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Abstract
By method of the pulsed laser deposition of Ni/C bilayer precursor films and annealing in N2, ambient amorphous SiO2 nanowires were prepared on fused quartz substrates. Field emission scanning electron microscopy images reveal that after annealing at the temperature of 1200°C for 1 h, the Ni/C films turn into dense nanowires with lengths and widths of several micrometers and tens of nanometers, respectively. Results of transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy fitted on it show that the nanowires are amorphous SiO2 nanowires and that they are grown in the Ni-leading solid-liquid-solid mechanism. Besides, C particles can promote SiO2 nanowires to grow longer in the forms of carbon and carbon monoxide.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays materials with the scale of nanometers have attracted great research attention for their large specific surface area and potential application such as catalyst and solar energy cells [1–5]. Among nano-materials, SiO2 nanowires show special properties like possible full-color display and high transmittance in visible light, which make them useful in probe of scanning near-field optical microscopes and optical fiber sensors in harsh environment [6–9]. Besides popularly applied sol-gel method [10], SiO2 nanowires are always fabricated by physical or chemical vapor deposition (PVD or CVD) method with metal catalyst in the vapor-liquid-solid or solid-liquid-solid (SLS) mechanism [11–15], besides, carbon/graphite powders can also catalyze its growth in the forms of carbon particles, CO or/and CO2 [16,17]. As for sol-gel method, the chemical reactions are easy to occur under lower temperature with the disadvantage of longer reaction duration and expensive or poisonous precursors. Meanwhile, for PVD and CVD method, they always need costly equipment and high vacuum.
In this article, SiO2 nanowires were grown through nickel and graphite powders co-catalyzing by the method of pulsed laser deposition (PLD) accompanied by N2 annealing. Among the fabrication of nano-materials, PLD is a simple and effective method which can synthesize multicomponent compounds with single target and it is easy to control the growth process through adjusting the parameters during film deposition. Besides, post-annealing in N2 atmosphere is also a commonly-used treatment in nano-materials growth for it can release the pressure and promote the growing of nucleus. In the article, the combination of the two process benefits the SiO2 nanowires grown. Firstly, uniform Ni/C bi-layer films were deposited on quartz substrate by PLD, followed by annealing in N2 ambient at the temperature of 1200°C. The morphology, structure and composition of the SiO2 nanowires were characterized, then the growth mechanism and the function of nickel and graphite powders were discussed in detail.
2. Experiment details
As is shown in Fig. 1, the growth of SiO2 nanowires can be divided into two steps, the fabrication of the precursor film and the formation of the nanowires. The precursor film was composed of nano-scale nickel and graphite powders which were deposited on fused quartz substrates with the method of PLD (Fig. 1(a)). In the PLD process, high purified Ni and C targets were ablated at a pressure of 2×10−3 Pa for 15 min by a pulsed laser produced by a Nd:YAG laser at the frequency of 10 Hz working at the wavelength of 532 nm, and the thickness of the as-deposited film was about 35 nm. After the formation of the Ni/C bi-layer films, the substrates were transferred into a furnace for annealing. The annealing process was operated under the temperature of 1200°C for 60 min in a nitrogen atmosphere with a flow rate of 1 L/min, for contrast, air was also chosen as the annealing ambient under the same condition. The annealing device can be schematically displayed in Fig. 1(b). Thus, dense and uniform SiO2 nanowires were formed.
Fig. 1. Schematic diagram of experimental set-ups for SiO2 nanowires growth, (a) the PLD equipment and (b) the annealing furnace. 1-focusing lens, 2-incident laser, 3-ablated targets, 4-substrates, 5-plume during PLD, 6-annealing tube, 7-heater, 8-samples, 9-annealing atmosphere flowThe morphology of the samples was characterized with field emission scanning electron microscopy (FESEM, Hitachi S4800), and the crystalline structures and composition of the nanowires were examined by transmission electron microscopy (TEM, Tecnai G2 F20-D436), high resolution TEM (HRTEM) and energy dispersive X-ray spectroscopy (EDXS) fitted on TEM.
3. Results and discussions
The morphologies of the samples before and after N2 annealing are shown in Fig. 2. The Ni/graphite precursor films are uniformly distributed on quartz substrate, which manifests that PLD is an effective method for fabrication of plat nanoscale films. There are some fluctuations on the films that were formed during the pulsed ablation of the nickel and carbon targets. After anneling at the temperature of 1200°C in nitrogen atmosphere for 1 h, SiO2 nanowires distributed on the quartz substrates and their lengths and widths were several micrometers and tens of nanometers, respectively. The existence of nickel in the precursor films and the growth of SiO2 nanowires after annealing under high temperature conform to the solid-liquid-solid (SLS) growth of 1-dimensional nanomaterials, in SLS growth, nickel nanofilms melt into nanoparticles at high temperatures and work as catalyst to form Ni-SiOx alloy, which will be discussed afterward.
In order to know more about the nanowires formed after N2 annealing, TEM, HRTEM and EDXS were carried out for the annealed samples and the results were displayed in Fig. 3. In TEM characterization, the nanowires were scraped from the substrates, dispersed in alcohol and transferred onto copper grids without carbon films. As can be seen from Fig. 3(a), the nanowires are coiled with each other and the widths of them are about 20-30 nm which are in line with the results from FESEM. The HRTEM morphology of the body part of the nanowires in Fig. 3(b) states that the nanowires are amorphous and the EDXS spectrum in Fig. 3(c) verifies the main elements made up the nanowires are Si and O and their semiquantitative atomic ratio is 1:2.3. From Fig. 3, it can be concluded that after annealing in N2 at the temperature of 1200°C for 1 h, the Ni/C films on quartz substrates turned into amorphous SiO2 nanowires with width of 20-30 nm. The superfluous O came from the air and the copper gird, besides, a small quantity of element Ni can be detected from the nanowires. Interestingly, in the TEM and FESEM morphologies of the nanowires (Fig. 3(a) and Fig. 2(b)), there are dark pellets on the tops of individual nanowires and EDXS characterization (not shown here) demonstrates the pellets mainly consist of Ni elements, which can be seen particularly in the inset of Fig. 3(a).
Fig. 3. (a) TEM, (b)HRTEM of the black circle point of (a) and (c)EDXS characterizations of the nanowires.
That nickel particles overwhelmed the top of the SiO2 nanowires describes that the nanowires are grown in the solid-liquid-solid (SLS) mechanism which is commonly existed in the nanomaterials fabrication in the presence of metal particles like Ni, Au and Ag, etc. [12–14]. In the SLS growth, the solid precursor films melted into liquid particles with grain size of tens of nanometers at high temperatures and the particles worked as catalyst to form alloys which were the basis of the nanowires, finally solid nanowires grew on the alloy particles. In the SiO2 nanowires growing here, the growth process can be divided into three steps as shown in Fig. 4: Fig. 4(a) shows the as-deposited Ni/C bi-layer films uniformly distributed on the quartz substrates, Fig. 4(b) displays the Ni/C films melt into liquid Ni/C alloy particles under the temperature of 1200°C, and in Fig. 4(c), SiO2 nanowires as long as several micrometers grow out on the basis of the alloy particles with nickel particles on the tops. Undoubtedly, the SiO2 nanowires grown in this article are in the SLS mechanism. Besides, the lengths of the silica nanowires can be controlled via changing the annealing durations within 60 minutes and this has been discussed elsewhere.
Fig. 4. Growth models for SiO2 nanowires. (a) the as-deposited bi-layer film, (b) the melted Ni-SiOx alloy particles and (c) the grown SiO2 nanowires.
The Ni-leading SLS growth was further proved by the FESEM morphology in Fig. 5(a) of the annealed sample without Ni particles in the deposition process. Without Ni particles, the Ni particles cannot form liquid catalyst nano-pellets and the Ni-SiOx alloys wouldn’t be formed, as a result, the SiO2 nanowires cannot grow up. There was only a flat surface on the substrate with random large particles in Fig. 5(a).
Fig. 5. FESEM morphologies of annealed samples with only. (a) C and (b) Ni particles in the precursor (black scale bar = 300 nm).
Then the function of graphite particles was studied. When only nickel particles were deposited on the quartz substrate, the morphology of the samples after annealing was shown in Fig. 5(b). As discussed before, nanowires can grow out by the leading of catalyst nickel, but the nanowires are only 500 nm long, which means carbon particles play an important role in the growing of SiO2 nanowires. It is stated that carbon can promote nanowire growth in the form of C, CO and/or CO2. It is thought that C and CO are the key elements in this experiment for the deposition and annealing process are performed in vacuum and CO2 may not generate. This conclusion was proved by the fact that nanowires couldn’t grow after annealing in the air because in this circumstance, carbon powders contact sufficiently with oxygen and will be oxidization into CO2. Based on the experiments and the discussions, the function of C in SiO2 nanowires growth can be described by the four following formula:
CO was generated by the incomplete combustion of carbon powders for the annealing process was conducted in N2 ambient (with some content of O2 left). SiO2 came from the fused quartz substrate, accompanied by the catalysis of Ni, SiOx nanowires grow up and grow longer under the participation of C and CO, after the annealing process was finished, SiOx nanowires would be oxidized into SiO2 ones when moved to air environment.
4. Summary
In conclusion, amorphous SiO2 nanowires were successfully fabricated by the co-catalysis of graphite and nickel particles. After C/Ni bilayers were deposited by PLD, the quartz substrates were annealed in a N2 ambient for 1 h at the temperature of 1200°C. The nanowires were crooked with length of several micrometers and width of tens of nanometers and they were characterized to be SiO2. In the growth of SiO2 nanowires, Ni functioned as liquid catalyst to form Ni-SiOx alloys and C catalyzed SiO2 nanowires to grow longer in the forms of C and CO.
Funding
National Natural Science Foundation of China (NSFC) (11275051); Shandong Academy of Sciences (SDAS) (2019QN0034); Program 973 (2012CB934303).
Acknowledgments
Many thanks to Mr. Leilei Guan from Fudan University for his help in the experiment process and results discussion.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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