Structure , Optical Properties , and Photocatalytic Activity towards H 2 Generation and CO 2 Reduction of GaN Nanowires via Vapor-Liquid-Solid Process

1 TU-NIMS Joint Research Center and Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China 2 Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 3 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 4Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), 92 Weijin Road, Nankai District, Tianjin 300072, China


Introduction
In view of the globally concerned energy shortage and environment pollution issues, photocatalysis using semiconductor and solar energy has been regarded as an ideal green chemistry technology [1][2][3].While TiO 2 -based photocatalysts were most popularly studied in last four decades [4], a huge number of new semiconducting materials have also been developed for photocatalytic applications.For example, a direct III-V group semiconductor GaN has been attracting extensive attention due to the following facts [5][6][7][8]: the band structure of GaN can encompass several possible redox potentials; in addition, the high stability of GaN makes it work well under various harsh conditions.
One-dimensional (1D) nanomaterials have been paid much attention in photocatalytic field owing to their advantageous properties compared to the particulate materials [9].On one hand, the surface-to-volume ratio can be enhanced due to the large surface area exposed on the nanowires, which in turn can act as the active sites for physical and chemical processes.On the other hand, 1D nanostructures provide the possibility of guiding the charge carriers transport in restricted pathways by limiting the transport in the transverse direction.By modifying the nanostructures with cocatalysts 2 International Journal of Photoenergy as the reactive sites, the charge carriers, especially electrons, are prone to accumulate at the reduction sites, thus enhancing the separation of the photogenerated electrons and holes [10].For above reasons, a lot of researches have focused on photocatalytic water splitting over GaN nanowires and great progresses have been achieved in recent years [11,12].For the synthesis of substrate-supported GaN nanostructures, the plasma-assisted molecular beam epitaxy (MBE) [13] and metal-organic chemical vapor deposition (MOCVD) [14] are widely used.However, the high cost and low production efficiency have limited their practical application.
In this paper, we report a simple and cheap chemical vapor deposition (CVD) method to prepare GaN nanowires in a tube furnace at ambient pressure.The growth of GaN nanowires by Au-catalyzed CVD process is based on the vapor-liquid-solid (VLS) mechanism [15].Photocatalytic activities of GaN nanowires for H 2 evolution and CO 2 reduction are performed and the effect of noble metal cocatalyst (e.g., Pt and Rh) on the photocatalytic activity is also clarified.

Fabrication of GaN
Nanowires.GaN nanowires were grown via a two-step process.Pieces of 2 × 2 cm 2 n-type Si (111) substrates (Sinopharm Chemical Reagent Co., Ltd) were ultrasonically cleaned in acetone and alcohol for 30 min each before rinsing with deionized water.Using an ion sputtering coater (SCB-12, KYKY Technology Development Ltd., China), a very thin gold film was coated on the substrates to catalyze the GaN nanowires growth.The Ga precursor, Ga 2 O 3 , was used as received without any further treatment.
As illustrated in Scheme 1, GaN nanowires were grown by means of chemical vapor deposition in a horizontal quartz tube furnace (Φ60 × 1000 mm, GSL-1200X, Hefei Kejing Materials Technology CO., LTD, China).The Si substrate was loaded into a polycrystalline alumina crucible, upright on the Ga source, and then the crucible was put into the middle of the quartz tube.Both the Ga precursor and the substrate were heated to 1000 ∘ C for 1 h and flowed with 60 mL/min NH 3 and then the reactor was naturally cooled down to the room temperature under flow of the carrier gas.

Materials Characterization.
The morphology and microstructure of the as-synthesized sample were observed on a field-emission scanning electron microscope (FE-SEM; S4800, JOEL, Japan, operated at 5 kV) equipped with an EDAX (Genesis XM2) attachment and a high-resolution transmission electron microscope (HRTEM; JEM-2100F, JEOL, Japan).Crystal structure was measured on an Xray diffractometer (XRD; Bruker D8, operated at 40 kV, 40 mA) using Cu K radiation ( = 1.54 Å).UV-vis diffuse reflectance spectra were measured at room temperature in the range of 220-800 nm on a UV-vis spectrophotometer (UV-2700; Shimadzu, Japan) and converted to absorbance by the Kubelka-Munk method.Photoluminescence (PL) spectra were acquired on a spectrofluorometer (Fluorolog-3 system, Horiba Jobin Yvon) with an excitation wavelength of 320 nm.

NH 3
Si substrate

Before reaction After reaction
Scheme 1: CVD reaction setup and growth mechanism for GaN nanowires by the vapor-liquid-solid process.

Photocatalytic Performance.
Photocatalytic water splitting was carried out in a quartz glass cell (220 mL) connected to a glass-closed gas circulation and evacuation system.A 300 W UV-enhanced Xenon lamp was used as the light source.To improve the photocatalytic activity, 0.5 wt% Rh and 0.5 wt% Pt were preloaded on the surface of GaN nanowires by the in situ photodeposition method: under light irradiation, an equivalent molar amount of Na 3 RhCl 6 or H 2 PtCl 6 in solution (50 mL water + 10 mL methanol) was reduced to metallic state and deposited onto catalyst surface, forming the Rh or Pt-loaded catalyst.Before reaction, the cell containing the reactant solution was well evacuated and then ∼2.5 kPa of argon gas was introduced as carrier gas.With increase of irradiation time, the gases evolved photocatalytically were mixed and circulated by an online electromagnetic circulation pump.The gases evolved were in situ analyzed with an online TCD gas chromatograph (Shimadzu GC-8AIT, argon carrier).
Photocatalytic CO 2 reduction was carried out in a quartz glass reactor, in which the GaN nanostructured film on the Si substrate with an area of 2 × 1 cm 2 was placed, and 3 mL of deionized water was injected.The 300 W UV-enhanced Xe lamp was used as the light source.The reactor was connected to a closed gas circulation and evacuation system.After evacuating the online system, high purity of CO 2 was purged into the reactor until a pressure of 80 kPa was achieved.The products from CO 2 reduction were detected by an online gas chromatograph (GC-14B, Shimadzu).

Results and Discussion
GaN nanowires grown on Si substrates are seemingly yellowish.By tuning the important growth parameters such as the starting materials, growth temperature, and the rate of NH 3 flow, a variety of GaN nanowires with different morphologies can be selectively prepared.Shown in Figure 1(a) is a typical FE-SEM image of GaN nanowires with lengths from several tens to hundreds micrometers and quite uniform diameter of ca.80 nm in average.Apparently, the aspect ratio of Figure 2 shows the XRD pattern of GaN nanowires grown on the Si substrates.All peaks except those from Si substrate (2 = 28.6 ∘ ) and Au catalyst (2 = 38.5 ∘ ) can be precisely indexed as the wurtzite GaN structure (JCPDS card, number 74-0243).The refined crystal parameters  =  = 3.189 Å and  = 5.166 Å are in good agreement with previously reported standard values, implying the good crystallinity of as-grown GaN nanowires.In fact, the sharp diffraction peak shown in the XRD pattern (see Figure 2) also confirms that the GaN nanowires are well crystallized.It should be noted that no characteristic peaks corresponding to impurities such as GaON [16] and the starting materials were present in the XRD patterns, further demonstrating the high purity of the GaN nanowires.UV-vis diffuse reflectance spectrum (DRS) and room temperature photoluminescence (PL) spectrum of the asprepared GaN nanowires were measured and the results are shown in Figure 3. From the UV-vis DRS spectrum, we can see a steep absorption edge located at ca. 355 nm, which is slightly shorter than that of the bulk GaN [17], probably due to the quantum confinement effect in nanomaterials.It should be pointed out that, in addition to the main absorption edge, International Journal of Photoenergy the long tail can also be observed in visible region.Highly likely, the long absorption tail resulted from the Si substrate and/or the possible plasmonic effect of Au nanoparticles.The room temperature photoluminescence (PL) is usually adopted to get the radiation-emission-related information such as the crystal defects and impurity levels in semiconductors.As shown in Figure 3, the emission peak centered at 362 nm in the PL spectrum is in good consistency with the standard peak [18] position of wurtzite GaN.Moreover, the sharp and smooth PL peak indicate the high crystalline quality of the as-prepared GaN nanowires.
Previous study has shown that GaN is a promising candidate to split water into H 2 since its conduction band edge of GaN is more negative than the H + /H 2 reduction potential [19].For the as-prepared GaN nanowires, photocatalytic H 2 evolution was performed in presence of methanol (50 mL H 2 O, 10 mL CH 3 OH) as the hole scavenger.In this reaction, 0.5 wt% Pt was in situ photodeposited as cocatalyst on the surface of GaN nanowires.As a comparison, GaN powders were also prepared by CVD method for photocatalytic H 2 evolution under the same conditions as mentioned above.Figure 4 shows the time courses of H 2 evolution on 0.5 wt% Pt/GaN-NWs and 0.5 wt% Pt/GaN-powder, respectively.With increase of irradiation time, H 2 evolution increased almost linearly.The rate of H 2 evolution over 5 h reaction with 0.5 wt% Pt/GaN-NWs was 6582 mol⋅h −1 ⋅g −1 , while that with 0.5 wt% Pt/GaN-powder was only about 25 mol⋅h −1 ⋅g −1 .Clearly, the 1-D GaN nanowires are much more advantageous over the GaN powders for photocatalytic H 2 evolution.
It is well known that the cocatalysts play a significant role in photocatalytic water splitting [20].The interfacial charge transfer from the flat band of GaN to the Femi level of Pt can drive the water splitting more efficiently [21].In this work, we also investigated the effect of another metal cocatalyst Rh, which has been proven to promote H 2 evolution in the Rh/Cr 2 O 3 core-shell structure.As shown in Figure 5, when the same amount of Pt and Rh was loaded, respectively, on the surface of as-prepared GaN nanowires with the same  irradiation area (∼2 × 1 cm 2 ), an obvious difference was found in H 2 generation under the same conditions: Pt was a more effective cocatalyst than Rh for H 2 evolution.It should be pointed out that the structural stability of GaN nanowires has been confirmed since no difference was detected on the XRD patterns of the samples before and after the photocatalytic reaction.
The photocatalytic reduction of atmospheric CO 2 into hydrocarbon fuels is also one of the main targets for conversion of solar energy into chemical energy [22].Herein, CO 2 reduction on Rh/GaN-NWs and on Pt/GaN-NWs was also conducted.Figure 6 shows the production of CH 4 and CO under UV-enhanced Xe lamp irradiation for 13.5 h.It was found that CH 4 and CO were the main products of CO 2 conversion, and the cocatalyst played a significant role in the selectivity of products.For Pt-loaded GaN nanowires, CH 4 was the main product with an average conversion rate of 1304.8 ppm⋅h −1 ⋅g −1 ⋅cm −2 , which was in a strong contrast to an average rate of 424.8 ppm⋅h −1 ⋅g −1 ⋅cm −2 for CO.For Rh-loaded GaN nanowires, both CO and CH 4 were the main products with a rate of 1084.6 ppm⋅h −1 ⋅g −1 ⋅cm −2 and 994 ppm⋅h −1 ⋅g −1 ⋅cm −2 , respectively.The abovementioned results implied that the cocatalysts played a crucial role in the final product of photoreduction reaction.The photocatalytic CO 2 reduction involves a series of multielectrons reaction.Carbon monoxide (CO) is the most common product in photoreduction reactions since this step only needs two electrons and protons and single electron transfer to CO 2 occurs at −0.53V NHE .However, to reform CH 4 with CO 2 , six more electrons and same amounts of protons are required with the redox potential at −0.24V NHE [23].The noble metal, Pt, with larger work function and lower Femi level than Rh, facilitates trapping the generated electrons more readily from the conduction band of GaN [20,24].Therefore, higher photocatalytic conversion of CO 2 to CH 4 is observed when loaded with Pt.

Conclusion
High quality single crystalline GaN nanowires are successfully synthesized through a vapor-liquid-solid process.Compared to the powder GaN samples also prepared by CVD method, GaN nanowires show significantly enhanced photocatalytic activities both for H 2 evolution from water splitting and for CO 2 reduction.Two noble metals Pt and Rh were photodeposited on the surface of GaN nanowires as cocatalysts in above reactions.It has been found that, for H 2 evolution, Pt/GaN nanowires show a higher photocatalytic activity than Rh/GaN nanowires, whereas, for CO 2 reduction, the cocatalyst plays an important role in controlling the multielectron reaction, that is, the final product selectivity: Pt/GaN was beneficial to reduce CO 2 to CH 4 while Rh/GaN produced nearly equal amount of CO and CH 4 .

Figure 1 :
Figure 1: (a) Low magnification SEM image; (b) high magnification SEM image of the GaN nanowires synthesized via a vapor-liquid-solid process; (c) conventional low magnification TEM image; and (d) HRTEM image of a GaN nanowire (inset: FFT diffraction pattern of the corresponding HRTEM).

Figure 2 :
Figure 2: XRD pattern and indices of the GaN nanowires grown on Si substrate by Au-catalyzed vapor-liquid-solid reaction.All peaks except those from Si (2 = 28.6 ∘ ) and Au (2 = 38.5 ∘ ) can be indexed as the wurtzite GaN structure.

Figure 3 :
Figure 3: UV-visible diffuse reflectance spectrum (red) and room temperature photoluminescence spectrum (black) of the as-grown GaN nanowires on Si substrate.

Figure 4 :
Figure 4: Photocatalytic H 2 generation over the 0.5 wt% Pt-loaded GaN nanowires and 0.5 wt% Pt-loaded GaN powder under a 300 W UV-enhanced Xenon lamp irradiation.