Aerobic Conditions Enhance the Photocatalytic Stability of CdS/CdOx Quantum Dots

Abstract Photocatalytic H2 production through water splitting represents an attractive route to generate a renewable fuel. These systems are typically limited to anaerobic conditions due to the inhibiting effects of O2. Here, we report that sacrificial H2 evolution with CdS quantum dots does not necessarily suffer from O2 inhibition and can even be stabilised under aerobic conditions. The introduction of O2 prevents a key inactivation pathway of CdS (over‐accumulation of metallic Cd and particle agglomeration) and thereby affords particles with higher stability. These findings represent a possibility to exploit the O2 reduction reaction to inhibit deactivation, rather than catalysis, offering a strategy to stabilise photocatalysts that suffer from similar degradation reactions.


Synthesis of ligand-free CdS quantum dots.
Ligand stripping of CdS-OA was carried out according to a literature procedure to form ligand-free CdS QDs (CdS-BF4). [2,3] CdS-OA in hexane (5 mL) was reduced to dryness and re-dispersed in a mixture of anhydrous CHCl3 (15 mL) and anhydrous DMF (1.9 mL) under a N2 atmosphere. The solution was then stirred in the presence of a 9 mL Et3OBF4 (1.0 M in acetonitrile) for 1 hour. Me3OBF4 (1 M in hexane) was added slowly until the particles precipitated (c.a. 3 mL). The precipitate was collected by centrifugation (6000 RPM, 3 min), and re-dispersed in a minimum of DMF and kept < 5°C. The QD concentration was determined from UV-visible spectroscopy based on the position and absorbance of the absorption maximum around 450 nm. [4] Full characterisation details (XRD, UV-visible absorption) can be found elsewhere. [3,5] Photocatalytic H2 evolution. Photocatalysis was carried out in a pyrex photoreactor thermostated at 25 °C. Solar irradiation was emulated by a solar light simulator (Newport Oriel, adjusted to irradiate 100 mW cm -2 photon intensity onto the photoreactor) equipped with an air mass 1.5 global (AM 1.5G) filter and a water filter (10 cm path length) to remove IR radiation.
In a typical experiment CdS-BF4 QDs in DMF (1 nmol) were transferred to a photoreactor and the DMF was removed at room temperature in vacuo while stirring. The electron donor and 2 mL of various concentrations of KOH were then added followed by 10 µL of an aqueous Co(BF4)2 solution (50 mM). The photoreactor was sealed with a rubber septum and purged with the desired gas mixture for 10 min, after which the vial was irradiated whilst stirring at 600 RPM. If aerobic conditions were required, the samples were left unpurged. To guarantee safe operation in the presence of O2, we employed a small photoreactor and kept the photoreactor away from sources of ignition at all times. The accumulation of H2 was quantified through periodic headspace gas analysis (50 μL) by gas chromatography. Experiments were repeated in triplicate and are given as a mean ± standard deviation, calculated as described below.
Photocatalysis under a constant flow of N2 or air was undertaken in the photocatalytic set-up described above with a gas inlet into the reaction solution of the photoreactor. Gas was purged into the vial at a constant flow of 3 mL -1 min -1 using a mass flow controller (Brooks Instruments). An outlet from the headspace provided a constant measure of the H2 evolution via gas chromatography.
Treatment of data. All analytical measurements were performed in triplicate and are given as the unweighted mean ± standard deviation (σ). σ of a measured value was calculated using equation (1).
Where n is the number of repeated measurements, x is the value of a single measurement and ̅ is the unweighted mean of the measurements.
The activity per weight of catalyst (mol H 2 g cat. -1 h -1 ) was calculated using equation 2 from the molar weight of the QD and co-catalyst.
Activity (mol H 2 g cat. Where n H 2 is the H2 produced (mol), rCdS is the radius of the QD (cm), ρCdS is the density of CdS (4.84 g cm −3 ), Na is Avogadro's number (mol −1 ), nQD is the number of moles of QD (mol), nco-cat. is the number of moles of added co-catalyst, Mco-cat. is the mass of co-catalyst (taken as 232.54 g mol -1 for Co(BF4)2), tirr. is the irradiation time of the sample (h).
Gas analysis. Gas chromatography (GC) was carried out on an Agilent 7890A gas chromatograph with a thermal conductivity detector. H2 was analysed using a HP-5 molecular sieve column (0.32 mm diameter) and N2 carrier gas with a flow rate of approximately 3 mL min −1 . The GC oven temperature was kept at 45 °C in both cases. Methane (2% CH4 in N2) was used as internal standard or external standard after calibration with different mixtures of known amounts of H2/O2/CH4. Transient absorption spectroscopy. TA spectra were obtained using an Ultrafast Systems Helios TA system with a Coherent Libra amplified Ti:sapphire laser system and Coherent OPerA optical parametric amplifier (OPA) pump/probe source. The samples were excited with ∼50-fs laser pulses generated by the OPA at a repetition rate of 1.1 kHz. TA spectra were obtained by time-delaying a broadband supercontinuum probe pulse that is overlapped in time and space with the femtosecond pump pulse. The supercontinuum is produced by focusing a small portion of the amplified laser fundamental into a sapphire plate. Multiwavelength TA spectra were recorded using dual spectrometers (signal and reference) equipped with fast Si array detectors. In all experiments, the excitation was achieved with 430 nm light at a power of 1 μJ. The data was fit with a multi-exponential decay function: I(t) = A1*e^(-k1*t) + A2*e^(-k2*t)… + baseline.
Raman analysis. Raman spectra were recorded using a confocal Raman spectrometer LabRam (Horiba Jobin Yvon) equipped with a liquid-nitrogen-cooled Symphony CCD detector (Horiba Jobin Yvon). A 514.73 nm line of an Ar ion laser (Coherent Innova 300c) was used as excitation wavelength. 10 µM solutions of CdS-BF4 QDs in 10 M KOH were measured in a rotating quartz cuvette with and without added EtOH. Accumulation time of the Raman spectra was 60 seconds. The laser power was set to 5 mW. The laser light was focused into the solution using a 20x objective (Nikon, 20x NA 0.5). For excitation, the 413 nm laser line of the Kr ion laser (Sabre) was used.