Multilayered WO₃ Nanoplatelets for Efficient Photoelectrochemical Water Splitting: The Role of the Annealing Ramp

Abstract

Multilayered WO 3 nanosquare platelet films were successfully grown on transparent TCO substrates by spray-coating WO 3 nanoparticles aqueous suspension prepared by the sol−gel method. This work assesses the influence of two annealing schemes in the photoresponse of WO 3 photoelectrodes with different film thicknesses. The photoelectrochemical characterization reveals that the slow-heating ramp produces a photoelectrode with an improved photocurrent density of 1.6 mA cm −2 at 1.23 V vs RHE. Comparing photoelectrodes with the same film thickness, the slow-heating ramp yields higher photocurrent densities, 80% more than the conventional fast-heating ramp. The effect of the annealing ramp on the morphology and crystalline-phase structure of WO 3 photoelectrodes is correlated with the photocurrent density. The slow-heating ramp annealing unveils film morphology with both higher porosity degree and higher nanosquare platelets dimensions. X-ray diffraction (XRD) structural analyses disclose that the films grow in monoclinic crystalline phase with a textural preferential direction [002], often related to improved photocurrent performances. The crystallite sizes and lattice microstrain are estimated using a simple X-ray diffraction broadening method, the Williamson-Hall analysis. A quantified correlation between the WO 3 lattice defects, intergrain strains, and performance is done. The proposed deposition method paves the way for producing efficient and scalable photoelectrodes of WO 3 for photoelectrochemical water splitting by using low-cost and simple manufacturing processes.

Preparation by electrophoretic deposition of molybdenum iodide cluster-based functional nanostructured photoelectrodes for solar cells

“…that absorbs light; once excited, charges are injected in the external circuit of the cell if the electron-hole recombination phenomena at interfaces are minimized. [1,[3][4][5][6][7][8][9][10][11][12][13][14][15][19][20][21]23,24,26,28,29,[32][33][34] The mediator redox (typically the I -/I 3 redox couple) has an electron shuttle role. It ensures the charge transport between the two electrodes (photoanode/counter-electrode or photoanode/photocathode) in the electrolyte (classically based on a nitrile solvent).…”

Charge Separation, Band-Bending, and Recombination in WO3 Photoanodes

“…25 This performance is also an outlier, with more publications reporting photocurrents closer to ~10-20% of the theoretical limit, and quantum yields far lower than 1, indicating severe recombination losses. 24,[26][27][28][29][30][31] Using transient absorption spectroscopy, we demonstrate herein that space-charge layer formation, achieved through electrolyte contact or applied bias in a three electrode cell, can significantly retard the ultrafast recombination processes that largely determine the efficiency of photoelectrochemical and photocatalytic performance. Figure 1a shows the transient absorption (TA) spectra of WO3 immersed in electrolyte (0.1 M H2SO4) in the absence of applied bias at different time delays after exciting with a 355 nm laser.…”

“…This performance is also an outlier, with more publications reporting photocurrents closer to ∼10–20% of the theoretical limit and quantum yields far lower than 1, indicating severe recombination losses (Figure S2). ,− Using transient absorption spectroscopy (TAS), we demonstrate herein that SCL formation, achieved through electrolyte contact or applied bias in a three-electrode cell, can significantly retard the ultrafast recombination processes that largely determine the efficiency of photoelectrochemical and photocatalytic performance.…”

Photoelectrochemical Water Splitting: Thermal Annealing Challenges on Hematite Nanowires

“…The NWs thickness ( L ) was determined from the SEM cross-sectional images (Figure S2). According to the literature, , it is expected that a reduced thickness makes the electrons’ pathway to reach the charge collector easier (due to the shorter distance), with less probability of electron–hole recombination. In this work, it was observed that L slightly decreases with the annealing temperature (from 330 ± 50 nm to 285 ± 37 nm for 550 and 800 °C, respectively).…”

“…A small narrowing of the peak is observed as the annealing temperature and time increase, more evident in the set of different temperatures. The meaning of the XRD peak broadening is indicative of grain size refinement, but it is also related to other structural disorders, such as dislocations, microstrain, or local strain heterogeneity. ,, From peak broadening analysis, crystallite size ( D XRD ) and microstrain (ε) were estimated, using the Williamson–Hall method: where w is the full width at half-maximum (fwhm) of Bragg peaks, θ is the diffraction angle, k is the Scherrer constant (0.94), and λ is the incident X-ray wavelength (1.540593 Å). From w ·cos­(θ) versus 4·sin­(θ) plots, D XRD and ε were estimated from the intercept and slope, respectively (Figure S4).…”