.	4861	tetracycline hydrochloride: (97.3)	[255]
4	[g-C₃N₄/polymethylmethacrylate (PMMA)]// [TiO₂/polyaniline (PANI)/PMMA]// [self-assembled 3, 4, 9, 10-perylene tetraformyl diimide (PDI)/PMMA] (TMOP)	tri-axial parallel electrospinning	simulated sunlight	536.7	Ciprofloxacin: (88.99), tetracycline hydrochloride: (91.15), chlortetracycline hydrochloride: (77.55), levofloxacin: (69.51), and colored dye methylene blue: (92.50)	[256]
5	SCN/NiS-1	hydrothermal	visible light (400 nm filter) irradiation	700.9	Rhodamine B (RhB): (98.5)	[257]
6	ZnS@Zn_0.58Cd_0.42S	hydrothermal	Xe lamp (CEL-PF300-T9, CEAU) with an AM1.5G filter	36000	Helianthine: (94.2)	[258]
7	NbO-BRGO	hydrothermal	300 W Xe > 400 nm	1742	crystal violet (CV): (97.6)	[259]
8	ZnIn₂S₄@SiO₂@TiO₂	sol–gel and solvothermal	300 W xenon lamp	618.3	methylene blue: (99.7)	[260]
9	Ag@TiO₂-P25-5%MoS₂	Combination of photocatalysts	solar simulator composed of two white light bulbs (60 watts)	1792	Ciprofloxacin: (75)	[261]
10	MoS₂/ZnO	hydrothermal	250 W metal halide lamp	235	Ciprofloxacin: (89)	[262]

This entry is adapted from the peer-reviewed paper 10.3390/catal13071102

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