Recent advance of graphene/semiconductor composite nanocatalysts: Synthesis, mechanism, applications and perspectives
Graphical abstract
Introduction
Energy crisis and global environmental destruction are the major issues to human society in recent decades [1], [2], [3], [4], [5], [6]. Thus, a green and efficient technology to solve above problems is urgently required [7], [8]. Since Fujishima discovered that hydrogen production occurred in TiO2 electrode with ultraviolet (UV) light irradiation in 1972, photocatalysis has attracting extensive interest as a green technology [9], [10], [11], [12]. As a promising technology, photocatalysis using photocatalysts to absorb and convert the renewable solar energy into chemical energy, which can be used in contaminants elimination and energy generation [13], [14]. It is considered to be an energy saving, environmental friendly and efficient technology [15], [16]. Accordingly, photocatalysis technology has a promising application prospect in the fields of environment and energy [17], [18].
Generally, the photocatalysis process (Fig. 1a) can be divided into three steps: (i) photocatalysts generate electrons (e−) and holes (h+) under light excitation whose energy is beyond the band gap energy of photocatalysts; (ii) e− transfers from valence band (VB) to conduction band (CB), leaving h+ in VB and (iii) e− and h+ as well as various free radicals (e.g., O2− and OH) react with target reactants (e.g., pollutants, CO2) through redox reaction. In addition to the light irradiation source, photocatalyst is an essential item in photocatalysis process. Up to date, a lot of photocatalysts have been explored and utilized, such as TiO2 [19], [20], ZnO [21], WO3 [22], CdS [23], metal-organic frameworks (MOFs) [24] and g-C3N4 [25], [26] etc. As a kind of typical and most commonly used photocatalyst, semiconductor has been greatly utilized in photodegradation of pollutants, water splitting as well as CO2 reduction for its cost effectiveness and high stability [27], [28], [29]. However, two major obstacles severely limit the use of photocatalysis technology: the rapid recombination of photogenerated electrons-holes and the insufficient utilization of light [30]. Several strategies have been utilized to solve the problems such as developing new photocatalysts, elements doping and coupling with other semiconductors [31], [32], [33]. For instance, Liu et al. [32] synthesized 3D N-TiO2-x/MoS2 core-shell nanostructures, which exhibited better photocatalytic performance than that of pristine TiO2 for the degradation of methyl orange and hydrogen evolution. Co-doping of N and Ti3+ and introduction of MoS2 were significant to narrow the band gap and promote the charge transfer. Owing to the synergistic effect between the narrow band gap and the heterojunction structure, the photogenerated e−-h+ pairs were effectively separated and transferred, which was the key factor to improve the photocatalytic activity.
Graphene (Gr), two dimensional (2D) network of hexagonal structured sp2-hybridized carbon atoms, which exhibits π-conjugated structure with sufficient delocalized electron clouds [34], is a rising star in photocatalysis in recent years which has aroused great research interest in the field of energy and environment [35]. Graphene has many outstanding properties (Fig. 1b) such as fast room-temperature mobility of electron (200,000 cm2 V−1 s−1), high electrical conductivity (~2.02 × 102 S cm−1), large theoretical specific surface area (2630 m2 g−1), good optical transmittance (~97.7%) and superior mechanical properties [36], [37], [38]. These unique properties of graphene are conducive to assembling semiconductor and receiving/transmitting photo-induced charge carrier. Pristine graphene sheets are chemically inert and hydrophobic as the lack of surface functional groups [39]. Graphene oxide (GO) and reduced graphene oxide (rGO) are two important derivatives of graphene. GO is the functionalized graphene with oxygen functional groups (epoxy and hydroxyl groups on the basal plane and carboxyl groups at the edges of graphene sheets), which potentially support GO to form stable suspensions in aqueous solution, but excessive O-bonded C atoms are existed in nonconducting σ states, resulting in the insulation of GO, which hinders the separation and transfer of charge carriers in the catalytic system [34], [40]. RGO derived from the reduction of GO, contains residual amounts of oxygenated functional groups and large populations of defects, which lead to considerable disruption of the 2D π-conjugation of the electronic structure and therefore decreases charge carrier mobility and electrical conductivity, rGO with less O-containing groups exhibits higher electrical conductivity more stable chemical properties than GO [41]. O-containing functional groups not only enhance the dispersibility of GO and rGO in solvents, but also serves as reactive sites for the nucleation and growth of nanoparticles, high electronegativity of oxygen atom can adsorb positively charged precursors through electrostatic interactions, leading to uniform distribution of other active components on GO and rGO with an intimate interface, which facilitates the preparation of various graphene-based semiconductors composites [34], [42]. Graphene materials can be used as support and co-catalyst for semiconductors, and the various functions of graphene in improving photocatalytic activity mainly as follows. (i) as an electronic medium which facilitates the transport of photo-induced carrier, inhibits the recombination of e−-h+; (ii) enhance the adsorption ability of photocatalysts and increase reaction site; (iii) expand light absorption range and intensity of semiconductor and (iv) serve as a co-catalyst to promote the hydrogen evolution. There has been great interest to couple graphene (along with GO and rGO) and semiconductor to develop graphene/semiconductor composite to improve photocatalytic activity toward environmental remediation and energy production [43], [44], [45].
Graphene/semiconductor composite nanocatalysts develop rapidly in recent years. The growth of scientific publications on the graphene/semiconductor composite in photocatalysis over the past decade can be seen in Fig. 1c, which indicates the importance and the great interest in this hot research field. Plentiful reports of graphene/semiconductor composites in photocatalysis applications have been published, including some reviews [34], [38], [41], [42], [46], [47]. However, these reviews either introduced only one aspect of photocatalysis applications of graphene/semiconductor composite or not detailed and comprehensive enough. For example, Rajasekhar et al. [38] summarized the applications of graphene/semiconductor nanocomposites in photocatalytic treatment of wastewater with synthetic dyes. Xie et al. [42] summarized the materials engineering of improving the electron transfer efficiency in graphene-based photocatalysts particularly, which could be instructive for constructing graphene-based photocatalysts with better performance. However, it did not systematically review the applications of graphene-based semiconductor photocatalysts in various fields. Zhang and co-workers [41] placed emphasis on maximization of the functions of graphene and optimization of semiconductors as well as interface engineering between graphene and semiconductors. Chen et al. [46] summarized the progress in the preparation and application of semiconductor/graphene photocatalysts about a decade ago. The above works reflected the rapid development of graphene/semiconductor composites, but they cannot provide a comprehensive and latest overview of the synthesis and applications graphene/semiconductor composites, as well as elaborate understanding the multiple and interesting roles of graphene in enhancing the photoactivity of composites. In view of the rapid development and extensive applications of graphene/semiconductor composite in the field of photocatalysis in recent years, an updated and comprehensive review is necessary. We do hope it is timely to promote further developments in this exciting area of research.
Hence, in this review, we provide a general overview of the latest research results of graphene/semiconductor composites nanocatalysts. Firstly, the synthetic methods of graphene/semiconductor composite were presented. Secondly, various functions of graphene in enhancing photocatalytic activity were introduced including enhance adsorptivity for reactants, extend light absorption range and intensity, act as a photosensitizer, as the electron transfer mediator, utilize as a co-catalyst and photothermal effect. Additionally, applications of graphene/semiconductor composites in photodegradation of pollutants, sterilization and disinfection, hydrogen production and reduction of CO2 as well as selective oxidation, nitroaromatics reduction, sensing, supercapacitors, battery and medical fields were summarized. Furthermore, most people only pay attention to the use of graphene in various aspects, most of the attention has been focused on the various uses of graphene and graphene/semiconductor composites, but there is little attention on the toxicity caused by the discharge into the environment after use. Therefore, this review also presented the toxicity of graphene and graphene/semiconductor composites. Finally, challenges and prospects in this emerging field are proposed as follows: (i) Enlarging the applicability of graphene/semiconductor composite photocatalysts; (ii) Fabricating photocatalysts that are easy to be recovered; (iii) In-depth exploration of photocatalytic disinfection mechanism; (iv) Using real waste water for experimental research and (v) Combining theory calculation with experimental studies.
This review would be able to act both as an objective, scientific interest introduction for newcomers in this field and as a reference for experienced researchers at the forefront of this research area. We do hope this paper will provide scientific reference on recognizing the multiple roles of graphene and taking advantages of graphene/semiconductor composite nanocatalysts to alleviate environmental pollution and generate energy on a large scale in the near future. In addition, it is expected that this review will enlighten the researchers that except for the properties and performance of graphene/semiconductor composites, the environmental risks after release also require attention.
Section snippets
Synthetic methods
The synthetic methods of graphene have been summarized by other authors [39] and will not be presented in this article. The synthetic methods have important experimental significance which affects the morphology, structure and photocatalytic efficiency of photocatalysts. In this section, various synthetic methods will be summarized to provide guidance for the preparation of graphene/semiconductor composite nanocatalysts. The main synthetic methods are hydrothermal method, solvothermal method,
Electron transfer mediator
One of the most serious barriers of photocatalysis technology is the rapid recombination of photogenerated e−-h+ in photocatalysts. The recombination includes bulk recombination and surface recombination, about 90% or more of the photogenerated electrons recombine within 10 ns [42]. When the electrons and holes pairs recombine, there are fewer electrons and holes to react with the reactants. Therefore, inhibiting the recombination of e−-h+ is the key factor to improve photocatalytic
Environmental remediation
With the deepening of industrialization, a variety of toxic and harmful pollutants continue to release into the environment which seriously threatening the environment and human health. The abatement of pollutants and the remediation of polluted environment are major problems concerned by the scientific community. Using of semiconductor photocatalysts has been proven to be a good strategy to remove pollutants, adding graphene to a semiconductor photocatalyst is beneficial to improve the removal
Toxicity of graphene and graphene/semiconductor composites
Graphene and its derivatives received widely interest on account of the outstanding properties. In addition, they have been well applied in photocatalysis, sensors, new energy and the biomedical sciences since their discovery in 2004 [202]. The main environmental impact of graphene and its derivatives is their toxic effect on organisms, as they are toxic to microbes, animal, plant cells, and even humans [203]. Accordingly, the toxicity and degradation are helpful to reveal the biosafety of
Conclusion and perspectives
Photocatalysis is a promising technology in the fields of environmental remediation and energy production. However, insufficient light utilization and rapid recombination of photogenerated e−-h+ limit its use. The good optical, thermal and mechanical properties of graphene make it an ideal matrix to support semiconductors. The addition of graphene can inhibit the agglomeration of semiconductor particles, enhance the absorption and utilization of light of semiconductors, and hinder the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The study was financially supported by the Program for Changjiang Scholars and Innovative Research Team in University (IRT-13R17), the National Natural Science Foundation of China (U20A20323, 51979103, 51679085, 51579096, 51521006, 51508177), the Fundamental Research Funds for the Central Universities of China (531107051205), the Funds of Hunan Science and Technology Innovation Project (2018RS3115, 2020RC5012), the Key Research and Development Project of Hunan Province of China (2017SK2241).
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These authors contribute equally to this article.