Phosphorus Cation Doping: A New Strategy for Boosting

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Phosphorus Cation Doping: A New Strategy for Boosting Photoelectrochemical Performance on TiO2 Nanotube Photonic Crystals Zhenzhen Li, Yanmei Xin, Wenlong Wu, Baihe Fu, and Zhonghai Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10688 • Publication Date (Web): 28 Oct 2016 Downloaded from http://pubs.acs.org on October 29, 2016

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ACS Applied Materials & Interfaces

Phosphorus Cation Doping: A New Strategy for Boosting Photoelectrochemical

Performance

on

TiO2

Nanotube

Photonic Crystals Zhenzhen Li, Yanmei Xin, Wenlong Wu, Baihe Fu, Zhonghai Zhang* †

School of Chemistry and Molecular Engineering, East China Normal University, 500

Dongchuan Road, Shanghai 200241, China. ABSTRACT Photoelectrochemial (PEC) water splitting is a promising technique for sustainable hydrogen generation. However, the PEC performance on current semiconductors need further improvement. Herein, a phosphorus (P) cation doping strategy is proposed to fundamentally boost PEC performance on TiO2 nanotube photonic crystal (TiO2 NTPCs) both in visible light region and full solar light illumination. The self-supported P-TiO2 NTPCs are fabricated using a facile two-step electrochemical anodization method and subsequent a phosphidation treatment. The Ti4+ is partially replaced by P cations (P5+) from the crystal lattice, which narrow the band gap of TiO2 and induce charge imbalance by the formation of Ti-O-P bonds. We believe the combination of unique photonic nanostructures of TiO2 NTPCs and P cation doping strategy will open up a new opportunity for enhancing PEC performance of TiO2-based photoelectrodes. KEYWORDS: Photoelectrochemical water splitting; Phosphorus cation; TiO2 nanotube; Photonic crystal 1.

INTRODUCTION

Solar energy collection, conversion and storage can be integrated in photoelectrochemical (PEC) water splitting process with semiconductors as photoelectrode.1 Various semiconductors with

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controlled nanostructures have been used as photoelectrodes,2-9 among them, titanium dioxide (TiO2) remains one of promising candidates due to its proper band-edge positions, high optical absorption, good optical and chemical stability, and low cost.10,11 However, the large band gap (3.2 eV for anatase and 3.0 eV for rutile) limits its optical absorption within ultraviolet (UV) light region, consequently minimizes its PEC conversion efficiency. Homogenous doping has been proposed as a useful way to modify the inner electronic structure of nanomaterials.12-14 With this method, two strategies have been investigated to expand the optical absorption of TiO2-based photoelectrodes into the visible-light region (accounts for ~40% of total solar energy). First, introducing nonmetal anions (such as C, N, F, and S) to form acceptor states above valence band.15–19 Substitutional N doping has been regarded as the most successful way to improve PEC performance in visible light region,20 however, defects from the N impurities localize the photoelectrons and suppress the electron transfer, which is detrimental to the PEC performance under full solar light illumination.21 Second, incorporating metal ions to form donor states below conduction band,22-24 however, which requires expensive ion-implantation facilities.25 Therefore, rational design doping strategy for improving PEC performance of TiO2based photoelectrodes is still a vital challenge. Inspired from the superior photocatalytic activity of nonmetal cation doping,26-28 we hypothesize which could be one of novel and promising strategies for enhancing PEC performance both in visible light and full solar light illumination. The doping of nonmetal cations will narrow the band gap of TiO2, and induce charge imbalance between Ti4+ and O2-, thus facilitates electron transfer and suppresses charge recombination. Among various nonmetal cations, phosphorous (P) distinguishes itself because of its much higher formation energy of 15.48 eV for substituting O in anatase TiO2 than the formation energy of 1.32 eV for substituting


2

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Ti, that is, the P is easier to incorporate into TiO2 lattice with Ti-O-P bond than to incorporate P3by replacing O2- for forming Ti-P bond.29-31 It is worth mentioning that few literatures have reported that P-doped TiO2 nanoparticles preparing by a conventional sol-gel method own superior photocatalytic activity,26-31 however, the P cation-doped TiO2-based photoelectrode for PEC water splitting has not been investigated and reported. Herein, as a proof-of-concept, we prepare the P cation-doped TiO2 nanotube photonic crystal (P-TiO2 NTPCs) photoelectrode for efficient PEC water splitting both under illumination of visible light and full solar light. The TiO2 NTPCs is selected as pristine candidate because of its hierarchical nanostructures, periodical top nanoring layer and highly-ordered vertical bottom nanotubes layer, which distinguishes the dual functions of amplifying optical absorption and facilitating electron transfer respectively.32 The ordered nanoring photonic layer on the top surface of TiO2 NTs help to harvest photons in a broad optical regions (UV, Visible, even near Infrared light), and the bottom vertical nanotubes act as “highway” of electron for efficient charge transfer, both of which induce enhancement of PEC performance. The TiO2 NTPCs is directly grown on titanium (Ti) foil by a wet-chemical route first, with a subsequent facile lowtemperature hosphidation process for uniform P doping. The self-supported nature of P-TiO2 NTPCs avoids the involving of conventional polymer binder, improves charge separation and expands carrier mobility for PEC water splitting. We believe the combination of unique photonic nanostructures of TiO2 NTPCs and effectiveness of P cation doping strategy will open up a new opportunity for enhancing PEC performance of TiO2-based photoelectrodes. 2.

Experimental section

2.1.Chemicals and materials


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All reagents are of analytical grade and used as received without any further purification. A 0.1 mm thick titanium foil was cut into pieces of 10 × 40 mm2. Ethylene glycol (EG), ammonia fluoride (NH4F), sodium hydroxide (NaOH), potassium chloride (KCl), and Sodium hypophosphite monohydrate (NaH2PO2) were purchased from Macklin Inc, Shanghai, China. All aqueous solutions were prepared using deionized (DI) water with a resistivity of 18.2 M Ω cm. 2.2.Preparation of the TiO2 NTPCs and P-TiO2 NTPCs The preparation processes of self-supported P-TiO2 NTPCs photoelectrode is illustrated in Scheme S1. Our previous reported two-step electrochemical anodization method32-37 was used to fabricate the TiO2 NTPCs with hierarchical top-photonic/bottom-tubular nanostructures. Prior to anodization, the Ti foils were totally washed by water and acetone, and dried in a pure nitrogen stream. The anodization was carried out in a conventional two-electrode system with the Ti foil as anode and Pt foil as cathode respectively. The anodization electrolytes consisted of 0.5 wt % NH4F and 2%(v/v) water in EG solution. The Ti foil was anodized at 60 V for 30 min in first anodization step, and then as-grown nanotube layer was removed in an ultrasonic bath. The Ti foil underwent second anodization at 30 V for 30 min. The as-anodized TiO2 NTPCs was annealed at 450 °C for 1 h in air. The one-step anodized TiO2 NTs samples were also prepared for comparison. The TiO2 NTPCs was loaded in a ceramic boat, and NaH2PO2 was used as phosphorus source, put 10 cm away from the TiO2 NTPCs in the upstream side. Afterwards, the two boats were put into a tube furnace, and the furnace was first purged with argon for 30 min. The phosphidation temperature and duration has been optimized to 400 °C and 1 h respectively. The Ar flow was maintained throughout the whole tempering process. In addition, the annealing process in air for prior crystallization of TiO2 NTPCs is important for stabilizing the


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photoelectrode, the direct phosphidation of as-anodized TiO2 NTPCs would result in a complete damage of the photoelectrode (Figure S1). 2.3.Materials Characterization. The morphologies of photoelectrodes were characterized by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM 2100). The crystalline structures of the phtoelectrodes were analyzed by X-ray diffraction (XRD) (Bruker D8 Discover diffractometer, using Cu Kα radiation (1.540598 Å)). The chemical compositions and status were determined from X-ray Photoelectron Spectroscopy (XPS) with an Axis Ultra instrument (Kratos Analytical) under ultrahigh vacuum (

Phosphorus Cation Doping: A New Strategy for Boosting

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