Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near

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Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near-Infrared Light Photoelectrochemical Water Splitting Yanmei Xin, Zhenzhen Li, Wenlong Wu, Baihe Fu, and Zhonghai Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01533 • Publication Date (Web): 05 Oct 2016 Downloaded from http://pubs.acs.org on October 11, 2016

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ACS Sustainable Chemistry & Engineering

Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near-Infrared Light Photoelectrochemical Water Splitting Yanmei Xin, Zhenzhen Li, Wenlong Wu, Baihe Fu and Zhonghai Zhang* School of Chemistry and Molecular Engineering, East China Normal University, Dongchuan Road 500, Shanghai 200241, China Correspondence

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ABSTRACT: Effective utilization of ultraviolet and visible light for hydrogen evolution in a photoelectrochemical (PEC) water splitting approach has been widely investigated, whereas, the infrared light, another major fraction of solar radiation (~50%), is rarely reported for implementing PEC water splitting application. In this paper, we first demonstrate the coupling of air and solution stable pyrite iron disulfide (FeS2) with hierarchical top-porous-bottom-tubular TiO2 nanotubes (TiO2 NTs) to realize high PEC performance not only in ultraviolet and visible light region, very interesting, but also in infrared light region with photocurrent enhancement by more than 3 order of magnitude compared to that of the pristine TiO2 NTs under illumination of near infrared light. The significant enhancement of PEC performance can be ascribed to the rational coupling of FeS2 with small band gap and TiO2 NTs with unique morphology and proper electronic features. We believe the proposed novel FeS2/TiO2 NTs photoelectrode has the

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potential to address the low efficiency of PEC water spitting in infrared light region issue, and thus can make a significant contribution in the field of energy conversion. KEYWORDS: FeS2; TiO2 nanotube; Photoelectrochemical; Water splitting; Infrared light INTRODUCTION Solar-driven photoelectrochemical (PEC) water splitting is one of the most promising strategies for integrating solar energy collection, conversion and storage in a single process, within which the solar energy is converted and contained in the simplest chemical bond, H-H.1 Many semiconductor materials (e.g., TiO2, ZnO, Fe2O3, WO3, Cu2O, CdS, and Ta2O5) have been extensively investigated as photoelectrodes in PEC system,2-14 among which TiO2 distinguishes itself due to its favorable band-edge positions, superior chemical and optical stability, low toxicity, and low cost. However, the PEC water splitting performance of TiO2 based photoelectrodes is still unsatisfying due to its large band gap (i.e., 3.2 eV for anatase and 3.0 eV for rutile), which limits its optical absorption within ultraviolet (UV) light region (less than 5% of the solar energy).15 Doping with non-metal or metal elements (e.g., N, F, C, Au, Co, Cu, Cr, Fe, Mn)16-28 and sensitization with small band gap semiconductors or chemical dyes are two strategies for expanding the light absorption of TiO2 into the visible light region (accounts for around 40% of the solar energy).29-35 However, the effective utilization of infrared light for PEC water splitting, another major fraction of solar radiation (~50%), is rarely reported because of lacking aqueous solution stable infrared light absorber materials. Very recently, air and aqueous solution stable pyrite iron disulfide (FeS2) has been successfully synthesized through a facile hydrothermal method.36,37 These previous results and literature works motivated us to employ the FeS2, one of the most attractive materials with small


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band gap of capability of near infrared light absorption, for PEC application in terms of cost, availability, and environmental compatibility.38,39 It has favorable physical properties, such as suitable band gap (Eg = 0.95 eV, indirect; 1.03 eV, direct), high light absorption coefficient (α > 105 cm-1 for hυ > 1.3 eV), high carrier mobility of 360 cm2 V-1 S-1, long minority carrier diffusion length (100-1000 nm), and according to the Schockley-Queisser model, a theoretical power conversion efficiency of 28% can be expected.40 However, currently, the single FeS2 material presents poor PEC performance due to its very small open circuit potential (OCP < 0.2 V).41 Therefore, we hypothesize to couple the FeS2 with a high OCP material, TiO2, to implement high PEC performance not only in UV and visible light region but also in infrared light region. Very recently, Wang and coworkers42 have combined the FeS2 nanoparticles (NPs) with TiO2 NPs film to prepare FeS2/TiO2 photoanode, and evaluated the PEC performance in an alkaline supporting electrolyte with sacrificial agent. However, the PEC performance of FeS2/TiO2 for real water splitting (no sacrificial agent), the electronic modification after FeS2 deposition on TiO2 are still not well investigated. Herein, top-porous-bottom-tubular TiO2 nanotubes (TiO2 NTs), prepared by two step electrochemical anodization method, have been rationally selected as pristine photoelectrode material owing to its facile preparation procedure, large surface area for contacting with supporting electrolyte, high light harvesting efficiency from its unique uniform nanostructure, and fast electron mobility induced by one-dimensional nanostructure.43-45 As a proof of concept, the FeS2 sensitized TiO2 NTs (FeS2/TiO2 NTs) serves as an efficient and stable photoelectrode for PEC water splitting under illumination of solar light, and very interestingly, the FeS2/TiO2 NTs presents the photocurrent enhancement by more than 3 order of magnitude compared to that of the pristine TiO2 NTs under illumination of near infrared light. Both TiO2 NTs and FeS2 are


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synthesized by a simple solution based approach instead of costly vapor deposition, it makes the overall synthesis highly cost effective and easily scalable. We believe the proposed novel FeS2/TiO2 NTs photoelectrode has the potential to address the low efficiency of PEC water spitting in infrared light region issue, and thus can make a significant contribution in the field of energy conversion. EXPERIMENTAL SECTION Chemicals and materials A 0.1 mm thick titanium foil (99.6%, Jinjia Metal, China) was cut into pieces of 40 × 10 mm2. Ethylene glycol (EG), ammonia fluoride (NH4F), hydrochloric (HCl), sodium chloride (NaCl), potassium chlorate (KCl), anhydrous sodium hydrogen phosphate (Na2HPO4), monopotassium phosphate (KH2PO4), phosphate buffer saline (PBS, pH = 7.4), acetone, iron(III) chloride(FeCl3), thiourea (CH4N2S) were purchased from Macklin Chemical and used as received. All aqueous solutions were prepared using deionized water (DI) with a resistivity of 18.2 MΩ cm. Preparation of TiO2 NTs and FeS2/TiO2 NTs

Scheme 1. Schematic diagram of the synthesis procedure of FeS2/TiO2 NTs. (a) Ti foil, (b) first anodized TiO2 NTs, (c) nanoconcaves left on Ti foil after ultrasonic removed the TiO2 NTs, (d) second anodized TiO2 NTs with top photonic layer, and (e) FeS2/TiO2 NTs.


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The design and fabrication strategy of the FeS2/TiO2 NTs photoelectrode is presented in Scheme 1. The TiO2 NTs were fabricated in a facile two-step anodization process on the basis of our previous reported method.44-46 Prior to anodization, the Ti foils (Scheme 1a) were first degreased by sonicating in ethanol and DI water, followed by drying in pure nitrogen stream. The anodization was carried out using a conventional two-electrode system with the Ti foil as an anode and a Pt foil as a cathode respectively. The electrolytes consisted of 0.5 wt% NH4F in EG solution with 2 vol% water. All the anodization was carried out at room temperature. In the firststep anodization, the Ti foil was anodized at 60 V for 30 min, and then the as-grown nanotube layer (Scheme 1b) was ultrasonically removed in deionized water, whereas left behind wellordered nanoconcaves on the Ti foil surface (Scheme 1c). The same Ti foil then underwent the second anodization at 60 V for 5 min to yield uniform TiO2 NTs with unique hierarchical nanostructure (Scheme 1d). After the two-step anodization, the prepared TiO2 NTs samples were cleaned with DI water and dried off with nitrogen gas. The as-anodized TiO2 NTs were annealed in air at 500 °C for 1 h with a heating rate of 5 °C min-1. Afterward, the FeS2 was deposited on the TiO2 NTs surface from a facile hydrothermal method (Scheme 1e). In details, the deposition of FeS2 on TiO2 NTs was carried out in a Teflonlined stainless steel autoclave at 25 mL capacity. In a typical procedure, 0.40 g FeCl3 were dissolved in 8 mL DI water. After stirring for 30 min, 8 mL of aqueous solution containing 0.20 g (NH2)2CS were added dropwise into the mixture. After 30 min stirring, the mixture was transferred into the Teflon-lined stainless steel autoclave and maintained at 180 °C for 12 h. Finally, the autoclave was cooled down to room temperature. The resulting products were collected and washed with DI water for several times. By tuning the concentrations of FeCl3 and (NH2)2CS, different deposition amounts of FeS2 can be synthesized and the effect of deposited


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FeS2 amount on TiO2 NTs were also investigated. For the convenience of discussion, we denoted the low amount of FeS2 on TiO2 NTs with 1-FeS2/TiO2 NTs, and high amount of FeS2 on TiO2 NTs with 2-FeS2/TiO2 NTs. Characterization of TiO2 NTs and FeS2/TiO2 NTs The morphologies of electrodes were characterized by scanning electron microscopy (SEM; Hitachi S4800) and transmission electron microscopy (TEM; JEOL JEM 2100). The crystalline structure of the electrodes was analyzed by X-ray diffraction (XRD) on a Bruker D8 Discover diffractometer using Cu Kα radiation (1.540 598 Å). Chemical compositions and status were analyzed by X-ray photoelectron spectroscopy (XPS) on an Axis Ultra instrument (Kratos Analytical) under ultrahigh vacuum (

Pyrite FeS2 Sensitized TiO2 Nanotube Photoanode for Boosting Near

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