Photoinduced Conversion of Methane into Benzene over GaN Nanowires Lu Li, †,‡ Shizhao Fan, ‡ Xiaoyue Mu, † Zetian Mi,* ,‡ and Chao-Jun Li* ,† † Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC H3A 0B8, Canada ‡ Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, QC H3A 0E9, Canada * S Supporting Information ABSTRACT: As a class of key building blocks in the chemical industry, aromatic compounds are mainly derived from the catalytic reforming of petroleum-based long chain hydrocarbons. The dehydroaromatization of methane can also be achieved by using zeolitic catalysts under relatively high temperature. Herein we demonstrate that Si-doped GaN nanowires (NWs) with a 97% rationally constructed m-plane can directly convert methane into benzene and molecular hydrogen under ultraviolet (UV) illumination at rt. Mechanistic studies suggest that the exposed m-plane of GaN exhibited particularly high activity toward methane C-H bond activation and the quantum efficiency increased linearly as a function of light intensity. The incorporation of a Si-donor or Mg-acceptor dopants into GaN also has a large influence on the photocatalytic performance. T he recent discovery of an enormous amount of shale gas is projected to change the landscape of the chemical industry, 1 since through suitable conversions shale gas may replace the dwindling petroleum resources as a carbon-based feedstock. 2-7 Unfortunately, due to the inert C-H bonds in methane, there has been no easy way of turning shale gas into synthetically useful compounds such as olefins and aromatics, 8 being especially hard for the latter. 9 This caused a major concern of a great shortage of aromatic compounds for shale-gas-based future chemical industry. So far, many efforts have been devoted to the formation of aromatics directly from methane and several efficient heterogeneous catalysts have been developed and well investigated, such as Mo, 10 Zn, 11 and Re 12 supported on ZSM-5 zeolites. However, an elevated temperature (>500 °C), owing to the large positive Gibbs free energy [eq 1], is required to promote the equilibrium conversions of methane. → + Δ = 6CH CH 9H , G 434 kJ/mol 4 6 6 2 (298 K) (1) Besides a thermal strategy, another promising approach is to use photoenergy to drive the conversion of methane. 13 Recently, Yoshida et al. 14,15 and Chen et al. 16,17 developed several photocatalysts for methane conversion, respectively, such as SiO 2 -Al 2 O 3 oxides with highly dispersed Ti, Ga 2 O 3 , and Zn + - ZSM-5 zeolite for the methane coupling reaction. However, none of these powdered photocatalysts previously reported can produce aromatic compounds and the methane conversion rate is still low. It is noted that most of these photocatalysts are insulator-supported (SiO 2 or zeolites) materials which have a large band gap and low optical absorption. Nevertheless, the widely used metal oxide semiconductors (such as TiO 2 , ZnO, and Cu 2 O) are not suitable supports for the methane conversion reaction, simply because they are not stable and the lattice oxygen can be abstracted by the produced hydrogen in the harsh gas- solid environment. 13,17 GaN, a well-known group III nitride semiconductor, has a direct energy band gap of ∼3.4 eV at rt, which can be further tuned across the entire solar spectrum by incorporating other elements. 18 Compared with metal oxide semiconductors, the controlled n- and p-type doping and the inherent chemical stability, due to the strongly ionic character of the atomic bonds, make GaN a suitable electronically active support for the photocatalytic reaction under harsh conditions. 19 Furthermore, compared with conventional powdered photocatalysts, nano- wires (NWs) are highly desirable due to their large surface-to- volume ratios, well-defined surface structures, and superior photoelectrical properties. 20 Consequently, we synthesized nondoped GaN NWs with lengths of 800 nm grown on silicon (111) substrate by plasma-assisted molecular beam epitaxy (MBE) under nitrogen-rich conditions. 21 Scanning electron microscopy (SEM, Figure 1a) and transmission electron microscopy (TEM, Figure 1b and 1c) images of the as- synthesized GaN NWs revealed that the NWs possess hexagonal cross sections and are vertically aligned to the substrate, with the morphology slightly tapered from top to bottom. The electron diffraction pattern (inset of Figure 1c) indicates that the wires are of single crystal wurtzite structure with the growth direction along the c-axis, with the top facet of the c-plane and lateral facet of m-plane. The diameter distribution of the top facets derived from SEM measurements fits well in a logarithmic normal distribution with a mean diameter of d NWs = 100 ± 5 nm (Figure 1d). Figure 1e shows the rt photoluminescence (PL) spectrum of GaN NWs with an intensive peak around 365 nm, corresponding to the band gap of 3.4 eV. To evaluate the photocatalytic performance of the GaN semiconductor comprehensively, we have also prepared GaN thin films (Supporting Information (SI) Figure S1) with a thickness of 650 nm grown on sapphire using AlN as the buffer layer and commercial powdered samples (Figure S2). As shown in Figure 1f, due to the orientated growth on the substrates, the only reflections of GaN NWs and thin films obtained from X-ray diffraction (XRD) measurements are 002 and 004, 22 which further confirms that the top facet of GaN NWs Received: January 14, 2014 Communication pubs.acs.org/JACS © XXXX American Chemical Society A dx.doi.org/10.1021/ja5004119 | J. Am. Chem. Soc. XXXX, XXX, XXX-XXX