Post‐Illumination Photoconductivity Enables Extension of Photo‐Catalysis after Sunset

Abstract

Cloud‐cover‐induced frequent and sharp dips in sunlight as well as diminished solar flux during the evenings of peak energy demand are major challenges in solar energy harvesting. Persistent and memory‐based photocatalysts that efficiently operate under low light fluxes beyond sunset, are a potential solution to address and mitigate these challenges. This review describes examples of persistent photocatalysis systems based on charge injection into multivalent charge storage materials that allow post‐illumination discharging and charge carrier generation that increase or maintain the catalysis rate. Persistent photocatalysis in defect‐laden charge storage materials with multivalent states combined with slow charge release associated with electronic persistent photoconductivity results in giant persistent photocatalysis that can last more than an hour after the illumination is shut off. Strategies are suggested to develop persistent photocatalysis by improved charge separation and present figures of merit for evaluating persistent photocatalysis efficiency and performance. Furthermore, this could enable the use of such material systems in environmental applications such as photocatalytic coatings for the remediation of air pollution that continue to function into the night and self‐cleaning applications that continue to disinfect during the night.

Photoirradiation-Induced Capacitance Enhancement in the h-WO3/Bi2WO6 Submicron Rod Heterostructure under Simulated Solar Illumination and Its Postillumination Capacitance Enhancement Retainment from a Photocatalytic Memory Effect

“…For an n-type semiconductor, the electric field in its surface space-charge region could cause photogenerated electrons to be swept away from the surface and excess photogenerated holes to be swept toward the surface, which subsequently reduces the surface-trapped electron density and increases the surface positive charge . For the h -WO 3 /Bi 2 WO 6 -20-1 sample, however, the localization of photogenerated electrons could reduce W 6+ to W 5+ and generate W 5+ band states below the conduction band edge . Thus, photogenerated electrons could transfer from its valence band into W 5+ band states, which subsequently increases the surface negative charge and causes its SPV signal to move to a less positive direction.…”

“…49 For the h-WO 3 /Bi 2 WO 6 -20-1 sample, however, the localization of photogenerated electrons could reduce W 6+ to W 5+ and generate W 5+ band states below the conduction band edge. 26 Thus, photogenerated electrons could transfer from its valence band into W 5+ band states, which subsequently increases the surface negative charge and causes its SPV signal to move to a less positive direction. Thus, the much weaker SPV signal of the h-WO 3 /Bi 2 WO 6 -20-1 sample indicated that more photo-generated electrons were transferred to W 5+ band states than that of the h-WO 3 sample.…”

“…To achieve the photocatalytic memory effect with an extended period of time, two working mechanisms should be combined in the photocatalyst system with both an oxidative state-based storage to build up a reservoir of charges and a kinetically slow charge release. 26 In these photocatalyst systems, photogenerated electrons/holes could be efficiently separated with proper material design strategies to enhance their photocatalytic performances under illumination, while part of them could in the meantime be stored in the energy-storing components of these photocatalysts. After the illumination was switched off, these stored charges could then be gradually released for a relatively long time to further generate radicals in the dark to keep their activities.…”

“…In recent years, a postillumination photocatalytic memory effect had been found in various semiconductor-based photocatalyst systems, which demonstrated persistent activity in the dark after the illumination was switched off for an extended period of time. − Thus, it could solve the lost intrinsic activity problem of traditional photocatalysts when the illumination was switched off and open up previously unexplored areas for photocatalysts, especially in environmental applications. To achieve the photocatalytic memory effect with an extended period of time, two working mechanisms should be combined in the photocatalyst system with both an oxidative state-based storage to build up a reservoir of charges and a kinetically slow charge release . In these photocatalyst systems, photogenerated electrons/holes could be efficiently separated with proper material design strategies to enhance their photocatalytic performances under illumination, while part of them could in the meantime be stored in the energy-storing components of these photocatalysts.…”

“…Subsequently, more photogenerated carriers could participate in the charging and discharging processes of the h -WO 3 /Bi 2 WO 6 supercapacitor to improve its capacitance tremendously (∼57% determined by the CV curve measurement at a scan rate of 1 mV/s) under simulated solar illumination. Furthermore, the h -WO 3 /Bi 2 WO 6 material system meets the requirement for potential photocatalysts with the photocatalytic memory effect, − which resulted in its desired capability to retain the photoinduced capacitance enhancement in the dark for a relatively long time. Even after the illumination was switched off for 5 h, it still retained ∼16% of its photoirradiation-enhanced capacitance.…”