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UDC 539.23 539.216.1
DOI: https://doi.org/10.18721/JPM.153.116
Morphology evolution of mesoporous silicon powder formed by Pd-assisted chemical etching at temperatures of 25 - 75 °C
O. V. Volovlikova 1 e, G. O. Silakov \ S. A. Gavrilov \ E. N. Lazorkina 1
1 National Research University of Electronic Technology, Moscow, Russia H [email protected]
Abstract: In this paper, we report the preparation of porous silicon powder by two-step Pd-assisted chemical etching with metallurgical grade polycrystalline silicon powder by varying the etching time from 30 to 90 minutes and solution temperature from 25 °C to 75 °C with and without thermal stabilization. A rapid temperature increase is observed with a maximum value of 95 — 100 °C in the case of etching without thermal stabilization. A high etching time of 90 minutes and a dissolution temperature above 50 °C have a negative effect on the formation of porous particles, which leads to the complete dissolution of silicon particles. The slow temperature growth for all initial temperatures in the case of the etching with thermal stabilization is observed. We established the positive effect of thermal stabilization in the process of etching on the thickness of the pores walls, reducing the uncontrollably growing rate of silicon etching, as a result, overetching of silicon.
Keywords: porous silicon powder, MACE, crystal size, Raman spectroscopy, thermal stabilization, temperature-time dependencies
Funding: The study was supported by a grant from the Russian Science Foundation (project no. 19-79-00205) in the part related to sample formation, investigations by Quantachrome Nova 3200e device, at the expense of State Task of the Ministry of Science and Higher Education of the Russian Federation 2020-2022 no. FSMR-2020-0018 in the part related to sample characterization using LabRAM HR UV-VIS-NIR.
Citation: Volovlikova O. V., Silakov G. O., Gavrilov S. A., Lazorkina E. N., Morphology evolution of mesoporous silicon powder formed by Pd-assisted chemical etching at temperatures of 25 — 75 °C, St. Petersburg State Polytechnical University Journal. Physics and Mathematics. 15 (3.1) (2022) 93-100. DOI: https://doi.org/10.18721/JPM.153.1l6
This is an open access article under the CC BY-NC 4.0 license (https://creativecommons. org/licenses/by-nc/4.0/)
Материалы конференции
УДК 539.23 539.216.1
DOI: https://doi.org/10.18721/JPM.153.116
Эволюция морфологии мезопористого порошка кремния, сформированного Pd-стимулированным химическим травлением при температурах 25-75 C
О. В. Воловликова 1 н, Г. О. Силаков ', С. А. Гаврилов ', Е. Н. Лазоркина 1
1 Национальный исследовательский университет «МИЭТ», Москва, Россия
Аннотация. В этой работе мы сообщаем о формировании порошка пористого кремния путем двухстадийного химического травления металлургического порошка поликристаллического кремния с использованием Pd путем изменения времени травления от 30 до 90 минут и температуры раствора от 25 до 75 ° C с термостабилизацией
© Volovlikova O. V., Silakov G. O., Gavrilov S. A., Lazorkina E. N., 2022. Published by Peter the Great St.Petersburg Polytechnic University.
и без нее. Наблюдается быстрое повышение температуры с максимальным значением 95 — 100°C в случае травления без термостабилизации. Длительное время травления 90 минут и температура растворения выше 50°C оказывают негативное влияние на формирование пористых частиц, что приводит к их полному растворению. Наблюдается медленный рост температуры для всех начальных температур в случае травления с термостабилизацией. Показано положительное влияние термостабилизации в процессе травления на толщину стенок пор, снижающее неконтролируемо растущую скорость травления кремния, и как следствие, перетрав кремния.
Ключевые слова: порошок пористого кремния, MACE, размер кристалла, рамановская спектроскопия, термостабилизация, температурно-временные зависимости
Финансирование: Исследование выполнено за счет гранта Российского научного фонда (проект № 19-79-00205) в части, связанной с формированием образцов, исследованием на приборе Quantachrome Nova 3200e, за счет государственного задания Министерства науки и высшего образования Российской Федерации 2020-2022 № FSMR-2020-0018 в части, связанной с определением характеристик пористого кремния с использованием LabRAM HR UV-VIS-NIR.
Ссылка при цитировании: Воловликова О. В., Силаков Г. О., Гаврилов С. А., Лазоркина Е. Н. Эволюция морфологии мезопористого порошка кремния, сформированного Pd-стимулированным химическим травлением при температурах 25 — 75 °C // Научно-технические ведомости СПбГПУ. Физико-математические науки. 2022. Т. 15. № 3.1. С. 93-100. DOI: https://doi.org/10.18721/ JPM.153.116
Статья открытого доступа, распространяемая по лицензии CC BY-NC 4.0 (https:// creativecommons.org/licenses/by-nc/4.0/)
Introduction
Increasing energy demand caused by technological advancement results in high consumption of non-renewable natural resources, in particular, natural gas. Depletion of these resources is one of the most significant global issues today. At present, hydrogen fuel cells are one of the most promising resources and energy-saving technologies. Their benefits include low emissions, high efficiency, portability, and silent operation [1]. One possible implementation is generating electricity by means of alcohol oxidation directly in a fuel cell [2, 3]. Recent scientific research results on alternative fuels have highlighted a growing interest in using porous silicon for miniaturized fuel cells and energy generators [4-6]. Potential benefits of using porous silicon for this purpose include several factors; in particular, high specific surface area and surface chemical reactivity, the possibility of modifying the surface morphology of the porous layers at the nano-and micro levels [7, 8]. In this regard, using porous Si powder allows increasing the specific surface area due to an extensive pore network [9] and higher cost-effectiveness thanks to replacing expensive single-crystal wafers with cheaper metallurgical Si powder.
Metal-assisted chemical etching (MACE) is a promising method for porous Si powder fabrication [10, 11] without an external current source. Various metals have been used for studying the MACE of Si powder [7, 12]. It has been shown that Ag [13, 14] and Fe [15] are among the most cost-effective and efficient. The implementation of MACE to Si powders provides the increase of pore diameter range and total porosity. Nonetheless, this results in the additional complexity of technology caused by an increased number of process parameters needing to be monitored. For instance, apart from the reactant concentration, etching duration, type and amount of metal, the formation of pores will also be affected by powder particle size. Furthermore, the etching of metallurgical-grade Si powder is accompanied by a number of difficulties, in particular, flotation, preventing the wetting of Si and hence uniform etching. This concern can be successfully remedied by adding an appropriate surfactant. Another general challenge consists of increasing the solution temperature that leads to a higher etching rate [11]. This causes undesirable effects such as overetching of small particles and underetching of large particles. Additionally, as shown earlier [16], the etching of Si powder with Ag and Fe as a catalyst is accompanied by the formation of silicate sediment. In order to avoid this phenomenon, we proposed using the Pd catalyst [17].
© Воловликова О. В., Силаков Г. О., Гаврилов С. А., Лазоркина Е. Н., 2022. Издатель: Санкт-Петербургский политехнический университет Петра Великого.
This study aims to adapt the Pd-assisted chemical etching for the metallurgical-grade Si powder so as to control the surface area and porous volume of the porous powder using different process duration and solution temperature; thereby decreasing the flotation and overetching effect using thermal stabilization of the solution during the etching.
Materials and Methods
Porous layers were formed at the surface of metallurgical-grade polycrystalline Si powder (Fig. 1, a) by Pd-assisted chemical etching. Mean particle diameter is 0.2 — 5 ^m. Si powder was treated in a mixture of 0.5 g/L PdCl2 and 0.65 M HC1 aqueous solutions for 30 min at 25 °C. Constant stirring was used to provide the uniform distribution of Pd particles on the surface of Si powder (Fig. 1, b). Etching of Si/Pd powder was carried out in a mixture of 40 % aqueous HF solution, 30 % aqueous H2O2 solution, and deionized water in a volume ratio of 25:10:4 25°C, 50°C, and 75°C both with and without thermal stabilization. Etching duration was 30 - 120 minutes. For thermal stabilization, a liquid thermostat was used with water as a heat carrying medium and a Teflon cell provided with a ventilated lid. The solution temperature was controlled using an external A>type thermocouple, covered with varnish for HF protection. Finally, the powders were rinsed with a mixture of deionized water and ethanol, centrifuged (MPW-351, Advanced Worldwide Technologies, Russia) and dried in room air for 24 hours.
The surface morphology of porous Si powder was analyzed using a scanning electron microscope (SEM; HeliosNanoLab 650, FEI Company, USA). Raman spectroscopy (LabRAM HR UV-VIS-NIR Raman Microscope, Horiba Scientific, Japan) was used to characterize porous silicon structures. The surface area and pore size of the porous structure formed by etching were measured using N2 adsorption-desorption isotherms on a Quantachrome Nova 3200e adsorption analyzer (Quantachrome Instruments, USA). Prior to measurements being taken, the porous Si samples were degassed at 300 °C for 3 hours to minimize contamination and water in the pores. Then, nitrogen adsorption and desorption isotherms were measured at 77 K. The specific surface area was calculated using BET (Brunauer-Emmett-Teller) method. The relative pressure interval was from 0.05 to 0.25. The BJH (Barrett-Joyner-Halenda) method was used for the calculation of the mesopore size distribution using the Kelvin model of pore filling [18, 19].
Results and discussions
Figure 1, c shows the micrographs of Si powder after 120 min etching in HF/H2O2/H2O at 25 °C.
As a result of the SEM studies carried out, it has been observed that porous silicon powders contain mesopores. Figure 2 shows the nitrogen adsorption/desorption isotherms at 77 K of the Si powder before etching and after etching at 25 °C, 50 °C and 75 °C throughout 30 — 90 minutes. The presence of mesopores is confirmed by a capillary-condensation hysteresis on the isotherms.
The specific surface area of the Si powder determined by the Brunauer-Emmett-Teller (BET) method was found to be 4.7 m2-g-1 for initial powder, 5.4 — 19.7 m2-g-1 for samples etched for 25 °C, 6 — 25.9 m2g— for samples etched for 50 °C, and 4.67 — 13.6 m2-g-1 for samples etched for 75 °C. The total volume of the Si powder determined by the Barrett-Joyner-Halenda (BJH)
a)
b)
5 jim
v . *
'ffBffj hHHI /H j' j
500 nm -
Fig. 1. SEM images of Si powder (a), Pd nanoparticles on silicon powder (b), porous silicon powder after 120 min etching (c)
a)
..V
I«
b)
i
Relative pressure, p/p0
Relative pressure, p/p0
c)
@ 20 0)
E 15
—•— 25 °C
▼ 50 °C
-<—75 5C
Relative pressure, p/p0
d)
@ 60. Q)
E
-25 °C\ 50 °C
it
J
' r
Relative pressure, p/p0
Fig. 2. Isotherms of N2 adsorption-desorption at 77 K for Si powders before etching (a) and after
etching for 30 (b), 60 (c) and 90 min (d)
method was found to be 17 cc^g-1 for initial powder, 43 — 196 cc-g-1 for samples etched for 25 °C, 18 — 45 cc-g—1 for samples etched for 50 °C, and 19 — 60 cc^g1 for samples etched for 75 °C. The pore size distribution plots obtained by the BJH method are shown in Fig. 3.
According to the data presented, all samples contain mesopores with diameters in the
a)
c 1.0x103
§ 8.0x104 g
2 6.0x1 o4
0 Q.
1 4.0x1 Û4
2 4)
•Ë 2.0X104
b)
■ 25 °C
2.0x10'3 - • 50 °C A - -
Iii ' ' £ —75 °C lêmm
P" c "Ö / X /
! -Ü 1.5x10"®- /
I £ . /
o ® 1.0x10"3 -o f V \
\m § 5.0x104 -
" ■----- ---■ b 0.0- y —* ■-» -■—
Diameter (nm)
Pore diameter (nm)
c)
■ 25 "C ___a
■ 75 °C A"'""
# /
- \ / /
4 \ . ; 1 ------4
5 10 15 20 25 30 35 40 45
Pore diameter (nm)
d)
5 10 15 20 25 30 35 40 45
Pore diameter (nm)
Fig. 3. Pore size distributions for Si powders before etching (a), after etching during 30 (b), 60 (c)
and 90 min (d)
range of 3 — 42 nm. Etching duration and solution temperature are known to be an essential parameter for the formation of porous materials especially when etching the powder, mainly due to problems such as flotation and possible overetching. The metallurgical-grade Si powders are dominated by pores with a diameter of 4 nm. Moreover, the samples after etching at 25, 50 and 75 °C are dominated by pores with a diameter of 21 — 41.67 nm, 3.65 — 31.1 nm, and 2.41 — 31.7 nm, respectively. The surface area of the porous powder and specific pore volume increase to a certain value with an increase in the solution temperature and etching duration. With further etching, the porous layer dissolves. The size of the pores increased and bond with the nearest pores with an increase in the duration of etching. Subsequently, the quantity of the pores with a big diameter decreases and the quantity of the pores with a small diameter increases.
Raman spectroscopy provides a swift and convenient method for the study of vibrational and structural properties of the porous material. It is also possible to obtain the average size of the crystals (pore walls) from the position of the phonon Raman peak [20, 21]. The shift (Aw) increases with increasing etching time without thermal stabilization. Thus, the maximum shift is 13.6 cm-1 for the etching duration of 30 min, 17.1 cm1 for 60 min, and 21.9 cm1 for 90 min at 25 °C. Increasing wavenumber shift indicates that the crystal size decreases, and thus, pore walls become thinner. At the same time, the etching duration of 120 min induces the formation of porous Si powder with Aw = 9.6 cm1. Fig. 4 shows the Raman spectra of porous Si powder formed by MACE with 60 min etching duration at 25 °C, 50 °C and 75 °C without thermal stabilization.
a)
b)
c)
14000 .'-J-- _____514,92 cm"'
12000
a 10000
........513,11 cm'1
I 8000 /A"""
■ \ 511.29 cm'1
1
2000 yV,,..
514,92 c 511,29 c
Wavenumbers, cm
490 495 500 505 510 515 Wavenumb
Fig. 4. Raman spectra obtained for Si powder etched in HF/H2O2/H2O with 60 min etching at 25 °C
(a), 50 °C (b), 75 °C (c)
The distribution of the size of powder particles is described by several spectra. Thus, the maximum shift Aw is 17.1 cm1 for 25 °C, 9.61 cm1 for 50 °C and 27.7 cm1 for 75 °C. For pure Si crystal and powder, a peak appears at 520.9 cm1. We used the equation (1) to calculate the size of the nanoparticles [22]:
Ara(D) = -A-(a/D)y, (1)
where Aw(D) is the Raman shift in nanostructures with diameter D, a is the lattice constant of silicon (0.543 nm) and A = 97.76 cm-1, y = 1.44 are the fit parameters that describe the phonon confinement in nanometric spheres of diameter D.
Table 1 shows crystal size of porous silicon powder after etching with different temperatures and durations with and without thermal stabilization. It was calculated by data of Raman spectra by Eq. 1.
The cryftal size decreases to 2 nm with increasing temperature of etching up to 50 °C and 75 °C without thermal ftabilization. Silicon powder completely dissolves, irrespective of the powder/etchant ratio at 75 °C and the etching duration of more than 90 min. According to the results on cryftal sizes obtained for the etching with thermal ftabilization, cryftal size at 25 °C is slightly higher than without thermal ftabilization. Cryftals 5 - 6 times larger in size were detected at 50 °C. Furthermore, there is no complete dissolution of Si powder at 75 °C.
Table 1
Crystal size of porous Si powder formed by MACE with and without thermal stabilization at different temperatures
Etching duration (t), min Crystal size (D), nm
Without thermal sta bilization
T = 25 °C T = 50 °C T = 75 °C
30 2-7 2 2
60 2-4 2 2
90 2-3 2 full etching
120 2-5 2 full etching
Etching duration (t), min Crystal size (D), nm
With thermal stabilization
T = 25 °C T = 50 °C T = 75 °C
30 6-10 2-13 2-11
60 2-6 3-10 3-8
90 2-6 2-10 2-8
120 4-13 2-6 2-4
Figure 5 shows plots of solution temperature as a function of etching duration at different initial temperatures. A rapid solution temperature increase is observed with a maximum value of 95 — 100 °C in the case of etching without thermal stabilization. The growth rates of temperature were found to be 16.8 °C/min, 30 °C/min, and 9.6 °C/min for initial temperatures of 25 °C, 50 °C and 75 °C, respectively. Following this, the temperature of the solution decreases monotonously and stabilizes at certain values. Moreover, the increase in temperature up to 90 — 100°C results in an uncontrolled dissolution of the powder and a decrease in the sample weight.
a)
Fig. 5. Temperature-time dependencies for solutions during the Si powder etching process at different initial temperatures without (a) and with thermal stabilization (b).
Metal-assisted chemical etching, in particular Pd-assisted chemical etching, is an exothermic process. The heating of the solution is always occurrent. The stated temperature of the etching process varies with the surface area of Si particles accessible to the etchant. Therefore, the higher the surface area of Si powder, the higher the etching temperature and consequently, the etching rate. Thermal stabilization (cooling) allows the limitation of the growth rate of temperature (Fig. 5, b), and thereby prevents overetching of powder particles. The slow temperature growth for all initial temperatures in the case of the etching with thermal stabilization is observed. The temperature changes were found to be from 25 °C to 41 °C, from 50 °C to 57 °C and from 70 °C to 75 °C for initial temperatures of 25 °C, 50 °C and 75 °C, respectively.
The overetching of the porous layer is not observed due to the absence of solution overheating. It has a positive effect on the surface morphology of the porous powder and material properties. Therefore, the thermal stabilization should be used during the etching of the metallurgical silicon powder in a solution containing HF/H2O2.
Conclusions
Thus, it was shown that silicon powder etching in HF/H2O2 leads to form porous layer with the specific surface area equaling 5.4 — 19.7 m2-g-1 for samples etched for 25 °C, 6 — 25.9 m2-g-1 for samples etched for 50 °C, and 4.67 — 13.6 m2-g-1 for samples etched for 75 °C. The silicon powder etching for 90 and 120 minutes at T = 50 °C and 75 °C without stabilization leads to its complete dissolution. A rapid increase in the solution temperature is observed with a maximum value of 95 °C (100 °C in the case of etching without thermal stabilization). The temperature growth rates were found to be 16.8 °C/min, 30 °C/min, and 9.6 °C/min for initial temperatures of 25° C, 50 °C and 75 °C, respectively. Using thermal stabilization changes the dissolution rate of the powder, which leads to formation of a porous layer with a crystal size 5 — 6 times higher than without thermal stabilization. The temperature changes were found to be from 25 °C to 41 °C, from 50 °C to 57 °C and from 70 °C to 75 °C for initial temperatures of 25 °C, 50 °C and 75 °C, respectively.
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THE AUTHORS
VOLOVLIKOVA Olga V.
[email protected] ORCID: 0000-0002-4165-1024
SILAKOV Gennady O.
[email protected] ORCID: 0000-0001-5215-603X
GAVRILOV Sergey A. [email protected]
ORCID: 0000-0002-2967-272X
Lazorkina Elena N.
[email protected] ORCID: 0000-0002-9155-0512
Received 08.05.2022. Approved after reviewing 08.07.2022. Accepted 08.07.2022.
© Peter the Great St. Petersburg Polytechnic University, 2022
