The Direct Hydrothermal Deposition of Cobalt-Doped MoS2 onto Fluorine-Doped SnO2 Substrates for Catalysis of the Electrochemical Hydrogen Evolution Reaction† 
Isolda Roger,£a Roberta Moca,£a Haralampos. N. Miras,a Kevin G. Crawford,b David A. J. Moran,b Alexey Y. Ganin*a and Mark D. Symes*a  
a WestCHEM, School of Chemistry, University of Glasgow, University Avenue, Glasgow, G12 8QQ, United Kingdom. 
b School of Engineering, University of Glasgow, Glasgow G12 8LT, United Kingdom.
* Email: mark.symes@glasgow.ac.uk; alexey.ganin@glasgow.ac.uk
£ These authors contributed equally to this work

EXPERIMENTAL SECTION
General Experimental Remarks: CoSO4·7H2O (99 %) and (NH4)6(Mo7O24)·4H2O (99 %) were purchased from Sigma Aldrich while SC(NH2)2 (99 %) was supplied by Alfa Aesar. All chemical reagents and solvents were used as purchased. All electrolyte solutions were prepared with reagent grade water (18.2 MΩ-cm resistivity), obtained from a Sartorius Arium Comfort combined water system. pH determinations were made with a Hanna HI 9124 waterproof pH meter. UV-Vis spectra were collected in the solid state on a Shimadzu UV-2101PC spectrophotometer. Fluorine-doped tin oxide on glass (FTO) coated plain float electrodes (7 ohms per sheet) were purchased from Hartford Glass Co., Inc. All other materials were obtained as stated in the text. Experiments performed at “room temperature” were carried out at 20 °C. 

Synthesis
Preparation of Co2Mo9S26-on-FTO: FTO substrates were prepared by being soaked for 10 minutes in a KOH / iso-propanol base bath, after which they were rinsed with a 1 M solution of HCl and a copious amount of distilled water. 300 nm-thick films of Co2Mo9S26-on-FTO were prepared by making a stock solution by mixing together 4.66 mL of an aqueous 50 mM solution of CoSO4·7H2O, 28 mL of an aqueous 7.1 mM solution of (NH4)6Mo7O24·4H2O, and 70 mL of an aqueous 50 mM solution of SC(NH2)2 with rigorous stirring (giving a Co : Mo : S ratio of 1 : 6 : 15). Hence this stock solution contained Co, Mo and S in the following overall concentrations: 2.3 mM Co, 13.6 mM Mo and 34.1 mM S. In a typical procedure, 15 mL of this stock solution was poured into a 20 mL Teflon liner, in which a 2.5 × 1 cm2 FTO substrate was placed with the conductive side facing down at an angle of ca. 45° angle (Fig. S1). The Teflon liner was then sealed inside a stainless steel reaction vessel and heated at a rate of 1 °C / min in a convection oven to 180 °C. After 72 hours at this temperature, the vessel was cooled at a rate of 10 °C / min to room temperature. The vessel was opened in air revealing a colourless solution with pH = ca. 8. The product was formed as a grey-black, translucent film on the conductive part of the substrate. The substrate with the film was washed with distilled water and dried in a desiccator over freshly regenerated silica gels at 100 °C. Subsequent annealing of these films at 300 °C under a stream of Ar did not produce any significant improvement in the electrochemical performance of these materials, and so all the results reported herein were obtained without any such annealing.
Thinner films (150 nm-thick) were prepared by the same method, except that the stock solution was prepared by mixing 4.66 mL of an aqueous 20 mM solution of CoSO4·7H2O, 28 mL of an aqueous 2.9 mM solution of (NH4)6Mo7O24·4H2O, and 70 mL of an aqueous 20 mM solution of SC(NH2)2 together with rigorous stirring (again giving a Co : Mo : S ratio of  1 : 6 : 15). 15 mL of this stock solution were then poured into a 20 mL Teflon liner containing an FTO substrate and heated as above. 

Morphological and Compositional Characterisation
Raman spectroscopy was carried out with a wavelength of 532 nm on Horiba Jobin-Yvon LabRam Raman HR800. To prevent degradation of the sample, a 10 % filter and 200 mm hole was used. The focus on the surface of the sample was achieved using the controls, with the aid of a microscope with 10× and 50× magnification. Before the measurement the instrument was calibrated using a piece of Si as a standard. Powder X-ray diffraction (PXRD) measurements were performed using a Panalytical XPert-pro diffractometer (CuKα radiation corresponding to λ= 1.54178 Å wavelength), operating in a Bragg-Bretano reflection geometry. Scanning electron microscopy was performed in conjunction with an Energy-dispersive X-ray (EDX) spectroscopy on a Philips XL30 ESEM with an attached Oxford Instruments x-act EDX detector. All the SEM pictures and the EDX analysis were recorded using a beam current of 20 kV. Initially, the SEM images were recorded by using the following magnifications: 20×, 100×, 800× and 4000×. The selected areas were then probed by EDX spectroscopy to obtain the elemental composition of the samples. Before running the EDX measurements a Cu foil standard was used for calibrating the measurements. X-ray photoelectron analysis (XPS): As-prepared Co-doped molybdenum sulfide-on-FTO samples were carefully packed and sent to the National EPSRC XPS Users' Service (NEXUS) at Newcastle University, UK. XPS spectra were acquired with a K-Alpha instrument (Thermo Scientific, East Grinstead, UK), using a micro-focused monochromatic AlKα source (X-ray energy 1486.6 eV, spot size 400 x 800 microns). The emission angle was zero degrees and the pass energy was 200 eV for surveys and 40 eV for high resolution. Charge neutralization was enabled. The resulting spectra were referenced to the adventitious C 1s peak (285.0 eV) and were analyzed using the free-to-download CasaXPS software package. Atomic Force Microscopy (AFM) measurements were obtained using a Bruker Dimension Icon AFM. Film steps relative to the FTO substrate were measured over a scan area of 20 µm × 6.7 µm. The step edges for AFM measurements were generated by electrolyzing films for extended time periods in 0.5 M H2SO4 according to the general electrochemical methods described below until limited exfoliation of the catalyst material from the FTO substrate had occurred. The relative height of the un-exfoliated catalyst material compared to the bare substrate exposed by exfoliation was then measured. Three different areas were analysed per sample and the data reported are average values for the step sizes that were measured. The thicknesses of the films were measured to ±50 nm thickness. For example, a step measurement on a 300 nm-thick film (taking a 4 μm cut each side of the step) gave a height of 270 nm. The film itself gave an average roughness of 26 nm, whilst for the substrate the roughness was 20 nm Ra.  Hence the roughness of the underlying FTO and the roughness of the deposit track reasonably closely. Errors associated with the AFM instrument itself were minimized by using a new tip for each measurement. Atomic absorption spectroscopy (AAS) was performed on a Perkin Elmer Aanalyst400 instrument as follows. To an accurate weight of material (carefully scraped from the FTO substrate) was added 5 mL of aqua regia and the sample was then boiled at 120 °C for 30 minutes, allowed to cool, and transferred to a 25 mL volumetric flask, rinsing all the glassware with deionized water. The samples were diluted to bring them into the linear range to be measured by AAS. A blank sample was also prepared using 5 mL of aqua regia. A series of standards were prepared in the range 0 – 5 mg/L for cobalt and 0 – 50 mg/L for molybdenum. The standards were prepared in water to match the diluted samples. Cobalt was measured at 240.7 nm using an air acetylene flame. Molybdenum was measured at 313.3 nm using a nitrous oxide / acetylene flame.

Electrochemical Methods
General Electrochemical Methods: Electrochemical studies were performed in a three-electrode configuration (unless otherwise stated) using a CH Instruments CHI760D potentiostat in 0.5 M H2SO4, unless otherwise stated. A large surface area carbon felt (Alfa Aesar) was used as the counter electrode (unless otherwise stated), and an Ag/AgCl (NaCl, 3 M) reference electrode (RE 5B, BASi) was used. Working electrodes were washed with deionized water prior to use. Carbon felt counter electrodes were not re-used. Three-electrode potentials were converted to the NHE reference scale using E(NHE) = E(Ag/AgCl) + 0.209 V. The active area of all working electrodes was on the order of 1 cm2.
Bulk electrolysis and linear sweep voltammetry: These were performed in a three-electrode configuration (unless otherwise stated) in single compartment electrochemical cells. Solutions were stirred, keeping the same stirring rate for all experiments. Where voltages have been corrected for ohmic resistances, the effective voltage (Veffective) is given by:
	Veffective = Vapplied – iR
where i is the current flowing through the cell and R is the resistance of the cell. Cell resistances were measured by the iR test function available on the CH potentiostats, using the general method developed by He and Faulkner. Briefly, the iR test function works by examining the current response to small step changes in voltage relative to a test potential at which no faradaic current flows. In our case, the step change (ΔV) was 0.05 V and the test potential was selected as 0 V vs. Ag/AgCl. The iR test function on the potentiostat then extrapolates the signal-averaged currents at 54 and 72 ps after the voltage-step edge backwards to obtain a current at t = 0, where this current can also be expressed as ΔV/R. R in this case is the solution resistance that is sought. The final parameter that the user must select with this function is the acceptable stability limit of the system at the value of R measured (“% overshoot”): in our case a value of 2% was chosen (default setting on the potentiostat). The error associated with this iR-correction is dominated by the error associated with gauging the resistance of the solution, where values were found to vary over a range of Rmeasured ± 3%. Linear sweep voltammograms were recorded at a scan rate of 2 mV s−1 unless otherwise stated.
Tafel Plots: Tafel plots were obtained in single chamber cells with stirring according to the general methods described above. Plots were generally collected by linear sweep voltammetry at a scan rate of 2 mV s−1, and the reported Tafel slopes are averages of several runs. Where specified, Tafel plots were also constructed by running bulk electrolyses at various potentials. In these cases, the current density was allowed to stabilize for 5 minutes at each potential before being recorded. The overpotentials reported have been corrected for resistive losses.
Gas chromatography
Gas chromatography was conducted in airtight cells according to the general electrochemical procedure given above and using an Agilent Technologies 7890A GC system. During electrolysis, the solution was stirred and the headspace was sampled by gas-tight syringe (volume taken per sampling event = 50 µL) and introduced onto the GC column by direct injection at various intervals. The column used was a 30 metre-long 0.320 mm widebore HP-molesieve column (Agilent). The GC oven temperature was set to 27 ºC and the carrier gas was Ar. The front inlet was set to 100 °C. The GC system was calibrated for H2 using certified standards of hydrogen at a range of volume % in argon supplied by CK Gas Products Limited (UK). Linear fits of volume % vs. peak area were obtained, which allowed peak areas to be converted into volume % of H2 in the cell headspace. Total system headspaces were calculated by filling the cells with water at room temperature. Charges passed were converted into expected volume percentages of hydrogen in the headspace by converting charges to an expected number of moles of gas (by dividing by 2F for H2, where F is the Faraday constant), and then taking the volume of 1 mole of an ideal gas at room temperature and pressure to be 24.5 L. Faradaic efficiencies were then calculated by taking the ratio of gas volume % based on the charge passed to the gas volume % measured by gas chromatography. Faradaic efficiencies were based on the total amount of charge passed, uncorrected for any background or capacitance currents. All gas determinations were performed at least twice, and average Faradaic efficiencies are reported in the main text.

REFERENCES