Article pubs.acs.org/Langmuir
Vertically Oriented TiOxNy Nanopillar Arrays with Embedded Ag Nanoparticles for Visible-Light Photocatalysis Weitao Jiang,†,‡ Najeeb Ullah,‡ Giorgio Divitini,‡ Caterina Ducati,‡ R. Vasant Kumar,‡ Yucheng Ding,† and Zoe H. Barber*,‡ †
State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, China Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, United Kingdom
‡
S Supporting Information *
ABSTRACT: We present a straightforward method to produce highly crystalline, vertically oriented TiOxNy nanopillars (up to 1 μm in length) with a band gap in the visiblelight region. This process starts with reactive dc sputtering to produce a TiN porous film, followed by a simple oxidation process at elevated temperatures in oxygen or air. By controlling the oxidation conditions, the band gap of the prepared TiOxNy can be tuned to different wavelength within the range of visible light. Furthermore, in order to inhibit carrier recombination to enhance the photocatalytic activity, Ag nanoparticles have been embedded into the nanogaps between the TiOxNy pillars by photoinduced reduction of Ag+ (aq) irradiated with visible light. Transmission electron microscopy reveals that the Ag nanoparticles with a diameter of about 10 nm are uniformly dispersed along the pillars. The prepared TiOxNy nanopillar matrix and Ag:TiOxNy network show strong photocatalytic activity under visible-light irradiation, evaluated via degradation of Rhodamine B.
1. INTRODUCTION TiO2 with tailored porosity, particle size in the nanometer range, and different nanostructures is attracting a lot of attention because of its potential use in photocatalysis and energy conversion.1,2 Use of high surface area TiO2 as a photocatalyst for pollutant degradation and water splitting is of particular interest.3−5 The basic mechanism is creation of an electron−hole pair by exciting an electron from the valence to the conduction band through light absorption. However, since the crystalline phases, rutile and anatase, of TiO2 have band gaps of 3.1 (400 nm in wavelength) and 3.2 eV (388 nm in wavelength), respectively, only a small fraction of the solar spectrum, restricted in the UV range, is absorbed. Great efforts have been devoted to extending photoabsorption to the visible region of the spectrum. Various metals6,7 and nonmetals such as carbon,8 nitrogen,9 or sulfur10 have been used as dopants to tune the TiO2 band gap and lead to visible light photocatalysis. Of all the doping elements, nitrogen (N) is particularly favored because its p state contributes to band-gap narrowing by mixing with the O 2p states.11,12 A lot of work has been done on Ndoped TiO2 nanopowder, nanorods, and nanotubes produced by solution-based synthesis.13−15 In this paper, we will illustrate a straightforward process to prepare vertically oriented TiOxNy nanopillar arrays. TiOxNy nanopillars have a band gap which can be tuned by our process to different wavelength within the range of visible light. A drawback of monolithic TiO2 as a photocatalyst is the high recombination rate of charge carriers. It is well known that deposition of noble metals on a semiconductor surface can © 2012 American Chemical Society
change the photocatalytic process by modifying the semiconductor surface properties and inhibiting recombination of carriers.6,7 The metals can enhance the yield of a particular product or rate of photocatalytic reaction. Silver is often applied to modify TiO2, because Ag nanoparticles can act as electron traps to slow the rate of electron−hole recombination. In this paper, we show that we can embed Ag nanoparticles into the prepared TiOxNy nanopillar structure by photoreduction of Ag+ (aq) under visible-light irradiation.
2. EXPERIMENTAL SECTION TiOxNy nanopillars are obtained by simple oxidation of TiN nanoporous film at elevated temperatures in oxygen or air. TiN nanoporous films were prepared by reactive dc magnetron sputtering of a high-purity Ti target in an ultrahigh-vacuum deposition chamber. A base pressure of ∼3.5 × 10−9 mbar was attained before introducing a mixture of high-purity N2 and Ar gases, containing 40% N2. Films were grown at a rate of ∼0.3 nm/s on Si substrates with a thermally grown oxide layer. Although no intentional substrate heating was employed, long exposure (up to 1 h) to the high power density sputtering plasma (∼ 6W/cm2 on the target surface, 4 cm from the substrate) increases the substrate temperature to 100 °C. All samples were rectangular with a size of 15 mm × 30 mm. To obtain TiOxNy films, the as-deposited TiN films were oxidized in a tube furnace at various temperatures in O2 or air. X-ray diffraction (D8 diffractometer, Bruker AXS) and fieldemission SEM (LEO VP1530) were used to characterize the film Received: September 15, 2011 Revised: January 9, 2012 Published: February 16, 2012 5427
dx.doi.org/10.1021/la203617u | Langmuir 2012, 28, 5427−5431
Langmuir
Article
Figure 1. (a and b) Structure of TiN film prepared by reactive dc sputtering. TiOxNy film prepared by oxidation of TiN film at (c) 650 and (d) 800 °C. (e) X-ray diffraction of the TiN and TiOxNy films. (f) PL spectra of the TiOxNy films following 2 h oxidation in O2. (g) Dependence of the PL spectra on the oxidation environments (in O2 and air) and oxidation time. (h) Thermogravimetric measurement in air at 900 °C, indicating replacement of N by O reaching saturation after 5 h oxidation. orientation and surface structure. A photoluminescence (PL) instrument (Accent RPM 2000) was used to measure the band gap with a 266 nm laser. In order to clarify the thermal oxidation process, a simultaneous DSC-TGA instrument (TA Instruments Q600) was employed. In order to obtain Ag-loaded TiOxNy films, each TiOxNy film was immersed into 30 mL of AgNO3 solution with a concentration of 0.05 M for 3 h and irradiated with a metal halide floodlight (RS, 500 W, center wavelength at 900 nm,
