Enhanced UV Photoresponsivity of ZnO Nanorods Decorated with Ag2S/ZnS Nanoparticles by Successive Ionic Layer Adsorption and Reaction Method

Recently, different kinds of energy band structures have been utilized to improve the photoelectric properties of zinc oxide (ZnO). In this work, ZnO nanorods were prepared by the hydrothermal method and then decorated with silver sulfide (Ag2S)/zinc sulfide (ZnS) via two-step successive ionic layer adsorption and reaction method. The photoelectric properties of nanocomposites are investigated. The results show that ZnO decorated with Ag2S/ZnS can improve the photocurrent of photodetectors from 0.34 to 0.56 A at bias of 9 V. With the immersion time increasing from 15 to 60 minutes, the photocurrent of photodetectors increases by 0.22 A. The holes in the valence band of ZnO can be transferred to the valence band of ZnS and Ag2S, which promotes the separation and suppresses the recombination of hole-electron pairs generated in ZnO. Moreover, electrons excited by ultraviolet (UV) light in Ag2S can also be injected into the conduction band of ZnO, which causes the photocurrent to increase more than the ZnO photodetector.


Introduction
Recently, increasing demand for clean energy, portable electronics, space and astronomical research, optical communications, and fire monitoring has caused UV photodetectors to be the subject of considerable attention due to their good flexibility, low-cost fabrication, and high sensitivity [1][2][3][4][5][6]. Many wide-bandgap semiconductors associated with this type of photodetectors have been explored [5,[7][8][9][10][11][12]. Among wide-bandgap materials, zinc oxide has become a good candidate for UV photodetection due to its wide bandgap (3.37 eV), high exciton binding energy (60 meV), high chemical and thermal stability, low cost, and strong emission at room temperature [13][14][15][16]. To improve the properties of ZnO, incorporating noble metal nanostructures, compounding with carbon nanomaterials, and decorating with semiconductor quantum dots are effective approaches [4,[17][18][19]. With decoration of semiconductors with different band structures, holes or electrons can be transferred from ZnO, which promotes separation and suppresses the recombination of hole-electron pairs generated in ZnO [20][21][22][23]. Ag 2 S as a direct, narrow bandgap (1.1 eV) semiconductor with optical absorption similar to silicon has attracted much attention as a promising candidate for photocatalysis and photoconduction [24][25][26]. Studies also proved that after accompanying Ag 2 S with ZnO, type-II heterojunctions are formed. Due to the different valence bands of Ag 2 S and ZnO, the holes in the valence band of ZnO can be transferred to the valence band of Ag 2 S. This process can promote separation and suppress the recombination of hole-electron pairs generated in ZnO, leading to the improved optoelectrical properties of ZnO [27][28][29][30]. Recently, Li and colleagues prepared Ag 2 S-coupled ZnO microspheres with 1.68 µA/cm 2 at 0.2 V under visible light [31]. Chen and colleagues prepared a Ag 2 S/ZnO core-shell nanoheterojunction with high photosensitivity in the wide spectral range from 400 to 1100 nm and a response time as short as 5 ms [32]. In this work, ZnO nanorods are prepared with the hydrothermal method. Then, ZnS and Ag 2 S are deposited via successive ionic layer adsorption and reaction method on the surface of ZnO nanorods. ZnS is an n-type [33][34][35] material, and Ag 2 S is a ptype [36][37][38][39] material. The nanocomposites demonstrate potential application in the fields of photodetection, photocatalysis, and solar cells [31,[40][41][42]. In this work, the optoelectrical properties of nanocomposites are investigated.

Materials and Methods
The ZnO nanomaterials in this paper were prepared by the hydrothermal method on glass substrates with a conductive thin film of indium-doped tin oxides (ITO) on one side. The size of substrates was 1 × 1 cm. Before preparation, the substrates were cleaned with ultrasound successively in acetone, ethanol, and deionized water for 30 min. To prepare ZnO seed layers on ITO substrates by the sol-gel dip-coating method, the substrates were immersed in precursor solution for 15 min, and then, the samples were dried for 15 min. This process was repeated six times. Finally, an annealing treatment was performed in air at 150 • C for 30 min. Then ZnO nanorod arrays were formed on the substrates in a solution consisting of 0.03 M zinc acetate dehydrate (Zn(AC) 2 ·2H 2 O) and 0.03 M hexamethylenetetramine (HMT) at 90 • C for 4 h. S 2− was incorporated into ZnO to form ZnS by immersing ZnO samples in aqueous solution containing 0.02 M Na 2 S for 5 min, 15 min, 30 min, 45 min, and 60 min, respectively and rinsing with pure ethanol. The ZnO nanorod arrays were decorated with Ag 2 S quantum dots (QDs) through the facile successive ionic layer adsorption and reaction (SILAR) method. ZnO samples were successively immersed in two different aqueous solutions, one containing 0.02 M Na 2 S and the other one containing 0.02 M AgNO 3 aqueous solution for 30 min and different time, respectively. During immersion, the solution should be stirred. After immersion, the samples were rinsed with pure ethanol to remove excess precursors and blown dry at room temperature. Then, the Ag 2 S-modified samples were fabricated as metal-semiconductor-metal (MSM) photodetectors with indium (In) electrode. Electrodes were exploited to form ohmic contact between them and the nanocomposites. One electrode was prepared on the ZnO seed layer and the other one was prepared on the top of nanorod arrays.
Surface morphologies of the nanocomposites were characterized using scanning electron microscopy (SEM, HITACH SU70, Tokyo, Japan). Finer details of the nanocomposites were characterized using transmission electron microscopy (TEM, FEI, Hillsboro, OR, USA) and high-resolution transmission electron microscopy (HRTEM, FEI, Hillsboro, OR, USA). The composition and bond band properties of the samples were measured by X-ray photoelectric spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA). Ultraviolet-visible spectroscopy (Shimadzu UV1700-visible spectrophotometer) was utilized to characterize the optical properties. I-V characterization of the as-synthesized devices was measured by an electrochemical workstation (CHI660e, Chenhua instruments Ins., Shanghai, China) with a three-electrode system under UV led (λ = 365 nm). The photoresponsivity spectrum of the devices was obtained by measuring the photocurrent (calibrated with a standard Si photodiode) under the illumination of a UV-enhanced Xe lamp spectrum from 300 to 600 nm using a scanning monochromator (DSR600, Zolix, Beijing, China).

Results and Discussion
The morphologies of ZnO and ZnS/ZnO heterojunctions array prepared by immersing ZnO samples in Na 2 S solution for 5, 15, 30, 45, and 60 min, respectively, are shown in Figure 1a-f. The hexagonal nanorods are not uniform, and the dominant diameter of the nanorods is about 260 nm. The XRD spectrum of the as-synthesized samples is shown in the inset of Figure 1a. Typical peaks belong to the wurtzite hexagonal phase of ZnO (JCPDS 36-1451), as presented in Figure 1a. When ZnO nanorods are immersed in Na 2 S solution, as shown in Figure 1b-f, the solution provides sulfide ions to react with zinc ions dissolved from the ZnO nanorods in order to form ZnS. During this process, the concentration of sulfide ions can be adjusted to influence the formation of ZnS. However, if the concentration is too large, many defects can be formed on the nanocomposites, leading to decreased efficiency. The formation of a ZnS shell can be determined by XPS spectra in Figure 2. The XPS wide-survey spectrum of the sample with an immersion time of 30 min is shown in Figure 2a. The characteristic peaks in the XPS spectrum can be assigned as Zn, S, C, or O, respectively. The ratio of S/Zn is about 0.16. No other impurity peaks can be found, showing that the obtained sample is of high purity. For ZnS/ZnO in Figure 2b, the S peak located at 162 eV corresponds to S 2p from ZnS. The above XPS analysis demonstrates the process of the formation of ZnS after immersion. Therefore, ZnO nanorods were covered by the ZnS shell through immersion.  Figure 1a. When ZnO nanorods are immersed in Na2S solution, as shown in Figure 1b-f, the solution provides sulfide ions to react with zinc ions dissolved from the ZnO nanorods in order to form ZnS. During this process, the concentration of sulfide ions can be adjusted to influence the formation of ZnS. However, if the concentration is too large, many defects can be formed on the nanocomposites, leading to decreased efficiency. The formation of a ZnS shell can be determined by XPS spectra in Figure 2. The XPS wide-survey spectrum of the sample with an immersion time of 30 min is shown in Figure 2a. The characteristic peaks in the XPS spectrum can be assigned as Zn, S, C, or O, respectively. The ratio of S/Zn is about 0.16. No other impurity peaks can be found, showing that the obtained sample is of high purity. For ZnS/ZnO in Figure 2b, the S peak located at 162 eV corresponds to S 2p from ZnS. The above XPS analysis demonstrates the process of the formation of ZnS after immersion. Therefore, ZnO nanorods were covered by the ZnS shell through immersion.  Based on the ZnS/ZnO nanocomposites, the ZnS/ZnO samples with the immersion time of 30 min in Na 2 S solution were then immersed in 0.02 M AgNO 3 aqueous solution for 15, 30, 45, and 60 min, respectively, which allows for the Ag 2 S/ZnS/ZnO nanocomposites to be obtained. The morphologies of the as-synthetized Ag 2 S/ZnS/ZnO nanorod arrays are shown in Figure 3a-d. It can be seen that after immersion, the nanorods were covered with spherical nanoparticles at the short immersion time. Because of the difference between the solubility product constant (K sp ) of ZnS (2.93 × 10 −25 ) and K sp of Ag 2 S (6.69 × 10 −50 ) [43], the cation exchange process occurs where zinc ions are replaced by silver ions, leading to the formation of Ag 2 S, which indicates that the spherical nanoparticles are Ag 2 S. With the increasing immersion time, the nanorods demonstrate a complete shape change from hexagon to ellipse. These results suggest that Ag 2 S was successfully deposited onto the surface of the nanorods. Based on the ZnS/ZnO nanocomposites, the ZnS/ZnO samples with the immersion time of 30 min in Na2S solution were then immersed in 0.02 M AgNO3 aqueous solution for 15, 30, 45, and 60 min, respectively, which allows for the Ag2S/ZnS/ZnO nanocomposites to be obtained. The morphologies of the as-synthetized Ag2S/ZnS/ZnO nanorod arrays are shown in Figure 3a-d. It can be seen that after immersion, the nanorods were covered with spherical nanoparticles at the short immersion time. Because of the difference between the solubility product constant (Ksp) of ZnS (2.93 × 10 −25 ) and Ksp of Ag2S (6.69 × 10 −50 ) [43], the cation exchange process occurs where zinc ions are replaced by silver ions, leading to the formation of Ag2S, which indicates that the spherical nanoparticles are Ag2S. With the increasing immersion time, the nanorods demonstrate a complete shape change from hexagon to ellipse. These results suggest that Ag2S was successfully deposited onto the surface of the nanorods.
In order to confirm the formation of Ag2S/ZnS/ZnO composites, TEM and HRTEM measurements were performed on the nanocomposite with the immersion time of 60 min, as illustrated in Figure 4. A large number of quantum dots evenly deposited onto the nanorod surface is shown in Figure 4a,b, which show that densely distributed QDs are formed on the surface of nanorods, and the diameter of the quantum dots is about 10-15 nm. Figure 4c shows the HRTEM image of the quantum dots deposited on the nanorod surface. It can be observed that the quantum dots have a spherical shape with a diameter of about 10 nm. The obvious lattice arrangement can be found in the quantum dots, and the d-spacing estimated to be 0.253 nm is indexed to the (−103) orientation of Ag2S crystalline.  In order to confirm the formation of Ag 2 S/ZnS/ZnO composites, TEM and HRTEM measurements were performed on the nanocomposite with the immersion time of 60 min, as illustrated in Figure 4. A large number of quantum dots evenly deposited onto the nanorod surface is shown in Figure 4a,b, which show that densely distributed QDs are formed on the surface of nanorods, and the diameter of the quantum dots is about 10-15 nm. Figure 4c shows the HRTEM image of the quantum dots deposited on the nanorod surface. It can be observed that the quantum dots have a spherical shape with a diameter of about 10 nm. The obvious lattice arrangement can be found in the quantum dots, and the d-spacing estimated to be 0.253 nm is indexed to the (−103) orientation of Ag 2 S crystalline.   To further investigate the structures of Ag 2 S/ZnS/ZnO nanocomposites, XPS measurements were measured. Figure 5a shows the Ag 3d region of the XPS spectra. The peak position of Ag 3d 5/2 is located at about 368 eV. This value is in good agreement with the reported values for Ag 2 S. Then, the peak area of Ag elements with different immersion time was calculated, as shown in Figure 5b. It is obvious that the Ag concentration increases with the increasing immersion time. Meanwhile, according to the XPS spectrum, the Ag/Zn ratio increasing from 0.08 to 0.22 indicates the same results. This result is consistent with the SEM images shown in Figure 3. Figure 5c shows the S 2p region of the XPS spectrum with the immersion time of 60 min. The black line represents the experimental data, and the red dots correspond to the fitted curve. Four labelled fitting Gaussian peaks were used to fit the experimental data. The binding energy of the S 2p 3/2 peak located at 161.5 eV is in accordance with the binding energy of ZnS. The lowest energy peak of S 2p 3/2 is located at 160.93 eV, which corresponds to Ag 2 S [44][45][46][47]. These results illustrate that Ag 2 S/ZnS/ZnO nanocomposites were formed after immersion. time was calculated, as shown in Figure 5b. It is obvious that the Ag concentration increases with the increasing immersion time. Meanwhile, according to the XPS spectrum, the Ag/Zn ratio increasing from 0.08 to 0.22 indicates the same results. This result is consistent with the SEM images shown in Figure 3. Figure 5c shows the S 2p region of the XPS spectrum with the immersion time of 60 min. The black line represents the experimental data, and the red dots correspond to the fitted curve. Four labelled fitting Gaussian peaks were used to fit the experimental data. The binding energy of the S 2p3/2 peak located at 161.5 eV is in accordance with the binding energy of ZnS. The lowest energy peak of S 2p3/2 is located at 160.93 eV, which corresponds to Ag2S [44][45][46][47]. These results illustrate that Ag2S/ZnS/ZnO nanocomposites were formed after immersion.  To investigate the optical properties of Ag 2 S/ZnS/ZnO nanocomposites, UV-vis absorption spectra from 350 to 600 nm were examined. For comparison, the spectrum of pure ZnO nanorods was also measured, indicated by the black line. Figure 6 shows the UVvis absorption spectra of Ag 2 S/ZnS/ZnO nanocomposites with various immersion time, revealing that the absorption edge of ZnO is extended to the visible region by decoration with Ag 2 S because of its narrow bandgap of~1.1 eV. When visible light is observable on the nanocomposites, hole-electron pairs are generated in Ag 2 S, leading to the absorption in visible light. With the increasing immersion time, more amounts of Ag 2 S are deposited, thereby causing the increase in absorption in the visible light region.
UV-vis absorption spectra of Ag2S/ZnS/ZnO nanocomposites with various immersion time, revealing that the absorption edge of ZnO is extended to the visible region by deco ration with Ag2S because of its narrow bandgap of ~1.1 eV. When visible light is observa ble on the nanocomposites, hole-electron pairs are generated in Ag2S, leading to the ab sorption in visible light. With the increasing immersion time, more amounts of Ag2S ar deposited, thereby causing the increase in absorption in the visible light region.  Figure 7e shows the photocurrent of photodetectors with various immersion time unde 9 V bias. It is observed that the photocurrent increases with the increase in immersion time. This phenomenon can be attributed to the replacement of ZnS by Ag2S. The photore sponsivity of the photodetector with the immersion of 60 min is shown in Figure 7f. It can be observed that the photoresponsivity increases for the Ag2S/ZnS/ZnO nanocomposite when compared with pure ZnO photodetectors not only in UV regions but also in th visible wavelength region. In addition, the UV-to-visible rejection ratio was improved from 1.48 to 1.82.  Figure 7f. It can be observed that the photoresponsivity increases for the Ag 2 S/ZnS/ZnO nanocomposites when compared with pure ZnO photodetectors not only in UV regions but also in the visible wavelength region. In addition, the UV-to-visible rejection ratio was improved from 1.48 to 1.82.
In order to explain the mechanism of photoresponse of Ag 2 S/ZnS/ZnO nanocomposites, the energy band schematic diagrams of different nanocomposites are shown in Figure 8. Considering the fact that not all amounts of ZnS are replaced by Ag 2 S, there are two types of energy bands in nanocomposites [48,49]. Figure Figure 8b shows the energy band schematic diagrams of Ag 2 S/ZnS/ZnO nanocomposites. It can be seen that the cascade structure represents the stepwise positions of band edges via the redistribution of ZnS and Ag 2 S in order to align Fermi levels. This structure is suitable for the injection of photogenerated electrons from Ag 2 S to ZnO and the transfer of holes from ZnO to Ag 2 S, and it is advantageous for the separation and transmission of hole-electron pairs. Generally, this structure can further increase the photocurrent compared with the ZnS/ZnO structure because of the injection of electrons. With the increasing immersion time, the first type is gradually replaced by the second type, which further increases the photocurrent. These results are consistent with the I-V characteristics of Ag 2 S/ZnO/ZnO nanocomposites with different immersion time. In order to explain the mechanism of photoresponse of Ag2S/ZnS/ZnO nanocomposites, the energy band schematic diagrams of different nanocomposites are shown in Figure 8. Considering the fact that not all amounts of ZnS are replaced by Ag2S, there are two types of energy bands in nanocomposites [48,49]. Figure 8a shows the energy band schematic diagrams of ZnS/ZnO. Under the illumination of 365 nm UV light, electrons are excited from the valence band to the conduction band of ZnO, resulting in an increase in the photocurrent. Because of the different valence bands of ZnS and ZnO, the holes in the valence band of ZnO can be transferred to the valence band of ZnS. This process can promote the separation of photogenerated hole-electron pairs and suppress their recombination in ZnO. Meanwhile, because the photon energy of UV light is smaller than the bandgap of ZnS, no electrons are excited to the conduction band in ZnS. Figure 8b shows the energy band schematic diagrams of Ag2S/ZnS/ZnO nanocomposites. It can be seen that the cascade structure represents the stepwise positions of band edges via the redistribution of ZnS and Ag2S in order to align Fermi levels. This structure is suitable for the injection of photogenerated electrons from Ag2S to ZnO and the transfer of holes from ZnO to Ag2S, and it is advantageous for the separation and transmission of hole-electron pairs. Generally, this structure can further increase the photocurrent compared with the ZnS/ZnO structure because of the injection of electrons. With the increasing immersion time, the first type is gradually replaced by the second type, which further increases the photocurrent. These results are consistent with the I-V characteristics of Ag2S/ZnO/ZnO nanocomposites with different immersion time.

Conclusions
In summary, Ag2S/ZnS/ZnO nanocomposites were prepared on ITO substrates via two-step facile successive ionic layer adsorption and reaction method with different immersion time. SEM and TEM images illustrate that the ZnS and Ag2S were evenly depos-