ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water

A robust precious metal-free photocatalyst system comprised of ligand-free ZnSe quantum dots and a phosphonic acid-functionalised Ni(cyclam) catalyst achieves efficient reduction of aqueous CO2 to CO.

Inductively-coupled plasma-optical emission spectroscopy (ICP-OES). ICP-OES was carried-out by the Microanalysis Services, Department of Chemistry, University of Cambridge using a Thermo Scientific iCAP 7400 spectrometer. Samples were digested in HNO 3 and diluted with ultrapure water to 1-10 ppm analyte. Blank samples of diluted HNO 3 were recorded as background.
External quantum efficiency (EQE). Photocatalysis samples were prepared as stated in the Experimental Section, but using an airtight, flat-sided quartz cuvette (1 cm path length) as the photoreactor. The cuvette was purged with CO 2 /CH 4 (2 %) and primed by irradiation for 2 h with a solar light simulator as stated above. The cuvette was then purged again with CO 2 /CH 4 (2 %) and irradiated with monochromatic light (λ = 400±5 nm, I = 1.0 mW cm -2 , A = 0.25 cm 2 ) using an LOT Quantum Design MSH-300 monochromator. Aliquots of headspace gas were taken periodically and analysed by gas chromatography. The EQE was calculated according to equation (2). EQE (%) = 2 × ×ℎ× irr × × × × 100 (2) Where n is the amount of produced CO or H 2 per time, N A is Avogadro's constant, h is the Planck constant, c is the speed of light, t irr is the irradiation time, λ is the irradiation wavelength, I is the irradiation intensity and A is the irradiated area.
Gas chromatography analysis. Gas chromatography was carried out on a Shimadzu Tracera GC-2010 Plus gas chromatograph kept at 130 °C using a barrier ionisation discharge (BID) detector and a molsieve column with He as the carrier gas. Methane (2 % CH 4 in CO 2 , BOC) was used as internal standard after calibration with different mixtures of known CH 4 /H 2 /CO compositions.
Infrared spectroscopy. IR spectra were recorded on a Thermo Scientific Nicolet iS50 FT-IR spectrometer. IR spectra of ZnSe-St and ZnSe-BF 4 were recorded in ATR mode by drying one drop of QD stock solution on an FTO-coated glass slide in vacuo.
Transmission electron microscopy (TEM). TEM images were collected using an FEI Phillips Technai F20 TEM, operating at an accelerating voltage of 200 kV located at the Electron Microscopy Suite of the Cavendish Laboratory, University of Cambridge.
Zeta potential. Zeta potential measurements of ZnSe-BF 4 (0.5 μM in water, pH adjusted to 5.5 with NaOH/HBF 4 ) in the presence of varying amounts of MEDA were conducted using a Malvern Zetasizer Nano ZS.
UV−Vis spectroscopy. UV−Vis spectra were recorded on a Varian Cary 50 UV−Vis spectrophotometer using quartz glass cuvettes (1 cm path length).
X-ray photoelectron spectroscopy. XPS spectra were recorded on an ESCALAB 250Xi located at the Optoelectronics group at the Cavendish Laboratory, University of Cambridge, operated by Chris Amey. Samples were prepared by drop-casting stock solutions of QDs on a Cu foil followed by drying in vacuo. The background of the spectra was subtracted and the spectra were subsequently fitted using PsdVoigt functions.
Treatment of data. All analytical measurements were performed in triplicate and are given as unweighted mean ± standard deviation (σ) unless otherwise stated. σ of a measured value was calculated using equation (3).
Where n is the number of repeated measurements, is the value of a single measurement and ̅ is the unweighted mean of the measurements. σ was increased to 5 % of ̅ in the event that the calculated σ was below this threshold. Lines between data points in Figures 3D, 4B and Figure S4 have been added to guide the eye.
Supporting Tables   Table S1. Attachment of different catalysts on ZnSe-BF4 based on ion-coupled plasma optical emission spectroscopy (ICP-OES). Samples (0.5 µM QD-BF4, 10 µM catalyst, in 26 mL 0.1 M aq. AA pH 5.5 under CO2) were stirred in the dark for 2 h, centrifuged and the precipitate digested in nitric acid.

Figure S2
. ATR-IR spectra of ZnSe quantum dots before (ZnSe-St) and after stripping (ZnSe-BF 4 ) and comparison with the spectra of DMF, zinc stearate and NaBF 4 . Signals assigned to residual stearate on ZnSe-BF 4 are highlighted with black arrows.  Figure S4. ATR-IR spectra of ZnSe-BF 4 modified with NiCycP. ZnSe-BF 4 QDs were incubated in aqueous NiCycP, washed with water to remove excess NiCycP and dried. Vertical arrows indicate bands assigned to adsorbed NiCycP by comparison with the spectra of blank ZnSe-BF 4 and neat NiCycP. Note the absence of B-F stretches (expected around 1000 cm -1 , cf. Figure S2), upon incubation of ZnSe-BF 4 in water. Figure S5. Long-term photocatalytic activity of ZnSe-BF 4 /NiCycP. Samples were re-purged with CO 2 after 20 h and 0.5 µM ZnSe-BF 4 , 10 µM NiCycP or nothing was added before irradiation was continued (0.5 µM QD, 10 µM NiCycP in 0.1 M aq. AA, pH 5.5 under CO 2 , 100 mW cm -2 , AM 1.5G, λ >400 nm, 25 °C).     The recovery of the bleach signal assigned to the trapped photoelectrons can be reasonably fitted to a minimum of a triexponential function (eq. 4).