Rapid flame induced dual-metal doping on WO3 electrode for boosting photo-electrochemical water oxidation
0 Abstract
A bidirectional co-doping of transition metal Fe and post-transition metal Sn on WO3 photoanode via a facile one step flame-doping process demonstrates the challenging amelioration of both thermodynamic charge migration and surface catalytic kinetics, achieving high-efficient photoelectrochemical (PEC) water oxidation reaction in a neutral pH. The direct flamethrower with rapid thermal flux effectively induces the bidirectional doping of Fe3+ and Sn4+ into WO3 without damaging its nanostructure and fluorine-doped tin oxide glass substrate. From the synergetic effect of the dual-metal doping, the photoinduced charge migration and the surface water oxidation kinetics are effectively ameliorated. As a result, the Fe/Sn co-doped WO3 photoanode shows significantly enhanced PEC response with 6.16-fold higher photocurrent density performance at 1.23 VRHE than bare WO3. This work highlights the facile metal atom co-doping method without affecting intrinsic properties of photoanode and substrate for boosting the PEC water splitting performance and solar fuel production.
Keywords
INTRODUCTION
As a promising strategy for green hydrogen production, extensive research has been conducted on photoelectrochemical (PEC) water splitting[1]. In order to activate the highly efficient solar energy conversion in the PEC cells, there are strong demands on resolving the sluggish kinetics of oxygen evolution reaction (OER) and ameliorating the poor conversion efficiency of the anode part for overcoming the energy barriers in the 4-electron steps of the water oxidation[2-5]. The performance of the PEC OER is determined by three fundamental factors: the light harvesting efficiency, which governs the number of photogenerated charge carriers; the charge transport efficiency, which encompasses the charge separation and collection processes including the critical aspect of charge recombination; and the charge transfer efficiency at the photoanode-electrolyte interface reaction sites[6-8]. In this context, n-type semiconductive transition metal oxides (TMO) have been explored as a potential photoanode material due to their cost-effectiveness, chemical stability, and suitable band alignment for the OER[9]. Among them, WO3 stands out as a promising candidate for the highly active photoanode due to advantages such as an appropriate energy band structure with band gap energy, moderate hole diffusion length, and high electron mobility[10-15]. However, the practical application of the WO3 faces challenges such as low charge transport efficiency due to deteriorated charge separation and collection by rapid charge recombination, low charge transfer efficiency resulting from poor surface OER kinetics, and instability caused by the defects-induced photo corrosion during the PEC operation in aqueous media[16,17]. Moreover, the PEC OER on the WO3 photoanode in the neutral pH environment critically suffers from extremely poor surface reaction kinetics than in an acid condition due to the d-band center of WO3 and its adsorption behavior of the intermediates during the 4-electron OER pathway[5,18-20]. Hence, improving the charge separation performance by suppressing the undesired recombination and enhancing the charge transfer efficiency to boost surface OER kinetics are critical for achieving the high efficiency of oxide-based PEC OER.
Among the various strategies to overcome the limitations of WO3[21-24], doping of metal elements in the WO3 nanostructure to produce a multi-metal oxide has been widely adopted since it is a facile process for improving the bulk properties of oxide photoanodes, where the charge carriers of the host material are increased by introducing heteroatoms from the doping process[25]. In general, doping can be categorized as ex-situ and in-situ, depending on whether the doping process is accompanied by the formation of the host material[26]. In the case of the in-situ doping, the fabrication process can be less complex than the ex-situ doping. However, the distributed dopants in the bulk of the host material may induce lattice distortion due to the size discrepancy between host atoms and dopants, acting as a charge recombination center[27-29]. Considering their obvious disadvantage, the ex-situ doping processes can be utilized as effective doping methods. Among the ex-situ doping methods, thermal diffusion-mediated doping is a facile and cost-effective approach for the doping of heteroatom metal elements into the TMO, but it suffers from prolonged annealing times and impurity injection control[30,31]. Furthermore, improving the surface OER kinetics remains a crucial challenge with the conventional thermal diffusion-mediated doping approach as it primarily focuses on bulk properties[32]. Therefore, a novel doping strategy that addresses the drawbacks associated with the conventional doping processes is necessary to achieve highly efficient PEC OER with WO3.
Herein, we introduced dual doping of 2D nanoflake structured WO3 with transition and post-transition metals using a facile ultrafast sol-flame doping strategy. The sol-flame doping can be performed by coating the dopant precursor in the form of sol and flame combustion treatment. Despite the short flame combustion process time for 40 s, the rapid thermal flux resulting from the high temperature of 1,000 °C introduced heteroatoms (Fe3+) from the overlaid precursor and induced effective bidirectional dual doping into WO3 by promoting the diffusion of Sn4+ derived from the fluorine-doped tin oxide (FTO) glass substrate without causing lattice distortion of the WO3 or damaging the substrate. As a result, Fe and Sn co-doped WO3 using sol-flame method (FL-doped WO3) exhibited outstandingly enhanced thermodynamic charge transport efficiency and catalytic kinetics transfer efficiency of 83% and 76%, respectively, compared to those of 65% and 41%, respectively, for Fe-doped WO3 using conventional thermal-mediated method (FN-doped WO3) and 55% and 22%, respectively, for bare WO3. Moreover, the charge recombination was effectively suppressed in the FL-doped WO3 and the charge carrier lifetime was also prolonged compared to that of FN-doped WO3 and bare WO3. Consequently, FL-doped WO3 achieved a significantly enhanced photocurrent density performance at 1.23 VRHE in pH 7.0 aqueous electrolyte, which was 6.16 times higher than that of bare WO3). Density functional theory (DFT) analysis confirms that the substitution doping of Fe and Sn atoms is induced on W sites. In addition, it reveals that Sn doping effectively improved charge transport efficiency and the co-doping of Fe and Sn enhanced surface OER kinetics of WO3. Our research results propose a novel doping strategy beyond conventional methods. The flame doping strategy can maintain the structural stability of the semiconductor while facilitating dual transition metal atom co-doping, effectively improving charge transport efficiency and surface OER kinetics. We expect that our flame doping method can be applied to enhance PEC performance by extending it to various transition metal atom dopants and semiconductors.
EXPERIMENTAL
Fabrication of the bare WO3 film
The bare WO3 2D nanosheet structure was prepared on FTO glass (15 Ω) by using hydrothermal method. First, the WO3 seed layer precursor was synthesized by adding 0.375 g of tungstic acid (99.9%, Sigma Aldrich) to 3 mL of hydrogen peroxide (H2O2, 30 wt%, Sigma Aldrich) and stirring at 140 °C until 1 mL of solution remained. Subsequently, the solution was dissolved in 9 mL deionized water (DI water) with
Fabrication of the FL-doped WO3 and FN-doped WO3
The FL-doped WO3 (Fe and Sn co-doped WO3 using sol-flame method) photoanode was synthesized through a sol-flame doping treatment. The Fe sol precursor was synthesized by adding 0.054 g of iron chloride hexahydrate (97%, Sigma Aldrich) and 33 μL of 6 M HCl to 20 mL of 2-metoxyethanol (99.8%, Sigma Aldrich) with 20 min stirring. Spin-coating was performed at 2,000 rpm for 40 s by dropping
PEC measurements
All PEC tests were measured using the CHI instrument and Gamry 600+ workstation in electrolyte with a three-electrode cell configuration. Platinum wire and Ag/AgCl electrodes were selected as the counter and reference electrodes, respectively. The electrolyte was 0.1 M potassium phosphate (KPi) buffer solution (pH 7.0). A solar simulator (ABET Technologies Solar Simulators) was applied as an artificial sunlight source for 1 sun illumination (100 mW cm-2). The 0.1 M KPi buffer solution with 0.5 M Na2SO3 was used as a hole scavenger (pH 7.74) to characterize the charge transport efficiency (ηtransport = Jscavenger/Jabs) and the charge transfer efficiency (ηtransfer = JKPi/Jscavenger). Here, Jabs refers to the photon absorption rate expressed by the maximum charge density obtained from the light absorptance. Electrochemical impedance spectroscopy (EIS) tests were performed using a three-electrode configuration, with platinum wire and Ag/AgCl electrodes as the counter and reference electrodes, respectively. The analysis was conducted with an AC voltage amplitude of 5 mV across a frequency range of 1 MHz to 0.01 Hz at an applied bias potential of
Photoluminescence and time-resolved photoluminescence measurements
The room-temperature photoluminescence (PL) spectra of WO3 photoanodes were obtained using a fluorescence spectrophotometer (FS5, Edinburgh). The system was equilibrated for 2 h before measurements. An excitation source with a 350 nm wavelength was used to excite the sample and 395 nm of long-pass filter was used. The time-resolved PL measurement (TRPL) was conducted using a Nd:YAG-based pulsed laser system (Surelite II-10, Surelite), which generates the fundamental wavelength at 1,064 nm (1 W) with a repetition rate of 10 Hz and a pulse width of 4-6 ns, along with its second (532 nm) and third (355 nm) harmonics. The third harmonic (355 nm) was then separated using the Surelite Separation Package (SSP) and directed into an optical parametric oscillator (Surelite OPO, Surelite) to produce the
Characterizations
The morphological properties and elemental mapping were evaluated by field emission scanning electron microscopy (FESEM, JSM-7600F) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2000). X-ray diffraction (XRD, D8 ADVANCE with Cu Kα radiation) was utilized to determine the crystal structure and phase formation of the sample. Additionally, the chemical bonding between the constituent elements was determined by X-ray photoelectron spectroscopy (XPS, ESCALAB250). Raman measurements were performed using a RAMAN spectrometer (DXR2xi) equipped with 532 nm laser and an electron multiplying charge-coupled device (CCD) detector. Optical properties were identified using an ultraviolet-visible (UV-Vis) spectrophotometer (SHIMADZU UV-3600i plus).
RESULTS AND DISCUSSION
Morphological and compositional analysis
Fe element was utilized as dopants that effectively enhance the PEC performance of WO3, taking advantage of their similarity in ionic radius to the host atom W6+ and their successful application in the n-type semiconductors[21,33,34]. Furthermore, we designed the co-doping for WO3 with Fe and Sn, utilizing the phenomenon of Sn diffusion derived from FTO at high temperatures through a flame treatment[35]. The sol-flame doping methodology involves a two-step process: initial coating of the substrate with the dopant precursor in solution form (sol), followed by a controlled flame combustion treatment that facilitates rapid incorporation of dopant elements into the host structure. This approach enables efficient doping while maintaining the structural integrity of the material. The synthesis method of FL-doped WO3 by the sol-flame method is illustrated in Figure 1A. The bare WO3 was synthesized on the seed layer-coated FTO substrate by hydrothermal synthesis method, followed by spin-coating with Fe sol and flame treatment to fabricate FL-doped WO3. In addition, to compare the FL-doped WO3 with the conventionally doped WO3, the FN-doped WO3 was prepared by the thermal-mediated doping using a furnace. The images of the synthesized photoanodes are exhibited in Supplementary Figure 1. Scanning electron microscopy (SEM) images of the FL-doped WO3 exhibit that the thin WO3 nanoflakes were densely cross-linked to form a film with a thickness of 2 μm [Figure 1B and C]. The bare WO3 and the FN-doped WO3 showed no significant differences compared to the FL-doped WO3. It indicates that the morphology and structure of WO3 were not significantly affected by doping [Supplementary Figure 2]. In HRTEM measurements, the d-spacing (0.364 nm) of the typical monoclinic WO3 (002) plane was observed in the FL-doped WO3, FN-doped WO3, and bare WO3 [Figure 1D and Supplementary Figure 3]. After the doping process, Fe-based material from the Fe sol was barely observed in the HRTEM. HRTEM line profile analysis shows that Fe and Sn existed in the FL-doped WO3 at 4.54% and 0.22% atomic percent, respectively[Figure 1E]. An inset figure in Figure 1E demonstrates that the detection of Sn. In addition, high-angle annular dark field (HADDF) and energy dispersive spectroscopy (EDS) mapping results validate the presence of Fe and Sn in FL-doped WO3 [Figure 1F]. Due to the high-temperature thermal shock and cooling process in sol-flame method, the diffusion of the dopants to the thermodynamically stable position was promoted[36,37], which induced the uniform distribution of Fe and Sn elements.
Figure 1. (A) Schematic illustration of the sol-flame doping method. (B) Top view and (C) cross section view of SEM images of the FL-doped WO3. (D) HRTEM images of the FL-doped WO3. (E) HRTEM line profile for the FL-doped WO3. (F) HAADF image and EDS mapping for the FL-doped WO3.
Crystallinity analysis
To investigate the effect of doping on the crystallinity of WO3, XRD patterns were measured. The three main peaks centered at 2θ = 23.1º, 23.6º, and 24.3º represent the (002), (020), and (200) planes of monoclinic WO3, respectively [Figure 2A]. The FL-doped WO3 and FN-doped WO3 exhibit monoclinic WO3 peaks similar to the bare WO3, indicating that additional heat treatment did not induce secondary phase formation of monoclinic WO3 [Figure 2B]. Furthermore, the intensity of the peaks increased after the additional heat treatment for doping, suggesting that the enhanced crystallinity of WO3 in the FL-doped WO3 and FN-doped WO3[38-42]. In particular, the rapid flame process effectively enhanced the crystallinity of WO3 in the FL-doped WO3 compared to the FN-doped WO3 photoanode synthesized by the conventional doping process. However, unlike the FN-doped WO3, the FL-doped WO3 exhibited a significant increase in the signal of the (002) and (020) planes. These dominant facet changes in the FL-doped WO3 are attributed to the reduction of short-distance structural disorder in WO3 due to flame treatment[43-45]. On the other hand, the FN-doped WO3 showed a similar peak tendency to the bare WO3 and a decrease in the signal of the (002) plane. This is because the short-distance structural disorder was generated on the surface of the FN-doped WO3 [Supplementary Figure 3]. The XRD data demonstrate that the high-temperature flame process preserves the intrinsic phase and enhances the crystallinity of the material, which affects the light harvesting efficiency, while allowing for effective doping.
Figure 2. (A) XRD patterns of the FL-doped WO3, FN-doped WO3, and bare WO3. (B) Enlarged XRD patterns on short range of 20°~32°. (C) Raman spectra of FL-doped WO3, FN-doped WO3, and bare WO3.
Raman spectroscopy measurements were also performed to investigate the influence of doping on the local structure of WO3 [Figure 2C]. The Raman band peaks centered at 272 and 326 cm-1 correspond to the bending modes of O-W-O, while the peaks centered at 714 and 806 cm-1 are related to the stretching modes of O-W-O[21,46]. No significant peak shift was observed in the FL-doped WO3 and FN-doped WO3 based on the bare WO3. The intensity of the peak signal at 610 cm-1, which is predicted to be residual hydrated WO3, decreased with additional heat treatment[46,47]. The reduction in intensity of each O-W-O stretching and bending mode peak after doping is attributed to the substitution of W by the Fe dopants in the FN-doped WO3, with a further decrease observed in the case of Fe and Sn co-doping in the FL-doped WO3. The details of substitution doping are addressed further in the DFT section. The Raman data indicate that doping has no significant influence on the local structure of WO3.
XPS analysis
XPS measurements were performed to investigate the surface chemical state of WO3 after additional heat treatment and to further confirm the presence of dopants. No significant changes in the C 1s spectra, including the C-C bond at 284.8 eV, indicate that the flame process using CH4 as a fuel was solely related to heat treatment and did not generate any by-products such as hydrocarbon materials on the surface
Figure 3. High-resolution XPS spectra of (A) O 1s, (B) W 4f, (C) Fe 2p and (D) Sn 3d for the FL-doped WO3, FN-doped WO3, and bare WO3.
Photoelectrochemical performance
Linear sweep voltammetry (LSV) measurements were performed in 0.1 M KPi (pH 7) to investigate the enhancement of PEC OER performance in WO3 by doping [Figure 4A]. The FL-doped WO3 exhibited a significantly enhanced photocurrent density of 1.45 mA/cm2, which is 6.16 times higher than the bare WO3 with a photocurrent density of 0.24 mA/cm2 at 1.23 VRHE. The FL-doped WO3 also outperformed the FN-doped WO3, which had a photocurrent density of 0.56 mA/cm2 at 1.23 VRHE. The same performance trend was observed in hole scavenger condition (pH 7.74) [Supplementary Figure 6]. The FL-doped WO3, FN-doped WO3, and bare WO3 showed photocurrent densities of 1.85, 1.37, and 1.15 mA/cm2, respectively, at 1.23 VRHE. Also, applied bias photon-to-current efficiency (ABPE) measurements confirm that the FL-doped WO3 can be efficiently operated in PEC OER [Supplementary Figure 7][50]. In order to analyze the factors contributing to the performance enhancement in detail, the light harvesting efficiency, charge transport efficiency (ηtransport), and charge transfer efficiency (ηtransfer), which influence the PEC OER performance, were examined. For light harvesting efficiency, absorptance (A) was derived from UV-vis measurements, which was calculated from the diffuse reflectance (R) and diffuse transmittance (T) of WO3 electrodes (A = 100%-R-T) [Figure 4B, Supplementary Figure 8A and B]. In the wavelength region beyond 450 nm, where the energy is higher than the band gap of WO3 electrodes, an increased light harvesting efficiency was observed for the FL-doped WO3 and FN-doped WO3 compared to the bare WO3, which well supported the XRD analysis. In the longer wavelength region above 450 nm, the FL-doped WO3 exhibited higher absorptance than the FN-doped WO3, which had only Fe doping. It indicates that Sn dopant acts as a shallow donor in the FL-doped WO3[21,51]. However, there was no significant change in the band gap
Figure 4. (A) PEC performance of WO3 electrodes. (B) UV-vis absorptance spectra of WO3 electrodes. (C) Charge transport efficiency and (D) charge transfer efficiency of WO3 electrodes. (E) EIS (Electrochemical Impedance Spectroscopy) measurement of the WO3 electrodes under 1 sun illumination (at 1.23 V vs. RHE, 106~0.01 Hz) and (F) Mott-Schottky plots of WO3 electrodes.
In order to clarify the role of the dopants, additional investigations were carried out on Sn-doped WO3, which was synthesized by the flame treated WO3 without Fe sol (FL-treated WO3, Supplementary Figure 9). XRD patterns of the FL-treated WO3 showed an overall improved crystallinity similar to the FL-doped WO3 and FN-doped WO3 [Supplementary Figure 10]. The FL-treated WO3 exhibited a weakened peak of the (002) plane, similar to the FN-doped WO3. It indicates that both conventional doping and flame doping strategies result in a weakened (002) plane when doping with a single element regardless of species. XPS spectra of FL-treated WO3 confirm the successful doping of Sn [Supplementary Figure 11]. The FL-treated WO3 showed a photocurrent density of 1.09 mA/cm2 at 1.23 VRHE and increased absorptance compared to the bare WO3 [Supplementary Figure 12]. The FL-treated WO3 had a similar absorptance shape to the FL-doped WO3, confirming that improving crystallinity through heat treatment enhances light harvesting efficiency and Sn dopant acts as a shallow donor. Therefore, the doping of Sn by the flame method efficiently improved the PEC OER, and the synergistic effect of Fe in the co-doping contributed to further boosting the performance by improving the ηtransport and ηtransfer. EIS measurements were performed to gain further insight into the ηtransport and ηtransfer enhancements [Figure 4E]. Based on the circuit model presented in the inset figure, the simulated line was calculated from the measured data. The series resistance (Rs) at the electrode-substrate interface, the charge transport resistance within the electrode (Rtr), and the charge transfer resistance at the electrolyte-electrode interface (Rct) were derived from the simulation. The FL-doped WO3 exhibited the lowest Rtr and Rct values compared to the FN-doped WO3 and bare WO3, indicating enhanced ηtransport and boosted surface OER kinetics, suppressing the charge recombination. These results are well-matched with the ηtransport and ηtransfer data shown in Figure 4C and D. The fitted parameters are presented in Supplementary Table 1. Mott-Schottky plot of WO3 electrodes is shown in Figure 4F, confirming an increase in donor density for the FL-doped WO3 and FN-doped WO3 after the doping process [Supplementary Table 2]. Moreover, the FL-doped WO3 exhibits a higher donor density compared to the FN-doped WO3, further validating the co-doping in the FL-doped WO3, as supported by TEM and XPS analyses. Moreover, the stability test exhibited that the current retention of the FL-doped WO3 was maintained for 24,000 s at the potential of maximum ABPE [Supplementary Figure 13].
Charge carrier dynamics
Furthermore, PL and TRPL measurements of WO3 photoanodes were carried out to investigate the charge carrier dynamics [Figure 5]. As depicted in Figure 5A, the FL-doped WO3 showed the most reduced PL intensity, following the FN-doped WO3. It indicates that the charge recombination is effectively suppressed in doped WO3, especially in the FL-doped WO3[52,53]. This improvement tendency was also observed in TRPL results [Figure 5B]. The FL-doped WO3 showed improved decay profile with prolonged photogenerated charge lifetime (7.48 ns) than FN-doped WO3 (6.32 ns) and bare WO3 (4.96 ns). The PL and TRPL results demonstrate that dual-metal doping by flame method effectively ameliorates the charge carrier dynamics, which contributes to the improvement of the charge transport efficiency. With the enhanced surface OER kinetics, the PEC OER performance of WO3 is efficiently boosted. These enhancements are illustrated in Supplementary Figure 14 with a simplified energy band diagram.
Figure 5. (A) PL intensities of FL-doped WO3, FN-doped WO3, and bare WO3. (B) Normalized time-resolved photoluminescence (TRPL) of FL-doped WO3, FN-doped WO3, and bare WO3. PL and TRPL were measured under room-temperature conditions. Detailed experimental information is provided in the experimental section.
DFT calculations investigations
To elucidate the performance enhancement when single Fe atoms and Sn, Fe co-doping are incorporated into WO3, DFT calculations were conducted. Our prior characterizations confirmed that the FN-doped WO3 involves simultaneous doping of Fe and Sn atoms, whereas the FL-doped WO3 contains only Fe. Defect formation energy calculations were performed to investigate the doping sites of Fe and Sn atoms in the structure. To minimize periodic interaction errors and depict a dilute doping concentration, a 2 × 2 × 2 expanded supercell from the bulk WO3 was utilized for structural modeling. Using the experimentally measured Fermi level, which is 2.42 eV away from the valence band maximum (VBM), as a reference point, the substitution of both Fe and Sn atoms for W atoms in WO3 is determined to be thermodynamically most stable [Figure 6A and B, Supplementary Figure 5]. Consequently, all subsequent calculations hereafter considered substitutional doping.
Figure 6. Defect formation energies of WO3 for (A) Fe and (B) Sn. (C) Unfolded band structures for bare WO3 and FL-doped WO3. The letter G in x-axis denotes Γ k-path. (D) Geometry-optimized structures for FN-doped WO3 and FL-doped WO3. The grey, blue, ochre, and red spheres represent W, Sn, Fe, and O atoms, respectively. (E) Calculated OER landscape for bare WO3, FN-doped WO3, and FL-doped WO3. (F) COHP results for bare WO3, FN-doped WO3, and FL-doped WO3.
Band structure calculations for the same 2 × 2 × 2 expanded supercell were then performed to probe the origins of increased ηtransport and improved electrical properties observed in electrochemical analysis. From Figure 3C, the higher intensity in FL-doped WO3 compared to FN-doped WO3 was observed, which indicates a higher amount of Fe atoms were introduced into the surface. Accordingly, one Fe atom was substituted in the FN-doped WO3 supercell model, while in FL-doped WO3, two Fe atoms and one Sn atom were substituted. In Figure 6C, bare WO3 without doping exhibited flat bands near the VBM around the G k-path. However, doping with single Fe atom in FN-doped WO3 [Supplementary Figure 15] and two Fe atoms and one Sn atom in FL-doped WO3 resulted in parabolic bands. The widely recognized relationship states that the second derivative of curves in the band structure is inversely proportional to the effective mass, and reduction of effective mass directly enhances mobility, consequently influencing conductivity[54,55]. Thus, the co-doping of Fe and Sn atoms affected the hole effective mass near the valence band top, exhibiting improved hole mobility and conductivity beyond all models, explaining the enhanced
Next, surface modeling was conducted to calculate the OER kinetics. As previously mentioned, the structures of FN-doped WO3 and FL-doped WO3 were modeled through substitutional doping, as illustrated in Figure 6D. In Figure 6E, bare WO3 exhibited weak adsorptions for all OER intermediates, with an overpotential of 1.60 eV, which is consistent with previous calculations[56,57]. In contrast, FN-doped WO3 showed overall stronger adsorptions for all intermediates, and FL-doped WO3, with the introduction of Sn atoms, exhibited an optimal adsorption strength compared to FN-doped WO3. As a result, FL-doped WO3 displayed an overpotential of 0.98 eV, making the OER kinetics more feasible compared to bare WO3
CONCLUSIONS
In summary, bidirectional dual-metal doping via facile flame doping method for enhancing the PEC performance of the WO3 photoanode was demonstrated. In contrast to the conventional thermal-mediated doping method, the rapid thermal flux with flame treatment induced the doping of Fe element from the Fe precursor solution and the diffusion of Sn element from the FTO substrate, which enables the bidirectional co-doping of Fe and Sn on the WO3 photoanode. XRD, XPS and Raman results confirmed that the flame treatment improved the crystallinity of the WO3 without affecting the intrinsic phase and local structure during co-doping. As a result, the FL-doped WO3 exhibited 6.16 times higher PEC OER performance compared to the bare WO3 with a challenging amelioration of both photoinduced charge migration, surface water oxidation kinetics, charge recombination, and charge carrier lifetime by co-doping in neutral electrolyte condition. The DFT calculation demonstrated the enhancement of the electrical properties by lowering effective mass, and optimized adsorptions of OER intermediates. Our report provides an effective doping strategy to TMO photoanode for enhancing the PEC performance.
DECLARATIONS
Authors’ contributions
Writing: Roh, S. H.; Kim, J.
Experimental analysis and investigation: Roh, S. H.; Kim, J.; So, W.; Li, Y.; Hong, W. T.; Kwon, H. M.
Validation: Jo, S. B.; Yang, W.; Oh, B. K.; Chung, C. H.; Park, J.
Review and editing: Ahn, C.; Kim, B. H.; Kim, J. K.
Supervision and project administration: Kim, J. K.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the government of the Republic of Korea (MSIT) (No. RS-2024-00467234), and by the NRF grant funded by the Korean government (MSIT) (No. RS-2024-00405818).
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
Supplementary Materials
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