Porous structure based on Fenton reaction-assisted chemical etching of commercial silicon powder and its application for electrocatalytic reduction of carbon dioxide

Silicon-based porous nanocomposites are considered promising as electrode materials for the photoelectrochemical reduction of carbon dioxide. However, the high cost of raw materials and tedious processing for building nanostructures may not be conducive to large-scale industrial applications in terms of cost. Herein, we would like to introduce a porous structure prepared by Fenton reaction-assisted chemical etching of low-cost commercial silicon powder in the mixed solution of hydrogen fluoride and hydrogen peroxide. These porous particles are further decorated with silver nanoparticles to explore their feasibility for photoelectrochemical reduction of carbon dioxide. As shown by experimental results, this silicon-based nanocomposite is capable of catalyzing the conversion of carbon dioxide into carbon monoxide. The low cost of commercial silicon powder (~$3000/ton) compared with that of silicon wafers also renders this method potential and feasible for large-scale production of silicon-based porous materials.


Introduction
Photoelectrochemical catalytic reduction, capable of converting carbon dioxide (CO 2 ) into compounds such as carbon monoxide (CO) to produce fuels or other high-value small molecular products, is one of the most promising solutions to global carbon balance and energy problems [1].The advantage of photoelectrochemical technology over electrocatalysis and photocatalysis is that this strategy not only uses the photogenerated carriers generated in the photocatalysis process to provide part of the reaction energy but also reduces the high potential required for electrochemical catalysis [2].The photogenerated electron-hole pairs are separated by the applied electric field, which improves the energy utilization efficiency in the photocatalytic process.
Silicon (Si) is the second most abundant element in the earth's crust after oxygen, which perfectly meets the requirement for sustainable and resource-friendly development.Due to its intrinsic semiconductor properties, Si-based materials can also be used as photocathodes and photoanodes in photocatalysis [3].However, the adsorption of water in aqueous electrolyte solution during photocatalytic conversion will lead to the occurrence of hydrogen evolution reaction (HER), which is a competitive reaction for the reduction of CO 2 [4].In addition to the potential for reduced selectivity due to the possible influence of the HER reaction, the high thermodynamic stability of CO 2 molecules makes the protonation or chemical bond-breaking process extremely difficult during the electrochemical reduction [5].In order to overcome these difficulties and improve the performance of the electrode, as well as the selectivity and conversion efficiency of CO 2 reduction, a variety of Si-based composite structures are used as catalysts for the photoelectrochemical catalytic reduction of CO 2 .For example, Kumar et al. reported the selective catalytic conversion from CO 2 to CO upon the surface of organic molecules loaded with Si substrates [6].In addition, using nanocomposites such as Si-based structures supported by metal nanoparticles is also an effective method.Roh et al. reported the conversion from CO 2 to mono-carbon and multi-carbon products via Si nanowire array-supported Cu nanoparticles [7].However, the costs of substrate materials (mainly Si wafers) are still high, and the processing steps for building nanostructures are usually tedious, often involving lithography and precious metal-assisted etching, which is not conducive to large-scale industrial applications in terms of cost [8].The development of a facial methodology for the fabrication of a semiconducting porous framework based on low-cost raw materials seems necessary.
The etching reaction of Si in the mixed aqueous solution of hydrogen fluoride (HF) and hydrogen peroxide (H 2 O 2 ) is frequently utilized in the semiconductor industry for wafer treatments.Although the standard redox potential in H 2 O 2 acidic media is +1.76 V, much higher than the minimal requirement for the oxidation of Si surface in HF-containing media (+0.7 V) [9], the kinetics is sluggish.For the chemical etching of Si, the hole injection is considered the rate-determined step in most cases [10], which is then highly related to the properties of the oxidants involved.Among all available oxidants, hydroxyl radicals (OH•) are second only to fluorine in oxidizing capacity, with a redox potential of 2.8 V high enough for the hole injection during the etching of Si.Despite its potential, which may interest both industrial applications and mechanism investigations, there are almost no publications explicitly reporting the etching of Si under the existence of OH•, according to our best knowledge.
In order to utilize OH• in acidic aqueous media, Fenton reactions involving iron-based materials and H 2 O 2 are extensively employed.Inspired by this, we would like to introduce an interesting Si-base porous material in this manuscript, which is prepared by Fenton reaction-assisted etching of commercial Si powder in the mixture of HF/H 2 O 2 .This Si-based porous structure is further integrated with Ag nanoparticles by electroless deposition for photoelectrochemical reduction of CO 2 , as a showcase for its feasibility in electrocatalytic applications.In addition to the electrocatalytic capability, the low cost of commercial Si powder (~$3000/ton) compared with that of Si wafer also renders this method potential and feasible for large-scale production of Si-based porous materials.As a result, we obtain a low-cost, large-scale production potential Si-metal nanocomposite structure that can efficiently utilize light and electrical energy input to catalyze the conversion of CO 2 into a C 1 product.

Experimental
In a typical run for the preparation of porous structures, 5 g of commercial Si powder (500 mesh, 99.99% Si) and 50 mg iron powder are added into 200 mL mixed solution of HF (4.8 M) and H 2 O 2 (5.0 M).The chemical etching is proceeded at 10℃ with agitation in a mechanically refrigerated cold bath for 12 hours.The product (FSiP) is collected by centrifugation after washing with deionized water and anhydrous ethanol several times.A control sample is also prepared without iron powder under the same condition (SiP).Then FSiP is put into the mixed solution of HF (4.8 M) and AgNO 3 (4 mM) for the incorporation of Ag nanoparticles.Finally, the sample (FSiP-Ag) is washed with deionized water and anhydrous ethanol and dried under at 60 ℃ under a vacuum for 24 hours.
Scanning electron micrographs and corresponding elemental mapping data (EDS) are collected under a ZEISS Sigma 300 with an accelerating voltage of 5 kV.The pore size distributions are evaluated by nitrogen adsorption (77 K) with a Quantachrome Autosorb-IQ3 system.The crystal structures of the prepared materials are characterized by a D8 ADVANCE Da Vinci X-ray Powder diffractometer (XRD).The chemical components of the sample surface are determined via an AXIS UltraDLD X-ray photoelectron spectrometer.The elemental analysis is performed on an iCAP 7600 ICP-AES.
All photoelectrochemical (PEC) measurements are carried out in our designed H-cell setup (100 mL for electrolyte with a 10 mL gas headspace).The H-cell is separated by a commercial nafion-117 exchange membrane (Dupont).100 mL 0.1M KHCO 3 is purified by electrolysis method for 12 h before use. 4 mg of FSiP is mixed with 375 μL distilled water, 125 μL ethanol, and 50 μL 5wt% nafion (D520), following a 30 min ultrasonic dispersion.100 μL of the catalyst ink was dropped onto a 1 cm × 1 cm hydrophilic carbon paper electrode (PVC-coated copper wire connected) and then air dried before testing the CO 2 reduction performance.A 1 cm × 1 cm Pt foil is introduced as a counter electrode, and a saturated calomel electrode (SCE) is used as a reference electrode.After fully assembled, the H-cell was bubbled with CO 2 for at least 30 minutes to reach CO 2 -saturated (pH=6.97).Also, the CO 2 was continuously bubbled at 20 sccm throughout the measurements under stirring (400 rpm).A 250 W Xe lamp illumination system (BBZM-I type, calibrated with a Si photodiode) is applied to provide 100 mW/cm 2 simulated sunlight on the photocathode during all electrolysis measurements.85% of iR compensation was contributed by the potentiostat (CHI660E).The SCE potentials are converted to the RHE by the following formula: E (vs.RHE) = E (vs.SCE) + 0.241 + 0.0592 * pH.The gas products were collected every 20 minutes in PE gas bags (Ningbo Hongpu Experiment Technology Company, 1 L).Gas samples are manually injected into gas chromatography (GC) systems for CO 2 (Agilent Technologies 7890B, with mass spectrometry Agilent Technologies 5977B MSD, 20 μL per test), H 2, and CO (Nexis GC-2030, with barrier discharge ionization detector, 1 mL per test) measurements.Standard calibration gas samples were used to quantify the gas samples acquired from PEC tests.

Results and Discussion
Figure 1 describes the overall process for the fabrication of FSiP-Ag porous composite.The Si-based porous framework (FSiP) is first prepared by the etching of Si powder in a mixture of HF and H 2 O 2 in the presence of a small amount of iron powder for triggering the Fenton reaction.In this acidic environment, ferrous ions can be supplied by iron powder, which later reacts with H 2 O 2 to form hydroxyl radicals (OHꞏ) [11].In this etchant, a porous structure can be formed through two mechanisms, which are dictated by two existing oxidants, namely H 2 O 2 and OHꞏ.H 2 O 2 is commonly used in the Si etching process, in which holes are injected into the surface of Si, followed by the dissolving of fluorinated compounds [12].On the other hand, OHꞏ, whose oxidation-reduction potential is much higher than H 2 O 2 , can be continuously generated through a Fenton reaction involving H 2 O 2 and ferrous ions and is expected to be capable of injecting holes into the surface of Si.The morphologies of samples are shown by the SEM images in Figure 2. In comparison with the surface morphology of pristine Si powder (Figures 2a and 2b), rich porous structures are formed on the surface of Si particles after etching with HF/H 2 O 2 solution in the presence of iron powder (Figure 2c).The BJH pore size distribution curves in Figure 3a show that fine porous frameworks with pore size mainly ranging from 2 nm to 4 nm are successfully prepared.In order to identify the role of OHꞏ in the etching process in HF/H 2 O 2 , the morphology of the control sample made from Si powder without adding iron powder (SiP) is also investigated.Although porous structures can be clearly observed in SiP (Figure 2d), the pore size distribution is different from that of FSiP, and no micropores are found, according to Figure 3. Therefore, it can be preliminarily inferred that the formation of these microporous structures is related to the hole injection by OHꞏ generated by the Fenton reaction in acidic HF/H 2 O 2 solution.
Owing to its higher specific area and more optimized pore size distribution, FSiP is chosen for the integration of Ag nanoparticles in the following experiments.Figures 2e and 2f show that Ag nanoparticles with sizes ranging from 20 to 80 nm can be homogeneously integrated into FSiP porous framework by commonly used electroless deposition, which is consistent with elemental mapping data given in Figures 2g and 2h.The crystal structures of pristine Si powder, SiP, and FSiP are characterized using XRD, and their diffraction data are shown in Figure 3b.The peaks located at 2θ values of 28.44, 47.30, 56.12, 69.13, 76.38, and 88.03° in all the samples are assigned to the (111), ( 220), (331), (400), (331), and (422) planes of silicon crystals, respectively [13].A comparison between the XRD data shows that the etching process has no effect on the main crystal structure, and no undesired compounds are found in FSiP with the introduction of extra iron powder, which is undoubtedly favorable for scaled production.  of Ag nanoparticles (4 mM and 8 mM AgNO 3 in electroless deposition).The Tafel slope of 149 mV dec -1 for FSiP-Ag with 8 mM AgNO 3 is close to the reported value for Ag catalysts, showing a reasonable electron transfer rate [14].Figure 4b also shows that the Faraday efficiency of CO 2 RR rises from 40% to 53% as the reduction potential increases from -0.3 V to -1.05 V.These results are comparable to that of Ag nanoparticles decorated Si photocathode made from monocrystalline Si wafer [15], demonstrating the feasibility of CO 2 RR catalyzed by FSiP-Ag based on low-cost Si powder.This can be attributed to its rich porous structures, which can facilitate the transportation of protons and improve the Faraday efficiency of CO 2 RR [15].

Conclusion
In conclusion, we prepare a large-scale production potential Si-Ag nanocomposites based on Fenton reaction-assisted chemical etching of low-cost commercial Si powder in HF/H 2 O 2 triggered by Fe (0).As a demonstration of concept and feasibility, this composite is shown to be capable of utilizing light and electrical energy input to catalyze the conversion of CO 2 into CO.Porous silicon powders with pore diameters mainly ranging from 2 nm to 4 nm are prepared by Fenton reaction-assisted HF/H 2 O 2 chemical etching.Silver nanoparticles are deposited onto the surface of porous Si powder by electroless method to catalyze the photoelectrochemical reduction of CO 2 to CO. PEC measurements reveal that the Si-Ag nanocomposites can reach the faradaic efficiency of CO up to 53% at -1.05V (vs.RHE).As a demonstration of concept and feasibility, this composite is shown to be capable of utilizing light and electrical energy input to catalyze the conversion of CO 2 into CO.

Figure 1 .
Figure 1.The fabrication process of Ag nanoparticles integrated Si-based porous framework.

Figure 3 .
Figure 3. (a) The pore size distribution curves of FSiP and SiP, (b) XRD patterns of pristine Si powder, SiP and FSiP.

Figure 4 .
Figure 4.The (a) LSV curves and Tafel plot of FSiP-Ag with different.(b) CO and H 2 Faraday efficiencies under different potentials.