Cu2O-SnO2 core-shell nanocatalysts for the photo/electrochemical CO2 conversion: synthesis standardization.

The Cu-Sn-based catalysts are an effective and cheap alternative to precious metal-based catalysts for the photo-electrochemical reduction of CO2 to valuable products such as syngas and other chemicals. Cu oxides have been chosen for their behaviour and properties in CO2 reduction. Synthesis method for core-shell Cu2O-SnO2 was reported by Zhang et al. (2019) and it consists of Cu2O wet precipitation and then SnO2 impregnation. To obtain the Cu2O nanocubes are used: CuCl2, NaOH and C6H8O6 (L-Ascorbic Acid); after, for SnO2 impregnation, are employed SnCl4·5H2O and NaCl as reagents. To control the thickness of the SnO2 shell a Cu2O - SnCl4·5H2O molar ratio of five is set. The aim of the thesis is to standardize the synthesis to obtain a more scalable way to get these core-shell structures. Peristaltic pumps are used to deliver NaOH, L-ascorbic acid, NaCl and SnCl4·5H2O solutions. The velocity of delivery of NaOH and NaCl aqueous solutions are fixed, while L-ascorbic acid aqueous solution and SnCl4·5H2O solution in ethanol are delivered at different velocities. Then, copper (I) oxide nanocubes and Cu2O-SnO2 core-shell structures have been characterized in the same way. X-ray diffraction allows to recognize crystalline phases and to evaluate the crystallite size by using the Scherrer equation. From Diffuse reflectance UV-vis (DR-UV-vis) spectra, band gap of the material is collected. SEM images show the morphology of the nanocrystals and EDX gives information about atomic compositions. Surface area is estimated by BET analysis and ECSA (electrochemical surface area) test and XPS analysis are performed to obtain surface-sensitive information about concentration, to investigate the main oxidation state of the elements and to check the real Cu2O – SnO2 ratio, turned out to be three, due to tin losses already observed in the reference paper. Also, the yield of Cu2O production is calculated for each Cu2O synthesis. The photo-electrodes, produced by spray deposition of the catalyst-based ink over carbon paper, are used to perform photoelectrochemical tests in a two chambers-cell. The tests performed are: Cyclic Voltammetries and Linear Sweep Voltammetries (in CO2 and N2 atmosphere, both in dark conditions and under illumination of the electrode with visible light), Electrochemical Impedance Spectroscopy to study the features of the electrode surface; the products of the catalysis are collected and quantified. The reproducibility of the Cu2O synthesis is checked by the XRD analysis: the recognized phase is the Cu2O in every powder, although some samples present CuO traces. The principal growth directions are the same, and the two major peaks ratio ((111) and (200) directions) is constant around 3.17 for each sample. Crystallite size decreases as the delivery velocity of L-ascorbic acid increases. Cu2O powders synthesized at the three major velocities have the bigger ECSAs and they are used as cores for the core-shell structures. Their XPS spectra show a Cu(I) percentage around 80% and adventitious carbon as contaminant. Starting by ECSAs, PEC tests are conducted on the most promising samples. The Cu2O-SnO2 ECSAs are SnCl4·5H2O delivery velocity dependant: the areas decrease as the velocity increases. Using the XPS peak of valence band region and energy band gap value from DR-UV-vis is possible to obtain the band structure of the material. In conclusion, the synthesis standardization is achieved for Cu2O while the impregnation needs to be further tested to obtain statistically significant data.