Rare Metals2019年第8期

Electrochemical preparation of silicon nanowires from porous Ni/SiO₂ blocks in molten CaCl₂

Sheng Fang Han Wang Juan-Yu Yang Shi-Gang Lu Bing Yu Jian-Tao Wang Chun-Rong Zhao

R&D Center for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals

作者简介：*Juan-Yu Yang e-mail: juanyuyang@163.com;yangjy@glabat.com;

收稿日期：23 March 2015

基金：financially supported by the National Natural Science Foundation of China(No.51404032 and No.51504032);the National High Technology Research and Development Program of China(No.2013AA050904);

Electrochemical preparation of silicon nanowires from porous Ni/SiO₂ blocks in molten CaCl₂

Sheng Fang Han Wang Juan-Yu Yang Shi-Gang Lu Bing Yu Jian-Tao Wang Chun-Rong Zhao

R&D Center for Vehicle Battery and Energy Storage,General Research Institute for Nonferrous Metals

Abstract：

Silicon nanowires(SiNWs)with diameter distributions ranging from 80 to 350 nm were prepared by electrochemical reduction of Ni/SiO₂ in molten CaCl₂.The effect of the content of nickel additives on the morphology of produced silicon was investigated.Large quantities of SiNWs are obtained by the electrochemical reduction of Ni/SiO₂ blocks with SiO₂ to Ni molar ratio of 20 and 10.Nickel additives repress the growth of irregular branches and promote longitudinal growth of SiNWs.Wire morphologies and surfaces are influenced by the electrolysis temperature.SiNWs become thicker with the increase of the electrolysis temperature.The optimum temperature to prepare single crystal SiNWs with high aspect ratio and extraordinary surface quality seems to be 1173 K.The amorphous layer of the silicon nanowire is thinner compared to the SiNW_s obtained from the pure SiO₂ pellets.The produced SiNWs show a photoluminescence emission peak at about 758 nm at room temperature.This work demonstrates the potentiality for the electrochemical reduction process to obtain large quantities of SiNWs with high quality.

Keyword：

Silicon nanowire; Nickel additives; SiO₂; CaCl₂; Electrochemical reduction;

Received： 23 March 2015

1 Introduction

Silicon nanowires (SiNWs) have received much attention because of their potential applications in electronics [ 1] ,optoelectronics [ 2] ,sensor applications [ 3] and lithium ion battery [ 4, 5] .Compared to silicon in flm form,SiNWs have the potential to provide better performance in these applications.To exploit this potential,SiNWs are needed in large quantities.Although,a great number of techniques including the chemical vapor deposition (CVD) approach [ 6] ,template-guided synthesis [ 2] ,oxide-assisted growth [ 7] ,laser ablation [ 8] ,supercritical fluid approach [ 9] and others [ 10] have been used to synthesis SiNWs,very few of them are able to produce SiNWs in bulk quantities.Thus,methods with the advantage of low-cost and bulk process capability should be investigated.

In 2003,preparation of pure silicon by the electrochemical reduction of solid SiO₂ in molten CaCl₂ was reported [ 11] .Both quartz and SiO₂ powders have been used as the raw materials [ 11, 12] .The formation of SiNWs by the electrochemical reduction of silica pellets obtained from nano-sized SiO₂ powders in molten CaCl₂ was reported in 2009 [ 13] .SiNWs also can be prepared by the electrochemical reduction of SiO₂ pellets with a tetropodlikc microstructure [ 14] .More recently,silicon nanowire arrays have been produced by the electrochemical reduction of a SiO₂ glass plate coated by nickel net [ 15] .By simplyincreasing the number of silica pellets or SiO₂ glass plates,gram quantity of nanowires can be produced in one turn electrolysis process.Moreover,as the raw material and the equipments are not expensive,the electrochemical reduction process would take the advantage of both cost and productivity.

However,the major disadvantage of this approach is the low quality of SiNWs.The SiNWs synthesized by the electrochemical reduction of SiO₂ have a low aspect ratio,high defects,irregular branches and relatively high content of oxygen [ 13, 14, 15] .These disadvantages hinder the application of the produced SiNWs in many fields,such as electronic devices and sensors.To solve the problem of irregul ar branches,Au nanoparticlcs were added to SiO₂ to promote longitudinal growth of the SiNWs [ 14] .This result indicates that adding metal powders to the SiO₂ pellets may be an approach to control the morphology and the structure of the prepared SiNWs.However,using nano-sized metal particles is not suitable for bulk production of SiNWs,since the cost of nanoparticles is very high.In addition,the high activity of nano-sized metal particles could make it difficult to handle during the mixing process.In this work,nickel powders with micrometer size were added to the SiO₂ powders to repress the growth of irregular branches and promote longitudinal growth of SiNWs.The effect of the content of nickel additives and the electrolysis temperatures on the microstructure of prepared SiNWs was studied and discussed.Furthermore,the optical property of the prepared SiNWs was investigated.

2 Experimental

SiO₂ powders (purity of 99.98%,particle size of25-30 nm) were mixed with distilled water to form silica sol,then nickel metal powders (T255,purity>99.7%,fisher sub-sieve size of 2.2-2.8μm) were added to the sol at the desired SiO₂ to Ni molar ratios of 20,10,8 and 7.The silica sol was dried at 353 K for 2 days and followed by ball milling.The preformed powders were hot pressed(compacting pressure of 10 MPa,hot-pressing temperature of 1223 K,dwell time of 75 min,purged with argon) into porous plates with the size of 53 mm×53 mm×5 mm and then cut into blocks with the size of10 mm×10 mm×5 mm.The blocks were drilled and threaded onto a molybdenum wire (purity of 99.95%,2.5 mm in diameter) to form Mo-Ni/SiO₂ contacting electrodes.All molten salt experiments were conducted in a scaled stainless steel vessel that was continuously purged with argon gas (purity>99.999%,O₂<2×10^-6,H₂O<1×10^-6 and flow velocity of 400 ml-min^-1) at desired temperatures (1123,1173,1223 and 1273 K).The graphite crucible was used as the container for the molten salt and the anode.A constant voltage (1.5 V) was applied for the electrochemical reduction of the Ni/SiO₂ blocks.Pre-electrolysis for removing impurities from the molten salt was carried out at 2.5 V for 1 h or longer between the graphite anode and a molybdenum rod cathode.The reduced blocks were raised from the molten salt and kept in the upper part of the reactor until the furnace temperature dropped to ambient temperature before removal for analysis.Solidified salt that adhered to the reduced blocks was removed by washing in distilled water in an ultrasonic bath.The powders obtained from the blocks were collected and dried under vacuum for analysis.

The synthesized powders were characterized by X-ray diffractometer (XRD,X’Pert PRO MPD),field-emission scanning electron microscope (FESEM,Hitachi S4800),transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM,FEI Tecnai F30),energy dispersive X-ray spectrum (EDS),and selected area electron diffraction (SAED).The room-temperature photoluminescence (PL) of the synthesized SiNWs was recorded on a JY-HR800 Raman laser spectrometer with a 523 nm emission line of Ar-ion laser.

3 Results and discussion

XRD patterns of the products from the electrochemical reduction of blocks with various molar ratios of SiO₂ to Ni are presented in Fig.1.Except one unconfirmed peak shown in Fig.ld,the diffraction lines are indexed to Si(JCPDS No.27-1402) and NiSi₂ (JCPDS No.43-0989),respectively.No diffraction line is indexed to nickel.It should be pointed out that nickel would react with prepared silicon and form the most stable Si-rich silicide at 1173 K according to the phase diagram.

SEM images of the products obtained by electrochemical reduction of Ni/SiO₂ blocks with different molar ratios of SiO2 to Ni are presented in Fig.2.The SiO₂ to Ni ratio.as expected,affects the product morphologies.Figure 2c,d presents the corresponding products for growth conditions of SiO₂ to Ni ratios of 7 and 8,respectively.In both cases,the nanowires are formed in extremely low yield or not at all.When the SiO₂ to Ni ratio increases to 10 and 20,large quantities of nano wires are produced in both cases.The nanowires produced by these two conditions can be classified into two different types of nanowires as presented in Fig.2a,b.Type I wires,as shown in Fig.2a,are curvy,highly branched and largely crooked.These wires have a similar morphology to the SiNWs prepared by the electrochemical reduction of pure nanometer SiO₂ pellets as reported in previous research [ 13] .Compared to the SiNWs obtained from the pure SiO₂ pellets,Type I wires show wider diameter distributions ranging from 50 to 400 nm.Since adding conducting additives to porous electrode can accelerate the reduction rate of SiO₂ [ 16] ,one may expect an increasing agglomeration of silicon atoms with the increase of the reduction rate.The increase of reduction rate could be a reason for the formation of thicker wires.The inhomogeneous distribution of nickel powders also gives rise to non-uniform distribution of diameters,because more silicon atoms could be formed at the nickel rich area.

Fig.1 XRD patterns of reaction products obtained by electrochem-ical reduction of Ni/SiO₂ blocks with various molar ratios of SiO₂ to Ni (20,10,8 and 7) at 1.5 V in molten CaCl₂ for 5 h at 1173 K

Fig.2 FESEM images of reaction products obtained by electrochemical reduction of Ni/SiO₂ blocks with various ratios of SiO₂ to Ni at 1.5 V in molten CaCl₂ for 5 h at 1173 K:a 20,b 10,c 8,and d 7

TypeⅡwires are long and straight as shown in Fig.2b.TypeⅡwires also have wide diameter distributions ranging from 80 to 350 nm.A large portion of TypeⅡwires (more than 97%) have average diameter distributions ranging from 80 to 250 nm.The length can be as long as 20μm and the aspect ratio can be as high as 100.Compared to SiO₂ to Ni ratio of 20,large quantities of TypeⅡwires are obtained when SiO₂ to Ni ratio is 10,which should be assigned to the content and distribution of nickel additives.Almost all of the nanowires obtained from the blocks with the SiO₂ to Ni ratio of 10 show straight morphology.In this case,nickel powders not only act as conducting points but also promote the growth of TypeⅡwires.Kinks are usually observed among Type II wires.In this system,the kink could be related to the structure of SiO₂ in porous electrode after immersed in molten CaCl₂ at 1173 K.Changing the morphology of produced silicon by adding metal particles to SiO₂ is not a well-known phenomenon but has been observed in previous work [ 14] .The high length-to-diameter ratio suggests that the silicon atoms are incorporated through the growing tip,which is the character of tip-led growth.If this was the case,nano-sized silicide droplets or particles would be formed.This is noteworthy here,since no nano-sized metal additives are used.

FESEM images of wires obtained from blocks with SiO₂to Ni molar ratio of 10 at different electrolysis temperatures are shown in Fig.3.SiNWs prepared at electrolysis temperatures of 1123 and 1173 K show average wire diameters of 50-100 and 80-250 nm,respectively.The nano wires become thicker with the increase of electrolysis temperature.When the electrolysis temperature increases to 1223and 1273 K,the diameters of silicon wires are in sub-micrometer scale as shown in Fig.3c,d,respectively.

Another effect of the temperature is to change the surface morphology and crystallinity of SiNWs.Figures 4a and 5a show the TEM images of SiNWs prepared at 1123and 1173 K,respectively.The SiNW synthesized at1123 K has a taper shape and a rough surface.The inset in Fig.4a represents the EBSD pattern which indicates low crystallinity of the SiNWs.An amorphous layer with about2-3 nm in thickness is observed on the nanowire body as indicated in Fig.4b.When the growth temperature increases to 1173 K,the nano wire body is characterized with a uniform diameter and smooth surface as shown in Fig.5a.It is a single crystal with cubic lattice structure as indicated by SAED pattern shown in the inset in Fig.5a.And the SAED pattern is resolved to show spots in (022),(200) and (111) plane families,with interplannar distances of 0.192,0.271,and 0.313 nm,respectively.HRTEMimage reveals well-defined lattice fringes of (200) and( ) planes,along with a smooth surface,as shown in Fig.5b.These results indicate a preferential (011) growth orientation,as indicated by the arrow.The SiNWs prepared at 1173 K show a much thinner amorphous layer (<1 nm)as shown in Fig.5b.Scan TEM images of these two nano wire bodies are shown in Fig.6 a,c,respectively.The elemental maps shown in Fig.6b,d represent no Ni in both of these two nanowire bodies,which validates the nanowires to be SiNWs.In contrast to the pure SiO₂ system,both of the two O profiles show no peaks with a gap in the center [ 13] .The amorphous layers of the SiNWs here are thinner compared to those of the SiNWs prepared by electrochemical reduction of pure SiO₂ pellets (about5-6 nm) [ 13] .These results demonstrate that the amount of O in the out layer is lower than that of the SiNWs prepared from electrochemical reduction of pure SiO₂ pellets.It seems that 1173 K is the optimum temperature to synthesis single crystal SiNWs with high aspect ratio and extraordinary surface quality.

Fig.3 FESEM images of SiNWs prepared by electrochemical reduction of Ni/SiO₂ blocks (SiO₂ to Ni ratio=10) at 1.5 V for 5 h in molten CaCl₂ at various temperatures:a 1123 K,b 1173 K,c 1223 K,and d 1273 K

Fig.4 TEM image (inset being SAED pattern) a and HRTEM image b of SiNWs prepared by electrochemical reduction of Ni/SiO₂ blocks(SiO₂ to Ni ratio=10) at 1.5 V for 5 h in molten CaCl₂ at 1123 K

Fig.5 TEM image (inset being SAED pattern) a and HRTEM image b of SiNWs prepared by electrochemical reduction of Ni/SiO₂ blocks(SiO₂ to Ni ratio=10) at 1.5 V for 5 h in molten CaCl₂ at 1173 K

The room-temperature PL spectrum of the SiNWs prepared by electrochemical reduction of Ni/SiO₂ blocks(SiO₂ to Ni ratio=10) in molten CaCl₂ at 1173 K for 5 h is shown in Fig.7.One broad and strong emission peak at about 758 nm is detected.The breadth of the PL peak may result from the wide diameter distribution of SiNWs [ 17] .The PL behavior of the synthesized SiNWs could be attributed to interface states between the amorphous sheath and the crystalline silicon core with some intrinsic defects [ 18] .

4 Conclusion

In this study,SiNWs with diameter distributions ranging from 80 to 350 nm were prepared by electrochemical reduction of the Ni/SiO₂ blocks in molten CaCl_2.The results show that the optimum SiO₂ to Ni ratio is 10 and the optimum temperature is 1173 K.This work indicates that SiNWs with equal quality to those synthesized by gasphase methods could be produced by the electrochemical reduction of solid SiO_2.The optical characterization suggests that the as-synthesized SiNWs have potential application in optoelectronics.The high length-to-diameter ratio suggests a tip-led growth process.To clarify the nucleation and growth process of SiNWs,further investigation is required.

Fig.6 Scan TEM images and corresponding line-scanning elemental maps:a,b SiNWs shown in Fig.4a and c,d SiNWs shown in Fig.5a

Fig.7 PL spectrum taken from SiNWs obtained from electrochem-ical reduction of Ni/SiO₂ blocks (SiO₂ to Ni ratio=10) at 1.5 V for5 h in molten CaCl₂ at 1173 K

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