Accurate exploration of synergistic thermal effect: Low-grade-heat-promoted charge recombination coupled with spherical incidence for efficient sunlight-driven photocatalysis.

The major objective of the emerging photo-thermo-catalysis is using waste heat to boost the photocatalytic reaction, especially that powered by sunlight. Because of the complex composition of light-intensity-dependent apparent activation energies, the issue that principally hinders the synergistic thermal effect to photocatalysis has hardly been accurately explored. In this work, by virtue of mutual match of theoretical simulation and experimental behaviors, we demonstrate that photocatalytic reaction rates exhibit a sensitively positive correlation with temperature under weak illumination, in which charge recombination predominates the rate-determining step of semiconductor-cocatalyst interfacial electron transfer. Under high-intensity irradiation, however, the aggravation of charge leakage inherently accompanied by thermionic emission severely weakens the synergistic thermal effect or even slows down the reaction by raising the temperature. Inspired by these, we manage to maximize the photocatalytic solar utilization by spherical incidence of sunlight with the assistance of low-grade heat.

A straightforward approach utilizing an exponential model to compensate for turbidity in chemical oxygen demand measurements using UV-vis spectrometry.

Recently, ultraviolet-visible (UV-vis) absorption spectrometry has garnered considerable attention because it enables real-time and unpolluted detection of chemical oxygen demand (COD) and plays a crucial role in the early warning of emerging organic contaminants. However, the accuracy of detection is inevitably constrained by the co-absorption of organic pollutants and turbidity at UV wavelengths. To ensure accurate detection of COD, it is necessary to directly subtract the absorbance caused by turbidity from the overlaid spectrum using the principle of superposition. The absorbance of COD is confined to the UV range, whereas that of turbidity extends across the entire UV-vis spectrum. Therefore, based on its visible absorbance, the UV absorbance of turbidity can be predicted. In this way, the compensation for turbidity is achieved by subtracting the predicted absorbance from the overlaid spectrum. Herein, a straightforward yet robust exponential model was employed based on this principle to predict the corresponding absorbance of turbidity at UV wavelengths. The model was used to analyze the overlaid absorption spectra of synthetic water samples containing COD and turbidity. The partial least squares (PLS) method was employed to predict the COD concentrations in synthetic water samples based on the compensated spectra, and the corresponding root mean square error (RMSE) values were recorded. The results indicated that the processed spectra yielded a considerably lower RMSE value (9.51) than the unprocessed spectra (29.9). Furthermore, the exponential model outperformed existing turbidity compensation models, including the Lambert-Beer law-based model (RMSE = 12.53) and multiple-scattering cluster method (RMSE = 79.34). Several wastewater samples were also analyzed to evaluate the applicability of the exponential model to natural water. UV analysis yielded undesirable results owing to filtration procedures. However, the consistency between the compensated spectra and filtered wastewater samples in the visible range demonstrated that the model is applicable to natural water. Therefore, this proposed method is advantageous for improving the accuracy of COD measurement in turbid water.

Revealing the Role of Electronic Doping for Developing Cocatalyst-Free Semiconducting Photocatalysts.

Developing cocatalyst-free photocatalysts is highly desired because it could avoid the very slow interfacial electron transfer that makes photocatalytic photon utilization a dilemma. However, even in the optimal case, photocatalysts without the use of cocatalysts deliver comparable performance only for conventional construction. We demonstrate here that electronic doping not only provides catalytically active sites in cocatalyst-free photocatalysts but also plays certain additional roles. These electronic states can efficiently channel the trapped electrons to the semiconductor surface without suffering from time-consuming detrapping and can facilitate the extraction of photogenerated holes. These features endow our demonstrated tungsten-doped CdS with evident superiority in photocatalytic performance over conventional counterparts loaded with platinum cocatalysts.

Revealing and Facilitating the Rate-Determining Step for Efficient Sunlight-Driven Photocatalysis.

Because of the complex composition of the apparent activation energy, the rate-determining step in a photocatalytic reaction like hydrogen evolution is still being explored even after sluggish oxygen evolution is replaced with efficient hole extraction. This issue severely limits the implementation of certain strategies like the synergistic thermal effect. Here, by developing a combined monitor method based on open-circuit potential decay, we demonstrate that semiconductor-cocatalyst interfacial electron transfer occurring on a decisecond to second time scale dominates photocatalytic hydrogen evolution. This time scale is approximately 6-12 orders of magnitude larger than the widely reported values of picoseconds to microseconds and is comparable to that predicted by Durrant et al. To improve photocatalytic hydrogen evolution, we manage to create more intermediate sites by electronically doping the semiconductor surface. This measure promotes semiconductor-cocatalyst interfacial electron transfer by charge recombination and makes the synergistic thermal effect very evident.