Materials: All chemicals used are commercially available and were used without any additional purification steps: Calcium silicide (CaSi2, technical grade) was purchased from Sigma-Aldrich Inc. Concentrated hydrochloric acid (HCl, 37% aqueous solution), toluene (99%), acetone (99%,) was purchased from Guangzhou Chemical Reagent Inc.. Ethyl acetate (EA, 99.5%), dimethyl sulfoxide (DMSO, 99.9%), n-butanol (99.7%), and 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%) were purchased from Aladdin Inc. Electronic-grade hydrofluoric acid (HF, 49% – 51% aqueous solution) was purchased from Macklin Inc. The toluene was distilled to remove water and stored in an nitrogen atmosphere before use.
Synthesis and Purification of Hydride-Terminated Silicane (HSi): 1.0 g of CaSi2 was weighted out in a nitrogen-filled glovebox and transferred to a 250-mL Schlenk flask equipped with a magnetic stirring bar. The flask was then cooled down to -30 °C using a low-temperature reactor (DHJF-4002, Greatwall Scientific Inc.). 100 mL of concentrated HCl aqueous solution was then added to the flask with mechanical stirring under constant nitrogen flow to initiate the sol-gel reaction. The reaction was maintained at -30 °C for 7 days. After that, the light-green colour product was isolated from the solution by vacuum filtration, washed with 15 mL of acetone and subsequently dried under vacuum at room temperature for 12 h. The dried product was finally transferred to a 20-mL glass vial and stored in a nitrogen-filled glovebox for further use.
The as-formed silicane was further purified by HF etching to remove silicon suboxide and other impurifies such as metal silicides. 250 mg of the dried product was transferred to a polyethylene terephthalate beaker equipped with a Teflon coated stir bar. 4 mL of ethanol and 4 mL of de-ionized water were added to the beaker with mechanical stirring to form a brown suspension. 3 mL HF aqueous solution was subsequently added into the mixture in ambient conditions under mechanical stirring to initiate the etching reaction (Caution! HF solution must be handled with extreme care). After 5 min, the colour of the suspension gradually changed to light green. ~60 mL of toluene was added to extract the hydride-terminated silicane from the aqueous layer. The product/toluene solution was obtained using a plastic pipette by multiple (i.e., 3*20 mL) extractions. The toluene was removed using a rotary evaporator and the product was dried under vacuum for 12 h.
Room-Temperature Synthesis of Crystalline Silicon (c-Si): 30 mg of HSi was transferred in a 100-mL Schlenk flask equipped with a magnetic stirring bar. Under constant nitrogen flow, 25 mL of DMSO was added into the flask to initiate the reaction. The reaction was maintained for 3 h at room temperature and the solution gradually turned from greenish colour to dark-gray. After that, the precipitate was isolated from the solution via vacuum filtration. 5 mL of EA was used to wash the solid for three times. The product was then dried under vacuum at 40 °C for 12 h before being transferred to a nitrogen-filled glovebox for storage.
Photoluminescence and Absorption Measurements: The absorption spectra of all samples were measured using obtained a Shimadzu UV-2450 spectrophotometer. Photoluminescence (PL) spectra of the silicane and c-Si powdery samples were recorded at room temperature using an FLS 980 spectrometer (Edinburgh Instruments) with a Xenon lamp as a continuous wave light source. The excitation wavelength was set as 365 nm for all samples.
Powder X-Ray Diffraction (PXRD) and X-Ray Photoelectron Spectroscopy (XPS) Measurements: PXRD patterns was collected with a Rigaku Smart Lab diffractometer (Bragg-Brentano geometry, Cu Kα1 radiation, λ = 1.54056 Å). The spectra were scanned between 2θ ranges of 10-80° with an integration of 600 s. XPS results were obtained using a VG Scientific ESCALAB 250 instrument, Thermo Fisher Scientific. CasaXPS software (VAMAS) was used to interpret high-resolution results. All spectra were internally calibrated to the C 1s emission (284.8 eV).
Hydrogen Generation Tests and Data Analysis: The hydrogen generation measurements were carried out in a sealed thick-walled reaction tube equipped with a rubber plug and a mechanical stirrer. 10.0 mL of the dried DMSO solution (or mixed DMSO/toluene solution for the concentration-dependent tests) was then added to the tube and the solution was bubbled with nitrogen flow for 20 min. The gas space above the solution was measured to be 52.5 mL using the water displacement method. 10.0 mg of freshly-etched HSi was transferred from an nitrogen-filled glovebox and added to the reaction tube to initiate the reaction with mechanical stirring. The gaseous analyte was extracted using a 1 mL GC syringe for each test.
The gaseous products were analyzed by Ruimin GC-2060 gas chromatography. A thermal conductivity detector (TCD) was used to detect hydrogen gas. Argon was used as the carrier gas. The oven temperature was kept at 80 °C. The TCD detector and injection port temperature were both kept at 120 oC.
The collected time-dependent hydrogen generation results fitted using the pseudo-second order kinetic equation:30,31
Which can be rearranged to obtain,
Fourier-Transform Infrared Spectroscopy (FT-IR) and Raman Spectroscopy: The FT-IR spectra were obtained using a Nicolet/Nexus-670 FT-IR spectrometer (ATR mode). The Raman spectra were collected using an Ocean Optics QEPRO high performance spectrometer. A 785nm solid-state diode laser (20.0 mW) was used to excite the sample.
Brunner−Emmet−Teller (BET) Measurements: The BET measurements with N2 adsorption isotherms (77 K) were carried out using a Quanta Chrome Autosorb-iQ2-MP instrument (Anton Paar QuantaTec Inc.). ~0.1 g of freshly prepared sample was transferred into the gas adsorption tube and degassed by built-in pretreatment facility automatically for 10 h at 100°C to obtain the activated sample.
Scanning Electron Microscopy (SEM) Imaging: The SEM images were obtained using a Regulus 8230 ultra-high resolution scanning electron microscope with an accelerating voltage of 5 keV.
Photocatalytic Synthesis of Hydrogen Peroxide (H2O2): In all photocatalytic experiments, 5 mg silicon-based catalyst was added into the mixed solution of ultrapure water (19 mL) and triethylamine (1 mL) with ultrasound for 15 minutes. The suspension was then transferred to a jacketed beaker, followed by O2 bubbling and continuous stirring for 30 min prior to photo-irradiation. A water-circulating system was used to keep the temperature of the reaction solution at 20 ± 0.1 °C. The light source was a xenon lamp (300 W), and the UV-light was cut by a filter (420 nm). The concentration of H2O2 was analyzed by a classic I3- method.32 Specifically, the supernatant was mixed with phthalic acid (0.1 M) and potassium iodide (0.4 M) in a volume ratio of 1:1:1. The absorbance at 355 nm was analyzed by UV-vis spectrophotometer after complete color development (30 min). The calibration curves are provided in Supplementary Figure 12, and a good linear relation (R2=0.999) is observed between absorbance and the H2O2 concentration.
Electron Paramagnetic Resonance (EPR) Studies: EPR experiments were performed on a Bruker EMX cw-X band spectrometer with a microwave frequency of 9.3 GHz in 77 K with a liquid helium flow cryostat. The power was kept at 2.19 mW. The spin concentration and g-value were calibrated by use of a standard sample (ultramarine blue diluted by KCl, g-value of 2.033). The data processing was performed with the WinEPR software package.
X-ray Photoelectron Spectroscopy (XPS) and Valance Band Analysis: The XPS data was internally calibrated to the C 1s emission (284.8 eV). The valance band was got by the intersection point of horizontal tangent from -2 to 0 eV and tangent from 0 to 4 eV.
Reaction of TEMPO with HSi: 30 mg of HSi was transferred in a 100-mL Schlenk flask equipped with a magnetic stirring bar. Under constant nitrogen flow, 25 mL of DMSO and 15 mg of TEMPO were added into the flask to initiate the reaction at room temperature. The solution gradually turned from greenish to dark-gray. After 3 h, the reaction was terminated and the precipitate was isolated from the solution via vacuum filtration. 5 mL of EA was used to wash the solid for three times. The product was then dried under vacuum at 40 °C for 12 h before being transferred to a nitrogen-filled glovebox for storage.
XANES Analysis: The Si K-edge XAS experiments were performed at the soft X-ray Microcharacterization Beamline (SXRMB) of the Canadian Light Source (CLS, Saskatoon, SK, Canada), with an energy resolution E/ΔE > 10000. The spectra were recorded with both total electron yield (TEY) and fluorescence yield (FLY) modes, which were normalized to the incident photon flux. XAS data were processed with Demeter (v.0.9.26), in which an Athena software was used for energy calibration (with Si standard) and spectral normalization.
Computational Simulation: All molecular calculations were done at DFT level using the CAM-B3LYP functional,33 cc-pVTZ basis set,34,35 Grimme’s empirical dispersion correction,36 and polarizable continuum model37 for the DMSO solvent. At optimized structures, hessian calculations were performed to ensure there was no (only one) imaginary frequency for stable (transition state) structures. Natural bond orbital38 (NBO) analyses were performed at the optimized structures to investigate their electronic structures. All molecular calculations were performed using the GAMESS-US program package.39 The solid state calculations were performed using the VASP-5.4.4 program package, with the projector augmented-wave (PAW) pseudopotentials and the PBE functional. The vaspkit program was used to generate the density of states, both total and projected.