Fabrication of ZnO@Ag3PO4 Core-Shell Nanocomposite Arrays as Photoanodes and Their Photoelectric Properties

In this study, we combine the methods of magnetron sputtering, hydrothermal growth, and stepwise deposition to prepare novel ZnO@Ag3PO4 core-shell nanocomposite arrays structure. Through scanning electron microscope (SEM) topography test, energy dispersive spectrometer (EDS) element test and X-ray diffractometry (XRD) component test, we characterize the morphology, element distribution and structural characteristics of ZnO@Ag3PO4 core-shell nanocomposite arrays structure. At the same time, we test the samples for light reflectance, hydrophilicity and photoelectric performance. We find that after deposition of Ag3PO4 on ZnO nanorods, light reflectance decreases. As the time of depositions increases, light reflectance gradually decreases. After the deposition of Ag3PO4, the surface of the sample shows super hydrophilicity, which is beneficial for the photoelectric performance test. Through the optical transient response test, we find that the photo-generated current reaches a maximum when a small amount of Ag3PO4 is deposited. As the time of depositions of Ag3PO4 increases, the photogenerated current gradually decreases. Finally, we conducted an alternating current (AC) impedance test and also verified the correctness of the photocurrent test. Therefore, the structure is expected to be prepared into a photoanode for use in fields such as solar cells.


Cleaning the Conductive Glass (FTO) Substrate
The FTO conductive glass was first ultrasonically cleaned with absolute ethanol for 10 min, and then ultrasonically cleaned with acetone for 10 min. Finally, the FTO conductive glass was ultrasonically cleaned with deionized water for 10 min and then dried.

Preparation of ZnO Seed Layer
In order to successfully grow ZnO nanorods on the surface of FTO, we first use magnetron sputtering to deposit a layer of ZnO seed on the FTO surface. The experiment was carried out under the conditions of a pressure of 4 × 10 −4 Pa, the argon gas flow rate of 40 sscm, then a radio frequency sputtering power of 60 W. We first pre-sputtered for 5 min and then officially sputtered each sample for 2 min. After the sputtering experiment was completed, the samples were taken out after cooling for 60 min.

Preparation of ZnO Nanorods Array
We used the hydrothermal method to grow ZnO nanorods on FTO with a deposited ZnO seed layer. In the experiment, we mixed hexamethyltetramine and zinc nitrate hexahydrate in a ratio of 1:1 as a precursor solution. The concentration of the solution was 40 mmol/L, the reaction temperature was 95 • C, and the reaction time was 4 h. After the reaction time was completed, we took the sample and washed it with deionized water for 3 times, then allowed it to dry naturally.
The reaction in the solution system was supposed to be as follows:

Preparation of Ag 3 PO 4 by Stepwise Dipping Method
In the experiment, we used the stepwise deposition method to prepare Ag 3 PO 4 material. We used AgNO 3 and Na 2 HPO 4 solutions as deposition solutions. The concentration and volume of the deposition solution were 0.02 mol/L and 200 mL, respectively. During each deposition process, the deposited sample was placed in the AgNO 3 solution for 30 min, then transferred to the Na 2 HPO 4 solution for 5 min, and finally rinsed 3 times with deionized water. After the experiment was completed, all samples were taken out and naturally dried in a fume hood for use.

Annealing Experiment
The sample was placed in an annealing furnace and heated from room temperature at a rate of 4 degrees Celsius per minute. And after the temperature was raised to 140 degrees Celsius, maintained the temperature for 80 min. The next step was to stop the program, then waited for it to slowly drop to room temperature automatically, and last took out the sample.

Characterization
The surface structure and morphological characteristics of the composite samples were characterized using a scanning electron microscope (ULTRA 55, Zeiss, Heidenheim, Germany). The samples were analyzed by X-ray diffractometry (D/max-1400, Japanese Science, Tokyo Metropolis, Japan). The light-reflecting properties of the samples were characterized by a solid ultraviolet-visible-near-infrared spectrophotometer (UV-3150, Shimadzu, Shanghai, China). The test wavelength range was 185-3300 nm with a resolution of 0.1 nm. The hydrophilic and hydrophobic properties of the samples were analyzed by a contact angle tester (DSA30, Kruss, Hamburg, Germany). The test contact angle ranged from 0 to 180 • with a resolution of ± 0.01 • .

Optical Performance Test
In the experiment, we used a sample with an effective area of 1 cm 2 on the working electrode. We used 0.1 mol/L Na 2 SO 4 solution (pH = 7) as the electrolyte, the electrode with 0.5 mm diameter platinum wire and the reference electrode with saturated calomel electrode (SCE). Finally we let the light source enter from the back of the sample. When testing the photo-generated current, we set the bias voltage to 0.3 V, the sampling time to 400 s, and the sampling interval to 40 s. When testing the alternating current impedance, we set the bias voltage to 0 and the frequency range was 10 −2 -10 5 Hz.

Results and Discussions
The hexamethylenetetramine in the solution is first hydrolyzed to form NH 3 .H 2 O. OH − and Zn 2+ form precipitated Zn(OH) 2 , the precipitate dissolves under the action of OH − , and finally forms ZnO through the action of OH − [15][16][17]. The formed ZnO finally forms ZnO nanorods along the ZnO seed layer on the FTO substrate. In the process of depositing Ag 3 PO 4 , the sample is first placed in AgNO 3 solution to fully absorb Ag + on the surface of the sample. In the Na 2 HPO 4 solution, Ag + and PO 4 3− ions are combined to form Ag 3 PO 4 precipitate deposit on the surface of ZnO nanorods.
In Figure 1, (A,B) are SEM images of FTO + ZnO nanorods. We can observe the large-area growth of ZnO nanorods and there is no connection between the nanorods.    [60,61]. As the increase of deposition times, the characteristic diffraction peak of the Ag 3 PO 4 gradually increases. By comparing each diffraction peak of the Ag 3 PO 4 , it can be seen that Ag 3 PO 4 grows uniformly in all directions.  Figure 4 shows the UV-visible reflectance spectrum of the sample. We observed a higher reflectivity of ZnO nanorods in the visible range. After deposition of Ag 3 PO 4 , the reflectance in the visible light band gradually decreases. Since ZnO nanorods have strong light absorption in the ultraviolet light band [62], we can see from Figure 4 that the light reflectance in the 220 nm-380 nm band is almost 0.
From Figure 4, we can observe that as the increase of deposition times of the Ag 3 PO 4 , the light reflection gradually decreases. When Ag 3 PO 4 is deposited 7 times, the light reflection of ZnO nanorods is minimized. Since Ag 3 PO 4 can effectively capture visible light and ultraviolet light of less than 530 nm, the light reflectance decreases rapidly around the 500 nm optical band. In general, the deposition of Ag 3 PO 4 on the surface of ZnO nanorods can effectively reduce the reflection of light.  Figure 5 shows the hydrophilicity of the sample. We observe that when only ZnO nanorods are grown on FTO conductive glass, the contact angle is about 13 • . This is because ZnO itself is a polar molecule and has certain hydrophilic properties. After depositing Ag 3 PO 4 , the contact angle is always 0 • no matter how many times it is deposited. Since ZnO is a polar molecule as a metal oxide, Ag 3 PO 4 is a metal inorganic salt and is also a substance having a polar structure. Polar molecules are charged due to internal imbalance, so the two are highly affinitive with polar water due to mutual electrostatic attraction. Therefore the static contact angle is always 0 • . The sample is completely hydrophilic so that it is sufficiently in contact with the solution.  Figure 6 shows a schematic diagram of the photocurrent generated by the ZnO@Ag 3 PO 4 heterojunction under illumination. When ZnO nanorods and Ag 3 PO 4 are in contact with each other, a heterojunction is formed. After the bias is applied, an energy band structure as shown in Figure 6 is formed. When light illuminates two materials, the electrons in the valence band of the two materials will transition to the conduction band. In addition, it will leave holes in the valence band. Under the applied bias voltage, the electrons flow to the conduction band of Ag 3 PO 4 and flow out through the external Pt electrode. The holes migrate from the Ag 3 PO 4 valence band to the ZnO valence band and are transported to the external circuit through the FTO conductive glass together with the holes on the ZnO valence band.   Figure 7, the current of each sample under illumination conditions is greater than that under dark conditions. Because the electrons on the valence band of Ag 3 PO 4 and ZnO cannot obtain sufficient energy in the dark condition, they cannot transit into the conduction band, and no photogenerated carrier is generated, and transient photocurrent cannot be formed. In addition, we also observe that the value of the photocurrent of Ag 3 PO 4 deposited on the surface of ZnO first increases and then decreases. When Ag 3 PO 4 is deposited once, the photocurrent value reach 1.3 mA/cm 2 . After a continuous photoresponse test, the photocurrent value is stable at around 1.0 mA/cm 2 . The photocurrent value decrease with the increase of the time of Ag 3 PO 4 deposited on the surface. When Ag 3 PO 4 is deposited 7 times, the photocurrent value dropped to about 0.4 mA/cm 2 . When there is only ZnO material, the light illuminates the sample and the electrons receive sufficient energy to transition to the conduction band. Because of the single material, the electron-hole pair recombination efficiency is very high, and the electrons in the conduction band release energy back into the valence band and recombine with the holes in the valence band [63][64][65]. After depositing a small amount of Ag 3 PO 4 on the surface of ZnO, Ag 3 PO 4 material can also effectively absorb photons and improve the utilization of light. After the heterojunction is formed between ZnO and Ag 3 PO 4 , the recombination of electron-hole pairs can be effectively suppressed, and the electron-hole pairs migration can be enhanced to promote the enhancement of photocurrent [66,67]. As the time of surface deposition of Ag 3 PO 4 increases, the increase in the number of surface Ag 3 PO 4 leads to the increase of surface resistance, which ultimately reduces the photocurrent.
In order to better reveal the principle of photocurrent response, we conducted AC impedance test results as shown in Figure 8. We observe that when only ZnO nanorods are used, the impedance value is very high, and the impedance value is greatly reduced after the deposition of Ag 3 PO 4 to form a heterojunction. As the time of depositions of Ag 3 PO 4 increases, the impedance value also increases. Comparing Figures 7 and 8, we observe that the impedance of the samples with only ZnO nanorods is higher than the impedance of the samples with multiple depositions of Ag 3 PO 4 , but the photocurrent value of the samples with only ZnO nanorods is higher than that of the samples with multiple depositions of Ag 3 PO 4 . Because a small amount of Ag 3 PO 4 is deposited on the surface, the formation of a heterojunction between ZnO and Ag 3 PO 4 can suppress the recombination of electron-hole pairs and accelerate the separation of electron-hole pairs, so that the photocurrent can be effectively enhanced by illumination. However, as the deposition of Ag 3 PO 4 on the surface of ZnO nanorods increases, Ag 3 PO 4 will cover the ZnO nanorods on a large surface, and the holes in the ZnO valence band will remain in the ZnO material. As the number of holes trapped in the ZnO nanorods increases, the photocurrent formed is weakened and the ZnO nanorods are also corroded. For samples with only ZnO nanorods, this is not the case. Therefore, we can obtain the best photoelectric performance of Ag 3 PO 4 deposited on the surface of ZnO nanorods once.

Conclusions
In this study, we successfully prepared ZnO@Ag 3 PO 4 core-shell nanocomposite arrays structure by the hydrothermal method and the stepwise deposition method. The morphology, structure and elements of the material were analyzed by SEM, XRD and EDS. Light reflection indicates that deposition of Ag 3 PO 4 on the surface of ZnO nanorods can effectively reduce the reflection of light. The deposition of Ag 3 PO 4 can simultaneously make the surface of the material superhydrophilic. When the ZnO@Ag 3 PO 4 core-shell nanocomposite arrays structure is used as a photoanode for photoelectric performance testing, through photocurrent response test, we find that depositing a small amount of Ag 3 PO 4 on the surface of ZnO nanorods can enhance photocurrent. When Ag 3 PO 4 is deposited once on the surface of ZnO nanorods, the transient photocurrent reached 1.3 mA/cm 2 , and the photocurrent was stabilized at 1.0 mA/cm 2 after repeated cycles. Through the alternating current (AC) impedance analysis, we find that the ZnO@Ag 3 PO 4 core-shell nanocomposite arrays have the lowest impedance when Ag 3 PO 4 is deposited once on the surface of ZnO nanorods. Therefore, the ZnO@Ag3PO4 core-shell nanocomposite arrays structure can be applied to high-speed photoelectric switches, solar cells, sensors and other fields.

Conflicts of Interest:
The authors declare no conflicts of interest.