Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights
Abstract
1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Optical Modeling and Thickness Evolution by Ellipsometry
3.2. Refractive Index and Optical Contrast
3.3. Morphology and Layer Integrity from Cross-Sectional FE-SEM
3.4. Crystalline Structure from X-Ray Diffraction
3.5. Vibrational Characterization by Raman Spectroscopy
3.6. Optical Transmittance and Band Gap Analysis
3.7. Electrochemical Behavior by Cyclic Voltammetry
3.8. Energy and Power Density Evaluation
3.9. Semiconductor Properties from Mott–Schottky Analysis
3.10. Semiconductor Properties from Electrochemical Impedance Spectroscopy
3.11. Quantification of the Interfacial Capacitive Contribution and Photoelectrochemical Implications
3.12. Capacitive vs. Faradaic Decomposition by the Dunn Method
3.13. Stability Monitoring of TiO2/WOx by EIS Before and After Chronoamperometry
4. Conclusions
Supplementary Materials
Author Contributions
Funding
Data Availability Statement
Conflicts of Interest
Abbreviations
WOx | Tungsten oxide |
TiO2 | Titanium dioxide |
PEC | Photoelectrochemical |
DC | Direct current |
FTO | Fluorine-doped tin oxide |
FE-SEM | Field-emission scanning electron microscopy |
XRD | X-ray diffraction |
OCP | Open-circuit potential |
EIS | Electrochemical impedance spectroscopy |
CV | Cyclic voltammetry |
MS | Mott–Schottky |
ND | Majority donor density |
ND,APP | Majority apparent donor density |
VFB | Flat-band potential |
Rs | Solution resistance |
Rp | Charge transfer resistance |
CPE | Constant phase element |
χ2 | Goodness of fit (chi-squared) |
Eg | Optical band gap |
Nd | Donor density |
Ps | Specific power |
Es | Specific energy |
AM 1.5G | Air mass 1.5 global (solar illumination standard) |
RHE | Reversible hydrogen electrode |
OER | Oxygen evolution reaction |
CA | Chronoamperometry |
Cdl | Double-layer capacitance |
ECSA | Electrochemically active surface area |
Cs | Specific capacitance |
Rf | Roughness factor |
fF | Faradaic fraction |
FRA | Frequency Response Analyzer |
sccm | Standard cubic centimeters per minute |
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Deposition Parameter | Value |
---|---|
Residual Pressure (Torr) | 1.0 × 10−5 |
Working Pressure (Torr) | 3.0 × 10−3 |
WOx Layer | |
DC Power (W) | 150 |
ΦAr (sccm) | 10 |
ΦO2 (sccm) | 10 |
Deposition Time (min) | 0, 5, 10, 20, 30 |
TiO2 Layer | |
DC Power (W) | 300 |
ΦAr (sccm) | 10 |
ΦO2 (sccm) | 2 |
Deposition Time (min) | 30 |
Samples | Medium Length (nm) |
---|---|
5 min | 29.5 ± 8 |
10 min | 47.9 ± 7 |
20 min | 38.5 ± 5 |
30 min | 46.5 ± 3 |
WO3 Deposition Time (min) | Structure | a (Å) | b (Å) | c (Å) | β (°) | V (Å3) | ρ (g.cm−3) | D (nm) | χ2 |
---|---|---|---|---|---|---|---|---|---|
0 | TiO2 | 3.7849 ± 5 | - | 9.5744 ± 29 | - | 137.16 ± 42 | 3.87 ± 1 | - | 4.74 |
5 | WO3 | 7.3286 ± 8 | 7.5465 ± 8 | 7.5076 ± 18 | 91.83 ± 2 | 415.00 ± 12 | 7.42 ± 2 | - | 2.01 |
TiO2 | 3.7923 ± 9 | - | 9.5538 ± 2 | - | 137.40 ± 4 | 3.86 ± 1 | |||
10 | WO3 | 7.3266 ± 15 | 7.5159 ± 12 | 7.6781 ± 16 | 90.39 ± 1 | 422.79 ± 14 | 7.29 ± 2 | 36 ± 3 | 2.11 |
TiO2 | 3.7840 ± 10 | - | 9.5421 ± 11 | - | 136.63 ± 5 | 3.88 ± 5 | - | ||
20 | WO3 | 7.2873 ± 2 | 7.5497 ± 3 | 7.6517 ± 4 | 90.65 ± 1 | 420.95 ± 3 | 7.32 ± 1 | 44 ± 5 | 1.88 |
TiO2 | 3.8175 ± 1 | - | 9.5034 ± 6 | - | 138.50 ± 1 | 3.84 ± 1 | - | ||
30 | WO3 | 7.3274 ± 1 | 7.5366 ± 2 | 7.7022 ± 2 | 90.67 ± 1 | 425.32 ± 2 | 7.24 ± 3 | 41 ± 1 | 1.97 |
TiO2 | 3.7930 ± 6 | - | 9.5576 ± 7 | - | 137.50 ± 3 | 3.86 ± 3 | - |
Samples | VFB (V) | Nd,app (×1019 cm−3) |
---|---|---|
0 min | 1.24 ± 2 | 10.26 ± 7 |
5 min | 0.074 ± 9 | 6.37 ± 3 |
10 min | 0.29 ± 1 | 3.68 ± 3 |
20 min | 0.56 ± 2 | 3.30 ± 4 |
30 min | 0.41 ± 2 | 8.32 ± 9 |
WOx | 0.36 ± 1 | 5.34 ± 3 |
Samples | CPE1 | CPE2 | Χ2 | ||||||
---|---|---|---|---|---|---|---|---|---|
Y0 (µS·sn) | n | Y0 (µS·sn) | n | ||||||
0 min | 22.3 | 144 | - | - | 15.2 | 0.933 | - | - | 0.300 |
5 min | 17.3 | 83.2 | - | - | 17.80 | 0.940 | - | - | 0.135 |
10 min | 17.70 | 13.2 | - | - | 25.00 | 0.918 | - | - | 0.233 |
20 min | 16.70 | 1.51 × 103 | - | - | 22.40 | 0.933 | - | - | 0.231 |
30 min | 13.90 | 355 | - | - | 27.30 | 0.946 | - | - | 0.052 |
WOx | 1.49 | 17.4 | 7.93 | 232 | 46.7 | 0.604 | 62.70 | 0.965 | 0.022 |
Samples | Rs1 ( | CPE1 | CPE2 | Χ2 | |||||
---|---|---|---|---|---|---|---|---|---|
Y0 (µS·sn) | n | Y0 (µS·sn) | n | ||||||
0 min | 20.5 | 0.658 | 1.49 | 7.51 × 10−3 | 46.1 | 0.964 | 111.0 | 0.807 | 0.060 |
5 min | 16.40 | 9.15 | - | - | 313.0 | 0.741 | - | - | 0.436 |
10 min | 3.66 | 7.66 × 10−3 | 9.54 | 6.20 | 59.0 | 0.703 | 340 | 0.807 | 0.005 |
20 min | 16.40 | 14.2 | - | - | 323.0 | 0.832 | - | - | 0.019 |
30 min | 13.7 | 8.05 | - | - | 386 | 0.866 | - | - | 0.009 |
WOx | 6.76 | 15.8 | 3.04 | 15.7 | 36.5 | 0.629 | 172.0 | 0.895 | 0.005 |
Samples (Without Lighting) | Cdl (µF) | ECSA (Cs = 25) cm2 | ECSA (Cs = 40) cm2 | ECSA (Cs = 45) cm2 |
---|---|---|---|---|
0 min | 4.17 × 10−2 | 1.67 × 10−3 | 1.04 × 10−3 | 9.26 × 10−3 |
30 min | 79.2 | 3.17 | 1.98 | 1.76 |
Samples (With Lighting) | Cdl (µF) | ECSA (Cs = 25) cm2 | ECSA (Cs = 40) cm2 | ECSA (Cs = 45) cm2 |
0 min | 70.5 | 2.82 | 1.76 | 1.57 |
30 min | 215.1 | 8.6 | 5.37 | 4.78 |
Samples | CPE | Χ2 | |||
---|---|---|---|---|---|
Y0 (µS·sn) | n | ||||
30 min-EIS (1) | 11.1 | 377 | 95.7 | 0.867 | 0.003 |
30 min-EIS (2) | 12.5 | 395 | 130 | 0.868 | 0.001 |
30 min-EIS (3) | 18.0 | 420 | 120 | 0.869 | 0.009 |
System | Electrolyte | J (1.23 VRHE (mA·cm-2)) | VFB (VRHE) | ND/NA (cm-3) | Reference |
---|---|---|---|---|---|
TiO2/WOx | 0.5 M Na2SO4 | ~0.19 | 0.41 | 8.32 × 1019 | This work |
WO3/BiVO4 | 0.5 M Na2SO4 | ~1.0 | — | — | [97] |
TiO2/BiVO4 (Ta:TiO2 NWs) | 0.5 M Na2SO4 | 2.1 | TiO2 ≈ 0.19; BiVO4 ≈ 0.08 | Ta:TiO2 ~1 × 1020 | [98] |
CuWO4/WO3/Ag | 0.5 M phosphate | 0.205 | -0.045 | ↑ vs. single phases | [100] |
WO3/Fe2O3 (nanorods, PVD/RF) | 0.1 M phosphate | 0.588 | 0.49 | ~9.57 × 1020 | [99] |
Cu2O/TiO2 (p–n) | 1 M NaOH | 0.139 | TiO2: 0.76; Cu2O: 0.54; Cu2/TiO2 –0.72; | Cu2O: 5.36 × 1018; TiO2: 1.06 × 1019; TiO2+C-NW: 1.30 × 1019 | [101] |
BiVO4/ZnO (3D bicontinuous) | 0.5 M Na2SO4 | 4.1 | — | — | [102] |
Fe2O3/TiO2 | 1 M NaOH | ~5.8 | — | — | [103] |
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Araujo, L.D.; Sartori, B.; Torres, M.D.M.; Graves, D.A.; Botan-Neto, B.D.; Murase, M.S.W.; Neto, N.F.A.; Leite, D.M.G.; Pessoa, R.S.; da Silva Sobrinho, A.S.; et al. Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights. Nanomanufacturing 2025, 5, 15. https://doi.org/10.3390/nanomanufacturing5040015
Araujo LD, Sartori B, Torres MDM, Graves DA, Botan-Neto BD, Murase MSW, Neto NFA, Leite DMG, Pessoa RS, da Silva Sobrinho AS, et al. Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights. Nanomanufacturing. 2025; 5(4):15. https://doi.org/10.3390/nanomanufacturing5040015
Chicago/Turabian StyleAraujo, Lucas Diniz, Bianca Sartori, Matheus Damião Machado Torres, David Alexandro Graves, Benedito Donizeti Botan-Neto, Mariane Satomi Weber Murase, Nilton Francelosi Azevedo Neto, Douglas Marcel Gonçalves Leite, Rodrigo Sávio Pessoa, Argemiro Soares da Silva Sobrinho, and et al. 2025. "Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights" Nanomanufacturing 5, no. 4: 15. https://doi.org/10.3390/nanomanufacturing5040015
APA StyleAraujo, L. D., Sartori, B., Torres, M. D. M., Graves, D. A., Botan-Neto, B. D., Murase, M. S. W., Neto, N. F. A., Leite, D. M. G., Pessoa, R. S., da Silva Sobrinho, A. S., & Pereira, A. L. J. (2025). Tuning Optical and Photoelectrochemical Properties of TiO2/WOx Heterostructures by Reactive Sputtering: Thickness-Dependent Insights. Nanomanufacturing, 5(4), 15. https://doi.org/10.3390/nanomanufacturing5040015