1. Introduction {#sec1}
===============

Surface-enhanced Raman scattering (SERS) is a no-label, lossless, and ultrasensitive spectral detection technique for identifying trace analytes and enables many applications, such as medicine, chemistry, biology, and so on.^[@ref1]−[@ref3]^ It is widely accepted that the SERS effect comes from two main factors: chemical enhancements (CM) and electromagnetic (EM) effect.^[@ref4],[@ref5]^ CM is based on the enhancement effect due to the charge transfer (CT) caused by the photoinduction of adsorbed molecules and surfaces.^[@ref6]−[@ref8]^ In contrast, the core of the EM effect is that the local surface plasma oscillations on the noble metal surface under the action of an EM field greatly enhance the Raman signal of the target molecule.^[@ref9]−[@ref11]^ To date, various noble metallic nanoparticles (NPs), for instance, Ag, Au, Pd, and Pt, have been extensively used as SERS substrates.^[@ref12]−[@ref19]^ However, these metal nanostructures do not find widespread use in realistic applications because of their high cost. Naturally, scientists have developed semiconductor materials as the SERS substrate,^[@ref20]−[@ref22]^ but the drawback is their lower SERS enhancement effect. Therefore, a precious metal was intentionally added to semiconductors to form hybrid semiconductor materials as SERS substrates, which combine the merits of both materials: the high SERS sensitivity of noble metals and the low cost of semiconductor materials. For example, Au/CdS, SiO~2~/Au, ZnO/Ag, Fe~3~O~4~/Au, and TiO~2~/Ag composites were employed as effective SERS substrates with high sensitivity,^[@ref23]−[@ref26]^ excellent detection limit, rapid response, and fingerprint effect. Unfortunately, the SERS study of such synergetic contribution of noble metals and semiconductors in hybrid nanocomposites is still relatively rare, especially for Cu~2~O.

Cu~2~O has a direct band gap of 2.2 eV and has good electrical conductivity as well as wide applications in magnetic field radiation, batteries, solar conversion, gas sensing, magnetic storage media, and catalysis.^[@ref27]−[@ref29]^ The SERS ability of Cu~2~O hydrosol was first put forward by Kudelski et al.^[@ref30]^ Shortly after, structures based on Cu~2~O NSs became a research hot spot because of their significant advantages with dense hot spots. However, the study on the synergetic effect of different noble metal (Au or Ag) hybrid cuprous oxides as SERS substrates has seldom been reported. In this work, we prepared highly uniform Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs as SERS substrates, and explored the SERS performance of R6G molecules. It is found that these substrates have ultrahigh SERS activity and reproducibility, on which the lowest detected threshold of R6G can be as low as 1.0 × 10^--12^ M. DDA simulation was performed to plot the EM field around Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs. It was further certified that the high Raman sensitivity due to EM enhancement mechanisms exhibited a strong electrical field surrounding the metal NPs and the region between Au (Ag) NPs and Cu~2~O NSs. In addition, CM was explored and the contribution of the CT process of MS molecules to SERS was investigated by the Herzberg--Teller theory.^[@ref31]^ The results show that the coupling between the semiconductor Fermi level and the metal work function is related to the CT process. This research is important for extending the applicability of Raman spectroscopy to various charge transfer problems on MS molecular surfaces and for developing novel MS Raman probes.

2. Results and Discussion {#sec2}
=========================

2.1. Morphology and Microstructure {#sec2.1}
----------------------------------

The scanning electron microscope (SEM) image of the Cu~2~O NSs is presented in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}a. The samples were observed to have a uniformly spherical morphology with an average diameter of about 200 nm. The surface of the Cu~2~O NS is rough and its corresponding high-resolution electron microscopy (HRTEM) image confirms its high-quality polycrystalline nature, as shown in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}b,c. The lattice spacing between the neighboring planes is 0.244 nm, corresponding to the (111) crystal plane of Cu~2~O. Its selected-area electron diffraction (SAED) (inset of [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}c) exhibits diffraction rings, indicating that the Cu~2~O NS has a polycrystalline structure.

![SEM, TEM, and HRTEM images of the Cu~2~O NSs. (a) SEM, (b) TEM, and (c) HRTEM images corresponding to the framed area of (b); the inset is the SAED pattern of an individual NS.](ao0c02301_0001){#fig1}

[Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}a shows the SEM image of the Cu~2~O/Au NSs. It is seen that their surfaces are rough with many Au NPs attached to them. [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}b illustrates the TEM image of the Cu~2~O/Au NSs; it was revealed that Au NPs with a diameter of about 16.76 nm were adsorbed on the surface of the Cu~2~O NSs. The HRTEM image further depicts that the lattice spacing between contiguous planes is 0.231 and 0.244 nm, corresponding to the (111) crystal plane of Au and the (111) plane of Cu~2~O respectively, as displayed in [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}c.

![SEM, TEM, and HRTEM images of the Cu~2~O/Au NSs. (a) SEM, (b) TEM, and (c) HRTEM images recorded on the framed area of (b).](ao0c02301_0009){#fig2}

Next, we analyzed the formation mechanism of the Cu~2~O/Au NSs. Cu~2~O can directly reduce AuCl~4~^--^ at room temperature because the standard reduction potential of Cu^2+^/Cu~2~O is 0.203 V and the standard reduction potential of AuCl~4~^--^/Au is 1.002 V.^[@ref32],[@ref33]^ In consequence, the following reaction occurs^[@ref36]^

The reduction reaction occurs on the surface of the NSs, so the resulting gold NPs can be in situ adsorbed on the surface of the Cu~2~O NSs. Subsequently, Cu~2~O/Au NSs were obtained.

The SEM image of the Cu~2~O/Ag NSs is visualized in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a,b. It is seen that the surface of the Cu~2~O/Ag NSs is rough and the distribution of Cu~2~O/Ag NSs on the Si substrate is uniform. The TEM image of the Cu~2~O/Ag NSs is presented in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}c. It should be noted that the sputtered Ag NPs with a diameter of 20.62 nm are evenly adsorbed on the upper surface of the Cu~2~O NSs. The HRTEM image further elucidates that the lattice spacing between neighboring planes is 0.230 and 0.244 nm, respectively, matching the (111) crystal plane of Ag and the (111) plane of Cu~2~O, as highlighted in [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}d.

![SEM, TEM, and HRTEM images of the Cu~2~O/Ag NSs. SEM (a, b) images of different magnifications, (c) TEM, and the (d) HRTEM image recorded on the framed area of (c).](ao0c02301_0010){#fig3}

[Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"} describes the X-ray diffraction (XRD) patterns of the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs. All of the peaks can be indexed to the cubic structure of Cu~2~O with lattice constants of *a* = *b* = *c* = 4.268 Å (JCPDS card no. 05-0667). The other four peaks can be assigned to the planes (111), (200), (220), and (311) of the Ag/Au cubic structure with *a* = *b* = *c* = 4.086 Å/*a* = *b* = *c* = 4.079 Å (JCPDS no. 04-0783/04-0784). Except these, no other impurities are detected.

![XRD patterns of the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs.](ao0c02301_0011){#fig4}

The composition of the obtained Cu~2~O/Au and Cu~2~O/Ag NSs was further analyzed by the EDX spectrum, as shown in [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"}. It proved that the former is made up of Au, Cu, and O elements and the latter consists of Ag, Cu, and O elements. The Si signal comes from the silicon substrate on which the sample is placed.

![EDX patterns of the NSs. (a) Cu~2~O/Au and (b) Cu~2~O/Ag.](ao0c02301_0012){#fig5}

The elemental composition and chemical states of the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs were explored by X-ray photoelectron spectroscopy (XPS), as shown in [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}. Calibration was based on C (1s) at 284.5 eV for XPS analysis to obtain accurate binding energy. The high-resolution Cu 2p XPS spectrum ([Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}b) shows two peaks of 952.11 and 932.18 eV, which are ascribed to Cu^+^ 2p~1/2~ and Cu^+^ 2p~3/2~, respectively, indicating the presence of Cu~2~O.^[@ref34]^ In addition, the Cu 2p binding energy of the Cu~2~O/Au and Cu~2~O/Ag NSs shifts to a higher energy than that of the bare Cu~2~O NSs, and the peak sites of Au 4f and Ag 3d differ from those of monometallic Au^0^ and Ag^0^.^[@ref35]^ This indicates that the charge distribution at the interface of Cu~2~O--Au (Ag) NSs has changed.

![XPS spectra of the Cu~2~O/Au and Cu~2~O/Ag NSs. (a) Survey scan, (b) Cu region, (c) Au region, and (d) Ag region.](ao0c02301_0013){#fig6}

2.2. SERS Study of the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs {#sec2.2}
-----------------------------------------------------------

A schematic diagram of the Cu~2~O (Cu~2~O/Au or Cu~2~O/Ag) NSs for SERS is illustrated in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}. To study the SERS properties of these substrates, R6G was selected as the probe. [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"} shows the SERS spectra of R6G with different concentrations on the different substrates. It was found that the SERS signals of R6G on the Cu~2~O (Cu~2~O/Au or Cu~2~O/Ag) NS substrate were still observable when its concentration was down to 1.0 × 10^--6^ M (1.0 × 10^--11^ or 1.0 × 10^--12^ M). The peaks were located at 611, 771, 1182, 1310, 1367, 1420, 1509, 1538, 1571, 1600, and 1650 cm^--1^, all of the peaks were consistent with the characteristic peak of R6G.^[@ref36]^

![Schematic illustration of SERS to target molecules on the substrates. (a) Cu~2~O NSs, (b) Cu~2~O/Au NSs, and (c) Cu~2~O/Ag NSs.](ao0c02301_0014){#fig7}

![SERS spectra of different concentrations of the R6G molecule on the different substrates. (a) Cu~2~O NSs, (b) Cu~2~O/Au NSs, (c) Cu~2~O/Ag NSs, (d, e), and (f) linear fit of the SERS intensity of the peak at 611 cm^--1^ vs R6G concentrations.](ao0c02301_0015){#fig8}

To evaluate its limit-of-detection (LOD) on these substrates, the calibration curve of the SERS intensity and concentration of the R6G solution at 611 cm^--1^is shown in [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"}d--f. The LOD values for R6G were identified to be 1.01 × 10^--7^, 8.07 × 10^--12^, and 1.13 × 10^--13^ M (*S*/*N* = 3)^[@ref37]^ on the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NS substrates, respectively. In addition, for practical application, the enhancement factor is another important parameter. Therefore, the characteristic peak at 611 cm^--1^ for R6G was also employed to assess it. The EF of our proposed SERS substrates was calculated using the equation^[@ref38]^where *I*~SERS~ and *I*~0~ are the SERS intensity of the R6G molecule and the Raman signal intensity of solid R6G under the same experimental conditions, respectively. *N*~0~ and *N*~SERS~ represent the number of R6G probe molecules in solid and SERS samples under laser irradiation during the test (details in the Supporting Information [Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf)). Based on our experimental results, the EFs for R6G are 3.87 × 10^3^, 1.25 × 10^8^, and 2.74 × 10^9^ on the Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NS substrates, respectively. The calculated details are shown in [Table S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf) (Supporting Information). In comparison with other hybrid semiconductor substrates, SERS detection parameters of various target molecules such as R6G and other analytes on the other substrates and our substrates are listed in [Table S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf) (Supporting Information). The results show that the EF and LOD on Cu~2~O/Ag NS substrate are obviously higher than those of other substrates. ([Table S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf), Supporting Information). The higher EF and lower LOD are related to the morphology of the Cu~2~O/Ag NSs with the rough surface because, given a certain volume, the sphere has the largest surface area, which is favorable for the capture of probe molecules.

In addition, 40 points were randomly selected from the Cu~2~O (Cu~2~O/Au or Cu~2~O/Ag) NS substrate to collect 3D Raman signals with an R6G concentration of 1.0 × 10^--4^ M (1.0 × 10^--8^ or 1.0 × 10^--11^ M) to validate the repeatability and uniformity of the substrate, as shown in [Figures [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}a, [10](#fig10){ref-type="fig"}a, and [11](#fig11){ref-type="fig"}a. All of the characteristic peaks are neatly arranged. Moreover, the relative standard deviations (RSDs) of the intensity are shown in [Figures [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}b--d, [10](#fig10){ref-type="fig"}b--d, and [11](#fig11){ref-type="fig"}b--d. RSDs of the peaks at 1367, 1509, and 1650 cm^--1^ are 14.42% (17.08 or 14.23%), 15.62% (16.93 or 14.07%), and 15.45% (17.37 or 15.15%), respectively. All calculated RSD values are lower than 18%,^[@ref39]^ which demonstrates the reproducibility of the Cu~2~O (Cu~2~O/Au or Cu~2~O/Ag) NS substrate.

![(a) 3D Raman spectra of 10^--4^ M R6G molecules at 40 randomly selected spots on the Cu~2~O substrate and (b--d) RSD values of the selected peaks at 1367, 1509, and 1650 cm^--1^.](ao0c02301_0016){#fig9}

![(a) 3D Raman spectra of 10^--8^ M R6G molecules at 40 randomly selected spots on the Cu~2~O/Au substrate and (b--d) RSD values of selected peaks at 1367, 1509, and 1650 cm^--1^.](ao0c02301_0002){#fig10}

![(a) 3D Raman spectra of 10^--11^ M R6G molecules at 40 randomly selected spots on the Cu~2~O/Ag substrate and (b--d) RSD values of selected peaks at 1367, 1509, and 1650 cm^--1^.](ao0c02301_0003){#fig11}

2.3. SERS Mechanism {#sec2.3}
-------------------

Wang et al. previously studied the SERS mechanism of Ge and Si nanomaterials and reported that the SERS activity of Ge and Si substrates was mainly caused by the CT effect between the semiconductor and the molecule.^[@ref40]^

For our case, involving the semiconductor-molecule model, the specific CT process is shown in [Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}a. In this process, from the Cu~2~O NSs to R6G, first the electrons in Cu~2~O NSs VB are stimulated by incident light. This creates the electrons in CB of Cu~2~O NSs CB and holes in VB. The excited electrons quickly transfer from the Cu~2~O NSs to the matching energy levels in the R6G mesosphere. They then go back to the Cu~2~O NSs and recombine with the hole. In this process, R6G molecules emit a Raman photon in some vibrational state.^[@ref41]^ Similar is the case for the CT process from R6G to the Cu~2~O NSs.^[@ref42]^ To further verify our SERS model above, the R6G molecules on Cu~2~O NS substrates with an excitation wavelength of 785 nm were tested, as shown in [Figure [13](#fig13){ref-type="fig"}](#fig13){ref-type="fig"}. The results show that under an excitation of 785 nm, the enhancement effect of R6G molecules on the Cu~2~O NSs is significantly lower than 532 nm because the excitation frequency (785 nm; 1.58 eV) is lower than the energy gap between the LUMO and HOMO of the probe molecule (R6G; (2.3 eV) and the band gap (2.2 eV) of the Cu~2~O NSs. Consequently, the CT process in R6G is prohibited under the excitation of 785 nm ([Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}b).

![Schematic of light-induced CT between the Cu~2~O NSs and the R6G molecule at different excitation wavelengths. (a) 532 nm and (b) 785 nm.](ao0c02301_0004){#fig12}

![SERS of the R6G spectrum (1.0 × 10^--3^ M) absorbed on the Cu~2~O NSs at 532 nm (red line) and 785 nm (black line) excitation wavelength.](ao0c02301_0005){#fig13}

To determine the CM mechanism on the SERS of the Cu~2~O substrate modified by noble metal, the SERS spectra of R6G on different substrates were compared. According to the Herzberg--Teller selection rule^[@ref31]^ (the details are provided in the [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf)), the (a″) vibration at 611 cm^--1^ is more selectively enhanced. The experimental results are shown in [Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}.

![Comparison between the SERS spectrum of the R6G molecule adsorbed on the Cu~2~O NSs, Cu~2~O/Au NSs, and Cu~2~O/Ag NSs at 532 nm excitation wavelength. (a) Comparison of the intensity of each peak position on different substrates, (b) comparison of the ratio of the peak intensity in the a″ vibration mode to the peak intensity in the a′ vibration mode on different substrates, and (c) comparison of the SERS spectrum on different substrates.](ao0c02301_0006){#fig14}

We can see that the a″ vibration mode at 611 cm^--1^ is enhanced relatively less compared to the a′ ones at 1367, 1509, and 1650 cm^--1^. The enhancement of the vibration mode (a″) is mainly caused by the CT process. More importantly, we compared the SERS intensity ratio of the a″ mode at 611 cm^--1^ to the a′ modes at 1367, 1509, and 1650 cm^--1^. It reveals that the ratio for Cu~2~O (Cu~2~O/Ag) NSs is significantly higher than that for Cu~2~O/Au NSs, as displayed in [Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}b. It illustrates that the contribution of the CT effect in the SERS spectrum is in the order Cu~2~O/Ag NSs \> Cu~2~O NSs \> Cu~2~O/Au NSs. Furthermore, UV--visible absorption spectroscopy (UV--vis) absorption spectra of the R6G molecule adsorbed on different substrates (Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs) were compared with those of the pristine counterparts and R6G, which is in accordance with the Raman intensity ratio ([Figure S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf), Supporting Information).

We further elucidate the CT contribution to SERS by the energy level diagram, as shown in [Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}a--b. The work function of Au (Ag) is 5.10 (4.26) eV, and the Fermi level of Cu~2~O (4.84 eV) is higher than that of Ag but lower than that of Au. Thus, the charge transfers from Cu~2~O to Au and from Ag to Cu~2~O until the Fermi levels of the two systems are consistent.^[@ref43],[@ref44]^

![(a, b) Charge transfer process between the interface of Cu~2~O and Au (Ag). (c, d) Schematic diagram of the light-induced CT process of Cu~2~O/Au (Ag) NSs and the R6G molecule under 532 nm excitation.](ao0c02301_0007){#fig15}

Based on the above discussion, we return to explain the CT process in SERS and the established MS-molecule model, as depicted in [Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}c,d. For the Ag (Au)--Cu~2~O-R6G system, the electrons in Ag (Au) were first stimulated by the laser, then the excited charge quickly transferred from Ag (Au) to the CB state of the Cu~2~O NSs. However, Ag (4.26 eV) and Au (5.1 eV) have different work functions, and the Fermi level of Cu~2~O is lower than that of Ag but higher than that of Au. In this case, the CT from Cu~2~O to Au dominates the CT process for the Au--Cu~2~O system, whereas the CT from Ag to Cu~2~O dominates the CT process for the Ag---Cu~2~O system. Thus, the CT process of Cu~2~O and R6G in the Au--Cu~2~O-R6G system is inhibited, while the CT process of Cu~2~O and R6G in the Ag--Cu~2~O-R6G system is enhanced. Sequentially, the two systems cause a significant difference in CT efficiency in the R6G molecular system.

To understand our experimental results, DDA simulation was performed to determine the electromagnetic field distribution (E-field) around the Cu~2~O NSs, Cu~2~O/Au NSs, and Cu~2~O/Ag NSs. According to the experimental results, Cu~2~O NSs with a diameter of about 200 nm were used as a model; this is consistent with the SEM image in [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"}. For Cu~2~O/Ag (Au) NSs, Ag (Au) NPs cover the surface of Cu~2~O NSs (see [Figures [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"} and [3](#fig3){ref-type="fig"}). [Figure S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf) shows the UV--vis spectra of Cu~2~O, Cu~2~O/Au, and Cu~2~O/Ag NSs. It can be seen that there are two absorption peaks at 479 and 619 nm for Cu~2~O NSs, at 482 and 616.5 nm for Cu~2~O/Au NSs, and at 507.5 and 615 nm for Cu~2~O/Ag NSs. On account of the dispersion properties of Cu~2~O that neither the refractive index nor the dielectric function is fixed with the wavelength and fluctuates greatly. Therefore, we choose the corresponding wavelengths of the two absorption peaks as the incident plane wave. The incident EM wave enters along the *y*-axis (*k* vector). The value of the dielectric constant is taken from Palik.^[@ref45]^

The E-field intensity distribution is displayed in [Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}. It varies greatly with the wavelength of the incident plane wave, which is consistent with the above experimental results. It is observed in [Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}c--f that due to the synergetic effect of the CT process, there is a highly localized strong E-field between the Ag (Au) NPs and Cu~2~O NSs. Significantly, the high-intensity E-field regions are distributed at the top of each Ag (Au) NP and in the junction region between the Ag (Au) NP and Cu~2~O NSs. This high-intensity localized E-field is very important for ultrasensitive Raman detection of molecules. Therefore, the SERS signal of R6G on the Cu~2~O/Ag (Au) NSs can be observed even with a concentration down to 1.0 × 10^--12^ (1.0 × 10^--11^) M.

![DDA simulations of the E-field distribution on (a, b) the Cu~2~O NSs at incident wavelengths of 479 and 619 nm, (c, d) the Cu~2~O/Au NSs at incident wavelengths of 482 and 616.5 nm, and (e, f) the Cu~2~O/Ag NSs at incident wavelengths of 507 and 615 nm, respectively. The light source is incident from the *y*-axis. The figure shows the *xz*-plane including the polarization direction in logarithmic coordinates.](ao0c02301_0008){#fig16}

This SERS substrate can be applied for the detection of other molecules, such as acridine orange, Sudan III, and so on.

3. Conclusions {#sec3}
==============

In summary, we have successfully prepared Cu~2~O NSs and Cu~2~O/Ag (Au) NSs by vacuum sputtering deposition (in situ reduction). The Cu~2~O NSs, Cu~2~O/Au NSs, and Cu~2~O/Ag NSs showed excellent SERS performances for detecting R6G with LODs of 1.01 × 10^--7^, 8.07 × 10^--12^, and 1.13 × 10^--13^ M, respectively. The extraordinary SERS phenomena were caused by two factors: (i) influence of the connection between the metal work function and semiconductor work function on the CT process of metal--semiconductor-molecular systems and (ii) the synergetic contributions of the strong E-field based on the E-field coupling effect between Ag (Au) and Cu~2~O NSs, and the surface plasmon resonance (SPR) effect of each Ag (Au) NP. The good SERS signal consistency and sensitive SERS detection performance revealed that Cu~2~O/Ag NSs possess great application prospects for biomedicine, environmental science, food safety, and so on.

4. Experimental Details {#sec4}
=======================

4.1. Materials {#sec4.1}
--------------

CuSO~4~·5H~2~O, NaOH, glucose, poly(vinylpyrrolidone) (PVP), ethylene glycol, and HAuCl~4~·4H~2~O were acquired from Tianjin Sailboat Chemical Reagent Technology Co., Ltd. The Ag particle was obtained from Beijing Rui New Material Technology Co., Ltd. All chemicals were analytically pure and could be used without any further purification.

4.2. Synthesis of the Samples {#sec4.2}
-----------------------------

### 4.2.1. Synthesis of Cu~2~O Nanospheres (NSs) {#sec4.2.1}

Cu~2~O NSs were synthesized by improving the glucose reduction method. The process is as follows: first, under an ultrasonic oscillation, 0.2496 g of CuSO~4~ and 0.04 g of PVP were sequentially dissolved in 50 mL of the ethylene glycol solution for 30 min. Second, 25 mL of an aqueous solution containing 0.1 g of NaOH was added to the above solution and stirred for 10 min to completely mix them. Then, a solution of glucose (6 g in 25 mL deionized water) was added for 15 min with slow stirring. Third, the obtained mixture was transferred to a water bath and kept at 80 °C for 1 h, then naturally cooled to room temperature. Finally, the resulting orange sediment was collected by centrifugation, washed several times thoroughly with deionized water and anhydrous ethanol, and then dried for 5 h in a vacuum oven at 55 °C. The product was Cu~2~O NSs.

### 4.2.2. Synthesis of Cu~2~O/Au NSs {#sec4.2.2}

The Cu~2~O NSs obtained by 0.01 g were dispersed in 10 mL of deionized water and then 3 mL of the HAuCl~4~ solution (0.1 wt %) was added to it under magnetic agitation. The solution color could be observed gradually changing from orange to red to black in a few seconds. The products were collected by centrifugation, washed several times with deionized water and anhydrous ethanol, and then dried in vacuum at 55 °C for 5 h.

### 4.2.3. Preparation of Cu~2~O/Ag NSs {#sec4.2.3}

The above-prepared Cu~2~O NSs (0.01 g) were dispersed into 50 mL of absolute ethanol in a beaker under ultrasonic oscillation for 30 min. Then, 30 μL of Cu~2~O NS ethanol solution was pipetted on the silicon wafer (1.0 ×1.0 cm^2^). After that, silver NPs were deposited on this silicon wafer by electron beam evaporation under vacuum condition (5 × 10^--4^ Pa in the cavity). Thus, Cu~2~O/Ag NSs were obtained.

4.3. SERS Measurements {#sec4.3}
----------------------

For the SERS measurements, the aforementioned Cu~2~O/Au and Cu~2~O/Ag NSs were dissolved in a certain amount of absolute ethanol to form a suspension. Afterward, 20 μL of the suspension was taken to disperse on the Si piece (0.5 × 0.5 cm^2^) and dried in air. Then, the substrate for SERS measurements was obtained by dripping 5 μL of Rhodamine 6G aqueous solution into the treated Si substrate and drying it.

4.4. Characterization {#sec4.4}
---------------------

Energy-dispersive spectroscopy (EDS) and SEM (Quanta 250ESEM, FEI) were used to investigate the surface morphology and elemental composition of the samples. The microstructure of the samples was further analyzed by using a JEOL JEM-2000EX TEM. XRD of the D/max-3B Rigaku model with Cu Kα radiation (λ = 1.5406 Å) was used to determine the phase identification of the sample. At room temperature, XPS from the PHI 5000 Versaprobe of UlVAC-PHI (Japan) was used to detect the sample chemical composition and chemical valence states.

The samples were dispersed in a certain amount of ethanol solution, and the UV--vis spectrometer (U-4100, Hitachi) was used to detect the absorption spectra of the samples in the wavelength range of 200--800nm.The probe molecular Raman spectral signal was measured using a confocal Raman spectrometer (Andor, England) with a laser excitation source wavelength of 532 nm and a laser power of approximately 4 mW. The acquisition time of each test was 10 s, and the stacking time was 3.

4.5. Simulation {#sec4.5}
---------------

DDA is a method developed in recent years to calculate the absorption, scattering, and electromagnetic field distribution of particles of arbitrary shapes and sizes, which has a strong advantage in calculating the interaction between the light and nanoparticles.^[@ref46]^ The DDA method considers the nanoparticles as cubes with N-polarizable points. Under the induction of the electric field of light, each small cube is polarized, and the induced electric field generated further affects the adjacent cubes.^[@ref47]^ The optical properties of the material can be obtained by self-consistent calculation of the whole system. There is no limit to which the grid is polarized, which means that the DDA method can represent multiple substances of arbitrary shape and composition. This paper used the DDA (DDSCAT code 6.0) algorithm to calculate the electromagnetic wave (light wave) and the arbitrary shape of the particle. It was written in Fortran by Draine & Flatau of Princeton University and runs on Linux. The program provides a variety of particle shapes (triangular prism, hexagonal prism, sphere, cuboid, cylinder, ellipsoid, and regular tetrahedron). For the particle shapes not set in the program, the user can customize the dipole array to generate this shape. The direction of polarization when light interacts with particles can be set by the angle. The parameters to be set in the program include the particle shape, effective radius of particles, number of dipoles, complex dielectric constant of particles varying with wavelength, etc. For Cu~2~O NS and Au and Ag NPs, the complex permittivity values of all simulation parameters are derived from Palik.^[@ref45]^

The Supporting Information is available free of charge at [https://pubs.acs.org/doi/10.1021/acsomega.0c02301](https://pubs.acs.org/doi/10.1021/acsomega.0c02301?goto=supporting-info).Enhancement factor calculations and details of the Herzberg--Teller selection rule as well as some figures (S1--S3) and tables (S1 and S2) related to this manuscript ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c02301/suppl_file/ao0c02301_si_001.pdf))

Supplementary Material
======================

###### 

ao0c02301_si_001.pdf

The authors declare no competing financial interest.

The author thanks teacher Liu Yanxiong of the Testing Center of Yunnan University for his help in the TEM experiment and schoolmate Yang Chengxiang for his help. We also thank the National Natural Science Foundation of China (11764046, 11304275, 11764047, and 61564010) for their support of this work.

SERS

:   surface-enhanced Raman scattering

DDA

:   discrete dipole approximation

EF

:   enhancement factor

LOD

:   limit-of-detection

SPR

:   surface plasmon resonance

R6G

:   Rhodamine 6G

PVP

:   poly(vinylpyrrolidone)

M

:   mol/L

NSs

:   nanospheres

NPs

:   nanoparticles

MS

:   metal--semiconductor

CT

:   charge transfer

EM

:   electromagnetic

CM

:   chemical enhancements

XRD

:   X-ray diffraction

SEM

:   scanning electron microscope

EDS

:   energy-dispersive spectroscopy

TEM

:   transmission electron microscopy

HRTEM

:   high-resolution electron microscopy

XPS

:   X-ray photoelectron spectroscopy

UV--vis

:   UV--visible absorption spectroscopy
