Facile and controllable surface-functionalization of TiO2 nanotubes array for highly-efficient photoelectrochemical water-oxidation
Graphical abstract
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).
References (39)
- et al.
A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production
Renew. Sustain. Energy Rev.
(2007) - et al.
Influence of surface-functionalized multi-walled carbon nanotubes on CdS nanohybrids for effective photocatalytic hydrogen production
Appl. Catal. B: Environ.
(2018) - et al.
A review on the effects of TiO2 surface point defects on CO2 photoreduction with H2O
J. Materiomics
(2017) - et al.
Experimental studies on vacancy induced ferromagnetism in undoped TiO2
Solid State Commun.
(2007) - et al.
Real surface area measurements in electrochemistry
J. Electroanal. Chem.
(1992) - et al.
Characterization of photoelectrochemical cells for water splitting by electrochemical impedance spectroscopy
Int. J. Hydrogen Energy
(2010) - et al.
Solar water splitting cells
Chem. Rev.
(2010) - et al.
Artificial photosynthesis: solar splitting of water to hydrogen and oxygen
Acc. Chem. Res.
(1995) - et al.
Efficient photochemical water splitting by a chemically modified n-TiO2
Science
(2002) - et al.
Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting
Chem. Soc. Rev.
(2014)
Metal oxide photoanodes for solar hydrogen production
J. Mater. Chem.
Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes
ChemSusChem
Computational insights into charge transfer across functionalized semiconductor surfaces
Sci. Technol. Adv. Mater.
Comparison of the photoelectrochemical behavior of H-terminated and methyl-terminated Si (111) surfaces in contact with a series of one-electron, outer-sphere redox couples in CH3CN
J. Phys. Chem. C
Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting
Nano Lett.
Rapid formation of a disordered layer on monoclinic BiVO4: co-catalyst-free photoelectrochemical solar water splitting
ChemSusChem
Enhancing Mo: BiVO4 solar water splitting with patterned au nanospheres by plasmon-induced energy transfer
Adv. Energy Mater.
On the role of oxygen defects in the catalytic performance of zinc oxide
Angew. Chem. Int. Ed.
Rapid and controllable flame reduction of TiO2 nanowires for enhanced solar water-splitting
Nano Lett.
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2021, Journal of Energy ChemistryCitation Excerpt :Photoelectrocatalytic water splitting to obtain solar fuels is one of the attractive strategies to address the environmental crises and energy shortage issues, which has attracted tremendous interest [1–5] since its discovery by Honda and Fujishima over TiO2 catalysts using ultraviolet (UV) irradiation in the 1970s [6]. Decades of research have generally considered the photocatalytic process in three steps: (1) photon absorption resulting in the generation of excited charge carriers, (2) the separation and diffusion of excited charge carriers to surface active sites, and (3) interfacial charge transfer between surface active sites and adsorbed chemical species [7]. For TiO2, the large band gap limits its optical absorption in the UV region.
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These authors contributed equally.