Light-Intensity-Dependent Semiconductor–Cocatalyst Interfacial Electron Transfer: A Dilemma of Sunlight-Driven Photocatalysis

Abstract

In photocatalytic reactions, the interfacial transfer of electrons from semiconductor nanostructures to cocatalysts is the key step that determines the utilization of photogenerated charges and is sensitively influenced by the behaviors of this electronic process. Under weak illumination, photocatalytic reaction rates deviate from linearity to incident light intensity (r = k ss·P inc α, with α → 0.5), because charge recombination predominates interfacial transfer. When the irradiation intensity is high, theoretically, thermionic emission would be the major electronic process (r = k te·P inc α, with α → 2). The ratio of photocatalytic reaction rate to incident light intensity that mainly reflects the energy utilization would encounter a minimum along the variation of irradiation intensity. This crucial relationship, however, has hardly been consciously considered. In this work, inspired by theoretical simulation, we demonstrate that sunlight-driven photocatalysis is generally on the bottom of the energy utilization curves for certain common semiconductors (CdS, TiO2, or C3N4).

Saw-Tooth Heat-Cycling Nitridation of Metallic Cu Yields First Photoactive p-Cu3N for PEC Applications

“…The J ph measured at 9.2 sun was three times larger than that produced at 1.1 sun. In general, the J ph follows a power-law dependence with P light ( J ph ∝ P light α ), where α = 1 means full utilization of incident photons into photocurrent. , Accordingly, J ph values were taken at 0.62 V RHE for different light intensities and plotted versus P light on a logarithmic scale. The data were well fitted using a power law that yields an exponent of α = 0.595.…”

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

“…Semiconductor–metal (SM) interfacial electron transfer is thermodynamically favorable; however, the formed potential barrier severely slows this electronic process (Figure ). , Generally, it is assumed that SM interfacial electron transfer can occur on a picosecond to microsecond time scale. ,− This time scale, however, is unreasonably small. For many semiconductors, hole extraction is far faster than bulk recombination. − This means that nearly all of the photogenerated holes can be separated from the semiconductor and the remaining electrons can survive on a millisecond to second or larger time scale.…”

“…SM interfacial electron transfer generally includes thermionic emission and trap state intermediate charge recombination that are connected in parallel (Figure ). The rate ( r iet ) of SM interfacial electron transfer can be given by because SM interfacial electron transfer is the rate-determining step, the photocatalytic utilization ( U ), which, unfortunately, exhibits a minimal value near the intensity of sunlight for some common systems (Figure S2), can be approximately given by where k te and k cr are the pseudo-rate constants for thermionic emission and charge recombination, respectively, after separation of the variable of incident photons ( P inc ). Thermionic emission, which numerically and also physically is the major approach for SM interfacial electron transfer under intense irradiation, displays inert or even negative correlation with temperature .…”

“…The rate ( r iet ) of SM interfacial electron transfer can be given by because SM interfacial electron transfer is the rate-determining step, the photocatalytic utilization ( U ), which, unfortunately, exhibits a minimal value near the intensity of sunlight for some common systems (Figure S2), can be approximately given by where k te and k cr are the pseudo-rate constants for thermionic emission and charge recombination, respectively, after separation of the variable of incident photons ( P inc ). Thermionic emission, which numerically and also physically is the major approach for SM interfacial electron transfer under intense irradiation, displays inert or even negative correlation with temperature . In contrast, charge recombination, which dominates the SM interfacial electronic process under weak illumination, exhibits an evident positive correlation with temperature .…”

“…Semiconductor−metal (SM) interfacial electron transfer is thermodynamically favorable; however, the formed potential barrier severely slows this electronic process (Figure 1). 22,23 Generally, it is assumed that SM interfacial electron transfer can occur on a picosecond to microsecond time scale. 21,24−28 This time scale, however, is unreasonably small.…”