Synthesis and Characterization of Nitrogen‐Doped Group IVB Visible‐Light‐Photoactive Metal Oxide Nanoparticles

Abstract

The incorporation of main group elements into wide-bandgap semiconductors has attracted significant interest because of the potential for modifying the physical properties of these semiconductors, as well as for applications such as photocatalysis, water splitting, and optoelectronics. Among the main group elements, nitrogen is believed to be the most favorable p-type dopant because of its similar size to oxygen, metastable AX center formation, and small ionization energy. For example, the incorporation of N into ZnO has been found to be a very promising way to make p-type ZnO, which represents a potentially useful alternative to toxic and expensive III–V semiconductors for applications such as light-emitting devices (LEDs). Another exciting example is visible-light-active TiO2, which is a promising material for applications such as water splitting and photocatalysis. Owing to its potential role in solving the inevitable global energy and environment crises, N-doped TiO2 has received a great deal of attention in recent years. Although several theoretical and experimental studies have validated the efficacy of the N doping of TiO2 for a variety of applications, the different approaches used for N doping have raised questions about the origin of the observed photocatalytic activity. In order to increase the understanding of N doping at the nanoscale level, it is imperative to carry out a systematic analysis of the photocatalytic activities of nanoparticles with different N-doping levels and metal oxide matrices to aid the development of a new generation of photocatalysts. However, from a survey of the literature, no such study has been conducted up till now. Here, we present the incorporation of controlled amounts of N into group IVB metal oxide (TiO2, ZrO2, and HfO2) nanoparticles via a reliable wet-chemical procedure under ambient conditions. Furthermore, we present the effects of doping on their optical and photocatalytic properties. The synthesis of N-doped group IVB metal oxides is based on our previously reported sol–gel approach. We have determined from the X-ray photoelectron spectroscopy (XPS) analysis of N-doped TiO2 [9,20] that coordinating the amine precursor to the Ti precursor (usually Ti[OCH2(CH3)2]4) is essential for the efficient N doping of metal oxides. In the present synthetic route, we first attach the amine to the central metal to form an amino-coordinated precursor complex; subsequently, we hydrolyze the precursor solution with distilled water. Under different pH conditions it is possible to obtain TiO2 slurries with different amounts of incorporated nitrogen. In addition, the particle size can be confined to the nanometer regime by controlling the rate of water addition. After washing, the dispersions are centrifuged and the obtained powders are dried under vacuum. The dried powders are yellow to orange in color depending on the doping level. Before characterization, the products are annealed at 200 °C for 1 h under vacuum to remove organic residues from the surface. The morphologies of the products have been studied by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). It has been found that N-doped TiO2 and ZrO2 are mostly amorphous when sintered at temperatures below 400 °C (Fig. 1A–C, lower curves); the XRD pattern of HfO2 shows clear diffraction peaks only at sintering temperatures above 600 °C. Figure 1A–C (upper curves) shows the XRD patterns of the sintered products. The XRD patterns of TiO2–xNx, ZrO2–xNx, and HfO2–xNx particles indicate anatase, cubic, and monoclinic structures for these materials, respectively (Joint Committee on Powder Diffraction Standards (JCPDS) Card Nos. 84-1286, 49-1642, and 78-0050, respectively). However, at these temperatures, the obtained N-doping levels of all the three materials are lower than 1 %, indicating that nitrogen is replaced by oxygen at elevated temperatures. In order to maintain a higher doping level, the sintering temperature has been set at 200 °C. The TEM studies show that the unsintered samples are amorphous and irregular in shape. Regular spherical nanocrystals start to appear when a sintering temperature of 150 °C is used. Therefore, the samples used for the optical and catalytic measurements described below are mixtures of crystalline and amorphous particles. Although we have not been able to completely avoid aggregation of the particles on the TEM grids, the particle size has been controlled in the range from 5 to about 50 nm and the particles are approximately spherical in shape, as shown in Figure 1D–F. The N-doping levels of these metal oxides have been investigated by core-level XPS. Figure 2A–C shows a series of fullscale XPS spectra of N-doped TiO2, ZrO2, and HfO2 nanoparticles. All these spectra show typical TiO2, ZrO2, and HfO2 features, as well as a clear N 1s binding energy peak around C O M M U N IC A IO N