Study On Corrosion Behavior Of Porous Silicon In Alkaline Solutions

The porosity and thickness of porous silicon are the two main parameters whichaffect the application and properties of porous silicon. As a result, it is very important tomeasure the two parametersaccurately. Among verious methods of determining theporosity and thickness of porous silicon, the weight loss measurement is generallyconsidered as a prefered measurement method due to its directness, simplicity, low costand without any assumption. The measurement is factly a destructive process. Usingthis method, porous silicon layers were removed from porous silicon samples, which isactually the serious corrosion and dissolution process of porous silicon in alkalinesolution. Moreover, the corrosion and dissolving process of porous silicon in alkalinesolution can directly affect the detection accuracy by weight loss measurements.Therefore, the corrosion behavior of porous silicon in different alkaline solution hasbeen systematically investigated in this paper. Meanwhile, the optimum condition ofmeasuring the porosity and thickness of porous silicon by weight loss measurementswas determined. In addition, a novel method was proposed to calculate the porosity ofporous layers. The main results are as follows:1) The morphology, microstructure, thermal stability, mass change, porosity andthickness change of different porous silicon samples fabricated by electrochemicalanodization have been characterized by SEM、XRD、DSC-TG、FT-IR and weight lossmeasurements, respectively. It was found that dissolution process of monocrystallinesilicon conformed Faraday’s laws of electrolysis, and the mass change(m1-m2) ofmonocrystalline silicon can be calculated by m1-m21.8582×10-6t1.0611×10-5J tandm-m.6937×10-6J1.0631×10-5126J tduring the preparation of porous silicon byelectrochemical etching. With anodizing time(10~60min) increasing, the degree of thecracking on porous silicon layer was more serious at high anodizing current density(＞30mA·cm-2). The porosity and thickness increased with anodizing current density andanodizing time increasing. The porosity and thickness of porous silicon prepared at thecurrent density of30mA cm-2for30min is26.33%and144.1μm, respectively. Inaddition, both the atmosphere and heating rate can affect the thermal stability of poroussilicon powder. In the air, N2and Ar atmosphere, the weight of the powder heated fromroom temperature to1250℃increased by72.62%,36.75%and2.15%, respectively.2) Different corrosion solutions were used to remove porous silicon layer, such as NaOH, KOH,TMAOH(CH3)4NOH), Ba(OH)2, Ca(OH)2, NaOC2H5, NaOCH3, Na2CO3,NaHCO3and NH3·H2O solution. It was found that the porous silicon layer can be onlycompletely removed by NaOH, KOH and TMAOH. The corrosion behavior of poroussilicon inNaOH, KOH and TMAOH solution were obviously affected by the solutionconcentration, ethanol(EtOH) additive and temperature. Moreover, the corrosionphenomenon, corrosion mechanism and corrosion parameters of porous silicon werediverse in different corrosion systems. The corrosion intermediate products of poroussilicon in TMAOH solution were mostly composed of [(CH3)4N]2SiO3and SiO2.3) The corrosion rate of porous silicon in alkaline solution increased with thetemperature increasing. As the concentration of NaOH and KOH increased, usingNaOH, KOH and TMAOH as the corrosion solutions, the corrosion rate and corrosioncurrent density subsequently increased and the corrosion potential shift towards morenegative values. The maximum corrosion rate of porous silicon in various concentrationof TMAOH solution can be obtained in0.5M TMAOH. The corrosion rate of poroussilicon(30mA·cm2-30min) corrosion in1.0M KOH,1.0M NaOH and1.0M TMAOHsolution at318K was322.4,296.4,196.7g·m-2·h-1, respectively. Meanwhile, thecorrosion potential and corrosion current density for porous silicon(30mA·cm2-30min)corrosion in1.0M KOH,NaOHand TMAOH solution at291K were-1.486,-1.474,-1.554V and2.427×10-5,2.349×10-5,6.743×10-6A·cm-2, respectively. In addition,adding ethanol into NaOH, KOH and TMAOH saolution all can make the corrosionpotential shifting towards positive values.4) The Arrhenius pre-exponential factor(A), activation energy(Ea), activationenthalpy(Ha) and activation entropy(Sa) of porous silicon corrosion were calculatedin the corrosion of1.0M NaOH、1.0M KOH、0.5M TMAOH、1.0M TMAOH、1.0M NaOH/EtOH(20%) and1.0M KOH/EtOH(30%), respectively. The A and Eaofporous silicon(30mA·cm2-30min) in1.0M NaOH and1.0M NaOH/EtOH (20%) were18.79kJ·mol-1、36.62kJ·mol-1and3.45×105g·m-2·h-1、4.35×108g·m-2·h-1, respectively.Though both the Eaand A can affect the corrosion rate, the study results reflect that theA was the main influencing factor of porous silicon corrosion in1.0M NaOH/EtOH(20%).5) The corrosion time, strength of alkaline solution, concentration of corrosionsolution, ethanol additive and temperature can affect the detection accuracy todetermine the porosity and thickness of porous silicon by weight loss measurements.The1.0M NaOH/EtOH(20%) can act as the optimal mixed solution to remove porous silicon layer at303~318K. In addition, the SEM method was proposed in this paper toeffectively measure the thickness and porosity of porous silicon.