1. Introduction

Raman scattering spectroscopy is a non-elastic scattering spectroscopic analysis method based on the energy band structure of materials. Due to the different molecular structures of different substances, their energy band structures vary, resulting in distinct Raman spectra. Consequently, Raman spectroscopy is widely employed for identification and analysis of material composition and molecular structure, becoming an efficient analytical tool [1]. However, the Raman signals of the target substances themselves are typically extremely weak, necessitating the utilization of SERS technology to amplify the Raman signals of the target substances [2]. SERS technology employs special substrate materials and structures that can significantly enhance the Raman signals of the target molecules [3], with enhancement factors often exceeding 10¹². This technique enables the detection of target substances at low concentrations and can even achieve single-molecule-level detection [4].Currently, SERS technology has been widely applied in various fields such as chemical analysis, medical diagnostics, food safety and environmental monitoring, demonstrating significant application potential and promising prospects. With continuous technological advancements, SERS technology will continue to drive the development of molecular structure analysis, providing us with more precise and efficient analytical tools [5].

The recognized mechanisms of SERS mainly include EM and CM [6]. In the SERS effect, EM plays a primary role, but CM also exhibits unique and specific enhancement effects. The synergistic interaction of these two mechanisms can achieve superior enhancement effects and reach lower detection limits [7].EM primarily achieves enhancement effects through localized surface plasmon resonance (LSPR) [8], which has a wide range of applications and can enhance the Raman signals of various molecules. In the EM mechanism, the incident laser generates a plasma effect on the surface, which absorbs more of the incident light, reduces reflection, strengthens scattering, and thus enhances the Raman effect [9]. However, this mechanism exhibits some limitations in terms of specificity. Meanwhile, CM achieves enhancement effects through charge transfer efficiency between the analyte and substrate [10]. CM mainly uses charge transfer to make it easier for electrons to enter the conduction band from the valence band of the measured substance, thereby allowing more electrons to transition back from the conduction band and generate Raman effects. This mechanism provides higher specificity enhancement, facilitating the detection and analysis of specific molecules. By combining EM with CM, better SERS effects can be achieved. EM offers broad applicability and high gain, while CM imparts specificity enhancement, leading to higher gains for specific molecules. This integrated approach not only increases the intensity of Raman signals but also further reduces the detection limit, enabling SERS technology to unleash its full potential in detecting substances at lower concentrations.

Noble metal materials were initially widely recognized for their application in SERS effects, particularly due to the outstanding SERS performance of noble metal nanostructures, such as Au/Ag NPs [11]. With deeper research into noble metal nanostructures and SERS mechanisms, researchers have been continuously exploring the application of other materials in SERS. For example, organic dyes [12], semiconductor nanostructures [13], and two-dimensional materials have also been utilized for SERS [14], and have shown certain enhancement effects. While exploring materials, the nanostructure of the substrate is also constantly being studied and improved, such as Pyramid structure [15], nanorod array [16], and nanotubes [17]. In particular, the application of semiconductor materials in the Raman field has experienced rapid development. Semiconductor materials have become highly promising SERS substrate materials due to their advantages of high chemical stability, good biocompatibility, high carrier mobility [18], and controllable preparation processes [19,20]. Semiconductor nanomaterials such as ZnO, ZnS, Pb₃O₄, CuO, and TiO₂ have demonstrated strong SERS signals, making them widely regarded as excellent SERS substrate materials [21].

In nanostructured semiconductor materials, the SERS mechanisms can be categorized into three cases: Firstly, charge transfer efficiency is one of the main sources of the SERS effect in semiconductors and belongs to the chemical enhancement mechanism. This enhancement effect is closely related to the energy level matching between the semiconductor substrate and the target molecules. Charge transfer efficiency occurs when the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the target molecule align with the conduction and valence band levels of the semiconductor. This process alters the polarizability tensor and electron density distribution of the molecule, resulting in a pronounced SERS effect. Secondly, when light is incident on the surface of semiconductor nanostructures, it can induce LSPR, leading to the generation of the SERS effect, which operates on a mechanism similar to that of metal materials. The third mechanism occurs when the dimensions of the nanostructures or cavities on the substrate are equal to the wavelength of the incident light, resulting in Mie scattering, which further improves the SERS performance of the substrate [22].One significant advantage of using semiconductors as Raman substrates is the ability to modify properties such as HOMO, LUMO, and surface charge density through various means, including adjusting nanostructure, doping, and introducing defects. By modifying these properties, the charge transfer efficiency and LSPR effects can be enhanced, leading to improved Raman enhancement and specificity [13].

While there have been numerous studies on the SERS effect of oxide semiconductors substrates, relatively few studies have focused on SERS performance of tantalum-based oxide materials (Ta₂O₅). However, tantalum-based oxide materials possess exceptional thermal stability and corrosion resistance, along with excellent optical and catalytic properties, thus holding significant potential in the field of SERS [23].Although there is relatively limited research on the SERS performance of tantalum-based oxide materials, existing studies have shown their potential in SERS applications. By manipulating the structure and surface properties of tantalum-based oxide materials, such as through nanoscale engineering, surface modification, or doping, their SERS performance can be further optimized to achieve higher sensitivity and specific enhancement [24,25]. Additionally, tantalum-based oxide materials can be combined with other functional materials to expand their applications in the field of SERS.

The application of Ta₂O₅ materials in the SERS field faces challenges such as relatively weak enhancement and the difficulty of achieving charge transfer efficiency due to its wide-bandgap semiconductor nature. To address these issues, this study focused on optimizing the SERS performance of Ta₂O₅ substrates through morphology control, band engineering, and material composites. In terms of morphology control, the preparation of urchin-like nano structures in Ta₂O₅ effectively increased the surface area and the number of active sites, thereby enhancing the SERS effect. In terms of band engineering, the self-doping effect was achieved in Ta₂O₅ structures prepared through hydrothermal reactions, resulting in a change in the charge density and band structure of Ta₂O₅, thereby regulating its Raman effect [26]. Finally, in the aspect of material composites, a composite structure was formed by combining Ta₂O₅ with Ag NPs that exhibit strong SERS activity. The SERS effect of Ag NPs was utilized, and additional modification sites were provided on the Ta₂O₅ surface, leading to synergistic enhancement.

In this study, a Ag NPs/Ta₂O₅ composite substrate was designed and prepared. With the synergistic enhancement of CM and EM, Three low concentration substances were detected, namely Rhodamine 6 G (R6G), crystal violet (CV), and methylene blue (MB), with detection limits of 10⁻¹³ M, 10⁻¹¹ M, and 10⁻¹¹ M, respectively. Moreover, the substrate exhibited good uniformity and strong stability, allowing for substrate recycling.

2. Experimental

2.1 Materials

Tantalum tablets (Ta, 99.9%), hydrofluoric acid (HF, AR, ≥ 40%), hydrogen peroxide solution (H₂O₂, AR, 30 wt. % in H₂O), silver nitrate (AgNO₃, AR, 99.8%) and polyvinylpyrrolidone (PVP, average Mw 10000) were purchased from Aladdin Co., Ltd. (Shanghai, China). Acetone (C₃H₆O, AR, ≥ 99.5%), ethylene glycol (C₂H₆O₂, AR, ≥ 99.5%) and ethanol absolute (C₂H₆O, AR, ≥ 99.7%) were bought from Sinopharm Chemical Reagent Co., Ltd. Deionized water is prepared by laboratory equipment.

2.2 Synthesis of Ag NPs and Ta₂O₅ nanostructures

Synthesis of Ag NPs was achieved through a chemical method [27]. Firstly, in a 20 mL round-bottom flask, an ethylene glycol solution was added and placed on a temperature-controlled magnetic stirrer. When the temperature reached 70 °C, 0.25 g PVP was added to the flask, followed by further heating to 135 °C. At this temperature, 0.05 g of AgNO₃ was added. The mixture was heated for 3 hours until it appeared milky white, after which the liquid was removed and allowed to cool. After cooling, 45 mL of acetone was added to the mixture for cleaning and centrifugation. After several cycles of cleaning and centrifugation, a solution of Ag NPs was obtained.

The Ta₂O₅ nanostructures were synthesized using a hydrothermal reaction method [28]. Firstly, a tantalum sheet with dimensions of 10 mm × 10 mm and a thickness of 1 mm was taken and subjected to grinding, polishing, and cleaning treatments. The treated tantalum sheet was then placed in a reaction vessel. Next, 5 mL of 30% mass concentration H₂O₂ and 1 mL of 40% mass concentration HF were injected into the reaction vessel. The reaction vessel was placed in a drying oven, and the oven temperature was set to 220 °C with a reaction time of 24 h. After the reaction was completed, the tantalum sheet was taken out and subjected to drying treatment. Multiple layers of Ta₂O₅ structures were observed on the surface of the tantalum sheet.

2.3 Preparation of the composite substrate

The preparation process of the composite substrate is shown in Fig. 1. The Ag NPs solution was subjected to 30 minutes of ultrasonic treatment to achieve uniform dispersion of the Ag NPs. Next, the Ta₂O₅ sample prepared by the hydrothermal reaction method was immersed in the Ag NPs solution and left to soak for 20 minutes. Subsequently, the sample was placed in a drying oven at a temperature of 60 °C Celsius for continuous drying for 24 h to obtain the composite substrate sample.

 

Fig. 1. Preparation process of Ag NPs/ Ta₂O₅ composite SERS substrate.

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2.4 Experimental instrument

Scanning electron microscopy (SEM, ZEISS Gemini Sigma 500) was employed to characterize the morphology and structure of Ag NPs, urchin-like Ta₂O₅ and the composite substrate. Energy-dispersive spectroscopy (EDS, Bruker QUANTAX EDS) was used to characterize the elemental composition of the composite substrate. The ultraviolet-visible spectrophotometer (UV-Vis, Alpha-1500) was used to measure the absorption spectrum of urchin-like Ta₂O₅ and determine the bandgap width. Ultraviolet photoelectron spectroscopy (UPS, Thermo Scientific Escalab Xi+) with an electron energy of 21.22 eV was utilized to determine the valence band position and Fermi level of the sample. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was employed to characterize the surface chemical composition and chemical states of Ta₂O₅. X-ray diffraction (XRD, Rigaku D/MAX-RB) was used to determine the crystal structure of urchin-like Ta₂O₅. Raman spectroscopy (Horiba HR Evolution 800) with a laser wavelength of 532 nm was employed to investigate the Raman properties of the composite substrate. Fifteen random locations were selected for Raman spectroscopy, and the average values were calculated.

3. Results and discussion

3.1 Characterization of Ta₂O₅ nanostructures and composite substrates

In the experiment, the Ag NPs were prepared using the afore-mentioned chemical method. In this method, ethylene glycol was used as a reducing agent to reduce AgNO₃ to Ag NPs at a temperature of 135 °C. Polyvinylpyrrolidone (PVP) was used as a capping agent to promote the formation of uniform Ag NPs structures [29]. From SEM images, the prepared Ag NPs showed a relatively uniform diameter distribution, mainly ranging from 80 nm to 120 nm, and exhibited good consistency (Fig. 2(a)). Incorporating these Ag NPs into the composite substrate would contribute to achieving significant Raman enhancement effects.

 

Fig. 2. (a-b) SEM image of Ag NPs, urchin-like Ta₂O₅. (c-d) SEM image of Ag NPs/Ta₂O₅ composite substrate. (e-f) TEM and HRTEM image of urchin-like Ta₂O₅. (g) electron diffraction pattern of urchin-like Ta₂O₅.

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The urchin-like Ta₂O₅ nanostructures were prepared using the hydrothermal method described in 2.2. In this process, H₂O₂ and HF were used to corrode the tantalum sheet, resulting in the formation of a layer of urchin-like Ta₂O₅ nanostructures on the tantalum surface. Due to the presence of a dense Ta₂O₅ protective layer on the tantalum surface, only a few acids can penetrate this barrier and corrode the tantalum sheet. Nitric acid, sulfuric acid, or even aqua regia, which are highly corrosive, cannot penetrate this protective layer. However, HF is an exception. The fluoride ions in HF are small enough to penetrate the oxide layer and react with the tantalum metal, forming a complex structure. Although HF is a weak acid, it is one of the few inorganic agents that can react with the tantalum sheet. The addition of H₂O₂ enhances the oxidizing properties of HF and promotes the formation of Ta₂O₅ nanostructures. The specific reaction equation is as follows [28]:

The Ta₂O₅ nanostructures formed by the hydrothermal reaction exhibit a multi-layered structure on a macroscopic scale. SEM images show that each layer is composed of Ta₂O₅ nanostructures with a sea urchin-like morphology (Fig. 2(b)). The nano-spikes of these urchin structures have a length of approximately 400 nm, while the base diameter is around 100 nm. The size of these nanostructures shows a relatively uniform distribution overall and exhibits good consistency.

Due to the unique structural features of the prepared Ta₂O₅, it is only necessary to immerse it in the Ag NPs solution, and the Ag NPs will spontaneously fill the gaps between the Ta₂O₅ nano-spikes, forming a tightly bonded Ag NPs/Ta₂O₅ composite substrate (Fig. 2(c), Fig. 2(d)). This processing method is relatively simple and does not require complex fabrication steps. The good bonding between the Ag NPs and the Ta₂O₅ nanostructures allows the composite substrate to exhibit highly consistent structural characteristics.

To further investigate the properties of Ta₂O₅ in urchin-like structures, the lattice structure of post-treated Ta₂O₅ nanospikes was characterized using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The HRTEM image (Fig. 2(f)) clearly shows the lattice spacing of Ta₂O₅ in the urchin-like structure. The hydrothermally synthesized Ta₂O₅ exhibits a single crystal structure predominantly growing along the (010) direction, with a lattice plane spacing of 0.385 nm, corresponding to the (010) crystal plane of orthorhombic Ta₂O₅ (JCPDS 79-1375) [30,31].

The results of EDS testing show that the distribution of oxygen and tantalum elements in the nanostructured Ta₂O₅ is extremely uniform (Fig. 3(a)), with a ratio close to 2:1 (Fig. 3(b)). Although this ratio does not exactly match the chemical formula of Ta₂O₅, subsequent XPS analysis reveals that this is due to the presence of a significant amount of oxygen vacancies in the nanostructured Ta₂O₅ after the hydrothermal reaction. Additionally, the use of hydrofluoric acid during the hydrothermal reaction results in a small amount of residual fluorine on the surface of the nanostructured Ta₂O₅. However, further X-ray diffraction (XRD) analysis confirms that these fluorine elements do not remain in the lattice. Furthermore, the content of fluorine is very low and does not significantly affect the properties of the sample.

 

Fig. 3. (a-b) EDS image of urchin-like Ta₂O₅. (c) UV-Vis patterns of Ag NP, R6G and R6G on Ag NPs/Ta₂O₅. (d) XRD patterns of urchin-like Ta₂O₅. (e-f) UV-Vis patterns of urchin-like Ta₂O₅ showing wavelength and energy.

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XRD analysis of the hydrothermally prepared urchin-like Ta₂O₅ sample revealed XRD patterns (Fig. 3(d)) corresponding to the lattice constants of orthorhombic Ta₂O₅ (JCPDS 79-1375) phase, which is consistent with the TEM results, confirming the prepared tantalum oxide as Ta₂O₅.Apart from these characteristic peaks, no other significant diffraction peaks were observed, indicating that no other by-products such as H₂TaO₆ and Ta₂O₅·nH₂O were formed during the hydrothermal reaction, or if present, they were present in only trace amounts. This also suggests that the fluorine element in hydrofluoric acid is only present on the surface of Ta₂O₅ nanospikes and hydrofluoric acid acts solely as an inorganic agent in the reaction, used for tantalum sheet etching, promoting the crystallization of Ta₂O₅ and the formation of the urchin-like structure, without being incorporated into the generated Ta₂O₅ lattice. The strongest peak in the XRD pattern corresponds to the (010) crystal plane, indicating a preferential growth along the b-axis, perpendicular to the (010) crystal plane. Other crystal planes such as (002), (012), (020), (2410) exhibit weaker crystallinity, showing smaller and broader peaks, consistent with the TEM results.

To measure the bandgap width and relative changes of the urchin-like Ta₂O₅ nanostructures, UV-Vis spectrophotometry was conducted on both purchased Ta₂O₅ powder and prepared Ta₂O₅ nanostructures in the wavelength range of 200 nm to 800 nm. As shown in Fig. 3(e) and Fig. 3(f), compared to the purchased Ta₂O₅ powder, the urchin-like Ta₂O₅ prepared through the hydrothermal reaction broadens the absorption frequency band, indicating a narrower bandgap width. By converting the energy-absorption spectra, an approximate bandgap width of 3.84 eV was calculated for the purchased Ta₂O₅ powder, while the bandgap width of the urchin-like Ta₂O₅ was approximately 2.78 eV, consistent with reported data on other hydrothermally synthesized Ta₂O₅ nanostructures [32]. The reduction in bandgap width is attributed to the excess tantalum sheet used in the experiment, which leads to the partial reduction of Ta⁵⁺ to Ta⁴⁺. The formation of Ta⁴⁺ is accompanied by the formation of oxygen vacancies. The presence of oxygen vacancies introduces defect energy levels, altering the overall band structure and significantly reducing the bandgap width [33,34]. Thus, it can be observed that the urchin-like Ta₂O₅ exhibits a significantly reduced bandgap width, thereby extending its absorption range for laser excitation, which positively contributes to achieving charge transfer efficiency and enhancing SERS performance of the substrate.

From Fig. 3(c), it can be seen that Ag NPs have a very wide spectral response, with absorption peaks around 450 nm. The absorption peak of R6G ranges from 500 to 550 nm. After dropping R6G onto the Ag NPs/Ta₂O₅ composite substrate, the absorption peak appears at around 500 nm. Due to the fact that the Raman spectrometer in the laboratory only has two types of lasers: 325 nm and 532 nm, a 532 nm laser was selected as the testing light source for this experiment, which is also the most common and effective Raman testing light source.

In order to gain a deeper understanding of the composition and band information of urchin-like Ta₂O₅, XPS characterization was performed on the purchased Ta₂O₅ powder and the prepared urchin-like Ta₂O₅. As shown in Fig. 4(a) and Fig. 4(d), both exhibit characteristic double peaks belonging to Ta4f. Notably, in the urchin-like Ta₂O₅, the characteristic peaks of Ta4f are observed to have a blue shift, indicating a higher binding energy. During the fitting process, it was observed that the Ta4f spectrum of urchin-like Ta₂O₅ exhibits, in addition to the double peaks at 26.3 eV (Ta^5 +4f5/2) and 28.2 eV (Ta^5 +4f7/2) attributed to Ta⁵⁺, two distinct double peaks located at 25.9 eV and 27.8 eV. The difference between these two peaks is similar to that of Ta⁵⁺, and their shapes are analogous. Through literature review, coupled with results from EDS and UV tests, it was confirmed that these double peaks are attributed to Ta⁴⁺ and represent Ta^4 +4f5/2 and Ta^4 +4f7/2. This finding indicates the presence of Ta⁴⁺ within the urchin-like Ta₂O₅ and further suggests the existence of oxygen vacancies [25].

 

Fig. 4. (a-b) XPS spectra of Ta4f, O1s of urchin-like Ta₂O₅. (c) XPS survey spectrum of Ag NPs/ Ta₂O₅ composite substrate. (d-e) XPS spectra of Ta4f, O1s of purchased Ta₂O₅. (f) Valence-band XPS spectra of purchased Ta₂O₅ and urchin-like Ta₂O₅. (g-h) UPS spectra of urchin-like Ta₂O₅.

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Analysis of the O1s XPS spectra (Fig. 4(b), Fig. 4(e)) revealed that the purchased Ta₂O₅ powder exhibited two peaks located at 529.9 eV and 531.6 eV, indicating the presence of oxygen in two forms corresponding to lattice oxygen and hydroxyl groups bound to the surface. In contrast, the urchin-like Ta₂O₅ displayed three peaks at 530.2 eV, 531.3 eV, and 532.4 eV, indicating the presence of oxygen in three forms: lattice oxygen, oxygen adsorbed by oxygen vacancies, and hydroxyl groups [35,36]. This indicates that the urchin-like Ta₂O₅ prepared through the hydrothermal reaction has a significant amount of oxygen vacancies and a certain amount of hydroxyl groups. The presence of oxygen vacancies and the unique nanostructure significantly influence the band structure and properties of Ta₂O₅, facilitating charge transfer efficiency and achieving enhanced Raman effects.

To accurately determine the position of the energy bands, UPS testing was conducted on the urchin-like Ta₂O₅ prepared through the hydrothermal reaction. The excitation energy of the instrument was set to 21.22 eV. According to the UPS spectrum (Fig. 4(g), relative to the vacuum level, the valence band maximum of the urchin-like Ta₂O₅ was located at -6.66 eV. Based on the UV testing results, it can be inferred that the conduction band minimum is located at -3.88 eV. According to the UPS spectrum with a bias voltage of -5 V (Fig. 4(h)), the work function of the urchin-like Ta₂O₅ was determined to be 3.75 eV (21.22 eV-17.47 eV), indicating that the Fermi level is at -3.75 eV. Analysis of the band structure reveals that the prepared urchin-like Ta₂O₅ has a Fermi level higher than the conduction band minimum, indicating a degenerate semiconductor. This is due to the presence of a significant amount of oxygen vacancies in the urchin-like Ta₂O₅ structure prepared through the hydrothermal reaction, which results in self-doping, increases the carrier concentration, provides more excited states, and further enhances the interaction with adsorbed molecules, facilitating charge transfer efficiency and enhancing the Raman signal of the analyte molecules [33].

3.2 Raman results

In this study, the Raman tests used a laser wavelength of 532 nm. Except for the test in Fig. 5(a), which used a laser power of 24 µW, the laser power used in other Raman tests was 4.8 µW. The enhancement effect of tantalum sheet and commercial tantalum oxide in Fig. 5(a) is very weak, it requires the use of higher power lasers to obtain Raman signals. If higher power lasers are also used for other tests, there will be a phenomenon of peak shaving in the spectrum at high concentrations of R6G, so a lower power laser was chosen. A widely used SERS probe molecule, R6G, was chosen for conducting comprehensive comparative tests on the SERS performance of tantalum sheet, commercial Ta₂O₅, urchin-like Ta₂O₅ structures, Ag NPs, and Ag NPs/Ta₂O₅ composite substrates. The results (Fig. 5(a)) showed that the Ag NPs/Ta₂O₅ composite substrate exhibited the strongest Raman enhancement, with higher peak intensity compared to the substrate with Ag NPs alone, and achieved an extremely low detection limit of 10⁻¹³ M (Fig. 5(c)) with an enhancement factor of up to 10¹⁰. This significant performance improvement is mainly attributed to the LSPR effect induced by Ag NPs and the energy level matching between the urchin-like Ta₂O₅ structures and R6G, promoting charge transfer efficiency. Furthermore, a comparative study revealed that the nanostructured Ta₂O₅ prepared through the hydrothermal reaction exhibited significantly better SERS performance than the commercially purchased Ta₂O₅ substrate. On one hand, the unique structure of nanostructured Ta₂O₅ can generate a stronger LSPR effect, which was verified by subsequent Comsol simulations. On the other hand, the nanostructured Ta₂O₅ structure has a larger specific surface area, and XPS results indicated the presence of a large number of oxygen vacancies on its surface. These characteristics allow the nanostructured Ta₂O₅ to modify the band structure and facilitate charge transfer efficiency.

 

Fig. 5. (a) The Raman spectra of R6G on Ta sheet, purchased Ta₂O₅, urchin-like Ta₂O₅, Ag NPs and Ag NPs/Ta₂O₅ composite substrate using a laser power of 24 µW. (b) Raman spectra of R6G on the Ag NPs/Ta₂O₅ composite substrates from 10⁻⁷ M to 10⁻¹² M using a laser power of 4.8 µW. (c) Ag NPs/Ta₂O₅ composite substrate's lowest detection limit for R6G. (d) The relationship between R6G concentration and the intensities of the Raman peaks at 611 cm^-1 and 1362 cm^-1. (e) Raman mapping image of R6G on the composite substrate. (f-g) Raman spectra of CV and MB on Ag NPs/Ta₂O₅ composite substrate at concentrations ranging from 10⁻⁷ M to 10⁻¹² M.

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Additionally, the relationship between the Raman signal intensity of the composite substrate and the concentration of R6G was tested (Fig. 5(d)). R6G was drop-cast onto the composite substrate in volumes of 2 µL over a concentration range of 10⁻¹² to 10⁻⁷ M, followed by Raman measurements. The test results showed a linear relationship between the logarithm of the intensity at 611 cm^-1 and 1362 cm^-1 peaks and the logarithm of the concentration, with correlation coefficients (R²) of 0.98162 and 0.98969, respectively. This provides a basis for quantitative measurement of the concentration of the analyte on the substrate to some extent. Raman mapping (Fig. 5(e)) revealed that the intensity of the 611 cm^-1 peak was relatively uniform within the tested range, indicating good uniformity of the substrate. This is mainly attributed to the urchin-like Ta₂O₅ structures tightly covering the entire surface of the substrate, with Ag NPs uniformly embedded in the gaps of the nano spikes. Therefore, the substrate exhibits good consistency and uniformity, resulting in good uniformity in Raman measurements.

By comparing and testing the SERS performance of tantalum foil, commercial Ta₂O₅, nanostructured Ta₂O₅, Ag NPs, and Ag NPs/Ta₂O₅ composite substrates, this study found that the Ag NPs/Ta₂O₅ composite substrate exhibited the highest peak intensity, superior to the use of Ag nanoparticles alone, and achieved a low detection limit of 10⁻¹³ M. The nanostructured Ta₂O₅ exhibited significantly better SERS performance than the commercial Ta₂O₅ substrate, attributed to its stronger LSPR effect and larger surface area, as well as the modification of the band structure by the presence of oxygen vacancies, facilitating charge transfer efficiency. Additionally, through testing the Raman signal intensity and R6G concentration of the composite substrate, quantitative measurement of the concentration of the analyte on the substrate can be achieved. Raman mapping results demonstrated good uniformity of the composite substrate, which is attributed to the urchin-like Ta₂O₅ structures covering the entire surface and the uniform embedding of Ag NPs in the gaps of the nano spikes, ensuring the consistency and uniformity of the substrate.

To validate the universality of the substrate enhancement effect, two dye molecules, CV (Fig. 5(f)) and MB (Fig. 5(g)), were tested. The test results showed that both CV and MB molecules exhibited clear characteristic peaks even at a concentration of 10⁻¹¹ M, indicating a strong enhancement effect of the substrate on these two dye molecules.

By calculating RSD parameters, the uniformity and consistency of the substrate can be characterized. Prepare 6 Ag NPs/Ta₂O₅ composite substrates, use 10⁻¹⁰ M R6G as probes, select 5 test points for each substrate, measure 30 sets of spectral data as seen in Fig. 5(d), and calculate the RSD value of 611 cm^-1 peak. The formula is as follows:

The SERS signal strength of R6G 611 cm^-1 peak is represented by I, and $\bar{I}$ represents the average value of all 611 cm^-1 peaks at the test points. The final measured substrate RSD value is about 9.16%, indicating that the Ag NPs/Ta₂O₅ substrate has good uniformity and reproducibility.

To quantify the sensitivity of the substrate, R6G was used as the probe and silica substrate as the benchmark. Calculate the EF for R6G using the following formula:

In the formula, ${I_{SERS}}$ and ${I_{Si{O_2}}}$ are the intensity of the SERS peak and the normal Raman intensity on SiO₂ substrate respectively, and ${N_{SERS}}$ and ${N_{Si{O_2}}}$ are the number of R6G molecules per unit volume of SERS and normal Raman scattering. As shown in Fig. 6(a), the peak strength of 10⁻² M R6G at 611 cm⁻¹ on SiO₂ is 836 and the peak intensity of 10⁻¹² M R6G on Ag NPs/Ta₂O₅ composite substrate is 1624. As a result, the average EF of Ag NPs/Ta₂O₅ composite substrate is 1.94 × 10¹⁰, which is higher than that of several Ta₂O₅-based or Ag NPs-based composite SERS substrates that have been reported, as shown in Table 1. As a result, the average EF of Ag NPs/Ta₂O₅ composite substrate was 1.94 × 10¹⁰, which is very excellent among several reported composite SERS substrates based on Ta₂O₅ or Ag NPs, as shown in Table 1.

 

Fig. 6. (a) The Raman spectra of 10⁻² M R6G on SiO₂ and 10⁻¹² M R6G on Ag NPs/Ta₂O₅. (b-c) The results of the 30-day stability test of the composite substrate. (d) SEM image of Ag NPs/Ta₂O₅ substrate after HNO₃ cleaning. (e) Comparison of Raman spectra before and after recycling treatment.

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Table 1. Sensitivity Comparison of Several Ta₂O₅-based or Ag NPs-based Composite SERS Substrates

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To verify the stability of the substrate, stability tests were conducted for a period of 30 days. After preparing the composite substrate, it was stored in the atmosphere at a temperature of around 25 °C at room temperature. Every 5 days, Raman spectra were measured using R6G samples at a concentration of 10⁻¹⁰ M, and the results are shown in Fig. 6(b). The intensity of the 611 cm^-1 peak was chosen for statistical analysis (Fig. 6(c)). In the initial 10 days, the peak intensity decayed rapidly and then stabilized. At the end of the 30-day test, the Raman signal remained above 80% of the initial peak intensity. This indicates that the urchin-like Ta₂O₅ structure in the composite substrate is highly stable, unaffected by oxidation, and exhibits excellent corrosion resistance and high-temperature stability, making it suitable for harsh detection environments, which is of great significance in practical applications. The main reason for the substrate decay is the oxidation of Ag NPs; however, due to the close coupling between Ag NPs and the urchin structure, it provides a certain level of protection, slowing down the oxidation process of Ag NPs. After the oxidation of Ag NPs, the substrate can be dissolved by immersing it in HNO₃, removing the oxidized Ag NPs. Since Ta₂O₅ has excellent acid corrosion resistance, the urchin-like Ta₂O₅ structure is not damaged (Fig. 6(d)). After ultrasonic cleaning with alcohol and deionized water, Ag NPs can be re-added to form a new composite substrate, achieving the reusability of the substrate. The Raman EF of the substrate produced after recycling can be restored (Fig. 6(e)).

3.3 Comsol simulation

To gain a deeper understanding of the electromagnetic enhancement mechanism of the composite substrate and assist in the experimental analysis, COMSOL software was used for simulation to study the electric field distribution of various structures under the excitation of a 532 nm laser [39]. Through these simulations, it is possible to explore the electromagnetic enhancement mechanism more comprehensively and provide guidance for optimizing the nanostructure of the substrate. Corresponding simulation parameters were set based on the actual situation of the experimental device and SEM images. The incident laser wavelength was set to 532 nm, and the diameter of the Ag NPs was 100 nm. The Ta₂O₅ nanostructure was simplified to a conical shape with a base diameter of 100 nm and a height of 400 nm, resembling the morphology of a urchin. By simulating this structure, corresponding electric field distribution images were obtained, as shown in the Fig. 7.

 

Fig. 7. Simulated electric field distribution of Ta₂O₅, Ag NPs, urchin-like Ta₂O₅ and Ag NPs/Ta₂O₅ composite substrates.

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From the results in Fig. 7(a), it can be observed that compared to Ta₂O₅ nanoparticles, the Ta₂O₅ nano-spikes can generate a stronger, more extensive, and more uniformly distributed excitation electric field. As a result, the Ta₂O₅ nano-spikes exhibit a stronger LSPR effect, leading to better enhancement. In Fig. 7(b), a significant LSPR effect can be observed between Ag NPs, leading to a strong local electric field. Figure 7(c) shows the simulation of a single Ta₂O₅ sea urchin-like nanostructure, revealing a stronger localized electric field at the base of the spikes, which is enhanced compared to individual nano-spikes but weaker than that between the Ag NPs. Figure 7(d) presents the simulation of the Ag NPs/Ta₂O₅ composite substrate structure. In this structure, there exists a strong and extensive excitation electric field between the Ag nanoparticles and the Ta₂O₅ nano-spikes, with the intensity even surpassing that between the Ag NPs. The exceptionally strong excitation field can be attributed not only to the unique sea urchin-like structure that tightly binds the Ag NPs and Ta₂O₅ nano-spikes but also, primarily, to the achievement of energy level alignment between the Ag NPs and Ta₂O₅ nano-spikes, facilitating charge transfer efficiency (CM). It can be concluded that the LSPR effect of composite substrates is very strong and superior to pure Ag NPs substrates, exhibiting a wide distribution range. Consequently, the Ag NPs/Ta₂O₅ composite substrate demonstrates excellent electromagnetic enhancement.

3.4 Theoretical analysis

The enhanced performance of the Ag NPs/Ta₂O₅ composite substrate is a result of EM and CM. The EM enhancement is primarily attributed to the LSPR effect of the Ag nanoparticles. The CM enhancement arises from the energy level alignment between R6G molecules, Ta₂O₅, and Ag NPs. The synergistic interplay between the EM and CM mechanisms significantly enhances the substrate's SERS effect, enabling highly sensitive detection and analysis of molecules.

Based on the EM mechanism, employing Comsol electromagnetic simulation technology allows us to draw the following conclusions: LSPR effects exist between the Ag nanoparticles and the Ta₂O₅ nanocones, as well as among the Ag nanoparticles themselves, resulting in strong localized electric fields and subsequent electromagnetic enhancement. In this process, a substantial portion of the incident light energy is absorbed by the surface plasmon waves, leading to a sharp decrease in reflected light energy and an increase in scattered light energy, significantly enhancing the Raman signal [9].

Based on the CM mechanism, charge transfer efficiency occurs between R6G and the composite substrate [24]. Through XPS and UPS testing results, it can be concluded that the valence band maximum (VB) and conduction band minimum (CB) of the urchin-like Ta₂O₅ structure are positioned at -6.66 eV and -3.88 eV relative to the vacuum level, respectively. The HOMO and LUMO energy levels of R6G are -5.70 eV and -3.00 eV, respectively. Due to the HOMO-LUMO gap of R6G, which is approximately 1.90 eV, being significantly smaller than the energy of the incident 532 nm laser (2.33 eV), there is a clear energy level matching between R6G and Ta₂O₅, allowing for electron transfer from R6G's HOMO level to Ta₂O₅'s conduction band and further to R6G's LUMO level. The Ta₂O₅ conduction band acts as a stepping stone for charge transfer efficiency and greatly facilitates the process. The energy level difference between the Fermi level of Ag nanoparticles (-4.84 eV) and the LUMO level of R6G and the conduction band minimum of Ta₂O₅, respectively, is approximately 1.84 eV and 0.96 eV, both significantly smaller than the energy of the incident laser (2.33 eV). Therefore, charge transfer efficiency also occurs between R6G and Ta₂O₅ and Ag nanoparticles(Fig. 8). This series of charge transfer efficiency effectively reduces the difficulty of electron transfer between R6G's HOMO and LUMO orbitals, increasing the scattering probability and enhancing the SERS effect of the substrate.

 

Fig. 8. The charge transfer efficiency between R6G, urchin-like Ta₂O₅ and Ag NPs.

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The urchin-like nanostructure of Ta₂O₅ also serves as a crucial foundation for achieving high sensitivity detection. It increases the substrate's surface area, providing more contact sites with R6G molecules. Moreover, the unique structure enables a tight integration between Ta₂O₅ and Ag NPs, facilitating charge transfer efficiency between the analyte molecules and the substrate, which contributes to enhanced Raman signals. Additionally, Ta₂O₅'s excellent optical properties, combined with its special structure, allow for multiple reflections and refractions of light, enhancing the utilization efficiency of the laser and increasing scattering, ultimately boosting the Raman signal [40].

4. Conclusions

In this study, urchin-like Ta₂O₅ nanomaterials were successfully synthesized using hydrothermal reactions and combined with Ag NPs to create SERS substrates for detecting low concentration target molecules such as R6G, CV, and MB. The urchin-like structure of the Ta₂O₅ nanomaterials, with its unique structural characteristics and self-doping effect, effectively modulated the band structure, thereby improving the charge transfer efficiency and surface plasmon resonance effect. This provides a crucial foundation for achieving high sensitivity in SERS detection. Furthermore, the constructed SERS substrates demonstrated strong quantitative capability, stability under strong acid and alkali conditions, and good stability and recyclability, making them highly promising for applications in complex environments such as wastewater monitoring.