Decoration of Si-nanowires-grafted Si micropillar array with Ag nanoparticles for photoelectrocatalytic dechlorination of 4-chlorophenol

Abstract

Use of Si materials as photoelectrodes in aqueous solutions is fundamentally required to inhibit the formation of an insulating SiO₂ layer as well as promote the transfer of photogenerated holes and electrons from the interior of Si to the solid–liquid interface. In order to meet these requirements, an Si material, with hierarchical structure and Si nanowires standing on the surface of Si micropillar array (SiNW/SiMP), was prepared for facilitating the transfer of photogenerated carriers to the solid–liquid interface. Decoration of SiNW/SiMP by Ag nanoparticles (Ag/SiNW/SiMP) via depositing Ag nanoparticles on the surface of SiNW/SiMP successfully restrained the generation of SiO₂ through preventing the contact of SiNW/SiMP with water. Pristine SiNW/SiMP photocathode exhibited enhanced photoelectrochemical activity with a photocurrent of approximately −31 mA cm⁻² at −1 V (vs. SCE), which was about one order of magnitude larger than that of SiMP; however, the photocurrent decayed with prolonged illumination. By comparison, Ag/SiNW/SiMP photocathode exhibited stable photocurrent of approximately −37.5 mA cm⁻² at −1 V (vs. SCE) during 10 cycles of CV testing, which was 21% higher than that of the pristine SiNW/SiMP. Ag/SiNW/SiMP photocathode exhibited excellent photoelectrocatalytic activity towards dechlorination of 4-chlorophenol (4-CP). Over 95% of 4-CP (initial concentration of 10 mg L⁻¹) was rapidly degradaded after 20 min, which were 3 and 1.1 times higher than those of the SiMP and SiNW/SiMP. The good reproducibility was verified by the results of six consecutive experiments.

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1. Introduction

Si material as a promising photocatalyst has attracted extensive attention in solar energy conversion because of its favorable band gap for optical absorption. However, it usually suffers from the problem of the formation of an SiO₂ layer on its surface when it comes in contact with water or oxygen. Since the SiO₂ layer could block the transfer of carriers through the solid–liquid interface,^1–3 the applications of Si material for water splitting^4,5 and pollutants degradation^6,7 in aqueous solutions are usually limited. Moreover, deep optical-absorption-depth for Si with the indirect band gap (more than 2 μm for doping Si⁸ and 200 μm for high-purity Si⁹) is required for obtaining efficient minority-carrier diffusion length. The photogenerated carriers in the interior of Si have to travel a long way to reach the solid–liquid interface, which increases recombination probability. In order to improve application of Si material in aqueous solutions, it is important to explore effective approaches of inhibiting the growth of SiO₂ layer and shortening the transfer distance of photogenerated carriers.

Three-dimensional hierarchical structures of Si materials have been reported to shorten the transfer distance of photogenerated holes and electrons between the interior of Si and solid–liquid interface^10–15 by way of providing ordered channels for aqueous solution infiltration. The attractive advantage of three-dimensional hierarchical structure is the improved photoelectrocatalytic activity of an Si material. As far as we know, few works focus on applying Si materials with hierarchical structure for pollutant control in water.

The oxidation passivation is another factor restricting the use of Si material in aqueous solutions. Methods of coating a protective layer on the surface of Si^16–20 have been reported to be able to overcome passivation of Si. In comparison with the reported protective layers, the Ag-nanoparticle layer exhibits excellent performance in protecting Si (underneath Ag-nanoparticle layer) from oxidation passivation. It is easy to be prepared by deposition process,^21,22 and more importantly, it possesses excellent electronic transport property.^23,24 Potential built-in electric field forms at the interface between Ag and Si, which can improve the separation of photogenerated electron and holes. Accordingly, an Ag-nanoparticle layer is considered to be a promising protection for Si materials against passivation.

In this study, decoration of an Si material with hierarchical structure (Si-nanowires-grafted Si micropillar array) by Ag nanoparticles (Ag/SiNW/SiMP) is reported. Ag nanoparticles deposited on the surface of Si-nanowires-grafted Si micropillar array (SiNW/SiMP) and the photoelectrocatalytic dechlorination of 4-chlorophenol (4-CP) using Ag/SiNW/SiMP as a photocathode was investigated. Ag nanoparticles are expected to isolate Si from aqueous solutions and thus the formation of SiO₂ can be effectively suppressed. The transfer distance of the photogenerated carriers, namely, the incident light penetration depth, will decrease from several micrometers to dozens of nanometers. This work is expected to provide reference for practical application of Ag/SiNW/SiMP in wastewater treatment and solar water splitting.

2. Materials and methods

2.1 Preparation of SiNW/SiMP

SiMP array was fabricated by etching Si wafer (p-type, (100) orientation, 0.001–0.005 Ω cm, 500 ± 20 μm) via an inductively coupled plasma (ICP) method.^25,26 For forming SiNW on its surface, the prepared SiMP array was first cleaned following a standard cleaning process,² and then dipped into the mixture solution of 4.2 M HF and 0.005 M AgNO₃, and etched for a period of time. Subsequently, the prepared SiNW/SiMP was rinsed with HNO₃ solution to remove the residual Ag and rinsed with ultrapure water and at last dried at room temperature under a stream of N₂.

2.2 Decoration of SiNW/SiMP with Ag nanoparticles

The SiNW/SiMP was put into mixture solution of 0.05 M HF and 0.001 M AgNO₃ for several minutes to deposit Ag nanoparticles, followed by washing it with ultrapure water to flush loosely bound Ag out. The drying of the prepared Ag/SiNW/SiMP was performed under a stream of N₂. The schematic of the process of decoration of SiNW/SiMP with Ag nanoparticles is shown in Scheme 1.

Scheme 1 The preparation process of Ag/SiNW/SiMP. Si wafer (a), SiMP (b), SiNW/SiMP (c), Ag/SiNW/SiMP (d) and its partial enlarged view (e).

2.3 Characterization and measurements

The morphologies of Ag/SiNW/SiMP were observed using scanning electron microscopy (FE-SEM S-4800, Hitachi, with Energy-dispersive X-ray spectroscopy (EDX)). The optical absorption was recorded on a Shimadzu UV-2450 UV/Vis spectrophotometer with fine BaSO₄ powder as the reference.

Photoelectrochemical measurements were performed in a three-electrode system in contact with a CHI 650B electrochemical station (CH Instruments, Shanghai Chenhua, China). The prepared Ag/SiNW/SiMP (active area 4 cm⁻²), a platinum foil and a saturated calomel electrode (SCE) were used as the working electrode, counter electrode and reference electrode, respectively. The volume of the quartz reactor was 50 mL and the incident light intensity was 100 mW cm⁻².

For the dechlorination experiment, the initial concentration of 4-chlorophenol was 10 mg L⁻¹. The concentration of 4-chlorophenol was measured by high performance liquid chromatography (HPLC, Waters 2695, Photodiode Array Detector 2996) with a C18 column. The mobile phase composed of methanol and water (v : v = 0.8 : 0.2) at the flow rate of 1.0 mL min⁻¹.

3. Results and discussion

3.1 Morphology of SiNW/SiMP

SiNW/SiMP prepared under various etching duration were characterized by SEM and the morphologies are shown in Fig. 1. As shown in Fig. 1a–f, SiNW appeared as the etching time prolonged from 10 min to 15 min. The density and length of the nanowires increased with the etching duration. When the etching time was up to 30 min, micropores instead of nanowires appeared on the surface of SiMP. Fig. 1g and h displayed the lateral views of pristine SiMP and SiNW/SiMP, respectively. The surface of SiMP was smooth (indistinct ripples from the ICP process^25,26) and vertically-aligned Si nanowire arrays distributed uniformly on the surface of SiMP after 25 min of etching. For brief description, the SiMP etched under a certain duration time was named as SiNW/SiMP-T (T = 10, 15, 20, 25, 30 min).

Fig. 1 SEM images: top views of SiNW/SiMP under various etching time (a–f); side views of SiMP (g) and SiNW/SiMP-25 (h).

3.2 Optical adsorption and photoelectrochemical performance of Si hierarchical structure

UV-vis diffuse absorption spectra were used to characterize the optical absorption of SiNW/SiMP. As shown in Fig. 2, the optical absorption of SiNW/SiMP increased with the etching duration time until 25 min. Further prolonging the etching time led to the decrease of the optical adsorption. The results indicate that etching duration affected formation of SiNW, and thus leading to different optical adsorption behavior of SiNW/SiMP. SiNW/SiMP-25 was found to be the best one in the present study.

Fig. 2 UV-vis diffuse reflectance spectra of SiNW/SiMP with various etching durations.

I–V curves were measured to evaluate the photoelectrochemical performance. The results are shown in Fig. 3a. The tendency of photocurrent change with the increase of etching duration was similar to that of optical adsorption. SiNW/SiMP-25 displayed photocurrent of approximately −31 mA cm⁻² at −1 V (vs. SCE), which was 10 times higher than that of SiMP (−3 mA cm⁻²). Both the improved light absorption as well as charge separation caused due to the shortening charge transfer distance was suggested to attribute to the enhanced photocurrent. However, the photocurrent was testified to be unstable in the cyclic voltammetry experiment (Fig. 3b), which gradually decayed with the prolongation of illumination. The experimental results proved that SiNW/SiMP photocathode was unstable in a solution and oxidation passivation of Si surface appeared under light illumination.

Fig. 3 (a) I–V curves of SiMP and SiNW/SiMP with various etching time; (b) cyclic voltammetry curves of SiNW/SiMP-25 (0.5 M H₂SO₄, 100 mW cm⁻² of light intensity).

3.3 Characterization and photoelectrochemical performance of Ag/SiNW/SiMP

In order to inhibit the oxidation passivation of SiNW/SiMP-25 in a solution, Ag nanoparticles were deposited on SiNW/SiMP-25. For brief description, Ag/SiNW/SiMP prepared under different deposition time was named as Ag/SiNW/SiMP-T (T = 1, 3 and 5 min). Fig. 4a displays the top view of Ag/SiNW/SiMP-1. A few Ag nanoparticles appeared on the surface of SiNW. With the increase of deposition time, the amount of Ag nanoparticles increased. Fig. 4b shows that Ag nanoparticles distributed successively and uniformly under deposition time of 3 min. Fig. 4c is the enlarged view of the Ag nanoparticles shown in the rectangular frame in Fig. 4b. Further extending the deposition time to 5 min, Ag nanoparticles aggregated into clusters (Fig. 4d), leading to the decrease of optical absorption of Ag/SiNW/SiMP. The elemental mapping of Ag/SiNW/SiMP-3 (displayed in Fig. 4e and f) proved that Ag nanoparticles were dispersed and did not cover overall surface of Si. Furthermore, the EDX spectrum (not shown) indicates that the percent concentration of Si and Ag is 96.3 and 3.4, respectively.

Fig. 4 SEM images of Ag/SiNW/SiMP with various deposition time: (a) 1 min, (b) 3 min, (c) is amplification of the rectangular frame in (b), (d) 5 min, and elemental mapping of Si(e) and Ag (f).

According to the SEM images and elemental mapping, Ag nanoparticles only cover the surface of Si partially. Although Si under Ag nanoparticles can avoid passivation, the exposed surface of Si will still be oxidated to SiO₂ once it comes in contact with water or oxygen. In this system, transparent SiO₂ acts as a window that allows incident light to pass through to induce sublayer Si, whereas Ag nanoparticles can play a role of the passage for photogenerated electrons transfer from Si to solution.

Fig. 5 shows the DRS spectra of Ag/SiNW/SiMP with various deposition duration of Ag nanoparticles. Compared with SiNW/SiMP, Ag/SiNW/SiMP samples displayed lower optical absorption intensity and it is evident that optical absorption decreased with the extension of Ag deposition time. It may be ascribed that Ag particles obstruct incident light.

Fig. 5 UV-vis diffuse reflectance spectra of Ag/SiNW/SiMP with various Ag deposition durations.

The photoelectrochemical properties of Ag/SiNW/SiMP were examined in 0.5 M H₂SO₄ solution. For various deposition durations of Ag, the photocurrents of Ag/SiNW/SiMP were different. As shown in Fig. 6a, Ag/SiNW/SiMP-3 photocathode exhibited the highest photocurrent of approximately −37.5 mA cm⁻² at −1 V (vs. SCE), which was 21% higher than that of SiNW/SiMP-25 (−31 mA cm⁻²). For Ag/SiNW/SiMP, a built-in electric field formed between Ag particles and Si hierarchical structure that helped to improve the separation of photogenerated electron and holes. The photocurrent attenuation of Ag/SiNW/SiMP-5 photocathode was possibly caused by the aggregation of Ag nanoparticles that prevented the incident light arriving at Si substrate.

Fig. 6 I–V curves (a) and cyclic voltammetry curves (b) of Ag/SiNW/SiMP samples (0.5 M H₂SO₄, 100 mW cm⁻² of light intensity).

Cyclic voltammetry curves were measured to evaluate the photoelectrochemical stability. Ag/SiNW/SiMP-3 exhibited the most stable photocurrent among the three Ag/SiNW/SiMPs (Fig. 6b). According to the results, 3 min was an optimal duration for Ag deposition. For the shorter deposition time, Ag particles were not enough to protect SiNW/SiMP from surface passivation. While for longer duration, the thicker Ag layer would hinder the optical absorption.

3.4 Photoelectrocatalytic dechlorination activity

Dechlorination of 4-CP was carried out to investigate the photoelectrocatalytic activity of Ag/SiNW/SiMP photocathode in an aqueous solution. The results are shown in Fig. 7a. After 60 min treatment, the dechlorination efficiencies of 4-CP were found as 7%, 15% and 35% in photolytic, adsorption and electrolysis processes (−0.6 V potential), respectively. In the photocatalytic process, about 70% of 4-chlorophenol dechlorination was achieved due to photocatalytic activity of Ag/SiNW/SiMP. When using Ag/SiNW/SiMP-3 as the photocathode, the highest dechlorination efficiency of 95% was exhibited, which were 1.4 and 2.7 times higher than those of photocatalysis alone and electrolysis alone, respectively.

Fig. 7 (a) Dechlorination of 4-chlorophenol over Ag/SiW/SiMP-3 in various processes, (b) dechlorination of 4-chlorophenol over SiMP, SiW/SiMP-25 and Ag/SiW/SiMP-3, (c) six consecutive photoelectrochemical processes using Ag/SiNW/SiMP-3 photocathode (−0.6 V vs. SCE, 0.05 M H₂SO₄, 100 mW cm⁻² of light intensity).

For comparison, the photoelectrocatalytic dechlorination of 4-CP using SiMP and SiNW/SiMP-25 photocathodes were also detected and the dechlorination efficiencies were 59% and 89% after 60 min of reaction, respectively (Fig. 7b), the value of which was more than 98% after 30 min of reaction using the Ag/SiNW/SiMP-3 photocathode. The kinetic constant of dechlorination was 0.104 min⁻¹, which was 3 times and 9 times higher than those of SiNW/SiMP-25 (0.033 min⁻¹) and SiMP (0.013 min⁻¹), respectively. The intermediate products of 4-CP dechlorination in the photoelectrocatalytic process using Ag/SiNW/SiMP-3 photocathode was identified via HPLC. In the original solution, only 4-CP was detected and phenol was detected as the main intermediate after 5 min of reaction. Its peak area was close to that of 4-chlorophenol. The peak area can be used to measure the concentration of 4-CP and phenol. With increasing reaction time, the peak of phenol appeared and the peak area of 4-CP decreased. Therefore, the intermediate product of 4-CP dechlorination is phenol. Furthermore, Ag/SiNW/SiMP-3 exhibited good repeatability after six consecutive experiments (Fig. 7c). The abovementioned results demonstrated that Ag/SiNW/SiMP was an effective photocathode for 4-CP dechlorination in a solution.

4. Conclusions

A Si material with hierarchical structure decorated by Ag nanoparticles was successfully prepared. The hierarchical structure can decrease the distance between the interior of Si and solid–liquid interface for photogenerated carriers, and consequently enhance the photocurrent of Si material. Furthermore, Ag nanoparticles shielded the Si material against oxidation passivation. The results of dechlorination experiments demonstrated the feasibility of using an Ag/SiNW/SiMP photocathode for rapid degradation of pollutants in aqueous solutions. This work is expected to open a new avenue for using hierarchical structure Si in an aqueous solution such as pollutants degradation, solar water splitting and CO₂ reduction.

Acknowledgements

This study was supported by the National Nature Science Foundation of China (No. 21377020) and the Program for Liaoning Excellent Talents in University (LJQ2014008).

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