Crystalline silicon (c-Si) has been widely used in semiconductor and energy-related industries. One of the biggest challenges of c-Si production is its high energy and environmental cost due to the requirement of high-temperature synthetic process and the accompanied intense greenhouse gas emission. Herein, we demonstrate a new process for preparing c-Si directly from hydride-terminated silicane (HSi) under mild conditions. We design a wet-chemistry approach that effectively creates Si-Si bonds between HSi flakes via nucleophilic attack by Lewis base reagent at room temperature. c-Si produced by such a chemical suturing process demonstrates promising capability in charge carrier migration and separation under visible light irradiation, which is of critical importance in photocatalysis. Compared to the existent c-Si manufacturing processes, the reported approach drastically reduces the energy consumption and provides a new strategy to tailor the electronic and photophysical properties of c-Si for optoelectronic and catalytic applications.
Article
Nucleophilic Attack Enables Chemical Suture of Crystalline Silicon at Room Temperature
https://doi.org/10.21203/rs.3.rs-1736191/v1
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nucleophilic attack
crystalline silicon
silicane
solid-state synthesis
surface chemistry
density functional theory calculations
Crystalline silicon (c-Si) is superior to other semiconductors in its low precursor cost, relatively simple fabrication procedures, and high material stability. These superiorities make it currently the best compromise as the workhouse semiconductor in modern industries. There has been a dramatic increase in demand of c-Si in the expanded electronic and photovoltaic markets. The supply of this treasurable material, however, is largely plagued by the intensive energy consumption and increased environmental impacts such as CO2 emission and wastewater generation in its synthesis.1–3
A large amount of energy is required to release silicon atoms from their precursors, which reconstruct to yield diamond-structured silicon. Figures 1a and 1b summarize the established methods of c-Si production. All these synthetic routes require high-temperature conditions. Deoxidization process is commonly used to produce metallurgical-grade c-Si by reducing silicon dioxide (SiO2) to yield elemental silicon. Due to the large binding energy of Si-O-Si network in SiO2, the reaction to produce the high purity level of c-Si requires substantial amount of reductant and energy (Fig. 1a). Pyrolysis is another established strategy to fabricate c-Si by the cleavage of reactive Si-H or Si-Cl bonds at high temperature to form c-Si with high purity (Fig. 1b). However, this reaction involves highly flammable silane molecules as reactants and proceeds at high temperature (> 500°C); the materials require special care during the transportation and the reaction.
Dehydrogenative coupling reactions are known to create Si-Si linkage in organosilane chemistry.4–11 The process generally involves the oxidative addition of the Si-H bonds with the predesigned molecular catalysis activation and the subsequent elimination of hydrogen (H2).4,6 Dehydrogenative coupling is thus potentially a powerful tool to prepare chemicals with Si-based backbones. Indeed, compounds with Sin units such as organodisilanes and linear polysilanes have been successfully synthesized through dehydrogenative coupling reactions.7,8,10 However, the crosslinking of Si-Si bonds through dehydrogenative reactions is difficult due to the significant steric hindrance of the bulky organic groups. On the other hand, volatile silanes without the bulky groups (e.g., SiH4 and HSiCl3) are too reactive to be used for such solution-based coupling reactions. The potential for dehydrogenative reactions to construct more complex, organized inorganic Si-based network (such as c-Si) have so far been largely ignored.
Two-dimensional (2D) nanomaterials can behave as the building blocks to construct three-dimensional (3D) bulk architectures.12–15 Silicane owns a sp3-hybridized single-layered Si backbone with surface passivation and thus its atomic arrangement is similar to that on the hydride-terminated c-Si(111) surface.16,17 Inspired by the organosilane coupling reactions promoted by Lewis base (LB) reagents, we sought an approach that would chemically “suture” hydride-terminated silicane (termed HSi henceforth) to form c-Si through dehydrogenative coupling reaction in a typical weak LB-type solvent. Our aim was to create covalent Si-Si bonds between HSi flakes through wet chemistry method at mild conditions (Fig. 1c).
In the proof-of-concept experiment, HSi was prepared using the chemical exfoliation of CaSi2 and purified by HF-etching, followed by multiple washing steps to remove unreacted precursors and impurities (See Methods). In a typical process of chemical suture of HSi, purified HSi was added to dimethyl sulfoxide (DMSO), a weak LB reagent, under a nitrogen atmosphere at room temperature. A substantial quantity of gas formed from the reaction, and the original greenish powders changed to shinny and dark-grey crystalloid pellets after the immersion in DMSO (Fig. 2a). The detailed morphology was verified by scanning electron microscopy (Figs. 2b, 2c, Supplementary Figs. 1 and 2). The original layered structure of HSi converted to micrometer-scaled crystalline trunks after the 12-h reaction with DMSO. The crystal evolution can also be monitored by the powdery X-ray diffraction (XRD). The signal at 2θ = ~15° in the XRD pattern of freshly prepared HSi is assigned to (001) lattice plane (see the inset of Fig. 2d). This signal disappeared after the DMSO treatment. It is important to note that c-Si and HSi share similar strong features in the XRD patterns (e.g., 28°, 47°, and 56°), which can be assigned to the common Si(111)-like facets.16
The generation of H2 was detected by gas chromatography (GC), confirming that the cleavage of Si-H occurs during the DMSO mixing process; in contrast, no observable H2 formation was noticed from the HSi powders or the controlled HSi/toluene solution (Fig. 2e), suggesting that the H2 formation is promoted by the presence of DMSO. Absorption and photoluminescence spectroscopies further revealed the bonding evolution during the DMSO treatment. The original absorption edge at 483 nm shifted to 1086 nm after the reaction (Fig. 2f), and the green photoluminescence (PL) at 495 nm is notably diminished (Supplementary Fig. 3). We ascribe this to the formation of c-Si which has a bandgap of ~ 1.1 eV, a significant reduction from the ~ 2.6 eV optical gap of silicane. Raman measurements confirm the presence of the distinguishable 2D Si six-member-ring (~ 380 and ~ 490 cm− 1) and Si-H vibration modes (~ 640 and ~ 730 cm− 1) in silicane, whereas only the bulk Si-Si signal at ~ 515 cm− 1 was observed in the treated sample (Fig. 2g). Similarly, the original Si-H stretching signal (~ 2100 cm− 1) disappeared from the Fourier transfer infrared (FT-IR) spectrum of the DMSO-treated sample (Fig. 2h). Both Raman and FT-IR results indicate that the DMSO treatment effectively cleaves Si-H bonds and converts the layered structure of silicane into bulk c-Si.
Synchrotron-based X-ray characterizations gave further insight into the impact of the oxidation state of silicon atoms and the crystal evolution induced by the DMSO treatment. The surface-sensitive total electron yield (TEY) X-ray absorption near-edge structure (XANES) spectra at the Si K-edge reflects the formation of surface oxide layer on both pristine and DMSO-treated samples (Supplementary Fig. 4), which is not unexpected because the surfaces of both HSi and c-Si are prone to oxidation.18,19 The total fluorescence yield (FLY) XANES spectra, on the other hand, reveals the bulk crystal information. The presence of the strong signal at ~ 1847 eV in both pristine and DMSO-treated samples is consistent with the feature of the standard SiO2 sample (Fig. 2i), indicating the presence of oxidation in both samples. The absorption onset energy of the Si 1s feature of HSi (1840.0 eV) is higher than that of c-Si (1839.1 eV). The high absorption onset energy in HSi may originate from the slightly positively charged Si atoms in HSi (χSi = 1.90 vs. χH = 2.20, and natural atomic charges calculated to be + 0.1 and − 0.1 e for Si and H (see Supplementary Note 1 and Note 2). The shoulder signal shifts by 0.9 eV following the DMSO treatment with a similar feature of the control c-Si sample and can be assigned to the resonance excitation of Si atoms in c-Si.20 These observations are in excellent agreement with XRD and Raman results, and together these findings suggest that the DMSO treatment leads to the complete conversion from the layered HSi structure to bulk c-Si crystalline.
Interestingly, it has been reported that DMSO, serving as a weak oxidizing reagent, can oxidize hydride-terminated porous silicon and form dimethyl sulfide (DMS) through a radical-driven mechanism.21 However, no DMS was detected from either the gas phase or the solution, suggesting that DMSO does not serve as the oxidant in this study. All these experimental results hint on a face-to-face suture of silicanes catalyzed by DMSO at room temperature, yielding c-Si and H2 gas.
To assure this is the case, we resourced to quantum chemistry calculations using density functional theory (DFT) method. A HSi(SiH3)3 molecule was used to model a HSi flake to study the suture mechanism. The reaction occurs locally at one Si site, which is surrounded by three Si atoms, and all the Si atoms are in sp3 hybridization. This model preserves the silicane chemical environment for the central Si and H atoms (Fig. 3a). With this model, we investigated the 2HSi(SiH3)3 = (SiH3)3Si-Si(SiH3)3 + H2 reaction, and the conclusions are transferrable to the suture of two silicanes flakes. The reaction was calculated to have \(\varDelta G\) = -15.7 kcal/mol when the translational and rotational contributions to the Gibbs free energies of the HSi(SiH3)3 monomer and the (SiH3)3Si-Si(SiH3)3 dimer are ignored. This approximation is reasonable since the HSi flakes, before and after they are connected by a Si-Si bond, cannot freely rotate and translate in the solution. All reported energies below were obtained under this approximation, unless further specified. Such a favorability is reduced to -8.5 kcal/mol, if the translational and rotational Gibbs energies of the H2 molecule are excluded. These computational results are in consistence with the observed mild exothermicity of the reaction, and the effusion of H2 gas itself contributes half of the thermodynamics favorability.
Without the presence of DMSO, the reaction barrier for the suture was calculated to be 62.7 kcal/mol, which is formidable and prevents the reaction from occurring (Fig. 3b). The reaction complex maintains a closed-shell electronic structure at the transition state (TS). We used symmetry-broken unrestricted open-shell DFT method to calculate the barrier and found it to be even higher (> 80 kcal/mol), excluding the possibility of a biradical TS. The closed-shell structure at TS corresponds to heterolytic partial cleavages of the two relevant Si-H bonds (Step i in Fig. 3b). The resultant hydridic and protonic H atoms readily form a H2 molecule, leaving the two Si centers to form a Si-Si bond and stitch up the silicanes (Step ii in Fig. 3b). The high barrier arises from the difficulty in the heterolytic cleavages of the Si-H bonds in the absence of catalyst, especially in the formation of the protonic H atom and the silicide center (the red arrow in Step i), in which the bond pair migrates in a counter-electronegativity manner.
The presence of DMSO lowers the barrier to 28.7 kcal/mol, a surmountable barrier at room temperature (Fig. 3b). DMSO attacks from the opposite direction of a Si-H bond using its O basic center, converting the Si to a siliconium center with a trigonal bipyramidal configuration (Step iii in Fig. 3b). The Si-H bond has been partially heterolytically cleaved. The hydridic center is ready to abstract a proton and thus induce the counter-electronegativity heterolytic cleavage of the Si-H bond of the other HSi(SiH3)3. Clearly, the inductions of the heterolytic cleavages of the two Si-H bonds, first by DMSO, then by the generated hydridic center, cause the 34 kcal/mol barrier reduction, which enables the reaction to occur slowly. The TS can be viewed as being stabilized by a 5-center-6-electron interaction across the O-Si+-H−-H+-Si− bridge (more details available in Supplementary Note 2 and Note 3). Crossing the TS, the H2 formation and Si-Si bond suture follow the same pathway as the uncatalyzed reaction, and the DMSO falls off (Step iv in Fig. 3b). The large availability of the DMSO solvent molecules guarantees that every Si site is accessible by these catalytic bases. The catalytic cycles hence continue and eventually results in the 2D-to-3D suture, as demonstrated in Fig. 3b. It is logical to expect that the basicity of a solvent and the steric accessibility of its basic site are the key to catalyze the suture process. It is noteworthy that the anisotropy of the DMSO in exhibiting its Lewis basicity facilitates the catalytic process. If the solution consists of a large amount of isotropic basic species, such as F−, it is likely that the two HSi bonds in Fig. 3 are heterolytically cleaved in the same Siδ−Hδ+ manner. The two hydridic H atoms will thus not form an effusive H2 molecule, and the reaction does not proceed. In the contrary, the DMSO solvent molecules can approach the backsides of the two Si-H bonds, which are to be sutured, through their oxygen end and methyl end, respectively, so that the Siδ−Hδ+ partial cleavage only occurs in one of the Si-H bonds, which is activated by the oxygen end.
We also performed calculations to investigate the possibility of the DMSO inserting its O atom to the Si-H bond and resulting in the Si-O-H moiety. As expected, this reaction is highly exothermic, with \(\varDelta G\) = -43.1 kcal/mol. However, Gibbs reaction barrier was estimated to be 39.1 kcal/mol high (See Supplementary Fig. 17). This reaction is thus kinetically non-competitive with the suture of HSi flakes, despite the thermodynamics sink of forming Si-O bonds. More details of our computational studies are given in Supplementary Notes 1 to 3.
The materials characterizations and control experiments lead us to an overall model of the LB-driven c-Si formation (Fig. 4a), beyond the understanding of the formation of each Si-Si bond that was gained by DFT calculation. The details of this model are presented in Supplementary Note 5. Overall, the suture starts at one pair of Si-H bonds on two HSi flakes, with the catalysis of DMSO (the first arrow in Fig. 4a). This first Si-Si bond, which we call anchored bond, brings nearby Si-H bonds of the two flakes close to each other and more suturing processes ensue. The suture propagates in two dimensions to completely stitch up the two flakes (the propagation arrows in Fig. 4a). Ideally, the other HSi flakes spectate the suture of the two flakes (see Supplementary Note 5 for more detailed discussion). After the suture of the first two flakes into the double-layered HSi2 intermediate structure, the third and fourth flakes are sutured with the first and second, respectively, on the two sides of the HSi2. Since the backsides of all Si-H bonds of the HSi2 are enclosed by Si-Si bonds and are not accessible to DMSO, the DMSO activation of Si-H bonds can only occur at the third and fourth flakes. On and on, the thickness of the HSin increases as n HSi flakes are sutured in this layer after layer manner, and eventually, the perfect c-Si is formed. Two HSi2 double-layered flakes cannot be sutured due to their DMSO-inaccessible backsides of Si-H bonds.
However, such an adiabatic (in thermodynamics sense) layering process hardly happens. This is because multiple anchored Si-Si bonds can be formed between several HSi flakes, which then evolve to some HSi2 layer fragments (see the “Reality” part of Supplementary Note 5). These layer fragments cannot be stitched up through the DMSO catalysis and therefore, the space between the HSi2 layer fragments eventually become enclosed cavities in the final product of c-Si. Some of them become pores at the surface of c-Si, if they are not enclosed in three directions.
The proposed mechanism suggests that the reaction rate to form the c-Si product and the properties of the product depend on LB concentration. In order to confirm this proposal, we implemented the dehydrogenative coupling reaction in solvents with various DMSO/toluene mixtures. Notable differences in the H2 production rate were observed (Fig. 4b), and the reactions with only 10% or 1% DMSO were far from completion even after 500 min. We attributed this to the reduced probability of the Si-H bond heterolytic cleavage by the lower concentration of LB. Interestingly, there is no obvious correlation between surface area, pore size, and DMSO concentration (Fig. 4d and Supplementary Fig. 5), while we expect enhanced porosity with a higher DMSO concentration. Clearly, the high DMSO concentration facilitates the formation of the enclosed cavities within the bulk of c-Si, instead of pores. The inner surfaces of these “bubbles” are inaccessible to the N2 gas in the isotherm experiment, and thus do not contribute to the measured surface areas. We observed that the DMSO serves as mild oxidizing agent in generating surface Si-O-Si bonds. This is verified by FT-IR results in Supplementary Fig. 7, which show clear signal of OSiSi-H bond stretching.
We applied electron paramagnetic resonance (EPR) analysis to further investigate the detailed surface states of c-Si. Two sets of strong signals can be found from all samples regardless of the concentration of DMSO used in the complete treatment (i.e., reaction for over 12 h, Fig. 4c). The strong absorption in the g = 2.0039 region is characteristic of the Pb defects attributed to the surface silyl radical groups.22–24 Another notable feature at g = 2.0111 can be assigned to the oxygen-related trap-hole centres (OHC) associated with the surface oxide species.25–27 Both absorptions exhibit obviously higher intensities for the sample prepared using 100% DMSO concentration. This is reasonable since the higher concentration of the catalyst results in a more rapid (i.e., less adiabatic) suture of HSi flakes into c-Si. Consequently, more enclosed cavities, and correspondingly a larger inner surface area and more radical sites in the inner surface result. Again, the inner surface of the enclosed cavities is inaccessible to the N2 adsorbate in measuring surface area.
The function of surface oxides and their derivatives (e.g., silyl and oxide radicals) on the photophysical properties of c-Si has been a subject of ongoing discussion.28,29 We therefore performed solid-state density functional theory (DFT) calculations on seven classes of surface state structures (Fig. 4e and Supplementary Fig. 8), including oxide/suboxides and silyl radicals, to investigate their impact on the electronic structures of c-Si. The projected density of state (pDOS, Fig. 4f and Supplementary Fig. 7) results clearly show that both states of valence band maximum (VBM) and conduction band minimum (CBM) are dominated by silicon contribution. The midgap state of the structure with a surface Si• is dominated by the pDOS of this Si atom, as expected. The midgap state of the structure with a surface O-Si• radical has not much contribution from the O atom. It mainly arises from the Si atom and its Si neighbors. Also, the midgap state is not far away from the original VBM (Fig. 4f). The surface O atom formally adopts a -2 valence state, while the unpaired electron is distributed around the nearby Si atoms. This is consistent with the large difference in electronegativities of O and Si and the easiness of electron migration in silicon (a semiconductor). These two midgap states are responsible for the two g-factor values shown in Fig. 4c. They are believed to effectively enhance the charge carrier migration under photoexcitation.
Hydrogen peroxide (H2O2) has been widely used in industries. However, its current synthetic approach (i.e., the anthraquinone process) causes large energy consumption and inevitably generates a significant amount of organic wastes. Motivated by the enhanced charge carrier migration of c-Si by the introduction of abundant surface radical states, we sought to investigate the possibility of the chemically sutured c-Si samples being used as the photocatalysis for the green production of H2O2. In a typical reaction, 5 mg of c-Si powders were dispersed in deionized water with continuous supply of oxygen (Supplementary Fig. 9). Triethylamine (TEA) and water served as the sacrificial agent and the hydrogen source, respectively. The oxygen reduction product, H2O2, was immediately formed when the solution was irradiated by the white light source (λex > 420 nm). The highest H2O2 yield was found on the c-Si formed by the treatment with 100% DMSO (Fig. 4g), which is expected to possess more enclosed cavities and larger inner surface area. The average yield for all measured 100% DMSO treated c-Si samples was 219.0 ± 4.9 µmol L− 1 under irradiation for 1 h at room temperature, over 35 × and 2.7 × higher than those of the reactions using HSi (6.2 ± 3.4 µmol L− 1) and commercial c-Si nanoparticles (81 ± 6.2 µmol L− 1), respectively. Considering the DMSO concentration does not significantly influence the band positions as verified by X-ray photoelectron spectroscopy (XPS, Supplementary Fig. 10), we attribute the yield improvement to the increased concentration of inner surface radical sites which facilitate the charge carrier migration and separation, and chemical adsorption of reactant and intermediate radical species (see Supplementary Note 6 for a detailed discussion based on solid state DFT calculations). The c-Si catalysts retained over 95% H2O2 production rate after 5 cycles (1 h per cycle) of photocatalytic reactions (Supplementary Fig. 11), indicating that the catalytic active sites and the structure of c-Si are sufficiently stable for efficient aqueous oxygen reduction reaction (ORR).
In the context of our observations, we provide two possible mechanisms of the photocatalytic H2O2 production (Supplementary Fig. 12). Two types of surface radical sites, siloxyl (≡ Si-O•) and silyl (≡ Si•) groups, of c-Si firstly react with H2O and/or O2, producing highly-active surface peroxide intermediates (•O-O-H or ≡ Si-O-O-H). Under the light irradiation, holes and electrons were separated and traveled to the surface of c-Si. The holes are abstracted by TEA to form TEA+, whereas the electrons further interact with the peroxide intermediates, leading to the formation of H2O2 and the re-generation of surface radical species.
In conclusion, we have developed a new wet chemistry route to prepare crystalline Si at room temperature. We achieved this via a Lewis-based-promoted chemical suture reaction on 2D silicanes. The basic catalyst heterolytically cleaves a Si-H bond in a Siδ+Hδ- manner, the hydridic H atom induces a counter-electronegativity Siδ-Hδ+ cleavage of a Si-H bond from another silicane, the hydridic and protonic H atoms form an effusive H2 molecule, and then the leftover silide and siliconium centers form a Si-Si bond between two silicanes. This whole process is thermodynamically favorable and features a surmountable energy barrier. Such a Si-Si bond formation propagates to suture silicanes into Si crystals. A higher concentration of the catalytic Lewis base facilitates the suture and the formation of crystalline Si with more abundant nanometer-scaled enclosed cavities. Although the inner surfaces of the enclosed cavities are not directly accessible to reactants, they provide inner surface radical sites that facilitate photoexcited charge separation and migration, which enhance the capability of the Si crystal as photocatalyst for oxygen reduction reactions. This is proved by the present work. Conceptually, this work demonstrates that the organosilane coupling strategy can be extended to inorganic synthesis of Si-based materials and provides an approach to further tailor the electronic and photophysical properties of silicon in optoelectronic and catalytic applications.
Supporting Information
Additional notes about the details of the theoretical studies on the DMSO-catalyzed c-Si formation mechanism and the influence of surface states on the electronic structure of c-Si; proposed mechanisms of the photocatalytic production of H2O2 promoted by the surface states on c-Si; additional chemical and photophysical characterizations of HSi and c-Si samples, including PL and FT-IR spectra, SEM, XPS, and BET isotherm tests.
Author Contributions
Z.Y. designed and directed this study. X.D. led the experimental work. X.D. and H.C. developed the synthetic recipe, prepared the materials, and carried out the characterizations and analysis. M.S. led the XAS measurement. J.L. obtained and analyzed the XAS data. T.Z. and G.Y. contributed to the computational studies. L.X., J.C.Y., and Y.W. performed the photocatalytic reactions and analyzed the results. T.Z. and Z.Y. drafted and finalized the reaction mechanism. X.D., T.Z., and Z.Y. wrote the manuscript. All authors read and commented on the manuscript.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (22175201), the Guangdong Natural Science Foundation (2019A1515011748), the Science and Technology Planning Project of Guangdong Province (2019A050510018). Z.Y. would like to acknowledge financial support from the Pearl River Recruitment Program of Talent (2019QN01C108) and Sun Yat-sen University. Y.W. would like to acknowledge the Research Grants Council of the Hong Kong Special Administrative Region (project no. 24304920). T.Z. is grateful to York University for the start-up grant (No. 481333) and the Natural Sciences and Engineering Research Council (NSERC) of Canada for funding (Grant No. RGPIN-2016-06276). T.Z. also thanks Compute Canada for providing computational resources. Part or all of the research described in this paper was performed using beamline SXRMB at the Canadian Light Source, a national research facility of the University of Saskatchewan, which is supported by the Canada Foundation for Innovation (CFI), the Natural Sciences and Engineering Research Council (NSERC), the National Research Council (NRC), the Canadian Institutes of Health Research (CIHR), the Government of Saskatchewan, and the University of Saskatchewan.
Conflict of Interest
A provisional patent application CN 202111355255.6 was filed on November 17, 2021 by the Sun Yat-sen University.
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.
- Wild-Scholten, M. d. & Alsema, E. A. Environmental Life Cycle Inventory of Crystalline Silicon Photovoltaic Module Production. MRS Online Proc. Libr. 895, 0895-G0803-0804 (2005).
- Alsema, E. A. & de Wild, M. J. Environmental Impacts of Crystalline Silicon Photovoltaic Module Production. MRS Online Proceedings Library 895, 305 (2006).
- Alsema, E. in Practical Handbook of Photovoltaics (Second Edition) (eds Augustin McEvoy, Tom Markvart, & Luis Castañer) 1097–1117 (Academic Press, 2012).
- Aitken, C. T., Harrod, J. F. & Samuel, E. Identification of some intermediates in the titanocene-catalyzed dehydrogenative coupling of primary organosilanes. J. Am. Chem. Soc. 108, 4059–4066 (1986).
- Corey, J. Y., Chang, L. S. & Corey, E. R. Dehydrogenative coupling of heterocyclic dihydrosilanes. Organometallics 6, 1595–1596 (1987).
- Brown-Wensley, K. A. Formation of Si-Si bonds from Si-H bonds in the presence of hydrosilation catalysts. Organometallics 6, 1590–1591 (1987).
- Chang, L. S. & Corey, J. Y. Dehydrogenative coupling of diarylsilanes. Organometallics 8, 1885–1893 (1989).
- Woo, H. G. & Tilley, T. D. Dehydrogenative polymerization of silanes to polysilanes by zirconocene and hafnocene catalysts. A new polymerization mechanism. J. Am. Chem. Soc. 111, 8043–8044 (1989).
- Sharma, H. K. & Pannell, K. H. Activation of the Si-Si Bond by Transition Metal Complexes. Chem. Rev. 95, 1351–1374 (1995).
- Rosenberg, L. Exploiting catalytic dehydrogenative coupling in the synthesis and study of polysilanes. Macromol. Symp. 196, 347–353 (2003).
- Liu, L. et al. Polymerization of silanes through dehydrogenative Si–Si bond formation on metal surfaces. Nat. Chem. 13, 350–357 (2021).
- Sudeep, P. M. et al. Covalently Interconnected Three-Dimensional Graphene Oxide Solids. ACS Nano 7, 7034–7040 (2013).
- Shehzad, K., Xu, Y., Gao, C. & Duan, X. Three-dimensional macro-structures of two-dimensional nanomaterials. Chem. Soc. Rev. 45, 5541–5588 (2016).
- Zhao, M.-Q. et al. Hollow MXene Spheres and 3D Macroporous MXene Frameworks for Na-Ion Storage. Adv. Mater. 29, 1702410 (2017).
- Li, W., Huang, Y., Liu, Y., Tekell, M. C. & Fan, D. Three dimensional nanosuperstructures made of two-dimensional materials by design: Synthesis, properties, and applications. Nano Today 29, 100799 (2019).
- Helbich, T. et al. Radical-Induced Hydrosilylation Reactions for the Functionalization of Two-Dimensional Hydride Terminated Silicon Nanosheets. Chem. Eur. J. 22, 6194–6198 (2016).
- Helbich, T. et al. Lewis Acid Induced Functionalization of Photoluminescent 2D Silicon Nanosheets for the Fabrication of Functional Hybrid Films. Advanced Functional Materials 27, 1606764 (2017).
- Liao, W. S. & Lee, S. C. Water-induced room‐temperature oxidation of Si–H and –Si–Si– bonds in silicon oxide. J. Appl. Phys. 80, 1171–1176 (1996).
- Buriak, J. M. Organometallic Chemistry on Silicon and Germanium Surfaces. Chem. Rev. 102, 1271–1308 (2002).
- Baba, Y., Shimoyama, I., Hirao, N. & Sekiguchi, T. Structure of ultra-thin silicon film on HOPG studied by polarization-dependence of X-ray absorption fine structure. Chem. Phys. Lett. 594, 64–68 (2014).
- Song, J. H. & Sailor, M. J. Dimethyl Sulfoxide as a Mild Oxidizing Agent for Porous Silicon and Its Effect on Photoluminescence. Inorg. Chem. 37, 3355–3360 (1998).
- von Bardeleben, H. J., Stievenard, D., Grosman, A., Ortega, C. & Siejka, J. Defects in porous p-type Si: An electron-paramagnetic-resonance study. Phys. Rev. B 47, 10899–10902 (1993).
- Schoisswohl, M., Cantin, J. L., von Bardeleben, H. J. & Amato, G. Electron paramagnetic resonance study of luminescent stain etched porous silicon. Appl. Phys. Lett. 66, 3660–3662 (1995).
- Pereira, R. N., Rowe, D. J., Anthony, R. J. & Kortshagen, U. Oxidation of freestanding silicon nanocrystals probed with electron spin resonance of interfacial dangling bonds. Phys. Rev. B 83, 155327 (2011).
- Stapelbroek, M., Griscom, D. L., Friebele, E. J. & Sigel, G. H. Oxygen-associated trapped-hole centers in high-purity fused silicas. J. Non-Cryst. Solids 32, 313–326 (1979).
- Ikeya, M. in New Applications of Electron Spin Resonance SiO2: Rocks, Faults and Sediments Ch. 9, 271–314 (WORLD SCIENTIFIC, 1993).
- Mashkovtsev, R. I. & Pan, Y. EPR study of new E′ centers in neutron-irradiated α-quartz. EPL 124, 54001 (2018).
- Lenahan, P. M. & Conley, J. F. What can electron paramagnetic resonance tell us about the Si/SiO2 system? J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.--Process., Meas., Phenom. 16, 2134–2153 (1998).
- El-Sayed, A.-M. et al. Theoretical models of hydrogen-induced defects in amorphous silicon dioxide. Phys. Rev. B 92, 014107 (2015).
- Ho, Y. S. & McKay, G. Pseudo-second order model for sorption processes. Process Biochem. 34, 451–465 (1999).
- Simonin, J.-P. On the comparison of pseudo-first order and pseudo-second order rate laws in the modeling of adsorption kinetics. Chem. Eng. J. 300, 254–263 (2016).
- Klassen, N. V., Marchington, D. & McGowan, H. C. E. H2O2 Determination by the I3- Method and by KMnO4 Titration. Anal. Chem. 66, 2921–2925 (1994).
- Yanai, T., Tew, D. P. & Handy, N. C. A new hybrid exchange–correlation functional using the Coulomb-attenuating method (CAM-B3LYP). Chem. Phys. Lett. 393, 51–57 (2004).
- Dunning, T. H. Gaussian basis sets for use in correlated molecular calculations. I. The atoms boron through neon and hydrogen. J. Chem. Phys. 90, 1007–1023 (1989).
- Woon, D. E. & Dunning, T. H. Gaussian basis sets for use in correlated molecular calculations. III. The atoms aluminum through argon. J. Chem. Phys. 98, 1358–1371 (1993).
- Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132, 154104 (2010).
- Tomasi, J., Mennucci, B. & Cammi, R. Quantum Mechanical Continuum Solvation Models. Chem. Rev. 105, 2999–3094 (2005).
- Reed, A. E., Curtiss, L. A. & Weinhold, F. Intermolecular interactions from a natural bond orbital, donor-acceptor viewpoint. Chem. Rev. 88, 899–926 (1988).
- Schmidt, M. W. et al. General atomic and molecular electronic structure system. J. Comput. Chem. 14, 1347–1363 (1993).
Yes there is potential Competing Interest.
A provisional patent application CN 202111355255.6 was filed on November 17, 2021 by the Sun Yat-sen University.
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