Temporally Distinguishing Photocatalytic Dynamics

Light can induce both photothermal and nonthermal catalytic activity in plasmonic nanoparticles, but the extent, timescale, and efficacy of these two mechanisms remain unresolved. Here, we introduce pump–pump photocatalysis, an adaptation of ultrafast excitation correlation spectroscopy in which incident laser pulses are split into two spatially and energetically equivalent pulses separated by a variable time delay. For a given reaction and catalyst, time-sensitive nonlinear enhancements in photocatalytic activity may be distinguished from time-insensitive responses as a function of time delay, pulse power, excitation wavelength, applied temperature, reactant pressure, and pulse asymmetry. In this way, the timescales of photothermal and nonthermal activity of any photocatalyst may be monitored on sub-picosecond to nanosecond timescales, and the conditions for optimal chemical reactivity may be discovered. To illustrate this technique, we perform ultrafast measurements of the ammonia decomposition reaction using a Cu–Ru antenna-reactor photocatalyst. Photothermal contributions exhibit little sensitivity to pulse delay, while nonthermal contributions are most apparent at low excitation intensity, moderate temperatures, and sub-nanosecond timescales. Here, nonlinear, nonthermal mechanisms enhance H2 production by a factor of up to eleven compared to when pulses overlap. This technique may be used to provide insightful in operando diagnostics and control of any photocatalyst for any chemical reaction while revealing the optimal distribution of light to maximize photocatalytic activity.