Heriot-Watt University Research Gateway A review of nanostructured non-titania photocatalysts and hole scavenging agents for CO2 photoreduction processes Citation for published version: Tan, JZY & Maroto-Valer, MM 2019, 'A review of nanostructured non-titania photocatalysts and hole scavenging agents for CO photoreduction processes', Journal of Materials Chemistry A, vol. 7, no. 16, pp. 9368-9385. https://doi.org/10.1039/C8TA10410G2 Digital Object Identifier (DOI): 10.1039/C8TA10410G Link: Link to publication record in Heriot-Watt Research Portal Document Version: Publisher's PDF, also known as Version of record Published In: Journal of Materials Chemistry A General rights Copyright for the publications made accessible via Heriot-Watt Research Portal is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. 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The well-documented TiO2 and modified TiO2-based photocatalysts have been shown to photoreduce CO2 into hydrocarbons. Meanwhile, there is also an increasing interest in the utilisation of non-titania based materials, namely metal sulphides, oxides, oxynitrides and nitrides, for CO2 photoreduction. Distinct from other published Received 29th October 2018 reviews, we discuss here recent progress made in designing metal sulphide, oxide, oxynitride and Accepted 18th December 2018 nitride photocatalysts for CO2 photoreduction through morphological changes, aiming at providing DOI: 10.1039/c8ta10410g Creative Commons Attribution 3.0 Unported Licence. a systematic summary of non-titania based materials for CO2 photoreduction. Furthermore, the rsc.li/materials-a introduction of hole scavengers in order to maximise the CO2 photoreduction efficiency is also reviewed. 1. Introduction concentrated mainly on modelling or process development. The use of TiO2 as a photocatalyst for CO2 reduction has been – Fossil fuels are currently unrivalled for energy generation, and extensively studied and has been reviewed elsewhere.6 10 our existing infrastructure is built to handle fossil fuels for However, the lack of systematic studies of non-TiO2 semi- transportation, heating and electricity.1 Our heavy reliance on conducting materials, namely metal sulphides, oxides, oxy- This article is licensed under a 2 fossil fuels results in annual emissions of 32 Gt of CO2. This is nitrides and nitrides, for CO2 photoreduction (CO2PR) has likely to increase to 36–43 Gt by 2035, subject to policies gov- inhibited the development of these photocatalysts compared to erning CO2 emissions and energy use, even with increasing titania-based photocatalysts. 3 ff Open Access Article. Published on 01 April 2019. Downloaded 4/16/2019 2:36:42 PM. renewable energy sources. To mitigate these environmental Although di erent photocatalysts (i.e., titania and non-titania issues as well as alleviate our dependence on fossil fuels, har- based semiconductors) have been proposed in the literature, the vesting the seemingly innite solar energy and storing it in the overall CO2PR conversion remains low especially under sunlight form of chemical fuels hold signicant promise to address irradiation, making the CO2PR system not practical for commer- ffi current and future energy demands. Moreover, the chemical cialisation. To further increase the e ciency of CO2PR, the industry and a vast amount of chemical products rely heavily on introduction of scavenging agents into the CO2PR system has been using fossil fuel feedstock. This further motivates the develop- proposed. However, so far, the introduction of hole scavenging ment of sustainable processes to generate fuels and chemical agents has not been systematically studied, though studies started feedstock from water and CO2 using solar energy. Such in the last century. Therefore, the necessity to systematically a process is akin to photosynthesis in nature, and therefore, it is scrutinise the recent development of non-TiO2 photocatalysts and referred to as the articial photosynthesis. hole scavenging agents for CO2PR is of great demand. Photoelectrocatalytic reduction of CO2 in aqueous suspen- There are enormous scienti c and technical challenges sions using semiconducting powders was rst proposed by involved in making even the simplest fuel, H2, and even more so 4 Inoue et al. in 1979. Later in 1987, the photocatalytic reduction for carbon-based fuels by means of CO2 photoreduction. Similar of CO2 to CH4 in the presence of H2O was proposed by Thampi to other photocatalytic processes, solar-driven photocatalytic 5 et al. Since then, an increasing number of studies on the conversion of CO2 in the presence of H2O to hydrocarbon fuels photo(electro)catalytic reduction of CO2 have been conducted uses semiconducting materials to harvest solar energy and (Fig. 1). Among these studies, almost 50% focused on the provides active sites to allow the photocatalytic conversion materials employed as photocatalysts for conversion of CO2 process to occur. The basic steps of the photocatalytic process under UV and/or visible irradiation. The rest of the studies can be summarised as follows: (1) generation of charge carriers (electron–hole pairs) by Research Centre for Carbon Solutions (RCCS), Heriot-Watt University, Edinburgh semiconducting materials upon absorption of photons with EH14 4AS, UK. E-mail: [email protected] appropriate energy from the irradiation of light, 9368 | J. Mater. Chem. A,2019,7,9368–9385 This journal is © The Royal Society of Chemistry 2019 View Article Online Review Journal of Materials Chemistry A (2) separation of charge carriers and their transportation to the hybrid organic–inorganic materials34–37) and other inorganic surface of the photocatalyst, and transition or main group metal oxides, sulphides, oxynitrides, (3) chemical redox reactions between the charge carriers and and nitrides. Since the use of carbon-based semiconductors for 30,34,38,39 the reactants. CO2PR has been reviewed elsewhere, these photo- CO2PR with H2O into fuels is illustrated in Fig. 2. TiO2 was catalysts are not be discussed herein. 5 the rst material used for CO2PR, and since then it has been Inorganic semiconductors, namely metal oxides, sulphides, widely used because of its abundance, availability, high chem- oxynitrides and nitrides, are among the rst semiconductors ical stability, low cost and non-toxicity.12 Despite the great effort made in the CO PR using TiO and its derivative materials, the 2 2 Prof M. Mercedes Maroto-Valer efficiency of the process remains low,7 mainly attributed to the (FRSE, FIChemE, FRSC, and following factors: FRSA) is the Assistant Deputy (a) Rapid recombination of photogenerated electron–hole Principal (Research & Innova- pairs;10 tion) and Director of the Research (b) Mild reducing power; Centre for Carbon Solutions The potential of the conduction band electrons is only (RCCS) at Heriot-Watt Univer- slightly more negative than the multi-electron reduction sity. She leads a multidisci- potentials of CO , thus providing a very small driving force, 2 plinary team of over 50 whereas the potential of the valence band holes is much more researchers developing novel positive than the water oxidation potential.7 solutions to meet the worldwide (c) Limited visible light absorption due to the wide bandgap demand for energy. Her team's (3.0–3.2 eV) of TiO .13,14 2 expertise comprises energy Strategies including doping,15,16 coupling with semi- Creative Commons Attribution 3.0 Unported Licence. generation, conversion and industry, carbon capture, conversion, conductors,17–19 dye sensitizing,20,21 surface modication22,23 etc. transport and storage, emission control, low carbon fuels, and low- have been extensively used to improve TiO photocatalysts and 2 carbon systems. She has over 450 publications, of which she edited are summarised elsewhere.9,14,24,25 However, the two most 4 books, and 32% of her publications are among the top 10% most commonly used methods for extending the absorption range to cited publications worldwide. Her research portfolio includes visible light, namely sensitization or doping, do not fully projects worth £35m, and she has been awarded a prestigious address the optical issue of wide bandgap materials. Sensitizing European Research Council (ERC) Advanced Award. She obtained agents (e.g., dyes or quantum dots) oen degrade when exposed a BSc with Honours (First Class) in Applied Chemistry in 1993 and to UV light and photogenerate oxidizing holes in TiO .7 Dopant 2 then a PhD in 1997 at the University of Strathclyde