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

Abstract

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.

Photocatalyst: To Be Dispersed or To Be Immobilized? The Crucial Role of Electron Transport in Photocatalytic Fixed Bed Reaction

“…Recently, we found that semiconductor–cocatalyst (SC) interfacial electron transfer in photocatalytic reactions occurs on a decisecond–second time scale (Figures a,d, , and S2). ,,− This value is 6–12 orders of magnitude larger than the widely believed picosecond–microsecond time scale and is comparable to the that of >1 s predicted by Durrant et al , This finding not only reveals that SC interfacial electron transfer is the rate-determining step in photocatalytic reactions but also provides rational elucidation to the very low utilization of incident photons . Interestingly, the time constants for SC interfacial electron transfer occurring in a fixed bed reaction is comparable to that in a slurry suspension (Figures and S2).…”

“…For many photocatalytic reactions of integrated HE with organic substance oxidation, the concentration of organic solute can be >1 mol/L and the maximal flux of reactant can be >2 × 10 –6 mol cm –2 s –1 , which is at least 1 order of magnitude larger than the photocatalytic reaction rate in the level of 10 –7 mol cm –2 s –1 under intense irradiation. The time scale for diffusion of reactant(s) can be only 0.1–0.5 s (Figure b), which is mainly smaller than the range from several deciseconds to several seconds for SC interfacial electron transfer (Figures d and ). ,,− ,, This suggests that mass transport cannot be an influencing factor in the photocatalytic fixed bed reactions.…”

“…The time (τ tp ) for transport of electrons through TiO 2 film is only 1.76 ms (Figure c), which is far smaller than the decisecond–second time scale for SC interfacial electron transfer (Figures and S2). ,,− These features suggest that transport of electrons in the TiO 2 films does not slow the reaction in the photocatalytic fixed bed reactors.…”

“…The time scale for diffusion of reactant(s) can be only 0.1−0.5 s (Figure 4b), which is mainly smaller than the range from several deciseconds to several seconds for SC interfacial electron transfer (Figures 2d and 3). 6,17,[28][29][30]32,35 This suggests that mass transport cannot be an influencing factor in the photocatalytic fixed bed reactions. Now let us discuss the electron transport.…”

Revealing the role of surface elementary doping in photocatalysis

“…[13][14][15] This measure can modify the electronic states that influence the chemical activity of the semiconductor surface, [16][17][18] and the electronic processes including electron transport in the interior of the semiconductor and interfacial transfer of electrons to the cocatalysts. 19,20 Since the increase in the density of electronic states will undesirably prolong the detrapping time during electron transport and aggravate the charge leakage, 16,[21][22][23][24][25][26][27][28][29] it is desired to facilitate the rate-determining step in the forward reaction without exacerbating the detrimental effect by accurate elementary doping.…”