Irreversible accumulated SERS behavior of the molecule-linked silver and silver-doped titanium dioxide hybrid system

In recent years, surface-enhanced Raman scattering (SERS) of a molecule/metal–semiconductor hybrid system has attracted considerable interest and regarded as the synergetic contribution of the electromagnetic and chemical enhancements from the incorporation of noble metal into semiconductor nanomaterials. However, the underlying mechanism is still to be revealed in detail. Herein, we report an irreversible accumulated SERS behavior induced by near-infrared (NIR) light irradiating on a 4-mercaptobenzoic acid linked with silver and silver-doped titanium dioxide (4MBA/Ag/Ag-doped TiO2) hybrid system. With increasing irradiation time, the SERS intensity of 4MBA shows an irreversible exponential increase, and the Raman signal of the Ag/Ag-doped TiO2 substrate displays an exponential decrease. A microscopic understanding of the time-dependent SERS behavior is derived based on the microanalysis of the Ag/Ag-doped TiO2 nanostructure and the molecular dynamics, which is attributed to three factors: (1) higher crystallinity of Ag/Ag-doped TiO2 substrate; (2) photo-induced charge transfer; (3) charge-induced molecular reorientation.

increasing the concentrations of AgNO3 as compared to that of the pure TiO2 sample, which is possible due to the fact that Ag + ions, deposited on the surface of TiO2 samples, suppressed the crystallization of the TiO2 anatase phase 3,4 . In addition, the anatase (101) peaks of TiO2 in the Ag/Ag-doped TiO2 substrates slight shifted to a smaller diffraction angle, indicating that the Ag/Ag-doped TiO2 nanostructure occurs a lattice distortion. It should be attributed to the diffusion and rearrangement of the Ti 4+ and O 2ions in the anatase TiO2 and the disturbed by the Ag + ions spreading into the anatase TiO2, leading to distortion in the crystal lattice of TiO2 3,5 . In spite of fact that the radius of Ag + ion (126 Å) is larger than that of Ti 4+ ion (68 Å), the Ag + ion could still enter into the crystal lattice of TiO2 to replace Ti 4+ ion by the solhydrothermal process, induced O vacancies or deficiencies of Ti 4+ , which results in a shifting to small angle, peak broadening and decline 4,5 .
Furthermore, X-ray photoelectron spectroscopy (XPS) analysis is performed for Ag-TiO2, TiO2 and Ag/Ag-doped TiO2 substrates prepared with 0.1, 0.3, 0.5 and 0.7 mM of AgNO3, as 4 shown in Supplementary Figure 3. The fully scanned spectra show that Ti, O and C elements exist in the pure TiO2, whereas, Ti, O, Ag and C elements exist in the Ag-TiO2 and Ag/Agdoped TiO2 substrates, in which no traces of any other impurity were observed, except for the adventitious carbon from ambient environment. To obtained further evidence about the interaction between the Ag and TiO2, the high resolution XPS spectra of Ag 3d, Ti 2p and O 1s are displayed in Supplementary Figure 3b-d. The Ag 3d5/2 and Ag 3d3/2 peaks in the Ag-TiO2 sample appeared at 368.2 and 374.2 eV of binding energies, respectively. This typical spin energy splitting of the 3d doublet (6 eV) proves that Ag certainly presents in the Ag-TiO2 sample in the form of metallic Ag 6 . And there is no Ag 3d peak in the TiO2 precursor due to the absence of Ag element. As compared to Ag-TiO2, Ag 3d peaks in the Ag/Ag-doped TiO2 samples showed widening and shifting to lower binding energy, suggesting the existence of silver oxidation states. The Ag 3d peaks in the Ag/Ag-doped TiO2 substrates were fitted by using software program XPSPeak 4.1, indicating that the chemical states of Ag exist mainly as Ag + (oxide) and Ag 0 (metallic Ag). It is particularly noted that the chemical state of Ag in the Ag/Ag-doped TiO2 sample prepared with 0.1 mM AgNO3 are mainly Ag + (oxide), which is ascribed to Ag + replacing Ti 4+ at TiO2 lattice site. With increasing the concentration of AgNO3, the area percentages of the chemical state of Ag 0 (metallic Ag) obviously increases. This is because the Ag + doping content reach saturation and the excess silver is deposited on the TiO2 in the form of metallic Ag. Thus, the prepared samples not only have doping of Ag + but also deposition of metallic Ag. Besides, the Ti 2p peaks at 458.2 and 464.0 eV correspond to Ti 2p3/2 and Ti 2p1/2, respectively 7,8 . For pure TiO2, the peak sitting between Ti 2p3/2 and Ti 2p1/2 lines is about 5.8 eV, suggesting the existence of the Ti 4+ oxidation state 8 . And the peak 5 positions of Ti 2p of Ag-TiO2 show a positive shift than that of pure TiO2, indicating a lower electron cloud density of the Ti atoms in the Ag-TiO2 sample 9,10 . It means that the Fermi level of Ag is lower than the conduction band of TiO2 so that the electron transfer can occur between TiO2 and the Ag deposited on the surface of TiO2 11,12 . However, compared to pure TiO2, the Ti 2p peaks in the Ag/Ag-doped TiO2 samples shifted into lower binding energy. By fitting XPS spectra, Ti 2p peaks of the Ag/Ag-doped TiO2 samples can be divided into Ti 3+ and Ti 4+ peaks.
During the sol-hydrothermal process, Ag + could disturb the formation of TiO2, that is, Ag + could replace Ti 4+ at TiO2 lattice site, resulting in some changes of Ti 4+ into Ti 3+ . Thus, the shifting of Ti 2p peak to lower binding energy is attributed to Ag + doping into the TiO2 so that the radius of Ti ion is expand and the electron moves far from the Ti nuclei 13  In Supplementary Figure 4, the SERS spectrum of 4MBA adsorbed on the Ag/Ag-doped TiO2 substrate is consistent with that of the previously reported for 4MBA adsorbed on Ag-TiO2 nanoparticles 14 . It can be found that at the same excitation wavelength, SERS spectrum of 4MBA is very similar to its normal Raman spectrum. But, the characteristic SERS peaks of 4MBA are observed to display slightly different from those of the normal Raman spectrum in frequency and intensity, which can be attributed to the different adsorption state of 4MBA molecule and the intrinsic properties of electromagnetic and chemical enhancement. Therefore, in our work, the reported spectra are the SERS spectra rather than normal Raman spectra. 14

Supplementary Note 2: Calculation of SERS enhancement factor
To evaluate the plasmonic properties of as-prepared Ag/Ag-doped TiO2 substrates, the enhancement factors (EFs) are calculated by using the following equation 17 where ISERS is the integrated intensity of a SERS mode such as the ring-breathing mode (ʋ(C-S)) at 1078 cm -1 , Ibulk is the intensity of the same mode in the Raman spectrum of 4MBA,

Supplementary Note 3: Control experiments for temperature effect
Firstly, a synthesized Ag/Ag-doped TiO2 substrate was cleaved into four pieces, then three of them were coated with 4MBA but the fourth without any coating, and namely sample A, B, C and D. Next, their irradiation time-dependent SERS spectra were measured as follow conditions. Sample A was kept at room temperature, sample B was heat and kept at 45 ºC, sample C was heat to 45 ºC and then cooled down to room temperature, sample D was heated to 45 ºC and then cooled down to room temperature before coated with 4MBA. As shown in Supplementary Figure 11a-c, the intensities of peaks at 1078, 88 and 148 cm -1 are irradiation time-dependent, respectively. The results of sample A and sample B confirm that the SERS intensity measured at higher temperature is stronger than that at room temperature. For sample C and sample D, the intensity of peak at 1078 cm -1 is like that of sample A at room temperature.
This indicates that there is no change of the SERS spectra of 4MBA even after the adsorbed molecule and the substrates went through the heating and cooling step, which confirms that the temperature-induced SERS enhanced behavior is a reversible process for 4MBA. Based

Supplementary Note 4: First principles calculation of Raman intensity for anatase TiO2
As well known, the Raman intensity of a vibration mode can be expressed as 33,34,35 : where N is the total number of certain vibration mode in the system, is the polarizability tensor invariant scalar given by: where is the average invariant of the polarizability tensor or also called the 'reduced trace of the matrix', and defined as: ̅ is the polarizability tensor directivity invariant or also known as the 'anisotropy' parameter of the matrix, and follows the formula: where , , , , , and are the polarizability tensor elements. These invariants are independent of the coordinate frame in which the matrix representation is given, i.e. independent of the orientation of the molecule.
To understand the Raman characteristics of anatase TiO2, we studied the correlation between the polarizability tensor of the anatase TiO2 and the concentration of VO defects by using density functional theory (DFT) calculations. As the material polarizability is always closely calculating the dielectric tensor [38][39][40] . And the calculated polarizability tensor of the anatase TiO2 with different VO defect concentrations were listed in Supplementary Table 3. As shown in Figure 5a of main text, a polynomial function was used to fit these data and the corresponding fitting curve was plotted by the following equation: where n is the number of the atoms in the TiO2 structure, denotes the concentration of VO defects. Therefore, combining Eq. (2), Eq (6) and Eq. (7), can be expressed as: Thus, the relationship between the Raman intensity and the concentration of VO defects is plotted in Figure 5b of main text. It can be found that the Raman intensity first decreases and then increases with decreasing the concentration of VO defects. The calculation displays a similar variation trend consisted with the experiment data and is well explanation for experimental results (Figure 4b). 28

Supplementary Note 5: Raman signal of Ag-Ag interaction
To study the temperature-dependent SERS characteristics of the peaks at 88 cm -1 ( Figure   4b), assigned to the Ag-Ag stretching vibration, the thermogravimetric and differential