Graphene-based Carbocatalysis: Synthesis, Properties and Applications

By Pinki Bala Punjabi, Rakshit Ameta and Sharoni Gupta

This book informs readers about recent advances in graphene carbocatalysis encapsulating the current developments in the syntheses, properties, characterizations, functionalization, and catalytic applications of graphene, its derivatives, and composites. It serves as a comprehensive primary reference book for chemistry and engineering students who are required to learn about graphene chemistry in detail. It also serves as an introductory reference for industry professionals and researchers who are interested in graphene research as well as its emerging applications in catalysis and beyond. Volume 2 presents information about the industrial applications of graphene-based materials. It starts with graphene-based photocatalysis and progresses into the electrochemical applications of related materials. Highlighted applications in this domain include the use of graphene for hydrogen production and in electrodes for electrochemical sensors. It also covers developments in graphene-based smart energy materials. The final chapter of the volume summarizes the future of graphene-based material technology.

• Language: English
• Publisher: Bentham Science Publishers
• Release date: Jun 5, 2023
• ISBN: 9789815136050

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Graphene–Based Photocatalysts

Jayesh Bhatt¹, Shubang Vyas¹, Avinash Kumar Rai¹, Neeru Madan¹, Rakshit Ameta², *

¹ Department of Chemistry, PAHER University, Udaipur (RAJ.) 313003, India

² Department of Chemistry, J. R. N. Rajasthan Vidyapeeth, Udaipur (RAJ.) 313001, India

Abstract

Graphene is a single layer of graphite with a unique two-dimensional structure with high conductivity, superior electron mobility, absorptivity, and specific surface area. The extraordinary mechanical, thermal, and electrical properties of graphene are due to long-range π conjugation. Due to these properties, graphene can be used in nanosystems and nano- devices. The photocatalytic efficiency of composites (semiconductor-based metal oxides and graphene-based photocatalysts) can be improved under visible light. Graphene behaves as an electron acceptor in these types of composite photocatalysts. Different types of graphene-based composites (graphene (G)-semiconductor, graphene oxide (GO)-semiconductor, and reduced graphene oxide (RGO)-semiconductor, where the semiconductor is TiO2, ZnO, CdS, Zn2SnO4, etc.) can be prepared through simple mixing and/or sonication, sol-gel process, liquid-phase, hydrothermal, and solvothermal methods. This chapter includes the most recent advances in different applications of graphene-based semiconductor photocatalysts for degrading various contaminants (treatment of waste water) and producing hydrogen (fuel of future) by photosplitting water, and photo-catalytically reducing carbon dioxide to energy-rich synthetic fuels (combating against global warming and energy crisis), etc.

Keywords: Graphene, Graphene Oxide, Graphene Reduced Oxide, Hydrogen, Photospliting, Photocatalysis.

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* Corresponding author Rakshit Ameta: Department of Chemistry, J. R. N. Rajasthan Vidyapeeth, Udaipur (RAJ.) 313001, India; E-mail: [e-mail]

INTRODUCTION TO PHOTOCATALYSIS

Photocatalysis is the process by which reactions are carried out in the presence of catalyst and light. The term photocatalysis is derived from the combination of two Greek words; the prefix the Photo and the suffix catalyst. Thus, it is a process where light is used to activate a substance (called photocatalyst), affecting the rates of a chemical reaction without participating in the chemical transformation.

Semiconductor photocatalysis is an emerging technology which has been applied for energy generation and environmental applications. Semiconductors are normally used as photocatalysts because there is a favourable combination of light absorption properties, electronic structure, excited-state, and a lifetime of charge transport characteristics. Various semiconductors have been used as photo- catalysts (such as TiO2, CdS, ZnO, SrTiO3, etc.) as they absorb the light (photon) with energy that is >band gap (energy gap). As a result, an electron from the valence band (VB) is promoted (excited) to its conduction band (CB); thus, generating an electro-hole (e--h+) pair. Here, the hole can oxidize, while the electron reduces any substrate (Fig. 1).

Fig. (1))

Generation of an e--h+ pair of semiconductors exposed to light.

The major advantages of heterogeneous photocatalysis process are:

Low cost,

High conversation efficiency,

High quantum yield,

High stability, and

High activity.

Along with this, there is a disadvantage of this process, and that is the recombination energy of e¯¯and h+. In this process, energy is lost in the form of heat. The efficiency of a photocatalyst rises with an increase in the number of active sites on that surface. On the other hand, the efficiency is decreased by these three important mechanisms of recombination:

Direct Recombination: Here, photoelectron in conduction band drops directly, occupying a vacant (unoccupied) state in the valence band, and combines with the hole simply by the electrostatic attraction.

Surface Recombination: It has selectively lower probability because surface species can utilize these photogenerated charge carriers (electron-hole) to drive the chemical reaction, and

Recombination at Recombination Centres: It is also called volume recombination, and is highly probable. Here, the recombination centres lie at lattice sites transition within the bulk of the crystal.

To overcome this problem of recombining charge carriers, there are three common methods to modify photocatalytic surfaces by increasing the charge separation and the lifetime. These are:

Surface sensitization,

Composite formation, and

Metallized semiconductor

Composites formation is useful when the energy of the irradiated light is not sufficient enough to excite an electron in a semiconductor because of its wide band gap. It is then coupled with another semiconductor with a small band gap; thus, the composite of these two semiconductors will increase efficiency by utilizing near UV, visible light or even sunlight (Fig. 2).

Fig. (2))

Composite of semiconductors.

This composite formation has two advantages. These are:

Increasing the response of semiconductors with a large band gap by coupling with other components with a small band gap, shifts the absorption range from ultra-violet to visible light and

Suppressing the recombination of an electron-hole by injecting the electrons from the higher conduction band of the small band gap component of composite into the conduction band of the large band gap component.

A composite of different semiconductors with graphene and graphene-based materials has been prepared to increase a photocatalyst's photocatalytic efficiency.

GRAPHENE, GRAPHENE OXIDE AND REDUCED GRAPHENE OXIDE

Diamond, graphite, graphene, fullerene (C60), carbon nanotubes, and carbon quantum dots are various allotropes of carbon. These allotropes can be quite commonly used as supporting materials for semiconductor photocatalysts and have found a wide range of applications in the degradation of organic pollutants, photosplitting of water (hydrogen generation), sensors, solar cells, carbon dioxide reduction to energy-rich molecule, etc.

Different photocatalytic nanocomposites based on graphene (G), graphene oxide (GO) and reduced graphene oxide (RGO) coupled with some semiconductors are important because they have desired physicochemical and optical properties such as more solar light harvesting, effective removal of pollutants, better separation efficiency of photocharge carriers, excellent stability, recyclability, etc.

The photocatalytic performance of recently developed G, GO, and RGO-based nanocomposites is the main focus of this chapter. It will be useful to workers in material science so that they can develop new G, GO and RGO-based photocatalysts with better activity and that too at low production costs.

Graphene is a single layer of graphite, and it possesses a unique two-dimensional structure with high conductivity, absorptivity, superior electron mobility and specific surface area. Apart from that, it can be obtained on a large scale and at a low cost. Graphene-based composite photocatalysts which contain heterostructure constituents have attracted attention of scientists all over the globe. A combination of different photocatalysts and graphene is expected to introduce some important properties of graphene into photocatalyst. Graphene oxide (GO) is considered a miracle material as it has a 2D honeycomb structure and it is also known for enhancing photocatalytic activity of different semiconductors. Graphene oxide is normally used as a cocatalyst in photocatalytic reactions, but little is known about the ability of GO to reduce CO2 photo-catalytically as a sole photocatalyst and the activation of light irradiation.

WASTE WATER TREATMENT

Scarcity of quality and quantity of water; both has becoming a burning problem all over the globe. Different organic contaminants constitute a major portion of water pollution. Advanced oxidation processes (AOPs) have been developed as green chemical technology to combat against such water pollution. Photocatalysis is also an advanced oxidation process, which treats polluted water in an eco-friendly manner.

Photocatalysis based on nanocatalysts is a quite promising and emerging method for the treatment of contaminated water. Solar photocatalysis is inexpensive, eco-friendly, and applicable on large scale.

Graphene

Dyes

Wu et al. [1] fabricated well-defined flake-like ZnO/graphene hybrid via a facile in situ hydrolysis-deposition route. Zinc acetate and graphene oxide (GO) were used as starting materials while ammonia was used as a precipitator and reductant. The photocatalytic activity of this photocatalyst was evaluated using brilliant red X-3B (X3B) dye as a model system. The photocatalytic activity of ZnO/graphene hybrid was found to increase first and then it decreased with increasing amount of GO. It was found that hybrid with 3 wt. % of GO showed the highest photocatalytic activity, which is three times higher than that of pure ZnO.

Gao et al. [2] prepared graphene oxide-Bi2WO6 composite by in situ hydrothermal reaction. Then graphene oxide was reduced by ethylene glycol, so as to give graphene-Bi2WO6 (G-BWO) composite. The graphene-Bi2WO6 photocatalyst showed enhancement in photocatalytic activity in the degradation of rhodamine B (RhB) in the presence of visible light (λ > 420 nm). It was revealed that photocatalytic activity was enhanced due to a negative shift in the Fermi level of G-BWO and the high migration efficiency of photoinduced electrons, which may reduce the charge recombination effectively.

ZnWO4/graphene hybride (GZW-X) photocatalysts were synthesized by Bai et al. [3] via a facile in situ reduction of graphene oxide and ZnWO4 in water. Degradation of methylene blue (MB) under UV and visible light both was observed in the presence of GZW-X photocatalysts with high efficiency. The photocatalytic efficiency of ZnWO4/graphene-0.2 wt% under visible-light and UV-light irradiation was found to be ~7.1 and 2.3 times than that of pristine ZnWO4, respectively. The visible photocatalytic activity was considered due to •OH and O2•–. They attributed enhancement of UV photocatalytic light activity in ZnWO4/graphene to the high separation efficiency of photoinduced electron–hole pairs resulting from the promotion of HOMO orbit of graphene in ZnWO4/graphene.

Fu et al. [4] prepared ZnFe2O4–graphene photocatalyst (which is magnetically separable also) with different graphene contents through a facile one-step hydrothermal method. First graphene sheets in this nanocomposite were exfoliated and then, these were decorated with zinc ferrite nanocrystals. It was reported that photodegradation rate of methylene blue was 88% on visible light irradiation in 5 min while in the presence of H2O2, it reached up to 99% after irradiation for 90 min. The ZnFe2O4–graphene served a dual function as the catalyst for photoelectrochemical degradation of MB and also for generation of a strong oxidant hydroxyl radical via photoelectrochemical decomposition of hydrogen peroxide under visible light irradiation as compared to ZnFe2O4 catalyst.

A novel Ag3PO4 photocatalyst showed high visible-light photocatalytic activity, but it is limited due to large crystallite size and severe photocorrosion. A new graphene-modified nanosized Ag3PO4 photocatalyst was prepared by Xiang et al. [5] via in situ growth strategy in an organic solvent. The nanosized Ag3PO4 graphene composite exhibited enhancement in visible-light photocatalytic activity and stability in the degradation of methylene blue in aqueous solution as compared with nanosized Ag3PO4 particles and large-sized Ag3PO4 particles–graphene composite. This enhanced stability and activity may be due to positive synergetic effects of the graphene sheets, and nanosized Ag3PO4 particles which include an increase in the number of active adsorption sites and suppression of charge recombination, reducing the formation of Ag nanoparticles.

Neelgund et al. [6] developed a novel method to synthesize graphene–ZnO composite through the reduction of graphite oxide followed by in situ deposition of ZnO nanoparticles. This graphene–ZnO catalyst was found capable of complete degradation of rhodamine B on exposure to natural sunlight. Its catalytic efficiency was found to be enhanced by sensitizing with cobalt phthalocyanine.

The hybrid composites of graphene decorated by large-sized CdS particles (G/M-CdS) were prepared by Lu et al. [7] via one-pot solvothermal route. The reduction of graphite oxide into graphene was accompanied by the generation of microsized CdS particles. It was reported that CdS particles formed on graphene sheets had an average size of approximately 640 nm. The as-prepared composite can remove rhodamine B from wastewater by adsorption.

The MoS2-graphene composites (MoS2/RGO) were synthesized by Komal et al. [8] through hydrolysis of lithiated MoS2 (LiMoS2). It was reported that MoS2/RGO composite exhibited an excellent photocatalytic performance than to Restack MoS2 and degraded about 90% of RhB within 30 min on visible light irradiation. Higher photocatalytic activity was attributed to the synergic effect between the MoS2 and RGO, which significantly reduced electron–hole pair recombination.

A BiVO4–graphene photocatalyst was prepared by Fu et al. [9] via a facile one-step hydrothermal method. It was observed that graphene sheets are exfoliated in this catalyst and decorated by leaf-like BiVO4 lamellas. It was revealed that BiVO4–graphene system showed significantly enhanced photocatalytic activity for degradation of methylene blue (MB), methyl orange (MO), rhodamine B (RhB) and active black BL-G (ABBLG) in water under visible light irradiation as compared to pure BiVO4 catalyst; may be due to the concerted effects of BiVO4 and graphene sheets or their integrated properties.

Zhang et al. [10] synthesized pure graphene flakes with different defects by CVD method to prepare the graphene/Cu2O composites. These composite photocatalysts were then used for degradation of methyl orange under visible light irradiation. It was observed that the catalytic activity was strongly dependent on the abundance of defects in graphene flakes. The highest activity was obtained (80.10%) in 30 min on light irradiation. It was indicated that the defects in graphene could facilitate to construct an efficient interface between graphene and Cu2O, which narrows down the band gap of original Cu2O semiconductor and inhibits the recombination of photo-generated electron–hole pairs.

A facile hydrothermal synthesis route was developed by Qu et al. [11] to prepare N and S, N co-doped graphene quantum dots (GQDs). Citric acid and urea or thiourea were used as the C, N and S sources, respectively. Both; N and S, N doped GQDs showed high quantum yields (78 and 71%). A broad absorption band was observed in the visible region in case of S, N co-doped GQDs due to doping with sulfur, which changes the surface state of GQDs. There was higher photocatalytic activity of S, N co-doped GQD/TiO2 composites for degradation of rhodamine B in the presence of visible light. The apparent rate of S, N:GQD/TiO2 was found to be 3 and 10 times higher than that of N:GQD/TiO2 and P25 TiO2 under visible light irradiation, respectively.

Chandel et al. [12] prepared visible-light-driven nitrogen-doped graphene (NG) supported magnetic ZnO/ZnFe2O4 (ZnO/ZF/NG) and ZnO/CoFe2O4 (ZnO/CF/NG) nanocomposites. The hydrothermal method was used for preparing nitrogen-doped graphene supported magnetic ZnO/ZF (ZnO/ZnFe2O4) and ZnO/CF (ZnO/CoFe2O4) nanocomposites while ZnO was synthesized via direct precipitation method. They evaluated their performances for mineralization of methyl orange and malachite green (MG) dyes in aqueous solution. It was found that the high surface area of ZnO/ZF/NG and ZnO/CF/NG was suitable for adsorptive removal of MG and MO dyes. The photodegradation performance of heterojunction photocatalysts was found to be higher than a bare photocatalyst in 140min under visible-light irradiation. It was observed that ZnO/ZF/NG and ZnO/CF/NG could be easily isolated from the aqueous solution by applying external magnetic field for 20 sec and 2min, respectively. The ZnO/ZF/NG and ZnO/CF/NG nanocomposites can be recycled about ten times.

Chen et al. [13] obtained CdS/graphene composites through a mild chemical reaction approach. It was indicated that CdS nanoparticles were dispersed on the surface of graphene nanosheets, and the composites exhibited improved specific surface area on addition of graphene. The catalytic activity was evaluated by monitoring the degradation of methyl orange. It was observed that 94% methyl orange was degraded over the composites after irradiation for 120min, which is higher than that of pure CdS, such excellent activity can be attributed to higher specific surface area, boosted light absorption capacity, high carrier separation efficiency, and enhanced redoxability of the carriers. Scavenging experiments indicated that superoxide anion radicals played a major role in this catalytic degradation.

Frindy et al. [14] hybridized hematite (α-Fe2O3) nanoparticles using a green hydrothermal method by loading of graphene obtained by the pyrolysis of biopolymers (chitosan, alginate, and carrageenan). It was reported that α-Fe2O3-G catalyst showed high catalytic activity of about 98% toward rhodamine-B in the presence of visible light irradiation. The size of the nanoparticles is affected by the presence of N and S heteroatoms on the surface of graphene. The specific surface area in the α-Fe2O3-G had a negative effect on catalytic performance. Hydroxyl radical was found to be the main reactive species. The reusability of α-Fe2O3-Gr was also tested, which indicated that the catalyst retained high activity and stability even after four cycles without affecting the morphology of catalysts. It was interesting to note that higher photocatalytic activity was observed by incorporating graphene into hematite nanoparticles as compared to loading of α-Fe2O3 on active carbons carbon nanotubes, and biochar.

Liang et al. [15] prepared visible-light-driven Z-scheme AgVO3/AgI graphene microtube (AgVO3/AgI@GM) using in situ ion exchange and hydrothermal methods. It was reported that structural and functional properties of AgVO3 and AgI were synergistically integrated to provide good conductivity. Here, AgVO3 nanowires were uniformly dispersed on the surface of graphene microtubules and this was attributed to the chelation of chitosan, which acted as source of Ag to generate AgI on the surface by an ion-exchange effect. It prevented AgVO3 and AgI agglomeration/shedding in photocatalytic applications and resulted in improved photocatalytic performance and cyclic stability. It was reported that hollow graphene microtube was beneficial for improving the mass-transport efficiency in photocatalytic processes; while its macrostructure was conducive for recycling. The AgVO3/AgI@GM exhibited significant photodegradation of methyl orange (93% removal rate in an hour) as well as a disinfection capacity for Escherichia coli (100% antibacterial efficiency in 45 min). It could also retain photocatalytic activity even after five cycles.

Rizal et al. [16] synthesized Ag/Mn3O4 composites with various concentrations of graphene via sol–gel method followed by hydrothermal synthesis. The degradation of organic dyes was also evaluated in the presence of organic acid and the order was:

The characterization results revealed that the Mn3O4 nanoparticles exhibited an excellent stability and crystalline structure and were successfully connected with Ag nanoparticles and graphene.

Graphene quantum dots (GQDs) are used as sensitizers for semiconductor catalytic materials and they show a significant enhancement in the catalytic performance of the catalyst. GQDs/CeO2 composites were prepared by Pei et al. [17] to improve the catalytic performance of CeO2. GQDs in the GQDs/CeO2 composites were uniformly coated around each CeO2 crystal, which increased the reactive sites and effective contact interfaces. As a result, extremely low electron-hole recombination rate was there; thus, optimal photoelectric performance could be achieved. It was proposed that photogenerated electrons were transferred from the GQDs conduction band to the conduction band of CeO2, which effectively increased the carrier lifetime. An enhanced photocatalytic activity was obtained for decomposition of rhodamine B under visible light. The reaction was about 13.8 times faster that with pure CeO2.

Wang et al. [18] synthesized ZnO, Ce doped ZnO (ZnCeO), ZnO-graphene (ZG) and Ce doped ZnO-graphene (ZGCeO) via a one-step hydrothermal method. As-prepared ZnO was found to have a hexagonal wurtzite crystal structure. The ZG and ZGCeO had flower-like and approximate flower-like structures as evident from SEM. The absorption edges had red-shifts and the absorption band was broadened in all doped or composite samples. The band gap energy of these samples decreased from 3.26 to 3.06 eV. It was reported that ZGCeO showed superior photocatalytic activity as compared to pure ZnO, ZnCeO and ZG. The

degradation rate of methylene blue was 99.17% in 90 min under simulated solar irradiation.

Wang et al. [19] prepared graphene-like carbon/TiO2 photocatalysts via a facile in situ graphitization approach. It was reported that graphene-like carbon/TiO2 photocatalyst with a monolayer carbon shell (0.468 nm) showed the highest photocatalytic activity, which is almost 2.5 times more than that of pristine TiO2 (P25) under UV light irradiation. A synergetic effect could assist in rapid photoinduced charge separation; thus, decreasing the recombination of electron-hole pairs. As a result, the number of holes participating in the photooxidation process was increased; thus, enhancing the photocatalytic activity.

A chemically bonded TiO2 (P25)-graphene nanocomposite photocatalyst was obtained by Zhang et al. [20] using a facile one-step hydrothermal method. Reduction of graphene oxide and loading of P25 both were achieved during the hydrothermal reaction. It was reported that as-prepared P25-graphene photocatalyst possessed high adsorption of methylene blue, light absorption range at a longer wavelength and efficient charge separation properties. A significant enhancement in the rate of degradation was observed with P25-graphene, as compared to bare P25 and P25-CNTs with the same carbon content.

A simple and straightforward strategy was developed by Fu et al. [21] to synthesize MnFe2O4–graphene photocatalysts (magnetically separable) with varied graphene contents. It was reported that graphene sheets were fully exfoliated and decorated with MnFe2O4 nanocrystals with an average diameter of 5.65 nm and a narrow particle size distribution. It was quite interesting to note that the combination of MnFe2O4 nanoparticles with graphene sheets exhibited high photocatalytic activity for the degradation of methylene blue under visible light irradiation although MnFe2O4 alone is photo-catalytically inactive. A strong magnetic property of MnFe2O4 nanoparticles can be used for magnetic separation. A significant enhancement in photoactivity can be attributed to the reduction of graphene oxide (GO), as the photogenerated electrons of MnFe2O4 can be transferred easily from the conduction band to the reduced GO, which effectively prevented a direct recombination of electrons and holes. It was confirmed that hydroxyl radicals played the role of the main oxidant in this system.

Hou et al. [22] synthesized graphene-supported zinc ferrite multi-porous microbricks hybrid via a facile deposition–precipitation reaction, followed by a hydrothermal treatment. It was reported that these microbricks hybrid exhibited higher photocatalytic activity for degradation of p-chlorophenol as compared to pure ZnFe2O4 multi-porous microbricks and ZnFe2O4 nanoparticles under the visible light irradiation (λ > 420 nm). This enhancement was attributed to the fast photogenerated charge separation and transfer due to the high electron mobility of graphene sheets, which has a high specific surface area, improved light absorption and multi-porous structure of the hybrid. It was revealed that the hydroxyl radicals were involved as the main active oxidant in this photocatalytic degradation.

The performance of a low bandgap composite of graphene nano-platelets (GNPs) and zirconium vanadate (ZrV2O7) was evaluated by Samy et al. [23] using an innovative photocatalytic reactor. The composite was coated on illuminated plates inside the reactor to evaluate its photocatalytic activity and stability. They used chlorpyrifos as a model pollutant. The degradation efficiencies using GNPs/ZrV2O7 were found to be 96.8, 95.2, 93.8, 92.8 and 91.0%, compared to 85.4, 84.6, 83.8, 82.6 and 81.8% in the case of bare ZrV2O7 in five consecutive runs.

Drugs

Moradi et al. [24] investigated sonophotocatalytic degradation of antibiotic sulfamethoxazole (SUX) in