InGaN-based photoanode with ZnO nanowires for water splitting
© The Author(s) 2016
Received: 28 September 2016
Accepted: 14 November 2016
Published: 14 December 2016
The water splitting properties of InGaN photoanodes equipped with ZnO nanowires were examined in this study. Over the solar spectrum range, the absorbance exhibited a remarkable increase due to the enhanced light absorption caused by the ZnO nanowires. By varying the ZnO nanowires length, the photo-to-current density of photoanodes was increased from 0.017 to 0.205 mA/cm2 at 1.23 V versus reversible hydrogen electrode. Consequently, the incident-photon-to-current efficiency was increased by a factor of 5.5 as the ZnO nanowires growth time increased from 2 to 4 h. The results of this research demonstrate the importance of light absorbance and the surface reaction sites of photoanodes on energy harvesting.
Materials such as Fe2O3, TiO2, ZnO, and InGaN have attracted significant attention in photoelectrochemical applications and solar energy storage because of their suitable band gaps for light absorption [1–4]. Over the past few decades, advances in solar cell and water splitting technologies have focused primarily on structure fabrication and catalyst application [5, 6]. InGaN-based materials have proven to be corrosion-resistant in many aqueous solutions, have been widely investigated, and have been used to produce efficient photoelectric devices with low dislocation density and high internal quantum efficiency (IQE). However, the water splitting mechanism of such devices is not yet fully understood [7–9]. Recently, researchers have attempted to use InGaN-based materials with micro- or nano-structures grown via molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD) for water splitting [10, 11]. By combining GaN materials with other catalysts, such as rare-earth materials, oxides, or metals, the efficiency of GaN materials can be further improved . Surface reaction sites are essential for carrier transportation and GaN materials’ photostability. Pu et al. have shown that Rh/Cr2O3 nanoparticles equipped InGaN/GaN nanowires with additional charge decay pathways can increase overall water splitting efficiency . However, this efficiency is not comparable to that of materials such as SnO2 and TiO2, because of the relatively low light absorption ratio of GaN materials [2, 14].
Normally, the light absorption ratio is as low as less than 20% around visible range. Moreover, the GaN material has a relatively long induced carrier lifetime and short carrier diffusion length, which makes the induced current density further lower than other material. The p-doping problem in p-GaN layer also constrains the performance of photoanode, which results in low hole mobility at room temperature. The influence of the light absorbance and surface area of GaN photoanode on photo-to-current efficiency have not been fully investigated as well.
Zinc oxide (ZnO), with a bandgap energy of 3.2 eV, has been reported to be a suitable model semiconductor for solar water oxidation because of its low onset potential and high electron mobility . Numerous strategies have been developed to adapt ZnO to water splitting, including fabrication of multiple semiconductor systems (Si/ZnO core/shell nanowires) to reduce the hole-electron recombination  and construction of one-dimensional (1-D) nanostructured ZnO-based electrodes with various morphologies for increased surface area and improved charge transportation and light trapping [17–19]. However, few studies related to GaN and ZnO material combinations for water splitting have been reported.
To increase the efficiency of InGaN-based thin film photoanodes, ZnO nanowires have been adapted to InGaN materials grown by the MOCVD method. The correlation between the efficiency of photoanodes and the ZnO nanowire growth time was studied by measuring the light absorption spectrum, the photocurrent density-versus-voltage (J–V) properties, and the incident-photon-to-current efficiency (IPCE).
2.1 Growth of InGaN-based thin film material
Prior to device fabrication, an Aixtron horizontal MOCVD reactor was used to grow broadband light absorption monolithic InGaN wafers on c-plane (0001)-patterned sapphire substrates. The precursors were trimethylgallium (TMGa), triethylgallium (TEGa), trimethylindium (TMIn), and ammonia (NH3). Silane (SiH4) and bis-cyclopentadienyl magnesium (Cp2 Mg) were used as n-type and p-type dopants, respectively. Before the deposition of a GaN nucleation layer, the sapphire wafer was thermally cleaned at 1150 °C in a H2 atmosphere for 10 min. A 30 nm-thick GaN buffer layer was then grown at 500 °C under a reactor pressure of 650 mbar, followed by deposition of a 3 μm-thick undoped GaN (uGaN) layer (Si-doped 8 × 1018 cm−3) at 1030 °C and a reactor pressure of 300 mbar. Individual In0.2Ga0.8N (3 nm)/GaN (15 nm) quantum well (QW) and quantum barrier (QB) were grown at temperatures of 735 and 815 °C. A thin 0.5 nm In0.06Ga0.94N wetting layer was inserted before the InGaN well layer, grown at the same temperature as the well. During the QW growth, the V/III ratio was set at 1.15 × 104, the reactor was 600 mbar, and the TMIn flows for the wetting layer and the InGaN quantum dots (QDs) layer were 8 and 68 μmol/min, respectively. The temperature was increased to 920 °C to grow a 20 nm pAlGaN electron blocking layer (EBL) and 200 nm pGaN layer (p-doping 3 × 1019 cm−3). Additional details are provided elsewhere .
2.2 Growth of ZnO nanowire material
ZnO nanowires were produced in an aqueous solution using a two-step process described elsewhere . A spin-coating method was used to prepare ZnO seeds on InGaN-based thin films. Zinc oxide particles were dropped on pre-cleaned InGaN-based thin films. After 30 min, the excess zinc oxide particles were removed using deionized (DI) water. The samples were placed on a hot plate to vaporize the water and increase the adhesion between the zinc oxide particles and the InGaN surface. ZnO nanowires were then grown by placing ZnO-seeded InGaN samples in solutions with 200 ml of water, 1.485 g of zinc nitrate hexahydrate (Zn(NO3)2 6H2O, 98% purity), and 0.7 g of hexamethylenetetramine (C6H12N4, 99% purity) at 90 °C. To evaluate the growth rate of the ZnO nanowires, different growth times were used for the different samples. The samples were then washed with acetone, ethanol, and DI water for 10 min to remove residual organics and growth solution.
2.3 Characterization of InGaN-based material and ZnO nanowires
Scanning electron microscope (SEM) images of InGaN structures were obtained with a field-emission SEM (S-4300). An acceleration voltage of 1.5 kV and an emission current of 1.5 μA were applied. X-ray diffractometry (XRD) was carried out to investigate the crystal quality of the InGaN samples and ZnO nanowires using the D1 system. The ultraviolet (UV) to visible spectra of the photoanodes were recorded using a Jasco V-650 spectrometer.
2.4 Photoelectrochemical measurements
3 Results and discussion
The photocurrent density versus time (J–T) under an externally applied potential for all samples at 1.23 V versus RHE is shown in Fig. 4b. The photocurrent density of the sample with the 4 h ZnO growth time was approximately 13 times higher than that of the bare InGaN-based thin film photoanode, which is consistent with the previously mentioned J–V results. The enhancement is attributable to the ZnO nanowires’ absorption and light trapping effect, as discussed before. It was observed that as the ZnO nanowire length increased, the photocurrent density of the photoanodes became more stable. As for bare InGaN-based thin film photoanodes, the overshoots caused by separation of photo-generated pairs of electrons and holes were severe. Holes can easily be accumulated at the photoanode surface.
The photoelectrochemical properties of InGaN-based thin film photoanodes with ZnO nanowires were studied in detail. The photocurrent density of InGaN-based thin films was found to be improved tremendously by the application of ZnO nanowires and to remain stable over long time periods. As a result, the IPCE value was also increased. The efficiency enhancement can be attributed to UV absorption by ZnO nanowires, the light trapping effect, and enlarged reaction sites on the thin film surfaces.
JK carried out the device fabrication, measurement and manuscript writing. VD assisted the growth of ZnO nanowires. HL and PL supplied the InGaN wafer. SM, YK and HC participated in the properties measurement. CK helped the setup of the measurement system. ZL and HL supervised the research. All authors read and approved the final manuscript.
This work was supported by the R&D program for Industrial Core Technology, through the Korea Evaluation Institute of Industrial Technology, funded by the Ministry of Knowledge Economy in Korea (Grant No. 10040225) and the National High Technology Program of China (Grant No. 2013AA03A101).
The authors declare that they have no competing interests.
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