Journal of The Electrochemical Society, 166 (15) H743-H749 (2019) H743 0013-4651/2019/166(15)/H743/7/$38.00 © The Electrochemical Society Sn Doping into Hematite Nanorods for High-Performance Photoelectrochemical Water Splitting Truong Thi Hien, Nguyen Duc Quang, Nguyen Manh Hung, Haneul Yang, Nguyen Duc Chinh, Soonhyun Hong, Nguyen Minh Hieu, Sutripto Majumder, Chunjoong Kim, z and Dojin Kim z Department of Materials Science and Engineering, Chungnam National University (CNU), Yuseong-gu, Daejeon 34134, Korea Photoelectrochemical water splitting is of great attention due to its environmental friendly generation of clean fuels. Hematite (α-Fe 2 O 3 ) is considered one of the promising candidates due to its intrinsic properties for the high performance photoelectrochemical electrode such as favourable bandgap (2.0–2.2 eV), a suitable energy band position, non-toxicity, low cost, and excellent chemical stability. Herein, we report about Sn-doped hematite nanorods and their implementation as photoanodes for photoelectrochemical water splitting. We provide the simple but efficient route to incorporate the Sn into the hematite without structural damage in the nanostructure and scrutinize the effect of Sn dopant on the photoelectrochemical activity of the hematite. By the two-step heat- treatment process, Sn can be successfully incorporated into the hematite, which reveals the enhanced photoelectrochemical responses compared with undoped hematite. We elaborate the effect of Sn dopant in the hematite on the photoelectrochemical activities, thereby the optimum concentration of Sn dopant can be suggested. In addition, the catalyst layer of the cobalt phosphate is introduced to further increase the photoelectrochemical performance of Sn-doped hematite nanorods. © 2019 The Electrochemical Society. [DOI: 10.1149/2.0621914jes] Manuscript submitted June 26, 2019; revised manuscript received August 26, 2019. Published October 17, 2019. Since the advent of the petroleum crisis in the 1970s, worldwide re- search has been focused on the conversion and storage of sustainable solar energy. 1 Photoelectrochemical (PEC) water splitting has been considered a promising technology to convert solar energy directly into chemical energy in the form of hydrogen and oxygen. 2,3 While various types of oxide have been investigated, hematite (α-Fe 2 O 3 ) has been considered a promising photoanode because of a favourable bandgap (2.0–2.2 eV) for the absorption of visible light, a suitable valence band position for water oxidation, non-toxicity, low cost, and excellent photoelectrochemical stability for long-term operation. 4–6 However, the practical use of hematite is limited by its intrinsic ma- terial properties. A hematite photoanode suffers from a low mobility of charge carriers, short lifetime of photoexcited charge carriers, and slow oxygen evolution reaction kinetics. 7–9 To solve these limitations for improved PEC performance, enormous effort has been focused on the modification of their structure via doping with various elements, such as Si, Ti, Al, Zn, Mo, Cr, and Sn, because elemental doping can enhance the photocatalytic activity of a-Fe 2 O 3 and extend the re- sponse of the semiconductor structure toward the visible region. 10–17 Among them, Sn-doped hematite photoanodes revealed a promising PEC performance. 18,19 Since the first attempt to improve the PEC performance of hematite by Sn doping was reported by Kennedy et al., 20 there have been con- tinuous reports on the effects of Sn doping of hematite toward en- hancement of PEC performance. 21 In particular, the source of Sn was the FTO glass substrate 19,22–24 reported that annealing process affected the PEC performance of a Sn-doped hematite crystalline film on an FTO glass substrate. Such Sn doping method requiring high-temperature for diffusion of Sn into the hematite had two major drawbacks. First, hematite nanostructures shrink and deform substan- tially by high-temperature annealing, which reduces the surface area of nanostructures. 23,24 Second, the progressive diffusion from the FTO substrate led to non-uniform distribution of Sn in the hematite. It was reported that the tip of the hematite nanorod (NR) having low Sn con- centration impaired the overall PEC performance of the hematite NR photoanode. 23 Recently, Li et al. 25 demonstrated a silica encapsula- tion of hematite NRs can preserve their morphology during the high- temperature calcination at 800°C while improving the uniformity of the dopant distribution along the nanorod growth axis rendering the optimal light absorption performance preserved. In this study, we developed a simple process to fabricate Sn-doped hematite nanorods with uniform dopant distribution while preserving the morphology to show large surface area for the maximum light ab- z E-mail: ckim0218@cnu.ac.kr; dojin@cnu.ac.kr sorption. By the simple two-step heat-treatment process, the nanorod structure of the hematite photoanode could be maintained, where the effect of Sn doping on the PEC performance was carefully investigated. In addition, the thin film of cobalt phosphate, Co-Pi, as the catalysis layer was formed on the hematite NRs, and its effect on enhancement of the PEC performance was investigated. Experimental Material.—Fluorine-doped tin oxide (FTO) glass substrates with a sheet resistance of 10 Ω/ were purchased from Taewon Sci- entific Co., Korea. Ferric chloride hexahydrate (FeCl 3 .6H 2 O 97%), urea (NH 2 CONH 2 ), tin(IV) chloride pentahydrate (SnCl 4 .5H 2 O 98%), cobalt(II) nitrate hexahydrate (Co(NO 3 ) 2 .6H 2 O 98%), monopotas- sium phosphate (KH 2 PO 4 ), ethanol (C 2 H 5 OH 99%), and sodium hy- drate (NaOH 97%) were purchased from Sigma Aldrich and used without any additional purification. Preparation of akaganeite nanorod film.—Akaganeite (β- FeOOH) nanorods were firstly synthesized on an FTO substrate via the hydrothermal method. A 120 mL capacity Teflon-lined stainless steel vessel was filled with 100 mL aqueous solution containing 2.43 g FeCl 3 .9H 2 O and 0.54 g NH 2 CH 2 COOH. Five pieces of clean FTO (2.5 cm × 2 cm) were placed into the autoclave. The vessel was sealed and heated at 100°C for 5, 7, 10, and 12 hrs in an oven and then allowed to be cooled down to room temperature. A uniform layer of akaganeite could be formed on the FTO substrate. The FeOOH-formed FTO was washed with deionized (DI) water, followed by drying in the oven at 80°C for 2 h. Preparation of Sn-doped hematite and pristine hematite nanorod films.—Twenty microliters of different concentrations of tin (IV) chlo- ride (SnCl 4 ), 0.025, 0.05, 0.1, 0.2, and 0.3 mol L −1 , in ethanol solution was drop-coated on to the FeOOH on FTO film (2.5 cm × 2 cm). The films were dried in the air to dry the solution on the film. Dried films were heated in air at 550°C with the ramping rate of 8.75°C min −1 for 2 h to form hematite. Then, the film was annealed in Ar at 750°C with ramping rate of 25°C min −1 for an additional 20 min, which allowed the hematite NRs to be doped by Sn ions. This sample is referred as Sn-Fe 2 O 3 (X M), where X is the concentration of SnCl 4 precur- sor solution. Therefore, five different samples, Sn-Fe 2 O 3 (0.025 M), (0.05 M), (0.1 M), (0.2 M), and (0.3 M) were prepared. The bare (un- doped) Fe 2 O 3 NR film was prepared directly from the FeOOH NR film via the same process but without the drop-coating of tin chloride solution, which is referred as pristine Fe 2 O 3 . ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 146.185.203.198 Downloaded on 2019-10-22 to IP