Recent Developments in the Use of Heterogeneous Semiconductor Photocatalyst Based Materials for a Visible-Light-Induced Water-Splitting System—A Brief Review

Abstract

Visible-light-driven photoelectrochemical (PEC) and photocatalytic water splitting systems featuring heterogeneous semiconductor photocatalysts (oxynitrides, oxysulfides, organophotocatalysts) signify an environmentally friendly and promising approach for the manufacturing of renewable hydrogen fuel. Semiconducting electrode materials as the main constituents in the PEC water splitting system have substantial effects on the device’s solar-to-hydrogen (STH) conversion efficiency. Given the complication of the photocatalysis and photoelectrolysis methods, it is indispensable to include the different electrocatalytic materials for advancing visible-light-driven water splitting, considered a difficult challenge. Heterogeneous semiconductor-based materials with narrower bandgaps (2.5 to 1.9 eV), equivalent to the theoretical STH efficiencies ranging from 9.3% to 20.9%, are recognized as new types of photoabsorbents to engage as photoelectrodes for PEC water oxidation and have fascinated much consideration. Herein, we spotlight mainly on heterogenous semiconductor-based photoanode materials for PEC water splitting. Different heterogeneous photocatalysts based materials are emphasized in different groups, such as oxynitrides, oxysulfides, and organic solids. Lastly, the design approach and future developments regarding heterogeneous photocatalysts oxide electrodes for PEC applications and photocatalytic applications are also discussed.

1. Introduction

The accessibility of an economical, clean, and copious energy source is one of the foremost challenges for humankind in the 21st century. Effective utilization of solar energy and converting it to use fuels that are more appropriate for storage and transportation is mandatory. Solar power provides a clean, renewable energy source with minimal ecological impact since it is a decentralized and virtually unlimited resource. In detail, the solar power reaching the earth’s surface is equivalent to that supplied by 130 million power plants of 500 megawatts [1,2,3]. However, the forthcoming use of solar energy as a major source to satisfy the worldwide requirements depends on accomplishing important milestones of a fundamental and technological nature. The feasibility of the enlargement of solar energy at the terawatt scale depends on finding cost-efficient compatible solutions to these difficulties [4,5,6].

The manufacture of solar hydrogen (H₂) through PEC water splitting as a proficient and renewable approach to arrest recurrent solar irradiance in the method of chemical fuels has drained remarkable consideration since the revolutionary report by Fujishima and Honda [7]. PEC water photoelectrolysis reaction is determined by solar energy generates an ecological approach to store energy in the covalent bonds of H₂ [8,9]. During earlier stages of research, stoichiometric water splitting was not successful in a theoretical way because of several difficulties, the simultaneous, reverse process, unstable nature of photocatalyst, poor variation in selecting photocatalyst and co-catalyst materials, etc. However, significant advancement has been executed, and many complications and hurdles have been overcome [9,10,11,12,13,14,15]. In this context, substantial research efforts have been carried out to upsurge the STH efficiency of those solar-driven photocatalysts. Thermodynamically, the conversion of one molecule of H₂O into H₂ and ½ O₂ is an uphill reaction with a great positive change in free energy change (ΔG° = 238 kJ/mol). In order to efficiently transform solar photons into usable fuels, n-type semiconductor materials can absorb a major portion of the spectral area, with appropriate energy band levels, and lower overpotential to execute a water photoelectrolysis reactions are mandatory. Extensive research efforts have been completed to promote viable n-type semiconducting electrodes that have emerged as photoanodes [14,15].

The oxygen evolution reaction (OER) is mutually kinetically and thermodynamically more exciting, and it hurts from severe oxidizing circumstances [16]. Moreover, the PEC water splitting system is one of the promising methods to examine the electron transfer process in producing oxygen (O₂) in water oxidation reaction to use solar energy [17,18,19,20,21]. In this regard, several oxide-based electrode materials have been investigated [22,23,24,25,26,27,28,29,30,31,32,33,34,35]. On the other hand, all these metal oxides have absorption activity restricted to the UV spectral region due to its wider band gap (>3.0 eV). This is a big drawback since the UV region comprises only 3–5% of all incident solar energy. Further, n-type oxide semiconductor materials such as Fe₂O₃, BiVO₄, and WO₃ photoelectrode materials have been extensively applied to gather the visible-light photons for highly proficient as well as steady PEC water splitting system and involve a large external bias [36,37,38,39,40,41,42,43,44,45].

It is challenging to develop cost-effective and steady photoelectrodes with STH efficiencies of over 10%. In this regard, a supreme semiconductor light absorber must satisfy a minimum of three simple necessities: (i) its bandgap must be narrow (varied between 1.8 and 2.4 eV) to display a strong light absorption, and its band energy levels must overlap water redox potentials for naturally splitting water into H₂ and O₂; (ii) it must possess great carrier mobilities, and thereby permit quick transport of photoinduced hole-electron pairs from the bulk to the surface; (iv) its surface assembly must be active for water adsorption and successive reactions; (v) it must display lower charge-transfer resistance at the interface. Moreover, highly stable, cost-effective, and ecofriendly photoelectrode materials are mandatory for solar photolysis.

Considering all these conditions for a material to carry out water photolysis (energy levels of the conduction band (CB) and valence band (VB) concerning oxidation and reduction potential of water), most heterogeneous semiconductor-based photocatalysts can also reduce the water. Figure 1 shows the energy for numerous oxide and heterogeneous photocatalysts oxide semiconductors with the water redox reactions, illustrating this fact.

The definite processes in PEC water photolysis are defined in Figure 2. In order to execute this, several steps are needed to gratify the execution of the overall PEC water photolysis reactions. These steps comprise (i) the capture of visible-light photons; (ii) separation of carriers; (iii) transportation of the separated electron/hole pairs through the charge transfer; which need to be catalytically active; and (iv) surface reactions. Both the potential of the excited charge carriers and suitable water oxidation kinetics are mandatory for proficient water photoelectrolysis reactions.

In recent years, organic semiconductors (OSCs) with heterojunctions have generally been utilized to construct cost-efficient and effective photocatalysts for photoenergy conversion systems [46,47,48]. In some cases, photocatalytic reactions via organic p–n bilayers react to the whole visible-light energy for the oxidative elimination of organic pollutants. In another instance, an organic p-n bilayer of phthalocyanine [MPc (M = H₂ or Zn), p-type] and perylene derivatives (3,4,9,10-perylenetetracarboxyl-bisbenzimidazole, PTCBI) or Pt-loaded fullerene (C₆₀, n-type) can act as a photocathode, whereas a reducing power can be produced at the C₆₀/water interface via a sequence of photophysical events in the bilayer [49,50,51,52]. Moreover, it was recently evidenced that an organic p-n bilayer can engage near-infrared energy to carry out photocatalysis [53]. In recent years, our group employed a wet-chemical process of a reprecipitation through a polar solvent in creating biphasic nanocrystals of aluminum phthalocyanine (AlPc)/cobalt phthalocyanine (CoPc) and C₆₀ [54,55,56], whereas earlier reprecipitation developed by Kasai et al. provide a single component of organic crystals whose size, however, is manageable among nanometers to micrometers [57,58,59].

(Oxy) nitrides are new kinds of materials recently employed as photoelectrode materials for water splitting systems. Oxynitride based photocatalyst tends to possess narrower bandgaps than the equivalent metal oxides since the M-N bond in oxynitrides has potential energy greater than that of the M-O bond in metal oxides. In particular, it must be noted that these kinds of materials vary from photocatalysts doped with N or sulfur [60]. Moreover, (Oxy)nitrides are well-known for their solidity against anodic dissolution in alkaline media, providing them appropriate materials for water splitting applications.⁶¹ Comparatively, certain (Oxy)nitrides photocatalysts can replace metal-oxide-based materials to absorb visible-light photons for water photolysis reactions [61,62,63]. Similarly, an organic solid/water interface in a p/n organic bilayer was established to be proficient of photoinduced redox reactions combined with photoconduction of the generated hole/electron pairs in the interior [50,64,65], whereas wide-range visible-light energy (λ < 750 nm) is accessible for the photogenerated redox reactions at the interface mentioned above.

It is projected that long-lasting struggles in this area will result in more fascinating approaches towards emerging durable and high-efficiency heterogeneous photocatalysts based electrodes for water oxidation shortly. This focused review illustrates recent findings in promoting and advancing heterogeneous photocatalyst materials for light-driven water splitting reactions. In this review, heterogeneous photocatalyst materials, such as organophotocatalysts, (Oxy)nitrides, oxysulfides, and other heterogeneous photocatalysts, are presented for solar-driven photocatalytic water splitting, concentrating on recent developments on heterogeneous photocatalyst materials. An outlook for the future growth of heterogeneous photocatalysts-based photoanodes is also projected.

2. (Oxy)nitrides Based Materials

Comparatively, metal nitride/oxynitride based photocatalysts are subjected to photodissociation and have not been employed for water photolysis reaction [66]. In particular, oxynitrides are exceptionally favorable photocatalysts due to their narrower bandgap and subsequent superior visible-light photons related to oxide-based materials. For instance, TaON, Ta₃N_5, and Ba(Ca, Sr)TaO₂N are the distinct classes of (Oxy)nitrides-based families. TaON, whose bandgap is varied from 1.9 to 2.5 eV and energy levels overlap with water redox potentials, is accepted as a more suitable material for developing highly active photoanodes.

The energy level positons of both TaON and Ta₃N₅ are more appropriate for water oxidation and reduction in visible-light conditions. Balaz and co-workers reported the CB and VB’s absolute positions of various kinds of characteristic TaON perovskites ATaO₂N (A = Ca, Sr, and Ba) and PrTaON₂ [67]. Moreover, TaON and Ta₃N₅ have been widely examined as solar-driven photoelectrode materials in water redox processes [68]. Particularly, above 10% quantum yield has been accomplished with TaON and Ta₃N₅ photocatalysts [69,70,71]. Domen and co-workers mostly investigated oxynitride based photoelectrodes as an alternative to oxide material, which is likely to arrest the incoming photons for water oxidation reactions [72,73,74]. In this regard, creating highly active oxynitride-based photoanodes is significant in achieving a high-efficiency PEC water photoelectrolysis process.

Electronic structures of oxynitrides were theoretically investigated in terms of the framework of density functional theory [75,76]. These results illustrate that the N2p orbital has greater potential energy than the O_2p orbital; it might be remarkable to engage a metal (Oxy)nitride as a photocatalytic material. In this context, (Oxy)nitrides display distinctive optical features, owing to O₂/N₃ ions’ interplay in the system [75,76]. Comparatively, (Oxy)nitrides of the metal system cause a red-shift in the material than oxide-based materials. This might be owing to incorporating nitrogen (N) into the lattices, whereas the extra energy levels in the host material band considerably affect the optical transition in the system [77]. Particularly, this originated owing to the lesser electronegativity of N to that of O. For instance, Figure 3 compares the schematic representation of the oxide-based NaTaO₃ and (Oxy)nitride BaTaO₂N, and both of them possess a similar perovskite structure [78].

Generally, the most vital parameter affecting photocatalysts’ visible-light-driven water oxidation efficiency is the amount of living photoinduced holes directed by the morphology and size of a photocatalyst [79]. In general, the photocatalytic performance can be promoted by means of photoelectrodes with huge surface area, higher dispersion, high crystallinity, reduced defect density, well-defined morphologies, and precisely exposed facets, permitting to decrease the amount of recombination sites and to highly photochemically active sites. In particular, the morphological and structural modification of oxynitrides considerably affects the photocatalytic activity for water oxidation. Also, the flux-assisted nitridation method is beneficial for enhancing the crystallinity, modifying the morphological as well as surface modification of oxynitrides [80,81]. It is also evidenced that the cocatalyst–photoelectrode interface and co-catalysts dispersion over electrodes play a significant part in oxynitride photocatalysis. For instance, Maegli et al. reported the controlled morphological and structural changes of LaTiO₂N/CoOx photoelectrodes for photocatalytic O₂ oxidation [82]. They also demonstrated that the flux method improved the crystallinity and higher nitrogen contents in LaTiO₂N than if prepared without flux. In particular, the skeletal and edgy morphology has higher CoOx catalyst dispersion on the LaTiO₂N electrode contained unsaturated bonds, thereby improving the photocatalytic O₂ evolution. Moreover, Kodera et al. demonstrated that flux treatment on BaNbO₂N photoelectrodes greatly influenced the morphological natures [83]. Prior to flux action, the BaNbO₂N morphological features were uncertain, with an accumulation of particle sizes varied from a few 10’ s nm to 100′ s nm. After the flux treatment, a noticeable modification in morphology happened. Figure 4 displays FE-SEM images of BaNbO₂N photocatalysts before and after flux action. After flux treatment, a cubic morphology was clearly noticed. Similar flux-influenced morphological modifications are also demonstrated for BaTaO₂N crystal structures [84]. Thus, the flux-aided nitridation process is mandatory to promote oxynitride’s photocatalytic features by enhancing crystallinity, tailoring the morphology and surface modifications, reducing the defect density.

Different (Oxy)nitrides materials have been considered to replace the oxide catalysts to capture the light-photons to produce H₂ and O₂ from water at the stoichiometric ratio [85,86,87,88,89,90]. Afterwards, these kinds of (Oxy)nitride-based photoelectron materials are well-known for PEC water oxidation, with nominal input [91,92]. A large quantity of such (Oxy)nitride photoanodes have been reported to date. Moreover, few of them attained superior incident photon-to-current efficiency (IPCE) efficiency of >10% with an applied bias lesser than the thermodynamically needed bias (1.23 V) for water photoelectrolysis. Further, in the case of wavelength, light photons with wavelengths up to 600 nm are accessible for PEC water oxidation with Ta3N5 [93] and LaTiO₂N [94]. In this regard, no narrower photoanodes (<2.0 eV~equivalent to 600 nm) are available to oxidize water without any externally applied bias has been demonstrated till now. Furthermore, while decreasing the photoanodes’ bandgap, reducing the driving powers for water redox reactions makes water electrolysis reactions more challenging.

Both the superior performances and stability of (oxy)nitrides for PEC water electrolysis rely on the effective transfer of the photoinduced holes from the photoelectrodes to reactants adsorbed on the surface to induce the water oxidation for OER. Building a catalytic active surface morphology for PEC water electrolysis is extremely required to advance surface charge transfer. In particular, the catalytic surface must provide the roles of gathering photoinduced holes, permitting ample adsorption of water molecules and making it a lower reaction kinetic barrier for the oxidation reaction. Despite its ability to capture solar energy and chemical inertness, the PEC behavior of (oxy)nitrides is bizarrely restricted due to its deprived photon absorption, the greater recombination rate of photogenerated carriers sluggish OER kinetics. To rectify these drawbacks, (oxy)nitrides photoanodes must be loaded with co-catalyst materials to promote light photon absorption. During the last decades, numerous works have been carried out to advance (oxy) nitride-based photoanodes with appropriate co-catalysts to efficiently utilize visible light photons [63,92]. Moreover, much attention has been paid to develop (oxy)nitride-based photoanodes loaded with cost-effective transition metal oxides. Nishimura al. demonstrated the TaON based (oxy)nitrides along with co-catalyst photoanodes and attained IPCE of 76% at 400 nm with a nominal external bias [92]. Subsequently, different (oxy)nitrides, such as LaTaO₂N [94], SrNbO₂N [95,96,97], and BaNbO₂N [98], among others, have been newly advanced as photoanodes [99,100,101,102,103].

Photoelectrodes with catalytic surface modification of several co-catalysts for water electrolysis, namely IrO₂, CoOx and “Co-Pi” have been deeply examined [102,103,104,105,106,107,108,109]. In particular, Abe and co-workers reported incorporating IrO₂.nH₂O nanoparticles over porous TaON photoanodes and have achieved the IPCE of 76% in an aq.Na₂SO₄ solution [109]. Further, it is the first report to demonstrate the PEC water splitting on TaON (oxy)nitride with a scannable production of O₂, as well as measured anodic photocurrent. Notably, Higashi et al. reported a methodology of pre-depositing earth plentiful CoO_x co-catalysts over TaON particles via impregnation before the electrophoretic deposition (EPD) of TaON particles [106]. Recent years, (Oxy)nitrides photoelectrode materials, namely LaTiO₂N/CoOx, BaTaO₂N/BaZrO₃, and BaNbO₂N [110,111,112,113], have been developed to exploit the visible-light photons absorption with the supporting sacrificial reagents. Much research has mainly focused on creating the visible-light active narrower bandgap semiconductors photoanodes (<2 eV). In this regard, Zhang et al. reported LaTaO₂N photocatalysts materials prepared through the flux technique. It also revealed enriched PEC behavior for the water oxidation reaction [114]. Quite recently, new kinds of rare earth hafnium (RHfO₂N) (Oxy)nitride were demonstrated for the photocatalytic water redox reactions [77,115]. The RHfO₂N perovskites (R = La, Nd and Sm) signified the GdFeO₃-type structure with a bandgap energy of 3.35, 3.40, and 2.85 eV. These hafnium-based materials photocatalytic results revealed RHfO₂N oxynitrides that have been successfully applied towards overall water splitting reactions. Also, Takata et al. have demonstrated and activated LaMg_xTa_1−xO_1+3xN_2−3x photocatalysts for photocatalytic water splitting by incorporating the co-catalyst of RhCrOx [116].

Niobium-based oxynitrides are of specific interest in water splitting owing to their narrow bandgap of 1.7 eV, while it was evidenced to be satisfactory for photocatalytic water photodissociation [117]. Maeda et al. first reported the narrower bandgap semiconductor of FTO/SrNbO₂N photoanodes (<2.0 eV) substrate via electrophoretic deposition (EPD) process and examined for water splitting reactions under neutral conditions.¹¹⁸ IrO₂ incorporated SrNbO₂N/FTO electrodes chronoamperometric (CA) investigation showed significantly improved PEC water oxidation behavior compared to SrNbO₂N/FTO as presented in Figure 5A. During the CA analysis, an external bias of 1.55 V_RHE was engaged under steady-state light irradiance (λ > 420 nm). The enhanced performance was accredited to the substantial role of adsorbed IrO₂ colloids over SrNbO₂N particles to suppress the material’s oxidative self-decomposition and thereby enhance the selectivity of photoinduced holes toward water oxidation. This is an efficient approach to improving the PEC performances in the lower potential region, thereby promoting water oxidation. The authors further evidenced the water oxidation by evaluating the stoichiometric generation of H₂ and O₂ upon irradiation and was shown in Figure 5B. Moreover, in terms of evolved H₂ and O₂, the authors estimated that 47% of the visible-light current might be credited to photogenerated water oxidation by IrO₂/SrNbO₂N.

In recent years, we also demonstrated the PEC activity of cobalt phosphate (CoPi)/La(Ta,Nb)O₂N and CoPi/ZnTaO₂N electrodes for PEC water oxidation in alkaline conditions [99,119], which showed up to a nearly four-fold advancement in the PEC activities at a lower oxidation potential. In particular, the loading of the CoPi over (Oxy)nitride photoanodes might promote the charge separation as well as the carrier collection created at the electrode surface, thereby improving PEC features of the photoanodes). We have also recently reported the CoPi loaded ZnLaTaON₂ photoanodes for PEC water oxidation performances under alkaline solution [120]. In all these cases, the photodeposition (PD) of CoPi onto (oxy)nitrides photoanodes under irradiation conditions were evidenced to promote the OER at the photocatalysts [121]. Further, the CoPi co-catalyst layer was incorporated over (oxy)nitrides by employing the in-situ PD approach to promote the OER half-reaction kinetics. We have also evidence that the CoPi deposition’s magnitude was determined by the electrodes’ morphology, time, and loading process method. The thicker CoPi incorporation over (Oxy)nitrides photoanodes resulted in the poorer PEC behavior for water oxidation. These facts can be illustrated that the photocatalytic water oxidation reaction happens at the CoPi/electrolyte interface. In the thicker CoPi, the photoinduced holes need to transfer between more number CoPi molecules and a CoPi/electrolyte interface. Subsequently, a decline in photocurrent is witnessed. However, in thin CoPi layer over (oxy)nitrides, associated cobalt ions on (Oxy)nitrides surface easily capture the photoinduced holes to yield the active catalytic species of cobalt (Co⁴⁺) mandatory for a water electrolysis reaction [122,123]. Likewise, amorphous Co(OH)_x has been used as a co-catalytic material for water oxidation reaction and can enhance the PEC stability as well as performances of Ta₃N₅ photoanodes [122,123,124,125], even though it is not as efficient as CoPi. One of the major roles for superior performance of CoPi co-catalyst in comparison with Co(OH)_x is the whole coverage of Co-Pi on the Ta₃N₅ photoanodes. Generally, in Co(OH)_x is incorporated over Ta₃N₅ photoanodes surface via the co-precipitation process; thereby, it is hard to obtain the homogeneous dispersion by PD method. Thus, the PD methodologies benefit from permitting the comparatively homogeneous wrapping of CoPi over the (Oxy)nitride surface of the photoanodes. These results show that the incorporation of CoPi over (Oxy)nitride has been an efficient route to enhance the PEC current gain via the PEC water oxidation reaction.

Like CoPi electrocatalysts, layered double hydroxides (LDHs) have been widely investigated as catalytic materials for water oxidation [126,127]. Particularly, LDHs attained by a solution exfoliation process revealed greater OER performances and improved durability compared to noble metal/metal oxide electrocatalysts. Mostly, LDHs co-catalysts were engaged to promote the PEC performances for water electrolysis reaction [128]. In this regard, Wang et al. reported the Ni–Fe LDHs incorporated Ta₃N₅ photoanodes array and are significantly enhanced stability and PEC performances for water oxidation [129]. Further, the author’s group deposited the Co(OH)x and CoPi electrocatalysts on sequence over Ni–Fe LDH/Ta₃N₅ photoanodes, further enriched the photocurrent. Liao et al. associated the effect of various co-catalysts (Co(OH)_x, Co₃O₄ and IrO₂) and electrolytes (NaOH and Na₂SO₄) on the photocurrent and superior durability of photoanodes comprising of nanoparticulate Ta₃N₅ [130].

Allam and co-workers recently reported a novel photoanode material of Ti-Pd mixed (Oxy)nitride films prepared through anodization method and attained a photocurrent density of 1.9 mA/cm² and a nearly five-fold enhancement in the efficiency compared to pure TiO₂ nanotubes [131]. Similarly, titanium oxynitride (TiON) emerges as a favourable candidate for light absorption and appropriate energy level positions for water splitting [132,133]. In particular, Soliman et al. fabricated the TiON nanotube arrays via NH₃ nitridation from anodized Ti foil. Very recently, Soliman et al. demonstrated the highly ordered Ag nanoparticles incorporated TiON nanotubes arrays for solar-driven water splitting [134]. Further, the author group investigated PEC properties of Ag/TiON nanotubes arrays resulting in an exceptionally superior photocurrent of 14 mA/cm² at 1.0 V SCE (Figure 6b), which is consistent with the absorption spectra (Figure 6a). The observed improvement of photocurrent over TiON counterparts is credited to co-operative features of Ag nanoparticles incorporation, N-doping, and the distinctive morphological characteristics of the fabricated NTs arrays. In detail, the IPCE measurements of Ag/TiON nanotube arrays with the potential between 0.2 V and 0.4 V (Figure 6d) evidences the maximum IPCE of 41% in the wavelength ranging between 450 and 510 nm and is comparatively superior to TiON nanotubes arrays (Figure 6c).

Table 1. Review on different kinds of (Oxy)nitrides based photoelectrodes with co-catalysts and their photoelectrochemical features for water splitting reactions.

Electrodes	Cocatalysts	Synthesis Method	Photocurrent(j)	Efficiency IPCE (@1.23 V vs. RHE)	Stability	Reference
LaTiOxNy	IrO2	magnetron sputtering	70 µA cm−2 at +1.0 V vs. Ag/AgCl	-	30 min	[90]
LaTiO2N	In2O3	Electrophoretic deposition (EPD)	0.61 mA cm−2 at 1.23 VRHE,	3.2% at 400 nm	30 min	[135]
SrNbO2N	IrO2	Polymer Complex(PC)/EPD	0.35 mA cm−2 at +1.2 V vs. RHE	0.2%	2h	[118]
TaON	CoOx/TiO2	screen-printing	0.7 mA cm−2 at +1.2 V vs. RHE	27% at with λ = 410 nm	15 h	[136]
BaTaO2N	CoOx/RhOx	EPD	-	10% at 600 nm	60 min	[137]
BaNbO2N	FeOx/Co(OH)x	particle transfer (PT)	5.2 mA cm−2 at 1.23 VRHE,	38% at with 460 nm	« 30 min	[138]
SrNbO2N	CoOx	magnetron sputtering	4.1 mA cm−2 at +1.2 V vs. RHE	10% λ = 400 nm	--	[139]
BaTaO2N	Co	PT	4.2 mA cm−2 at 1.2 VRHE	24% with λ = 400 nm	6 h	[140]
SrNbO2N	CoPi	flux treatment/PT method	1.5 mA cm−2 at 1.23 VRHE	-	2 h	[141]
BaNbO2N	-	Flux treatment/PT	5 mA cm−2 at 1.23 VRHE	-	-	[142]
CaNbO2N	Co-Pi/Al2O3	solid state/PC	70 μA cm−2 at 1.23 VRHE	-	30 min	[109]
CaTaO2N-A	NiB	EPD	40 μA cm−2 at 1.23 V versus RHE	-	-	[143]
SrNbO2N-NRs	-	PT	1.3 mA cm−2 at 1.23 VRHE,	-	15 min	[144]
SrNbO2N	Co3O4	EPD	2.0 mA cm−2 at 1.23 VRHE	12% with 420 nm	30 min	[145]

summarizes studies on different (Oxy)nitride-based electrode materials for solar light-driven water splitting reactions. The electrodes presented in Table 1 and the co-catalysts’ deposition significantly improved the PER performance with higher durability for solar-water splitting. In recent years, (Oxy)nitrides-based electrode materials have gained much consideration. They must be investigated in numerous ways, namely morphological features, co-doping strategies, defect induced features, surface activation, co-catalyst loading, etc. These examinations on the (Oxy)nitride photoelectrode materials basically lead to a more detailed understanding of their effective usage in the water-splitting system.

3. Oxysulfide Based Photocatalyst

Oxysulfides are new kinds of fascinating photocatalytic materials in heterogeneous based semiconductors photocatalysts. Similar to (Oxy)nitrides, the main idea of combining sulfur into oxides is basically the same as that for N incorporation. Particularly, the oxysulfides recognized as stable photocatalyst form than its corresponding metal oxides. Its energy level positions are appropriate for water photoelectrolysis reactions. In particular, oxysulfides comprise S as a constituent element that forms the tops of the VB. Thus, the sulfur loading usually reduces the bandgap compared to its counterparts of the corresponding oxide by developing shallower filled energy states than O 2p states. Thereby, photoinduced holes can travel smoothly in the VB of the material, predominantly beneficial for water oxidation concerning the 4-electron transfer. Till now, overall water splitting through an independent oxysulfide is yet to be developed. In contrast, few oxysulfides based photocatalysts are applied to PEC water electrolysis reaction with bias assistance, tandem style, or Z-scheme, water splitting.

Ishikawa et al. reported the stable oxysulfide of Sm₂Ti₂S₂O₅ with a wide absorption band in the visible-light region (λ < 600 nm) and band edge potential are more appropriate for overall water splitting [146]. These oxysulfide photocatalysts can be attained by annealing the mixture of equivalent precursors under vacuum and subsequently processed for sulfurization through H₂S gas [147]. Density function theory calculations further evidence that the S_3p atomic orbitals constitute the upper region of the VB of Sm₂Ti₂S₂O₅ and provide a significant influence in lowering the bandgap energy to its counterpart of metal oxide (Sm₂Ti₂O₇). Further, metal sulfides such as CdS are not appropriate to engage as a catalytic material for water oxidation due to the integral unstable nature of these electrode materials in the reaction through the photoinduced generation of holes [148,149]. In contrast, Sm₂Ti₂S₂O₅ performed as a relatively stable photoelectrode material for water electrolysis reaction. Similar to (Oxy)nitride, the photooxidation performance of the oxysulfides will be improved by incorporating the colloidal IrO₂ over Sm₂Ti₂S₂O₅, thereby increasing the quantum efficiency for O₂ generation to 1.1% [150,151]. Moreover, physical characterization evidence that the decomposition of oxysulfide in bulk and at the surface, in precise the oxidation of S²⁻ species, does not happen throughout the photooxidation of water. Quite recently, Ma et al. reported the Sm₂Ti₂S₂O₅ via novel flux method and demonstrated its photocatalytic activity is promoted by incorporating IrO₂ co-catalyst and Pt over oxysulfides. Further, Z-scheme visible-light assisted water splitting into H₂, and O₂ was evidenced by applying the oxysulfides photocatalyst for H₂ production combined with an H-CS-WO₃ photocatalyst (shown in Figure 7) [152]. These results conclude that the oxysulfide Sm₂Ti₂S₂O₅ behaves as a highly stable photoelectrode material for water redox reactions under visible-light illumination. Similarly, other kinds of Ln₂Ti₂S₂O₅ (Ln: Pr, Nd, Gd, Tb, Dy, Ho, and Er) oxysulfides with similar layered perovskite structure are recognized as active and stable electrode materials for water photoelectrolysis reactions with supporting electrolyte comprising a sacrificial electron donor or acceptor [153]. Likewise, Zhang et al. developed that Ln₂Ti₂S₂O₅ compounds (absorption edge at ca. 600 nm) evidenced photocatalytic behavior for water electrolysis reaction with appropriate support sacrificial reagents [154]. Moreover, this is the first Ln₂Ti₂S₂O₅ oxysulfide to behave as a photocatalyst for the water oxidation reaction. Similarly, Ogisu and co-workers developed the visible-light responsive La₅In₃S₉O₃ and La₃GaS₅O₁₄ oxysulfide photocatalysts and investigated water electrolysis reaction with a proper hole or electron scavenger(s) under irradiation conditions [155,156,157].

These oxysulfides based electrodes reveal remarkable PEC behaviors. Usually, p-type semiconductors behave as photocathodes, and generate H₂ at the boundary between the semiconductor and the electrolyte. In particular, most of the sulfides are naturally an n-type semiconductor, while Cu⁺ -containing sulfides tend to behave as p-type semiconductors. In this regard, Liu et al. reported the Cu⁺-containing oxysulfide La₅Ti₂CuS₅O₇ photocathode via the particle transfer method. In contrast, doped oxysulfides show an 8-fold enhancement compared to their counterparts for PEC water splitting reaction [157]. Likewise, Ma et al. reported the La₅Ti₂CuS₅O₇ oxysulfides enabling to act both as photocathodes and photoanodes by tuning its work functions and employing them as electrodes in the water-splitting reaction [158]. Similarly, Hisatomi et al. reported the new kinds of oxysulfides La₅Ti₂Cu_1−xAg_xS₅O₇ photocathodes, functioning at the operative potential for PEC H₂ evolution under solar-light irradiance [159].

4. Other Heterogeneous Photocatalysts

As an alternative to oxides, Yoshimizu attempted new kinds of silicide based photocatalyst (Iron disilicide β-FeSi₂) composed of abundant, durable, and non-toxic elements [160]. In particular, the author investigated the β-FeSi₂ catalyst material for H₂ evolution catalysts because it possesses appropriate conduction band energy levels and succeeded an obvious quantum efficiency of 24% upto 950 nm support of a sacrificial agent. Similarly, Akiyama et al. succeeded in fabricating the semiconducting β-FeSi₂/Si composite through the vapor-liquid-solid method. They investigated the photocatalytic H₂ behavior with the support of the formaldehyde as a sacrificial agent [161]. As an H₂ evolution photocatalyst, β-FeSi₂ is projected to be sensitive to near-infrared light (>1300 nm), which is the longest wavelength of light to be used. Recently, Akiyama et al. fabricated the β-FeSi₂/SiC composite powder and evolved H₂ under the visible light photons with methyl alcohol as a sacrificial reagent [162].

Similar to heterogeneous photocatalysts, metal-free photocatalysts are favorable photocatalytic materials that have not been widely investigated. On this phenomenon, Wang et al. demonstrated a new class of metal-free graphitic C₃N₄ (g-C₃N₄) based material and employed as photocatalysts for water splitting reactions under visible-light photons [163,164]. In this regard, few kinds of metal-free photocatalytic materials are presently available: g-C₃N₄, C₃N₃S₃ [165], a-sulfur [166], red phosphorus [167], black phosphorus [168], and organic ones as below. However, all these metal-free photocatalysts require scarce/expensive noble metal co-catalysts to be active for PEC water splitting reactions [169]. Fortunately, boron carbide and hexagonal boron carbon nitride (h-BCN) are fascinating new kinds of proficient metal-free and light-active photocatalysts for water splitting reaction. Liu et al. developed the boron carbides as photocatalytic materials for H₂ production. They achieved H₂ production of 14.5 µmol h⁻¹ g⁻¹ without any co-catalysts [170].

5. Organic Photocatalyst (Organophotocatalyst)

Generally, organic materials have run after inorganic ones that exhibit high electromagnetic properties due to high electron density and various elements such as conductions, magnetism, optics, etc. Organophotocatalysis based system exhibits an innovative technique for water splitting reactions exploiting energy over the whole range of visible light.

Primary reports designated photocathodes made from conducting polymers, namely polyacetylene [171,172,173], polypyrrole [174], polyaniline [175,176], poly(3-methylthiophene) [177] or poly(3-hexylthiophene), P3HT [178,179], phthalocyanine [180,181,182,183,184]. Organic photocatalyst without electrode also has a long history using poly(p-phenylene) [185,186,187] etc. Conversely, these photocatalytic materials did not comprise any junction between OSCs and charge separation from the Schottky junction between p-type OSC and electrolyte and kinetically low efficiency.

The closest research field of organic photovoltaic (OPV) devices to organic photocatalyst becomes more than 14% power conversion efficiency from the sunlight [188], and organic photocatalysts would have promising high efficiency in the near future if the stability problem is solved. Besides general merits in organic materials such as fabrication process varieties, flexibility, low density, and solubility, there are great merits of huge variety to control electronic properties in light absorption with a pi-system, conducting polymers nanostructures, e.g., synthetic metals or phase separations of bulk heterojunction [189,190,191,192,193,194,195]. As shown, the first p-n junction OPV [196] exhibited unidirectional electron-hole transfer path at both interfaces and the potential difference is derived by the photo illumination as visible-light-induced charge creation is succeeded at the boundary between the two OSCs as shown in Figure 8. Upon absorption of visible-light photons in one of the OSCs, an exciton is created and then transported to the junction with another OSC. At this point, the exciton dissociates, resultant in the arrival of a hole in one OSC called the donor, and an electron in the other OSC called the acceptor. In photovoltaic systems, charge carriers then transport to the electrodes where charges have been collected. These concepts are useful to design an efficient PEC system. By choosing a bilayer system, photoinduced charge separation becomes efficient in the case of PEC also [52]. Furthermore, various modifications gave more efficiency, such as p-n interface [197] and liquid/electrolyte interface [198]. Low cost and water-based synthesis of p-n junction like nanoparticles are possible to mass productively [46,47,48]. By combining polymer substrate, macroscopic separation of redox site is possible [49], and multilayerization of the photocatalyst film [47] exhibit more efficient utilization of natural light. The utilization of longer wavelength reaching to near-infrared is also attractive to utilize the sunlight [48,199]. Very recently, lateral charge separation to the substrate direction was discovered for step shape electrode using Kelvin probe force microscopy [200]. It is noteworthy that phthalocyanine has another role as a catalyst. One of the evidential phenomena is P3HT/PCBM photoanode [201,202], where coating with co-catalyst phthalocyanine enhanced photoanodic current. A recent observation by Kelvin force probe microscopy reveals a dipole formation between the co-catalyst and bulk heterojunction (BHJ), suggesting efficient charge separation for multi-electron transfer [202].

The main concern is the arrangement of the two different things since excitons can only diffuse in the range of few tens of nm for the duration of their lifetime, i.e., beforehand spontaneous recombination happened. As a result, the two OSCs must be either loaded as two very thin layers, probably with an interlayer comprising both constituents, resulting in a mixture of the two materials creating molecular p-n junctions all over the bulk layer BHJ. More importantly, BHJ is made between a polymer or an array of small molecules, performing as the light absorber, and a C₆₀ derivative [195] typically. In order to employ such organic junctions in direct PEC water splitting, there are similarities in the continuous processes as above, while dissimilar conditions must be seen at the interface between the organic layer with a liquid electrolyte.

OER has a more crucial issue that strongly relates to stability and efficiency problems due to multielectron oxidation and co-existing thermodynamically easy path of photoanode etching. However, OER in an aqueous medium was validated with organic polymer catalysts/electrolyte solution junctions, obtaining photocurrent densities in the range of a few microamperes [203,204,205], or more bias potential [206]. Saline oxidation is one of the smart systems as alternative oxidation of chloride ions [207,208]. Coating of the metal oxide layer is another choice for long term stability [209,210,211,212,213,214].

Pt nanoparticles are good HER catalyst and were incorporated over the uppermost of an organic p/n bilayer electrode of small moieties (metal-free phthalocyanine, H₂Pc/C₆₀), photocurrent values grasped several cents of µA.cm⁻² and the faradic efficiency for H₂ evolution in aqueous media reached nearly 90% (Figure 9) [45]. H₂ generation at Pt-attached C₆₀/water interface utilizes electrons from the CB of the C₆₀ layer. While the lowest of the CB of C₆₀ is less negative than H₂ evolution, the production of 2-electron reduced C₆₀²⁻ species was demonstrated spectroscopically after a 20-min induction period.

The organic electrode presented in Figure 10 comprises photoelectrodes based on OSC to oxidize thiol [215], based on TiO₂ to oxidize water [216], hydrazine oxidation [217] or borane-ammonia [218]. By employing photoelectrodes, H₂ can be produced without additional electric bias. In this case, concomitant H₂ evolution takes place at the Pt electrode without any input. Our research group recently investigated the photocatalysis system comprising an organic p/n bilayer for the stoichiometric degradation of hydrazine into N₂ and H₂ [218]. In this regard, these works illustrate the H₂ generation by reducing power photoinduced at the PTCBI in an organic p/n bilayer.

Our research group recently reported the PEC system containing an organic-based photocathode of a p/n bilayer (ITO/ZnPc/C₆₀–Pt) alongside a WO₃ photoanode, where a stoichiometric generation of H₂ and O₂ was initiate to happen (Figure 11) [220]. In this system, oxidation and reduction powers can be markedly produced at the tungsten oxide photoanode and organo-photocathode, correspondingly. In this context, electron transport from tungsten oxide CB to the hole continued p-type VB in the organic p/n bilayer can proficiently happen for executing the PEC method, accordingly effecting a high concentration of carriers, which is readily existent for rate-limiting O₂ evolution at photoanodes for effective charge separation. Finally, combining the two different water splitting materials is an efficient approach to developing a Z-scheme photocatalytic system.

The recent trend to use OSM is to be covered with a thin inorganic layer, for example, amorphous molybdenum sulfide (a-MoS_x) as HER catalyst as an alternative of Pt [221,222]. Photocathodes fabricated via spray deposition of a combination of a-MoS_x and TiO₂ on the BHJ layer obtained a photocurrent density of 200 µA·cm⁻² at 0 V_RHE and up to 300 µA·cm⁻² at –0.4 V_RHE. Another inorganic layer is TiO_x layer on P₃HT:PCBM BHJ layer and the catalytic film [223]. Moreover, metallic Al/Ti interfacial layers resulting in an enhancement in the photocurrent nearly about the 8 mA·cm⁻² at 0 V vs. RHE. A 50-nm-thick C₆₀ layer also functions as an interfacial layer, with a current density attaining 1 mA·cm⁻² at 0 V vs. RHE [224]. Also, reduced graphene oxide, WO₃ and NiO, CuI, polyaniline underlayers are effective for persistent PEC features in acidic aqueous electrolytes [225,226,227,228,229,230]. The use of α-sexithiophene (α-6T) and chloroboron subphthalocyanine (SubPc) combined with C₆₀ as the electron extracting layer gave more H₂ evolution [231]. MoS₃ coating gave 3 mW/cm² at 0 V vs. RHE [225]. Tandem cells are known to obtain high Open circuit voltage (Voc) in a solar cell. For water splitting, such high Voc is necessary, at least 1.23 V theoretically. Applications by the use of high Voc with the tandem structure are reported [231,232,233,234,235,236,237,238]. There are several review papers about using advanced organic and carbon-based materials [239,240,241,242,243,244,245,246,247,248], while the present paper focused only on the heterogeneous organic semiconductor photocatalysts.

Among these heterogeneous photocatalysts, the conjugated microporous polymers (CMPs) have been recognized as photocathodes for H₂ evolution from water to support a sacrificial electron donor. Sprick et al. also investigated various kinds of CMPs by varying monomer linker length and were investigated for photocatalytic H₂ evolution reactions [248]. Like CMPs, covalent organic frameworks (COFs) are also an emergent class of crystalline polymers comprised of organic units connected via covalent bonds to produce porous networks. Very recently, Sick et al. reported first-time COFs as a new kinds of porous electrode materials of BDT-ETTA (benzo[1,2-b:4,5-b′]-dithiophene-2,6-dicarboxaldehyde (BDT); 1,1′,2,2′-tetra-p-aminophenylethylene, ETTA) for water-splitting reaction [249]. Further, these materials satisfy the necessities for light-driven water reduction application, which comprise effective light-harvesting, appropriate energy level positions, and stability.

6. Conclusions and Future Outlook

Energy harvesting technology from the limitless solar irradiation source has been recognized as a sustainable solution for the global energy crisis. In this review, the visible-light-driven water-splitting system has significantly improved since H₂ is a water-splitting reaction expected to support future societies. In this context, a series of heterogenous photocatalyst materials have been examined for PEC water splitting reaction. Special consideration has been paid to organophotocatalysts based electrodes for the applications of solar-driven water splitting reactions. (Oxy)nitrides, oxysulfides, and organophotocatalysts have the potential to acts as stable photoelectrodes for water redox reaction under visible-light illumination, have been shown to PEC behavior without noticeable degradation. In some cases, heterogenous semiconductor photocatalyst systems need to feature organophotocatalysts along with a simple conventional oxide and were evidenced to be effective for the stoichiometric decomposition of water into H₂ and O₂. Over the past few decades, the available photocatalysts for visible-light-driven water splitting has mostly comprised of metal oxides. The development of heterogenous photocatalysts attempting to attain the same role has probably given a boost to research on photocatalytic material for solar energy conversion.

In summary, over the past decades, (oxy)nitride and oxysulfides based photoelectrodes have been faced a high-speed advancement in terms of both the stable electrochemical nature as well as the activity of PEC water oxidation. Understanding the mechanism assembled by investigating this kind of photoanodes will cover a solid path for advancing other types of proficient photoanodes. In all these systems, the photoelectrode materials, which cannot merely contribute to the complete decomposition of water, can play active parts. The usage of these novel kinds of heterogeneous photocatalyst based materials and suitable electrocatalyst materials have more advantages in gathering solar energy. PEC water decomposition is already recognized as the most promising methodologies for obtaining solar hydrogen. In this perspective, developing new kinds of heterogeneous photocatalyst will be an efficient strategy for creating a commercially viable water-splitting system.

During the past few years, considerable research efforts have applied photoelectrochemistry for other reactions besides water splitting. In this context, the conversion of CO₂ into value-added organic molecules has gained enormous interest in recent years. Carbon dioxide reduction reaction (CO₂RR) is far more complicated than water splitting. Still, it permits the opportunity of extracting the greenhouse gas CO₂ from the atmosphere and turning it into value-added fuels. Commercially, products such as methane or ethanol generation via CO₂RR have been explored [250,251]. Conversely, the major limits for CO₂RR are finding suitable photoelectrodes and robust semiconductors to be kept in direct contact with the reaction, as the higher required reductive potentials (>1 V) improve materials corrosion [252,253]. Despite the described potentialities for the usage of photoelectrochemistry in H₂ production and CO₂RR, it has not been possible to demonstrate an application capable of competing economically with conventional procedures that are frequently used e.g., methane reforming permits obtaining H₂ at € 3/kg. In this regard, to be capable of developing alternate methods built on the usage of renewable energies such as photoelectrocatalysis, it is essential to promote efficient and robust devices created using cost-effective most-abundant electrode materials (like heterogeneous photocatalyst), demanding optimization approaches like the tandem arrangement of photoelectrodes to relax the stringent conditions required for effective PEC value-added solar fuel production. Indeed, PEC water splitting development will help to identify and adapt photoelectrodes for other electrochemical reactions.