Functions of CNMs in Photocatalytic H2 Generation: History
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Contributor:
Muhammad Asghar Rasool
,
Rabia Sattar
,
Ayesha Anum
,
Sami A. Al-Hussain
,
Sajjad Ahmad
,
Ali Irfan
,
Magdi E. A. Zaki

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.

  • semiconductor
  • photocatalysis
  • water splitting
  • carbon nanomaterials

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 [1][2][3][4][5]. 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 [6][7][8]. 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. [3] 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. [9] 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. [10] also used rGO additive to stabilize the very thin nanorods of Zn0.5Cd0.5S with a corresponding upsurge in photocatalytic H2 evolution.

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 [11][12]. Noble metals are expensive and rare. Thus, economical cocatalysts are needed for economic photocatalytic systems. Carbon compounds are effective cocatalysts for photocatalytic H2 evaluation [13][14]. 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 [15][16]. Khan et al. [13] 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 [17]. 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 [17]. A hybrid rGO/ZnxCd1−xS photocatalyst prepared first via coprecipitation and then a hydrothermal reduction approach elevates the cocatalytic performance for H2 production [18]. 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. 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 [19][20][21]. 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. [22] 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 [19]. 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. [23] 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 [21][24]. 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) [24].

4. Photocatalyst

Carbon-based nanomaterials are effective photocatalysts for H2generation, according to theory and experiment [25][26][27], 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 [28][29], as its CB minimum consists of anti-bonding π* orbital; at pH = 0, it has a potential of −0.52 eV [30]. 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 [31]. 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 [27]. Meng et al. [29] 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 [26][32][33]. 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 [26]. 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 [32].

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 [34][35]. Ye et al. [36] 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 [37]. In a typical experiment [38], 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 1.
Table 1. Role of carbon nanomaterials in photocatalysis.
Function Type Carbon Nanotubes Graphene Fullerenes Graphene Oxide Graphitic Carbon Nitride Carbon Quantum Dots
Supporting material ✔ ✔  × ✔ ✔ ✔
Photocatalyst ✔ ✔ ✔ ✔  × ✔
Cocatalyst ✔ ✔ ✔ ✔ ✔ ✔
Photosensitizer  × ✔ ✔ ✔ ✔ ✔
Bandgap narrowing effect ✔ ✔  × ✔ ✔ ✔

This entry is adapted from the peer-reviewed paper 10.3390/catal13010066

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