Enhancement of Photoelectrochemical Performance of Ag@ZnO Nanowires: Experiment and Mechanism

This paper focuses on the enhancement of photoelectrochemical (PEC) performance of uniform silver nanoparticles-decorated ZnO (Ag@ZnO) nanowires, which have been synthesized by two-step chemical vapor deposition to prepare ZnO nanowires then magnetron sputtering method to deposit Ag nanoparticles. Moreover, we analyzed the mechanisms of the PEC behavior of the Ag@ZnO nanowires. The PEC characteristics show that the current density of Ag@ZnO nanowires increased comparing to that of unmodified ZnO nanowires. The optimized content of the Ag-decorated ZnO photoelectrode is up to the maximum photocurrent density of 24.8 μAcm at 1 V vs. Ag/AgCl, which was almost four times than that of the unmodified ZnO photoelectrode. Based on the surface plasmon resonance (SPR), effect of Ag nanoparticles was enhanced PEC performance of the Ag@ZnO nanowires. Because SPR effect of Ag nanoparticles extended the light absorption and enhanced the separation efficiency of the photogenerated electron-hole pairs. The remarkable PEC properties offer metals-semiconductor compound nanostructures materials as a promising electron source for high current density applications.

In a previous report, uniform silver nanoparticlesdecorated ZnO (Ag@ZnO) nanowires composite which were successfully synthesized by two-step CVD and MS method are reported [54,55]. We studied the structure and optical properties of Ag@ZnO nanowires by magnetron sputtering method at sputtering time of 80, 100, and 120 s. However, there are no detail reports on the study of deposition parameters, which will impact on the film properties. The analysis about the PEC behavior of the Ag@ZnO nanowires is seldom reported. In the present paper, using transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques, the observed morphology and structure of Ag@ZnO nanowires were analyzed. The PEC performance of the samples was investigated. Ag@ZnO nanowires exhibit excellent PEC performance compared to unmodified ZnO nanowires, which are likely to be the potential applications in photoelectric devices.

Experimental Section
2.1. Grown of Samples. The Ag@ZnO nanowires were synthesized on quartz substrates by two-step CVD and MS method. The ZnO nanowires were grown by the CVD, and then Ag nanoparticles were deposited on ZnO nanowires using the MS system. As shown in Figure 1, ZnO NWs decorated with Ag nanoparticles were manufactured in the two steps of CVD and MS. The precursor is the powder of ZnO (1 g) on 1D nanostructure for nanowires. It was placed into an alumina boat and loaded into the tubular furnace. The Au-coated quartz substrates were placed downstream from the powders. The furnace was heated to a preset temperature (1250°C). Ar was used as carrier gas during growth at the constant flow rate and pressure: 100 sccm, 50 Pa for ZnO nanowires. Afterwards, the furnace was naturally cooled down to room temperature. Radio frequency target (RFT) is an Ag disc of 60 mm in diameter with a purity of 99.99%. The base pressure in the deposition chamber and radio frequency power were 6.0 × 10 -4 Pa and 3 W, respectively. The growth of sample proceeded in the growth ambient with the Ar of 20 sccm at a constant working pressure of 1.0 Pa, and the sputtering times are 80,100, and 120 s, respectively.

Characteristic of Samlpes.
The morphologies and components of Ag@ZnO and unmodified ZnO nanowires were observed by using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX), and TEM. The crystallinity was analyzed by using XRD equipped with monochromated Cu Ka irradiation. PEC properties of sample photoanodes were carried out in a three-electrode cell at room temperature. Figure 2 illustrates the schematic diagram of experimental setup. Briefly, 0.05 mol/L Na 2 SO 4 solutions were used as electrolytes, through which nitrogen was bubbled. The illumination source was a 200-W Xe arc lamp.   To confirm the attachment between nanoparticles and ZnO nanowires on the surface, TEM was used, and the observed results were shown in Figures 4(a)-4(c). The figures are the TEM images of a single silver composite ZnO nanowire after ultrasonic dispersion. As can be seen from Figure 4(a), only a small number of silver nanoparticles were attached to the surface of ZnO nanowires when the sputtering time was 80 s. With the continuous increase of sputtering time of silver particles, the particle distribution density and particle size on the surface of ZnO nanowires gradually increased. When the sputtering time was 120 s, the silver particles on the surface appeared clustering. As shown in the magnification of surface particles of Ag/ZnO under highpower electron microscope, Figure 4(a) shows that the silver sputtering time was the shortest, and the crystal array of ZnO could be seen most clearly. The SAED pattern indicates that the crystal lattice of ZnO is clear and the lattice spacing is 0.28 nm, which corresponds to the diffraction crystal surface (100) of ZnO. As the sputtering time was prolonged, the number of silver particles on the surface of the nanowires increased, covering the surface of ZnO, and the crystal texture was not as clear as that in Figure 4(b). But the ZnO lattice arrangement can also be seen clearly in Figure 4(b). Moreover, in Figure 3(c), in addition to the (100) crystal plane, there is also a ZnO (110) crystal plane with lattice spacing of 0.16 nm and a silver diffraction ring.

Results and Discussion
The XRD was used to study the structure of ZnO nanowires which were modified or not with Ag. As shown in  . The XRD pattern depicts a set of well-defined diffraction peaks which indicated the crystalline nature of the as-synthesized nanowires, in contrast with the other diffraction peaks, because Ag and Zn exist in the synthesized Ag@ZnO nanowires. The XRD patterns illustrated that the peaks at two theta (2θ) values of 38.24°can be attributed to the prominent (111) diffraction patterns of nanocrystalline Ag particles. When the Ag content was further increased, the diffraction peak intensity of ZnO slightly decreased, while the diffraction peak intensity of Ag increased, which was due to the increase of covered surface by the Ag particles and consistent with SEM and EDX analysis. Moreover, the diffraction peaks in the XRD figure are clear and sharp, indicating that the crystallinity is good, the crystal defects are few, and no impurity peaks proves that the purity is very high.
The typical EDX results of the unmodified ZnO and Ag@ZnO nanowires were plotted in Figures 6(a)-6(d). From EDX spectra, there are clear peaks of the element (O and Zn for unmodified ZnO; O, Zn, and Ag for Ag@ZnO). The EDX data confirmed that the unmodified ZnO nanowires con-tained 63.61 at. % zinc and 39.39 at. % oxygen. And the Ag@ZnO nanowires contained 0.31 at. %, 0.96 at. %, and 1.61 at. % silver. The atomic percent of Ag was gradually increased with the sputtering time being longer. It is worth mentioning that with the increase of silver sputtering time, the content of zinc atoms in the sample material gradually decreases while the content of oxygen atoms gradually increases. In theory, in ZnO material, the atomic ratio of Zn and O should be 1 : 1, but intrinsic donor-type defects of the ZnO exist, namely the oxygen vacancy and zinc clearance, causing more zinc and O atoms than the theoretical value. In this experiment, sputtering time increasing of silver particles made the ratio of zinc and O atoms that were gradually adjusted from greater than 2 : 1 to approximately 1 : 1, so that it was based on silver nanoparticles composite, improved the intrinsic defects of zinc oxide.

Photoelectrochemical
Performance. The PEC performance of unmodified ZnO and Ag@ZnO nanowires photoanodes has been studied by evaluating the current density-voltage (J-V) characteristics under light illumination. The J-V characteristics of the sample photoanodes are shown in Figure 7. As can be seen from Figure 6(a), for the 1.0 V vs Ag/AgCI, the photocurrent density of A@ZnO nanowires reached the highest value of 24.8 μAcm -2 , which is almost six times higher than that of the unmodified ZnO photoelectrode  Journal of Nanomaterials photocurrent density of 4.8 μAcm -2 . Dark scan linear sweep voltammagrams from -0.5 to +1.5 V showed a small current in the range of 0 to 40 μA/cm 2 . When the Ag nanoparticles were modified, the SPR effect of Ag particles enhanced PEC properties. In a previous report, the charge transport within doped or decorated ZnO nanostructure is more efficient compared to ZnO nanostructured electrodes [56][57][58][59].  Figure 7(a) shows the photocurrent response of Ag/ZnOcoated nanowires with different silver contents. As can be seen from the Figure 7, with the increase of silver content, the photocurrent gradually increased. Among them, the optical response of 0.31 at. % Ag/ZnO is poor, the photocurrent density is very weak, and there is a certain response delay. When the silver content is 0.96 at. %, the delayed response disappears. When the silver content is 1.61 at. %, the photocurrent density increased significantly, which was about 1 mA/cm 2 . It is proved that the increase of particle density and size on the surface of nanowires leads to better photosensitivity. As Ag particles are combined with ZnO nanowires, there is electron transfer when they contact with each other.
At this time, the more Ag content there is, the more electrons it will carry. Under the action of external electric field, electrons will transfer to the opposite electrode, thus enhancing the photocurrent density.
The Ag@ZnO nanowires (for 0.96 at. % and 1.61 at. %) were selected for LSV test, and the test results are shown in Figure 7(b). At the same voltage, the current is stronger when there is more silver particles or light. The enhancement of photocurrent for Ag@ZnO nanowires was attributed to the following three factors: firstly, the local field is enhanced by SPR effect of Ag nanoparticles; secondly, the high separation efficiency of photogenic electron-hole pair; and thirdly, due to the interaction between ZnO and Ag, and reduce the charge resistance and charge compound rate. Figure 7(c) shows EIS test diagrams of Ag@ZnO nanowires with and without light. The arc radius of high silver content nanowires is smaller than that of silver content nanowires, and the arc radius with light is smaller than that without light. Since the arc radius in the figure reflects the resistance of the interface layer occurring on the electrode surface, the presence of Ag particles accelerates the electron-hole pair separation of ZnO nanowires, and the interface charge transfer is faster. The interaction between Ag and ZnO can effectively improve the separation and transfer efficiency of electron-hole pairs in ZnO and effectively inhibits the electron-hole pair     Journal of Nanomaterials recombination simultaneously. The results show that this method can effectively inhibit the electron-hole pair recombination and improve the photocatalytic activity of Ag@ZnO nanowires. We believe that Ag@ZnO nanowires can be used as effective materials to enhance photocurrent. Based on the above discussions, the electron-transfer mechanism of the Ag@ZnO nanowires photoanode in PEC is just showed in Figure 7(d). Under the illumination, the electrons are excited to the CB of ZnO, and the same number of holes are created in the in the VB. The energy level of the bottom of the CB of ZnO is higher than the newly formed Fermi energy of the Ag@ZnO nanowires, so the photoexcited electrons would be transferred from ZnO to Ag, driven by the energy difference to enhance the separation efficiency of the photogenerated electron-hole pairs and reduces carrier recombination. Meanwhile, due to SPR excitation, Ag nanoparticles absorb the resonant photons to generate hot electrons, and hot electrons were transferred to the CB of ZnO.

Conclusions
In summary, the morphologies, structure, and PEC properties of successfully synthesized uniform Ag@ZnO nanowires by two-step CVD and MS methods were investigated. Our results indicate that the ZnO nanowires are highly crystalline with a lattice fringe of 0.28 nm, which corresponds to the (0002) planes in the ZnO crystal lattice. The average diameter of the Ag nanoparticles in the ZnO nanowires is estimated as 34.86 nm. With the Ag nanoparticles decoration, visible emission peaks increased. Significantly, Ag@ZnO nanowires exhibit excellent PEC performance compared to unmodified ZnO nanowires. The PEC studies indicate that all electrodes are resistive in nature. The optimized content of the Ag@ZnO nanowires photoelectrode achieved the maximum photocurrent density of 24.8 μAcm -2 at 1 V vs. Ag/AgCl in 0.5 M Na 2 SO 4 , which was almost four times than that of the ZnO nanowires photoelectrode under the same conditions. The enhancement of the Ag@ZnO nanowires PEC performance was attributed to the SPR effect of Ag nanoparticles, which extended the light absorption and enhanced the separation efficiency of the photogenerated electron-hole pairs. Our results indicate that Ag@ZnO nanostructures show great potential as flat panel displays, high brightness electron sources, and photoanodes in PEC devices.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.