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Article

Microemulsion-Based Synthesis of Highly Efficient Ag-Doped Fibrous SiO2-TiO2 Photoanodes for Photoelectrochemical Water Splitting

by
Samia Arain
1,
Muhammad Usman
2,
Faiq Saeed
3,
Shouzhong Feng
4,
Waheed Rehman
5,
Xianhua Liu
5,* and
Haitao Dai
1,*
1
Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin 300072, China
2
Center of Excellence on Catalysis and Catalytic Reaction Engineering, Department Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand
3
Tianjin Key Laboratory of Molecular Optoelectronic Science, Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China
4
Anhui Zhongyi New Material Science and Technology Co., Ltd., Chuzhou 239599, China
5
School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(1), 66; https://doi.org/10.3390/catal15010066
Submission received: 3 December 2024 / Revised: 2 January 2025 / Accepted: 5 January 2025 / Published: 13 January 2025
(This article belongs to the Special Issue Catalytic Properties of Hybrid Catalysts)
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Abstract

:
Fibrous SiO2-TiO2 (FST) is one of the most promising materials for advancing photoelectrochemical water-splitting technology due to its cost-effectiveness and environmental friendliness. However, FST faces intrinsic limitations, including its low conductivity and wide bandgap. In this study, significant progress was made in modifying FST to overcome some of these limitations. This work involved synthesizing a new photoanode made of Ag-doped FST utilizing the microemulsion process. The Ag-doped FST was characterized using XRD, FTIR, UV–Vis, DRS, N2 adsorption–desorption, FESEM, TEM, and XPS. The results confirmed the formation of a continuous concentric lamellar structure with a large surface area. The addition of Ag species into the FST matrix caused interactions that reduced the bandgap. The Ag-doped FST photoanode exhibited an impressive photocurrent density of 13.98 mA/cm2 at 1.2 V (vs. RHE). This photocurrent density was notably higher than that of FST photoanodes, which was 11.65 mA/cm2 at 1.2 V (vs. RHE). Furthermore, the conduction band of Ag-doped FST is positioned closer to the reduction potential of hydrogen compared to that of FST, SiO2, and TiO2, facilitating rapid charge transfer and enabling the spontaneous generation of H2. The fabrication of Ag-doped FST provides valuable insights into the development of high-performance photoanodes for PEC water splitting.

Graphical Abstract

1. Introduction

The escalating pace of global climate change has led to heightened apprehension regarding ecological imbalances, greenhouse gas emissions, and the elevation of sea levels. These worries have been amplified by the growth in population and economic development [1,2,3], which underscore the urgent need for renewable, carbon-neutral, and environmentally friendly energy sources [4,5]. Photoelectrochemical (PEC) water splitting has emerged as a promising technique for utilizing solar energy to produce H2 (a clean and renewable fuel source) from water. The efficiency of H2 generation relies heavily on the performance of the semiconductor photoanodes, which play a critical role in separating and transporting the electron–hole pairs created by light, improving the overall efficiency [6,7,8,9].
Different semiconductor materials such as titania (TiO2), hematite (α-Fe2O3), bismuth vanadate (BiVO4), and graphitic carbon nitride (g-C3N4) have been explored for PEC water splitting [10,11,12,13]. TiO2 is particularly notable among these materials because of its low cost, exceptional photoactivity, and ability to remain stable when exposed to light [14]. Specifically, mesoporous TiO2, with its large pore volume, environmental friendliness, and ordered porous structure, provides significant photocatalytic performance because of enhancing mass transfer and its large number of reactive surface sites [15,16]. Despite these advantages, TiO2 suffers from limitations such as a wide bandgap, a small surface area, and a rapid recombination rate of photogenerated holes (h+) and electrons (e−) [17]. Moreover, the band edge potential of TiO2 is not consistently optimal for H2 production, and the ineffective charge transfer at the electrolyte interface hampers the kinetics of the process. Thus, improving TiO2 photoanodes is critical for optimizing PEC water-splitting efficiency [18].
Different methods and approaches have been employed to enhance TiO2, including combining it with porous silica (SiO2), modifying its morphology, and introducing metal doping. Studies have shown that mesostructured silica with titania nanoparticles with large surface areas and small bandgaps can significantly improve processes such as dye photodegradation [19]. Similarly, incorporating SiO2 into zirconia also facilitated enhanced photodegradation [20]. TiO2 nanotubes doped with Si demonstrated superior PEC performance compared to undoped TiO2 due to better optical absorption and charge separation [21].
Several researchers have explored the use of nanoparticles like Pt, Pd, Au, and Ag to enhance the photocatalytic efficiency of semiconductor materials for photoelectrochemical water splitting. Among these nanoparticles, Ag has gained prominence due to its cost-effectiveness and greater stability [22]. Ag nanoparticles can exhibit localized surface plasmon resonance (LSPR), a property that enhances light absorption and generates hot electrons, which significantly boost the efficiency of visible light harvesting: an important factor in photoelectrochemical (PEC) water splitting [23]. Moreover, Ag doping introduces localized electronic states within the bandgap of the material, reducing the bandgap energy and enabling the more efficient absorption of visible light, thereby improving the material’s photocatalytic performance [24]. Additionally, Ag acts as an electron trap, preventing the recombination of photogenerated charge carriers, which enhances charge separation. Therefore, the synergistic effects between Ag and the fibrous silica-titania (FST) matrix optimize the material’s photocatalytic properties, with the porous silica structure improving light scattering and the TiO2 component contributing excellent photocatalytic behavior [25].
The conventional routes for integrating TiO2 with SiO2, such as chemical vapor deposition and sol–gel processes, have limitations due to different factors like the unwanted impurities, complex procedures, and high costs of the processes [26,27]. Recently, fibrous SiO2 has gained attention due to its unique features, such as discrete channels, broad pore widths, and exceptional thermal stability [28,29]. A fibrous SiO2-TiO2 (FST) material was developed, with a distinctive structure that effectively degraded pollutants like ibuprofen and 2-chlorophenol [30,31].
In the current work, Ag-doped FST synthesized through microemulsion was investigated as a photoanode for PEC water splitting. This is the first time Ag-doped FST has been employed as the photoanode for PEC water splitting. The physicochemical and electrical characteristics of Ag-doped FST were examined through XRD, N2 adsorption–desorption, FTIR, FESEM, TEM, Mott–Schottky, and EIS. Subsequently, a correlation between these characteristics and the performance of the photoelectrochemical system was established to enhance water-splitting efficiency.

2. Results and Discussions

2.1. Crystal Structure Analysis of FST and Ag-Doped FST Photoanodes

X-ray diffraction (XRD) patterns were obtained to analyze the phase composition and crystallinity of the FST and Ag-doped-FST photoanodes, which are presented in Figure 1. All samples exhibited anatase as the only crystalline phase, with no diffraction peaks corresponding to silica or silicates, indicating that the silica was in an amorphous state. The anatase phase’s high thermal stability, provided by silica’s presence, was confirmed by the absence of peaks corresponding to rutile, even after calcination at 700 °C. This stabilization was likely due to the high dispersion of TiO2 aggregates within the amorphous silica network. The diffraction peak at 2θ = 25.3° corresponds to the 101 planes of anatase TiO2 [32]. Consequently, distant peaks were observed for both photoanodes at the following angles: 25.4°, 36.9°, 37.8°, 38.5°, 48.2°, 53.9°, 55.3°, and 63.2°. These associated peaks were attributed to the crystallographic plane of (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), and (2 0 4), respectively, indicating the anatase phase of TiO2 [33]. A small diffraction peak was observed at 2θ = 23°, further indicating the presence of the amorphous SiO2 phase in the FST photoanode. This also confirmed the successful synthesis of FST [34,35]. Moreover, due to the interference with TiO2 framework by fibrous SiO2, it was noted that the TiO2 peak in FST widened and somewhat diminished.
The broad peak of FST indicated that it had a smaller crystallite size than TiO2. The broad peak at 23° was ascribed to the KCC-1 support, and the peaks at 2θ = 38.2°, 44.3°, 64.5° 77.6°, and 81.6° were indexed to the (1 1 0), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) lattice planes of metallic Ag (JCPDS card no. 4-783). There was no prominent Ti species peak due to Ti’s highly dispersed status [36,37]. Additionally, the diffraction peaks at 2θ = 38.2° and 51.2°, corresponding to the (004) and (200) planes of metallic Ag, respectively, indicated the presence of metallic silver.

2.2. High Surface Area and Pore Characteristics of Ag-Doped FST Through Nitrogen Adsorption

Figure 2 represents the nitrogen adsorption–desorption isotherm for Ag-doped FST, and it showed a high surface area, indicated by the steep initial rise in the adsorption isotherm at low relative pressures, suggesting a large number of adsorption sites, which corresponds to a high surface area. In addition, the initial increase in adsorption was typically due to monolayer adsorption on the surface of a material. At intermediate pressures (0.2 < P/P0 < 0.8), the adsorption increased more rapidly, suggesting multilayer adsorption and capillary condensation in the mesopores, while, at high relative pressures (P/P0 > 0.8), there was a steep increase in adsorption, often indicating the filling of macropores (pores with diameters > 50 nm) or external surface adsorption. The SBET of FST and Ag-doped FST were 338.2 m2g−1 and 233.4 m2g−1 respectively. In addition, Figure 2 shows that the Ag-doped FST had a cumulative pore volume of 0.6318 cm3g−1, which was 1.87 times greater than that of the bare FST (0.3363 cm3g−1) reported in the literature [38]. The enhanced pore volume of the Ag-doped FST proved that the AgNO3 treatment disrupted the FST matrix, leading to the generation of gaps, which in turn increased the porosity. This higher pore volume provided more active sites for the redox reactions at the interface, which facilitated the enhancement of photocatalytic activity. A larger pore volume can not only facilitate the diffusion of free radicals but also enhance the light absorption capability by producing more photogenerated charge carriers. The combination of micro- and mesopores leads to the effective charge diffusion of the photogenerated charge carriers, which can then be transferred and used effectively. Furthermore, the inset graph shows the pore size distribution derived from the adsorption data and shows that a pore size distribution of 10 nm was predominant in the sample. These properties suggest that Ag-doped FST has a well-defined mesoporous structure, making it suitable for applications such as catalysis, adsorption, and photocatalysis [39,40,41].

2.3. Functional Group Dynamics: FTIR Spectra of SiO2, TiO2, and Ag-Doped FST

Figure 3 depicts the transmission spectra of the Ag-doped-FST, FST, SiO2, and TiO2 obtained through FTIR analysis within the wavelength range of 400–1600 nm. The FTIR of SiO2 exhibits prominent characteristic peaks. Interestingly, there is an O–H stretching band at 1280 cm−1, attributed to the presence of –OH groups and free water. Moreover, the Si-O-Si asymmetric stretching band appears at 1077 cm−1. The external Si-OH group can be observed at 785 cm−1, while the Si-O-Si asymmetric stretching and bending modes appear at 453 cm−1.
Similar functional groups were observed for FST, indicating that SiO2 was intact with TiO2. The band of FST at 1270 cm−1 was observed, suggesting the preservation of SiO2’s characteristics. It is also clear from the spectra that TiO2 possessed a broad band in the range of 900 to 450 cm−1 [39], with specific peaks at 859 cm−1 and 643 cm−1, which could be attributed to the vibrational absorption of the Ti-O-Ti links. For Ag-doped FST, additional bands were observed at 1162 cm−1, 1060 cm−1, 797 cm−1, 775 cm−1, 657 cm−1, 515 cm−1, and 449 cm−1. These bands were attributed to the presence of the silver dopant, indicating interactions between the Ag and the FST framework. The presence of these new bands suggested modifications of the chemical structure, possibly due to the introduction of Ag, which could interact with the existing functional groups and alter the vibrational characteristics of the material. By comparing the spectra of Ag-doped FST, FST, TiO2, and SiO2, the interactions and modifications due to the doping as well as the integration of TiO2 and SiO2 were evident, showcasing the changes in the functional groups and their vibrational properties.

2.4. Chemical Oxidation State Analysis

XPS analysis was used to measure the chemical surface states and composition of the Ag-doped FST catalysts. Figure 4a presents the Ti peaks observed at 456.0 eV, 458.8 eV, 461.8 eV, and 464.6 eV, attributed to Ti3+ 2p3/2 and Ti4+ 2p3/2. The doublet peaks corresponded to the spin-orbit split components of Ti 2p. The peak positions were indicative of titanium in the Ti4+ oxidation state, likely in the form of TiO2. The blue shift change in the Ag-doped FST was attributed to the decrease in the electron density, confirming the successful doping of Ag in the FST. Figure 4b presents the Si peaks at 103.2 eV, 103.4 eV, 104.0 eV, and 104.3 eV. Si 2p peaks typically split into two components, 2p1/2 and 2p3/2, due to spin-orbit coupling, with a characteristic energy difference of approximately 0.8 eV. The peaks at 103.2 eV and 104.0 eV observed in our XPS analysis corresponded to the Si 2p3/2 and Si 2p1/2 states, indicative of silicon in the SiO2 environment. The additional peaks at 103.4 eV and 104.3 eV likely arose from slight variations in the chemical environment of silicon, such as interactions with the FST framework.
Figure 4c presents O 1s with peaks at 529.2 eV, 530.4 eV, 533.0 eV, and 533.1 eV, which correspond to different oxygen environments. The peak at 529.2–530.4 eV can be attributed to the lattice oxygen in metal oxides (e.g., TiO2, SiO2), while the peaks at 533.0–533.1 eV are characteristic of the oxygen in metal oxides, thus confirming the presence of TiO2 or SiO2 [42]. This red shift in the spectrum of O1s was attributed to the decrease in the electron density and the presence of hydroxyl groups in addition to adsorbed oxygen. Moreover, this also confirmed that the substantial doping of Ag led to the change in the electron density of the O1s spectrum.
Figure 4d, for Ag 3d (Ag-doped FST), represents two peaks at binding energies of 365.5 eV and 371.6 eV, corresponding to Ag 3d5/2 and Ag 3d3/2, respectively. These peaks indicate the presence of silver in the sample. The separation between the two peaks (~6.1 eV) is consistent with the spin-orbit splitting of Ag 3d. The intensity ratio and position confirmed the presence of metallic silver (Ag) within the sample. Therefore, it could be inferred from the spectra that the synthesized sample contained Ag, SiO2, and TiO2, with surface-adsorbed species.

2.5. Surface Morphology of Ag-Doped FST

The surface morphology of the photoanodes was observed through FESEM and TEM, with results shown in Figure 5a,b and Figure 5c,d respectively. Figure 5a,b illustrates the microstructure of the Ag- doped FST, depicting spherical particles with a rough surface texture, indicating the presence of Ag. It can also be observed from the micrographs that the particles were closely packed together to form a dense cluster. The structure was notably rough and porous, which potentially enhanced properties like catalytic activity or surface area. The particle size range of 250 to 280 nm highlighted the uniform size distribution, therefore depicting a well-controlled synthesis process. The TEM micrographs presented in Figure 5c,d offer a detailed overview of the morphology of the single particles of Ag-doped FST. The particle exhibits a branched or dendritic morphology, indicative of a high surface area [43]. Additionally, it shows the fine features and permeable structure, which may enhance its functional capabilities and prove advantageous for a wide range of applications. The rough texture of the particles could be ascribed to the presence of Ag within the sample. Figure 5e,f exhibits the HRTEM images with lattice fringes with interplanar spacings of 0.135 nm and 0.242 nm, which correspond to the (200) crystal plane of Ag species and the (101) of TiO2, respectively. Figure 5g indicates the polycrystalline nature of the photocatalyst and the very good distribution of Ag on the FST, as proved by the selected area electron diffraction (SAED) pattern.
The high-angle annular dark field image, elemental mapping image, and EDX spectrum of the Ag-doped FST are shown in Figure 5h,i and Figure S1, respectively. The HAADF image in Figure 5h provides an enhanced contrast of heavier elements, making the Ag doping sites more visible against the lighter matrix. Figure 5i shows the combined elemental mapping of an Ag-doped FST particle. The overlay of the distinct components shows the uniform and consistent distribution of the dopant and matrix elements. The elemental mapping depicted in Figure 5j illustrates the spatial distribution of silicon (Si) within the particle. The consistent pink coloring reflects the uniform dispersion of silicon, implying that silicon was a predominant constituent of the FST matrix. The elemental map of oxygen (O), shown in green in Figure 5k, illustrates the homogeneous distribution of oxygen throughout the structure. This suggests that oxygen was evenly integrated into the matrix, most likely as part of the SiO2 or TiO2. Figure 5l represents the elemental map for titanium (Ti) in red. The scattered red spots indicate the presence of titanium in smaller quantities, potentially as isolated dopants or part of a mixed oxide phase. Figure 5m displays the elemental map for silver (Ag), highlighted in yellow. The yellow dots signify the existence of silver, which seems to be evenly distributed throughout the particle. This dispersion of particles is essential for applications that need homogenous doping to achieve maximum performance [44,45].

2.6. UV–Visible Spectroscopy

UV–visible spectroscopy is the most widely used technique to investigate the optical properties of particles. Figure 6 illustrates the UV–Vis absorbance spectra of the Ag-doped FST, FST, SiO2, and TiO2. The UV–Vis absorbance spectra show that the Ag-doped FST exhibited the highest absorbance in the UV region (250–400 nm), driven by the localized surface plasmon resonance (LSPR) of the silver nanoparticles. The red curve represents undoped FST, which absorbs light in the UV range but with a lower intensity compared to Ag-doped FST. SiO2 (blue curve) showed relatively low absorbance, with a sharp decline after 300 nm, indicating that it primarily absorbed in the deep UV region. TiO2 (green curve) had slightly higher absorbance than SiO2; however, it was less intense than that of the Ag-doped FST. SiO2 (blue curve) showed relatively low absorbance, with a sharp decay after 300 nm, indicating that it primarily absorbed in the deep UV region. With a peak absorbance of about 350 nm, TiO2 (green curve) had a slightly greater absorbance than SiO2 but a lower absorbance than the FST materials. This, combined with the reduced bandgap energy, enhanced the visible light absorption and photocatalytic efficiency compared to that of FST, SiO2, and TiO2.

2.7. Optical Properties and Mott–Schottky Analysis of Ag-Doped FST

Figure 7a displays the Tauc plot used to determine the optical bandgap of the Ag-doped FST, FST, SiO2, and TiO2. The data clearly show that the absorption edge for the Ag-doped FST occurred at around 2.3 eV, suggesting a smaller bandgap compared to pristine FST (~2.8 eV), SiO2 (~3.2 eV), and TiO2 (~3.0 eV). The Ag ions in the FST matrix introduce localized electronic states within the band structure, narrowing the energy gap between the valence and conduction bands. This reduces the bandgap energy, as evidenced by the shift in the absorption edge and the Tauc plot analysis, enhancing visible light absorption. Moreover, Ag acts as an electron trap, capturing the photogenerated electrons and preventing their recombination with holes, which prolongs charge carrier lifetime and improves charge separation efficiency. Furthermore, silver nanoparticles exhibit localized surface plasmon resonance (LSPR), generating an electromagnetic field that enhances the excitation of charge carriers and facilitates their transfer across the semiconductor–electrolyte interface. These newly formed states enable electronic transitions to occur at lower energy levels, therefore expanding the range of wavelength that may be absorbed into the visible area. This feature is especially beneficial for photocatalytic applications since it enables the material to use a wider range of the solar spectrum. The enhanced light absorption capacities can augment the formation of electron–hole pairs, enhancing the material’s photocatalytic activity [46,47,48].
From the comparison of the bandgap energies of the Ag-doped FST with those of SiO2 and TiO2, it is clear that doping has a stronger impact on the FST matrix. SiO2 and TiO2, having bandgaps of roughly ~3.2 eV and ~3.0 eV, respectively, do not undergo significant changes in their original states. The large bandgap values of SiO2 and TiO2 suggest that they mostly absorb ultraviolet (UV) light, which restricts their effectiveness when exposed to visible light. On the other hand, Ag-doped FST has a smaller band gap, which allows it to use visible light effectively. This attribute renders it a superior option for applications such as visible-light-driven photocatalysis or photoelectrochemical water splitting [49]. Through the optimized integration of silver into the FST framework, the optical properties of the material were enhanced, providing improved performance for energy and environmental applications.
Figure 7b represents the Mott–Schottky plot, providing valuable insights into the semiconductor characteristics of the Ag-doped FST, FST, SiO2, and TiO2 photoanodes. The linear regions observed in the plots of inverse capacitance squared (1/C2) versus potential (V vs. RHE) are indicative of the formation of a depletion region, a hallmark of semiconductor behavior.
The slope of these linear regions is directly related to the doping density and carrier concentration within a semiconductor. It is evident from the plot that all four materials demonstrated semiconductor properties within a certain range of electrical potential. The addition of Ag species to the FST resulted in a more pronounced incline in the linear area, indicating a higher concentration of charge carriers or higher efficiency in separating charges. SiO2 and TiO2 had reduced gradients, suggesting decreased carrier densities. Therefore, it can be concluded that Ag-doped FST demonstrates exceptional optical and electrochemical characteristics, including a larger bandgap energy and increased carrier density. The observations emphasize the improved performance of Ag-doped FST as a result of doping, indicating its potential as a highly effective material for photocatalytic and electrochemical applications.

2.8. Boosted PEC Efficiency of Ag-Doped FST

The PEC performance of the Ag-doped FST surpassed that of FST, SiO2, and TiO2 due to its enhanced photocurrent density and lower charge transfer resistance. The comparison of the performance metrics of the Ag-doped FST and the other materials is summarized in Table 1.
As shown in Table 1, the Ag-doped FST exhibited superior PEC performance compared to FST and the other materials, with the lowest charge transfer resistance and the highest photocurrent density. This performances significantly surpass those of previously reported materials based on FST, TiO2 and silicon for PEC water splitting (Table S1). These enhancements are attributed to Ag doping, which improves charge separation and facilitates light absorption in the visible range. Figure 8a represents the current density (J) as a function of the applied potential (E) for the Ag-doped FST, FST, SiO2, and TiO2, providing insight into the performance of the PEC process. It was evident that the photoanodes showed a very minute current density in the dark, indicating the importance of light for electrocatalytic water splitting. The photocurrent densities of the FST photoanode were 2.49, 4.62, 7.96, and 11.65 mAcm−2 at 0.3, 0.6, 0.9 and 1.2 V (vs. RHE), respectively. Consequently, the current densities of the Ag-doped FST were 3.38, 6.0, 11.42, and 13.98 mAcm−2 at 0.3, 0.6, 0.9, and 1.2 V (vs. RHE), respectively. These results indicate that the photocurrent densities of the Ag-doped FST were 1.2-fold higher than the FST photoanode at 1.2 V (vs. RHE), which could be attributed to the doping of Ag, which acts as an electron trap and causes surface plasmonic effect to enhance the charge separation capacity of photogenerated electrons at the interfaces and increase the surface redox reactions. In addition, the Ag-doped FST showed the lowest onset potential, implying a lower energy barrier for charge transfer and potentially the better utilization of the solar spectrum [50].
The redox behavior of the Ag-doped FST, FST, SiO2, and TiO2 was evaluated using CV. Figure 8b presents the current density as a function of the applied potential for the forward and reverse scans. All materials exhibited the characteristic behavior of a CV curve, displaying hysteresis between the forward and backward scans [43,51]. Consequently, larger hysteresis lines were observed for the Ag-doped FST than for the FST and other materials. This is attributed to the better transfer and utilization of light-generated charge carriers, which surpasses the recombination of the charge carriers. Figure 8c demonstrates the applied bias photon to current efficiency (ABPE) measurements and the photocurrent generated under the different applied voltages. The ABPE values for the FST, SiO2, and TiO2 were 6.21%, 4.33%, and 3.81% at 0.5 V (vs. RHE), respectively. Ag-doped FST achieved the highest ABPE value of 7.37% at 0.5 V (vs. RHE), representing a 1.12-fold increase compared to the standalone FST photoanode. The incorporation of Ag nanoparticles into the FST photoanode enhanced light absorption due to localized surface plasmon resonance (LSPR) effects, therefore increasing the generation of photoexcited electrons and holes. These nanoparticles also facilitated better charge separation and reduced the recombination rates. Also, the fibrous structure of FST, combined with the conductive nature of Ag, improved charge transport, resulting in more efficient electron transfer processes. Moreover, the synergy between silica and titania in the FST matrix further enhanced the PEC performance since silica provided a porous structure to improve light scattering and adsorption, while titania offered excellent photocatalytic properties.

2.9. Enhanced Charge Transfer Efficiency in Ag-Doped FST: EIS Nyquist Plot Analysis

Figure 9 presents the Nyquist plots of Ag-doped FST, FST, SiO2, and TiO2. The Ag-doped FST exhibited the smallest semicircle among all materials, depicting superior charge transfer efficiency at higher frequencies including the electrolyte resistance (Rs) and transfer resistance (Rct) at the electrode/electrolyte interface, therefore leading to enhanced photocurrent generation. The measured Rs values of all the materials were similar. The calculated Rct values of FST, SiO2, and TiO2 were 1125 Ω, 622 Ω, and 1496 Ω, respectively. Moreover, the Ag-doped FST showed the smallest Rct value of 311 Ω, attributed to the higher ion diffusion rates. Ag doping enhances electronic and ionic processes by introducing localized surface plasmon resonance (LSPR), which generates hot electrons that improve conductivity and facilitate electron transfer. These results are in alignment with the high current densities obtained through the above characterizations and can be attributed to the Ag doping, which probably increases the conductivity of the material and improves electron mobility, thereby lowering the recombination of charge carriers [52,53,54]. The EIS analysis further confirms that the Ag-doped FST nanocomposite is a promising light-active photoelectrode material that exhibits efficient and improved photoelectrochemical water splitting.

2.10. PEC Stability

Another essential factor for the PEC performance is the long-term stability of the photoanode. Figure 10a depicts the i–t curves of the Ag-doped FST, FST, SiO2, and TiO2 after they underwent testing for 16 h at 1.23 V (vs. RHE) under continuous illumination. The LSV measurements are in good agreement with the photocurrent densities. The Ag-doped FST and FST exhibited higher and more stable photocurrent densities compared to SiO2 and TiO2, indicating that even after the addition of fibrous SiO2 and Ag doping, the FST retained favorable durability during the PEC water splitting process. This enhanced performance suggests that the interfacial interaction between silver, silica, and titania improves the charge transfer efficiency and stability of the photoanodes; therefore, it can be deduced that Ag-doped FST nanocomposite is a viable material for high-performance, light-active photoelectrodes in PEC water-splitting applications [55,56,57,58]. Moreover, Figure 10b presents the XRD results of the Ag-doped FST before and after the stability experiment, which confirms the presence of its crystalline structure after the 16 h long stability test. This shows the Ag-doped FST can be used as a reliable and reusable electrocatalyst for photo electrocatalytic water splitting.

2.11. Plausible Mechanism

Based on all the results described above, a reasonable mechanism is presented below for photoelectrochemical water splitting. Mott–Schottky plots (Figure 7b) were utilized to calculate the position of (CB) from the flat band potential values obtained. As illustrated in Figure 11, the conduction band (CB) potentials (vs. NHE) of the FST and Ag-doped FST were determined to be −0.023 V and 0.157 V, respectively, while their valence band (VB) potentials (vs. NHE) were measured at 2.77 V and 2.143 V, respectively. The photoanode absorbs solar radiation, causing the movement of electrons from the valence band (VB) to the conduction band (CB) in the Ag-doped FST material, generating an electron–hole pair in the photoanode. Electrons in the conduction band (CB) become excited and migrate toward the surface of the photoanode, whereas positively charged holes (h+) remain in the valence band (VB) of the photoanode. The excited electrons move from the photoanode to the photocathode. The presence of positively charged holes (h+) in the valence band (VB) of the Ag-doped FST photoanode facilitates the oxidation of water molecules, while the electrons reaching the photocathode reduce protons (H+) to form hydrogen gas [59,60]. The water-splitting reactions are as follows.
At photoanode: 2H2O → O2 + 4H+ + 4e−
At photocathode: 4H+ + 4e− → 2H2
Overall reaction: 2H2O → 2H2 + O2
In summary, doping the FST matrix with Ag modified the electronic structure and reduced the bandgap from 2.8 eV to 2.3 eV. Additionally, it promoted more efficient charge separation and minimized the recombination of electron–hole pairs. Due to the suitable band alignments and energy levels, the Ag-doped FST improved the efficiency of splitting water using PEC methods.

3. Materials and Methods

3.1. Materials

Commercial Titania (JRC TiO2; 99.5%) was purchased from Sigma-Aldrich (Saint. Louis, MO, USA); butanol (C4H10O; 99.5%), urea (CH4N2O; 99%), sodium sulfate (Na2SO4; 99.0%), and toluene (C6H5CH3; 99.8%) were supplied by Merck China (Shanghai, China); tetraethyl orthosilicate (TEOS; 98%) was obtained from Merck Schuchardt OHG (Hohenbrunn, Germany); cetyltrimethylammonium Bromide (CTAB; 98%) was procured from Fisher Chemical (Shanghai, China). Silver nitrate (AgNO3; 97%) powder was purchased from China.

3.2. Fabrication of Ag-Doped FST Photoanode

FST was synthesized using a microemulsion method and the detailed methodology is given in earlier work [30]. Initially, an even mixture of cetyltrimethylammonium bromide, distilled water, and urea was stirred vigorously at room temperature. Following that, butanol and toluene were incorporated, and the mixture was continuously agitated. After 20 min, TiO2 seed was added, followed by tetraethyl orthosilicate, and the mixture was vigorously agitated. The mixture was subsequently heated via hydrothermal treatment utilizing microwave radiation for 4 h. The white precipitate was produced by centrifuging the resultant solution, rinsing it with distilled water and acetone, and drying it under an atmospheric environment at 393 K. Finally, the obtained product was calcined at 823 K for 3 h and labeled as FST. The synthesized FST was then physically combined with 0.035 g of silver nitrate in distilled water, and the mixture was stirred for 30 min. The solution was autoclaved for 2 h at 120 °C. The Ag-doped FST was applied on a 1 cm × 1 cm glass surface coated with fluorine tin oxide (FTO) employing the carbon paint method. In the end, the Ag-doped-FST photoanode was pre-dried on a hot plate at 50 °C for 10 min, followed by drying overnight (Figure 12).

3.3. Physiochemical Characterization

The crystalline structure of the photoanodes was analyzed using X-ray diffraction (XRD). Cu Kα radiation was used, and the measurements were taken at 2 theta angles ranging from 20° to 65° using a Bruker D8 Advance X-ray instrument (Bruker, Billerica, MA, USA). Raman spectroscopy (Sunterra, Bruker, Ettlingen, Germany) was used at room temperature with a 785 nm laser diode and 10 mW excitation. The surface area of the photoanodes was determined using the multipoint Brunauer–Emmet–Teller (BET) method through nitrogen adsorption-desorption analysis. The pore size distribution was obtained using the Barrett–Joyner–Halenda (BJH) theory with a SA3100 instrument (Beckman Coulter, Brea, CA, USA). Field-emission scanning electron microscopy (FESEM) (JEOL JSM-6701F, JEOL Ltd., Akishima, Japan) and transmission electron microscopy (TEM) (Philips EM420, Philips Electron Optics, Eindhoven, The Netherlands) were used to assess the morphological structures of the photoanodes. The functional groups were identified through Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer Spectrum GX FTIR Spectrometer (PerkinElmer, Waltham, MA, USA). Utilizing X-ray photoelectron spectroscopy (XPS), the oxidation state and electron movement of the photoanode were assessed with a PHI5000 Versa Probe II (ULVAC-PHI, Kanagawa, Japan). UV–Vis diffuse reflectance spectra were measured on a Lambda 950 UV–vis/DRS spectrophotometer (Perkin Elmer, Shelton, CT, USA) at room temperature in the 200–800 nm range to determine the bandgap of the photoanodes. Photoluminescence (PL) was used to investigate the properties of the photogenerated charge separation and transfer using a Quanta Master TM 60 Fluorescence Spectrophotometer (Photon Technology International, Burlington, NJ, USA).

3.4. PEC Measurements

For the PEC water-splitting activity, 0.1 M Na2SO4 solution was used with a three-electrode configuration. This included an Ag/AgCl reference electrode, a synthesized photoanode as the working electrode, and platinum as the counter electrode. The measurements of both dark and illuminated currents were taken using a potentiostat (PGSTAT128N, Metrohm Autolab, Utrecht, The Netherlands) equipped with a 350 W xenon lamp and 1.5 AM filter. An external bias of 1.5 V vs. Ag/AgCl was used, and the scan rate was set to 0.01 V s−1. The charge carrier densities and conduction band were calculated using Mott–Schottky analysis at 1 kHz. This investigation was undertaken to acquire a greater understanding of the charge transport behavior of the synthesized photoanodes. Electrochemical impedance spectrum (EIS) Nyquist plots were generated by applying 10 mV sinusoidal fluctuations at frequencies ranging from 0.1 Hz to 100 kHz. The data were converted to the reversible hydrogen electrode (RHE) using Equation (4) [61].
ERHE = EAg/AgCl + 0.0591 pH + 0.1976
Equation (5) displays the equation for calculating the ABPE.
A B P E = 1.23 − V b × J p P
where Vb is the bias potential applied, Jp displays the photocurrent density, and P is the intensity of light in (mWcm−2).

4. Conclusions

This study demonstrates a remarkable enhancement in a Ag-doped FST photoanode for PEC water splitting, with several key factors contributing to its superior performance. The introduction of Ag nanoparticles significantly boosts the photoanode’s light absorption through localized surface plasmon resonance, leading to the higher generation of photoexcited electrons and holes. This is reflected in the substantial increase in the photocurrent density observed in the LSV. The enhanced charge separation and reduced recombination rates are further supported by the CV data. The unique fibrous structure of FST, combined with the conductive properties of Ag, facilitates improved charge transport and electron transfer processes. This structural and compositional synergy between silica and titania in the FST matrix, along with the Ag doping, contributes to optimized PEC performance. Silica’s porous nature enhances light scattering and adsorption, while titania imparts excellent photocatalytic properties. Additionally, the stability of the Ag-doped FST photoanode is notably improved under operational conditions, with a durability test showing minimal degradation over 16 h of continuous illumination. The Ag nanoparticles play a crucial role in enhancing stability by mitigating the photo corrosion of the titania matrix. Overall, the Ag-doped FST photoanode demonstrates substantial improvements in both photocurrent density and operational stability, highlighting the effectiveness of integrating Ag doping with the distinctive structural attributes of FST to advance water-splitting technologies.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15010066/s1, Figure S1: EDX spectrum of Ag-doped FST; Table S1: Summary of previous literature reports based on Ag-doped FST, FST, SiO2 and TiO2 for PEC water splitting. References [62,63,64,65] are cited in Supplementary Materials.

Author Contributions

Investigation, methodology, writing—original draft, S.A., M.U. and F.S.; resources, S.F. and W.R.; conceptualization, funding acquisition, supervision, writing—review and editing, X.L. and H.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work was partially financially supported by the National Natural Science Foundation of China (grant Nos. 62375200 and 61975148), the Tianjin graduate Research Innovation project (grant No. 2022BKY065), and the Independent Innovation Fund of Tianjin University (grant No. 2024XSU-0006).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest

Author Shouzhong Feng was employed by the company Anhui Zhongyi New Material Science and Technology Co. Ltd. (Chuzhou, China). The remaining authors declare that this research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

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Catalysts 15 00066 g001
Figure 1. XRD patterns of Ag-doped FST and those of the standard JCPDS cards (FST, SiO2, and TiO2).
Figure 1. XRD patterns of Ag-doped FST and those of the standard JCPDS cards (FST, SiO2, and TiO2).
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Figure 2. Adsorption–desorption isotherms of Ag-doped FST.
Figure 2. Adsorption–desorption isotherms of Ag-doped FST.
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Figure 3. FTIR spectra of TiO2, SiO2, FST, and Ag-doped FST.
Figure 3. FTIR spectra of TiO2, SiO2, FST, and Ag-doped FST.
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Figure 4. XPS spectra of Ag-doped FST: (a) Ti 2p, (b) Si 2p, (c) O 1s, (d) Ag 3d.
Figure 4. XPS spectra of Ag-doped FST: (a) Ti 2p, (b) Si 2p, (c) O 1s, (d) Ag 3d.
Catalysts 15 00066 g004
Catalysts 15 00066 g005
Figure 5. FESEM images of (a) Ag-doped FST and (b) FST at 500 nm; TEM image of (c) Ag-doped FST at 100 nm and (d) Ag-doped FST at 50 nm; (e,f) HR-TEM of Ag-doped FST and FST; (g) SAED pattern of Ag-doped FST; (h) STEM; (i) Combined mapping and (j–m) elemental mapping of Si, O, Ti, and Ag.
Figure 5. FESEM images of (a) Ag-doped FST and (b) FST at 500 nm; TEM image of (c) Ag-doped FST at 100 nm and (d) Ag-doped FST at 50 nm; (e,f) HR-TEM of Ag-doped FST and FST; (g) SAED pattern of Ag-doped FST; (h) STEM; (i) Combined mapping and (j–m) elemental mapping of Si, O, Ti, and Ag.
Catalysts 15 00066 g005
Catalysts 15 00066 g006
Figure 6. UV–visible spectra of Ag-doped FST, FST, SiO2, and TiO2 photoanodes.
Figure 6. UV–visible spectra of Ag-doped FST, FST, SiO2, and TiO2 photoanodes.
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Catalysts 15 00066 g007
Figure 7. (a) Tauc plot and (b) Mott–Schottky plot analysis of TiO2, SiO2, FST, and Ag-doped FST.
Figure 7. (a) Tauc plot and (b) Mott–Schottky plot analysis of TiO2, SiO2, FST, and Ag-doped FST.
Catalysts 15 00066 g007
Catalysts 15 00066 g008
Figure 8. (a) LSV curve, (b) CV curve, and (c) photocurrent density curves of Ag-doped FST at different applied potentials.
Figure 8. (a) LSV curve, (b) CV curve, and (c) photocurrent density curves of Ag-doped FST at different applied potentials.
Catalysts 15 00066 g008
Catalysts 15 00066 g009
Figure 9. EIS Nyquist plot of Ag-doped FST, FST, SiO2, and TiO2 photoanodes.
Figure 9. EIS Nyquist plot of Ag-doped FST, FST, SiO2, and TiO2 photoanodes.
Catalysts 15 00066 g009
Catalysts 15 00066 g010
Figure 10. (a) Long-term stability of Ag-doped FST, FST, SiO2, and TiO2 photoanode; (b) XRD spectrum of Ag-doped FST before and after use.
Figure 10. (a) Long-term stability of Ag-doped FST, FST, SiO2, and TiO2 photoanode; (b) XRD spectrum of Ag-doped FST before and after use.
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Catalysts 15 00066 g011
Figure 11. Schematic illustration of a plausible mechanism of FST and Ag-doped FST.
Figure 11. Schematic illustration of a plausible mechanism of FST and Ag-doped FST.
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Catalysts 15 00066 g012
Figure 12. Schematic diagram of fabrication of FST and Ag-doped FST.
Figure 12. Schematic diagram of fabrication of FST and Ag-doped FST.
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Table 1. Comparison of PEC parameters of Ag-doped FST, FST, SiO2, and TiO2.
Table 1. Comparison of PEC parameters of Ag-doped FST, FST, SiO2, and TiO2.
ParameterAg-doped FSTFSTSiO2TiO2Remarks
Bandgap Energy (eV)2.52.83.23.0Ag doping narrows the bandgap, enhancing visible light absorption
Photocurrent Density (mAcm−2 at 1.2 V vs. RHE)13.9811.65--Ag-doped FST shows ~1.2-fold improvement over FST
Charge Transfer Resistance (Rct, Ω)31111256221496Ag-doped FST has the lowest Rct, indicating superior charge transfer efficiency
Surface Area (m2g−1)233.4---Higher surface area enhances the number of active sites for redox reactions
ABPE (%) at 0.5 V vs. RHE7.376.214.333.81Improved applied bias photon-to-current efficiency due to Ag doping
Onset Potential (V vs. RHE)LowerHigher--Lower onset potential indicates reduced energy barrier for charge transfer
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Arain, S.; Usman, M.; Saeed, F.; Feng, S.; Rehman, W.; Liu, X.; Dai, H. Microemulsion-Based Synthesis of Highly Efficient Ag-Doped Fibrous SiO2-TiO2 Photoanodes for Photoelectrochemical Water Splitting. Catalysts 2025, 15, 66. https://doi.org/10.3390/catal15010066

AMA Style

Arain S, Usman M, Saeed F, Feng S, Rehman W, Liu X, Dai H. Microemulsion-Based Synthesis of Highly Efficient Ag-Doped Fibrous SiO2-TiO2 Photoanodes for Photoelectrochemical Water Splitting. Catalysts. 2025; 15(1):66. https://doi.org/10.3390/catal15010066

Chicago/Turabian Style

Arain, Samia, Muhammad Usman, Faiq Saeed, Shouzhong Feng, Waheed Rehman, Xianhua Liu, and Haitao Dai. 2025. "Microemulsion-Based Synthesis of Highly Efficient Ag-Doped Fibrous SiO2-TiO2 Photoanodes for Photoelectrochemical Water Splitting" Catalysts 15, no. 1: 66. https://doi.org/10.3390/catal15010066

APA Style

Arain, S., Usman, M., Saeed, F., Feng, S., Rehman, W., Liu, X., & Dai, H. (2025). Microemulsion-Based Synthesis of Highly Efficient Ag-Doped Fibrous SiO2-TiO2 Photoanodes for Photoelectrochemical Water Splitting. Catalysts, 15(1), 66. https://doi.org/10.3390/catal15010066

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