Transient Absorption Microscopy Maps Spatial Heterogeneity and Distinct Chemical Environments in Photocatalytic Carbon Nitride Particles

Abstract

Limitations in solar energy conversion by photocatalysis typically stem from poor underlying charge carrier properties. Transient Absorption (TA) reveals insights on key photocatalytic properties such as charge carrier lifetimes and trapping. However, on the microsecond timescale, these measurements use relatively large probe sizes ranging in millimetres to centimetres which averages the effect of spatial heterogeneity at smaller length scales. A home-built Transient Absorption Microscopy (TAM) setup is reported and used to study single particles of carbon nitride (CN_x), an emerging photocatalyst. For the first time, to the best of the authors' knowledge, µs-s timescales are explored within individual particles to gain a more complete understanding of their photophysics. The dynamics of trapped charges are monitored, enabling measurement and quantification of heterogeneity in the transient absorptance signal of individual CN_x particles and within them. Particle-to-particle heterogeneity in the trapped charge density is observed, while spatial heterogeneity in lifetimes within a particle is revealed using a smaller probe beam with a ≈5 µm diameter. Overall, the observations suggest that contributions from different local environments independently influence charge trapping at different timescales. TAM on the micron and microsecond spatiotemporal resolution will aid in tackling design questions about optimal chemical environments for the promotion of photoactivity.

Keywords: carbon nitride; charge carrier dynamics; photocatalysis; transient absorption microscopy.

Figure 1

a) Components and setup of the µs‐DR‐TAM used for our experiments including two probe CW light sources: Broadband light (Option 1) and laser diode light source (Option 2). b) Microscopic image of a single CN_x particle (scale bar 100 µm). c) TA signal from a single CN_x particle measured. Modeled beam profiles for d) broadband light and e) laser diode light source. Abbreviations in the schematic: THG – Third‐Harmonic Generator, OF – Optical Filter, Off‐axis PM – off–axis parabolic mirror, Mono. – Monochromator, PD – photodetector, Refl. Obj. – Reflective Objective.

Figure 2

a) Plots of TA signals from two different particles when probed using the broadband light source (red and blue traces), error bars are shown as the shaded regions and are generated using standard deviations from triplicate measurements. b) absorptance(10 µs) versus area of particle A) t_50% versus area of particle.

Figure 3

Plots of TA decays at two different spots (red and blue traces) measured with a) broadband light probe on particle A, b) laser diode probe on particle A, and c) laser diode probe on particle B. The t_50% values have been included in each plot in the respective trace color.

Figure 4

Plots of TAM maps of 100 µm × 100 µm areas using different representations. Combined false color images representing absorptance(10 µs) as brightness and t_50% as color for a) particle A and d) particle B. t_50% values represented using blue to red colors for b) particle A and e) particle B. Absorptance(10 µs) values represented using a grayscale for c) particle A and f) particle B. Each square pixel has a side length of 25 µm and the size of the probe (5 µm FWHM) with which each pixel was measured is shown as dotted circles.

Figure 5

Schematic showing two sets of chemical environments (shown as sets of blue–purple and orange–red regions) each influencing one parameter (absorptance(10 µs) or t_50%) for both particles (A and B). Each of the circular spots represents the 250 µm FWHM spot. The yellow grid represents the dimensions of the TAM maps (100 µm × 100 µm) constructed using the 5 µm FWHM laser diode measurements. For simplicity, the four colors represent defects that give rise to extreme values (high and low) of both parameters.