Surface-enhanced Raman scattering (SERS) as a probe for detection of charge-transfer between TiO₂ and CdS nanoparticles

Abstract

Charge-transfer processes of coupled semiconductor nanoparticles play an essential role in many application fields of semiconductors. It has been difficult to characterize the charge-transfer process between two semiconductor nanoparticles. Herein, we propose a new analytical tool with the chemical mechanism of Surface-Enhanced Raman Scattering (SERS) to provide information about the charge-transfer process in coupled semiconductor nanoparticle systems. In this work, two assemblies (namely, TiO₂–MBA–CdS and CdS–MBA–TiO₂) have been fabricated as models to explore the charge-transfer process between TiO₂ and CdS nanoparticles by SERS. 4-MBA works as both the linker of these two semiconductors and the probe of SERS to reveal the charge-transfer process. Under excitations at 633 and 785 nm, which are far from the surface plasmon resonance of the CdS and TiO₂ nanoparticles in the assemblies, there are some obvious changes in the peak intensities and peak shifts in the SERS spectra after introducing CdS to the TiO₂/MBA system and TiO₂ to the CdS/MBA system. The differences of the peak intensities in the TiO₂–MBA–CdS and CdS–MBA–TiO₂ assemblies show different charge-transfer processes. We have found that the dipole moment of a MBA molecule greatly influences the charge-transfer process between the TiO₂ and CdS nanoparticles.

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Introduction

Coupled semiconductor–semiconductor, and coupled semiconductor–molecule structures play important parts in solar cells^1–3 and photocatalysis.^4–6 Interparticle electron transfer between dissimilar semiconductor particles is an ongoing research topic. Part of the motivation for such studies is to enhance charge separation and an attempt to increase the lifetime of charge carriers, thereby improving the conversion efficiency of solar cells and the photocatalytic efficiency of various catalytic reactions. To enrich practical applications based on these coupled semiconductor–semiconductor and coupled semiconductor–molecule structures, many efforts have been made to study the feasible charge-transfer process involved. Hyun et al.⁷ reported the injection of photoexcited electrons from colloidal PbS into TiO₂ nanoparticles for sensitizing solar cells. Bessekhouad et al.⁸ used CdS/TiO₂ and Bi₂S₃/TiO₂ to analyze the charge-transfer mechanism in the photocatalytic degradation of organic pollutants. Lee et al.⁹ investigated the electronic states that occur between N719 and TiO₂ surfaces in dye-sensitized solar cells. However, it is difficult to observe the charge-transfer process between micro and nanoscale particles especially when the particles are less than 10 nm.

It is a good choice that the process of photoelectric reaction reflected through the optical phenomenon or spectrum, and SERS provide such a way. The chemical enhancement mechanism (CM)^10–13 of SERS involves a charge-transfer process between the substrate and the adsorbate. Typically, chemical enhancement is a resonance Raman-like process, which is sensitive to the polarizability change of probe molecules. Benefiting from the widespread research of the chemical mechanism (CM) of SERS, new approaches for observing the charge-transfer process between different kinds of materials have been developed by detecting the Raman spectra of molecules assembled on them. According to the CM mechanism, the charge-transfer between absorbed molecules and a substrate can lead to selective enhancement of some characteristic peaks. In recent years, numerous experimental approaches have been developed to understand the charge-transfer process between a metal and a semiconductor by the SERS technique. The structures of metal nanoparticles coupled with a semiconductor by a bridge molecule were formed to investigate the charge-transfer process between metal nanoparticles and semiconductors. It has been reported in the literature that a group of silver-deposited TiO₂ nanoparticles with a series of Ag contents have been prepared to validate the charge-transfer process that occurs in the Ag–TiO₂ nanoparticles.¹⁴ TiO₂–MBA–Ag(Au) and Ag(Au)–MBA–TiO₂ sandwich nanostructured assemblies have been synthesized by a self-assembly method to explore the direction of the charge-transfer between metal nanoparticles and semiconductors.¹⁵ Furthermore, three different kinds of nanostructures (ZnO–PATP–Ag, Au–ZnO–PATP–Ag, and Cu–ZnO–PATP–Ag) fabricated by self-assembly methods were used to investigate the photo-induced charge-transfer process in metal–semiconductor contacts by the SERS technique.¹⁶ The charge-transfer processes in Ag/N719/TiO₂¹⁷ and TiO₂/N3/Ag¹⁸ dye-sensitized solar cell models have also been studied with SERS spectra.

Due to the existence of metal nanoparticles in the systems mentioned above, the SERS intensity of the bridge molecules absorbed between the metal and semiconductor nanoparticles is greatly enhanced by localized surface plasma resonance (LSPR).^19,20 In these systems, we can easily obtain the charge-transfer process information among the metals, molecules and semiconductors because of the largely enhanced Raman signal of the bridge molecules. However, it is difficult to obtain the Raman signal of the molecule on a semiconductor which only comes from the CM, especially on coupled semiconductors, unless the structure, size, or calcination temperature of the semiconductor is adjusted. To obtain more details about the charge-transfer process between coupled semiconductors, a system without metal nanoparticles must be designed.

In response to the important position of coupled wide band gap semiconductors and narrow band gap semiconductors in solar energy utilization,^21–24 the coupling of two semiconductor particles may offer an opportunity to sensitize a wide band gap semiconductor material by another one with a narrower gap.²⁵ Indeed, interparticle electron transfer has been demonstrated in a large number of combinations, including CdS to ZnO,²⁶ CdS → Ag₂S,²⁷ CdS → AgI,²⁸ ZnS → AgI, ZnS → Ag₂S,²⁹ ZnS → Cd₃P₂, ZnS → TiO₂, and ZnS → ZnO.²⁶ The charge-transfer from CdS to TiO₂ particles has been investigated more widely than in any of the other systems. The conduction band of CdS lies above that of TiO₂, and therefore, electron transfer from the former to TiO₂ particles is energetically allowed. It should be pointed out that, only appropriate sizes of the two coupled semiconductors will produce strong SERS signals. According to a previous study of the relationship between semiconductor size and SERS intensity,^30,31 appropriate sizes of TiO₂ (about 8 nm) and CdS (about 5 nm) were chosen to study the charge transfer process between them.

Since the –SH group of the MBA molecule has the ability to combine with semiconductors,^32,33 it is possible to assemble different ordered semiconductor–MBA–semiconductor systems by adding different kinds of semiconductors. MBA is a polar molecule that greatly influences the photo-induced charge-transfer process by polarization direction, and it is advantageous to discuss the charge-transfer between coupled semiconductors. Herein, CdS–MBA–TiO₂ and TiO₂–MBA–CdS systems were fabricated by a self-assembly method, and SERS spectra were measured with different excitation wavelengths. We deduced the direction of charge-transfer through the energy band locations and work functions of the two semiconductors and compared the intensity of the characteristic peak (b2/a1) from the SERS spectra of MBA between the two systems. Through the analysis of the SERS spectra, we determined the charge-transfer between TiO₂ and CdS nanoparticles coupled with MBA molecules, showing the utilization of SERS in the field of semiconductor materials.

Experimental section

Chemicals

4-Mercaptobenzoic acid (MBA) and titanium(IV) butoxide were purchased from Sigma-Aldrich and used without further purification. All other chemicals (anhydrous ethanol and nitric acid were obtained from Beijing Chemical Works, and cadmium nitrate and sodium sulfide were obtained from Tianjin Guangfu Fine Chemical Research Institute) were employed without further purification. Distilled and deionized water from a Milli-Q-plus system with a resistivity of >18.0 MΩ were used in aqueous solution.

Synthesis of TiO₂ NPs

In this work, the preparation method for titanium dioxide nanoparticles is the same as that described in the previous literature,^34,35 which employed a sol-hydrothermal method. First, 5 ml titanium(IV) butoxide was mixed with 5 ml anhydrous ethanol, and then this mixed solution was added dropwise to another mixed solution (20 ml anhydrous ethanol, 5 ml ionized water and 1 ml 70% nitric acid) under rough stirring at room temperature. After stirring for two hours, the hydrolyzed titanium(IV) butoxide changed to a light yellow transparent solution. Second, the as-prepared sol was kept at 160 °C for 6 h in a stainless-steel vessel and then cooled to room temperature. The sol-hydrothermal product was dried at 60 °C for 24 h. Finally, TiO₂ NPs were obtained by calcining the product at 450 °C for 2 h.

Synthesis of CdS NPs

The synthesis of cadmium sulfide nanoparticles was followed by a method reported in the previous literature,³⁰ and sodium sulfide was slowly added dropwise to an aqueous solution of nitric acid at room temperature. After thorough mixing, the samples were stirred for 2 h at room temperature. The obtained CdS microcluster solution was centrifuged, and the precipitate was rinsed with distilled water once more.

Preparation of TiO₂/MBA and CdS/MBA systems

The TiO₂/MBA system was prepared by the following steps. Briefly, 20 mg of the prepared TiO₂ NPs was dispersed in an MBA ethanol solution (10⁻³ M) and the mixture was stirred for 7 h. Subsequently, the product was centrifuged and rinsed with anhydrous ethanol once more. After drying the system, TiO₂/MBA powder was obtained. The method for the preparation of the CdS/MBA system was the same as that of the TiO₂/MBA system; 20 mg of the prepared CdS NPs was dispersed in an MBA ethanol solution (10⁻³ M) and the mixture was stirred for 7 h. Subsequently, the product was centrifuged and rinsed with anhydrous ethanol once more. After drying, CdS/MBA powder was obtained.

Preparation of TiO₂/MBA/CdS and CdS/MBA/TiO₂ systems

The method for the preparation of the TiO₂/MBA/CdS system is as follows: 20 mg of the above-mentioned TiO₂/MBA powder was dispersed in 20 ml ethanol solution, and then the solution was added to 20 mg of the as-prepared CdS NPs and the mixture was stirred for 7 h. After this, the product was centrifuged and rinsed with anhydrous ethanol once more. We obtained the TiO₂/MBA/CdS powder when the system was dried completely. The method for the preparation of CdS/MBA/TiO₂ was similar to that of the TiO₂/MBA/CdS system, except for the order in which the semiconductors were added. The schematic of the whole assembly process of the two systems (TiO₂–CdS and CdS–TiO₂) is shown in Fig. 1.

Fig. 1 Schematic diagram of the preparation process of the two systems. (1) TiO₂/MBA/CdS and (2) CdS/MBA/TiO₂.

Instrumentation

The crystal structures of the TiO₂ and CdS samples were determined by X-ray diffraction (XRD) using a Siemens D5005 X-ray powder diffractometer with a Cu Kα radiation source at 40 kV and 30 mA. The surface morphologies of the samples were examined using a JEM-2100 transmission electron microscope (TEM) operating at an accelerating voltage of 200 kV. UV-Vis diffuse reflectance spectra (UV-Vis DRS) were recorded on a Hitachi UV-4100 UV-Vis spectrophotometer. Raman spectra were obtained by using a Horiba-Jobin Yvon LabRAM ARAMIS system with a resolution of ca. 4 cm⁻¹. Radiation wavelengths of 633 and 785 nm from an air-cooled HeNe narrow bandwidth laser were used as the excitation source. Data acquisition was the result of the signal 60 s accumulations for the MBA molecules in different assemblies at ambient temperature.

Results and discussion

Measurement of TEM and UV-Vis DRS spectra

Fig. 2 shows typical TEM images of TiO₂ NPs, TiO₂/MBA, TiO₂/MBA/CdS, CdS NPs, CdS/MBA, and CdS/MBA/TiO₂. Fig. 2a shows that the diameter of the TiO₂ NPs is 8 ± 0.51 nm, and the crystal type is an anatase phase (as shown in Fig. S1, ESI†), which is consistent with the literature;³² the observed lattice spacing of 0.351 nm in Fig. 2a corresponds to the (101) plane of anatase (JPCDS 21-1272). After the adsorption of the 4-MBA molecules, no remarkable change is observed. It can be seen from Fig. 2d that the size of the CdS NPs is 5 ± 0.38 nm, and that the lattice spacing of 0.328 nm matches the (111) spacing of the cubic lattice (as shown in Fig. S2, ESI†).³⁶ The sandwich structures, which are assembled between two semiconductors, exhibit strong interactions between the 4-MBA molecules and semiconductor nanoparticles. The 4-MBA molecules are combined with the TiO₂ nanoparticles by the thiol group in the TiO₂/MBA system.³²

Fig. 2 TEM images of (a) TiO₂ NPs, (b) TiO₂/MBA, (c) TiO₂/MBA/CdS, (d) CdS NPs, (e) CdS/MBA, and (f) CdS/MBA/TiO₂.

Therefore, in the TiO₂/MBA/CdS sandwich system, connective 4-MBA molecules should combine with TiO₂ NPs through the thiol group of the molecules due to the formation of Ti–S bonds, which can result in the blue shift of the photoabsorption threshold from the TiO₂ band–band transition (as shown in Fig. 3) when connected to the CdS NPs by carboxyl groups. However, the case is the opposite for the CdS/MBA/TiO₂ system (i.e., connective 4-MBA molecules combine with CdS NPs through the thiol group of the molecules due to the formation of Cd–S chemical bonds when connected to the TiO₂ NPs by carboxyl groups).³⁷

Fig. 3 UV-Vis DRS spectra of the TiO₂ NPs, CdS NPs and different assemblies.

Fig. 3 shows the UV-Vis DRS spectra of the TiO₂ NPs, CdS NPs and different assemblies. The samples (TiO₂, TiO₂–MBA and TiO₂–MBA–CdS) exhibit a wide optical absorption below 400 nm, which is due to the band–band electron transition of TiO₂ NPs in accordance with their band gap at ca. 3.2 eV. A long tail from 400–500 nm matches the optical absorption related to the surface state of TiO₂ NPs. When TiO₂/MBA is compared with pure TiO₂ NPs, the optical absorption threshold of the TiO₂ band–band transition of the TiO₂/MBA sample shows a slight blue shift, and the photoabsorption (approximately 400–500 nm), which is consistent with the surface state of TiO₂, is also enhanced. In addition, its optical response is extended to approximately 570 nm. These changes can be ascribed to the interaction between the adsorbed molecules and the TiO₂ substrate because of the formation of a charge-transfer complex by the Ti–S bond.³² Similar changes in photoabsorption have been previously reported by Rajh et al.³⁸ in the charge-transfer complexes of enediol molecules with TiO₂ NPs. After introducing CdS, the system shows an obvious absorption in the region of 390–490 nm because of the existence of CdS. Moreover, we can also see the wide optical absorption of pure CdS in the region of 390–490 nm. The changes in the absorption peaks in the UV-Vis DRS and X-ray photoelectron spectra (XPS) of the charge-transfer systems (as shown in Fig. S3, ESI†) show that the TiO₂–MBA–CdS coupled semiconductor system had been fabricated.

From Fig. 3, the band edge absorption of pure CdS is approximately 510 nm (2.4 eV), which is close to the reported value.³⁹ After modification of the MBA molecules, the photoabsorption threshold of CdS shows a slight blue shift, which can be attributed to the influence of the molecules adsorbed on CdS NPs. After introducing TiO₂, the photoabsorption threshold continues to blue shift, and the peak at 290 nm is similar to that of pure TiO₂. Therefore, we can draw a conclusion from the changes of the absorption peaks of the UV-Vis DRS and the XPS spectra of the charge-transfer systems (as shown in Fig. S4, ESI†) that the CdS–MBA–TiO₂ system is formed.

SERS spectra of 4-MBA in charge-transfer assemblies

To avoid error from the laser power in this experiment (excited at 633 and 785 nm), we normalized all the SERS spectra using the intensity ratio of each peak to the highest peak (1592 cm⁻¹).

The SERS spectra measurements of the TiO₂/MBA and TiO₂/MBA/CdS CdS/MBA and CdS/MBA/TiO₂ systems with excitation at 633 nm are shown in Fig. 4a. In addition, the Raman shift and the assignment of the bands are listed in Table 1. For the TiO₂/MBA assembly, two strong peaks at approximately 1592 and 1075 cm⁻¹ are assigned to the aromatic totally symmetric stretching mode ν(CC) and the in-plane ring breathing mode coupled with ν(C–S). However, the peak assigned to the in-plane ring breathing mode shifted to 1080 cm⁻¹ in the CdS/MBA assembly. Other weak bands are also observed at approximately 1147 (ν15, b2) and 1182 cm⁻¹ (ν9, a1) in the TiO₂/MBA assembly, which are attributed to the C–H deformation modes. In the CdS/MBA assembly, however, we identify a band at 1142 cm⁻¹ (ν15, b2), which is attributed to the C–H deformation modes. Meanwhile, there is a relatively strong peak at 1411 cm⁻¹ due to the COO⁻ symmetric stretching mode in the TiO₂/MBA assembly, which can be observed at 1406 cm⁻¹ in the CdS/MBA assembly.

Fig. 4 (a) SERS spectra of the TiO₂/MBA, TiO₂/MBA/CdS, CdS/MBA and CdS/MBA/TiO₂ systems at 633 nm excitation; (b) the SERS intensity ratio of the peaks at 1147 and 1075 cm⁻¹ as well as 1411 and 1075 cm⁻¹ in the four systems.

Table 1 Wavenumbers and assignment of bands in the SERS spectrum of the MBA molecule^a

Wavenumbers (cm^−1)		Band assignments
On TiO2	On CdS
a ν, stretching; for ring vibrations, the corresponding vibrational modes of benzene and the symmetry species under C2v symmetry are indicated.
1075	1080	In-plane ring breathing + ν(C–S) a1
1147	1142	C–H deformation modes ν15, b2
1182	1182	C–H deformation modes ν9, a1
1411	1406	b(O–H) + ν(C-ph) + 19a + ν(CO2), b2
1592	1592	Totally symmetric ν(CC), a1

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After introducing CdS into the TiO₂–MBA assembly (from Fig. 4a red line), the Raman band at 1411 cm⁻¹, the characteristic vibration of 4-MBA, visibly shifts to approximately 1406 cm⁻¹, and the peak at 1147 cm⁻¹ (ν15, b2) slightly shifts to approximately 1145 cm⁻¹. Meanwhile, the Raman band at 1080 cm⁻¹ (C–S, a1) obviously moves to 1076 cm⁻¹, and the bands at 1142 cm⁻¹ (ν15, b2) and 1406 cm⁻¹ (ascribed to the COO⁻ symmetric stretching mode) move to 1143 cm⁻¹ and 1047 cm⁻¹ after the introduction of TiO₂ into the CdS/MBA assembly, respectively. These peak shifts can be attributed to the interaction between TiO₂ and CdS. Interestingly, the peak intensity at 1147 cm⁻¹ (ν15, b2) is not distinctly changed after introducing CdS into the TiO₂/MBA assembly. However, the peak at 1142 cm⁻¹ (ν15, b2) in the CdS/MBA assembly became obviously enhanced after the introduction of TiO₂.¹⁵

According to the charge-transfer model of Lombardi,¹³ only the totally symmetric vibrational modes of probe molecules are expected to be enhanced via the Franck–Condon contribution, and the Herzberg–Teller effect can enhance both totally and non-totally symmetric vibrational signatures in the SERS spectra. That is, non-totally symmetric vibrational signatures can only be enhanced by the CT mechanism through the Herzberg–Teller contribution. The intensity ratios of the peaks at the b2 mode and the a1 mode can directly identify the charge-transfer progress between the molecules and semiconductor nanoparticles.^40,41 In this work, we choose two peaks at 1147 cm⁻¹ (the b2 mode) and 1075 cm⁻¹ (the a1 mode) as the reference peaks for comparison, and we plot the intensity ratio of these two peaks in Fig. 4b. From Fig. 4b (black line), it is clear that the ratio of the b2 and a1 modes in TiO₂/MBA/CdS slightly decreases compared to that of the TiO₂/MBA assembly. Nevertheless, the intensity ratios of the peaks at 1147 cm⁻¹ and 1075 cm⁻¹ in CdS/MBA/TiO₂ obviously increase compared to the CdS/MBA assembly. Furthermore, the non-totally symmetric vibrational signatures in the CdS/MBA/TiO₂ assembly, which can be efficiently enhanced by the charge-transfer process, are obviously stronger than those of the TiO₂/MBA/CdS assembly.

To confirm the phenomenon, we also plot the intensity ratio of the two peaks at 1411 cm⁻¹ (the b2 mode) and 1075 cm⁻¹ (the a1 mode), as shown in Fig. 4b (red line), and the same conclusion is obtained. These results demonstrate that the assembly sequence method largely influences the charge-transfer degree, which means that the assembly direction of the MBA molecules directly controls the charge-transfer process. The same phenomenon can be seen in the Raman spectra under excitation at 785 nm; the Raman spectra and the intensity ratio of the peaks at 1147 cm⁻¹ and 1075 cm⁻¹, and 1411 cm⁻¹ and 1075 cm⁻¹ are shown in Fig. S5 (ESI†). It can be seen that the COO⁻ group shows excitation wavelength dependence in the four systems as shown in Fig. 4b and Fig. S5b (ESI†), which is more obvious in the CdS–MBA system.

Notably, the ratio of I₁₄₁₁ and I₁₀₇₅ in CdS–MBA is greatly larger compared to I₁₁₄₇ and I₁₀₇₅. The COO⁻ group, which gives a band at 1411 cm⁻¹, possesses a strong electron attraction capability. When the molecules get absorbed on the CdS, the strong electronegativity of COO⁻ caused an increase in the electron cloud around the carboxyl group, which is obviously larger than that of the C–H (peak at 1147 cm⁻¹). The apparent charge-transfer degree of the COO⁻ group is larger than that in the C–H group, which causes a larger ratio of I₁₄₁₁ and I₁₀₇₅ in the CdS–MBA than the ratio of I₁₁₄₇ and I₁₀₇₅. At the same time, the electronegativity of TiO₂ is dramatically larger than that of CdS. In the CdS–MBA assembly, the electron cloud around the carboxyl group is much stronger than that in the TiO₂–MBA and TiO₂–MBA–CdS assemblies. The ratio of I₁₄₁₁ and I₁₀₇₅ in CdS–MBA is much larger than those in TiO₂–MBA and TiO₂–MBA–CdS, which provides the importance of the assembly direction of the MBA molecules between the two kinds of semiconductors.

CT mechanism of MBA in semiconductor–semiconductor systems

In this experiment, we formed semiconductor–molecule–semiconductor sandwich structures with two kinds of semiconductor nanoparticles; they are CdS, with the valence band (VB) at 6.23 eV and the conduction band (CB) at 3.98 eV, and TiO₂ with the VB at 7.4 eV and the CB at 4.2 eV as shown in Scheme 1.² The molecule connecting these two semiconductor nanoparticles is MBA, with its highest occupied molecular orbital (HOMO) at 8.48 eV and its lowest unoccupied molecular orbital (LUMO) at 3.85 eV.⁴²

Scheme 1 Energy levels of TiO₂, 4-MBA and CdS.

To further confirm the charge-transfer process in these sandwich structures, the work functions of the semiconductors were compared. According to the previous literature, the work function of CdS is 4.5 eV, which is lower than that of TiO₂, whose work function is 4.7 eV,^43,44 meaning that, when these two semiconductors form a charge conduction system, the electrons must flow from the smaller work function material to the larger work function material, and in these two systems, the flow occurs in the direction from CdS to TiO₂.

Additionally, the surface-state energy level (ESS) of CdS is 5.13 eV,⁴⁵ which is 1.1 eV higher than the VB level of CdS and 0.93 eV lower than the CB level of TiO₂. The ESS level of CdS forms a bridge from the VB of CdS to the CB of TiO₂. In our experiment, the laser wavelengths used here are 633 nm and 785 nm, and the corresponding energies of the photons are 1.96 eV and 1.58 eV, respectively. Obviously, the energy of the laser photons is larger than the band gap from the VB of CdS to the ESS of CdS, or the ESS of CdS to the CB of TiO₂. When these sandwich structures are excited by the laser, electrons in the VB of CdS could be excited into the ESS of CdS, and then, jump to a higher level, e.g., the CB of TiO₂. In these assemblies, meanwhile, the MBA molecules function as a bridge between the semiconductors and play an important role as a transmitter. From the intensity ratio of the peaks attributed to the b2 mode and peaks attributed to the a1 mode of these two sandwich assemblies, the assembly direction of MBA between these two semiconductors deeply influences the charge-transfer process. That is, the thiol group of MBA tends to preferentially connect with the surface of the semiconductors. When the order of the assembly process is TiO₂–MBA–CdS, the thiol group of MBA attached to the TiO₂ nanoparticles, while the carboxyl group of MBA attached to the CdS nanoparticles. The strong electronegativity of the carboxyl group in the MBA molecule forms a negative center, when electrons transfer from CdS to TiO₂, and the negative center of MBA hinders the transmission process of the electrons and causes a weaker selective enhancement of the b2 mode. In contrast, when the assembly order is CdS–MBA–TiO₂, the thiol group of MBA attached to the CdS nanoparticles, and meanwhile, the thiol group of the MBA molecule forms a positive center and promotes the electron transfer from CdS to TiO₂ due to a stronger selective enhancement of the b2 mode.

Conclusion

In this work, two systems (TiO₂/MBA/CdS and CdS/MBA/TiO₂) have been fabricated to explore the charge-transfer process at the interface between TiO₂ and CdS nanoparticles by the SERS technique. According to the energy levels and work function of TiO₂ and CdS, the charge-transfer direction of the TiO₂–MBA–CdS and CdS–MBA–TiO₂ systems is from CdS to TiO₂ under laser excitation wavelengths at 633 and 785 nm. The inner polarity of the molecule (4-MBA here) can dramatically influence the charge-transfer process when it is assembled at the interface between two kinds of semiconductors. By changing the assembly direction of the 4-MBA molecules, we formed two different kinds of sandwich assemblies. According to the analysis of the SERS spectra of these systems under excitation wavelengths at 633 and 785 nm, the peak assigned to the b2 mode was significantly enhanced in the CdS–MBA–TiO₂ system, but in the TiO₂–MBA–CdS system, it was barely enhanced because the dipole direction of the MBA molecules hindered the charge-transfer process from CdS to TiO₂, and the charge-transfer process was slightly weaker in TiO₂–MBA–CdS assemblies. These results provide details about the charge-transfer process at the interface of the semiconductor nanoparticles, and also prove that the SERS technique has surprising application prospects in the field of nanomaterials.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

The research was supported by the National Natural Science Foundation (Grants 21773080, 21711540292, and 21773079) of the People's Republic of China.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj04003f

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