SEM was used to investigate the morphology of the ZnO nanowires. Figure 1 shows the high-density vertically aligned ZnO nanowires at different growth times. The ZnO nanowire densities were almost the same at all growth times, which suggests that the seed layer distribution determines the ZnO nanowire density, as shown in Fig. 1a–d. The seed layer can be observed easily in the side-view images in Fig. 1e, f. No obvious change was evident for growth times from 2 to 5 h, nor did the diameters of the nanowires change noticeably, whereas the lengths of the ZnO nanowires increased gradually from 1 to 3 μm. After 4 h of growth, the diameter of a single ZnO nanowire was approximately 250 nm. The ZnO nanowire density can be tuned by changing the seed layer density and deposition time (thereby controlling the DI water exposure time).
The crystalline properties of ZnO nanowires grown on InGaN-based thin films were investigated by the XRD method. As samples with different growth times had similar XRD spectra, only the XRD spectra with and without ZnO nanowires are presented here (see Fig. 2a). The intrinsic sharp diffraction peaks of ZnO materials can be clearly observed and correspond to the peaks of a wurtzite hexagonal structure. The GaN intrinsic diffraction peaks indicate that the thin film produced was of high quality and was oriented in the (0001) direction. The layer information for this innovative photoanode is shown in Fig. 2b. The layers visible in the figure, from the bottom to the top, are sapphire, InGaN/GaN thin film, and ZnO nanowires.
The optical spectra of the samples were examined using a Jasco V-650 spectrophotometer. As Fig. 3a shows, the transmittance of the 0 h ZnO growth sample was lower than that of the 2 and 4 h ZnO growth samples over a wavelength of 400 nm, while the transmittance exhibited the opposite trend at wavelengths below 400 nm. The transmittance value increased by approximately 5% on average, and there was an obvious difference between the 2 h ZnO growth and 4 h ZnO growth samples. The reflectance spectra confirmed that, with ZnO nanowires on the surface, less reflective light was observed over the solar spectrum range, as shown in Fig. 3b. As a result, samples with ZnO nanowires exhibited better light absorbance—more than 80% from 300 to 400 nm—than samples without ZnO nanowires, as shown in Fig. 3c. The light trapping effect and UV light absorption induced by ZnO nanowire material together influenced the photoelectrochemical performance of the samples. In addition, ZnO nanowires on InGaN-based thin film surfaces can effectively enhance the reaction sites and carrier diffusion efficiency during the water splitting process. After ZnO nanowire growth, multiple-layer metals of Cr/Pt/Au with thicknesses of 10/100/1000 nm, which are commonly utilized in InGaN-based devices, were deposited to produce ohmic contact on the partly dry etched nGaN surface.
The J–V properties of the samples with and without ZnO nanowires were studied under one sun illumination with a sweep rate of 10 mV/s, as shown in Fig. 4a. The dark current density was negligible for all of the samples. The photocurrent density of the bare InGaN-based thin film at a potential of 1.23 V (vs. RHE) was approximately 0.015 mA/cm2. For the sample with 2 h of ZnO nanowire growth, the photocurrent density was significantly increased to 0.077 mA/cm2 under the same conditions. The photocurrent density was further increased as the ZnO nanowire growth time increased to 4 h. The photocurrent density was enhanced for two reasons. First, ZnO nanowires on InGaN-based thin film surfaces can effectively increase light trapping, which results in greater light absorption of the device. At the same time, ZnO nanowires have an intrinsic light absorption in the UV range. Second, InGaN-based thin films with ZnO nanowires on their surfaces have larger surface areas than bare InGaN-based thin films. More reaction sites are involved in the photo-to-current reaction under sunlight exposure, which also facilitates the energy conversion efficiency.
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 IPCE curves of samples with ZnO growth time of 2 and 4 h were measured at 1.23 versus RHE, as shown in Fig. 5a. For InGaN-based thin film with a 2 h ZnO growth time, the IPCE value reached a maximum of 0.8% at a wavelength of 370 nm. For the InGaN-based thin film with a 4 h ZnO growth time, the IPCE value increased to 4.4% at a wavelength of 370 nm. The wavelength absorption range for these two samples was 350–430 nm. Because of the low light absorption rate of the QW region, no distinct IPCE peak was observed at approximately 470 nm, which is consistent with the indium composition of the single QW. The absorbance spectra indicated that the light absorption increased from 60 to 80% around the UV range. However, the photocurrent density and IPCE enhancements were much higher, which means that carrier diffusion and reaction sites also play key roles in the behavior of InGaN-based photoanodes. This finding should be confirmed by other measurement methods in the future. The simplified band diagram is demonstrated in Fig. 5b. Without bias, the generated carriers face large potential when flow from the active region to the P or N electrode separately. Under the bias, the band incline efficiently increases the carriers overflow the potential barriers.