Na2Ti3O7@RF@Ag Heterostructures as Efficient Substrates for SERS and Photocatalytic Applications

A multi-step procedure was effectively employed to synthesize innovative three-dimensional (3D) heterostructures encompassing sodium titanate (Na2Ti3O7) nanowire cores, an intermediate resorcinol–formaldehyde (RF) layer, and outer silver (Ag) nanoparticle sheaths, referred to as Na2Ti3O7@RF@Ag heterostructures. Initially, a one-step hydrothermal technique facilitated the direct growth of single-crystal Na2Ti3O7 nanowires onto a flexible Ti foil. Subsequently, a two-step wet chemical process facilitated the sequential deposition of an RF layer and Ag nanoparticles onto the Na2Ti3O7 nanowires at a low reaction temperature. Optimal concentrations of silver nitrate and L-ascorbic acid can lead to the cultivation of Na2Ti3O7@RF@Ag heterostructures exhibiting heightened surface-enhanced Raman scattering (SERS), which is particularly beneficial for the detection of rhodamine B (RhB) molecules. This phenomenon can be ascribed to the distinctive geometry of the Na2Ti3O7@RF@Ag heterostructures, which offer an increased number of hot spots and surface-active sites, thereby showcasing notable SERS enhancement, commendable reproducibility, and enduring stability over the long term. Furthermore, the Na2Ti3O7@RF@Ag heterostructures demonstrate remarkable follow-up as first-order chemical kinetic and recyclable photocatalysts for the photodecomposition of an RhB solution under UV light irradiation. This result can be attributed to the enhanced inhibition of electron–hole pair recombination and increased surface-active sites.

Sodium titanate (Na 2 Ti n O 2n+1 , where 2 ≤ n ≤ 9) has garnered considerable attention as a promising material due to its notable attributes: a high level of chemical inertness, nontoxicity, ion exchange capabilities, and cost-effectiveness [29][30][31].This versatile substance finds applications in chemical absorption, photocatalysis, supercapacitors, and sodium-ion batteries [32][33][34][35][36][37].Numerous synthesis methods have been employed to fabricate sodium titanate nanostructures [38][39][40].The hydrothermal method has emerged as a preferred choice due to its simplicity and cost-effectiveness, making it widely used for obtaining a diverse range of nanostructures [35,41].One-dimensional sodium titanate nanostructures adorned with Ag nanoparticles via an ion-sputtering technique have recently emerged as highly effective SERS substrates, showcasing exceptional detection sensitivity, stability, and uniformity [28].This limitation stems from the necessity to conduct the deposition of Ag nanoparticles under high-vacuum conditions, which are energy-intensive and not environmentally friendly.
Prior investigations have often resorted to thermal evaporation or ion-sputtering techniques to achieve the uniform deposition of Au or Ag nanoparticles on one-dimensional semiconductor nanostructures to fabricate 3D SERS substrates [42][43][44].These methods can effectively produce high-performance SERS substrates but fall short regarding energy savings and carbon reduction [45].Moreover, the literature dedicated to the development of SERS substrates on one-dimensional semiconductor nanostructures through the application of a resorcinol-formaldehyde (RF) resin layer, a strategy aimed at diminishing the reliance on noble metal nanoparticles, has been relatively scant [46,47].In earlier studies, resorcinol exhibited versatility by serving as a critical reactant in forming the RF layer and playing an essential role in passivating the surfaces of metal nanoparticles to prevent agglomeration [48,49].Furthermore, resorcinol's capabilities extended to acting as a reducing agent, facilitating the conversion of metal salts into their nanoparticle counterparts [50,51].This innovative approach promises effective SERS substrates and aligns with sustainability goals by reducing the carbon footprint associated with traditional high-vacuum methods.
This study details the fabrication of Na 2 Ti 3 O 7 @RF@Ag heterostructures achieved through straightforward hydrothermal and wet chemical techniques, aiming to enhance their photocatalytic and SERS capabilities.The optimization of AgNO 3 and L-ascorbic acid volumes was explored to maximize SERS enhancement, specifically for detecting rhodamine B (RhB) molecules.The Na 2 Ti 3 O 7 @RF@Ag heterostructures were thoroughly characterized to reveal their morphology, crystal structure, and chemical composition using microscopic and spectroscopic techniques.These heterostructures offer improved separation of photoelectron-hole pairs and increased surface-active sites, enhancing their photocatalytic efficiency and reusability.Additionally, the Na 2 Ti 3 O 7 @RF@Ag heterostructures present an advantageous geometry for the uniform deposition of Ag nanoparticles, facilitating the creation of high-performance SERS and photocatalytic substrates.

Results and Discussion
Figure 1 presents a schematic representation of the fabrication process for cultivating Na 2 Ti 3 O 7 @RF@Ag heterostructures.Initially, a straightforward hydrothermal technique was utilized to nurture Na 2 Ti 3 O 7 nanowires on a Ti foil at 220 • C for 5 h.Subsequently, a self-assembled APTMS ((3-Aminopropyl)trimethoxysilane) monolayer was employed to enhance the hydrophilic properties of Na 2 Ti 3 O 7 nanowires, allowing for the uniform decoration of the RF layer [47].Lastly, Na 2 Ti 3 O 7 nanowires coated with an RF layer, in conjunction with L-ascorbic acid, facilitated the reduction of Ag + to Ag, culminating in the formation of Na 2 Ti 3 O 7 @RF@Ag heterostructures.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 16 decoration of the RF layer [47].Lastly, Na2Ti3O7 nanowires coated with an RF layer, in conjunction with L-ascorbic acid, facilitated the reduction of Ag + to Ag, culminating in the formation of Na2Ti3O7@RF@Ag heterostructures.
Figure 1.Synthetic scheme of the preparation of the Na2Ti3O7@RF@Ag heterostructures.
The FESEM morphology (Figure 3a) of the Na2Ti3O7 nanowires reveals a high-density distribution of these nanowires that have grown across the entire surface of the Ti foil, displaying a random orientation.Figure 3b displays a FETEM image of a Na2Ti3O7 nanowire featuring an intermediate diameter of approximately 105 nm.Additionally, Figure 3c reveals a selected area electron diffraction (SAED) pattern of a Na2Ti3O7 nanowire, presenting a well-defined arrangement of single crystal dots.This single-crystal-dot array corresponds to the crystal structure of the Na2Ti3O7 nanowire and aligns with the monoclinic Na2Ti3O7 crystal phase (JCPDS Card No. 72-0148).The HRTEM image (Figure 3d) demonstrates an index lattice spacing of 0.314 nm, which agrees with the lattice spacing of the (111) plane of Na2Ti3O7.In addition, the FETEM-EDS mapping images (Figure 3e) prove the homogeneous distribution of Ti, O, and Na elements in the nanowire.The calculated atomic percentages of the Na2Ti3O7 nanowires from the FETEM-EDS mapping images reveal the following compositions: Na 7.9%, Ti 25.1%, and O 67%.    2b shows the XRD pattern of Na 2 Ti 3 O 7 @RF heterostructures of Na 2 Ti 3 O 7 nanowires without any distinctive peaks.Notably, the presence of Ag nanoparticles within the Na 2 Ti 3 O 7 @RF@Ag heterostructures (Figure 2c) is evident from the faint peaks at 2θ values of 38.2 • , 44.4 • , 64.6 • , and 77.6 • , corresponding to the (111), (200), (220), and (311) crystal planes of cubic-phase Ag (JCPDS Card No. 87-0720).This outcome verifies the absence of impurities within the Na 2 Ti 3 O 7 @RF@Ag heterostructures.
The FESEM morphology (Figure 3a) of the Na 2 Ti 3 O 7 nanowires reveals a high-density distribution of these nanowires that have grown across the entire surface of the Ti foil, displaying a random orientation.Figure 3b displays a FETEM image of a Na 2 Ti 3 O 7 nanowire featuring an intermediate diameter of approximately 105 nm.Additionally, Figure 3c reveals a selected area electron diffraction (SAED) pattern of a Na 2 Ti 3 O 7 nanowire, presenting a well-defined arrangement of single crystal dots.This single-crystal-dot array corresponds to the crystal structure of the Na 2 Ti 3 O 7 nanowire and aligns with the monoclinic Na 2 Ti 3 O 7 crystal phase (JCPDS Card No. 72-0148).The HRTEM image (Figure 3d) demonstrates an index lattice spacing of 0.314 nm, which agrees with the lattice spacing of the (111) plane of Na 2 Ti 3 O 7 .In addition, the FETEM-EDS mapping images (Figure 3e) prove the homogeneous distribution of Ti, O, and Na elements in the nanowire.The calculated atomic percentages of the Na 2 Ti 3 O 7 nanowires from the FETEM-EDS mapping images reveal the following compositions: Na 7.9%, Ti 25.1%, and O 67%.  Figure 4a reveals an FETEM image of a Na2Ti3O7@RF heterostructure with a coreshell structure.Figure 4b shows an SAED pattern of a Na2Ti3O7@RF heterostructure which only presents a single-crystal-dot array aligned with the monoclinic Na2Ti3O7 crystal phase (JCPDS Card No. 72-0148).The HRTEM image (Figure 4c) reveals the presence of an approximately 4.02 nm thick amorphous RF layer decorated on the Na2Ti3O7 nanowire.Figure 4a reveals an FETEM image of a Na2Ti3O7@RF heterostructure with a coreshell structure.Figure 4b shows an SAED pattern of a Na2Ti3O7@RF heterostructure which only presents a single-crystal-dot array aligned with the monoclinic Na2Ti3O7 crystal phase (JCPDS Card No. 72-0148).The HRTEM image (Figure 4c) reveals the presence of an approximately 4.02 nm thick amorphous RF layer decorated on the Na2Ti3O7 nanowire.Figure 4a reveals an FETEM image of a Na 2 Ti 3 O 7 @RF heterostructure with a coreshell structure.Figure 4b shows an SAED pattern of a Na 2 Ti 3 O 7 @RF heterostructure which only presents a single-crystal-dot array aligned with the monoclinic Na 2 Ti 3 O 7 crystal phase (JCPDS Card No. 72-0148).The HRTEM image (Figure 4c) reveals the presence of an approximately 4.02 nm thick amorphous RF layer decorated on the Na 2 Ti 3 O 7 nanowire.Additionally, a lattice fringe characterized by an interplanar spacing of 0.314 nm corresponds to the (111) plane of the monoclinic Na 2 Ti 3 O 7 crystal phase (JCPDS Card No. 72-0148).The FETEM-EDS mapping images (Figure 4d) still exhibit the heterostructure's homogeneous Ti, O, and Na distribution.This observation verifies that the presence of the RF layer on the Na 2 Ti 3 O 7 nanowire does not induce any alterations in the material's elemental composition.
Molecules 2024, 29, x FOR PEER REVIEW 5 of 16 Additionally, a lattice fringe characterized by an interplanar spacing of 0.314 nm corresponds to the (111) plane of the monoclinic Na2Ti3O7 crystal phase (JCPDS Card No. 72-0148).The FETEM-EDS mapping images (Figure 4d) still exhibit the heterostructure's homogeneous Ti, O, and Na distribution.This observation verifies that the presence of the RF layer on the Na2Ti3O7 nanowire does not induce any alterations in the material's elemental composition.Figure 5a,b presents the FETEM images of a Na2Ti3O7@RF@Ag heterostructure grown using 0.15 mL AgNO3 and 3 mL L-ascorbic acid volumes.The FETEM images can reveal that the Ag nanoparticles' size is about 3-10 nm, and they are completely decorated on the Na2Ti3O7@RF heterostructure.The SAED pattern (Figure 5c) of a Na2Ti3O7@RF@Ag heterostructure displays a combination of a single-crystal-dot array and a polycrystalline ring.The presence of a single-crystal-dot array in the SAED pattern of Na2Ti3O7@RF@Ag heterostructure corresponds to the crystalline structure of Na2Ti3O7 nanowire, which aligns with the monoclinic Na2Ti3O7 crystal phase (JCPDS Card No. 72-0148).This alignment indicates the well-preserved crystal structure of Na2Ti3O7 even after the deposition of Ag nanoparticles.The Ag nanoparticles' distinctive appearance exhibited a concentric ring pattern, corresponding to the diffraction from the (111) plane of the cubic Ag crystal structure (JCPDS Card No. 87-0720).The HRTEM image of the Na2Ti3O7@RF@Ag heterostructure (Figure 5d) shows two visible lattice fringes exhibiting an interplanar spacing of 0.235 nm which can be confidently attributed to the (111) lattice plane of the cubic Ag (JCPDS Card No. 87-0720).This observation underscores the successful deposition of cubic-phase Ag nanoparticles within the heterostructures.The FETEM-EDS mapping image (Figure 5d) provides valuable insights into the composition of the Na2Ti3O7@RF@Ag heterostructure.This composition analysis reveals the presence of Ti, O, Na, and Ag elements within the Na2Ti3O7@RF@Ag heterostructure, indicating the successful synthesis of this heterostructure through a two-step wet chemical process on the Na2Ti3O7 nanowire.Figure 5a,b presents the FETEM images of a Na 2 Ti 3 O 7 @RF@Ag heterostructure grown using 0.15 mL AgNO 3 and 3 mL L-ascorbic acid volumes.The FETEM images can reveal that the Ag nanoparticles' size is about 3-10 nm, and they are completely decorated on the Na 2 Ti 3 O 7 @RF heterostructure.The SAED pattern (Figure 5c) of a Na 2 Ti 3 O 7 @RF@Ag heterostructure displays a combination of a single-crystal-dot array and a polycrystalline ring.The presence of a single-crystal-dot array in the SAED pattern of Na 2 Ti 3 O 7 @RF@Ag heterostructure corresponds to the crystalline structure of Na 2 Ti 3 O 7 nanowire, which aligns with the monoclinic Na 2 Ti 3 O 7 crystal phase (JCPDS Card No. 72-0148).This alignment indicates the well-preserved crystal structure of Na 2 Ti 3 O 7 even after the deposition of Ag nanoparticles.The Ag nanoparticles' distinctive appearance exhibited a concentric ring pattern, corresponding to the diffraction from the (111) plane of the cubic Ag crystal structure (JCPDS Card No. 87-0720).The HRTEM image of the Na 2 Ti 3 O 7 @RF@Ag heterostructure (Figure 5d) shows two visible lattice fringes exhibiting an interplanar spacing of 0.235 nm which can be confidently attributed to the (111) lattice plane of the cubic Ag (JCPDS Card No. 87-0720).This observation underscores the successful deposition of cubic-phase Ag nanoparticles within the heterostructures.The FETEM-EDS mapping image (Figure 5d) provides valuable insights into the composition of the Na 2 Ti 3 O 7 @RF@Ag heterostructure.This composition analysis reveals the presence of Ti, O, Na, and Ag elements within the Na 2 Ti 3 O 7 @RF@Ag heterostructure, indicating the successful synthesis of this heterostructure through a two-step wet chemical process on the Na 2 Ti 3 O 7 nanowire.X-ray photoelectron spectroscopy (XPS) is a powerful tool for investigating the chemical states of elements in Na2Ti3O7@RF@Ag heterostructures (0.15 mL AgNO3 and 3 mL Lascorbic acid volumes), providing valuable insights into surface chemistry.The XPS survey spectrum (Figure 6a) shows the existence of Na, Ti, C, O, N, and Ag in the Na2Ti3O7@RF@Ag heterostructures.The high-resolution (HR) XPS spectrum of Na 1s (Figure 6b) exhibits a peak at 1071.4 eV, confirming the valence state of Na as +1 [52,53].The presence of Ti 4+ species is confirmed by Ti 2p doublets with binding energies of 458.5 eV and 464.6 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively, as shown in Figure 6c.Two additional minor peaks were detected at 459.1 and 463.0 eV, consistent with the presence of the Ti-C bond, confirming the robust adhesion of the RF layer to the surface of the Na2Ti3O7 [54].The C 1s spectrum of the RF layer (Figure 6d) is deconvoluted, revealing four distinct peaks centered at 284.2, 285.0, 285.7, and 286.1 eV which can be attributed to C-C, Ti-C, C-O, and C=O bonds, respectively [55,56].The O 1s spectrum of the RF layer (Figure 6e) can be successfully deconvoluted with three discernible peaks at 530.6, 532.5, and 532.9 eV which are assigned to the C=O, Ti-O, and C-O groups, respectively [56,57].The N 1s spectrum of the RF layer (Figure 6f) can also successfully deconvolute into two distinct peaks at 398.4 eV and 401.2.6 eV, corresponding to C-NH-C and C=N-C, respectively [58].The HRXPS spectrum of Ag 3d (Figure 6g) can be observed at binding energies of 367.8 and 373.8 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively.The 6 kV binding energy difference between these peaks confirms the presence of an Ag 0 state [59].X-ray photoelectron spectroscopy (XPS) is a powerful tool for investigating the chemical states of elements in Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL AgNO 3 and 3 mL L-ascorbic acid volumes), providing valuable insights into surface chemistry.The XPS survey spectrum (Figure 6a) shows the existence of Na, Ti, C, O, N, and Ag in the Na 2 Ti 3 O 7 @RF@Ag heterostructures.The high-resolution (HR) XPS spectrum of Na 1s (Figure 6b) exhibits a peak at 1071.4 eV, confirming the valence state of Na as +1 [52,53].The presence of Ti 4+ species is confirmed by Ti 2p doublets with binding energies of 458.5 eV and 464.6 eV, corresponding to Ti 2p 3/2 and Ti 2p 1/2 , respectively, as shown in Figure 6c.Two additional minor peaks were detected at 459.1 and 463.0 eV, consistent with the presence of the Ti-C bond, confirming the robust adhesion of the RF layer to the surface of the Na 2 Ti 3 O 7 [54].The C 1s spectrum of the RF layer (Figure 6d) is deconvoluted, revealing four distinct peaks centered at 284.2, 285.0, 285.7, and 286.1 eV which can be attributed to C-C, Ti-C, C-O, and C=O bonds, respectively [55,56].The O 1s spectrum of the RF layer (Figure 6e) can be successfully deconvoluted with three discernible peaks at 530.6, 532.5, and 532.9 eV which are assigned to the C=O, Ti-O, and C-O groups, respectively [56,57].The N 1s spectrum of the RF layer (Figure 6f) can also successfully deconvolute into two distinct peaks at 398.4 eV and 401.2.6 eV, corresponding to C-NH-C and C=N-C, respectively [58].The HRXPS spectrum of Ag 3d (Figure 6g) can be observed at binding energies of 367.8 and 373.8 eV, corresponding to Ag 3d 5/2 and Ag 3d 3/2 , respectively.The 6 kV binding energy difference between these peaks confirms the presence of an Ag 0 state [59].
SERS activity is intricately linked to the morphology, size, and density of Au and Ag nanoparticles, which collectively govern the efficiency and performance of SERS measurements [60,61].This investigation assessed the SERS performance of the Na 2 Ti 3 O 7 @RF@Ag heterostructures synthesized under various reaction conditions, using RhB as the target molecule.Figure 7a shows the SERS spectra of Na 2 Ti 3 O 7 @RF@Ag heterostructures grown via different Ag-nanoparticle-deposition methods, such as the wet chemical method (only 0.15 mL of AgNO 3 ; 0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid) and ion-sputtering method (Ag-ion-sputtering time of 90 s), which were immersed in the RhB solution (10 −6 M) for 1h.The primary vibrational modes for the characteristic peaks of the RhB molecule are aromatic C-C stretching (1076 cm −1 ), C-H in-plane bending (1197 cm −1 ), C-H bending (1279 cm −1 ), the stretching vibration of bridge C-C aromatic bonds (1360 cm −1 ), aromatic C-C bending (1508 cm −1 ), C-H stretching (1527 cm −1 ), aromatic C-C bending, and C=C stretching vibration (1647 cm −1 ), respectively [62,63].This study chose a Raman peak at 1647 cm −1 as the reference point to assess SERS activity because it does not readily overlap with the SERS substrate or the solvent.The Raman intensity of RhB is significantly boosted in the presence of Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid) compared to the Na 2 Ti 3 O 7 @RF@Ag heterostructures (only 0.15 mL of AgNO 3 ) and the Na 2 Ti 3 O 7 @RF@Ag heterostructure (Ag-ion-sputtering time of 90 s).The efficacy of the Na 2 Ti 3 O 7 @RF@Ag heterostructures for RhB detection was assessed using the SERS enhancement factor (EF), determined by the formula EF = I SERS × C 0 /I 0 × C SERS .Here, I SERS and I 0 represent the SERS (with Na 2 Ti 3 O 7 @RF@Ag heterostructures) and standard Raman intensities of RhB, while C SERS and C 0 denote the RhB concentration on the Na 2 Ti 3 O 7 @RF@Ag heterostructures (10 −6 M) and bare Ti foil (10 −4 M) [64,65].The calculated maximum EF values for the RhB Raman band at 1647 cm −1 are 5.00 × 10 6 (0.15 mL AgNO 3 ), 1.09 × 10 7 (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid), and 2.88 × 10 6 (Agion-sputtering time of 90 s), respectively.The Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL AgNO 3 and 3 mL L-ascorbic acid) are 2.18 and 3.78 times higher than the Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 ) and Na 2 Ti 3 O 7 @RF@Ag heterostructures (Ag-ionsputtering time of 90 s), respectively.SERS activity is intricately linked to the morphology, size, and density of Au and Ag nanoparticles, which collectively govern the efficiency and performance of SERS measurements [60,61].This investigation assessed the SERS performance of the Na2Ti3O7@RF@Ag heterostructures synthesized under various reaction conditions, using RhB as the target molecule.Figure 7a shows the SERS spectra of Na2Ti3O7@RF@Ag heterostructures grown via different Ag-nanoparticle-deposition methods, such as the wet chemical method (only 0.15 mL of AgNO3; 0.15 mL of AgNO3 and 3 mL of L-ascorbic acid) and ion-sputtering method (Ag-ion-sputtering time of 90 s), which were immersed in the RhB solution (10 −6 M) for 1h.The primary vibrational modes for the characteristic peaks of the RhB molecule are aromatic C-C stretching (1076 cm −1 ), C-H in-plane bending (1197 cm −1 ), C-H bending (1279 cm −1 ), the stretching vibration of bridge C-C aromatic bonds (1360 cm −1 ), aromatic C-C bending (1508 cm −1 ), C-H stretching (1527 cm −1 ), aromatic C-C bending, and C=C stretching vibration (1647 cm −1 ), respectively [62,63].This study chose a Raman peak at Figure 7b shows the SERS intensity of an RhB solution (10 −6 M) at 1647 cm −1 obtained from Na 2 Ti 3 O 7 @RF@Ag heterostructures grown using different volumes of AgNO 3 .The most prominent Raman intensity is observed at 0.15 mL of AgNO 3 .As the volume of AgNO 3 increases, there is a gradual decrease in the Raman intensity.As the SERS enhancement effect remained relatively low, this research introduced different volumes of L-ascorbic acid to expedite the formation and deposition of Ag nanoparticles, as shown in Figure 7c.The most prominent Raman signal is observed for 3 mL of L-ascorbic acid.This trend may be attributed to the more effective distribution of hot spots achieved with Molecules 2024, 29, 218 8 of 16 3 mL of L-ascorbic acid, significantly enhancing Raman signal intensity.This result is consistent with previous FETEM observations.To confirm the effectiveness of the Ag nanoparticle preparation method employed in the wet chemical method compared to the ion-sputtering method, this study used the ion-sputtering method to deposit Ag nanoparticles onto Na 2 Ti 3 O 7 @RF heterostructures at different times.Figure 7d reveals the SERS intensity of an RhB solution (10 −6 M) at 1647 cm −1 obtained from Na 2 Ti 3 O 7 @RF@Ag heterostructures grown using different Ag-ion-sputtering times.Hence, it can be determined that the most significant SERS enhancement effect was achieved with an ion-sputtering time of 90 s.This result is much lower than for Na 2 Ti 3 O 7 @RF@Ag heterostructures prepared via wet chemical methods (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid).In order to understand the differences, Na 2 Ti 3 O 7 @RF@Ag heterostructures (Ag-ion-sputtering time of 90 s) were further analyzed through FETEM.Figure 7e displays a FETEM image that depicts a size range of Ag nanoparticles, varying from approximately 3 to 21 nm.While the Ag nanoparticles are thoroughly dispersed within the Na 2 Ti 3 O 7 @RF heterostructure, their size distribution appears irregular.This non-uniformity in size has consequences for the hot spot distribution on the heterostructure's surface, reducing its SERS enhancement effect [24,66].Hence, the simplicity and remarkable efficiency of this study's Ag nanoparticle preparation method can be further validated.
Figure 7b shows the SERS intensity of an RhB solution (10 −6 M) at 1647 cm −1 obtained from Na2Ti3O7@RF@Ag heterostructures grown using different volumes of AgNO3.The most prominent Raman intensity is observed at 0.15 mL of AgNO3.As the volume of AgNO3 increases, there is a gradual decrease in the Raman intensity.As the SERS enhancement effect remained relatively low, this research introduced different volumes of L-ascorbic acid to expedite the formation and deposition of Ag nanoparticles, as shown in Figure 7c.The most prominent Raman signal is observed for 3 mL of L-ascorbic acid.This trend may be attributed to the more effective distribution of hot spots achieved with 3 mL of Lascorbic acid, significantly enhancing Raman signal intensity.This result is consistent with previous FETEM observations.To confirm the effectiveness of the Ag nanoparticle preparation method employed in the wet chemical method compared to the ion-sputtering method, this study used the ion-sputtering method to deposit Ag nanoparticles onto Na2Ti3O7@RF heterostructures at different times.Figure 7d reveals the SERS intensity of an RhB solution (10 −6 M) at 1647 cm −1 obtained from Na2Ti3O7@RF@Ag heterostructures grown using different Ag-ion-sputtering times.Hence, it can be determined that the most significant SERS enhancement effect was achieved with an ion-sputtering time of 90 s.This result is much lower than for Na2Ti3O7@RF@Ag heterostructures prepared via wet chemical methods (0.15 mL of AgNO3 and 3 mL of L-ascorbic acid).In order to understand the differences, Na2Ti3O7@RF@Ag heterostructures (Ag-ion-sputtering time of 90 s) were further analyzed through FETEM.Figure 7e displays a FETEM image that depicts a size The uniformity and reusability of SERS substrates are crucial factors in real-world SERS applications [25,47].In order to assess the uniformity of the Na 2 Ti 3 O 7 @RF@Ag heterostructures, ten random spots were chosen on the substrate loaded with RhB solution (10 −6 M) for an SERS analysis.Figure 8a shows the SERS spectra of an RhB solution (10 −6 M) acquired from these ten positions on the substrate.The Raman signals' peak positions and intensities exhibit remarkable uniformity across various locations on the the Na 2 Ti 3 O 7 @RF@Ag heterostructures.This outcome further substantiates the effective enhancement of Raman signals achieved by the Na 2 Ti 3 O 7 @RF@Ag heterostructures.The reusability of the Na 2 Ti 3 O 7 @RF@Ag heterostructures is a pivotal concern in this study.In order to address this issue, RhB molecules were systematically eliminated from the Na 2 Ti 3 O 7 @RF@Ag heterostructures using photocatalytic degradation facilitated by exposure to UV light for 1h. Figure 8b reveals the Raman spectra of Na 2 Ti 3 O 7 @RF@Ag heterostructures initially immersed in an RhB solution with a 10 −7 M concentration for 1 h.These spectra are presented before and after five consecutive UV light exposure cycles.These observations emphasize the remarkable reusability of the Na 2 Ti 3 O 7 @RF@Ag heterostructures, which consistently retain a comparable Raman signal strength even after undergoing five repeated cycles.This result reconfirms their enduring efficacy and suitability for practical applications.In addition, this occurrence can be attributed to the unique structural configuration of the Na 2 Ti 3 O 7 @RF@Ag heterostructures, which provide an augmented quantity of hot spots and surface-active sites.As a result, Na 2 Ti 3 O 7 @RF@Ag heterostructures exhibit remarkable SERS enhancement, excellent reproducibility, and sustained stability over an extended period.
usability of the Na2Ti3O7@RF@Ag heterostructures is a pivotal concern in this study.In order to address this issue, RhB molecules were systematically eliminated from the Na2Ti3O7@RF@Ag heterostructures using photocatalytic degradation facilitated by exposure to UV light for 1h. Figure 8b reveals the Raman spectra of Na2Ti3O7@RF@Ag heterostructures initially immersed in an RhB solution with a 10 −7 M concentration for 1 h.These spectra are presented before and after five consecutive UV light exposure cycles.These observations emphasize the remarkable reusability of the Na2Ti3O7@RF@Ag heterostructures, which consistently retain a comparable Raman signal strength even after undergoing five repeated cycles.This result reconfirms their enduring efficacy and suitability for practical applications.In addition, this occurrence can be attributed to the unique structural configuration of the Na2Ti3O7@RF@Ag heterostructures, which provide an augmented quantity of hot spots and surface-active sites.As a result, Na2Ti3O7@RF@Ag heterostructures exhibit remarkable SERS enhancement, excellent reproducibility, and sustained stability over an extended period.The SERS spectra were acquired for freshly synthesized SERS substrates after their immersion in different concentrations of RhB solution for 1 h, followed by air-drying.This assessment aimed to determine the low detection limit of the Na2Ti3O7@RF@Ag heterostructures for RhB solutions.Figure 9a displays the SERS intensities of the Na2Ti3O7@RF@Ag heterostructures at 1647 cm −1 over a concentration range from 10 −6 M to 10 −10 M of RhB solution.As the RhB concentration decreases, the SERS signal intensities at 1647 cm −1 gradually diminish.When the RhB solution concentration decreases below 10 −10 M, the SERS signal at 1647 cm −1 becomes challenging to effectively detect for Na2Ti3O7@RF@Ag heterostructures.Nevertheless, even at a concentration as low as 10 −10 M, the SERS signal at 1647 cm −1 can still be reliably detected using Na2Ti3O7@RF@Ag heterostructures.To ascertain the versatility of the Na2Ti3O7@RF@Ag heterostructure across various organic dyes, this study investigated the SERS effect of different methylene blue (MB) concentrations.Figure 9b displays the SERS intensities of Na2Ti3O7@RF@Ag heterostructures at 1623 cm −1 over a concentration range from 10 −6 M to 10 −10 M of MB solution.As the concentration of the MB solution drops below 10 −10 M, effectively detecting the SERS signal at 1623 cm −1 becomes challenging for Na2Ti3O7@RF@Ag heterostructures.The SERS spectra were acquired for freshly synthesized SERS substrates after their immersion in different concentrations of RhB solution for 1 h, followed by air-drying.This assessment aimed to determine the low detection limit of the Na 2 Ti 3 O 7 @RF@Ag heterostructures for RhB solutions.Figure 9a displays the SERS intensities of the Na 2 Ti 3 O 7 @RF@Ag heterostructures at 1647 cm −1 over a concentration range from 10 −6 M to 10 −10 M of RhB solution.As the RhB concentration decreases, the SERS signal intensities at 1647 cm −1 gradually diminish.When the RhB solution concentration decreases below 10 −10 M, the SERS signal at 1647 cm −1 becomes challenging to effectively detect for Na 2 Ti 3 O 7 @RF@Ag heterostructures.Nevertheless, even at a concentration as low as 10 −10 M, the SERS signal at 1647 cm −1 can still be reliably detected using Na 2 Ti 3 O 7 @RF@Ag heterostructures.To ascertain the versatility of the Na 2 Ti 3 O 7 @RF@Ag heterostructure across various organic dyes, this study investigated the SERS effect of different methylene blue (MB) concentrations.Figure 9b displays the SERS intensities of Na 2 Ti 3 O 7 @RF@Ag heterostructures at 1623 cm −1 over a concentration range from 10 −6 M to 10 −10 M of MB solution.As the concentration of the MB solution drops below 10 −10 M, effectively detecting the SERS signal at 1623 cm −1 becomes challenging for Na 2 Ti 3 O 7 @RF@Ag heterostructures.
In order to evaluate the temporal stability of the Na 2 Ti 3 O 7 @RF@Ag heterostructures, RhB and MB solutions, each with a concentration of 10 −6 M, were analyzed after preparing the SERS substrate and storing it in the dark for different durations of 1, 8, 15, 22, and 29 days.Figure 9c,d present the SERS intensities of Na 2 Ti 3 O 7 @RF@Ag heterostructures at 1647 cm −1 (RhB) and 1623 cm −1 (MB) for these different detection times, respectively.Remarkably, even after 29 days of storage, the intensities of the SERS signal at 1647 cm −1 (RhB) and 1623 cm −1 (MB) retain substantial levels of 60.4% and 48.9%, respectively.This result can verify that the Na 2 Ti 3 O 7 @RF@Ag heterostructures exhibit long-term stability.
The primary absorption peak at 552 nm for RhB gradually diminishes as the duration of UV light irradiation increases [67,68].Notably, there is no shift in the primary peak of RhB, indicating that the process primarily involves decomposing the benzene/heterocyclic rings [69].In order to assess photocatalytic activity, a plot of (C/C 0 ) versus time is presented in Figure 10a.The photocatalytic efficiency values of various samples, including Na 2 Ti 3 O 7 nanowires, Na 2 Ti 3 O 7 @RF heterostructures, Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid), and Na 2 Ti 3 O 7 @RF@Ag heterostructures (Ag-ion-sputtering of 90 s), are 76.3, 83.0, 86.5, and 79.7%, respectively.The Langmuir-Hinshelwood model is widely employed to assess the photodegradation kinetics of organic dyes in aqueous solutions.The reaction-rate constants (k) for different samples, including Na 2 Ti 3 O 7 nanowires, Na 2 Ti 3 O 7 @RF heterostructures, Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL AgNO 3 and 3 mL L-ascorbic acid), and Na 2 Ti 3 O 7 @RF@Ag heterostructures (Agion-sputtering of 90 s), are determined to be 0.0079, 0.00966, 0.01112, and 0.00846 min −1 , respectively, as depicted in Figure 10b.These findings suggest that the Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid) exhibited approximately 1.4 and 1.3 times greater efficiency compared to Na 2 Ti 3 O 7 nanowires and Na 2 Ti 3 O 7 @RF@Ag heterostructures (Ag-ion-sputtering of 90 s).The Na 2 Ti 3 O 7 @RF@Ag heterostructures reveal exceptional efficiency as reusable photocatalysts for the photodecomposition of RhB solutions under UV light irradiation.This outcome can be ascribed to the heightened suppression of electron-hole pair recombination and the increased availability of surfaceactive sites.In order to evaluate the temporal stability of the Na2Ti3O7@RF@Ag heterostructures, RhB and MB solutions, each with a concentration of 10 −6 M, were analyzed after preparing the SERS substrate and storing it in the dark for different durations of 1, 8, 15, 22, and 29 days.Figure 9c,d present the SERS intensities of Na2Ti3O7@RF@Ag heterostructures at 1647 cm −1 (RhB) and 1623 cm −1 (MB) for these different detection times, respectively.Remarkably, even after 29 days of storage, the intensities of the SERS signal at 1647 cm −1 (RhB) and 1623 cm −1 (MB) retain substantial levels of 60.4% and 48.9%, respectively.This result can verify that the Na2Ti3O7@RF@Ag heterostructures exhibit long-term stability.
The primary absorption peak at 552 nm for RhB gradually diminishes as the duration of UV light irradiation increases [67,68].Notably, there is no shift in the primary peak of RhB, indicating that the process primarily involves decomposing the benzene/heterocyclic rings [69].In order to assess photocatalytic activity, a plot of (C/C0) versus time is presented in Figure 10a.The photocatalytic efficiency values of various samples, including Na2Ti3O7 nanowires, Na2Ti3O7@RF heterostructures, Na2Ti3O7@RF@Ag heterostructures (0.15 mL of AgNO3 and 3 mL of L-ascorbic acid), and Na2Ti3O7@RF@Ag heterostructures The reusability of photocatalysts is of the utmost importance as it directly impacts their effectiveness in photocatalytic processes.In this study, an RhB solution with Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid) was subjected to UV light irradiation for 3 h.Subsequently, the substrate was thoroughly rinsed twice with copious amounts of de-ionized water in preparation for the next cycle of photodegradation.Over four cycles, the photocatalytic efficiency values of the Na 2 Ti 3 O 7 @RF@Ag heterostructures on Ti foil were found to be 80.3, 80.4, 77.4, and 78.3, respectively, as depicted in Figure 11a.The results reveal that the Na 2 Ti 3 O 7 @RF@Ag heterostructures exhibit superior reusability for decomposing the RhB solution.Furthermore, an XRD analysis of Na 2 Ti 3 O 7 @RF@Ag heterostructures (0.15 mL of AgNO 3 and 3 mL of L-ascorbic acid) after undergoing four cycles of recycling (Figure 11b) indicate no significant crystalline alterations, substantiating the enduring nature of the heterojunction.This analysis underscores the exceptional and sustained photocatalytic degradation efficiency of Na 2 Ti 3 O 7 @RF@Ag heterostructures.
Molecules 2024, 29, x FOR PEER REVIEW 11 of 16 Na2Ti3O7@RF@Ag heterostructures reveal exceptional efficiency as reusable photocatalysts for the photodecomposition of RhB solutions under UV light irradiation.This outcome can be ascribed to the heightened suppression of electron-hole pair recombination and the increased availability of surface-active sites.The reusability of photocatalysts is of the utmost importance as it directly impacts their effectiveness in photocatalytic processes.In this study, an RhB solution with Na2Ti3O7@RF@Ag heterostructures (0.15 mL of AgNO3 and 3 mL of L-ascorbic acid) was subjected to UV light irradiation for 3 h.Subsequently, the substrate was thoroughly rinsed twice with copious amounts of de-ionized water in preparation for the next cycle of photodegradation.Over four cycles, the photocatalytic efficiency values of the Na2Ti3O7@RF@Ag heterostructures on Ti foil were found to be 80.3, 80.4, 77.4, and 78.3, respectively, as depicted in Figure 11a.The results reveal that the Na2Ti3O7@RF@Ag heterostructures exhibit superior reusability for decomposing the RhB solution.Furthermore, an XRD analysis of Na2Ti3O7@RF@Ag heterostructures (0.15 mL of AgNO3 and 3 mL of Lascorbic acid) after undergoing four cycles of recycling (Figure 11b) indicate no significant crystalline alterations, substantiating the enduring nature of the heterojunction.This analysis underscores the exceptional and sustained photocatalytic degradation efficiency of Na2Ti3O7@RF@Ag heterostructures.The reusability of photocatalysts is of the utmost importance as it directly impacts their effectiveness in photocatalytic processes.In this study, an RhB solution with Na2Ti3O7@RF@Ag heterostructures (0.15 mL of AgNO3 and 3 mL of L-ascorbic acid) was subjected to UV light irradiation for 3 h.Subsequently, the substrate was thoroughly rinsed twice with copious amounts of de-ionized water in preparation for the next cycle of photodegradation.Over four cycles, the photocatalytic efficiency values of the Na2Ti3O7@RF@Ag heterostructures on Ti foil were found to be 80.3, 80.4, 77.4, and 78.3, respectively, as depicted in Figure 11a.The results reveal that the Na2Ti3O7@RF@Ag heterostructures exhibit superior reusability for decomposing the RhB solution.Furthermore, an XRD analysis of Na2Ti3O7@RF@Ag heterostructures (0.15 mL of AgNO3 and 3 mL of Lascorbic acid) after undergoing four cycles of recycling (Figure 11b) indicate no significant crystalline alterations, substantiating the enduring nature of the heterojunction.This analysis underscores the exceptional and sustained photocatalytic degradation efficiency of Na2Ti3O7@RF@Ag heterostructures.

Syntheses of Na 2 Ti 3 O 7 Nanowires
A 0.25mm thick titanium (Ti) foil was cut to the desired dimensions 0.5 cm × 0.5 cm or 1.5 cm × 2.5 cm.The substrate underwent ultrasonic treatment in ethanol and 1.0 M HCl for 10 min each, effectively removing organic contaminants and oxide layers.After each treatment, it was thoroughly rinsed with ethanol and dried using an air purge.Subsequently, the cleaned substrate was placed in separate 50 mL Teflon-lined stainless-steel autoclaves containing a 0.375 M NaOH solution (20 mL) and heated to 220 • C for 5 h.Finally, the substrate was washed with de-ionized water and ethanol and dried via air purge.

Syntheses of Na 2 Ti 3 O 7 @RF@Ag Heterostructures
The substrates featuring Na 2 Ti 3 O 7 nanowires underwent a surface hydrophilicity enhancement process by immersing them in a 50 mL ethanol solution containing 5 mM APTMS for 6 h at room temperature.Subsequently, the substrates were thoroughly rinsed with ethanol and de-ionized water and dried using an air purge.For the growth of the RF layer on the Na 2 Ti 3 O 7 nanowires, the substrates were placed in a 40 mL aqueous solution containing 16.37 mg of resorcinol, 0.0284 mL of NH 4 OH, and 0.0444 mL of CH 2 O.This mixture was vigorously stirred for 6 h at 50 • C. Next, the substrate was rinsed with ethanol thrice and dried at 60 • C for 2 h.To deposit Ag nanoparticles on the Na 2 Ti 3 O 7 @RF heterostructures, the substrate was immersed in a 50 mL aqueous solution with different volumes of AgNO 3 (5 mM) and L-ascorbic acid (50 mM) under vigorous stirring for 1 h.Finally, the resulting substrate underwent ethanol rinses and was dried at 60 • C for 2 h.

Characterization
The as-prepared SERS substrates underwent a comprehensive analysis to investigate their microstructures and elemental composition.Field-emission scanning electron microscopy (FESEM) was employed using a Hitachi S-4800 instrument (Tokyo, Japan).Field-emission transmission electron microscopy (FETEM) was also utilized with a JEOL JSM-2100F apparatus (Tokyo, Japan) equipped with energy-dispersive X-ray spectroscopy (EDS).X-ray diffraction (XRD) was performed using a Bruker (Billerica, MA, USA) D8 SSS instrument based in the United States to characterize the crystal structures of the as-prepared substrates.The chemical states of the elements within the Na 2 Ti 3 O 7 @RF@Ag heterostructures were determined through X-ray photoelectron spectroscopy (XPS), conducted using a ULVAC-PHI PHI 5000 VersaProbe instrument (Chigasaki, Japan).Furthermore, SERS measurements were carried out using a micro-Raman identify spectrometer (MRI532S) provided by Protrustech in Taiwan.These measurements utilized an excitation wavelength of 532 nm, with a laser beam diameter of 3.3 mm and a resolution of 2.2 cm −1 .

SERS and Photocatalytic Measurement
For SERS measurement, a micro-Raman identify spectrometer with 532 nm excitation wavelength employed 1 mW laser power and 0.15 s detector integration time.The SERS properties of the Na 2 Ti 3 O 7 @RF@Ag heterostructures were evaluated, immersing substrates in RhB solutions and drying them for 1 h at room temperature in the dark.For photocatalytic measurements, photocatalyst activity was assessed by degrading an RhB solution (0.08 mM) without pH adjustment under UV light (253.7 nm, 10 W, Philips, Amsterdam, The Netherlands) irradiation.Concentrations were measured with a DR/UV-Vis spectrometer (Hitachi U-2900, Tokyo, Japan), and efficiency was calculated as C/C 0 , where C 0 and C represent initial and final RhB concentrations.

Figure 1 .
Figure 1.Synthetic scheme of the preparation of the Na 2 Ti 3 O 7 @RF@Ag heterostructures.

Figure 10 .
Figure 10.(a) Photocatalytic activities and (b) kinetic linear simulation curves of as-prepared photocatalysts under UV light irradiation.

Figure 11 .
Figure 11.(a) The reusability of the Na2Ti3O7@RF@Ag heterostructures under UV light irradiation.(b) XRD spectra of Na2Ti3O7@RF@Ag heterostructures before cycling and after the fourth cycle.

Figure 10 .
Figure 10.(a) Photocatalytic activities and (b) kinetic linear simulation curves of as-prepared photocatalysts under UV light irradiation.

Figure 10 .
Figure 10.(a) Photocatalytic activities and (b) kinetic linear simulation curves of as-prepared photocatalysts under UV light irradiation.

Figure 11 .
Figure 11.(a)The reusability of the Na2Ti3O7@RF@Ag heterostructures under UV light irradiation.(b) XRD spectra of Na2Ti3O7@RF@Ag heterostructures before cycling and after the fourth cycle.

Figure 11 .
Figure 11.(a) The reusability of the Na 2 Ti 3 O 7 @RF@Ag heterostructures under UV light irradiation.(b) XRD spectra of Na 2 Ti 3 O 7 @RF@Ag heterostructures before cycling and after the fourth cycle.