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Article

Ag Nanoparticles Synthesized on Black-Titanium Dioxide by Photocatalytic Method as Reusable Substrates of Surface-Enhanced Raman Spectroscopy

by
Tianze Cong
,
Yifeng Zhang
,
Hui Huang
,
Chengwei Li
,
Zeng Fan
and
Lujun Pan
*
School of Physics, Dalian University of Technology, No. 2 Linggong Road, Ganjingzi District, Dalian 116024, China
*
Author to whom correspondence should be addressed.
Chemosensors 2022, 10(11), 441; https://doi.org/10.3390/chemosensors10110441
Submission received: 28 September 2022 / Revised: 22 October 2022 / Accepted: 23 October 2022 / Published: 25 October 2022
(This article belongs to the Special Issue Nanocomposites for SERS Sensing)
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Abstract

:
The construction of excellent surface-enhanced Raman spectroscopy (SERS) substrates needs rationally designed architectures of noble metals or semiconductors. In this study, Ag nanoparticles (NPs) are densely and uniformly synthesized on the surfaces of black-titanium dioxide (b-TiO2) NPs through a facile two-step photocatalysis method. The b-TiO2 improved the utilization efficiency of natural sunlight by the extension of light absorption from the ultraviolet (UV) to the visible (Vis) region. First, Ag seeds were densely grown in a short time on the surfaces of b-TiO2 NPs under the irradiation of UV light. Then, Ag NPs were grown slowly and uniformly from the Ag seeds under the irradiation of Vis light. The as-prepared Ag/b-TiO2 with high sensitivity achieved a limit of detection as low as 10−12 M for rhodamine 6G. Meanwhile, the substrate showed reusability due to the high photocatalytic ability of b-TiO2. The Ag/b-TiO2 SERS substrate achieves SERS detections of organic pollutants, such as hydroquinone, p-phenylenediamine, and terephthalic acid, indicating that this substrate possesses potential applications in food safety and environmental monitoring.

1. Introduction

As one of the most powerful analytical techniques for the detection of target analytes, surface-enhanced Raman spectroscopy (SERS) has received wide recognition in many frontier fields [1,2,3,4,5,6]. To date, two main enhancement mechanisms of SERS are widely accepted [7,8]. One is the electromagnetic mechanism (EM), which is most commonly associated with the localized surface plasmon resonance of noble metal nanostructures [9,10]. The other is the chemical mechanism (CM), which depends on the charge-transfer (CT) processes [11]. It is widely believed that the EM plays a greater role than the CM in SERS [12]. Therefore, noble metal nanostructures have been currently fabricated as SERS substrates, in particular Ag and Au with multifarious morphology, such as Au/Ag nanoparticles, nanorods, and nanostars [13,14,15,16,17,18,19]. However, the high cost, poor biocompatibility, and strong spectral background of noble metal substrates have limited their practical applications [20,21]. Although semiconductor materials have the potential to address these shortcomings, they exhibit low SERS enhancement [22,23]. Thus, noble metal-semiconductor nanocomposites have attracted great attention to make full use of their respective advantages [24,25,26].
Titanium dioxide (TiO2), a type of semiconductor, is widely applied to fabricate SERS substrates due to its nontoxicity, chemical stability, low cost, environmental protection capability, and photocatalytic ability [27,28]. Noble metal nanoparticles are capable of being synthesized onto TiO2 nanostructures by a photocatalytic process to simplify the preparation process. Fu at el. synthesized Ag NPs on the surface of TiO2 film by a photocatalytic method for SERS detection [29]. Jing et al. synthesized Ag NPs on a TiO2 nanotube array for the detection of antibiotic residue in milk [30]. Xu et al. fabricated a TiO2@Ag nanostructure as a SERS substrate to specifically recognize cancer cells [31]. However, the preparation of noble metal-TiO2 SERS substrates by the photocatalytic method still faces some challenges: (i) the wide band gap (3.2 eV) of TiO2 limits the utilization of visible (Vis) light, which makes TiO2 much less effective under sunlight [32,33]; (ii) the photocatalytic reactions under ultraviolet (UV) irradiation are very fast, which makes it difficult to regulate the synthesis process of Ag NPs, leading to the inhomogeneity of SERS substrates.
Recently, black-titanium dioxide (b-TiO2) has attracted great attention owing to the extension of light absorption from the UV to the Vis region through the formation of oxygen vacancies and Ti3+ defects on the surface of TiO2. Gao et al. synthesized b-TiO2 for hydrogen evolution at a rate of 1.882 mmol·g−1·h−1 under Vis light irradiation, which benefited from the expansion of the optical response range and the enhanced photocatalytic performance of b-TiO2 [34]. Zhang et al. prepared a b-TiO2 nanosheet array for photocatalytic reduction from CO2 to CO at a rate of 128.5 µmol·g–1·h–1 under Vis light irradiation [35]. Zhang et al. synthesized b-TiO2/carbon fiber composites to improve adsorption ability and photocatalytic efficiency for the degradation of methylene blue [36]. These results indicate that b-TiO2 is capable of improving the utilization efficiency of natural sunlight. Meanwhile, it has been found that the photocatalysis rate of b-TiO2 under UV irradiation is higher than that under Vis irradiation [36,37]. Thus, noble metal NPs are capable of being densely synthesized on the surface of b-TiO2 under UV light irradiation by the photocatalysis method. On the contrary, the rate of synthesis of noble metal NPs is delayed under Vis light irradiation, which furnishes an occasion to control the reaction process of NPs.
Herein, a reusable SERS substrate is fabricated by synthesizing Ag NPs onto the surface of b-TiO2 using the two-step photocatalytic method. The oxygen vacancies and Ti3+ defects on the surface of b-TiO2 extend light absorption from the UV to the Vis region. It is achieved to regulate the size dimension and distribution of Ag NPs on the surfaces of b-TiO2 NPs by utilizing UV light irradiation to improve the growth sites of Ag seeds and Vis light irradiation to tardily synthesize Ag NPs through the photocatalysis process. Furthermore, the Ag/b-TiO2 substrate shows reusable capability owing to the high photocatalytic ability of b-TiO2. This paper provides a novel idea for the fabrication of high-performance SERS substrates.

2. Experimental

2.1. Materials and Instruments

The morphology of TiO2, b-TiO2, and Ag/b-TiO2 was characterized by a scanning electron microscope (SEM, NOVA NanoSEM 450, FEI, Hillsboro, USA). A transmission electron microscope (TEM, JEM-F200, JEOL, Tokyo, Japan) was utilized to characterize the microstructure of as-prepared TiO2, b-TiO2, and Ag/b-TiO2 samples. Raman spectroscopy (Renishaw in Via plus, 532 nm) was used to characterize the structure of TiO2 and b-TiO2 samples and the SERS performance of Ag/b-TiO2. X-ray diffraction (XRD-7000S, Shimadzu, Tokyo, Japan) was utilized to characterize the phase structure of TiO2, b-TiO2, and Ag/b-TiO2 samples. UV light was provided by a low-pressure mercury lamp (Philips TUV, Amsterdam Noord-Holland, Holland). Vis light was provided by a xenon lamp (Zolix LSH-X150, Zolix, Beijing, China) equipped with a 400 nm filter. The distance between the UV/Vis lamp and the substrates was approximately 10 cm.

2.2. Synthesis of TiO2 Samples

TiO2 samples were prepared by an ordinary sol-gel method. Firstly, 1.5 w% hydrochloric acid was added to 50 mL deionized water and stirred for 30 min at room temperature. Then, 50 mL of tetrabutyl titanate was gradually added to the aqueous solution, and white precipitates appeared. After stirring for five hours, the white precipitates were washed with deionized water 3 times and dried at 70 °C for 2 h. Lastly, the samples were annealed in air at 450 °C for 2 h to form the TiO2 NPs with an anatase phase. The samples were stored in a dry environment for subsequent experiments.

2.3. Synthesis of b-TiO2 Samples

First, 500 mg TiO2 NPs were mixed with 1.5 g NaBH4 samples, followed by grinding for 10 min. The mixture was transformed into a quartz boat and annealed at 350 °C in a nitrogen atmosphere for 2 h to fabricate b-TiO2 samples. The obtained b-TiO2 was rinsed with deionized water 3 times and dried at 70 °C for 2 h to remove excess NaBH4.

2.4. Synthesis of Ag/b-TiO2-UV, Ag/b-TiO2-Vis, and Ag/b-TiO2-UV-Vis Substrates

Ag NPs were synthesized on the surface of b-TiO2 by the photocatalytic method. Firstly, 20 mg b-TiO2 NPs were added to a 40 mL Ag NO3 solution with a concentration of 100 mM and slowly stirred. Then, the mixture was irradiated with UV/Vis light for different lengths of time to fabricate Ag/b-TiO2-UV/Ag/b-TiO2-Vis substrates. The Ag/b-TiO2-UV-Vis substrate was fabricated by a similar method—the samples were irradiated under UV light for 5 min, then under visible light irradiation for different lengths of time. Finally, these samples were rinsed with deionized water 3 times and dried at 70 °C for 2 h.

2.5. SERS Measurement

Rhodanmine6G (R6G) with well-established vibrational features was used as a probe molecule to characterize the SERS performance of the Ag/b-TiO2 substrate. Firstly, Ag/b-TiO2 samples were mixed with R6G aqueous solution at different concentrations. Then, the mixture was stirred for 5 min and kept static for 2 h. Finally, a glass tube was inserted into the mixed solution. Then, the samples were entered into the glass tube by a siphon. Raman spectra were recorded under the same conditions, as follows: the excitation wavelength of the laser was 532 nm, with a laser spot size of ~2 μm; the laser power was 72.1 μW; a 50× objective was used to focus the laser beam and collect the Raman signals; the accumulation time was 10 s. The Raman spectra of each sample were collected at 10 randomly selected points. The Raman spectra of terephthalic acid, hydroquinone, p-phenylenediamine, and methylene blue (MB) were obtained under the same experimental conditions.

3. Results and Discussion

Figure 1 shows the fabrication process of Ag/b-TiO2 samples as SERS substrates. The electronic band below the conduction band induced by Ti3+ results in the Vis light activity of b-TiO2, which means the Vis light is able to be utilized in the photocatalysis process to synthesize Ag NPs. Firstly, the prepared b-TiO2 samples are immersed in AgNO3 solution in a dark room. Then, the mixture is slowly stirred under UV/Vis light irradiation. Then, Ag/b-TiO2 samples are cleaned with deionized water and stored for SERS detection. It is found that the photocatalysis rate under UV irradiation is higher than that under Vis irradiation, which might be due to the more photogenerated electron transfer paths induced by the intermediate electronic bands under UV irradiation. Thus, a two-step photocatalysis method is proposed to fully utilize the UV and Vis irradiation characteristics to synthesize Ag NPs onto b-TiO2: firstly, abundant Ti3+ and oxygen defects induce a large number of Ag seeds under UV irradiation; then, the Ag seeds are grown slowly and uniformly to Ag NPs under Vis irradiation. By the two-step photocatalysis method, the as-prepared Ag/TiO2 is able to simultaneously improve the sensitivity and uniformity of SERS substrates.
Figure 2 shows the surface morphology of the TiO2 and b-TiO2 NPs by SEM and TEM images. Figure 2a shows the TiO2 NPs synthesized by the solvent method. The average size of the TiO2 NP samples is ~82 nm, obtained by counting 50 different TiO2 NPs. Figure 2b shows the surface morphology of the b-TiO2 samples. The average size of the b-TiO2 NP samples is ~80 nm, obtained by counting 50 different b-TiO2 NPs. The b-TiO2 NPs show analogous structures and dimensions to TiO2 NPs, indicating that the structure of TiO2 NPs is not destroyed in the annealing process. In order to explore the reason for the change from white TiO2 to b-TiO2, TEM images were characterized to analyze the nanostructure of the TiO2 and b-TiO2 samples. Figure 2d shows that TiO2 NPs with good crystallinity are coated by amorphous shells. The lattice distance of 0.35 nm corresponds to the (101) planes of anatase TiO2. The amorphous shells formed by the oxygen vacancies and Ti3+ defects supply abundant growth sites for Ag seeds. Figure 2c shows the morphologies and microstructures of Ag/b-TiO2 NPs. Figure S1 shows the element mapping images of Ag, Ti, and O of Ag/b-TiO2 samples. It is found that Ag NPs are homogeneously distributed on the surfaces of b-TiO2 NPs. Figure 2e,f show the TEM images of Ag/b-TiO2 NPs. The lattice distance of 0.24 nm corresponds to the (111) planes of Ag. It is found that Ag NPs are substantially deposited onto the surfaces of b-TiO2 NPs by the photocatalysis process, forming a large number of nanogaps between Ag NPs to enhance the SERS sensitivity of the substrate.
Figure 3a shows the Raman spectra of TiO2 and b-TiO2. The peaks at ~145, ~396, ~517 and ~638 cm−1 correspond to the anatase crystal phase of TiO2. It is found that the strongest mode of b-TiO2 at 145 cm−1 exhibits a shift, which is attributed to the oxygen vacancies and defects of b-TiO2. The Ag NPs were synthesized on the surface of b-TiO2 by the photocatalytic method. The XRD patterns were collected to characterize the structural information of the TiO2, b-TiO2, and Ag/b-TiO2 samples, as shown in Figure 3b. The peaks at 2θ = 25.2°, 37.9°, 48.1°, 53.8°, 55.1°, and 62.8° are attributed to the (101), (004), (200), (105), (211) and (204) diffractions of the anatase TiO2 (JCPDS card No. 84-1286), respectively [34]. Furthermore, the peak at 2θ = 44.3° is attributed to the (200) diffraction of Ag, confirming that Ag was successfully deposited onto the surface of b-TiO2. Figure S2 shows the UV–Vis absorption spectroscopy of TiO2 and b-TiO2. The TiO2 sample shows an absorption edge at ~400 nm, corresponding to ~3.2 eV. The absorption edge of b-TiO2 is extended to ~450 nm, indicating the reduction of the band gap of the b-TiO2 sample. Meanwhile, b-TiO2 has the potential to absorb the entire range of solar spectra. Figure 3c shows the element contents of Ag in Ag/b-TiO2 and Ag/TiO2 samples under the same synthesis conditions. It was found that the element contents of Ag in the Ag/b-TiO2 substrate are several times those in the Ag/TiO2 substrate, resulting from the fact that a large number of growth sites of Ag seeds in the amorphous surface of b-TiO2 improves the deposition efficiency of Ag NPs on the surface of b-TiO2 during the photocatalysis process. The results of SEM, TEM, XRD, and Raman spectra clearly indicate that the Ag/b-TiO2 sample is successfully fabricated.
In order to characterize the SERS performance of the Ag/TiO2 substrate and the Ag/b-TiO2 substrate, the different samples under UV light irradiation were mixed into the R6G solution with a concentration of 10−7 M. Figure 4a and Figure S3 show the Raman signal intensities at the 612 cm−1 peak, detected by Ag/b-TiO2 and Ag/TiO2 substrates with different UV light irradiation times, ranging from 5 to 40 min. It is found that both of the substrates with an irradiation time of 20 min show the highest SERS sensitivity. Figure 4b shows the Raman signal intensities of R6G detected by the Ag/b-TiO2 substrate and the Ag/TiO2 substrate. Figure 4c shows the Raman intensities at 612, 774, 1187 and 1364 cm−1 peaks of the Ag/b-TiO2 substrate and the Ag/TiO2 substrate. It is indicated that the Ag/b-TiO2 substrate shows higher SERS sensitivity due to the higher density of Ag NPs in the Ag/b-TiO2 substrate. Meanwhile, Ag NPs are capable of being synthesized onto the surface of b-TiO2 by the photocatalytic method under Vis light due to the structural characteristics of b-TiO2. Figure 4d shows the Raman signal intensities at the 612 cm−1 peak, detected by Ag/b-TiO2 substrates with different Vis light irradiation times, ranging from 1 to 32 h. It is found that the Ag/b-TiO2-Vis substrate with an irradiation time of 16 h shows the highest SERS sensitivity. The comparison of SERS performance between Ag/b-TiO2-UV and Ag/b-TiO2-Vis shows that the sensitivity of the Ag/b-TiO2-UV substrate is higher than that of the Ag/b-TiO2-Vis substrate. However, the uniformity of the Ag/b-TiO2 substrate is unsatisfactory, which limits its application in SERS detection. The relative standard deviations (RSDs) of the Ag NP size in the Ag/b-TiO2-UV and Ag/b-TiO2-Vis samples are calculated to be 55% and 23%, respectively, indicating that the inhomogeneous characteristics of the Ag/b-TiO2-UV substrate is due to the fast growth rate of Ag nanoparticles under UV light irradiation. For Ag/b-TiO2-Vis samples, the synthesis rate of Ag onto the surface of b-TiO2 is very slow, resulting in the uniform distribution of Ag NPs. However, the low density of Ag NPs results in reduced SERS sensitivity.
To simultaneously improve the sensitivity and uniformity of the SERS substrate, a two-step photocatalytic method is provided: firstly, Ag seeds are synthesized on the surface of b-TiO2 under UV light irradiation; then, Ag NPs are slowly grown under Vis light irradiation. Ag NPs are densely and uniformly distributed on the surface of b-TiO2 by this two-step photocatalytic method, forming a large amount of “hot spots” on the SERS substrate. Figure 4e shows the comparison of SERS performance between Ag/b-TiO2-UV and Ag/b-TiO2-UV-Vis. The Raman signal intensities at the 612 cm−1 peak of R6G (10−7 M) are collected from 30 random sites on the Ag/b-TiO2-UV and Ag/b-TiO2-UV-Vis substrates. The RSD values of the Ag/b-TiO2-UV and Ag/b-TiO2-UV-Vis substrates are calculated to be 35% and 11%, respectively. It is shown that the RSD of the Ag/b-TiO2 substrate is improved, with the same SERS sensitivity, by the UV–Vis photocatalysis process. Due to the extension of light absorption from the UV to the Vis region through the formation of oxygen vacancies and Ti3+ defects, compared with TiO2, b-TiO2 shows peculiar properties in the fabrication process of SERS substrates. The addition of vacancies and defects increases the growth sites of Ag seeds, which improves the SERS sensitivity of the substrate. The ability to utilize visible light moderates the growth process, which not only improves the uniformity of the SERS substrate but also provides a presumable scenario of utilizing sunlight in the fabrication process of the SERS substrate. Figure 4f shows the SERS detections of R6G solutions with concentrations ranging from 10−9 to 10−12 M, detected by the Ag/b-TiO2-UV-Vis substrates. The enhancement factor is calculated to be ~1010 (details in supporting information). The Raman spectra of MB molecules with different concentrations are also shown in Figure S4. The characteristic SERS peaks of MB at 1394 and 1629 cm−1 correspond to C-H in-plane ring deformation and C-C ring stretching, respectively. The intensity of the Raman signal decreases with the decrease in MB concentration from 10−7 to 10−11 M. It is also observed that the Raman signals show a sufficient signal-to-noise ratio to push the limit of detection down to 10−11 M for MB molecules using the as-prepared Ag/b-TiO2 substrate.
Owning to the two-step photocatalytic methods, the as-prepared Ag/b-TiO2 substrate, with high sensitivity and homogeneity, is capable of being applied in practical fields. Figure 5a shows the Raman spectra of terephthalic acid with concentrations ranging from 10−5 to 10−7 M and the characteristic SERS peaks of terephthalic acid at ~868 and ~1450 cm−1, ascribed to the out plane bending of the C-H and O-H hydrogen bond bending vibration modes, respectively [38]. Figure 5b shows the Raman spectra of hydroquinone, with concentrations ranging from 10−5 to 10−7 M. The characteristic SERS peaks of hydroquinone at ~1180 and 1612 cm−1 are ascribed to C-H bending and C-C stretching, respectively [39]. Figure 5c shows the Raman spectra of p-phenylenediamine, with concentrations ranging from 10−5 to 10−7 M. The characteristic SERS peaks of p-phenylenediamine at ~1611, 1523 and 843 cm−1 are ascribed to the stretching of aromatic C-C, C-N stretching, and the symmetric ring breathing modes, respectively [40]. These results prove the potential of the as-prepared Ag/b-TiO2 substrate in practical detection.
Due to the high photocatalytic activities of the b-TiO2 samples, organic pollutants are initially adsorbed on the surface of b-TiO2 and further degraded into small molecules (such as carbon dioxide and water) under UV light irradiation to realize the self-cleaning process [33,41,42]. In order to verify the self-cleaning ability of Ag/b-TiO2, the R6G solution with a concentration of 10−7 M was used as a probe molecule. Firstly, the Ag/b-TiO2 samples were mixed in the R6G solution. Then, the Ag/b-TiO2 samples were transformed onto a glass plate. After the Raman detection, the substrate was irradiated under UV light for 1 h. Finally, the substrate was used to collect the Raman signal and immersed into a new R6G solution. The above process was repeated four times, and the results are shown in Figure S5. It is shown that the Raman signal of R6G was collected, but the R6G signal disappeared after UV light irradiation. The Raman signal of R6G was redetected after the substrate was immersed in a new R6G solution, indicating that the disappearance of the Raman signal of R6G is due to the photocatalytic activity of the Ag/b-TiO2 substrate rather than the loss of SERS activity. These results confirm that the substrate achieves the self-cleaning process and has the ability for repeated Raman detection.
In order to explore the SERS enhancement mechanism of the as-prepared Ag/b-TiO2 substrate, finite-different time-domain (FDTD) and density functional theory simulations were used to analyze the electromagnetic field enhancement and adsorption ability. The boundary condition was set as the perfectly matched layer. The incident light was set as a plane wave with a 532 nm wavelength, with propagation in the x-direction and polarization in the z-direction. The spatial parameters were set as follows: time step was 0.0006 fs; the mesh size was 1 nm; the background index was 1.0 (air). Figure 6a shows the great enhancement of the electromagnetic field between two Ag NPs on the surface of b-TiO2, which is called the “hot spot”. Due to the dense and uniform distribution of Ag NPs on the surface of b-TiO2, a large number of “hot spots” are formed in the three-dimensional space, thus improving the sensitivity of the substrate. The comparison of Ag-based SERS substrates with the FDTD simulation is shown in Table S1. To verify the SERS enhancement mechanism of the as-prepared Ag/b-TiO2, the CT degree (ρCT) was calculated to quantitate the contribution of CT. The CT degree is capable of being defined by the following equation [43]:
ρ C T = R 1 + R
where R is the ration of the intensity of a non-totally symmetric line, b2, to that of a totally symmetric line, a1. The SERS spectra of 4-mercaptobenzoic acid (4-MBA) molecules are detected to measure the degree of the CT contribution. The 1587 cm−1 (C-H, b2) and 1080 cm−1 (C-S stretch, a1) peaks of 4-MBA were chosen to calculate the intensity ratio R [44]. Figure S6 shows the ρCT of Ag/b-TiO2-UV-Vis with different irradiation times. The value is ~0.2, which means that the SERS enhancement mechanism is attributed to the EM.
Meanwhile, Figure 6b,c show the adsorption energy of a hydroquinone molecule adsorbed on the surface of TiO2 and the surface of Ag, calculated by the density functional theory. The Ag (101) surface was modeled by a p(4 × 4) periodic supercell slab with nine atomic layers, and the TiO2 (111) surface was modeled by a p(6 × 6) periodic supercell slab with six atomic layers. The vacuum space between the slab and its periodic image was 15 Å. The size of the Ag (101) supercell was 17.334 × 16.343 × 36.556 Å3, and the size of the TiO2 (111) supercell was 16.02 × 20.42 × 30.173 Å3. All DFT simulations were performed with the software CP2K [45,46,47,48,49]. The orbital transformation procedure was used for wave function optimization and self-consistent field convergence. The PBE functional was used with the D3 dispersion correction scheme [50,51]. DZVP-MOLOPT-SR-GTH basis sets were used alongside plane waves expanded to a 600 Ry energy cutoff. Electronic cores were represented by Geodecker–Teter–Hutter pseudopotentials [52,53]. The 4d, 5s electrons of Ag, the 3s, 3p, 3d, 4s electrons of Ti, the 2s, 2p electrons of O, the 2s, 2p electrons of C, and the 1s electrons of H were treated as valence. In static calculations, the geometries were optimized by the Broyden–Fletcher–Goldfarb–Shanno minimizer. For each slab model, the last three layers in the bottom were fixed during calculations. According to the calculation, the adsorption energy values of a hydroquinone molecule on the surface of Ag and the surface of TiO2 were −1.1 and −2.7 eV, respectively. These results show that hydroquinone molecules tend to be adsorbed on the surface of TiO2, indicating that TiO2 is not only used as a carrier for Ag NPs to construct the 3D nanostructured SERS substrates but also to enrich hydroquinone molecules between Ag NPs, where the electromagnetic fields are greatly enhanced.

4. Conclusions

In this paper, Ag/b-TiO2 substrates are fabricated by the two-step photocatalytic method. It is proved that Ag NPs are densely and uniformly synthesized onto the surface of b-TiO2 as a SERS substrate with high sensitivity and homogeneity. The Ag/b-TiO2 substrate realized the limit of detection for R6G down to 1 × 10−12 M. The value of RSD for SERS detection was 11.2%. FDTD and density functional theory simulations were simulated to analyze the SERS enhancement mechanism of Ag/b-TiO2. It is confirmed that b-TiO2 not only improves the adsorption ability of target molecules but also enriches molecules at the “hot spots” between Ag NPs. For practical applications, the Ag/b-TiO2 substrates achieve SERS detections of three kinds of organic pollutants, including hydroquinone, p-phenylenediamine, and terephthalic acid.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/chemosensors10110441/s1, Figure S1: Element mapping images of Ag, Ti, and O in Ag/b-TiO2 samples; Figure S2: UV-Vis absorption spectra of TiO2 and b-TiO2; Figure S3: The SERS sensitivities of Ag/TiO2-UV with different synthesis time; Figure S4: Raman spectra of MB molecules with different concentrations from 10−7 to 10−11 M; Figure S5: The self-cleaning ability of Ag/b-TiO2 susbtrate; Figure S6: The ρCT value of the Ag/b-TiO2-UV-Vis with different irradiation time; Table S1: The comparison of Ag-based SERS substrates simulated by FDTD. Supplementary References: [54,55,56,57,58,59,60,61].

Author Contributions

Conceptualization, T.C. and L.P.; methodology, T.C., H.H. and C.L.; software, T.C. and Y.Z.; validation, T.C., H.H. and Z.F.; formal analysis, T.C. and L.P.; investigation, T.C. and H.H.; resources, Y.Z. and Z.F.; data curation, T.C.; writing—original draft preparation, T.C. and L.P.; writing—review and editing, T.C. and L.P.; visualization, T.C. and C.L.; supervision, L.P. and Z.F.; project administration, C.L.; funding acquisition, L.P., Z.F. and C.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (no. 51972039, 52272288, 51803018), LiaoNing revitalization Talent Program (no. XLYC1902122), Fundamental Research Funds for the Central Universities (no. DUT21JC06), and the China Postdoctoral Science Foundation (no. 2021M700658).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic representation of the fabrication of Ag/b-TiO2.
Figure 1. Schematic representation of the fabrication of Ag/b-TiO2.
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Figure 2. (a) SEM image of TiO2. (b) SEM image of b-TiO2. (c) SEM image of Ag/b-TiO2. (d) TEM image of b-TiO2. (e,f) TEM images of Ag/b-TiO2.
Figure 2. (a) SEM image of TiO2. (b) SEM image of b-TiO2. (c) SEM image of Ag/b-TiO2. (d) TEM image of b-TiO2. (e,f) TEM images of Ag/b-TiO2.
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Figure 3. Structural characterizations of TiO2, b-TiO2, and, Ag/b-TiO2 samples. (a) Raman spectra of TiO2 and b-TiO2. (b) XRD patterns of TiO2, b-TiO2, and, Ag/b-TiO2 samples. (c) The comparison of Ag content between Ag/TiO2 and Ag/b-TiO2.
Figure 3. Structural characterizations of TiO2, b-TiO2, and, Ag/b-TiO2 samples. (a) Raman spectra of TiO2 and b-TiO2. (b) XRD patterns of TiO2, b-TiO2, and, Ag/b-TiO2 samples. (c) The comparison of Ag content between Ag/TiO2 and Ag/b-TiO2.
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Figure 4. SERS performance of Ag/b-TiO2. (a) The SERS sensitivities of Ag/b-TiO2-UV with different synthesis times. (b,c) The comparison of SERS sensitivities between Ag/b-TiO2 and Ag/TiO2. (d) The SERS sensitivities of Ag/b-TiO2-Vis with different synthesis times. (e) The homogeneity characterizations of Ag/b-TiO2-UV and Ag/b-TiO2-UV-Vis. (f) SERS detection of R6G molecules with concentrations ranging from 10−9 to 10−12 M.
Figure 4. SERS performance of Ag/b-TiO2. (a) The SERS sensitivities of Ag/b-TiO2-UV with different synthesis times. (b,c) The comparison of SERS sensitivities between Ag/b-TiO2 and Ag/TiO2. (d) The SERS sensitivities of Ag/b-TiO2-Vis with different synthesis times. (e) The homogeneity characterizations of Ag/b-TiO2-UV and Ag/b-TiO2-UV-Vis. (f) SERS detection of R6G molecules with concentrations ranging from 10−9 to 10−12 M.
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Figure 5. SERS detection of organic pollutants. (a) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of terephthalic acid. (b) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of hydroquinone. (c) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of p-phenylenediamine.
Figure 5. SERS detection of organic pollutants. (a) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of terephthalic acid. (b) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of hydroquinone. (c) SERS spectra obtained using different concentrations, from 10−5 to 10−7 M, of p-phenylenediamine.
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Figure 6. SERS enhancement mechanism of Ag/b-TiO2. (a) Two adjacent Ag NPs on the surface of b-TiO2 are simulated by FDTD. The illustration is an enlarged image. (b) Density functional theory simulation of a hydroquinone molecule adsorbed on the surface of Ag. (c) Density functional theory simulation of a hydroquinone molecule adsorbed on the surface of TiO2.
Figure 6. SERS enhancement mechanism of Ag/b-TiO2. (a) Two adjacent Ag NPs on the surface of b-TiO2 are simulated by FDTD. The illustration is an enlarged image. (b) Density functional theory simulation of a hydroquinone molecule adsorbed on the surface of Ag. (c) Density functional theory simulation of a hydroquinone molecule adsorbed on the surface of TiO2.
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Cong, T.; Zhang, Y.; Huang, H.; Li, C.; Fan, Z.; Pan, L. Ag Nanoparticles Synthesized on Black-Titanium Dioxide by Photocatalytic Method as Reusable Substrates of Surface-Enhanced Raman Spectroscopy. Chemosensors 2022, 10, 441. https://doi.org/10.3390/chemosensors10110441

AMA Style

Cong T, Zhang Y, Huang H, Li C, Fan Z, Pan L. Ag Nanoparticles Synthesized on Black-Titanium Dioxide by Photocatalytic Method as Reusable Substrates of Surface-Enhanced Raman Spectroscopy. Chemosensors. 2022; 10(11):441. https://doi.org/10.3390/chemosensors10110441

Chicago/Turabian Style

Cong, Tianze, Yifeng Zhang, Hui Huang, Chengwei Li, Zeng Fan, and Lujun Pan. 2022. "Ag Nanoparticles Synthesized on Black-Titanium Dioxide by Photocatalytic Method as Reusable Substrates of Surface-Enhanced Raman Spectroscopy" Chemosensors 10, no. 11: 441. https://doi.org/10.3390/chemosensors10110441

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