Solar Energy Materials & Solar Cells xxx (xxxx) xxx Please cite this article as: Suhee Kang, Solar Energy Materials & Solar Cells, https://doi.org/10.1016/j.solmat.2020.110890 0927-0248/© 2020 Elsevier B.V. All rights reserved. CO 2 selectivity of fower-like MoS 2 grown on TiO 2 nanofbers coated with acetic acid-treated graphitic carbon nitride Suhee Kang , Haritham Khan , Caroline Sunyong Lee * Department of Materials and Chemical Engineering, Hanyang University, South Korea A R T I C L E INFO Keywords: TiO 2 nanofbers MoS 2 Graphitic carbon nitride Photocatalytic CO 2 reduction CO 2 selectivity CO 2 adsorption S-scheme ABSTRACT Activated CO 2 adsorption sites are crucial for improving selectivity in photocatalytic CO 2 reduction. Co-catalysts incorporating rare or noble metals have previously been required to achieve high CO 2 selectivity (S CO2 ); thus, noble-metal-free catalysts with high S CO2 are desirable but challenging to realize. We introduced S-scheme heterojunction using noble-metal-free TiO 2 /MoS 2 /graphitic carbon nitride (g-C 3 N 4 ) with a strong redox ability showing S CO2 > 90%. This heterostructure improved CO 2 conversion, to levels 3.1 times higher than that of the g-C 3 N 4 alone and exhibited suffcient kinetic overpotential (0.18 eV) to produce signifcant amounts of CH 4 . When the proportion of g-C 3 N 4 was optimized, the specifed TiO 2 /MoS 2 /g-C 3 N 4 achieved high S CO2 (~90%) due to its improved CO 2 adsorption, in turn due to the improved specifc surface area and pore size distribution attributable to amino (NH 2 ) groups of g-C 3 N 4. We introduced, a novel, noble-metal-free TiO 2 /MoS 2 /g-C 3 N 4 heterostructure that maximizes the number of CO 2 adsorption sites and charge carriers separation through interconnected components, and thus increases S CO2 . 1. Introduction There has been a dramatic increase in studies reporting methods of CO 2 reduction aimed at avoiding severe environmental issues, such as accelerated global warming due to the rapid consumption of fossil fuels [1]. Among these methods, photocatalytic CO 2 reduction has attracted particular attention recently; if the unlimited resource of solar energy can be utilized, renewable energy could be produced without the requirement for any additional energy from electricity [2]. C1 products, such as CO, CH 4 or CH 3 OH, can be obtained from photocatalytic CO 2 through conversion of solar light into chemical fuels [3]. Generally, CO 2 is a highly stable molecule with thermodynamic and kinetic barriers so that weak CO 2 adsorption and fast electron-hole recombination are observed during photocatalytic CO 2 reduction [4]. To enhance CO 2 adsorption while maximizing photocatalytic CO 2 selectivity (S CO2 ), incorporating noble metal complexes into photocatalysts is a useful approach, due to their excellent charge separation. Recently, Wang et al. reported the highest S CO2 value, of >99%, using TiO 2 nanocrystals/Au nanoparticles covered with g-C 3 N 4 nanosheets [5]. Moreover, Li et al. reported >99% S CO2 for Ag/TiO 2 nanoparticle/g-C 3 N 4 heterostructure, attributed to the surface plasmon effect of the noble metal used as a co-catalyst [6]. These superior CO 2 selectivities were achieved by incorporating noble metals into the heterostructures. However, these high-cost noble metal photocatalysts are diffcult to apply in artifcial photosynthesis felds commercially [7]. Thus, developing a well-designed heterostructure with high S CO2 that is free of noble metals is challenging but desirable. TiO 2 is a functional semiconductor used in various photocatalytic applications due to its high chemical stability, relatively low fabrication cost, and non-toxic materials [8–11]. Despite these excellent properties, TiO 2 has a wide bandgap and responds mainly to ultraviolet (UV) ra- diation, which comprises less than 5% of the solar spectrum. Its bandgap energy must be adapted so that it absorbs the entire range of the solar spectrum. Moreover, the geometry of the structures used as catalysts signifcantly affects the active sites [12]. TiO 2 nanoparticles are usually used for their catalytic activities [13,14]. However, these zero-dimensional nanoparticles have problems of agglomeration, dramatically reducing the number of adsorption sites and thus decreasing its surface area. These agglomerated nanoparticles have resulted in a poor electron transfer system with relatively lower pho- tocurrents and poor photocatalytic CO 2 reduction, compared to those for one-dimensional structures. Therefore, one-dimensional structures such as nanorods, nanowires, or nanofbers, with improved electron pathway should be considered for photocatalytic CO 2 reduction materials due to * Corresponding author. E-mail address: sunyonglee@hanyang.ac.kr (C.S. Lee). Contents lists available at ScienceDirect Solar Energy Materials and Solar Cells journal homepage: http://www.elsevier.com/locate/solmat https://doi.org/10.1016/j.solmat.2020.110890 Received 12 August 2020; Received in revised form 23 October 2020; Accepted 17 November 2020