Modified TiO2 photocatalyst for CO2 photocatalytic reduction_ An overview Contents lists available at ScienceDirect Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou Review Article Modified TiO2 photocatalyst for CO2 photocatalytic reduction: An overview Hamidah Abdullaha,b, Md. Maksudur Rahman Khanb, Huei Ruey Ongb, Zahira Yaakoba,⁎ a Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi, Selangor, 43600, Malaysia b Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Gambang, Kuantan, Pahang, 26300, Malaysia A R T I C L E I N F O Keywords: Carbon dioxide (CO2) Photocatalyst Titanium dioxide (TiO2) A B S T R A C T The photocatalytic pathway to reduce carbon dioxide (CO2) to fuel, an artificial photosynthesis process, is a futuristic and ultimate way to combat the energy crisis and CO2 emission issues. The most widely used catalyst for photocatalytic reactions is titanium dioxide (TiO2) due to its availability, chemical stability, low cost and resistant to corrosion. Although TiO2 photocatalyst suffers due to its wide band gap (only can be activated under ultraviolet light irradiation) and high electron-hole recombination rate, it remained as a precursor for the de- velopment of visible light responsive materials for CO2 reduction through different modifications, such as doping of metal, nonmetal, semiconductors etc. There is a significant improvement in CO2 conversion using the visible light responsive TiO2 based catalysts. The product distribution due to the photocatalytic reduction of CO2 highly depends on the band gap and band edges of the catalyst. The understanding in the mechanistic pathway of CO2 reduction is very important to design the catalyst for the production of desired product. This present paper provides an overview of research and development of TiO2 based photo-catalysts for CO2 reduction and focuses on the improvement of the photocatalyst based on the band gap engineering, charge transfer and CO2 adsorp- tion. Moreover, the challenges and future prospect in the developing modified TiO2 for photocatalytic reduction of CO2 has also been discussed. 1. Introduction Photocatalysis can be defined as the acceleration of a photo-reaction by the presence of a catalyst [1]. The catalyst in photo-reaction is able to absorb a photon from light and generate electron-hole pairs. This photo-generated electron and hole at the conduction band (CB) and at the valence band (VB) of the catalyst are used for reduction and oxi- dation reaction, respectively. Photocatalytic reactions are widely used in water splitting to produce hydrogen and oxygen [2], dye degradation [3,4] and CO2 reduction [5,6]. The most widely used photocatalyst for the photocatalytic reactions is titanium dioxide (TiO2) due to its availability, chemical stability, low cost and resistance to corrosion [7]. However, using TiO2 comes with its own limitations. It is only active when irradiated by UV light, due to its wide band, which makes it not as effective under sunlight, since the solar spectrum only consists of about 4% of UV light. Adding to that, TiO2 has a high electron/hole pair re- combination rate compared to the rate of chemical interaction with the adsorbed species for redox reactions [8]. All of these traits, makes the development of a visible light driven photocatalyst with a lower re- combination rate of photo-generated electron-hole pairs highly desir- able. When it comes to the development of TiO2 catalyst, a lot of sig- nificant studies has been made. Various techniques have been employed to make the catalyst be able to absorb a photon when in visible light region and have a low electron-hole recombination rate. To name a few, all of the following techniques, such as doping [9], non-metals doping [10], coupling with others semiconductor [11,12], co-doping [13,14] and surface modification via organic materials [15,16] have been at- tempted. Water splitting [13], dye degradation [14] and CO2 reduction [17] have been reported to benefit from using modified TiO2 photo- catalyst compared to TiO2 due to its higher photocatalytic activity. And since the reduction of CO2 to hydrocarbon has great potential of pro- viding the world with an alternative fuel and solve our woes in regards to CO2 emission, which incidentally, is one of the principal culprits in causing global warming, methods for photocatalytic reductions of said CO2 merits more attention. However, the modifications to the TiO2 photocatalyst that makes it successfull in water splitting and dye de- gradations under visible light irradiation fails to achieve the same success in CO2 reduction. This is mostly due to the catalyst require- ments, in terms of CB and VB edge, being vastly different for CO2 re- duction than for water splitting or dye degradation. The position of CB and the VB of the semiconductor can be a tool in determining the re- duction and oxidation capabilities of the photoelectrons. The photo- generated electrons and holes are efficiently utilized if the reduction potential and oxidation potential of the reaction is less negative and less http://dx.doi.org/10.1016/j.jcou.2017.08.004 Received 7 March 2017; Received in revised form 30 July 2017; Accepted 4 August 2017 ⁎ Corresponding author. E-mail address: zahirayaakob65@gmail.com (Z. Yaakob). Journal of CO₂ Utilization 22 (2017) 15–32 2212-9820/ © 2017 Elsevier Ltd. All rights reserved. MARK http://www.sciencedirect.com/science/journal/22129820 http://www.elsevier.com/locate/jcou http://dx.doi.org/10.1016/j.jcou.2017.08.004 http://dx.doi.org/10.1016/j.jcou.2017.08.004 mailto:zahirayaakob65@gmail.com http://dx.doi.org/10.1016/j.jcou.2017.08.004 http://crossmark.crossref.org/dialog/?doi=10.1016/j.jcou.2017.08.004&domain=pdf Modified TiO2 photocatalyst for CO2 photocatalytic reduction: An overview Introduction Reaction involved in photocatalytic reduction of CO2 TiO2 catalyst for photocatalytic reduction of CO2 Modified TiO2 catalyst for photocatalytic reduction of CO2 Effect of dopants on band gap modification Cation doped-TiO2 Anion doped-TiO2 Anion-anion doped-TiO2 Effect of dopant on charge carrier mobility Cation doped-TiO2 Anion doped-TiO2 Cation-cation doped-TiO2 Effect of Cation-anion co-doping on TiO2 Effect of other semiconductor on TiO2 Enhancement of CO2 adsorption by surface modification Photocatalytic reduction of CO2 by modified TiO2 Mechanism of CO2 photoreduction Conclusions, challenges and future prospects in developing modified TiO2 photocatalyst for photocatalytic reduction of CO2 Acknowledgements References