Research Article pubs.acs.org/journal/ascecg
Conversion of Bulk Metallurgical Silicon into Photocatalytic Nanoparticles by Copper-Assisted Chemical Etching Baoqin Guan,† Yu Sun,‡ Xiaopeng Li,*,§ Junna Wang,§ Si Chen,† Stefan Schweizer,§ Yong Wang,*,† and Ralf B. Wehrspohn*,§,∥ †
Department of Chemical Engineering, School of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai, P. R. China 200444 ‡ Department of Materials Engineering, Hanyang University, Ansan, Gyeonggi-do 426-791, Korea § Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Halle 06099, Germany ∥ Fraunhofer Institute for Microstructure of Materials and Systems (IMWS), Halle 06120, Germany S Supporting Information *
ABSTRACT: Low-grade metallurgical silicon (MG-Si, purity ∼98%−99%, $1/kg) with annual production over six million tons is an attractive feedstock to produce active photocatalysts. However, MG-Si is known as an electronically dead material due to serious charge recombination associated with high metal impurity levels. Upgrading MG-Si close to solar grade is essential to achieve desired performance; nevertheless, the traditional silicon refinement process is cost ineffective, has high energy consumption, and causes environmental pollution. Here, we address this critical issue by employing a roomtemperature one-step Cu-assisted chemical etching (CuACE) process, which successfully purifies MG-Si into active photocatalysts. We discover that the use of reducing agent (H3PO3) instead of commonly employed oxidant (H2O2) in the etchant system induces a novel phenomenon called “chemical cracking effect”. This effect significantly decreases the granularity of bulk MG-Si particles and simultaneously exposes fresh surfaces carrying impurities to the acids. This induces CuACE with promising purification rates, where major removal efficiencies of metal impurity reach 98.2% for Fe, 62.6% for Ca, and 61.0% for Al. Also, purified MG-Si exhibits excellent photocatalytic activity toward methyl orange (MO) degradation. Our approach provides new insights into metal-assisted chemical etching (MACE) of dirty silicon and opens a path for utilization of MG-Si in heterogeneous photocatalysis. KEYWORDS: Metallurgical silicon, Metal-assisted chemical etching (MACE), Purification, Silicon nanoparticles, Photocatalytic degradation
■
INTRODUCTION
refinement of MG-Si at a lower cost while reducing process complexity.6−9 There are two concurrent industrial routes of purification of Si, of which the traditional “Siemens” process is one of them. It involves the gasification, distillation, and chemical vapor deposition (CVD) of silicon. However, this process is energy intensive, where the gasification step amounts already to more than 200 kWh/kg.2 Additionally, the process is not environmentally benign because it produces highly toxic and flammable intermediate chlorosilane products. If compared with the “Siemens” route, the recently developed metallurgical route is characterized by higher production volume and energy efficiencies. However, the method still requires elevated temperatures and complicated multivacuum processes.7,8
Silicon plays a fundamental role in modern microelectronics and the solar cell industry due to its abundance, nontoxicity, and advanced processing infrastructure. It is well known that metal impurities create deep traps in the silicon bandgap, which significantly affect its photovoltaic and photocatalytic properties.1 For example, metallurgical silicon (MG-Si, purity ∼98%− 99%) as raw materials for solar grade silicon (SG-Si, 99.9999% (6N)) and electronic grade silicon (EG-Si, >99.9999999% (9N)) is seen as electronically dead due to the presence of heavy contaminants based on various transition metals.2,3 With respect to this, the maximum tolerated levels of metal impurities for the SG-Si raw material is around 25 ppm (Al) and less than 5 ppm for other metals.4 However, the production cost of SG-Si is still a major constraint for the photovoltaic industry, where the purification step imposes substantial costs as well as the generation of carbon emissions.5 Therefore, tremendous efforts have been devoted for the purification and © XXXX American Chemical Society
Received: June 29, 2016 Revised: September 5, 2016
A
DOI: 10.1021/acssuschemeng.6b01481 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering
to develop an alternative cost-effective metal catalyst method coupled with a more simplified etching process. To proceed, we tried to address aforementioned scientific issues as well as bridge the technological gap by adopting a one-step Cu-assisted chemical etching new method (CuACE, 1 h, 25 °C). Through comparing two published CuACE processes based on CuACE-I (Cu(NO3)2-HF-H3PO3-C2H5OH-H2O) and CuACE-II (Cu(NO3)2-HF-H2O2-C2H5OH-H2O),23,24 we realized that by introducing a reducing agent (H3PO3) instead of commonly utilized oxidant (H2O2) in the etchant system, a new reaction is induced. This phenomenon called “chemical cracking effect” significantly decreases the granularity of the original MG-Si particles and simultaneously exposes fresh surfaces carrying the impurities to the action of acids. Consequently, metal impurities could be removed efficiently to yield upgraded MG-Si particles with excellent photocatalytic activities toward degradation of methyl orange (MO). By contrast, though CuACE-II was also shown to be efficient in removing metal impurities, the upgraded MG-Si particles are still electronically dead without the presence of the “chemical cracking effect”.
The wet chemical route performed at low temperatures is a promising process since it consumes less energy as well as employs simpler equipment. So far, there are two main developed wet chemical routes: (i) acid leaching (also known as hydrometallurgical refining),9−13 and (ii) metal-assisted chemical etching (MACE).14−16 The acid leaching method has been employed for Si purification from 1919 in the manufacturing of radar components, and after nearly a century, it became an important industrial technique. For instance, Santos et al. upgraded MG-Si from ∼98% to 99.9% by combing HCl (16%, 5 h, 80 °C) with HF (2.5%, 2 h, 80 °C).10 Zhang et al. applied aqua regia solution (6 h, 70 °C) to purify MG-Si using metal with enhanced removal rates (79% for Fe, 77% for Al, 45% for Ca).11 Kim et al. obtained 99.99% pure Si through leaching MG-Si (∼99.74%) with an acid mixture composed of HNO3, HF, and CH3COOH (25 h).9 Recently, Zong et al. reported a remarkable purification rate of low-grade Si from 84% to 99.999% by combining high-energy mechanical ball milling (HEMM, 1000 rpm, Ar atmosphere) with acid leaching (HF, HCl, HNO3) under ultrasonication (2 h, 60 °C).12 It is worth noting that acid leaching, however, only removes the impurity phase originally exposed on the top surface of the MG-Si particles. According to Zong et al, no obvious purification effect should be observed if the average size of the particles is above 500 μm.12 Therefore, it could be predicted that finer particle size would increase the extraction efficiency of metal impurities. However, this should also increase the cost of investment (e.g., HEMM), energy consumption, and possibly decrease the production yield. Since its discovery in 2000 by Li et al.,17 metal-assisted chemical etching (MACE) has become widely employed for the production of nanostructures such as porous silicon and silicon nanowires. On the other hand, MACE of MG-Si was reported by us in 2013,14 where we observed that MACE can not only remove metal impurities from the top surface of the MG-Si particles but also can simultaneously create nanostructures as well as remove metal impurities from inside the core of MG-Si particles thanks to in-depth penetration of the Ag catalysts. Such unique etching features make the MACE method advantageous over the traditional acid leaching technique. Moreover, with the assistance of metal catalysts, the MACE process proceeds faster than the acid leaching technique even using less etching time (0.15−1 h) and lower temperature (∼25 °C).14 This results in reduced investment costs as well as shortened production time periods. Apart from enhanced purification, the nanostructured and upgraded silicon may not only serve as a source for further ingot growth but also opens up new applications such as photocatalysts due to both the enhanced light absorption and the efficient charge carrier separation. However, there are still many issues remaining to be solved with MACE including the mechanism of action. As some metal impurities are more sensitive to acidic treatment, it is normal to observe a purification effect to some extent. However, the extent of purification effect is still unknown in order to upgrade electronically dead MG-Si to nanostructured photoactive materials. Also, what would be the dominant factor in the mixed etchant system determining the photocatalytic activity? From the standpoint of technology, previous works are exclusively based on Ag-assisted chemical etching (AgACE), involving mainly two steps: metal deposition and electroless chemical etching.18−22 Therefore, in order for MACE to compete with the traditional acid leaching process, it is urgent
■
EXPERIMENTAL SECTION
Material Preparation. Coarse metallurgical-grade silicon powder (MG-Si, purity
