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Review

An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production

1
Department of Chemistry, The University of Lahore-Sargodha Campus, Sargodha 40100, Pakistan
2
Hamdard Institute of Pharmaceutical Sciences, Islamabad Campus, Hamdard University of Pharmaceutical Sciences, Islamabad 44000, Pakistan
3
Department of Chemistry, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 13623, Saudi Arabia
4
Department of Chemistry, UET Lahore, Faisalabad Campus, Faisalabad 37630, Pakistan
5
Department of Chemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan
*
Authors to whom correspondence should be addressed.
Catalysts 2023, 13(1), 66; https://doi.org/10.3390/catal13010066
Submission received: 28 November 2022 / Revised: 12 December 2022 / Accepted: 15 December 2022 / Published: 29 December 2022
(This article belongs to the Special Issue Nanomaterials for Photocatalysis)

Abstract

:
At present, the energy shortage and environmental pollution are the burning global issues. For centuries, fossil fuels have been used to meet worldwide energy demand. However, thousands of tons of greenhouse gases are released into the atmosphere when fossil fuels are burned, contributing to global warming. Therefore, green energy must replace fossil fuels, and hydrogen is a prime choice. Photocatalytic water splitting (PWS) under solar irradiation could address energy and environmental problems. In the past decade, solar photocatalysts have been used to manufacture sustainable fuels. Scientists are working to synthesize a reliable, affordable, and light-efficient photocatalyst. Developing efficient photocatalysts for water redox reactions in suspension is a key to solar energy conversion. Semiconductor nanoparticles can be used as photocatalysts to accelerate redox reactions to generate chemical fuel or electricity. Carbon materials are substantial photocatalysts for total WS under solar irradiation due to their high activity, high stability, low cost, easy production, and structural diversity. Carbon-based materials such as graphene, graphene oxide, graphitic carbon nitride, fullerenes, carbon nanotubes, and carbon quantum dots can be used as semiconductors, photosensitizers, cocatalysts, and support materials. This review comprehensively explains how carbon-based composite materials function as photocatalytic semiconductors for hydrogen production, the water-splitting mechanism, and the chemistry of redox reactions. Also, how heteroatom doping, defects and surface functionalities, etc., can influence the efficiency of carbon photocatalysts in H2 production. The challenges faced in the PWS process and future prospects are briefly discussed.

Graphical Abstract

1. Introduction

The global energy crisis and environmental challenges are crucial to producing clean, environmentally friendly energy. As pollution levels rise and the supply of fossil fuels declines, the market for green energy is escalating. Most of the world’s power comes from fossil fuels, which pollute the atmosphere and contribute to global warming [1,2,3]. When fossil fuels are burned, 7.8 billion metric tons per year of CO2 and other pollutants are released [4,5]. Fossil fuels account for practically all of the world’s energy demands (86.4%) [6]. New energy sources can reduce or replace fossil fuels. Wind, hydroelectric, solar, geothermal, and other renewable energies are green and eco-friendly [7]. Solar energy can be used worldwide to generate electricity and heat. The Earth’s atmosphere absorbs 3.85 × 1024 J of solar power annually. Currently, 2% of solar energy is converted into green energy via various methods [8]. Photocatalytic materials are essential for converting solar energy directly into fuel [9]. Hydrogen generation as a future renewable, green fuel requires a quantum-efficient, stable photocatalyst [10]. Hydrogen has the potential to replace fossil fuels as an energy source. Water electrolysis requires voltages greater than 1.23 eV to produce hydrogen, owing to slow oxygen reactions [11]. Hydrogen is 2.75 times as energetic as hydrocarbon fuels and 2.40 times as active as methane. Hydrogen-powered fuel cells could produce both electricity and water. Scientists are using new methods and technologies with the defined requirements of green and cost-effective ways to create and store hydrogen energy. Introducing nanotechnology results in increased hydrogen generation and storage capacity [12]. Hydrogen can be produced through autothermal reforming, steam reforming, water electrolysis, and partial oxidation. Hydrogen generation systems include electrochemical, photochemical, and solar-assisted thermochemical systems [13]. In 1972, Fujishima and Honda devised a photocatalytic water-splitting (PWS) method using UV light for the first time. Since then, researchers have studied H2 production using semiconductor photocatalysts [14]. Photocatalysis-based hydrogen production by direct water-splitting (WS) is an auspicious and renewable solution to the existing energy shortage [15]. The generation of hydrogen (H2) and oxygen (O2) can be accomplished by PWS when exposed to sunlight. Previous research on PWS focused on enhancing the photoactive material’s effectiveness and durability [16]. Inorganic photocatalysts use light afbsorbers to generate electron-hole pairs, whereas cocatalysts reduce and oxidize water to create hydrogen and oxygen [17]. Metal-based photocatalysts are in high demand but are costly, corrosive, and potentially hazardous [18]. The MnO2 nanosheet material is promising for electrochemical energy storage, electrocatalysis, and photocatalysis [19]. TiO2 is the photocatalytic semiconductor that has received the most research over the past few decades. TiO2-carbon composite displays significantly improved photocatalytic performance under solar irradiation [20]. TiO2/MnO2 nanocomposites have exceptional photocatalytic activity and solar energy consumption due to their small energy bandgap, negatively charged surface, and large specific surface area. Metal-based photocatalysts are being phased out in favor of other metal-free photocatalysts owing to their high band gap (~3.5 eV) [21,22]. Photocatalysts still face difficulties because of low quantum yield, excessive charge recombination, and limited light absorption [23]. To address the former requirements, one must look for inexpensive, broad-spectrum materials with effective internal charge transfer [22].
To improve photocatalysis, scientists are turning to carbon-based nanomaterials (CNMs) (Figure 1) such as graphene [24], carbon nanotubes (CNTs) [25], graphitic carbon nitride (g-C3N4) [26], fullerene (C60) [27], carbon dots (CDs) [28], and carbon nanofibers (CNFs) [29], etc. Carbon-based nanomaterials show exceptional physicochemical properties, including thermal and chemical stability, high surface area, and better electrical, mechanical, and optical properties (Table 1). A variety of approaches are frequently used to fabricate CNMs such as arc discharge, chemical vapor deposition (CVD), laser ablation, and ball-milling [30]. Carbon nanomaterials have small band gaps, unique morphologies, and controlled structures. Low-dimensional materials are attractive possibilities for photocatalyst development [9]. Though CNMs own very little intrinsic photocatalytic activity, they can improve the efficiency of the active site of the photocatalyst as a carrier. Combining CNMs with a photocatalyst can improve the reagent’s active center’s adsorption site. Carbon-based nanomaterials with electron-hole pairs can also serve as electron acceptors or transmission channels to limit photoexcitation. Photocatalysts benefit from their ability to reduce particle aggregation and enhance particle distribution. The photothermal effects or photosensitivity of CNMs can be enhanced in the presence of light. Carbon-based nanomaterials can be a better option for PWS because they have many benefits, including low costs and a huge reserve. Researchers have extensively examined CNMs for H2 production via light photocatalysis. Photocatalysts for H2 production, such as carbon dots, are frequently employed. Progress has been made in CNM-photocatalyzed H2 production.
This review describes the role of CNMs in the WS process for hydrogen synthesis in detail. The mechanism and redox reaction involved are also discussed. The study also focuses on the factors that influence photocatalytic activity. The challenges and potential prospects of the PWS process are briefly discussed.

2. Mechanism of Photocatalytic Water Splitting (PWS)

Fujishima and Honda were the first to decompose water using solar radiation successfully. TiO2 and Pt can serve as anode and cathode electrodes in a photoelectrochemical cell. Under UV irradiation, this mechanism could divide water into hydrogen and oxygen [32]. A few years later, Bard applied the basic principles of this technology to develop the photocatalysis method. Since then, tremendous effort has gone into designing semiconductors that can dissolve water into H2 and O2 when exposed to light [33,34,35,36]. Photocatalysis begins with the photo-absorption of semiconductor photocatalysts. Under solar irradiation, holes are generated in the valence band VB that excite electrons to the conduction band CB. To interact with specific reagents, these charge carriers separate and move to the semiconductor surface [37,38]. For optimal photocatalytic activity, a semiconductor must have a bandgap bigger than 1.23 eV [39,40,41]. Then, the photo-excited electrons and holes must be quickly separated and migrated. On the photocatalyst surface, there must be a large number of adsorption sites and active centers [42]. A lower CB minimum than water’s reduction potential (RP) is required for hydrogen evolution, while a higher VB maximum is required for oxygen evolution during photocatalysis [43,44,45]. Three significant steps involved in photocatalysis are: (1) semiconductor photocatalysts absorb light to produce electron-hole pairs; (2) charge separation and surface migration in semiconductor photocatalysts; and (3) redox reaction happens for WS, electrons reduce water molecules to H2, and holes oxidize them to O2 [46,47,48]. The separation of hydrogen and oxygen from water is an endothermic process. Gibbs’s free energy change for WS is +273.2 kJ mol−1, according to Equation (3), equivalent to 1.23 eV per moved electron [39,49].
H2O(l) → H2(g)+ ½ O2 (g), ΔG = +273 kJ mol−1
In semiconducting material, the conduction band CB must be more negative than the H+/H2 redox potential. In contrast, the valence band VB of the semiconducting material must be more positive than the electrode potential of O2/H2O, i.e., 1.23 eV [39], as shown in Figure 2.
However, unintentional electron-hole recombination is an essential concern in this method, substantially lowering photocatalytic efficacy [50,51,52]. The separation of electrons from holes can be improved by using CNMs to speed up carrier transit. Because of their high conductivity, CNMs can capture electrons driven by light in the CB of loaded semiconductors. When it comes to working function, CNMs perform significantly better than load semiconductors [40,53,54]. As a result, electrons are either trapped on the CNM’s surface or are promptly transferred to the active site via the CNM’s conductive surface [55]. Carbon nanomaterials display excellent semiconductor properties, including GO, rGO, CNTs, CDs, g-C3N4, and C60. Developing a Schottky barrier between the metal catalyst and the CNMs semiconductor facilitates electron-hole pair separation during the deposition of the metal catalyst [56,57,58,59]. The difference in Fermi level between metal catalysts and CNMs semiconductors promotes charge flow until Fermi equilibrium, band bending, and Schottky barriers. Schottky barrier creation can efficiently restrict photogenerated electron backflow, permitting more electrons to contribute to hydrogen evaluation reaction (HER).
DFT calculations help better to understand photocatalytic HER on CNMs at the molecular and atomic levels. DFT’s primary objective is to explore the active sites/defects available on the surface of CNMs. For example, among different types of CNMs, graphene might have a lot of edges or defects at its active site. The heteroatom interfaces with different materials and replaces the carbon atom [60]. The H2 production mechanism can be described through theoretical construction and computational analysis. DFT investigations focus on the calculations related to electron density, structure elucidation, charge distribution, and energy of reaction [61].

3. Functions of CNMs in Photocatalytic H2 Generation

3.1. Structural Support to Improve Structure Sustainability

In semiconductor photocatalysis, different carbon compounds such as CNTs, CDs, graphene, GO, and activated carbon (AC) serve as structural support [62,63,64,65,66]. Carbon nanomaterials offer a larger surface site for the dissemination and immobilization of nanoparticles (NPs). These inert and thermally stable CNMs maintain their structure and properties when coupled with NPs. Numerous defect and active sites (oxygen-based functional groups) present in carbon materials can be exploited to initiate the development and anchoring of homogenous nanoparticles. The carbon materials’ light weight is essential for their use as structural support. It is now also possible to produce activated carbon, carbon fibers, CNTs, and graphene on a considerable scale and inexpensively due to the copious amount of carbon in the earth’s crust. The presence of these carbon components can change nucleation and growth rates, slowing down semiconductor nanoparticle aggregation and strengthening their architectures [67,68,69]. This section discusses how carbon-based materials stabilize nanostructures. Thus, structural stability applies to component geometries and morphologies. Extremely small nanoparticles, clusters, or other nanostructures that are dispersed and stabilized can substantially increase structural strength. As a result, semiconductor nanoparticles possess a large surface area, numerous surface defects, and a superior contact interface with carbon materials for increased charge transfer. Therefore, carbon-based hybrid semiconductor photocatalysts are more effective at producing H2 than pure semiconductor photocatalysts. The surface chemistry of carbon materials is greatly impacted by interactions between semiconductor nanoparticles and carbon material. Wang et al. [64] proposed that hydroxyl, carboxyl, carbonyl, and epoxy groups increased the hydrophilic property of the CNTs surface by first treating them with HCl and HNO3. The ZnxCd1−xS nanoparticles were then supported by the functionalized CNTs using a solvothermal technique with the help of the raw ingredients including zinc acetate, cadmium chloride, and thiourea. The well-organized Zn0.83Cd0.17S nanoparticles on the CNTs surface have a diameter of about 100 nm. If CNTs were not there, these nanoparticles would stick together. The excellent dispersion improves the interfacial surface of the Zn0.83Cd0.17S/CNT photocatalyst. Since the conduction band of the photocatalytic material is more populated than the Fermi level of CNTs, photoexcited electrons will move to the surface of the nanotubes. Thus, the aforesaid phenomenon can efficiently separate photoinduced charge carriers at the photocatalyst’s interface. In addition, CNT-incorporated Zn0.83Cd0.17S nanocomposite has a lower band gap value than pure Zn0.83Cd0.17S. Under 300 to 800 nm illumination, Zn0.83Cd0.17S/CNT nanocomposite generates 6.03 mmol·h−1g−1 of H2, which is 1.5 times more than the pristine Zn0.83Cd0.17S.
Li et al. [70] prepared CdS-coupled graphene nanosheets via a hydrothermal approach using GO as supporting material, Cadmium acetate for Cd2+ precursor, and DMSO as the solvent as well as source of S2−. Solvothermal heating turned GO into graphene with CdS clusters on its surface. The interactions between nanoparticles and graphene can be strengthened by physical adsorption, the electrostatic force of attractions, and charge transference. The homogenous CdS cluster distribution on graphene, the CdS/graphene photocatalyst, has a greater specific surface area. Uniform CdS cluster distribution is attributed to the more effective transport of photoexcited charges to graphene. At 1.0 wt% graphene, 1.12 mmol·h−1 of hydrogen is generated, which is 4.87 times than the neat CdS. Shen et al. [71] also used rGO additive to stabilize the very thin nanorods of Zn0.5Cd0.5S with a corresponding upsurge in photocatalytic H2 evolution.

3.2. Cocatalyst

Cocatalysts in a photocatalytic system enhance H2 evaluation efficacy. Metals such as Pt, Au, or Pd have a more significant work function than semiconductors, making them excellent cocatalysts. Cocatalysts improve charge separation, provide catalytic sites, reduce over-potential, and minimize H2 activation energy [72,73]. Noble metals are expensive and rare. Thus, economical cocatalysts are needed for economic photocatalytic systems. Carbon compounds are effective cocatalysts for photocatalytic H2 evaluation [74,75]. The Fermi level of carbonaceous compounds is lesser than CB but RP is greater than H+/H2. Carbon compounds have advantages similar to noble metal cocatalysts, with increased active sites and a localized photothermal effect owing to increased surface area and extended light absorption intensity [76,77]. Khan et al. [74] fabricated a CNTs/CdS/TiO2/Pt photocatalyst via the hydrothermal method. Pt was designed onto TiO2 via photodeposition whilst CdS via hydrolysis. This photocatalyst generates H2 at a greater rate under visible radiations, using sodium sulfide and sulfate sacrificial agents. In the first step, the electrons and holes are separated between the TiO2 and CdS interface. The photoelectron moves from the TiO2 to the CNT surface and then Pt catalyst. The Pt nanoparticles work as cocatalysts due to their lower Fermi level of CNTs as compared to TiO2. Both Pt and CNTs are promising photocatalysts and can improve the photo-catalytic H2 evaluation of CdS/TiO2. Preferably, using 0.4 and 4 wt.% of Pt and CNTs, respectively, enhances the photocatalytic performance by 50%. A graphene/MoS2/TiO2(GMST) photocomposite was prepared hydrothermally using Na2MoO4, H2NCSNH2, and graphene oxide precursors at 210 °C and utilized as an efficient material for photocatalytic H2 production [78]. It was exposed by using ethanol as a sacrificial agent and TiO2 as the photocatalyst while graphene and MoS2 functioned as cocatalysts. The GMST photocatalyst exhibited a high rate (~165.3 mol·h−1) of H2generation at 5.0 and 0.5 wt.% of graphene and MoS2, respectively, having 9.7% quantum efficiency (QE) at 365 nm [78]. A hybrid rGO/ZnxCd1−xS photocatalyst prepared first via coprecipitation and then a hydrothermal reduction approach elevates the cocatalytic performance for H2 production [79]. A high rate of H2 evaluation (~1824 mol·h−1g−1) was observed for this optimized photocatalyst using 0.25 wt.% of rGO, which has 23.4% apparent QE at 420 nm under solar irradiation. Liu et al. integrated NH3-treated CDs into g-C3N4 by heating CDs and urea for 3 h at 550 °C, where the carbon dots were first synthesized electrochemically and then hydrothermally treated with ammonia. This g-C3N4/CDs hybrid photocatalyst was found to be efficient in WS under solar light for the evolution of H2 and O2. A maximum QE of 16% was achieved using a g-C3N4/CD catalyst at 420 ± 20 nm. Actually, the photocatalytic action of g-C3N4 at the first stage caused water to be split into H2O2 and H2, then, at the subsequent stage, the carbon dot-catalyzed breakdown of H2O2 into H2O and O2.

3.3. Photosensitization

Carbon compounds with semiconducting or dye-like characteristics can function as a photosensitizer in some cases, enhancing the photoresponse of a broad bandgap of the semiconductor photocatalyst by inducing additional photogenerated electrons [80,81,82]. The emission of photoelectrons from the material can overlap with semiconductor absorption and are responsible for the transfer of the resonance energy from the carbon material to the coupled semiconductor. As the result, the LUMO of the carbon material becomes more negative in comparison to the coupled semiconductor’s CB. Wang et al. [83] prepared a graphene/ZnS (G-ZS) photocatalyst via the hydrothermal method for photocatalytic H2 evolution under visible light. A graphene-supported ZnS photocatalyst evaluated 7.42 mol·h−1g−1 hydrogen at 0.1 wt.% graphene, which was found to be 8 times that of the pristine ZnS. Since visible light cannot excite ZnS, the photoelectron most likely originates from graphene and migrates to the CB of ZnS [80]. The rGO/Pt/TiO2 photocatalyst is an excellent photosensitizer and evaluates H2 at a rate of 11.24 mmol·h−1g−1. Carbon dots are excellent candidates for use as photosensitizers in photocatalytic reactions due to their high photo-absorption and photoluminescence properties. For instance, Martindale et al. [84] fabricated a carbon dot (photosensitizer)-based Ni-bis- (diphosphine) (NiP) photocatalyst for H2 generation under visible light. To produce this photocatalyst, 30 nmol of NiP and 2.2 nmol of CD are used in 0.1M EDTA solution with pH 6; a photocatalyst g-CQDs/NiP produces H2 at 398 mol·h−1g−1. The apparent QE at 360 ± 10 nm was observed ∼1.4%. Carbon dots can convert near-infrared photons for semiconductor photocatalysis due to their photoluminescence up-conversion property [82,85]. Hydrogenated TiO2(H-TiO2)-based photocatalysts were prepared with carbon quantum dots (CQDs) under bath reflux. This photocatalyst is UV-visible-NIR compatible and showed remarkable photocatalytic H2 production capability under the illumination of a 300 W Xe arc lamp. For this photocatalytic system, Pt was used as the cocatalyst and methanol as the sacrificial agent across a wide spectral range. The higher H2 generation rate of 7.42 mmol·h−1g−1 was observed in comparison to H-TiO2 nanobelts (6.01 mmol·h−1g−1) [85].

3.4. Photocatalyst

Carbon-based nanomaterials are effective photocatalysts for H2generation, according to theory and experiment [86,87,88], owing to the semiconducting properties of CDs, CNTs, graphene, GO, rGO, and C60. Such a nanocarbon of the semiconductor sort can have a greater number of negatively charged LUMO sites than H+/H2 RP. Reduced graphene oxide rGO is a commonly used photocatalyst [89,90], as its CB minimum consists of anti-bonding π* orbital; at pH = 0, it has a potential of −0.52 eV [40]. Density functional theory (DFT) studies explain the electronic structure of graphene oxide by alternating the relative ratio of -OH and epoxy groups present on the surface. The resulting electronic structure shows the photocatalytic hydrogen, and oxygen evaluation reaction occurs [91]. Teng’s group reported that GO synthesized by a modified Hummers’ method had an apparent direct Eg value of 3.3–4.3 eV and an indirect Eg of 2.4–3.0 eV. Graphene oxide (GO) is comprised of graphene molecules of different oxidation stats and can produce H2 under UV/Visible illuminations. In a 20% methanol aqueous solution with 0.5g GO and no cocatalyst, mercury lamp irradiation yielded 17,000 mmol·h−1 H2 [88]. Meng et al. [90] prepared a p-MoS2/n-rGO photocatalyst with p-type MoS2 deposited on the surface of n-type rGO. Under solar irradiation and ethanol as sacrificial agents, this photocatalyst exhibited higher H2 evaluation activity than bare MoS2 and MoS2/rGO. This is because, as shown by the photoelectrochemical experiment, the p-MoS2/n-rGO junctions are very good at separating charges. Zhu’s team compared H2 production activity for neat CDs and CD-based composite materials [87,92,93]. An H2 evaluation at a 423.7 mol·h−1g−1 was achieved using carbon dots in pure water without needing a cocatalyst. Carbon dots were created hydrothermally from MWCNTs oxide [87]. In methanol sacrificial agent and Pt cocatalyst, a hybrid carbon nanodot/WO3 photocatalyst produces H2 at 1330 mol·h−1g−1 under xenon lamp irradiation [92].

3.5. Band Gap Narrowing Effect

Chemical bonds can form when semiconductor photocatalysts and carbon compounds make strong contacts (e.g., metal O C bonds). Chemical bonding reduces photocatalyst band gaps, increasing H2 production [94,95]. Ye et al. [96] reported the hydrothermal fabrication of graphene/CdS (0.01:1) and CNTs/CdS (0.05:1) hybrid materials with the photocatalytic H2 evaluation of and 52 mol·h−1, respectively. The addition of graphene or CNTs into CdS resulted in the narrowing of the Eg values of these hybrid photocatalysts and consequently led to superior photocatalytic H2 generation as compared to pristine CdS. This was in addition to the benefit of more effective charge separation. Bi2WO6 is often not used for water-splitting H2 generation, due to its smaller RP and comparatively less negative CB. However, coupling Bi2WO6 nanosheets with graphene, the CB of Bi2WO6 becomes more negative, and the feasibility of photocatalytic H2 evaluation increases [97]. In a typical experiment [98], Bi2WO6 nanoparticles were produced via sonication onto the graphene’s sheets using GO, HNO3, (NH4)10W12O41, and Bi(NO3)3·5H2O, followed by calcination at 450°C for 3h in an inert environment. Raman and XPS research validated the chemistry between Bi2WO6 and graphene. After coupling with graphene, Mott–Schottky calculations showed that the RP of CB of Bi2WO6 increased from +0.09 V to −0.30 V as compared to the standard hydrogen electrode. As a result, 0.03 g of Bi2WO6/graphene photocatalyst was used to produce 159.2 mol·h−1 of H2at 420 nm in an aqueous solution of methanol.
In conclusion, in several aspects, carbon materials can significantly enhance the H2 reaction rate compared to semiconductor photocatalysts. The inclusion of various carbonaceous materials coupled with semiconductor photocatalysts can result in a variety of proficient effects including structural support for improved structure constancy, electron collection, the reduction of the recombination rate of photo-stimulated charge pairs, photocatalyst, Eg narrowing outcome cocatalyst, and photosensitization. It is essential to remember that certain carbon materials can potentially play many functions throughout the entire photocatalytic process. Moreover, due to some limitations imposed by its structure and properties, a particular type of carbon material might not be able to fulfill all the functions. The potential characteristics of various CNMs are summarized in Table 2.

4. Carbon-Based Nanomaterials for Photocatalytic H2 Evaluation

4.1. Graphene-Based Composite Materials

Graphene is an allotropic form of carbon composed of monoatomic thick layers organized in a honeycomb pattern [99,100]. Its exceptional thermos-mechanical, electrical, catalytic, and optical features led to its use in different practical potentials [101]. Graphene is a potential candidate for photocatalysis applications owing to its large surface area and exceptionally high electron transference capability [102]. Graphene can be prepared differently, but frequently used methods include chemical vapor deposition (CVD), chemical and electrochemical exfoliation, and graphene oxide reduction [103,104]. Graphene oxide is an essential derivative of graphene with electrical and electronic properties. A pure graphite crystal structure must be oxidized to produce graphene oxide [105]. Graphene oxide sheets have several oxygenated functional groups on their surface which display distinct features from those of pristine graphene [106]. In addition, the average production cost of GO is less than that of G and numerous other nanomaterials. According to some studies and reports, the self-agglomeration of individual graphene-based photocatalysts prevents them from ever reaching their full potential in terms of efficiency [107].
Incorporating other nanomaterials into graphene layers is a promising technique to improve graphene layer exfoliation and characteristics [108]. These extraordinary features are particularly advantageous for fabricating photocatalytic nanocomposites based on graphene. Graphene-based hybrid nanocomposites have been identified as an essential material for O2 and H2 evaluation reactions [109]. The first principal calculations comprehensively explain the mechanism of PWS. The process begins with photoexcited electrons entering the graphene’s LUMO and transporting the surface via π* orbitals, resulting in the most significant possible charge separation. Carbon materials are excellent electron acceptors and transit channels for limiting photoinduced electron-hole pairs and improving the photocatalytic H2 reaction rate [110].
Metal-doped graphene materials possess excellent photocatalytic capabilities due to maximum charge separation between VB and CB. TiO2 is the most researched photocatalytic material on account of its thermodynamically stability, high oxidizing power, cost-effectiveness, and comparatively low toxicity [111]. The two most common phases of TiO2 are rutile and anatase, with the Eg values of 3.2 and 3.0 eV, respectively. The UV light irradiation at wavelengths lower than 380 nm, which makes up around 1/20th of the entire solar spectrum on the Earth’s surface, can only excite TiO2 [112]. The integration of TiO2 photocatalysts onto graphene is drawing interest. It offers the chance to address all three methods of photocatalytic development simultaneously: enhanced contaminant adsorptivity, simple charge transit separation, and expanded light absorption range [113]. A G/TiO2 composite photocatalyst with 5% graphene produced by the hydrothermal method, upon irradiation with UV-visible light without a cocatalyst, generates hydrogen at 86 mmol·h−1g−1 [114]. Nitrogen-doped graphene N/G is getting more attention because its physical, chemical, biological, and material science properties are much better than those of pure graphene [115]. Chemical vapor deposition is the most frequently used technique for the fabrication of N/doped graphene. In the CVD method, N and graphene precursors are utilized, and ammonia is a widely used source for N precursors [116,117]. When a neodymium-doped yttrium aluminum garnet (ND-YAG) laser shines on nitrogen-doped graphene in a methanol sacrificial agent, hydrogen produces at 6 mmol·h−1. The photocatalytic efficiency of a nitrogen-doped semiconductor is increased because it has both n- and p-type conductivities [118,119]. Cu2O is a promising photocatalyst because it is cheap, non-toxic, and has a narrow 2.2 eV band gap. Still, the photogenerated electrons and holes in Cu2O are easily recombined, which is a critical issue regarding redox reaction progress. Graphene/Cu2O hybrid material prepared by the CVD method also displays excellent semiconducting properties and evaluates hydrogen efficiently [120]. Cu2O/doped graphene composite produces hydrogen at a rate of 19.5 mmol·h−1 under visible light without needing a cocatalyst or sacrificial agent [121]. Park et al. synthesized Pt-TiO2, GO-TiO2, and a ternary hybrid of GO with Pt and TiO2 for PWS hydrogen generation under UV irradiation. Also, they found that the combination of GO and Pt on TiO2 could improve the H2 activity, while GO acted as a cocatalyst [122]. Xiang et al. employed microwave-assisted hydrothermal treatment to develop TiO2 nanosheets modified by GO, and the prepared hybrid material demonstrated improved photocatalytic H2 generation in the presence of methanol (sacrificial agent). Surprisingly, in the absence of the Pt cocatalyst, the H2 generation rate climbs 736 mmol·h−1g−1 with 3.1% quantum efficiency, exhibiting 41 times more performance than individual TiO2 nanosheets [123]. The hydrogen efficiency of a binary and ternary hybrid composite (TiO2/rGO, TiO2/Pt, and TiO2/rGO/Pt) is evaluated based on the charge-carrying capacity of reduced graphene oxide (rGO) and the efficiency of platinum nanoparticles as electron traps for photogenerated electrons. Compared to bare TiO2, adding rGO to TiO2 practically doubles the hydrogen generation rate (1.95%), and the TiO2/Pt photocatalytic system performed best, with a 15.26 factor increase in hydrogen production compared to TiO2 [124]. Specific to these materials is their ability to efficiently absorb visible light, due to their narrow bandgap and CB edge being more negative as compared to the RP of hydrogen positive ions. Pure CdS particles, on the other hand, still have several issues limiting their ability to produce H2 [125,126]. Using a solvothermal process to synthesize graphene/CdS clusters, cadmium disulfide can be spread out well, making the photocatalyst’s surface area bigger. Hydrogen was evolved at a rate of 70 mol·h−1 using a xenon lamp irradiation source, graphene as a cocatalyst, and 0.1 M and 0.05 M of Na2S and Na2SO3, respectively, as sacrificial agents [96]. Another important n-type semiconductor photocatalyst, Bismuth tungstate (Bi2WO6) with a narrow Eg of 2.69 eV, could have ability for absorbing photons from visible light. It has been much used as a photocatalyst to break down organic pollutants [127,128]. Because of its lower CB value, Bi2WO6 is an auspicious candidate for hydrogen generation from WS. When Bi2WO6 and graphene are combined by the electrodeposition method, the latter’s CB shifts to a more negative state, which positions the former as the superior photocatalyst. Under the influence of ultraviolet light, hydrogen can be produced photocatalytically in the presence of methanol at a rate of 159.2 µmol·h−1 [98]. In recent times, MoS2 has gained widespread interest as an effective cocatalyst for the hydrogen evolution process. A nanocomposite of rGO/CdS was prepared via the solvothermal approach, employing DMSO as a reducing agent and S2− source as well [70]. (NH4)2MoS4 was used as a precursor of MoS2, and was selectively photo-deposited on the surface of rGO by changing the pH of the solution. There have been no previous studies on the controlled photo-deposition of MoS2 on the photocatalyst’s surface for H2 generation [69]. For PWS, a graphene/MoS2/TiO2 hydrothermally produced composite is employed. The effectiveness of the combined photocatalytic process is improved by the use of both graphene and MoS2. With a xenon arc lamp, graphene/MoS2/TiO2 generates hydrogen at a rate of 165.3 µmol·h−1g−1 in aqueous methanol [78]. GO/ZnCl2/Na2S demonstrates photocatalytic efficiency that is eight times greater compared to pristine GO. Without a sacrificial agent, this combination generates hydrogen having a rate of 7.42 mol·h−1 when exposed to visible light [83].
ZnxCd1−xS is a phenomenal material with photosensitive and photocatalytic properties. The heterojunction of graphene with Zn0.8Cd0.2S synthesized by the co-precipitation hydrothermal process displays photocatalytic hydrogen production [129]. Under the influence of solar irradiation, it can produce hydrogen at a rate of ~1824 µmol·h−1 without any cocatalyst, while sodium sulfide and sulfates are provided as sacrificial agents [78]. Ternary photocatalysts (NiS/ZnxCd1−xS/rGO) are synthesized having enhanced photocatalytic H2 production efficacy under simulated solar radiation. One of its key features is this photocatalyst’s co-loading of NiS and rGO on NiS/ZnxCd1−xS. rGO can provide an active reduction site for synthesizing H2 and behave as an electron accumulator and transporter. NiS can assemble photoexcited holes between NiS and ZnxCd1−xS nanostructures because of nanoscale p-n junctions. Photogenerated electrons and holes are easily separated, leading to strong photocatalytic H2 generation. Without a cocatalyst and sacrificial agent, an rGO/NiS/Zn0.5Cd0.5S photocatalyst synthesized via the co-precipitation method and hydrothermal treatment generates hydrogen at 375.5 mol·h−1 [130]. Lin et al. synthesized a ternary photocatalyst UiO-66/CdS/rGO by photodeposition, with a 13.8 times higher photocatalytic hydrogen production rate compared to pure CdS, and even demonstrated significant advantages over the commendably ideal composite material P25/CdS/rGO [131]. rGO was used to stabilize the ultra-thin Zn0.5Cd0.5S nanorods with an increased photocatalytic H2 evolution of 1.12 mmol·h−1. Compared to pure CdS particles, this rate was 4.87 times higher. TiO2 doped with 5 wt.% graphene exhibits a hydrogen production rate of 8.6 mmol·h−1, which is found to be 1.9 times more than the neat P25. The hydrogen evolution rate of 40.9 mmol·h−1 under visible illumination was observed for the ZnIn2S4 photocatalyst with 1 wt.% loading of rGO using lactic acid (sacrificial agent). However, pure ZnIn2S4 has the rate of only 9.5 mmol·h−1. With an ideal concentrations of rGO (0.25 wt.%) and NiS (3 mol%), the fabricated ternary hybrid photocatalyst displayed the maximum rate of H2 production (375.7 mmol·h−1) [132,133]. A tabularized summary of graphene-based photocatalysts for H2 production is given in Table 3.

4.2. Carbon Nanotubes (CNTs)

Iijima discovered carbon nanotubes for the first time in 1991 [134,135]. Among all the allotropes of carbon, the most promising and investigated allotropes are CNTs, exhibiting sp2 hybridization. Few materials in nanotechnology are as unique as CNTs [136]. These are renowned for their electric conductivity and chemical stability and have been used as one of the most promising catalytic support materials [137,138,139,140,141,142]. Carbon nanotubes are classified as either single-walled or multi-walled CNTs, based on the number of graphitic layers present [143,144]. Several methods for manufacturing carbon nanotubes have been devised, including arc discharge, CVD from hydrocarbons, and laser ablation, etc. Arc discharge is one of the primeval processes of synthesizing CNTs [145]. The arc discharge method uses the thermal dispersion of graphite electrodes in plasma. Chemical vapor deposition uses hydrocarbon gases to develop single- and multi-walled carbon nanotubes [146,147,148]. Carbon nanotubes are promising catalysis materials and can be used as catalysts, additives, or support. A number of metals and their oxides are found to be excellent catalysts [149,150] for the generation of hydrogen on the surface of CNTs, synthetic fuel [151], as well as the intermediates that are used in the production of polymers and artificial fibers [152,153,154]. Carbon nanotubes’ poor dispersion in solvents results from their low chemical compatibility, exacerbated by their poor van der Waals interactions and hydrophobicity [155,156]. However, because of their vast functionalization capabilities, they are more soluble in a wide range of solvents [157].
Regarding photocatalysis, CNTs are attractive carbon semiconductors due to their unique electrical properties, such as high conductivity and electroluminescence, as well as their vast surface area and high tensile strength [158,159]. Coupling CNTs with other catalytic materials can improve their photocatalytic activity. The solvothermal decomposition of a single-source molecular precursor in the presence of ethylene glycol resulted in a CNTs/CdS composite [160]. CdS is an advanced semiconducting substance that, in its purest state, can evaluate hydrogen at a rate of 14.5 µmol·h−1 g−1 with 0.1 M Na2S and 0.05 M Na2SO3 used as a sacrificial agent under a xenon lamp. The CdS/CNTs composite material improves hydrogen photocatalytic activity and yields 52 µmol h−1 g−1 of hydrogen under identical conditions [96]. Iron oxide nanoparticles have excellent adsorption, biocompatibility, and electrocatalytic activity. Although iron oxide nanoparticles have several advantages, they are limited in electrochemical sensors due to poor dispersion and high levels of aggregation [161,162]. Multi-walled carbon nanotubes/iron oxide nanocomposites MWCNTs/Fe2O3 can be prepared by the in situ flame synthesis method and are an efficient photocatalyst for hydrogen synthesis [163]. The photoelectrochemical H2 efficacy of MWCNTs/Fe2O3 nanocomposites is 85 L h−1 m−2 [164]. TiO2 absorbs only a little percentage of solar UV radiation and endures photogenerated charge carrier recombination. TiO2 has been modified with carbonaceous materials (Figure 3), absorbing more solar energy and slowing down the electron-hole recombination process [165]. Compared to a pure TiO2 semiconductor, the photocatalytic material consists of functionalized carbon nanotubes and titanium dioxide (TiO2/FCNTs) prepared by the arc discharge method and shows significantly increased hydrogen production at a rate of 2134 mmol·h−1 g−1 under solar irradiation and glycerol as a sacrificial agent. [63,166].
Kang et al. reported the hydrothermal synthesis of the CuO/NiO photocatalyst with CNTs as a structural support. Herein, the CuO/NiO nanomaterial provides active reaction sites, Eosin Y acts as a sensitizer, and triethanolamine (TEOA) is utilized as a sacrifice reagent for photocatalytic H2 evolution under visible light. A rate of H2 evolution of ~1.0 mmol·h−1 g−1 was achieved under optimum conditions [167]. Surface-functionalized carbon nanotube-cadmium sulfide SF/CNTs/CdS composites displayed higher photocatalytic H2 production activities when exposed to solar irradiation than pure CdS nanorods (2.5 mmol·h−1 g−1). The composite material SF/CNTs/CdS evaluates H2 at a rate of 6.1 mmol·h−1 g−1, when platinum is utilized as a cocatalyst, and solar light is used as the radiation source [168]. Another phenomenal nano-semiconducting material, zinc ferrite ZnFe2O4 coupled with CNTs (CNTs/ZnFe2O4), produces hydrogen at 18 mmol·h−1g−1 under a mercury lamp irradiation source using aqueous methanol sacrificial agent [169]. Compared to single and two-component systems, anisotropic ternary core-shell Zn0.5Cd0.5S/TiO2/MWCNTs photocatalysts exhibit significantly higher photocatalytic activity. Under solar irradiation, composite material Zn0.5Cd0.5S/TiO2/MWCNTs generates hydrogen at a rate of 21.9 mmol·h−1g−1without the need for a sacrificial agent or catalyst [170]. Alternatively, metal sulfide, ZnS was extensively considered for catalytic H2 generation. However, its wide Eg restricts the light response in the visible range. To move this Eg into the visible region, more effort has been put into doping ZnS and creating solid solutions of Cd1−xZnxS. Additionally, it has been demonstrated that sulfide solid solutions such as Cd1−xZnxS are effective photocatalysts for H2 production from WS under visible range [171,172,173]. The CNTs/Cd0.1Zn0.9S photocatalyst is prepared by a simple hydrothermal method, and is an excellent material for H2 evaluation in the visible spectrum [80]. As with the pure Cd0.1Zn0.9S semiconductor, the CNTs/Cd0.1Zn0.9S composite evaluates hydrogen with a 78.2 mmol·h−1g−1 rate of reaction in the absence of a cocatalyst and sacrificial agent [79]. A well-assembled layer of Zn0.83Cd0.17S nanostructures with ~100 nm diameter was fabricated onto the CNT surface. The Zn0.83Cd0.17S/CNTs photocatalyst has a higher interfacial area owing to the excellent dispersion of nanotubes. As the potential of the CB of Zn0.83Cd0.17S is more negative as compared to the nanotubes’ Fermi level, the photoexcited electrons will travel from this material to the surface of the CNTs. The benefit described above can thus effectively separate photo-stimulated charge transporters at the interface of nanotubes and Zn0.83Cd0.17. The CNT-coupled nanocomposite’s band gap was also tuned to a lower value when compared to neat Zn0.83Cd0.17S. H2 was produced 1.5 times as quickly by the CNTs-incorporated nanocomposite with a photocatalytic rate of~6.03 mmol·h−1 g−1 [174]. A solvothermal technique was used to create a Zn0.83Cd0.17S nanocomposite modified with carbon nanotubes. At the optimal amount (0.25 wt.%) of CNTs, the prepared Zn0.83Cd0.17S/CNTs nanocomposites exhibit the maximum photocatalytic performance when exposed to a Xe lamp, with a 5.41 mmol·h−1 g−1 reaction rate [175]. Using irradiation ranging from 300 to 800 nm, the Zn0.83Cd0.17S/CNTs nanocomposite produced 1.5 times as much H2 per hour as pure Zn0.83Cd0.17S. The photocatalytic H2 generation rate of the CNTs/Cd0.1Zn0.9S composite was 78.2 mmol·h−1 at 420 nm. Only 24.1 mmol·h−1 of pure Cd0.1Zn0.9S was produced [176,177]. Also, a ternary photocatalyst Co coupled with N-doped CNTs was prepared by heating cobalt acetate and melamine together at 800 °C. This composite material, Co/N-CNTs, produces H2 at a 14.7 mmol·h−1 g−1 reaction rate [178]. A tabularized summary of CNT-based photocatalysts for H2 generation is given in Table 4.

4.3. Graphitic Carbon Nitride (g-C3N4)

Over the last decade, g-C3N4has been intensively studied as a promising semiconductor. This material has mechanical properties similar to diamond due to its cyamelurine (C6N7)-consisted units and strong covalent interactions between C and N atoms. In addition, it is thermally, chemically, and photochemically stable too [180,181]. g-C3N4 is a conventional semiconducting material with an Eg value of 2.7 eV. Its semi-conductive properties are utilized in photocatalytic HER and CO2 reduction reactions as well [182,183,184]. g-C3N4 is nontoxic and has a high level of biocompatibility, so it has minimal adverse environmental impacts [185] and can be cost-effectively fabricated on the massive scale [186]. It has various benefits for photocatalytic HER over other standard photocatalysts, including a strong light absorption capacity, greater surface area, exceptional superficial features, superior elemental stability, and cost effectiveness [187,188,189,190]. For the production of g-C3N4 heterojunctions, various procedures have been used due to the material’s extraordinary structural stability. There are multiple methods to accomplish this, including ion exchange, electrodeposition, co-precipitation, electrospinning, and the high-temperature solid-phase process [191,192]. Some inexpensive and commonly used nitrogen precursors are urea, cyanamide, dicyandiamide, and melamine as building blocks in the thermal condensation process to make g-C3N4 [42,193,194]. However, the photocatalytic performance of g- C3N4 is less than other photocatalysts despite its use with different photocatalysts. The recombination rate of e/h+ for g-C3N4 is very high, which limits its capacity to reduce H2 [65]. It can absorb light only at 460 nm, limiting its ability to gather considerable amounts of radiation. The covalently bonded hydrogen over the surface of g-C3N4 is also accountable for the lowered rate of H2 production in the 0.1–4.0 mmol·h−1g−1 range [159,195]. Hydrogen generation from aqueous environments, the photocatalytic reduction of CO2 and other organic/inorganic pollutants, NOx degradation, and the detection of O2 activation sites have all relied heavily on g-C3N4 [196,197,198,199,200,201]. g-C3N4 as a comparatively innovative catalyst in the photocatalysis development process utilized to degrade water was firstly reported by Wang et al. [202,203]. However, in g-C3N4 there is an insufficient transference of photoexcited e(s) and h+(s), and the strong recombination of e/h+ pairs restricts the application of the photocatalyst [204,205]. It is possible to overcome it by the doping of the g-C3N4 structures with various metals and nonmetals [206,207] and heterojunction photocatalysts formed by associating with metal and semiconductor nanoparticles [208,209].
MoS2/g-C3N4 composite heterojunctions are suitable candidates for photocatalytic H2 evaluation reactions [210,211,212]. MoS2 has recently become a popular photocatalyst and electrocatalyst because of its low bandgap, which matches the solar spectrum, and its ability to resist photo-corrosion. Therefore, MoS2 is generally stated to be a semiconductor cocatalyst. Lu et al. used the ultrasonic agitation method to make g-C3N4/MoS2 to avoid the contamination of organic pollutants such as methyl orange (MO), ciprofloxacin (CPFX), and tetracycline (TCY). Impregnating MoS2 into mesoporous g-C3N4 resulted in a high-performance heterojunction for producing H2 [213,214,215]. The effectiveness of TiO2 as a photocatalyst has led many researchers to rely solely on it for their PWS experiments. Combining TiO2 with another semiconductor material increases its efficiency by a factor of a hundred or more. The thermal condensation of the TiO2/g-C3N4 nanocomposite with TEOA sacrificial agent produces 286 mmol·h−1g−1 of H2 [216], whereas the hydrogen evaluation rate of pure g-C3N4 is 54.13 mmol·h−1g−1 [217]. Ag-loaded g-C3N4 has recently gained popularity as a photocatalyst in many fields. Ag quantum dots/g-C3N4 composite was fabricated via a one-step mix-calcination technique by Chen et al. for PWS. The maximum HER efficiency was increased by 4.6 times in comparison to g-C3N4 under a visible range [218]. Under visible light irradiation, Sun et al. used binary photocatalysts of Ag/g-C3N4nanostructures to remove NOx from the air [219]. Graphene has gained significant consideration in nanomaterials for its distinct electronic characteristics, greater surface area, and elastic structure. The electronic structure of g-C3N4 can be efficiently altered by the flawless in-plane combination of g-C3N4 and graphene via continuous π conjugation interactions, where graphene’s exceptional electron capability contributes to unconstrained in-plane e/h+ segregation and transference [220,221]. G/g-C3N4 composite evaluates hydrogen at 0.451 mmol·h−1 g−1 [222]. Rh has been extensively studied as an active component for water hydrolysis among metals. It is widely known as a catalyst support and significantly impacts the catalytic activity of supported metal nanoparticles (NPs). Here, g-C3N4 was made easily and in large amounts by burning urea in the air. The resultant material was used to help stabilize Rh-NPs so they could be used to turn water into hydrogen. The g-C3N4/Rh nanocomposite produces hydrogen at 60 µmol h−1 g−1 [17]. Metal-free catalysts can be retained by doping semiconductors with nonmetal impurities, which can then be used to improve their photocatalytic properties by altering the semiconductors’ electronic structures and morphology [223]. In the periodic table, boron, carbon, and nitrogen all share the same atomic size, making it possible to introduce B atoms into the framework of g-C3N4 [224]. Boron-doped g-C3N4 nanosheets can be synthesized using microwave heating, a fast and green method [225]. The B/g-C3N4 composite generates hydrogen at 94 µmol·h−1g−1 without cocatalyst and sacrificial agent [226]. Carbon nanostructures that have been doped with N atoms have been found to have improved electron transfer and a wider range of visible light adsorption properties [227,228]. The nitrogen-doped graphitic carbon nitride N/g-C3N4 photocatalyst prepared by the hydrothermal method displays enhanced photocatalytic hydrogen production at 44.28 µmol h−1 g−1 [3].
Hydrogen was produced more efficiently in photocatalytic systems using nickel compounds such as nickel oxide [229], nickel sulfides [230], nickel phosphide [231], and nickel pyrophosphate [232], all of which were loaded onto carbon nitride as photocatalysts for the decomposition of water [233]. A photocatalyst synthesized by the thermal polymerization of amorphous nickel pyrophosphate (a-Ni2P2O7) supported onto the g-C3N4 generates hydrogen at 207 µmol h−1 g−1 [232]. Tungsten oxide WO3 has received much attention because it absorbs visible light, is chemically stable, and moves electrons well [234]. In recent years, WO3 has been used in many different areas, such as photocatalysts [235], gas sensors [236], electrochromic displays [237], etc. The VB and CB positions of g-C3N4 are higher as compared to neat WO3, so their band structures are similar [238]. A hydrothermally produced tungsten oxide composite with g-C3N4WO3/g-C3N4 has a high H2 evaluation capacity of 963 mmol·h−1g−1 in the existence of TEOA sacrificial agent [239]. In the same way, WO3/g-C3N4/Ni(OH)x, made in two steps (calcination and photodeposition) using TEOA as a sacrificial agent, can produce 576 mmol·h−1g−1 of hydrogen [240]. Cheng et al. produced 400 mmol·h−1g−1 of H2 by thermally treating WO3/g-C3N4 nanomaterial with TEOA sacrificial agent [241]. A HER of 1853 mmol·h−1g−1 was achieved using the same graphitic carbon-nitride heterojunction with tungsten oxide after adjusting the synthesis conditions [242]. Under visible light irradiation, the optimal heterostructures of g-N3N4 with 10wt. % loadings of nanodiamonds (NDs) exhibit HER of 59.1 mmol·h−1 [243]. The boron-doped carbon nanodiamonds with g-C3N4 heterojunction (B/CNDs/g-C3N4) have an optimum HER and apparent QE of 96.3 mmol·h−1 and 6.91%, respectively, at 420 nm, which is approximately 5 and 2 times greater than the pure g-C3N4 nanosheets, 18.2 mmol·h−1 and 3.92%, respectively [244]. Furthermore, loading 8 percent Ni3C@Ni NPs with a nickel shell-layer thickness of 15 nm on g-C3N4 resulted in the maximum HER of 11.28 µmol h−1 compared to the binary g-C3N4/Pt nanostructure. A higher HER over g-C3N4 proved that Ni3C@Ni mounted a more effective cocatalyst than the well-known noble metal Pt [245]. A tabularized summary of g-C3N4-based photocatalysts for H2 generation is given in Table 5.

4.4. Fullerenes (C60)

Buckminsterfullerene C60, discovered in 1985, is considered the most representative zero-dimensional carbon substance. Fullerenes are CNMs with varying structural forms (hollow sphere, ellipsoid, etc.) [247]. Fullerenes are good building blocks for supramolecular assemblies and functional micro- and nanostructures due to their well-defined structures, high electron-receiving ability, and electron-transport capacity. The orderly structure of fullerene molecules can potentially affect or improve fullerene micro/nanostructure (FMNSs) properties [248,249,250]. Various techniques, such as arc discharge, laser ablation, chemical vapor deposition (CVD), the laser irradiation of polyaromatic hydrocarbons (PAHs), and the resistive arc heating of graphite, can all be utilized in the synthesis of fullerenes. In the beginning, fullerene was produced by laser-vaporizing carbon in an atmosphere devoid of oxygen, but this method only produced a small amount of the material [31,251]. For the large-scale production of fullerene C60, an electrical arc is used to heat graphite while a laser irradiates polycyclic aromatic hydrocarbons. A supersonic expansion nozzle and a pulsed laser can be used to create fullerene on graphene (G) coupled with helium gas. Carbon is vaporized off a spinning graphite sheet using a focused pulsed laser and transferred to a high-density helium stream. Fullerenes and their derivatives are used in organic photovoltaics [252], antioxidants [253], biopharmaceuticals [31], polymer additives [254], catalysts [255], water purification protection catalysts [256], portable power devices [257], and automotive components [258]. In addition, they are ideal for storing medical and gas supplies. Many advantages come with fullerenes. However, their dispersion and solubility in solution are poor. Fullerene derivatives’ photocatalytic characteristics have been extensively explored as a result. Bai et al. created a photocatalyst to remove organic dyes by replacing the fullerene with a hydroxyl group, which was then combined with titanium dioxide [259]. In recent years, scientists have found encouraging results from fullerene-based composite nano-photocatalytic materials in removing organic contaminants, photocatalytic hydrogen synthesis, photovoltaic cells, photocatalytic organic synthesis, and sterilization [259,260,261,262,263]. Several fullerene/semiconductors have been developed for photocatalytic wastewater treatment and WS, including C60/TiO2, C60/ZnO, C60/CdS, C60/g-C3N4, and others. Fullerene/semiconductor photocatalysts can be produced using various synthetic techniques, such as simple adsorption, hydrothermal/solvothermal synthesis, the sol-gel method, and mechanical ball-milling [264]. Chai et al. synthesized a TiO2/C60/dCNTs photocatalyst, and when it was exposed to UV light, it was found to improve photocatalytic H2 production. It produced H2 at a rate of 651 mol h−1 using 5 wt% loading of C60. That is 2.9 times as much as bare TiO2 [54]. Regarding dye degradation and H2 production, Tahir et al. have recently stated that the WO3/C60 nanocomposite has a greater photocatalytic efficiency [265]. Shahzad et al. synthesized a WO3/fullerene photocatalyst using a simple photodeposition method using 1.5% Ni3B/Ni(OH)2 as a cocatalyst for H2 production. The photocatalytic efficiency of WO3/C60/Ni3B/Ni(OH)2 in H2 generation was exceptional, reaching 1578 mol h−1 g−1 [266]. Song et al. produced a range of C60-modified Fe2O3 polymorph photocatalysts utilizing a simple adsorption technique. These as-prepared samples displayed enhanced photocatalytic efficiency for H2 production at 0.3218 mmol·h−1 g−1 under xenon lamp irradiation [267].
Although Cr2O3 nanoparticles are stable and cheap, the 3.4 eV bandgap value limits their applications in H2 production [268,269]. The high surface area of fullerene also provides more photocatalytic sites for the photocatalysts. This way, a metallic oxide modified with C60 could be an ideal system for improving photoinduced electron migration efficiency. Cr2−xFexO3-type photocatalysts are synthesized using a simple absorbing process and used in photocatalytic hydrogen evolution. The photocatalytic H2 evolution rate of 0.2205 mmol·h−1 g−1 is significantly enhanced in the as-prepared C60/Cr2−xFexO3 nanocomposites in the presence of a TEOA sacrificial agent under xenon lamp irradiation [270]. A tabularized summary of fullerene-based photocatalysts for H2 production is given in Table 6.

4.5. Carbon Quantum Dots (CQDs)

Carbon quantum dots are a novel and economical type of quasi-spherical nanocarbon particle with a diameter of less than 10 nm [274,275,276]. Carbon quantum dots, a fluorescent nanomaterial class, have recently received much attention due to their distinct optical and photodegradation features compared to organic dyes [277,278]. Researchers are looking into how CQDs can be used as catalysts. Much research and development have gone into making fluorescent nanoparticles that are non-hazardous. Top-down and bottom-up methods are used to synthesize CQDs. Top-down methods include laser ablation, arc discharge, electrochemical and plasma therapy. Examples of bottom-up techniques include pyrolysis, microwave-assisted, chemical oxidation, reverse micelle, and others. Carbon quantum dots (CQDs) are efficient photosensitizers that produce hydrogen in an aqueous solution using solar radiation [84]. Carbon quantum dots exhibited photocatalytic activity in the visible region beyond >455 nm and maintained their overall photocatalytic activity for at least one day under full solar spectrum illumination. There is no evidence to support using CQDs for photocatalytic methods that are not constrained by cost, toxicity, or scalability issues [279]. Carbon quantum dots can be used in numerous photocatalytic processes with their high specific surface area and electron storage capacity. Carbon quantum dots in photocatalysis can create more active reaction sites by improving light absorption across a more comprehensive wavelength range [45,280,281]. Carbon quantum dot/semiconductor composites formed predominantly by physical adsorption were used to study the photocatalytic degradation of organic dye pollutants in the liquid or gas phase when exposed to visible and infrared light. It is critical to the photocatalytic activity of these composites that the CQDs in these composites have a conjugated structure [282,283]. Composites containing copper and CQD were more efficient at releasing H2 than pure copper nitrate (Cu-NPs). The maximum rate of H2 evolution for a sample containing 15.6 wt% CQDs was 64 mmol·h−1g−1 [284]. There has been research into the photodegradation of MB by nanocomposite TiO2-CQDs. An enhanced photocatalytic activity is achieved by increasing electron energy in the TiO2-CQDs nanocomposite under illumination by a xenon lamp [285]. That is the first report of a novel and auspicious photocatalyst based on the surface plasmon resonance (SPR) of Ag NPs and the conversion property of CQDs to enhance the broad range of light absorption and photogenerated charge transference of g-C3N4 in photo-driven HER [276]. Yang et al. created g-C3N4 heterostructures (CCTs) by polymerizing freeze-dried urea and CQD precursor at high temperatures. During the co-condensation process, they discovered that implanted CQDs might actively induce tube assembly. That results in creating micro-regional heterostructures, facilitating the passage of photoelectron carriers. The photovoltaic properties of the CCTs were excellent compared to the photoelectrochemical properties of pure g-C3N4 and CQD-adsorbed g-C3N4 (CQD/g-C3N4) nanosheets [286]. Yu et al. described a new CQDs/ZnFe2O4 composite photocatalyst produced through a modest hydrothermal process initially. The CQDs/ZnFe2O4 (15 vol %) composite exhibits an approximately 8-fold more significant transient photocurrent response than ZnFe2O4; this shows that electron-hole pairs generated by photosynthetic processes have higher transfer and separation efficiencies [287]. For the formation of H2, Zhu’s group studied both pure CDs and composite photocatalysts based on carbon dots. Pure CDs, made hydrothermally from MWCNTs, photocatalyzed the pure water with HER of 423.7 mmol·h−1 g−1 in the absence of any cocatalyst. The Z-scheme carbon nanodots/WO3 hybrid photocatalyst produced H2 at a rate of 1330 mmol·h−1 g−1 under xenon lamp illumination, with methanol serving as a sacrificial agent and Pt serving as the cocatalyst [87,92,93]. Yu et al. reported the preparation of CQD-modified P25-TiO2 composites (CQDs/P25) via the one-step hydrothermal method. The CQDs/P25 showed superior photocatalytic H2 evolution than the pure P25 without use of any other noble metal cocatalyst under UV-Vis and visible light irradiation (l > 450 nm) [281]. The photocatalytic material 5% CDs/NiCo2O4 was created hydrothermally and then calcined. This composite photocatalytic material utilized uniformly loaded CDs to decrease the recombination of photoexcited charges and accelerate the photoinduced electron transference. In the CDs/NiCo2O4 composite (CDs, 5%), the optimum HER at normal atmospheric pressure is 62 µmol·h−1g−1 and OER is 29 µmol·h−1 g−1, which is approximately four times greater than the pristine NiCo2O4 material [288].
Carbon dot-modified TiO2NPs and nanotubes produced H2 at the rate of 75.5 mmol·h−1 g−1 and 246.1 mmol·h−1 g−1. One percent Carbon fiber CFs in the CuO/CFs/TiO2 composite photocatalyst resulted in a substantially higher photocatalytic HER of 2000 mmol·h−1 g−1 in 10% ethanol solution under a 300W xenon lamp, more than 45 times than TiO2 and 2 times more than CuO/TiO2. Under visible-light irradiation, a 0.50 wt% CFs/TiO2 photocatalyst produced 6.03 mmol·h−1 g−1 of H2 compared to 0.71 mmol·h−1 g−1 for CdS/TiO2. Photocatalysts that use CNMs as their support materials have a new lease of life thanks to single-atom technology [145]. The HER of the N-doped CDs (NCDs)-based photocatalyst (Pt/NCDs/TiO2) is 175.3 mmol·h−1 cm−2, which is 17 and 4 times more than neat TiO2 and NCDs/TiO2, respectively, for the photocatalyst created by adding a single Pt atom to NCDs and then hybridizing it with TiO2. The Pt atomic efficiency is maximized, and charge transferability is increased by the interaction of the single Pt atom with N/CDs [289].
Zinc sulfide-carbon dot ZnS/CD nanocomposites display a greater hydrogen evolution rate than the bare ZnS nanospheres, up to 3.2 times higher than that rate. The hydrogen production rate of ZnS nanospheres is 198 mmol·h−1 g−1 and increases in all the ZnS/carbon nanomaterials [15]. A tabularized summary of CQD-based photocatalysts for H2 production is given in Table 7.

5. Factors Influencing Properties of Carbon-Based Nanomaterials in Photocatalytic H2 Evaluation

Understanding the factors and problems that influence the performance of carbon-based photocatalysts for H2 evaluation reactions is critical. The photocatalyst performance of H2 production is highly dependent on the dimensions of the carbonaceous materials, the types of defects present, the number of layers, chemical functionalities, the conductivity of the carbon material and the interfacial contact area amid semiconductor and carbon materials. To maximize photocatalytic H2 generation using carbon materials, reasonable procedures are employed to regulate the carbon materials’ structural and property effects.

5.1. Doping

The doping of pristine CNMs with heteroatom can increase their activity as catalysts. By introducing defect sites, heteroatoms can change the physiochemical properties of CNMs and improve active reaction sites [292]. Doping carbon materials with heteroatom also controls their electrical conductivity, electronic structure, and metal or semiconductor-like properties [293]. Doping CNMs with a heteroatom induces a positive or negative charge in the carbon atoms near the dopants, depending on their relative placement and mismatch in electronegativity. That, in turn, causes a shift in the charge distribution in CNMs. Charge variation impacts many physical and chemical properties, including effective surface area, active reaction sites, and electrical conductivity. It can govern charge transfer on the catalytic surface and affect the CNM-based photocatalyst performance [294]. The technology that regulates and detects the atomic level, composition, and catalysis center is still a mystery, despite the progress that has been made in heteroatom doping. Nonmetallic elements are the main focus of the most recent research on heteroatom doping in CNMs for photocatalytic H2 generation. Heteroatom doping introduces defects and vacancies in nanomaterials. It is challenging to detect the specific heteroatom, vacancies, or defects that contribute to CNM catalysis [118,295]. Using a modified Hummer’s process, N-doped graphene quantum dots were fabricated by oxidizing GO in NH3 at 500 °C. The nitrogen-doped GO material is metal-free photocatalysts and exhibits p- and n-type conductivities, constructing a charge transfer route within the molecule using the Z-scheme. The p-type conductivity produced in oxygen functional groups promotes hydrogen evolution, whereas n-type conductivity promotes oxygen evolution. However, the activity was not relatively as high as that of the composite photocatalyst made of N-doped graphene oxide [119]. A facile synthesis of phosphorous-doped graphene P/G through the thermal breakdown of H2PO4 at 900 °C under an argon atmosphere for photocatalytic H2 production is reported [295]. The p-doping led to the transformation of graphene with a zero bandgap into semiconducting graphene, which displayed photocatalytic activity in both the visible and ultraviolet spectra. Triethanolamine as sacrificial agent and Pt as cocatalyst produced 282 mol h−1 g−1 H2 under UV-Vis irradiation.

5.2. Defects

The photocatalytic efficacy of the carbonaceous catalyst can be improved by using defects present on the material surface, particularly the instinct defects of the catalyst [296]. Nanomaterials contain defects on multiple scales, including point, line, plane, and volume. In addition, the point defects can be further subdivided according to their location and composition into vacancies, interstitial atoms, and impurities [297]. Realistically, it is not easy to produce defect-free CNMs. The defects in the material can affect the CNMs’ electrical and surface properties. The photocatalytic reaction is greatly influenced by the distribution of electron density and charge in CNMs due to defects. The active sites created by these defects can be used as active centers for photocatalytic reactions [298]. It is also possible that the presence of oxygen functionalities and other non-metallic moieties in CNMs lends even more credence to their use for effective H2 generation [299]. Some advanced CNMs own many instinct defects. Defects in graphyne, the new “wonder” material following graphene, alter the efficacy of its photocatalytic hydrogen evolution, although there is not enough study on this. Finding innovative, controllable approaches to address problems caused is more crucial; many incisive and exciting studies have been published; it is still tough to prepare CNMs with different defects under controlled conditions. Controlling defects may make things easier to understand their impact on photocatalytic performance. Most of the reported defective CNMs up to this point were created by annealing or etching [300,301]. Still, it is challenging to discuss the relationship between defects and photocatalytic activity in detail since these techniques cannot precisely control the defects produced.

5.3. Dimensions

The efficacy of a photocatalytic material is highly correlated with the size of the materials. Based on their size, zero-dimensional CNMs have dual photocatalytic and co-catalytic applications. For example, CQDs’ photocatalytic properties are typically related with their optical possessions of light absorption and scattering. Decreasing the size of CQDs probably increases charge recombination because e/h+ pairs are created in a compact volume [302,303]. Developing CQD-modified nanocomposite materials will limit control recombination. It is a productive approach to getting over 0D CNMs restriction. Due to their high specific surface area, the 1D CNMs photocatalytic systems have numerous advantages over the 0D CNM system. In 1D CNMs, many active centers are available for photocatalytic reactions because of their exceptional light absorption and scattering efficiency.
Additionally, high-efficiency charge transfer is possible with 1D CNMs, favorably affecting the speed of hydrogen evolution [304,305]. However, the current methods of producing and exploiting 1D material for photocatalytic H2 production involve essential modifications for practical application. The 2D CNMs possess high photochemical stability, specific surface area, exceptional mechanical and optical properties, and the potential to be used in advanced photocatalytic systems. Improving H2 generation efficiency and mass transfer performance are the biggest challenges in 2D CNM photocatalysis [9,306]. 2D CNMs possess several advantages over 0D and 1D CNMs in developing and producing efficient photocatalytic H2 systems. The 2D CNMs can shift the borders of the conduction band and valence bands’ borders and efficiently absorb solar radiation, resulting in the recurrent reflection and scattering of photons [245]. The 0D and 1D CNMs only transfer charge one way, whereas 2D CNMs transfer charge in multiple directions, which makes H2 evolution much more efficient. The photoinduced electron mobility at the photocatalyst surface is affected by the thickness of 2D CNMs, making it an important parameter to evaluate. For 2D photocatalytic systems, this is the fundamental issue.

6. Conclusions

The major challenges of the present day include climate change and the energy crisis. Researchers are looking for energy sources other than fossil fuels, because fossil fuels contribute to the production of greenhouse gases. Hydrogen is a renewable source of energy with widespread potential; however, there are challenges associated with its industrial production and storage. The photocatalytic synthesis of hydrogen from carbon materials is an interesting and potentially useful energy technology. There has been extensive research into the potential of carbon materials, including CNTs, graphene, graphene oxide, graphitic carbon nitride, C60, carbon quantum dots, etc., to outperform semiconductor photocatalysts in H2 production. Carbon-based nanomaterials require additional reliable and low-cost preparation methods for photocatalytic H2 evolution. This review explains how carbon-based composite materials improve semiconductor photocatalysts for PWS as catalyst support, an electron acceptor and transference canal booster, cocatalyst, photosensitizer, photocatalyst, and Eg narrowing effect. This review comprehensively explains how carbon-based composite materials function as a photocatalytic semiconductor for hydrogen production, the WS mechanism, and the chemistry of redox reactions. Also, how heteroatom doping, defects, and surface functionalities, etc., can influence the efficiency of carbon photocatalysts for H2 production. The challenges faced by the PWS process and future prospects are briefly discussed.

Author Contributions

Conceptualization, R.S.; A.I. and M.E.A.Z.; project administration, M.E.A.Z.; supervision and resources, R.S.; data curation and visualization, A.A.; writing—original draft preparation, M.A.R., S.A.A.-H., A.I. and S.A.; writing—review and editing, M.A.R., R.S., A.I., and M.E.A.Z.; funding acquisition, S.A.A.-H., A.I. All authors have read and agreed to the published version of the manuscript.

Funding

The authors extend their appreciation to the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University for funding this work through Research Group No. RG 21-09-77.

Data Availability Statement

Data are available in the manuscript.

Acknowledgments

Authors acknowledge the support and literature facilities provided by the Government College University Faisalabad (GCUF)-Faisalabad and UOL Sargodha Campus-Sargodha-Pakistan.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structures of different Carbon-based nanomaterials. Reprinted/adapted with permission from Ref. [31]. Copyright © 2020, Elsevier.
Figure 1. Structures of different Carbon-based nanomaterials. Reprinted/adapted with permission from Ref. [31]. Copyright © 2020, Elsevier.
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Figure 2. Photocatalytic water-splitting (PWS) mechanism.
Figure 2. Photocatalytic water-splitting (PWS) mechanism.
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Figure 3. The CNTs/TiO2 composite for hydrogen production.
Figure 3. The CNTs/TiO2 composite for hydrogen production.
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Table 1. Physical and chemical properties of CNMs.
Table 1. Physical and chemical properties of CNMs.
MaterialYoung’s ModulusThermal Conductivity (W/mk)Tensile StrengthBandGap (eV)Charge Career Mobility (cm2/V·S)
Graphene0.5–1.0 TPa5000130 GPa02 × 105
Carbon Nanotubes1–5 TPa350013–53 GPa1.11 × 105
Graphitic Carbon Nitride210–320 GPa3.5–7.625 GPa2.7-
Fullerenes61–110 GPa0.14–2.8758–71 MPa1.8-
Carbon Quantum Dots0.14–4.48 GPa0.460–69 MPa3.7310−2–10−6
Table 2. Role of carbon nanomaterials in photocatalysis.
Table 2. Role of carbon nanomaterials in photocatalysis.
Function TypeCarbon NanotubesGrapheneFullerenesGraphene OxideGraphitic Carbon NitrideCarbon Quantum Dots
Supporting material×
Photocatalyst×
Cocatalyst
Photosensitizer×
Bandgap narrowing effect×
Table 3. Graphene-based composites and H2 evaluation rate.
Table 3. Graphene-based composites and H2 evaluation rate.
Composite MaterialCocatalystSacrificial AgentIrradiation SourceRate of H2 ProductionReference
TiO2/doped graphenegraphene0.1 M Na2S and 0.04 M Na2SO3Visible light86 mmol·h−1 g−1[114]
N/doped grapheneNoneMethanolND-YAG laser6 mmol·h−1 g−1[118]
Graphene/Cu2ONoneNoneVisible light19.5 mmol·h−1 g−1[121]
Graphene/CdSgrapheneNa2S and Na2SO3Xenon lamp70 µmol h−1 g−1[96]
G/Bi2WO6NoneMethanolVisible light159.2 µmol h−1 g−1[98]
G/MoS2/TiO2MoS2EthanolXenon arc lamp165.3 µmol h−1 g−1[78]
rGO/Zn0.8Cd0.2SNoneNa2S and Na2SO3Solar irradiation1824 µmol h−1 g−1[78]
GO/ZnCl2/Na2SgrapheneNoneVisible light7.42 µmol h−1 g−1[83]
rGO/NiS/Zn0.5Cd0.5SNoneNoneSolar irradiation375.5 µmol h−1 g−1[130]
Table 4. Rate of H2 production by CNT-based photocatalysts.
Table 4. Rate of H2 production by CNT-based photocatalysts.
Composite MaterialCocatalystSacrificial AgentIrradiation Source Rate of H2 ProductionReference
CNTs/CdSNone Na2S and Na2SO3Xe lamp52 µmol h−1 g−1[96]
ZnO/CNTsPtWater-methanol (1:4)UV-visible1 µmol h−1 g−1[157]
TiO2/FCNTsNone GlycerolSolar light2134 mmol·h−1 g−1[63]
CuO/NiO/CNTsCuO/NiOTriethanolamineVisible 1 mmol·h−1 g−1[167]
Ascorbic acid/CNTs/CdSPtNone Sunlight 120.1 mmol·h−1 g−1[168]
CNTs/ZnFe2O4NoneMethanolMercury lamp18 mmol·h−1 g−1[169]
Zn0.5Cd0.5STiO2/Multi-walled CNTsNone None Solar light21.9 mmol·h−1 g−1[179]
CNTs/Cd0.1Zn0.9SNoneNoneNone78.2 µmol h−1 g−1[79]
Co/NCNTsNoneNoneVisible light14.7 mmol·h−1 g−1[178]
Zn0.83Cd0.17S/CNTsNoneS2−, SO32−UV-visible6.03 mmol·h−1 g−1[64]
Table 5. H2 production rate with reported g-C3N4-based photocatalysts.
Table 5. H2 production rate with reported g-C3N4-based photocatalysts.
Composite MaterialMorphologySynthesis MethodSacrificial AgentH2 ActivityReference
g-C3N4NanoparticlesChemical exfoliationTriethanolamine54.13 mmol·h−1 g−1[217]
N/g-C3N4NanoparticlesHydrothermal methodTriethanolamine44.28 mmol·h−1 g−1[3]
TiO2/g-C3N4NanoparticlesThermal condensationTriethanolamine286. mmol·h−1 g−1[216]
G/g-C3N4Nanoparticles-Methanol0.451 mmol·h−1 g−1[222]
Rh/g-C3N4Nanoparticles-Methanol60 mmol·h−1 g−1[17]
B/g-C3N4Nanoparticles-Triethanolamine94 mmol·h−1 g−1[226]
g-C3N4/a-Ni2P2O7)Nanoparticles Solvothermal methodTriethanolamine207 mmol·h−1 g−1[232]
WO3/g-C3N4NanoparticlesHydrothermal, calcinationTriethanolamine963 mmol·h−1 g−1[239]
WO3/g-C3N4/Ni(OH)xNanoparticlesCalcination, photodepositionTriethanolamine576 mmol·h−1 g−1[240]
WO3/g-C3N4NanoparticlesOne-pot preparationTriethanolamine400 mmol·h−1 g−1[241]
WO3/g-C3N4NanoparticlesPost-thermal treatmentTriethanolamine1853 mmol·h−1 g−1[242]
g-C3N4/WO3/BCSNanoparticlesOne-pot heat method20 vol% Triethanolamine2500 mmol·h−1 g−1[246]
Table 6. Fullerene-based materials and H2 production rate.
Table 6. Fullerene-based materials and H2 production rate.
PhotocatalystFunction of NMsLight SourceSacrificial AgentH2 Production RateReference
C60/MoS2PhotosensitizerXenon lampTriethanolamine6.89 mmol·h−1 g−1[262]
WO3/C60/Ni3B/Ni(OH)2NoneNoneNone1578 mol h−1 g−1[266]
C60/Fe2O3PhotosensitizerXenon lampTriethanolamine0.3218 mmol·h−1 g−1[267]
C60/Cr2–x/FexO3PhotosensitizerXenon lamp Triethanolamine0.2205 mmol·h−1 g−1[270]
C60/CdSPhotosensitizerXenon lamp10 vol% lactic acid and 1wt%Pt1.73 mmol·h−1 g−1[271]
C60/TiO2PhotosensitizerXenon lampTriethanolamine0.5792 mmol·h−1 g−1[272]
C60/graphene/g-C3N4PhotosensitizerLED irradiationTriethanolamine0.0946 mmol·h−1 g−1[273]
Table 7. H2 production rate of carbon quantum dot-based composites.
Table 7. H2 production rate of carbon quantum dot-based composites.
PhotocatalystCocatalystSacrificial AgentLight Source H2 ActivityReference
CQDsNoneWaterXenon lamp423.7 µmol h−1 g−1[87]
CQDs/WO3PtMethanolXenon lamp1330 mmol·h−1 g−1[92]
ZnS/CDsNoneWater Xenon lamp630.6 mmol·h−1 g−1[15]
CuO/CF/TiO2CuOEthanolXenon lamp2000 µmol h−1 g−1[290]
CdS/TiO2NoneNoneVisible light0.71 µmol h−1 g−1[291]
C60/CdS/TiO2NoneNoneVisible light6.03 µmol h−1 g−1[291]
CQDs/-TiO2PtMethanolXenon lamp7.42 mmol·h−1 g−1[85]
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Rasool, M.A.; Sattar, R.; Anum, A.; Al-Hussain, S.A.; Ahmad, S.; Irfan, A.; Zaki, M.E.A. An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production. Catalysts 2023, 13, 66. https://doi.org/10.3390/catal13010066

AMA Style

Rasool MA, Sattar R, Anum A, Al-Hussain SA, Ahmad S, Irfan A, Zaki MEA. An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production. Catalysts. 2023; 13(1):66. https://doi.org/10.3390/catal13010066

Chicago/Turabian Style

Rasool, Muhammad Asghar, Rabia Sattar, Ayesha Anum, Sami A. Al-Hussain, Sajjad Ahmad, Ali Irfan, and Magdi E. A. Zaki. 2023. "An Insight into Carbon Nanomaterial-Based Photocatalytic Water Splitting for Green Hydrogen Production" Catalysts 13, no. 1: 66. https://doi.org/10.3390/catal13010066

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