Enhanced Photoelectrochemical Behavior of H-TiO2 Nanorods Hydrogenated by Controlled and Local Rapid Thermal Annealing

Nanoscale Research Letters, May 2017

Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core—disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO2 have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO2 and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO2 nanorods grown on F:SnO2 (FTO) substrate in order to address the degradation issue of FTO in the typical TiO2 nanorods/FTO system observed in the conventional non-RTA treated approaches. Without the FTO degradation in the RTA approach, we systematically studied the intrinsic relationship between the annealing temperature, structural, optical, and photoelectrochemical properties in order to understand the role of the disordered shell on the improved photoelectrochemical behavior of H-TiO2 nanorods. Our investigation shows that the improvement of PEC performance could be attributed to (i) band gap narrowing from 3.0 to 2.9 eV; (ii) improved optical absorption in the visible range induced by the three-dimensional (3D) morphology and rough surface of the disordered shell; (iii) increased proper donor density; (iv) enhanced electron–hole separation and injection efficiency due to the formation of disordered shell after hydrogenation. The RTA approach developed here can be used as a suitable hydrogenation process for TiO2 nanorods/FTO system for important applications such as photocatalysis, hydrogen generation from water splitting and solar energy conversion.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://link.springer.com/content/pdf/10.1186%2Fs11671-017-2105-x.pdf

Enhanced Photoelectrochemical Behavior of H-TiO2 Nanorods Hydrogenated by Controlled and Local Rapid Thermal Annealing

Wang et al. Nanoscale Research Letters Enhanced Photoelectrochemical Behavior of H-TiO Nanorods Hydrogenated by 2 Controlled and Local Rapid Thermal Annealing Xiaodan Wang Sonia Estradé Yuanjing Lin Feng Yu Lluis Lopez-Conesa Hao Zhou Sanjeev Kumar Gurram Francesca Peiró Zhiyong Fan Hao Shen Lothar Schaefer Guenter Braeuer Andreas Waag a.waag@tu- Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core-disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO2 have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO2 and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO2 nanorods grown on F:SnO2 (FTO) substrate in order to address the degradation issue of FTO in the typical TiO2 nanorods/FTO system observed in the conventional non-RTA treated approaches. Without the FTO degradation in the RTA approach, we systematically studied the intrinsic relationship between the annealing temperature, structural, optical, and photoelectrochemical properties in order to understand the role of the disordered shell on the improved photoelectrochemical behavior of H-TiO2 nanorods. Our investigation shows that the improvement of PEC performance could be attributed to (i) band gap narrowing from 3.0 to 2.9 eV; (ii) improved optical absorption in the visible range induced by the threedimensional (3D) morphology and rough surface of the disordered shell; (iii) increased proper donor density; (iv) enhanced electron-hole separation and injection efficiency due to the formation of disordered shell after hydrogenation. The RTA approach developed here can be used as a suitable hydrogenation process for TiO2 nanorods/FTO system for important applications such as photocatalysis, hydrogen generation from water splitting and solar energy conversion. H-TiO2 core-shell nanorods; Hydrogenation; Rapid thermal annealing; TEM/EELS; Optical absorption; PEC property - Background Recently, H-doped TiO2 (H-TiO2) has triggered broad research interest due to its enhanced photocatalytic properties. Chen et al. reported that H-TiO2 nanoparticles were obtained by high-pressure hydrogen gas annealing. The nanoparticles contained an unique crystalline core and a disordered shell homo-interface which led to a narrowed band gap and enhanced photocatalytic behavior [1]. Wang et al. reported a new approach assisted by hydrogen plasma to synthesize H-doped black titania with core-shell nanostructures, superior to the high pressure hydrogenation process [2]. So far, the photocatalytic reports of H-TiO2 in literature are mainly limited to nanoparticle systems [3]. There are only a few investigations about H-TiO2 films on conductive substrates which can be used as photoanodes for three-electrode photoelectrochemical (PEC) studies [3]. Notably, highly oriented H-TiO2 nanorods (NRs) and nanotubes (NTs) have been demonstrated to be highly efficient photoanodes for solar light driven water splitting [4, 5]. Such an unidirectional nanostructure decouples the processes of light absorption and charge collection, which can benefit the charge carrier separation and transport [6–8]. However, the progress of hydrogen processing methods and their influence on the structural, optical, and photoelectrochemical behaviors of H-TiO2 is rarely reported due to lack of a practical hydrogenation method with excellent controllability on the processing parameters. Wang et al. reported a pioneer work of H-TiO2 nanorods grown on the fluorine-doped tin oxide (F:SnO2; FTO) substrate by high temperature hydrogen gas annealing in a tube furnace [4]. They studied the relation between the annealing temperature and photoelectrochemical properties. Due to the degradation issue of FTO substrate, the photocurrent density decreases at hydrogenation temperatures beyond 350 °C, an intrinsic relation between the annealing temperature and photoelectrochemical properties of H-TiO2 could not be indicated. The degradation issue of H-TiO2 nanorods/FTO material system will restrict its applications such as photocatalysis, hydrogen generation from water splitting and solar energy conversion. Since the hydrogen treatment can strongly influence the structural and photocatalytic properties of H-TiO2 [9], a precise control of processing parameters (temperature, time, flux etc.) will play an important (...truncated)


This is a preview of a remote PDF: https://link.springer.com/content/pdf/10.1186%2Fs11671-017-2105-x.pdf

Xiaodan Wang, Sonia Estradé, Yuanjing Lin, Feng Yu, Lluis Lopez-Conesa, Hao Zhou, Sanjeev Kumar Gurram, Francesca Peiró, Zhiyong Fan, Hao Shen, Lothar Schaefer, Guenter Braeuer, Andreas Waag. Enhanced Photoelectrochemical Behavior of H-TiO2 Nanorods Hydrogenated by Controlled and Local Rapid Thermal Annealing, Nanoscale Research Letters, 2017, pp. 336, Volume 12, Issue 1, DOI: 10.1186/s11671-017-2105-x