Mechanism of Laser-Induced Bulk and Surface Defect Generation in ZnO and TiO₂ Nanoparticles: Effect on Photoelectrochemical Performance

Abstract

Laser processing of neat and gold-nanoparticle-functionalized ZnO and TiO2 nanoparticles by nanosecond-355-nm or picosecond-532-nm light enabled control of photocurrent generation under simulated sunlight irradiation in neutral aqueous electrolytes. We obtained more than twofold enhanced photoelectrochemical performance of TiO2 nanoparticles upon irradiation by picosecond-532-nm pulses that healed defects. Laser processing and gold nanoparticle functionalization of ZnO and TiO2 nanomaterials resulted in color changes that did not originate from optical bandgaps or crystal structures. Two-dimensional photoluminescence data allowed us to differentiate and quantify surface and bulk defects that play a critical yet oft-underappreciated role for photoelectrochemical performance as sites for detrimental carrier recombination. We developed a detailed mechanistic model of how surface and bulk defects were generated as a function of laser processing parameters and obtained key insights on how these defects affected photocurrent production. The controlled healing of defects by pulsed-laser processing may be useful in the design of solar fuels materials. Pulsed lasers are powerful tools for the time-efficient preparation and/or modification of functional materials. 14-21 Recent investigations have shown that laser-modified TiO2 particles can be used to improve the light-driven water splitting to form hydrogen 22 or

Perspective of Surfactant‐Free Colloidal Nanoparticles in Heterogeneous Catalysis

“…In case ps‐532 nm laser pulses were applied, strong quenching of the photocurrent is observed with increasing Au NPs mass loading while nearly no effect of the Au NPs mass load is observed with ns‐355 nm pulses. Similar results were found in case of Au/ZnO catalysts in the same study . The latter is explained with a dominant formation of surface‐near defects in case of ps‐532 nm pulses, with Au NPs acting as main absorption centers due to the strong surface plasmon resonance near 532 nm .…”

“…Regardless of the deposition efficiency‐structure‐correlation not yet being fully understood in the electrostatic repulsive regime, a broad variety of laser‐generated catalysts (including adsorbed multi‐metallic nanoparticles) is available to date especially when employing the electrostatic attraction regime . Representative examples being available for studies in photo‐ (Figure a, b, f), redox‐ (Figure c, d) and electrocatalysis (Figure e) are shown in Figure and explained below. The presented strategy enables a general concept for the quantitative deposition of surfactant‐free NPs on support materials in high loadings (up to more than 60 wt%) and material yields (up to 100 %) without altering preadjusted support materials due to nanoparticle formation processes.…”

“…Copyright 2017 American Chemical Society. f) AuNP on TiO 2 and AuNP on ZnO after pulsed laser post‐processing (PLPP) inducing a defined number of pulses per particle and cycle in water for fundamental photocatalysis studies . The abbreviations “ns” and “ps” mark the sample series treated with nanosecond, 355 nm or picosecond, 532 nm laser pulses, respectively.…”

“…al aimed to gain more insight into the laser‐based alteration of the defect structure as well as its impact on different catalytic reactions. In their recent study, TiO 2 /P25 (as well as ZnO) was loaded with 1 wt% and 3 wt% of Au NPs, respectively, treated with picosecond 532 nm and nanosecond 355 nm laser pulses in a liquid jet setup (similar to Figure c), as described in Refs [107,256]. The setup allows to precisely correlate the applied mass‐specific laser energy and potentially change the catalyst properties (see Figure f) with the number of laser irradiation cycles (see Figure ).…”

“…The latter is explained with a dominant formation of surface‐near defects in case of ps‐532 nm pulses, with Au NPs acting as main absorption centers due to the strong surface plasmon resonance near 532 nm . In turn, ns‐355 nm laser pulses are absorbed throughout the whole Au/TiO 2 catalyst mainly causing bulk defects due to high thermal load and subsequent isochoric melting …”