Elsevier

Journal of Catalysis

Volume 365, September 2018, Pages 138-144
Journal of Catalysis

Facile and controllable surface-functionalization of TiO2 nanotubes array for highly-efficient photoelectrochemical water-oxidation

https://doi.org/10.1016/j.jcat.2018.06.022Get rights and content

Highlights

  • Development of facile and effective surface-functionalization via TiCl3 solution treatment.

  • Dual functionality; generation of oxygen vacancies and deposition of nano-branches.

  • Simultaneous improvement of charge transport and transfer efficiencies in TiO2 nanotubes.

  • Significantly improved PEC water-oxidation performance; 2.25 mA cm−2 at 1.23 VRHE.

Abstract

We report facile and effective surface-functionalization of TiO2 nanotubes array (NTs) via a TiCl3-mediated solution treatment and its effects on the charge transport and transfer properties for photoelectrochemical (PEC) water-oxidation. TiO2 NTs with ∼5 μm length were prepared by hydrothermal-etching a TiO2 nanorods array. Subsequently, TiO2 NTs were treated with an aqueous TiCl3 solution at 80 °C to generate surface oxygen vacancies and to deposit a TiO2 nano-branch layer on the side-walls of TiO2 NTs, and these modifications were confirmed by X-ray photoelectron spectroscopy and transmission electron microscopy. Through electrochemical impedance spectroscopy analysis, we found that the TiCl3-mediated surface-functionalization of TiO2 NTs significantly improves the charge carrier transport and transfer properties, owing to the increase in the charge carrier density (due to the generation of surface oxygen vacancies) and surface roughness (due to the formation of nano-branches), respectively. The TiCl3 treatment considerably improves the incident photon-to-current conversion efficiency (IPCE) and photocurrent density of TiO2 NTs (especially at low-bias potentials) during the PEC water-oxidation, and the treated material demonstrates a maximum IPCE of ∼93% and a photocurrent density of ∼2.25 mA/cm2 at 1.23 V versus the reversible hydrogen electrode.

Introduction

Photoelectrochemical (PEC) water-splitting using solar energy is a promising route to produce hydrogen fuel for clean and sustainable energy future [1], [2], [3]. Currently, a key challenge in the development of commercially viable PEC devices is the unavailability of photoanodes that are highly active, stable, and cost-effective [1]. Metal-oxides such as TiO2, Fe2O3, and WO3 are potential photoanodes owing to their excellent chemical stability, catalytic activity, suitable band-edge position, and natural abundance [4], [5], [6], [7]. However, the PEC performance of metal-oxide photoanodes is still insufficient for practical application owing to their inherent material limitations including poor charge transport (electrical conductivity) and sluggish oxygen evolution reaction (OER) kinetics [6].

Surface-functionalization through chemical reactions or physical adsorption is a promising approach to enhance the inherent material properties because it not only allows for the enhancement of the activity and kinetics at the surface but also for the improvement of the bulk charge transport for PEC water-splitting [8], [9], [10], [11], [12], [13], [14]. For instance, the generation of oxygen vacancies was demonstrated to improve both the surface catalytic activity and charge transport properties of metal-oxide photoanodes, leading to enhanced PEC water-oxidation performance [12], [15], [16], [17], [18]. Additionally, adsorption of functional ligands and deposition of additional layers affect the adsorption of water molecules, catalytic activity, and even stability of metal-oxide photoanodes [8], [19]. A number of surface-functionalization methods have been developed to enhance surface properties of metal-oxide photoanodes (e.g., coating/doping [20], [21], adsorption/grafting of molecules/polymers [19], and reducing via thermal/electrochemical/solution treatments [12], [22]). In particular, thermal activation under reducing or oxygen-deficient environments (e.g., H2, CO, and inert gases) at high temperature (>500 °C) is a common method for generating oxygen vacancies in metal-oxide photoanodes [22]. Previously, we reported on a flame method (i.e., rapid annealing using a high-temperature flame under reducing atmosphere) to effectively generate surface oxygen vacancies in the TiO2 nanorods-array photoanode without damaging the nanorods or substrate [16]. We could control the concentration of oxygen vacancies by modulating flame-annealing parameters such as the fuel-to-air equivalence ratio and annealing time. However, the thermal activation and flame methods are energy-intensive, require specific setup, and they increase the processing cost and are limited by scalability/reproducibility issues. In this regard, it is highly desirable to develop a novel method that is facile, scalable, and feasible at low temperature, for example, a solution-based method, for improving the PEC performance of metal-oxide photoanodes.

In this study, we report a TiCl3-solution-based surface-functionalization method for TiO2 nanotubes array (NTs), which imparts dual-functionalities, i.e., the method not only enables the formation of oxygen vacancies at the surface but also the deposition of a nano-branch layer on the side-walls of the NTs. We show that oxygen vacancies in TiO2 NTs increase the charge carrier concentration and the nano-branch layer increases the surface roughness, thereby improving charge carrier transport and transfer properties of TiO2 NTs, respectively. Importantly, the surface-functionalized TiO2 nanotubes array (S-NT) photoanode demonstrates a significantly improved photocurrent density of 2.25 mA/cm2 at 1.23 V versus reversible hydrogen electrode (RHE), which is three times higher than that of pristine TiO2 NTs photoanode.

Section snippets

Preparation of TiO2 NRs and NTs

Fluorine-doped SnO2 (FTO, Pilkington, TEC-8) substrates were sequentially cleaned in acetone, deionized (DI) water, and ethanol under ultrasonication and dried in a nitrogen stream. Before the growth of TiO2 NRs, a seed layer of TiO2 nanoparticles was deposited on the cleaned FTO substrate by a spin-coating process [20]. The coating solution was prepared by dissolving 0.85 ml of titanium butoxide (TBOT, 97%, Aldrich) in a mixture containing 50 ml of isopropyl alcohol (IPA, 99.5%, Samchun),

Preparation of surface-functionalized TiO2 NTs

Fig. 1 schematically illustrates the preparation of TiO2 NTs and its surface-functionalization by hydrothermal etching followed by the TiCl3 solution treatment. First, vertically-aligned TiO2 NRs (average length, ∼5 μm) with a [0 0 1] growth direction were synthesized on TiO2-NP-coated FTO substrates at 160 °C by a hydrothermal method (Figs. 1(a) and S1) [23], [25]. TiO2 NRs were then hydrothermally etched with aqueous HCl to obtain TiO2 NTs through anisotropic etching/corrosion. During the

Conclusion

In summary, we presented a facile and effective surface-functionalization method that enables not only the introduction of a controllable amount of oxygen vacancies but also deposition of a nano-branch layer on TiO2 NTs. The TiCl3-mediated solution treatment at 80 °C significantly improves the PEC water-oxidation performance of TiO2 NTs because the generated oxygen vacancies enhance the charge transport efficiency (∼99%) by increasing the donor concentration and the nano-branch layer enhances

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1C1A1A01053785 and No. 2017R1A2B3010927).

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