Facet-Dependent Photoreduction on Single ZnO Crystals

Photocatalytic reactions occur at the crystal–solution interface, and hence specific crystal facet expression and surface defects can play an important role. Here we investigate the structure-related photoreduction at zinc oxide (ZnO) microparticles via integrated light and electron microscopy in combination with silver metal photodeposition. This enables a direct visualization of the photoreduction activity at specific crystallographic features. It is found that silver nanoparticle photodeposition on dumbbell-shaped crystals mainly takes place at the edges of O-terminated (0001̅) polar facets. In contrast, on ZnO microrods photodeposition is more homogeneously distributed with an increased activity at {101̅1̅} facets. Additional time-resolved measurements reveal a direct spatial link between the enhanced photoactivity and increased charge carrier lifetimes. These findings contradict previous observations based on indirect, bulk-scale experiments, assigning the highest photocatalytic activity to polar facets. The presented research demonstrates the need for advanced microscopy techniques to directly probe the location of photocatalytic activity.

The resulting crystals were thoroughly washed with milli Q water and dried overnight in an oven at 80 °C prior to use.

Synthesis ZnO microrods 2
14.9 g of ZnO(NO 3 ) 2 ·6H 2 O was dissolved in 50 mL of milli Q water at room temperature, resulting in a 0.5 M zinc nitrate aqueous solution. The pH of the solution was increased to pH 12 by adding dropwise a 1.5 M aqueous KOH solution, followed by vigorous stirring for 10 min. The white slurry mixture was transferred into a 30 mL stainless steel Teflon-lined autoclave and the hydrothermal reaction was conducted at 180 °C in an oven for 20 hours.

S3
After the reaction was completed, the suspension was cooled down and the final product was collected by pressure filtration. The white powder was thoroughly washed with milli Q water and dried overnight in an oven at 120 °C for 12 hours.
XRD spectra of the dumbbell and microrod samples Graph S1. XRD spectrum of the synthesized dumbbell crystal sample.
Graph S2. XRD spectrum of the synthesized microrod crystal sample.

Glass cleaning and sample preparation for iLEM experiments
The glass coverslips were cleaned with Milli-Q ultrapure water (Millipore) followed by thermal treatment at 460°C overnight. In order to spread the ZnO crystals uniformly over the glass surface, the coverslip was placed in an UV-ozone photo reactor (PR-100) for several minutes to make the surface more hydrophilic. A suspension of 1 mg/mL of the ZnO crystals in ultrapure water was shaken thoroughly and dispersed in an ultrasonic water bath for 1 minute. Then, 15 µL of this suspension was dropcasted on top of the glass coverslip, resulting in a thin and uniform layer of ZnO crystals after drying in the dark.

Photodeposition of silver nanoparticles on the ZnO crystal surface
After obtaining initial SEM micrographs on the ZnO crystal of interest, the sample chamber was vented to atmospheric pressure and a droplet of 1 mM aqueous silver nitrate was added on the top of the envisioned ZnO crystal on the glass coverslip. The ZnO photocatalyst was subsequently illuminated for 20 seconds with 365 nm UV light from an LED source outside the vacuum chamber coupled in via an optically transparent window in the chamber door.
Afterwards, the liquid was carefully removed before pumping down the EM chamber and recording new SEM images.

Glass cleaning and sample preparation for single-particle photoluminescence experiments
The glass coverslips were purchased from Matsunami Glass and cleaned by sonication in a 20% detergent solution (As One, Cleanace) for 6 hours, followed by five times sonication in warm water for 30 minutes. Finally, the coverslips were rinsed with Milli-Q ultrapure water (Millipore). 15 µL of the well-dispersed 0.5 mg/mL aqueous suspensions of the ZnO crystals S6 was drop-casted on the cleaned coverslip. The coverslip was dried in the dark to immobilize the particles on the surface.

Single-particle photoluminescence measurements by confocal microscopy
Single-particle photoluminescence images and decay profiles of the ZnO samples were recorded using an objective-scanning

Photoluminescence decay profile analysis
Since the decay profiles correspond to a biexponential decay, the amplitude weighted average lifetime (τ amp ) is determined as the average charge carrier lifetime along the length of the crystals. The analysis is performed using SymPhoTime (PicoQuant, Germany) and is based on the following formula: τ corresponds to the decay components and A to the respective amplitudes.
S7 Figure S1. SEM images of typical dumbbell-shaped ZnO microcrystals as prepared.  ZnO microrod, a more homogeneous silver nanoparticle photodeposition (inset a.1) is observed on the seemingly defect free crystal side faces (inset b.1). Figure S5. SEM images of typical rod-like ZnO crystals as prepared. S10 S11 Figure S6. Data of the photoluminescent lifetime experiments that are represented in Figure 3 in the main article. Data includes, from top left to bottom right, the transmission image, the PL lifetime image with the probed positions (Both included in the manuscript as well), the PL S12 lifetime image with the emission maxima for the probed positions, the measured lifetime decay data, the table with the fitted parameters and the obtained PL emission spectra with the wavelengths of the corresponding emission maxima