Two-Dimensional Materials and Composites as Potential Water Splitting Photocatalysts: A Review

Hydrogen production via water dissociation under exposure to sunlight has emanated as an environmentally friendly, highly productive and expedient process to overcome the energy production and consumption gap, while evading the challenges of fossil fuel depletion and ecological contamination. Various classes of materials are being explored as viable photocatalysts to achieve this purpose, among which, the two-dimensional materials have emerged as prominent candidates, having the intrinsic advantages of visible light sensitivity; structural and chemical tuneability; extensively exposed surface area; and flexibility to form composites and heterostructures. In an abridged manner, the common types of 2D photocatalysts, their position as potential contenders in photocatalytic processes, their derivatives and their modifications are described herein, as it all applies to achieving the coveted chemical and physical properties by fine-tuning the synthesis techniques, precursor ingredients and nano-structural alterations.


Introduction
Today, energy is one of the utmost critical environmental, economic and political issues due to the contributing factors of continuously increasing global population, escalating urbanization and growing energy consumption. These dynamics are compelling researchers and front-runners in political, social, environmental and industrial fields towards a shift from conventional energy production methods towards more innovative, sustainable, efficient, safe and environmentally benign technologies and materials. The total energy consumption of the entire globe is 18.5 TW per year currently, which is expected to rise to 40.8 TW by 2050 [1]. Approximately 85% of total energy usage comes from fossil fuel. It is estimated that there are sufficient fossil fuel resources to meet global energy demands for several centuries [2]. However, irreversible climate change due to generation of greenhouse gases, depletion of fossil fuel reserves, geopolitical conflicts and other environmental concerns are continuous threats for secure and sustainable energy supply. Therefore, the world needs to devise advanced, sustainable methods and sources competitive to fossil fuels in terms of efficiency and economics while ensuring minimal environmental damage [3]. The sun is an everlasting source of inexhaustible energy with which to produce hydrogen, which proves to be potential candidate to meet secure energy demands due to environmental friendliness, high energy value and the possibility of onsite production [4]. The amount of solar energy reaching earth is around 9.5 × 10 16 J/s; that is 10 4 times the total usage by entire human population, endorsing its candidature for potential sustainable energy sources [5]. The major research concern for harnessing solar energy is the production of solar fuel. Conventional technologies of solar energy production suffer from many downsides, such as the limited storage capacity for the energy, the intermittent availability in certain areas and the economic feasibility of the process. The steam reforming process is traditionally used for hydrogen production from fossil fuels, which is not considered as sustainable in the long haul due to massive carbon dioxide emission and depleting resource trends. Catalytic steam reforming using fossil fuels is virtually the only industrial process responsible for producing the entire hydrogen supply of the world today, with electrolysis contributing only a meager 4% via water electrolysis. There is a growing interest in generating hydrogen from sustainable and renewable resources using environmentally friendly methods to deliver inexpensive and clean energy [6]. Photocatalytic water splitting activity is one of the key research areas for onsite hydrogen production and storage [7][8][9][10][11][12][13][14][15][16]. It is one of the emerging process for harnessing solar energy into fuel in the form of hydrogen without any use of fossil fuels or emission of carbon dioxide. There is a growing interest in the field of semiconductor photocatalysis [17] which could be used for water dissociation into constituent oxygen and hydrogen gases using solar energy, which is abundantly available all over the world. Solar energy conversion in the form of hydrogen via photocatalytic water splitting has become an attractive subject [18].
The prerequisites to an energy efficient, cost effective and operationally feasible photocatalyst are the ability to effectively harvest and utilize photons in the visible light region to generate electron-hole pairs; lower band gap energies; unimpeded mobility of charge carriers; a low recombination rate of electron-hole pairs; and a greater surface area, resulting in a higher number of exposed active sites. The earliest discoveries fulfilling these requirements comprised granular semi-conducting materials and their derivatives, such as ZnO and TiO 2 [19]. But these conventional photocatalysts have several shortcomings, such as: (1) Their higher band gap energy, making the materials suitable for mainly UV light absorption instead of visible light [20]; (2) Only having the ability to catalyze either water oxidation or reduction at a time, leading to their unsuitability to act as dual function overall water splitting catalysts [21]; (3) Their higher charge recombination rates in bulk medium and on the surface, resulting in lower activity [22]; (4) Inaccessibility of active sites lying in bulk material [23].
Further research and development in this regard led to the conclusion that the nano-materials are better photocatalysts because of their high surface area property [24,25]. Among nanomaterials, 2D materials are promising photocatalyst alternatives due to their sheet-like structures; electrical, thermal and mechanical properties; the operational advantages of being easily handleable; having the ability to form integrated composites with highly photoactive co-catalysts; and having higher surface to volume ratios. The bi-layer and mono-layer 2D nanocrystals and nanosheets provide the basis for highly exposed active sites and faster charge transfer schemes [26], while the doped precious and photo-active metals and their related compounds offer the added compensation of higher light absorption capacity and lower band gap energy [27,28].

Photocatalysis: Theoretical Digest
Hydrogen production by photocatalytic water splitting has proven to be a superlative solution for resource and economics related problems in novel hydrogen production methods. The process is centered on a semiconducting catalyst material and sunlight to convert the light energy to chemical energy, also termed "artificial photosynthesis." The photons of light having higher energy than the energy gap of semiconducting material, when irradiated on the photocatalyst material, results in photo generated charge carriers usually termed the electron hole pair. This generation of electron-hole pairs in the conduction band and valence band acts as the foundation for redox reactions, as indicated below [29,30]: Overall: The oxidation and reduction half reactions at anode and cathode, respectively, under acidic (low pH) solutions proceed as: Oxidation: Under basic (high pH) conditions, the chemical reactions are represented as: Oxidation: Reduction: The holes generated in the valence band resultantly are responsible for oxidation of adsorbed species (water in this case) releasing oxygen, and the agitated electrons from the conduction band are the reducing agents to produce hydrogen from in-contact water molecules as shown in  The holes generated in the valence band resultantly are responsible for oxidation of adsorbed species (water in this case) releasing oxygen, and the agitated electrons from the conduction band are the reducing agents to produce hydrogen from in-contact water molecules as shown in Figure 1.  [23].) The production of charge carriers, such as electron and hole ones, is a fundamental step for all photocatalytic processes; other crucial factors include the width of material band gap and the energy levels of both conduction and valence bands of the material. For a feasible reaction to occur, the conduction band of the electron acceptor species should be more positive than the conductive band of the semiconductor photocatalyst, whereas the valence band of the electron donor type must be more negative than the semiconductor's valence band, and the minimum band gap must be greater The production of charge carriers, such as electron and hole ones, is a fundamental step for all photocatalytic processes; other crucial factors include the width of material band gap and the energy levels of both conduction and valence bands of the material. For a feasible reaction to occur, the conduction band of the electron acceptor species should be more positive than the conductive band of the semiconductor photocatalyst, whereas the valence band of the electron donor type must be more negative than the semiconductor's valence band, and the minimum band gap must be greater than 1.23 eV [31]. As a result of the interface electron transfer process, the redox reaction takes place and the photolysis of water molecules occur.

1.
To catalyze the splitting of water at the interface of electrolyte and electrode, the charge carriers are required to be transferred to catalyst surface once the electron-hole pairs are created. The quick Catalysts 2020, 10, 464 4 of 35 recombination of photo-generated electron-hole pairs ( Figure 2) releasing heat or photon energy before they can catalyze the redox reactions is a major challenge in this step, requiring a high degree of crystallinity [32].

2.
The predilection for semiconductor materials to work under the ultraviolet (UV) light is another major challenge, as only about 4% of solar energy is comprised of UV light. It is advantageous for photocatalysts to work under visible light, which requires the band gap to be in visible range.

3.
Photo-corrosion and catalyst decay are also among the limitations of the photocatalytic splitting of water. TiO 2 , ZrO 2 , KTaO 3 , SrTiO 3 and BiVO 4 are among the notable contenders for photocatalytic water splitting because of having band gaps around 1.23 eV. Typical sulfide-based photocatalysts, such as cadmium sulfide (CdS), have a tendency to undergo decay under operating conditions due to oxidation of sulfide into elemental sulfur at the same potentials that are used for water splitting, requiring the use of certain sacrificial reagents, e.g., sodium sulfide, to control any sulfur lost [33]. (Reprinted with permission from [34]. Copyright (2019), Elsevier).

Two Dimensional Materials as Photocatalysts for Water Splitting
A good photocatalyst must possess the characteristics of photo-activity, thermal stability, photo-stability, chemical inertness, low cost, non-toxicity and post-reaction recoverability [35]. To tackle the environmental pollution factor, metal-free catalysts are always the first priority for synthesize with Catalysts 2020, 10, 464 5 of 35 maximum efficiency. The first and second generation of catalysts include combinations of inorganic materials [36] and various oxides and sulfides of metals, but the main problem of these materials is the toxic and corrosive nature [33,37]. Among various other photocatalyst materials available, 2D materials are promising candidates for the water splitting applications, because of merits, such as: (1) The adjustable number of layers, so that the band gap and photo-absorption can be tuned [38].
(2) The sheet-like ultra-thin structure facilitating the charge transport towards surface, resulting in lower recombination rate [39]. (3) The enhanced specific surface area leading to greater exposure of active sites on surface and better catalytic performance [40].
The fundamental strategies to synthesizing 2D nanosheets comprise top-down and bottom-up methods, which vary on the basis of starting precursors and treatment methods also shown in Figure 3.

Two Dimensional Materials as Photocatalysts for Water Splitting
A good photocatalyst must possess the characteristics of photo-activity, thermal stability, photostability, chemical inertness, low cost, non-toxicity and post-reaction recoverability [35]. To tackle the environmental pollution factor, metal-free catalysts are always the first priority for synthesize with maximum efficiency. The first and second generation of catalysts include combinations of inorganic materials [36] and various oxides and sulfides of metals, but the main problem of these materials is the toxic and corrosive nature [33,37]. Among various other photocatalyst materials available, 2D materials are promising candidates for the water splitting applications, because of merits, such as: (1) The adjustable number of layers, so that the band gap and photo-absorption can be tuned [38].
(2) The sheet-like ultra-thin structure facilitating the charge transport towards surface, resulting in lower recombination rate [39]. (3) The enhanced specific surface area leading to greater exposure of active sites on surface and better catalytic performance [40].
The fundamental strategies to synthesizing 2D nanosheets comprise top-down and bottom-up methods, which vary on the basis of starting precursors and treatment methods also shown in Figure  3. Till now, various 2D materials have been discovered by researchers, such as graphene, graphitic carbon nitride, black phosphorus/phosphorene and hexagonal boron nitride. All these materials have proven potential for water splitting activity by photocatalysis [42]. 2D materials show unusual structural, electronic and optical properties [43]. However, some limitations have been observed in terms of reaction completion due to the recombination rate of the electrons and holes which carry out reactions. In the search for alternatives to metal-based systems, metal-free elementals and compound photocatalysts have recently been developed. Red-P, alpha-sulfur and boron are notable examples reported in the literature [44]. However, these elemental photocatalysts exhibit low photocatalytic activity. There are various techniques that researchers employ to enhance activity majorly, including doping, sensitization, tuning morphology and making heterostructures with other materials [45]. The search for a non-toxic, inexpensive, corrosion resistant photocatalyst with a focus on the design and efficiency of photocatalyst, while assessing the activity for economical and scalable hydrogen generation, is ongoing. Table 1 summarizes the discussed literature in a comprehensive manner. The main focus of this paper is to go over 2D materials, their modifications and the different techniques that have been employed to enhance their photocatalytic water splitting performances. Said materials include, but are not limited to, graphitic carbon nitride, graphene, black phosphorus, metal Till now, various 2D materials have been discovered by researchers, such as graphene, graphitic carbon nitride, black phosphorus/phosphorene and hexagonal boron nitride. All these materials have proven potential for water splitting activity by photocatalysis [42]. 2D materials show unusual structural, electronic and optical properties [43]. However, some limitations have been observed in terms of reaction completion due to the recombination rate of the electrons and holes which carry out reactions. In the search for alternatives to metal-based systems, metal-free elementals and compound photocatalysts have recently been developed. Red-P, alpha-sulfur and boron are notable examples reported in the literature [44]. However, these elemental photocatalysts exhibit low photocatalytic activity. There are various techniques that researchers employ to enhance activity majorly, including doping, sensitization, tuning morphology and making heterostructures with other materials [45]. The search for a non-toxic, inexpensive, corrosion resistant photocatalyst with a focus on the design and efficiency of photocatalyst, while assessing the activity for economical and scalable hydrogen generation, is ongoing. Table 1 summarizes the discussed literature in a comprehensive manner. The main focus of this paper is to go over 2D materials, their modifications and the different techniques that have been employed to enhance their photocatalytic water splitting performances. Said materials include, but are not limited to, graphitic carbon nitride, graphene, black phosphorus, metal phosphides and metal organic frameworks. Figure 4 presents the bandgap energies of all potential 2-D materials. phosphides and metal organic frameworks. Figure 4 presents the bandgap energies of all potential 2-D materials.

Graphitic Carbon Nitride and Derivatives
Graphitic carbon nitride is metal-free polymeric catalyst comprised of repeating triazine units. It is proving to be a highly efficient catalyst for photocatalytic activity with band gap of 2.56-2.70 eV. Due to highly efficient performance with an environmentally friendly nature, it is one of the most highly esteemed materials for water splitting. Due to some of its limitations, research is underway to make it more feasible and economical with improved activity. Different techniques, such as morphological alteration by changing temperature, modifying g-C 3 N 4 with some metal and doping, for making heterostructures, have been investigated [79][80][81]. These modifications are targeted toward achieving a highly efficient catalyst by lowering the recombination rate of electrons and holes, improving value and improving environmental friendliness as much as possible [82]. 2D nanomaterials are being fabricated by facile and reliable methods: top-down ones include thermal exfoliation, chemical exfoliation, mechanical cleavage and selective etching; the bottom up approaches include chemical vapor deposition and wet synthesis methods. The most employed technique for the fabrication of 2D g-C 3 N 4 is thermal treatment [83]. This facile method is easy to scale up for industrial level synthesis. However, in order to improve the photocatalytic performance, different techniques are employed and are under investigation to get maximum efficiency of a given material. Different temperature controlled synthesis techniques have been investigated to alter the morphology of the material [84]. Quan Gu investigated the morphological evolution of graphitic carbon nitride nanostructures and also evaluated their photocatalytic activities under visible light. The materials were synthesized at different temperatures, and showed different morphologies, optical properties, energy band structures and photocatalytic activity for water splitting. By changing the temperature, different structures, such as nanosheets, nanotubes and nanoparticles, were observed. BET surface area increased dramatically from 4.5 m 2 /g of bulk g-C 3 N 4 to 210.1 m 2 /g of modified sample obtained at 540 • C. The exfoliated structures showed better activity for water splitting; i.e., 297 and 120 µmol/g·h for HER and OER respectively [85]. Zhi -An Lan modified g-C 3 N 4 with bromine to investigate the change in properties. It was observed to get enhanced optical, conductive and photocatalytic properties of g-C 3 N 4 . The optimal catalyst CNU-Br 0.1 demonstrated more than two times higher H 2 evolution activity than pure CNU [86]. Junqing Yan fabricated g-C 3 N 4 /TiO 2 heterojunction by the hydrothermal method to get good production and separation of charge carriers. The fabricated TiO 2 /g-C 3 N 4 samples are active in the hydrogen evolution reaction of water splitting with the best performance of 700 µmol/g·h and better photo-stability [87,88]. Recently, Yuhao Yang proposed a 2D/2D heterojunction strategy in order to compensate for low efficiency of charge separation in ultra-thin g-C 3 N 4 2D sheets. The high surface area interface of g-C 3 N 4 nanosheets and 2D TiO 2 , coupled with favorable band structure, reliability of operation and stability of TiO 2 nanosheets resulted in high H 2 evolution rates and a robust, recyclable photocatalyst [89]. Guigang Zhang conducted some experimental studies of various redox cocatalysts, reporting g-C 3 N 4 modified with Pt, PtO x , and CoO x . The results proved that cocatalysts, especially Pt/g-C 3 N 4 , show potential for water splitting activity without the use of a sacrificial agent [88]. Yaping Zeng fabricated NiTiO 3 /g-C 3 N 4 heterostructured material by the sol gel method. It was observed that NiTiO 3 had a suitable band gap and unique photo response in visible light range. It showed three times the activity of NiTiO 3 /g-C 3 N 4 hybrid catalysts, as compared to pure g-C 3 N 4 and 3 wt%, the catalysts presented optimal activity. This is ascribed to the strong light absorption in the visible region and the prolonged recombination rate of electrons and holes [90].
Xiaojie She aimed at the template free synthesis of porous ultrathin nonmetal doped g-C 3 N 4 nanosheets for increased H 2 production. The formation of an ultrathin structure and the introduction of oxygen proved favorable for the enhancement of the photocatalytic performance. The average H 2 evolution rate of came out at ≈ 3786 µmol/g·h; that is five times that of bulk g-C 3 N 4 photocatalyst material. This increased activity was endorsed due to more adsorption and active sites, the enhanced redox ability and improved electron transportability due to the introduction of the electrophilic groups (C-O, C=O and COOH) [91]. Mohammad W. Kadia et al., in their work, claimed to increase the efficiency of water splitting using visible light, and also increased the charge separation using mesoporous g-C 3 N 4 which was coated over by WO 3 . They further claim that by coating a semiconductor surface with a metal-oxide, it can capture the surface electrons, and hence can increase the charge separation. The parameters affected the WO 3 /g-C 3 N 4 in charge separation ability, mesoporous structure gap between valence and conduction band high surface area [92]. Recently, a step-heterojunction scheme was demonstrated by Junwei, Fu. et al. [69], whereby 2D/2D WO 3 /g-C 3 N 4 photocatalyst was developed by ultrasonic exfoliation of WO 3 and dual-step thermal etching of g-C 3 N 4 in bulk forms. The resulting material showed 1.7 times higher H 2 production activity by barricading the charge recombination. This advancement is owed to the fact that the electronic transfer on interface of both materials from g-C 3 N 4 to WO 3 balanced the higher fermi level of g-C 3 N 4 and the lower fermi level of WO 3 . This mechanism creates a continuous electron transfer trend and simultaneously prevents the useful charge carriers (electrons from CB of g-C 3 N 4 and holes from VB of WO 3 ) from recombining. KNbO 3 has been researched extensively in terms of photocatalytic activity for water splitting, but it has low quantum efficiency, showing water splitting activity only under UV light irradiation. It has been used to enhance g-C 3 N 4 activity by making heterojunction. Dongbo, Xu. et al., in their research, presented the synthesis of g-C 3 N 4 /KNbO 3 heterojunction composites by the hydrothermal method and used them for water splitting and hydrogen production using sunlight. The g-C 3 N 4 and KNbO 3 had better performances for water splitting by more than two and 1.8 times that of their pure forms respectively [93].
Seza, A. et al. presented microwave assisted synthesis of g-C 3 N 4 /SnO 2 by simple pyrolysis of urea. It is the first ever synthesis of g-C 3 N 4 using microwave assistance, wherein urea is converted to g-C 3 N 4 nanorod, and then the uniform distribution of SnO 2 over the surface makes it a promising material for visible light photoactivity to be used for water splitting [94]. Jingran Xiao, et al., prepared g-C 3 N 4 on ZnO wires, and then incorporated nano clusters of Pt over the surface. Both the g-C 3 N 4 and ZnO act as photo anodes while Pt carries the charges generated; i.e., electrons generated. This showed great improvement in the photoactivity in sun light; the efficiency increased by 9.5 times that of ZnO pure and 4.5 times that of g-C 3 N 4 /ZnO. The recombination of electron holes further reduced by the newly formed anode with Pt over surface. Composites of ZnO and g-C 3 N 4 were prepared and nano-clusteres were decorated over the surface, which eventually increased the performance [95]. Feng Guoa et al., formed another heterojunction of g-C 3 N 4 with CoO; studies showed that this metal oxide decoration over the surface of graphite carbon nitrite increased the photo activities of the newly formed photocatalyst.
The formation of this hetero-conjunction catalyst was done through the "solvothermal" process. Among all of the combinations, 30 wt% CoO and g-C 3 N 4 had H 2 evolution rates of 50.2 µmol/g·h and 27.8 µmol/g·h for O 2 evolution [96]. Kelin He et al. fabricated a nano-heterojunction co-catalyst system based on the doping of hexagonal Ni 3 C nanoparticles produced by thermolysis at low temperature on the base g-C 3 N 4 nanosheets via simple grinding method. The catalyst comprising of over 15 wt% Ni 3 C content showed the hydrogen production rate of 1518 µmol/g·h when irradiated with visible light and an apparent quantum yield 116 fold greater than pure g-C 3 N 4 ; i.e., 0.4% at 420 nm successfully showing better results than noble metal 0.5 wt% Pt/g-C 3 N 4 sample. This relatively low cost, good performance, non-noble metal addition of co-catalyst enables a decrease in hydrogen production over-potential, improvement in oxidation kinetics and effective charge carrier separation [97]. Another Non-noble metal alternative with superior hydrogen evolution rate of 169 µmol/g·h has been reported by Lu Chen et al. The technique centered on the combination of Ni 3 N nanoparticles and g-C 3 N 4 nanosheets via a hydrothermal method and subsequent annealing step which ensures stabilization of Ni 3 N structures on g-C 3 N 4 and maintains the hydrogen productivity. The notable advantages of this material include the lower cost, higher throughput than 3 wt% Pt/g-C 3 N 4 material, multiple cycles operation, better carrier separation and visible light operation. These properties are attributed to the optimal loading and distribution of Ni 3 N active co-catalyst medium and better stability of Ni 3 N phase with a minimal conversion to less active Ni or Ni(OH) 2 states [98]. Based on the single layer fabrication and promising scalable strategy of single layer SnS 2 nanosheets of 3-atom thickness exhibiting above 38% water splitting efficacy under visible light by Yongfu Sun et al. Shao-hua Chen et al. further investigated the prospects of coupling SnS 2 nanosheets with g-C 3 N 4 to overcome the performance limitations of afore-mentioned research. The results were conformed with the intended improvements of better charge carrier separation and band alignment owing to the weak Vander Walls heterojunction between SnS 2 and g-C 3 N 4 [99]. Another approach using environmentally benign materials and methods was introduced by Rongchen Shen et al., in which the hybridization of Co 2 P nanoparticles formed from phosphorization technique of cobalt metal organic framework and g-C 3 N 4 nanosheets. The resulting g-C 3 N 4 /Co 2 P/K 2 HPO 4 material formed by simple grinding method exhibited a 556 times superior H 2 production rate compared to simple g-C 3 N 4 , i.e., 556 µmol/g·h, owing to the lower hydrogen production over potentials, better light absorbance, diminished charge carrier combination tendency and provision of an alternate reaction pathway by proton-reduction [100]. A recent approach to produce cobalt phosphide (CoP)/g-C 3 N 4 cocatalysts has been presented by Xiao-Jun Sun et al., involving dual-step calcination of ZIF-67. The calcination was carried out to formulate CoP nanoparticles, which were found to boost the hydrogen release rate to as high as 201.5 µmol/g·h with a meager NPs loading of 1.42 wt%. ZIF-67 was formulated by the traditional method and g-C 3 N 4 by melamine thermal condensation; they were subsequently combined, ground and calcined at a high temperature. The H 2 release rate exhibited a directly proportional relation with increasing ZIF-67 concentration until 5 wt% (1.42% CoP), indicating deterrence in ability to absorb light. Moreover, the lower water contact angle of the hybrid material than those of the individual components conforms to the assumption of better water absorption ability and reduction of adsorbed proton specie. These factors, along with the notably lower charge transport resistance indicated by photocurrent experiments, contribute to the exemplary alliance and compatibility of CoP and g-C 3 N 4 as photocatalysts [101]. Another methodology to incorporate graphite carbon nitride as a root material was verified by garnishing carbon based quantum dots on g-C 3 N 4 nano-tubes by thermal co-polymerization method. Yang Wang et al. reported the resultant hydrogen release rate as high as 3538.3 µmol/g·h due to reduced charge recombination and greater light capturing capabilities, demonstrating 10.94% quantum yield. The co-condensation of these components encourages tube arrangement and micro-sized composite structure to stimulate transfer of photoelectrons. Introduction of C-QDs notably shifted the pure g-C 3 N 4 nano-sheet structure towards nano-tube structure, because of the tough amide bonds created among C-QDs and urea precursor. Furthermore, the band gap of untreated base material was shifted from 2.70 to 2.60 eV due to the structural change and multi-reflective containment of incident light radiations in the nano-tubes [102]. Nan Wang et al., in their research, developed MnO 2 with a co-modified nanocarbon tube with a g-C 3 N 4 ternary composite. Different properties were studied, and the compound was characterized by different conventional techniques. It was observed that this ternary compound shows more active photocatalysis in contrast with carbon nanotubes (CNTs)/C 3 N 4 . Here, CNTs capture the electrons generated when light falls over the surface of this catalyst, which resultantly increases the rate of water splitting; hence, more hydrogen is formed. Hydrogen production is increased with rate of 4067 µmol/g·h [103]. Figure 5 shows the composite g-C 3 N 4 /GO with MoS2 as potential photogenerated carrier separator.

Graphene-Based Photocatalysts for Water Splitting
Another metal-free photocatalyst for photocatalytic water splitting activity is graphene. It has gotten immense attention recently. In 2004, demonstration of the "sticky-tape peeling" of atomically thin sheets of carbon atoms from a mass of graphite brought a revolution in 2D materials research. This thinnest-ever material is known as "graphene" [105]. It is characterized by excellent chemical, mechanical and electrical properties [106]. For example, graphene is 100 times harder than steel; superior to copper in terms of electrical and thermal conductivity; and is flexible and transparent. Proposed future applications include uses ranging from computer chips and flexible displays to batteries and fuel cells [107].
Graphene is proving to be a potential candidate for the production of hydrogen, due its low cost

Graphene-Based Photocatalysts for Water Splitting
Another metal-free photocatalyst for photocatalytic water splitting activity is graphene. It has gotten immense attention recently. In 2004, demonstration of the "sticky-tape peeling" of atomically thin sheets of carbon atoms from a mass of graphite brought a revolution in 2D materials research. This thinnest-ever material is known as "graphene" [105]. It is characterized by excellent chemical, mechanical and electrical properties [106]. For example, graphene is 100 times harder than steel; superior to copper in terms of electrical and thermal conductivity; and is flexible and transparent. Proposed future applications include uses ranging from computer chips and flexible displays to batteries and fuel cells [107].
Graphene is proving to be a potential candidate for the production of hydrogen, due its low cost and environmentally friendly nature, and shows unique charge carrier mobility properties, large specific surface area (single layer: 2650 m 2 /g) [108], high transparency, structural flexibility, chemical stability and unique optoelectronic properties [109]. Graphene has been established as a viable and efficient material choice for improved catalytic operation with various other semi-conducting materials [29] due to its extended two-dimensional sp 2 hybrid carbon network framework structure. It offers excellent electron transporting and accepting characteristics, due to which it behaves as a multi-dimensional passageway for electrons, and hence, helps to effectively separate the photo-generated charge carriers. However, the possibility and efficacy of graphene to transport electron holes is yet to be explored and studied. Moreover, a great number of established studies have been concentrated on combinations of a single type of nanoparticle and graphene film, for use as photocatalysts to improve hydrogen gas production ( Figure 5); but the incorporation of two or more components as co-catalysts on graphene for greater selectivity and improved performance has not been achieved. Graphene can be produced via top down or bottom up approach ( Figure 6) and synthesis methodology effects the intrinsic property of the graphene. The atomically thin and flexible graphene layers can not only provide a support for dispersing metallic or oxide nanoparticles and provide a highly conductive matrix, but also can induce easier electron transfer from the conduction band of semiconductors to graphene due to large energy level offset formed at the interface, leading to an efficient charge separation. provide a highly conductive matrix, but also can induce easier electron transfer from the conduction band of semiconductors to graphene due to large energy level offset formed at the interface, leading to an efficient charge separation. The different structures of graphene have been explored to enhance the photocatalytic performance of graphene. Graphene can be only used with a combination of other materials due to its lack of a natural band gap. Yibo Yan et al. prepared functionalized graphene quantum dots to improve performance [111]. Since the development of photocatalytic technology, TiO2 has been a photocatalyst due to its high efficiency, low cost and good stability. TiO2/graphene composites are currently being considered as one of the promising candidates for photocatalytic applications. The study by Angel Pérez del Pino et al. is focused on developing an inexpensive, metal-free, eco-friendly and benign photo-active catalyst while eliminating unsafe metals and reagents with superior efficiency and practicability. Laser-exposure leads to the development of defects in lattice structure, crinkles and holes, nitrogen integration, reduction of GO recognized by a drop-in oxygen atoms attached to GO and a rise in carbon content. It was further ascertained that the proliferation in Ncontent in GO structure results in better charge segregation, hydrogen liberation and photo-catalytic efficacy. Thus, it was logically concluded that the favorable factors for improving performance are nitrogen content assimilated in graphene and a degree of reduction, because redox reactions are favorably promoted by carbon adjacent to pyridinic-N. A hydrogen gas production rate of up to 89.29 The different structures of graphene have been explored to enhance the photocatalytic performance of graphene. Graphene can be only used with a combination of other materials due to its lack of a natural band gap. Yibo Yan et al. prepared functionalized graphene quantum dots to improve performance [111]. Since the development of photocatalytic technology, TiO 2 has been a photocatalyst due to its high efficiency, low cost and good stability. TiO 2 /graphene composites are currently being considered as one of the promising candidates for photocatalytic applications. The study by Angel Pérez del Pino et al. is focused on developing an inexpensive, metal-free, eco-friendly and benign photo-active catalyst while eliminating unsafe metals and reagents with superior efficiency and practicability. Laser-exposure leads to the development of defects in lattice structure, crinkles and holes, nitrogen integration, reduction of GO recognized by a drop-in oxygen atoms attached to GO and a rise in carbon content. It was further ascertained that the proliferation in N-content in GO structure results in better charge segregation, hydrogen liberation and photo-catalytic efficacy. Thus, it was logically concluded that the favorable factors for improving performance are nitrogen content assimilated in graphene and a degree of reduction, because redox reactions are favorably promoted by carbon adjacent to pyridinic-N. A hydrogen gas production rate of up to 89.29 µmol/L was recorded [112]. Shixiong Min et al. prepared TiO 2 -rGO-CoO x advanced photo-catalyst via two step hydrolysis-thermal process to separately integrate TiO 2 and CoO x species on rGO sheets. Despite the direct non-participation of rGO as a catalyst in redox processes in water splitting, its incorporation results in noteworthy increases in gas production rates and catalytic activity due to its quality of greater electron and hole kinetics, translating to lower charge combination and transportation of electrons from TiO 2 , and hole transport to CoO x [113]. Cheng, P., and Yang, Z. et al., improved the performance of typical photocatalyst TiO 2 by combining it with graphene, which has unique properties, such as flexible structure, huge specific surface area, high transparency and electron mobility, making it a worthy contender to be combined with TiO 2 to improve its photoactivity. The experimental results specified the solvothermal reduction of graphene oxide to graphene sheets, thereby augmenting the light absorption ability of P25-GR nanocomposites and also their charge separation efficiency. Graphene played the role of photogenerated electron acceptor due to its 2D π-conjugation structure and helped as an effective transporter in the separation of electron-hole pairs. The hybrid displayed superior activity towards the evolution of hydrogen from methanol solution under the illumination caused by Xe-lamp when compared to bare P25 due to improved ability for light absorption and a lower electron-hole pair recombination rate (Figure 7). The examination of photocatalytic activity of P25-GR nanocomposites based on graphene content affirmed a mass ration of 0.5 wt% GR to P25 as the optimum, with further addition of graphene leading to a decreased photocatalytic activity [39]. Li et al. worked to enhance photo-activity of traditional TiO 2 -rGO promoter and introduce an innovative method to fabricate these materials which is environmentally friendly, single-step, intense chemical-free and upgrades the gas production capability of the system by dropping band gap energy. The absence of surface Ti 3+ species and its presence in the bulk of the sample results in better performance with respect to H 2 liberation, creation of an alternate e-conduit, modifying the reaction from UV to the visible spectrum and absorbing charge transporters. Materialization of titanium-carbon bonds leads to reducing the band gap of titanium oxide, expanding the photosensitivity range, declining the charge conduction resistance and facilitating conduction between rGO and TiO 2 (Figure 8). Hydrogen productivity grew with increasing rGO content and laser exposure duration. Peak H 2 formation rate recorded was 16000 µmol/g·h and solar transformation efficiency was 14.3%, which were the subsequent outcomes of strongly coupled TiO 2 and rGO components [91]. solution under the illumination caused by Xe-lamp when compared to bare P25 due to improved ability for light absorption and a lower electron-hole pair recombination rate (Figure 7). The examination of photocatalytic activity of P25-GR nanocomposites based on graphene content affirmed a mass ration of 0.5 wt% GR to P25 as the optimum, with further addition of graphene leading to a decreased photocatalytic activity [39]. Li et al. worked to enhance photo-activity of traditional TiO2-rGO promoter and introduce an innovative method to fabricate these materials which is environmentally friendly, single-step, intense chemical-free and upgrades the gas production capability of the system by dropping band gap energy. The absence of surface Ti 3+ species and its presence in the bulk of the sample results in better performance with respect to H2 liberation, creation of an alternate e-conduit, modifying the reaction from UV to the visible spectrum and absorbing charge transporters. Materialization of titanium-carbon bonds leads to reducing the band gap of titanium oxide, expanding the photosensitivity range, declining the charge conduction resistance and facilitating conduction between rGO and TiO2 ( Figure 8). Hydrogen productivity grew with increasing rGO content and laser exposure duration. Peak H2 formation rate recorded was 16000 μmol/g·h and solar transformation efficiency was 14.3%, which were the subsequent outcomes of strongly coupled TiO2 and rGO components [91].  Sankeerthana Bellamkonda et al. studied unique photocatalytic composites of graphene and carbon nanotube amalgamations of semiconductor materials. The linking of reduced TiO 2 -x facets with carbon matrix exhibited the extraordinary photoactivity for the dissociation of water to produce hydrogen at room temperature. These nanohybrid composites produced hydrogen at 29,000 µmol/g·h, with 14.6% solar energy conversion efficiency, to hydrogen. The augmented photoactivity of this nanohybrid composite is ascribed to O 2 vacancies on the surface, the presence of Ti 3+ sites in the TiO 2 lattice and the interfacial interaction between TiO 2 and graphene [114]. Multi-layers of graphene result in increased light absorption, and the formation of 111 oriented gold nanoplatelets on multilayer graphene was analyzed by Diego Mateo et al. The orientation of incorporated Au NPs was detected using XRD and EDX. The hydrogen and oxygen production rates noted were 12,000 and 9000 µmol/g·h respectively. When compared with previous literature using similar materials, it was concluded that the enhanced rates were a result of using specialized one-step synthesis process along with the highly oriented attachment of Au particles on graphene material, which affected the strength of interactions, ability of charge separation and the photo-catalytic performance [115]. Mohamed, Mokhtar et al. found that energy conversion efficiency is also prohibited in most of these metal oxides due to their high band gap energies (E g ). To mitigate such problems, spinel families, AFe 2 O 4 (A = Co, Cu, Zn) have been examined as photocatalysts for H 2 production; particularly, manganese ferrite (MnFe 2 O 4 ) hybridized with graphene/graphene oxide to decrease E g and particles size, and to enhance both electronic conductivity and surface area. Enhanced light absorption ability of these hybrids induced more charge carriers, thereby increasing their apparent quantum efficiency to work as photocatalysts for water splitting. A nanospindle-shaped GO1:Mn1 hybrid absorbed more visible light for the stated purpose of generating high current density values exceeding traditional semiconductors, which improved its performance as a photocatalyst [116]. Katsuya Iwashina et al. studied the photo-catalytic activity for water splitting using the rGO-TiO 2 composite (oxygen-producing promoter) as an electron-transfer facilitator and sulfide-metal materials as hydrogen-producing promoters. Sulfide metal materials in unaided form are unfitting for direct use for water-splitting promoters due to issues relating to photo-corrosion. However, when combined with semiconducting materials for hydrogen production and using rGO-TiO 2 functionalized material for oxygen liberation, efficient gas production was observed. This scheme is also feasible for use in solar-aided reactions. Loading of noble metal co-catalysts with sulfide metals affected the activity owing to the particle sizes and extents of distribution. Moreover, the incorporation of platinum via adsorption showed the highest photo-catalytic character [117]. Zhenhua Pan et al. made RGO overcome the charge transfer challenge between loaded catalyst specks on a conductive sheet by introducing reduced graphene oxide as an arbitrating redox binder. The degree of reduction is an established factor in determining the equilibrium between hydrophilicity and conductive ability necessary for a photo-active material, which was ascertained in this case by XPS and Raman spectroscopy. The water dissociation rate was enhanced 3.5 fold by the incorporation of rGO in catalytic sheets, when compared to those that were devoid of rGO under identical circumstances, due to its inherent property of assisting in charge transmission. The role of Au as mechanical support and protection against self-oxidation was reiterated by results. Moreover, rGO helped in robust fixation of other active particulates on the Au sub-layer. Increasing its content to 1.25% (optimum) led to gradual growth in effectiveness, but a further rise in GO content caused the undesired excessive coverage of active species. Furthermore, the samples with superior dispersion of particulates presented an extra boost in efficiencies by as much as 1.7 times [118]. Graphene oxide (GO) based hybrid materials could be employed in several applications, including hydrogen generation. Carbon-based 2D structures processed by graphene oxide and its reduced form have controllable, layered structures; huge surface areas; and tunable optical, electronic and magnetic properties. Certain orthodox photocatalytic materials, such as Pt, TiO 2 , ZnS and CdS, could be dispersed onto these GO/rGO matrices, resulting in efficient hybrids for less energy intensive H 2 production via photocatalytic water dissociation reaction. Doping of metal ion, particularly Co 2+ and Al 3+ , considerably enhanced the production of hydrogen by rGO, indicating that efficient photocatalyst could be synthesized using simple hybrids of rGO and transition metal ions [119]. A novel combination of materials was studied by Xiujun Fan, et al. Nano-crystals of three transition metal carbides were enclosed in thin graphene nano-ribbons via hot filament chemical vapor deposition technique to obtain highly pure Fe, Co and Ni carbides supported on the peaks of graphite thorn-like upright nano-ribbon structure. The stimulating hierarchical structure produced inspired a comprehensive and relative study of the sizes, shapes and other dispersion factors of carbides on 2D graphene vertical conical arrays. The GNRs not only act as a supporting substrate to enhance stability but also prevent the aggregation of carbides on the surface, holding them well into place. The porous channels also expedite the electron transport to the core, causing a distinct decline in conduction resistance. The suitability of these materials for ORR has been indicated by ample reduction of oxygen at low Tafel slopes of 39, 41 and 45 mV/dec for Fe, Co and Ni carbide GNRs, respectively-comparable to 32 mV/dec value for Pt/C electrode. The HER current densities produced by M 3 C-GNRs (M = Fe, Co, Ni) were 166.6, 79.6 and 116.4 mA/cm 2 at 200 mV over-potential. These operative statistics can be ascribed to the coupled carbide-graphene effects of high roughness factors, large active area exposure and superior conducting ability and porosity of graphene [120].

Phosphorene
Phosphorene is a two dimensional (2D) material with a band gap of 0.3 to 2 eV from bulk phosphorus ( Figure 9) to single layer phosphorene. The band gap of phosphorene is tunable depending on applied strain. The properties of phosphorene change with change in stress, number of layers and applied electric field [121,122]. It consists of singled-layer black phosphorus, is similar to graphene, is included in the family of monolayer-flatland materials and arranged in a hexagonal puckered lattice [50,123]. phosphorus ( Figure 9) to single layer phosphorene. The band gap of phosphorene is tunable depending on applied strain. The properties of phosphorene change with change in stress, number of layers and applied electric field [121,122]. It consists of singled-layer black phosphorus, is similar to graphene, is included in the family of monolayer-flatland materials and arranged in a hexagonal puckered lattice [50,123]. The advantage of phosphorene over graphene is that it shows a band gap. Black phosphorous was discovered around 100 years ago, but the discovery of phosphorene is just over four years old. Therefore, very little experimental research of phosphorene has been reported [124]. A monolayer of black phosphorous (BP) was synthesized using the sticky-tape technique used earlier for graphene; the material was titled phosphorene. Phosphorene has the promising properties of niche 2D materials. It shows the quantum confinement effect in a direction perpendicular to the 2D plane, which induces distinct electronic and optical properties; natural passivation of the surface without any dangling bond; no lattice mismatch concerns for assembling vertical heterostructures with other 2D materials; a high specific surface area; and a strong interaction with light. These properties are considered extremely necessary for photocatalytic applications. Additionally, phosphorene has a The advantage of phosphorene over graphene is that it shows a band gap. Black phosphorous was discovered around 100 years ago, but the discovery of phosphorene is just over four years old. Therefore, very little experimental research of phosphorene has been reported [124]. A monolayer of black phosphorous (BP) was synthesized using the sticky-tape technique used earlier for graphene; the material was titled phosphorene. Phosphorene has the promising properties of niche 2D materials. It shows the quantum confinement effect in a direction perpendicular to the 2D plane, which induces distinct electronic and optical properties; natural passivation of the surface without any dangling bond; no lattice mismatch concerns for assembling vertical heterostructures with other 2D materials; a high specific surface area; and a strong interaction with light. These properties are considered extremely necessary for photocatalytic applications. Additionally, phosphorene has a tunable band gap and shows anisotropic properties that make it suitable for a variety of applications [125]. Black phosphorous is the most stable allotrope of phosphorous. It has layered in-plane strong bonds along with weak van der Waals interlayer interactions. This layered structure provides the opportunity for fabrication of very thin layers of phosphorous by exfoliation from a bulk size. The techniques which have been reported for the fabrication of phosphorene so far are based on micromechanical cleavage using sticky-tape and solvent exfoliation methods. Single layer phosphorene has also been fabricated by combining mechanical exfoliation with plasma thinning technique. This sticky-tape micro mechanical exfoliation can yield phosphorene of high crystal quality, but with very low product density. In order to avoid chemical degradation, black phosphorus must be exfoliated using anhydrous and oxygen-free solvents such as acetone, chloroform, isopropyl alcohol or dimethyl form-amide. The technique of exfoliation with sonication results in low cost and a high yield of 2D phosphorene. It also seems feasible as a process for development in large scale production. However, it becomes practical only if there is a good matching of the surface tension between the layer crystal and the solvent, and if the solvent can stabilize the nanosheet to evade restacking and aggregation. Phosphorene shows a highly anisotropic structure in nature. The monolayer structure of black phosphorus is a 2D material; however, its excitonic properties closely resemble quasi-1D material, such as carbon nano-tubes. Similar to bulk black phosphorus, phosphorene shows quantum confinement perpendicular to the 2D plane. The band gap of phosphorene can be modulated appropriately for photon absorption in the ultraviolet to the near-infrared region of the solar spectrum. Apart from theoretical studies, there is actually very little published work that proves the water splitting capability of phosphorene, conclusively, at the laboratory scale [126]. However, some work has been done for phosphorene hybrids with various oxides, graphene and few sulfides. The hybrid of black phosphorous and TiO 2 has been reported as a photocatalyst in the quest for improved activity as shown in Figure 10. It was demonstrated that when bare BP was irradiated by visible light, it increased its degradation after various runs. However, when TiO 2 was loaded to form TiO 2 /BP hybrid, the degradation was reduced by 90%. The reason for the improved stability can be described as due to the substitution of Ti atoms into the BP matrix to make BP resistant to moisture or oxygen. This study suggests that alteration of phosphorene with semiconductor materials might be a solution to increase the stability of phosphorene to ambient degradation and phosphorene can better be used as a photocatalyst [127]. In addition to BP/TiO 2 , BP/SnO 2 , BP/WO 3 and other oxides also need to be investigated, as no research has been published yet. Yongqing Cai et al. reported that graphene could be suitable as a supporting layer for encapsulating phosphorene to improve interaction and provide collaborative effects. This hybrid combination needs in depth study to find conclusive findings [128]. The eminent ability of black phosphorus nano-sheets to employ the solar energy efficiently (approximately 75%) in photocatalytic operation and the superior charge separation capacity of cobalt phosphide (CoP) can effectively be integrated in a single material, as supposed by Bin Tian, et al. [129]. The synthesis procedure consists of a straightforward solvo-thermal step involving white phosphorus and a Co-source (Co (NO 3 ) 2 ·6H 2 O) instead of the relatively tedious, expensive and complicated procedure of bulk BP synthesis and later exfoliation to nano-sheets. The special effects of CoP addition on BP resulted in manifold increase in H 2 release rate, 30 before and 735 µmol/g·h after CoP addition. MoS 2 /phosphorene and WS 2 /phosphorene hybrids demonstrate type II heterostructures. Type II is more expedient for electron-hole separation than type I and type III. The band gap of MoS 2 /phosphorene varies between 0.59 to 1.06 eV, whereas it varies between 0.90 to 1.44 eV for the WS 2 /phosphorene hybrid [130]. It seems that MoS 2 /phosphorene cannot be used as a water splitting photocatalyst because its band gap is much below the thermodynamics band gap requisite of 1.23 eV for a photocatalyst [131]. Very little work has been done for phosphorene/sulfide hybrids. Other potential 2D sulfides/phosphorene hybrids, such as CdS/phosphorene, CdSe/phosphorene and WS 2 /phosphorene, are yet to be explored. However, sulfide compounds are easily degradable in water, as they get irradiated by light. Hence, along with band gap congruency, stability is also major drawback with sulfides photocatalyst materials [132].

Metal Phosphides
Phosphides of many transition metals are emerging as very popular photocatalysts due to their ability to adsorb hydrogen atoms and release H2 gas, easily proving themselves to be very practical HER (hydrogen evolution reaction) catalysts, as confirmed by Shi and Zhang [133]. Furthermore, their tuneability to perform as dual function catalysts has been investigated via co-catalysis mechanism to enhance both HER and OER processes by incorporation of metal hydroxo/oxo groups on material surfaces. Nickel, cobalt and iron phosphides and their binary and ternary derivatives have been tried and tested by many researchers for photocatalytic applications and have showed promising outcomes. The P-content in the metal catalyst material has been suggested as a major deciding factor

Metal Phosphides
Phosphides of many transition metals are emerging as very popular photocatalysts due to their ability to adsorb hydrogen atoms and release H 2 gas, easily proving themselves to be very practical HER (hydrogen evolution reaction) catalysts, as confirmed by Shi and Zhang [133]. Furthermore, their tuneability to perform as dual function catalysts has been investigated via co-catalysis mechanism to enhance both HER and OER processes by incorporation of metal hydroxo/oxo groups on material surfaces. Nickel, cobalt and iron phosphides and their binary and ternary derivatives have been tried and tested by many researchers for photocatalytic applications and have showed promising outcomes. The P-content in the metal catalyst material has been suggested as a major deciding factor in determining the HER activity of material, as it is found to be responsible for the catalyst's interaction and affinity with H atoms. This trend has been reported by Yuan Pan et al. [134] and Sengeni Anantharaj, et al. [135], who confirmed the direct relation between HER catalytic activity and P content; i.e., an increase in the P ratio results in a corresponding increase in activity as well as in release rate. The OER's (oxygen evolution reaction) activity depends principally on the optimal bond energy values of the interaction between catalytic material and reaction intermediates (oxide, peroxide and hydro peroxide). Nickel phosphide was investigated as a dual functional photocatalyst by Marc Ledendecker, et al. [136]. It showed high activities for both HER and OER, attaining 10 mA/cm 2 under 1.7 V. Ni 5 P 4 nanostructure was grown directly on Ni-sheets by an extremely straightforward process involving the heating of elemental phosphorus and Ni-foil sheet under inert conditions. The material showed good performance for the especially imperative OER kinetics, which is normally a challenging task in photocatalysis. This feature can be the consequence of formation of highly active NiOOH intermediate deposition on the surface which contributes to reduce the operation over-potential, the shift in electronic structure, superior current density and stable operation in a 20-h test conditions in acidic environment [136]. An alternate variation of nickel phosphide, Ni 2 P as a potential electrode catalyst was suggested by Lucas A. Stern, et al. [137], wherein the active specie was found to promote both the HER and OER kinetics presenting an economically feasible and efficient single-material substitute catalyst. The Ni 2 P nanoparticles and nanowires were fabricated through simple and scalable heating of the precursor ingredients, both of which showed comparable OER activities. The detailed structural studies showed the shell/core structure of NiO x /Ni 2 P which contributes prominently in the process as the Ni 2 P core offers an electronic passageway to the NiO x shell, thereby providing better diffusive characteristics and improved activity as compared to equivalent sized NiO x nanoparticles.
The reported values of current density 10 mA/cm 2 achieved with overpotential were as insignificant as 290 mV for OER and 10 mA/cm 2 , with 136 mV overpotential for HER in acidic medium driving the developed catalyst material towards the mainstream photocatalytic water splitting systems as a promising and practical candidate material for both anodic and cathodic reactions [137]. Apart from nickel, other transition metal phosphides have also been investigated for catalysis to release hydrogen and oxygen from water by using incident light source. Cobalt phosphides emerged as a stimulating opportunity in this regard, and several reviewers and researchers have explored the utility of different forms and combinations of Co phosphides. Yang Y, et al. [138] formulated cobalt centric thin sheets via an intricate multistep process involving plasma cleaning of glass plate substrate, sputter-coating conduction nano-layers of chromium (10 nm) and gold (40 nm), Co-layer deposition by aqueous plating solution of CoSO 4 , formation of porous thin Co-oxide layer by anodic treatment of Co-layer under static voltage and then finally chemical vapor deposition of phosphorus for conversion to Co-phosphide while conserving the regular porous film (500 nm thickness). The ratio of film active area to the electrode surface area (denoted by roughness factor) was calculated as a parameter to endorse the high activity of material without the necessity of external additives. Hence, the proposition was confirmed by an observed increase in RF from six (compact-before anodic treatment) to 95 (highly porous). The operational tests showed steady operation and an exceptional HER activity of 30 mA/cm 2 at overpotentials of 175 mV (in acidic condition) and 430 mV (under basic conditions) which is even higher than most noble metal based contemporary catalysts, such as CoP nanoparticles (20 mA/cm 2 ), Co/CNT (10 mA/cm 2 ), MoP (10 mA/cm 2 ), Ni 2 P (20 mA/cm 2 ) and C 3 N 4 (10 mA/cm 2 ). Similarly, notable results were observed in terms of OER activity as well; i.e., current density of 30 mA/cm 2 at 330 mV overpotential which is again higher than most up-to-date studied catalysts, such as CoP nanoparticles (25 mA/cm 2 ), Au/Co 3 O 4 (25 mA/cm 2 ) and most ternary selections as well [138]. A valuable comparative analysis of different raw materials for CoP fabrication and their effectiveness in terms of HER efficiencies as scrutinized by Afriyanti Sumboja, et al. [139] to provide an outlook for large-scale and commercially applicable source of CoPs manufacture. Salts such as cobalt acetate and cobalt acetylacetonate were phosphodized by sodium hypophosphite in a single-step. The CoP formed from the former ingredient exhibited better HER promotion (overpotentials: 160 mV in acidic, 175 mV in basic media) due to greater phosphide fraction (92.5%) and greater surface area (ECSA: 10-15 cm 2 ). While the latter being a readily oxidizable salt produces a greater fraction of metallic Co rather than phosphide (63.5% CoP), lower surface area (ECSA: 7-12.5 cm 2 ) and greater overpotential (169 mV in acidic, 188 mV in basic media) [139]. The application of Co-phosphide nanosheets as HER promoter in 1M KOH media has been studied by Liang Su,et al. [140] wherein the novel approach of effects of surface refurbishment of CoP nanosheets are exposed. The CoP nanosheets were conventionally synthesized by phosphidation of Cobalt oxides and then engineered electrochemically to promote an irrevocable and stable transformation into Co(OH) x /CoP hybrid configuration that displayed excellent HER catalysis producing 10 mA/cm 2 with 100 mV overpotential as compared to the untreated CoP nanosheet overpotential of 180 mV. The dispersion of hydroxyl group on the nanosheet surface as an outcome of 10 hours' exposure to 20 mA/cm 2 exhibit a coarser surface, greater turnover frequency of 0.234 per second and reliable operation even after 5000 cycles. These improvements in performance by the surface modification are attributed to the arrangement of Co(OH) x complexes which are efficient promoters of the breakdown of water into molecular components. As this stage is the rate-determining step of HER mechanism, this surface makeover of CoP nano-sheets proves to be highly favorable for the process and the copiously released H-atoms interact with adjoining CoP molecules to release hydrogen gas readily. The merger of both Co(OH) x and CoP specie is most beneficial for the process as compared to single-component systems as established by the comparative assessment of Co(OH) x /CoP (100 mV overpotential), untreated CoP (180 mV) and C 3 N 4 (300 mV) samples [140]. CoP nano-rods supported on Nickel foam as reported by Yun-Pie Zhu, et al. [141] is a novel strategy to fabricate dual-function HER and OER promoters with high efficacy, high porosity, enhanced diffusive transport and surface area (148 m 2 /g) while maintaining high conductivity. The synthesis procedure revolves around potentio-deposition of nanorods on Ni-foam while the CoP deposited is produced in situ, facilitated by direct electron passageways provided by Ni-foam. Current densities of 10,100 and 800 mA/cm 2 at 54, 121 and 235 mV overpotentials for HER and 100 mA/cm 2 for overall cell voltage of 1.62 V were accomplished which are even more efficient than Pt and IrO 2 system [141]. Ternary variants, such as Ni-Co-P have also been developed successfully to integrate the desired properties of component materials. Yingjie Li, et al. [142] cultivated Ni-Co-P nano-sheet arrangements on Ni foam by two stage hydrothermal and phosphorization scheme. The synthesized material demonstrated extraordinary performance for both HER and OER catalysis even when compared with the traditional commercially used noble metal (Pt/C, Ir/C) electrodes and the separately tested Ni 2 P and Co 2 P nano-sheet arrays produced by the same method. The overpotential values for Ni-Co-P ternary electrodes for HER and OER were 133 and 308 mV respectively necessary to operate at 50 mA/cm 2 and the cell voltage required was as minimal as 1.77 V. The remarkable activity and stability of material can be credited to marginal electron resistance, normalized hydrogen binding forces, material's inherent high activity and a distinctive super-aerophobic nano-framework [142].
A similar arrangement was prepared by Liang H, et al. [143] with plasma-facilitated growth of Ni-Co-P on Ni foam, concluding in a menial over-potential of 32 mV for HER and 280 mV for OER with 10 mA/cm 2 current density and 1.58 V cell potential. Bi-functional high porosity Ni-Co-P nano-sheet catalyst supported on carbon-fiber paper was developed by Rui Wu, et al. [144] by electrodeposition resulting in identical sheet arrangements with subsequent annealing to generate spongy Ni 0.1 Co 0.9 P structure. The annealing temperature, Ni: Co ratio of 1:9 and the precursors were deciding factors for optimal synthesis of active material. The detailed characterization results indicated that resulting material operated at high turn-over frequency (0.24 per sec for H 2 , 0.12 per sec for O 2 ), reliable stability (20 h operation, 1000 cycles), better electron transmission ability and interactive sites contributed by Ni fraction and high efficiency. Cell voltage required for operation at 10 mA/cm 2 was a competitive value of 1.89 V and the over-potential was 125mV as compared to 174 and 196 mV for CoP and Ni 2 P electrodes [144]. An ample amount of laudable work in the ternary Ni-Co-P catalyst and its composites category can be mentioned such as NiCoP/Carbon cloth composite as dual function catalyst reported by Cheng Du, et al. [145], with overpotentials as petite as 44 and 62 mV for HER and OER respectively at 10 mA/cm 2 current density and 1.77V cell potential for 100 mA/cm 2 output paves the way for practical non-noble metal electrode materials. NiCoP/r-GO composite for stimulating both HER and OER synthesized by Jiayuan Li, et al. [146] presented 10 mA/cm 2 current density with cell voltage 1.59V with stable operation for more than 75 hrs. The coupling of Co-doping in Ni 2 P and their combination with rGO resulted in a boost in electron transport, catalytic activity and surface arrangement of active spots. An elegant and novel heterostructure involving NiCoP@Cu 3 P nano-sheets was cultivated on Cu foam by Xingxing Ma, et al. [147] as a favorable candidate for both HER and OER catalysis showcasing 54 mV and 309 mV over-potentials for HER and OER at 10 mA/cm 2 . Similarly an intricate system of NiCoP nano-sheets grown on N-doped Carbon-coated Ni foam as formulated by Miaomiao Tong, et al. [148] through hydrothermal and phosphorization route required 31.8 mV and 308.2 mV for HER and OER to accomplish 10 mA/cm 2 current density. The durable performance and activity experiments showed comparable outcomes to the commercial Pt/C and Ru 2 O catalysts.

Metal Organic Frameworks and Derivatives
Metal organic framework or MOF is the peculiar category of crystalline synthetic materials that has claimed a prominent position in the league of modern multi-functional materials due to the distinctive properties of high porosity, adaptable pore structures, larger surface area and amendable elemental compositions. MOFs comprise of a network assembly of metal centers linked to organic bridging ligands which result in a variety of geometries, compositions and pore spacing dependent upon the synthesis procedure, conditions and initiators. The unique metal-centric interlinked geometric morphology allows MOF applications in various fields including, but not limited to, gas adsorption and separation, catalysis, drug delivery systems, optics and sensors [149][150][151][152][153][154][155][156][157] and the possibilities and opportunities of its efficient utilization and improvements are continuously being analyzed by researchers every day [158,159]. Numerous MOF-based catalyst materials have been synthesized, characterized and upgraded for photocatalytic water splitting reactions in recent times [160,161] with a focus to develop novel and feasible bi-functional promoters to make the concept of commercial solar hydrogen production from water a practical reality [162]. The standard practices to convert MOFs into practical photocatalysts comprise of inclusion of light-sensitive building blocks into the MOF backbone for instance aminoterephthalates [163,164] and porphyrins [165], Doping with any active metal or noble metal in order to enhance catalytic capacities [166], inorganic nano-particles encapsulated in MOFs [167], assembling ternary composites and introduction of electron donor or trapping materials to prevent charge recombination challenges [168]. A primary perspective of efforts in this regard will be presented herewith. The long-standing fact of high activity of noble metals in water splitting systems photocatalytically was incorporated into MOF-based materials by Meng Lan, et al. [169] to generate 3 types of Pt-tempered heterojunctions; i.e., Pt-ZnS-CoS, Pt-Zn 3 P 2 -CoP and Pt-ZnO-Co 3 O 4 . The synthesis procedure involved the ternary steps of Zn-Co ZIF preparation by previously reported methods [170][171][172][173], the sulfidation, phosphorization or oxidation of base ZnCo ZIF for each sample separately and the subsequent doping of Pt nano-particles. The twin-metal ZIF backbone facilitates the light absorption ability, increases the active area contact and favors charge carrier transportation and separation, while the dispersed Pt particles capture the electrons and motivate the hydrogen production. The resulting hydrogen release rates by above listed hetero-junctions are 8210, 9150 and 7800 µmol/g·h respectively. In the structural analysis, it was noted that the polyhedral MOF shape and porosity was retained after Pt-doping as well and the operational durability was noted to be unaffected even after 5 cycles. Cheng Wang, et al. [174] reported the fabrication of two kinds of Zr-carboxylate MOFs by intricate methods and their subsequent center-doping with Pt-nanoparticles whereby the produced Pt@MOF material displayed the ability to catalyze HER, recyclable nature by simple centrifugation and the turnover number of 7000. Another example of incorporating Pt into MOF units for enhanced activity has been reported by Dengke Wang, et al. [175] wherein, the Pt nano-particles were disseminated uniformly via a novel photo-reduction method in MIL-100(Fe) MOF which has formerly been established as a water stable and photo-excitable material [176]. The introduction of optimal fraction of Pt (0.8%) in MOF structure verified the hydrogen release increase from 5.9 to 109 µmol/g·h (18.5 times of pristine MOF). This prime value is observed due to the collaborating mechanism of electrons generation by photo-active MOF structure and the immediate electron seizing by central Pt also acting as the prime H 2 production site. This coordinated functioning results in enhanced charge separation without compromising on the exposed excitation area of for light. Qin Liu,et al. [177] used a permeable Ni-foam as a substrate to utilize its conductive capacity, high exposed surface region and ease of electronic diffusion through the channels towards the active dispersed Ni-MOF arrays which allowed the current capacity to reach 100 mA/cm 2 at 320 mV over-potential and turnover frequency of 0.25 mol/s O 2 . The in-situ hydrothermally grown Ni-MOF nano-sheet arrays on Ni-foam was analyzed thoroughly and compared to commercial Ru 2 O/NF OER catalyst and simple NF (Nickel foam). The results displayed the trends of lower Tafel slope 123 mV/dec than NF (193 mV/dec) but higher than Ru 2 O/NF (72 mV/dec) due to the different OER mechanisms followed by these materials. Faradaic efficiency measured for oxygen release by contrasting the theoretical and practically released oxygen was nearly 100% with a stable operation for 20 h test time endorsing its practicality as a successful OER promoter. In 2016, ultra-thin nano-sheets of bi-metallic NiCo MOF were prepared with a purpose to promote oxygen discharge from water by Shenlong Zhao, et al. [178]. Utilization of nano-sized ultra-thin MOF sheets introduces the merits of lower mass transport resistance, better electronic pathways for conduction, superior exposure to the remote active sites and un-bound highly active free metal sites which act in combination with the intrinsic effectiveness of Ni and Co base metals in OER catalysis. The 2D NiCO, ultra-thin MOF nano-sheets supported on Co-sponge exhibited reassuring performance with over-potential 189 mV at 10 mA/cm 2 , continuous and stable operating time of 200 h and Faradaic efficiency of 99.3%. These parameters verified the hypothesized collective benefits of incorporating two-metals in MOF network and the working decision to use ultra-thin MOF structure on a porous support. A similar study using a different combination of metals, i.e., Ni-Fe ultrathin (≈3.5 nm) MOF nano-sheets, was conducted by jingjing duan, et al. [179]. This Ni-Fe combo also showed superior analytical parameters due to the identical structural and elemental choices as previously listed work. The over-potential observed in this case was 240 mV for 10 mA/cm 2 with noteworthy turnover frequency of 3.8 per sec and stable efficient operation for 20000 s. A three-component engineered MOF structure was developed by Rui Lin, et al. [180] that combined established photo-active ingredients to make a strong partnership of UiO-66 MOF, CdS and reduced graphene oxide. Inspired to resolve the electron-hole recombination challenge in CdS photocatalysts, combining CdS/rGO composite with UiO-66 MOF not only resolved this issue but added the benefits of its high contact area, considerable thermal stability [181] uniform pore channels and proven photo-active nature of UiO-66 for hydrogen production [182]. Consequently, the UiO-66/CdS/1%-rGO combination showed the best charge separation capacity as compared to other proportions of rGO and the hydrogen release 13.8 times superior to pristine CdS. Another variation of ternary MOF photocatalyst is based on MoS 2 /UiO-66/CdS fusion structure [183] which delivered 32,500 µmol/g·h H 2 release rate. A CdS/MOF combination was developed to promote H 2 evolution reaction by Jiao He, et al. [184] with the unusual idea of supporting CdS active material on photocatalytically inert MIL-101 MOF defining its sole purpose of providing greater exposed active area. MIL-101 has Cr centric structure with benefits of excellent air, water and heat stability. The supported CdS@MOF formed a crystalline cubic shape and the highest H 2 release rate was noted for 10 wt% proportion on CdS reaching upto 75,500µmol/g.h. Another specimen using MIL-101 was prepared by amine sensitization along with Pt co-catalyst doping in 2014 to obtain greater hydrogen efficiency and stability [185]. Furthermore, Co-oxide restrained in MIL-101 was prepared by intricate method involving double solvo-thermal technique to create a robust material (TOF: 0.012/sec per Co-atom) with a rich oxygen productivity (88%) [186]. The inventive strategy to produce a photo-sensitive MOF combination was explored to directly synthesize the MOF using dye-resembling organic azobenzene tetra carboxylic acid as an organic ligand and the metal (Gd) salt as the central backbone element. Xiaojun Sun, et al. [187] used a solvo-thermal method to synthesize yellow colored Gd-MOF crystals which were later also metal-laden with Ag (1-2%) as a second catalyst. The experimental comparison showed 154.2 µmol/g·h H 2 release from pristine Gd-MOF and 212 µmol/g·h from Ag (1.5%)/Gd-MOF under UV-Vis exposure due to the introduction of electron-trapping noble metal nature. Ag proved to be a suitable and feasible option as a noble metal dopant due to its cost effectiveness, lower electron transport resistance and decreased charge re-combination by capturing excited electron species released from photoactive MOF [188]. An alternate approach to integrate the qualities of superior photo-sensitivity, high HER activity and larger exposed surface area is to prepare syndicate of materials serving these purposes as formulated by Jiao He, et al. [189] wherein, a light sensitive Cr MOF UiO-66 was loaded with Pt nano-particles for HER activity and rhodamine B (RhB) dye for photo-activation. The impact of this fusion is clearly evident from H 2 evolution rates whereby pure UiO-66 displayed no perceptible H 2 release, Pt/UiO-66 showed 3.9 µmol/g·h and adsorbed RhB/Pt/UiO-66 showed 116 µmol/g·h which is 30 times greater than un-treated Pt/UiO-66. A different photo-active MOF variant MIL-125-NH 2 for H 2 production from water can be used as the base material with a MoS integration, as discovered by Tu N. Nguyen, et al. [190], to produce H 2 evolution rate as high as 2094 and 1454 µmol/g·h with quantum productivity 11% and 5.8% using two different kinds of MoS clusters; i.e., Mo 3 S 13 and IT-MoS respectively. MoS 2 is an inherently photo-active compound [191] and the sources of light responsive behavior are the vacancy defects and sulfur-edges only [192,193] while the basal faces do not contribute in catalysis and the size of particles is inversely related to the HER activity [194] Robust interaction between the supporting MOF and MoS result in higher exposed active area, lower electron conduction resistance and high stability. In an innovative approach to replicate the active nodular-structure of [NiFe] hydrogenase complex [195], a two-dimensional layered photo-responsive MOF [Ni 2 (PymS) 4 ] n was fabricated via cautiously supervised multi-step hydrothermal process [196]. The resultant material showed robust water and air stability, bi-atomic Ni webbed morphology with 4 PymS − linkers and turnover frequency of 10.6 per hour and hydrogen release upto 6017 µmol/g·h using TEA as electron source, flourescien as photo-activator and white LED or visible light as source. The pH of solution, nature of sensitizer, nature of electron source and amount and particle size of catalyst were observed to be the deciding factors to determine the practical competence of the catalyst. A dual function catalyst with the aim of promoting both HER and OER components of the water dissociation process has many advantages and has been a center of research since decades now. A MOF-based route to achieve this goal was proposed by Yang An, et al. [169]. Pt and CoPi components were dispersed onto the innately photo-active MIL-125 (Ti) MOF [131] to boost hydrogen and oxygen discharge and suppress charge recombination tendency by the deposited components. The experimental H 2 and O 2 liberation rates under UV-Vis exposure were 42.33 and 21.33 µL/h. Separate influence of trace materials on the half-reactions were confirmed by appraising the MIL-125 (Ti) with CoPi@MIL-125 (Ti) and Pt@MIL-125 (Ti), which verified that the OER activity of CoPi-activated sample amplified to 307 µL/h from 143 µL/h, whereas in case of HER, introduction of Pt on MOF increased the activity 25 fold. Furthermore, the order of Pt and CoPi doping is a significant factor as a higher activity was noted when Pt was loaded first. Recently, an aluminum-centric MOF devised from 2-aminoterephthalic acid (ATA) was recognized as an active oxygen evolution promoter and modification of this material with Ni assimilation resulted in a dual purpose (HER and OER) overall water dissociation catalyst [197]. The half reaction analysis validated the coordinating nature of both Al and Ni components as OER for Al-ATA-Ni MOF was 5166.7 µmol/g·h contrasted with 550 µmol/g·h and HER for Al-ATA-Ni MOF was 1200 µmol/g·h and the material without Ni was found inactive towards hydrogen production. A bipyridine-centered MOF-253 was modified by confining Pt complex into the network with the intention to promote HER and simultaneously act as a photo-exciter [198]. The base MOF-253 possesses the anticipated merits of high exposed area and promising stability [199], which is evident in the resulting compound and the addition of Pt component translates to five times increase in photo-activity. Another Ti-centric MOF was amended by incorporating Ru-complex established on the similar strategy to promote HER in visible light presence [200]. Despite being a relatively new concept and material, 2D MOFs have found various application in electrocatalysis, photocatalysis and heterogeneous catalysis in general [201]. The focus on optimization of manufacturing procedure, product stability, surface coating and immobilization techniques and scalability can bring this novel category of materials into limelight for future catalytic processes due to the inherent benefits of exposed active sites, turnover frequencies, effective coordinated properties of metal and organic ligands and easy handling capability.

Conclusions and Future Outlook
A tremendous amount of research and development has been carried out to improve, study and implement various strategies in photocatalytic water splitting using different categories of materials, composites, heterojunctions and immobilizing schemes, as summarized in Table 2 [202,203]; however, the use of 2D materials, their combinations and derivatives is a developing concept in photocatalytic water splitting research, which poses itself as an excellent opportunity for advancement and further exploration of innovative materials. With tunable and unique architectures, distinctive features and desirable structural intricacies being the specialties of 2D materials, it seems imperative to delve into this arena for progressive photocatalysis systems. Keeping these positive outlooks and prospects in view, this review highlights the evolution of modern nanomaterials, 2D photocatalysts and MOF derivatives to attain the coveted properties of low charge recombination, high photo absorbance activity, efficient electron conductivity, fast kinetics and large surface area for water splitting to produce hydrogen. It has been concluded that production of hydrogen can be enhanced using various techniques, including doping with noble metal particles, surface functionalization, nano-heterostructures, synthesis-controlled morphology and surface decoration of nano-particles in 2D cages. The most common backbone materials noted in this regard are the graphitic carbon nitride, and graphene, which serve as supporting pillars and high area foundations in case of composite materials and hetero-structural modifications. Meanwhile, phosphorene has emerged as a relatively novel material and limited research has been done to explore the possibilities of practical application of this material. Metal phosphides and metal organic frameworks have been extensively incorporated in 2D heterostructures and as supporting cocatalysts to improve performance and functional enhancement of base materials and assure performance improvement with further research and experimentation. With the acknowledgement of the fundamentally essential properties displayed by the 2D base materials and their variants, as discussed in this article, it is safe to conclude that photocatalytic research and the commercial application of this promising technology has found 2D materials and their hybrids to be promising and stimulating candidates. However, despite the progress in research, the challenges associated with 2D materials present an opportunity to explore improvements in existing 2D materials and the development of new alternatives with novel electronic and structural features, such as electrochemically reduced thin films (e.g., α-Fe 2 O 3 ) [28], layered oxyhalides (FeOBr, Bi 4 VO 8 Cl, etc.), thiophosphates (CoPS 3 ), metal chalcogenides, 2D MOFs and metal-free semiconducting materials [46]. Moreover, the setbacks associated with cost effective, bulk production of these ultrathin photocatalysts limits the scalability for prospective commercial operation which needs to be addressed and resolved. The odds of bulk scale synthesis from nano-layered and non-layered materials to manufacture 2D thin sheets must be improved to make thickness-controlled and efficient 2D photocatalysts. Debottlenecking of issues related to stability in water splitting reaction conditions, such as agglomeration, corrosion and oxidation, must be prioritized to explore surface deformation control and reduction of surface energies. Fast charge carrier re-combination could be mitigated by sacrificial agents, but better practicality demands investigation of alternative solutions, such as 2D/2D, 2D/1D and 2D/0D heterojunctions to introduce interfacial charge separation. Modern systems and industries thrive on the basis of technology; thus, optimization and simulation tools must be introduced to foster research and development in photocatalysis and eliminate the gap between theoretical and real-time reaction systems.