Constructing Er-Doped ZnO/CuS/Au Core-Shell Nanowires with Enhanced Photocatalytic and SERS Properties

: In this study, we fabricated Er-doped ZnO/CuS/Au core-shell nanowires using two-step wet chemical methods and an ion-sputtering method on a glass substrate as a bifunctional photocatalytic and surface-enhanced Raman scattering (SERS) substrate. The characteristic properties of as-prepared photocatalysts were conﬁrmed by scanning electron microscopy, X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, DR/UV-Vis spectroscopy, and photoluminescence spectroscopy. Compared with Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires, Er-doped ZnO/CuS/Au core-shell nanowires exhibited remarkably photocatalytic activity to degrade acid orange 7 solutions under blue LED light. These results ascribed to the Er-doped ZnO/CuS/Au core-shell nanowires can enhance the visible-light absorbance and the separation efﬁciency of photogenerated electron-hole pairs, inducing their higher photocatalytic activity under blue LED light. In addition, Er-doped ZnO/CuS/Au core-shell nanowires exhibit high sensitivity, a low detection limit (10 − 6 M), uniformity, recyclability, and stability of SERS performance for detected acid orange 7. on the Er-doped ZnO/CuS/Au core-shell nanowires to Er-doped ZnO/CuS core-shell nanowires, Er-doped ZnO/CuS/Au core-shell nanowires be photocatalytic of the separation of photogenerated electron-hole

Traditional physical techniques (adsorption on activated carbon, coagulation by chemical agents, ion exchange on synthetic adsorbent resins, etc.) can generally be used efficiently to remove dyes [21,22]. Advanced oxidation processes have been described as efficient procedures for obtaining high oxidation yields from several organic compounds such as

Results and Discussion
The surface morphology of Er-doped ZnO nanowires and Er-doped ZnO/CuS coreshell nanowires were characterized by field-emission scanning electron microscopy (FE-SEM). Figure 1a shows the tilt-view FESEM image of vertical-aligned Er-doped ZnO nanowires directly fabricated on the glass substrate with the ZnO seed layer by a facile wet chemical process at 90 • C for 3 h. Er-doped ZnO nanowires exhibited a sharp surface uniformly with an average diameter and length of 100-200 nm and 3-4 µm, respectively. Then, the equal molar of copper(II) acetate and thioacetamide were selected as the Cu and S precursors to grow CuS nanoparticles on the Er-doped ZnO nanowires via a wet chemical process at 90 • C for 2 h. Figure 1b-e displays the tilt-view FESEM images of Er-doped ZnO/CuS core-shell nanowires which were synthesized at the different concentrations of CuS precursor (2.5, 5, 10, 20 mM). As the concentration of CuS reaction precursors increases, the thickness of the Er-doped ZnO nanowires coated with CuS nanoparticles increases gradually. However, when the concentration of CuS reaction precursor exceeds 20 mM, CuS nanoparticles tend to self-aggregate rather than deposit on the Er-doped ZnO nanowires.
The crystal structure of as-prepared Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires were analyzed via an X-ray diffractometer. Figure 2a reveals   The crystal structure of as-prepared Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires were analyzed via an X-ray diffractometer. Figure 2a    The crystal structure of as-prepared Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires were analyzed via an X-ray diffractometer. Figure 2a    Er-doped ZnO/CuS core-shell nanowire is shown in Figure 3c. The interlayer spacings of Er-doped ZnO/CuS core-shell nanowire are obtained at 0.261 nm and 0.280 nm, which can be corresponded to the d-spacing of the (002) lattice plane of the hexagonal ZnO crystal (JCPDS Card No. 75-0576) and (102) lattice plane of the hexagonal CuS crystal (JCPDS Card No. 65-3928), respectively. In addition, the EDS mapping analysis of an Er-doped ZnO/CuS core-shell nanowire (Figure 3d) can demonstrate that the core and shell are mainly composed of ZnO and CuS, respectively. Furthermore, the content of Er was about 0.76 at.%. Therefore, it can be observed that the composition and morphology of the Erdoped ZnO/CuS core-shell nanowires are consistent. Recently, Au nanoparticles deposited on the heterostructures can improve their photocatalytic activity by generating the Schottky barrier to suppress the recombination of electron-hole pairs and increase visible light absorption [34,35]. The FETEM image ( Figure  4a) of an individual Er-doped ZnO/CuS/Au core-shell nanowire was fabricated at the Au deposition time of 30 s. Figure   Recently, Au nanoparticles deposited on the heterostructures can improve their photocatalytic activity by generating the Schottky barrier to suppress the recombination of electron-hole pairs and increase visible light absorption [34,35]  The photocatalytic efficiency of as-synthesized photocatalysts with the same area (2.5 cm × 1.5 cm) was evaluated by degrading acid orange 7 (AO7) under blue LED light. Figure 5a shows the plots of C/C0 in the degradation of AO7, where C0 is the initial concentration of AO7 and C is the instantaneous concentrations of the AO7, respectively. Thus,  The photocatalytic efficiency of as-synthesized photocatalysts with the same area (2.5 cm × 1.5 cm) was evaluated by degrading acid orange 7 (AO7) under blue LED light. Figure 5a shows the plots of C/C 0 in the degradation of AO7, where C 0 is the initial concentration of AO7 and C is the instantaneous concentrations of the AO7, respectively. Thus, it can be ascertained that the photodegradation rate of AO7 followed the order of Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor) > Er-doped ZnO/CuS core-shell nanowires (5 mM CuS precursor) > Er-doped ZnO/CuS core-shell nanowires (2.5 mM CuS precursor) > Er-doped ZnO/CuS core-shell nanowires (20 mM CuS precursor) > Er-doped ZnO nanowires > ZnO nanowires (without Er-doped) > blank (without photocatalysts) after the irradiation time of 3 h. The plots of C/C 0 vs. t reveal that the photocatalytic degradation follows pseudo-first-order reaction in the as-synthesized photocatalysts, as shown in Figure 5b. From the plot of −ln (C/C 0 ) vs. t, rate constant (k) can be determined. The rate constant (k) was calculated to be 0.0000326 (blank), 0.000483 (ZnO nanowires), 0.000665 (Er-doped ZnO nanowires), 0.00626 (Er-doped ZnO/CuS core-shell nanowires (2.5 mM CuS precursor)), 0.00827 (Er-doped ZnO/CuS core-shell nanowires (5 mM CuS precursor)), 0.01004 (Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor)), and 0.000885 min -1 (Er-doped ZnO/CuS core-shell nanowires (20 mM CuS precursor)), respectively. It is obtained that Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor) exhibit the best photocatalytic activity rather than the above five as-synthesized photocatalysts.
In order to further enhance the photocatalytic activity of Er-doped ZnO/CuS coreshell nanowires, the ion-sputtering method can be used to deposit Au nanoparticles on the Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor) at the different Au-sputtering times under blue LED light, as shown in Figure 5c. As a result, it can be ascertained that the photodegradation rate of AO7 followed the order of Er-doped ZnO/CuS/Au core-shell nanowires (30 s) > Er-doped ZnO/CuS/Au core-shell nanowires (45 s) > Er-doped ZnO/CuS/Au core-shell nanowires (15 s) > Er-doped ZnO/CuS coreshell nanowires (without Au nanoparticles). Thus, the Au nanoparticles decorated on the Er-doped ZnO/CuS core-shell nanowires at the different Au-sputtering times can exhibit a slightly higher photocatalytic efficiency than that of Er-doped ZnO/CuS core-shell nanowires. Figure 5d shows the plots of −ln (C/C 0 ) vs. t of Er-doped ZnO/CuS coreshell nanowires at the different Au-sputtering times, for which the rate constant (k) was calculated to be 0.01004 (Er-doped ZnO/CuS core-shell nanowires (without Au nanoparticles)), 0.01405 (Er-doped ZnO/CuS/Au core-shell nanowires (15 s)), 0.02087 (Er-doped ZnO/CuS/Au core-shell nanowires (30 s)), and 0.01517 min -1 (Er-doped ZnO/CuS/Au core-shell nanowires (45 s)), respectively. A simple ion-sputtering method to deposit 30 s Au nanoparticles on Er-doped ZnO/CuS core-shell nanowires can provide about a 2.08 times higher rate constant. The increase in photocatalytic activity in the case of Au nanoparticles shall be ascribed to restrict the recombination of electron-hole pairs under blue LED light. The dark reaction/adsorption data of as-prepared photocatalysts (ZnO nanowires, Er-doped ZnO nanowires, Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor), and Er-doped ZnO/CuS/Au core-shell nanowires (30 s)) is shown in Figure 5e. The results show that the as-prepared photocatalysts are not evident for the dark adsorption, which may be ascorbic to the photocatalysts directly grown on the glass substrates to reduce the dark adsorption. In order to further enhance the photocatalytic activity of Er-doped ZnO/CuS coreshell nanowires, the ion-sputtering method can be used to deposit Au nanoparticles on the Er-doped ZnO/CuS core-shell nanowires (10 mM CuS precursor) at the different Ausputtering times under blue LED light, as shown in Figure 5c. As a result, it can be ascertained that the photodegradation rate of AO7 followed the order of Er-doped ZnO/CuS/Au core-shell nanowires (30 s) > Er-doped ZnO/CuS/Au core-shell nanowires (45 s) > Er-doped ZnO/CuS/Au core-shell nanowires (15 s) > Er-doped ZnO/CuS coreshell nanowires (without Au nanoparticles). Thus, the Au nanoparticles decorated on the Er-doped ZnO/CuS core-shell nanowires at the different Au-sputtering times can exhibit The UV-Vis diffuse reflectance spectra (DRS) of the Er-doped ZnO nanowires, Erdoped ZnO/CuS core-shell nanowires, and Er-doped ZnO/CuS/Au core-shell nanowires are compared in Figure 6a. In contrast to Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires, the as-prepared Er-doped ZnO/CuS/Au core-shell nanowires exhibit the stronger absorption from the UV-to visible-light regions. This result proved that the sequential deposition of CuS nanoparticles and Au nanoparticles could effectively improve the light absorption ability of Er-doped ZnO nanowires to enhance their photocatalytic activity. Furthermore, photoluminescence (PL) measurement is an efficient and suitable tool to understand the improved charge carriers separation rate of the as-prepared Erdoped ZnO/CuS/Au core-shell nanowires over Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires, as shown in Figure 6b. The decrease in PL intensity can enhance the separation efficiency of charge carriers [36]. The weaker UV-and visible-light emission intensity of the Er-doped ZnO/CuS/Au core-shell nanowires indicates a lower recombination rate for the photogenerated electron and hole pair than Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires. Therefore, the optical absorption and emission properties for the Er-doped ZnO/CuS/Au core-shell nanowires shall be benefited from visible-light-driven photocatalytic applications. are compared in Figure 6a. In contrast to Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires, the as-prepared Er-doped ZnO/CuS/Au core-shell nanowires exhibit the stronger absorption from the UV-to visible-light regions. This result proved that the sequential deposition of CuS nanoparticles and Au nanoparticles could effectively improve the light absorption ability of Er-doped ZnO nanowires to enhance their photocatalytic activity. Furthermore, photoluminescence (PL) measurement is an efficient and suitable tool to understand the improved charge carriers separation rate of the as-prepared Er-doped ZnO/CuS/Au core-shell nanowires over Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires, as shown in Figure 6b. The decrease in PL intensity can enhance the separation efficiency of charge carriers [36]. The weaker UVand visible-light emission intensity of the Er-doped ZnO/CuS/Au core-shell nanowires indicates a lower recombination rate for the photogenerated electron and hole pair than Er-doped ZnO nanowires and Er-doped ZnO/CuS core-shell nanowires. Therefore, the optical absorption and emission properties for the Er-doped ZnO/CuS/Au core-shell nanowires shall be benefited from visible-light-driven photocatalytic applications. The results prove that the as-prepared Er-doped ZnO/CuS/Au core-shell nanowires have better photocatalytic performance than the nanopowder type of commercially available photocatalysts (ZnO and P25 TiO2) and CuS under blue LED light, as shown in Figure   Figure 6. (a) UV-vis diffuse reflectance spectra and (b) PL spectra of Er-doped ZnO nanowires, Er-doped ZnO/CuS core-shell nanowires, and Er-doped ZnO/CuS/Au core-shell nanowires.
The results prove that the as-prepared Er-doped ZnO/CuS/Au core-shell nanowires have better photocatalytic performance than the nanopowder type of commercially available photocatalysts (ZnO and P25 TiO 2 ) and CuS under blue LED light, as shown in Figure 7a. As a result, it can be ascertained that the photodegradation rate of AO7 followed the order of Er-doped ZnO/CuS/Au core-shell nanowires > CuS nanopowder > P25 TiO 2 nanopowder > ZnO nanopowder. Figure 7b shows the plots of −ln (C/C 0 ) vs. t of commercial and as-prepared photocatalysts, for which the rate constant (k) was calculated to be 0.0004878 (ZnO nanopowder), 0.0006895 (P25 TiO 2 nanopowder), 0.00325 (CuS nanopowder), and 0.02087 (Er-doped ZnO/CuS/Au core-shell nanowires), respectively. Thus, the Er-doped ZnO/CuS/Au core-shell nanowires reveal a better photodegradation rate of AO7 than commercial P25 TiO 2 , ZnO, and as-synthesized CuS nanopowders under blue LED light. The excellent photocatalytic activity of Er-doped ZnO/CuS/Au core-shell nanowires might be ascribed to the unique heterostructure, which can benefit the separation of charge carriers and presents better stability, which prevents aggregation from improving the photocatalytic activity. In order to understand the reusability of the as-synthesized Er-doped ZnO/CuS/Au core-shell nanowires under blue LED light, the Er-doped ZnO/CuS/Au core-shell nanowires were reused for the photodegradation of AO7. Figure 7c shows that the photocatalytic efficiency of AO7 after five cycles remained 87.9%. This result can demonstrate that Er-doped ZnO/CuS/Au core-shell nanowires contribute to future applications in actual photocatalysis. The degree of mineralization of AO7 solution with Er-doped ZnO/CuS/Au core-shell nanowires was estimated by determining the decrease in the total organic carbon (TOC) for the reaction solution, as shown in Figure 7d. It can be seen that the TOC has gradually decreased with irradiation time.
ZnO/CuS/Au core-shell nanowires were reused for the photodegradation of AO7. Figure  7c shows that the photocatalytic efficiency of AO7 after five cycles remained 87.9%. This result can demonstrate that Er-doped ZnO/CuS/Au core-shell nanowires contribute to future applications in actual photocatalysis. The degree of mineralization of AO7 solution with Er-doped ZnO/CuS/Au core-shell nanowires was estimated by determining the decrease in the total organic carbon (TOC) for the reaction solution, as shown in Figure 7d. It can be seen that the TOC has gradually decreased with irradiation time. Studying the active species formed in the photocatalytic process is crucial to understand the photocatalytic mechanism of Er-doped ZnO/CuS/Au core-shell nanowires. Studying the active species formed in the photocatalytic process is crucial to understand the photocatalytic mechanism of Er-doped ZnO/CuS/Au core-shell nanowires. Therefore, the effects of some scavengers on the photodegradation of AO7 were investigated to evaluate the photocatalytic mechanism. Herein, isopropyl alcohol (IPA), triethanolamine (TEOA), silver nitrate (AgNO 3 ), and L-ascorbic acid (AA) were adopted as quenchers of the hydroxyl radicals (•OH), holes (h + ), electrons (e − ), and superoxide radical anions (•O 2-), respectively [37,38]. Figure 8a shows that the photocatalytic efficiency decreases from 99.2% to 57.5% and 57.2% by adding TEOA and AA. This result demonstrates that •OH and •O 2play as the essential radicals for the photocatalytic degradation of AO7 under blue LED light. As in the above results, the photocatalytic mechanism of Er-doped ZnO/CuS/Au core-shell nanowires is proposed and presented in Figure 8b. For ZnO, the valence and conduction bands were 2.9 and −0.32 eV (vs. NHE), respectively [39]. For CuS, the valence and conduction bands were 1.53 and −0.5 eV (vs. NHE), respectively [40].

Preparation of Er-Doped ZnO Nanowires
The spin coating and thermal annealing processes were used to fabricate the ZnO seed layer onto the glass substrate. A wet chemical method was used to synthesize Erdoped ZnO nanowires on the glass substrate with a ZnO seed layer. In a typical synthesis process, 0.297 g Zn(NO3)2·6H2O, 0.140 g C6H12N4, and 0.016 g Er(NO3)3·5(H2O) were dissolved in 105 mL ultrapure water followed by the addition of 1 mL 1,3-diaminopropane and vigorous stirring for 15 min. The above solution was transferred into a 150 mL sealed bottle in which the glass substrate with a ZnO seed layer was fixed on the wall and heated Due to Er-doped ZnO/CuS/Au, core-shell nanowires with unique three-dimensional heterostructures shall be beneficial to use as a high-performance SERS substrate. Herein, we used Er-doped ZnO/CuS/Au core-shell nanowires with different Au-sputtering times to evaluate the optimal SERS substrate. Figure 9a displays the Raman spectra of AO7 molecules deposited on the Er-doped ZnO/CuS/Au core-shell nanowires with the Ausputtering times of 15, 30, and 45 s, respectively. The prominent characteristic Raman peaks of AO7 are the vibrational peaks at 1098, 1209, 1339, 1420, 1456, and 1602 cm −1 [41]. Furthermore, the Raman intensity of the Er-doped ZnO/CuS/Au core-shell nanowires decreased significantly with increased Au-sputtering times. This result demonstrated that Er-doped ZnO/CuS/Au core-shell nanowires with the Au-sputtering time of 15 s could provide a better hot spot distribution and surface-active sites to enhance the intensity of the Raman signal. Figure 9b displays the Raman spectra of AO7 deposited on the Er-doped ZnO/CuS/Au core-shell nanowires with the Au-sputtering times of 15 s at the different concentrations (10 −4 to 10 −6 M). The intensity of Raman peaks decreased significantly as the concentration of AO7 solution decreased. The AO7 solution with the lowest detected concentration was 10 −6 M. This result proves that the Er-doped ZnO/CuS/Au core-shell nanowires shall be helpful in highly sensitive SERS-base chemical sensing.
The uniformity and reusability of the SERS substrate have a profound impact on its practical application. Herein, we measured the Raman spectra (Figure 9c) from 10 random sites to investigate the uniformity of Er-doped ZnO/CuS/Au core-shell nanowires with deposited AO7 solution (10 −4 M). This result demonstrates that Er-doped ZnO/CuS/Au core-shell nanowires reveal a relatively consistent Raman intensity. Figure 9d shows the Raman spectra of Er-doped ZnO/CuS/Au core-shell nanowires with deposited AO7 solution (10 −5 M) measured during the five recycling processes. The AO7 could be entirely removed by rinsing and being irradiated UV light. Furthermore, the Raman intensity of Er-doped ZnO/CuS/Au core-shell nanowires at 1339 cm -1 for AO7 decreased by less than 25% over five cycles. This result reveals that the Er-doped ZnO/CuS/Au core-shell nanowires exhibit highly efficient reusability.

Preparation of Er-Doped ZnO Nanowires
The spin coating and thermal annealing processes were used to fabricate the ZnO seed layer onto the glass substrate. A wet chemical method was used to synthesize Erdoped ZnO nanowires on the glass substrate with a ZnO seed layer. In a typical synthesis process, 0.297 g Zn(NO3)2·6H2O, 0.140 g C6H12N4, and 0.016 g Er(NO3)3·5(H2O) were dissolved in 105 mL ultrapure water followed by the addition of 1 mL 1,3-diaminopropane and vigorous stirring for 15 min. The above solution was transferred into a 150 mL sealed bottle in which the glass substrate with a ZnO seed layer was fixed on the wall and heated

Preparation of Er-Doped ZnO Nanowires
The spin coating and thermal annealing processes were used to fabricate the ZnO seed layer onto the glass substrate. A wet chemical method was used to synthesize Er-doped ZnO nanowires on the glass substrate with a ZnO seed layer. In a typical synthesis process, 0.297 g Zn(NO 3 ) 2 ·6H 2 O, 0.140 g C 6 H 12 N 4 , and 0.016 g Er(NO 3 ) 3 ·5(H 2 O) were dissolved in 105 mL ultrapure water followed by the addition of 1 mL 1,3-diaminopropane and vigorous stirring for 15 min. The above solution was transferred into a 150 mL sealed bottle in which the glass substrate with a ZnO seed layer was fixed on the wall and heated at 90 • C for 3 h. The Er-doped ZnO nanowires were washed with ultrapure water and ethanol, and dried at 60 • C for 1 h.

Preparation of Er-Doped ZnO/CuS/Au Core-Shell Nanowires
CuS film was synthesized onto the Er-doped ZnO nanowires through a wet chemical method. In a typical synthesis process, the different concentrations of CuS precursor (equal molar Cu(CH 3 CO 2 ) 2 · H 2 O and CH 3 CSNH 2 ) were dissolved in 50 mL ultrapure water by vigorously stirring for 15 min. The above solution was transferred into a 60 mL sealed bottle in which the Er-doped ZnO nanowires were fixed on the wall and heated at 90 • C for 2 h. The Er-doped ZnO/CuS core-shell nanowires were washed with ultrapure water and ethanol, and dried at 60 • C for 1 h. Au nanoparticles were used to decorate the Er-doped ZnO/CuS core-shell nanowires to form Er-doped ZnO/CuS/Au core-shell nanowires by an ion sputtering system (Cressington 108) under the different deposition times.

Photocatalytic Activity Test
The photocatalytic activities of photocatalysts were evaluated via the degradation of acid orange 7 (AO7) solution (0.047 mM) without adjusting the pH value. In a typical photocatalytic process, a blue light LED (λ max = 465 nm, 100 W) lamp was used as a visible light source. The concentration of the AO7 solution was analyzed by using a DR/UV-Vis spectrometer (Hitachi U-2900, Tokyo, Japan) to record the change in the characteristic absorption band (λ = 465 nm). The photocatalytic efficiency of the photocatalysts under blue LED light is defined as C/C 0 , in which C 0 and C are the initial concentration and instantaneous concentration of the AO7 solution, respectively.

Conclusions
This study demonstrates that two-step wet chemical and ion-sputtering methods can be used to fabricate Er-doped ZnO/CuS/Au core-shell nanowires as a high-performance photocatalytic and SERS substrate. The effects of different concentrations of CuS precursor and Au-sputtering times on the Er-doped ZnO/CuS/Au core-shell nanowires for photocatalytic performance were systematically investigated. Compared to Er-doped ZnO nanowires or Er-doped ZnO/CuS core-shell nanowires, Er-doped ZnO/CuS/Au core-shell nanowires can be favorable to improve photocatalytic degradation of AO7 solution under blue LED light by increasing the separation of photogenerated electron-hole pairs and their light-harvesting ability. In addition, Er-doped ZnO/CuS/Au core-shell nanowires exhibited good sensitivity, reproducibility, and reusability for detected AO7 solutions. Furthermore, the Er-doped ZnO/CuS/Au core-shell nanowires revealed a low cost and sample approach, high visible-light-induced photocatalytic activity, and high photocatalytic and SERS reusability, which shall be beneficial for different kinds of applications.