SiO2 composites and Si powders

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 1505–1510 www.elsevier.com/locate/ceramint

Synthesis and photoluminescence of Si3N4 nanowires from La/SiO2 composites and Si powders Feng Wangn, Xiaofang Qin, Lixia Yang, Yanfeng Meng, Lixiang Sun School of Chemistry and Materials Science, Ludong University, Yantai 264025, China Received 21 July 2014; received in revised form 27 August 2014; accepted 13 September 2014 Available online 28 September 2014

Abstract Large-scale Si3N4 nanowires were successfully synthesized by the reaction of La/SiO2 composites and Si powders in N2 atmosphere at 1200 1C for 6 h. The products were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The results show that the products mainly consist of hexagonal Si3N4 nanowires and a small amount of Si2N2O. The Si3N4 nanowires have a length of several hundreds of microns and a diameter of 100–200 nm, and Si3N4 nanowires grow along [100] direction. The growth of Si3N4 nanowires follows vapor–solid (VS) mechanism, and the growth process of the nanowires is simply discussed. The photoluminescence (PL) spectrum of Si3N4 nanowires at room temperature shows four emission peaks at 406, 472, 495 and 605 nm. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Electron microscopy; C. Optical properties; D. Si3N4; Nanowires

1. Introduction One-dimensional (1D) nanomaterials, such as nanowires, nanotubes, nanorods, nanobelts, nanowhiskers and nanocables, have attracted considerable attention owing to their excellent mechanical, optical, electronic, physical and chemical properties and wide potential applications [1–3]. Over the past several years various methods have been used to synthesize 1D nanomaterials, including chemical vapor deposition (CVD) [4], thermal evaporation method [5], carbothermal reduction [6], hydrothermal synthesis [7], direct nitridation method [8], combustion synthesis [9], and so on. As an advanced engineering ceramic and semiconductor material silicon nitride (Si3N4) has excellent properties, such as high strength, high hardness, corrosion resistance, excellent chemical stability, wide band-gap (
5.3 eV), good resistance to thermal shock and oxidation [10–12], and so on. Among the various Si3N4 materials, Si3N4 nanowires can be used as promising candidates for fabricating solar-blind photodetectors n

Corresponding author. Tel.:þ86 535 6672176. E-mail address: [email protected] (F. Wang).

http://dx.doi.org/10.1016/j.ceramint.2014.09.085 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

owing to the fast response and recovery as well as the good reversibility between the high and low conductivity states [13]. Recently, the synthesis of Si3N4 nanowires has received extensive attention and many routes have been proposed to prepare Si3N4 nanowires. Kim et al. prepared α-Si3N4 nanowires directly through the reaction of silicon substrate and NH3, using gallium (Ga), gallium nitride (GaN), and iron (Fe) nanoparticles as catalysts [14]. Zou et al. synthesized singlecrystalline α-Si3N4 nanowires by the reaction of Mg3N2 and SiCl4 in an autoclave at 600 1C [15]. Yang et al. reported the synthesis of Al-doped Si3N4 nanowires by catalyst-assisted pyrolysis of polyaluminasilazane precursors using FeCl2 powders as catalyst [16]. Zhang et al. prepared α-Si3N4 nanowires with diameters of a few tens of nanometers by combustion synthesis at 2800 1C [13]. Chen et al. fabricated α-Si3N4 nanowires via nitriding cryomilled nanocrystalline Si powders at 1300 1C for 12 h [17]. Besides, Lin et al. synthesized ultralong α-Si3N4 nanowires using N2 as the nitrogen source, SiO or a mixture of Si and SiO2 as the silicon source, N2 and Ar as the barrier gas, and CH4 as the reducing gas by a simple catalyst-free CVD route under superatmospheric pressure conditions [18]. Kusunose et al. obtained β-Si3N4 nanowires

F. Wang et al. / Ceramics International 41 (2015) 1505–1510

by the carbothermal reduction and nitridation of a homogeneous mixture of SiO2, carbon, and a small amount of cobalt via VLS mechanism [19]. However these methods have different disadvantages, such as high-cost, strict equipment requirement, high reaction temperature (2800 1C), and other reaction conditions. In this paper, we report a simple and low-cost method for the large-scale synthesis of Si3N4 nanowires by the reaction of La/SiO2 composites and Si powders in N2 atmosphere at 1200 1C for 6 h. The PL spectrum of Si3N4 nanowires at room temperature shows four emission peaks at 406, 472, 495 and 605 nm. 2. Experimental procedure 2.1. Preparation of Si3N4 nanowires Detailes of the synthesis process was described as follows. First, 1.57 g lanthanum nitrate hexahydrate (La(NO3)3  6H2O, AR) was dissolved in absolute ethanol (25 ml) under electromagnetic stirring. Tetraethoxysilane (TEOS, AR 25 ml) and oxalic acid solution (4.8%, 4 ml) were slowly added into the above solution. Silica sol was obtained by stirring the solution at room temperature for 24 h. Hexamethylenetetramine solution (35.7%, 2 ml) was added to speed up the gelation of the silica sol. The silica xerogel was obtained after the gel was dried at 110 1C for 12 h. The xerogel was fired in air at 700 1C for 3 h to obtain La/SiO2 composites. The mixture of La/SiO2 composite and Si powders according to equal molar ratio of SiO2 and Si was placed in an alumina tube furnace. The alumina tube furnace was heated to 1200 1C with a flowing nitrogen atmosphere (200 ml/min) and maintained at this temperature for 6 h. Then the system was cooled down to room temperature naturally under the protection of nitrogen. Finally a large amount of the white cotton-like products was obtained in the alumina boat, as shown in Fig. 3a.

Fig. 1. The strong and sharp diffraction peaks indicate that the products have an excellent crystalline structure. The diffraction peaks marked with α can be indexed as those from hexagonal α-Si3N4 with lattice constants a¼ 7.7553 Å and c¼ 5.6225 Å, which are consistent with the standard values (JCPDS 41-0360, a¼ 7.7541 Å and c¼ 5.6217 Å) [13]. The peaks marked with β can be identified as those from hexagonal β-Si3N4 with lattice constants a¼ 7.6036 Å and c¼ 2.9084 Å (JCPDS 33-1160, a¼ 7.6044 Å and c¼ 2.9075 Å) [20]. Therefore, the present Si3N4 is a mixture of α- and β-Si3N4 phases. In addition the diffraction peaks at about 2θ¼ 19.21, 19.91, 26.61, 37.11 and 37.71 marked with ● in the XRD pattern can be attributed to the Si2N2O phase (JCPDS, 47-1627) [21,22], which may be related to the formation mechanism of Si3N4 nanowires. Besides, two strong diffraction peaks at about 2θ=22.11 and 28.51 marked with n in the XRD pattern can be ascribed to cristobalite SiO2 phase (JCPDS, 39-1425) [23] which may originate from unreacted La/SiO2 composites. Further characterization of the white cotton-like products was made with FTIR. Fig. 2 shows the FTIR spectrum of the products. As seen in Fig. 2 there are two absorption bands at 800–1100 cm  1 and 400–600 cm  1, which correspond to the stretching vibration absorption peaks and the bending vibration of Si–N bond of Si3N4, respectively 12000

α α

10000

Intensity (a.u.)

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α

α

α

8000

0 10

2.2. Characterization of products

β

α

β

4000 2000

βα

α

6000

β

β

α

20

β β αα

β

30

40

50

β β β αα β α α

60

70

80

2Theta (deg.)

Fig. 1. XRD pattern of the white cotton-like products prepared from the reaction of the La/SiO2 composite and Si powders in N2 atmosphere. 60

406

Transmittance (%)

The crystalline phase of the products was characterized by X-ray powder diffraction (XRD) with a Rigaku D/max2500VPC diffractometer using CuKα radiation. IR spectrum of Si3N4 was recorded on a Fourier transform infrared (FTIR, MAGNA550, KBr) spectrometer in the 400–2400 cm  1 range. The morphology and structure of the products were examined by scanning electron microscopy (SEM, JSM-5610LV), and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G20) with energy dispersive X-ray (EDX) spectroscopy. Photoluminescence (PL) spectrum of the products was measured in an LS55 fluorescence spectrometer with a Xe lamp at room temperature.

50

453 677 492 575

40

1632 851 1038 894 936

30

3. Results and discussion A large amount of the white cotton-like products was obtained by the reaction of the La/SiO2 composite and Si powders at 1200 1C for 6 h in N2 atmosphere. The crystalline phase of the products was investigated by XRD, as shown in

20 2400

2000

1600

1200

800

400

Wavenumbers (cm-1) Fig. 2. FTIR spectrum of the white cotton-like products prepared from the reaction of the La/SiO2 composite and Si powders in N2 atmosphere.

F. Wang et al. / Ceramics International 41 (2015) 1505–1510

[14,24,25]. The band at 575 cm  1 is the characteristic absorption peak of β-Si3N4 [26,27]. The absorption peaks at 936, 851 and 492 cm  1 resulted from α-Si3N4 [26–29]. In addition, the weak absorption peak at 894 cm  1 is generally assigned to the characteristic absorption peak of Si2N2O [29]. The absorption peak at 1632 cm  1 is attributed to the absorbed water during the examination [30]. The XRD and FTIR results reveal that the products mainly consist of Si3N4 and a small amount of Si2N2O. After the furnace was cooled to room temperature, a large amount of the white cotton-like products was found which covered the alumina boat. An optical photograph of the white cotton-like products is shown in Fig. 3a. The white cotton-like product is about 1–2 mm in thickness. The morphology of the white cotton-like Si3N4 products was further characterized by SEM. Fig. 3b indicates a typical SEM image of Si3N4 products at a low magnification. From the SEM images, it can be seen that the products are mainly composed of large-scale Si3N4 nanowires. The nanowires are very dense, fine and long. Besides, there are some bulk particles among Si3N4 nanowires. Based on the XRD and FTIR, these bulk particles should be Si2N2O. The magnified SEM images (Fig. 3c and d) reveal that the Si3N4 nanowires have a length of several hundreds of microns and a diameter of 100–200 nm. The surfaces of Si3N4 nanowires are very smooth and clean without any particles. The structures of Si3N4 nanowires were further characterized by TEM, as shown in Fig. 4. TEM image of an individual nanowire is shown in Fig. 4a. The nanowire is straight and uniform structures are seen along the nanowire with a diameter of about 170 nm and smooth surfaces. The corresponding EDX spectrum (Fig. 4b) suggests that the nanowire is composed of only Si and N elements with atomic ratio of about

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3:4, which is in good agreement with the atomic ratio of Si3N4. The Cu element in the EDX spectrum originates from the copper grid supporting samples. HRTEM images of Si3N4 nanowires are shown in Fig. 4c and d. In HRTEM images (Fig. 4c and d) the spacing distances between two adjacent fringes perpendicular to the growth direction are 0.671 nm, corresponding to (100) planes of hexagonal α-Si3N4. This shows that the Si3N4 nanowires grow along [100] direction. From the HRTEM images it can be also seen that the Si3N4 nanowires possess a perfect crystal structure with few structural defects, including dislocations and stacking faults. The Si3N4 nanowires sheathed with an amorphous layer consisting of SiO2 or Si2N2O are not found, suggesting that Si2N2O alone exists in the present sample. To explain the growth mechanism of Si3N4 nanowires, there are two well-accepted growth models: vapor–solid (VS) model [31] and vapor–liquid–solid (VLS) model [32]. No catalytic droplets are found at the top of the nanowires, which implies that the growth of the Si3N4 nanowires does not follow the traditional VLS mechanism in the present experiment. However, La catalyst still plays an important role on the formation of the Si3N4 nanowires. The growth process of Si3N4 nanowires can be described as follows. First, a solid–solid reaction between the mixture of SiO2 and Si powders would happen at reaction temperature (1200 1C), which can be shown by the following equation [33]: SiO2(s) þ Si(s)-2SiO(g)

(1)

Due to the presence of metal La, the NRN bond is easily broken to form N atoms with higher activity by Eq. (2) when N2(g) diffuses to the surface of La/SiO2(s).

4 mm

Fig. 3. (a) Optical photographs of the white cotton-like products; (b) a typical SEM image of Si3N4 nanowires with low magnification; and (c, d) SEM images of Si3N4 nanowires with high magnification.

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100 nm

[100]

0.671 nm 0.671 nm

[100]

5 nm

5 nm

Fig. 4. (a) TEM image of an individual Si3N4 nanowire; (b) the corresponding EDX spectrum of Si3N4 nanowires in (a); and (c, d) HRTEM images of Si3N4 nanowires with the spacing distances of 0.671 nm, corresponding to (100) planes of hexagonal α-Si3N4.

N2(g)-N(a) þ N(a)

(2)

where (a) represents atomic species. With the reaction (1) and (2), the dissociated nitrogen atoms react with SiO(g) and replace oxygen in SiO to form Si2N2O and Si3N4 as follows: 2SiO(g) þ 2N(a)-Si2N2O(s) þ 1/2O2(g)

(3)

3Si2N2O(s)þ 2N(a)-2Si3N4(s) þ 3/2O2(g)

(4)

3SiO(g) þ 4N(a)-Si3N4(s) þ 3/2O2(g)

(5)

SiO(g) þ 1/2O2(g)-SiO2(s)

(6)

With the elapsing of the reaction time in N2 atmosphere the stepwise replacments will progress from SiO to Si2N2O, then to Si3N4. Finally, Si3N4 nanowires and a small amount of Si2N2O are formed. The above results and discussions suggest that the VS mechanism governs the growth of Si3N4 nanowires. Due to the relatively low reaction temperature in the present experiment, La–Si–O liquid alloy droplets cannot be formed. Hence, the growth of Si3N4 nanowires followed the VS growth mechanism rather than the VLS model in the presence of metal La catalyst. The PL properties of the Si3N4 nanowires were started out at room temperature with an excitation wavelength of 325 nm. Fig. 5 displays the PL spectrum of the Si3N4 nanowires. From

the PL spectrum, it can be seen that the Si3N4 nanowires exhibit four emission peaks centered at about 406 nm (3.05 eV in photon energy), 472 nm (2.63 eV), 495 nm (2.51 eV) and 605 nm (2.05 eV). Clearly these bands are not from the direct band gap emission, but from a deep-level or trap-level state. According to the calculation by Robertson and Powell the existing defect levels in the gap of Si3N4 include RSi–SiR, QN–, RSi0, and RSi–, which can be defined by four types of defects of the Si–Si bond, the N–N bond and the Si and N dangling bonds [34,35]. In addition there are two nitrogen defect states that give rise to levels within the gap, namely N4þ and N02, which have been calculated to be near the conduction and valence band, respectively [35,36]. Therefore the strong emission peak at 406 nm may be caused by the electronic transition of RSi0-Ev, or recombination either from the conduction band to the N02 levels or the valence band to the N4þ levels, which is similar to the previously reported PL peaks observed in α-Si3N4 whiskers [37,38]. From the XRD (Fig. 1) and FTIR (Fig. 2) results, it is noted that there are Si2N2O and SiO2 in the products. The presence of the oxygen vacancy in Si2N2O and SiO2 will inevitably lead to Si–O–Si and N–Si–O defective gap states, which lead to the electronic transitions RSi0-Si–O–Si and RSi0-N–Si–O, giving rise to the emissions around 2.51 eV and 2.63 eV, respectively [39,40]. Previous studies

F. Wang et al. / Ceramics International 41 (2015) 1505–1510

Intensity (a.u.)

406

472

400

440

495

480

605 520

560

600

640

Wavelength (nm) Fig. 5. PL spectrum of Si3N4 nanowires at room temperature.

show that the peak at 605 nm (2.05 eV) is possibly caused by the electronic transition of Ec-Si0, where Ec is the conductionband bottom [39]. The PL property of Si3N4 nanowires indicates that they have potential applications as optical materials. 4. Conclusion In summary, large-scale Si3N4 nanowires have been synthesized by the reaction of La/SiO2 composites and Si powders in N2 atmosphere. The Si3N4 nanowires have a length of several hundreds of microns and a diameter of 100–200 nm. HRTEM images indicate that Si3N4 nanowires grow along [100] direction. In the present experiment, the growth of Si3N4 nanowires follows VS mechanism and the stepwise replacements of oxygen in SiO. The PL spectrum of Si3N4 nanowires at room temperature shows four emission peaks at 406, 472, 495 and 605 nm. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 51102128 and 21103080), the Natural Science Foundation of Shandong Province (Nos. ZR2011EL005 and ZR2010BL023), and the Natural Science Foundation of Ludong University (No. LY2010006). References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (1999) 435–445. [2] W.S. Shi, Y.F. Zheng, N. Wang, C.S. Lee, S.T. Lee, A general synthetic route to III–V compound semiconductor nanowires, Adv. Mater. 13 (2001) 591–594. [3] X.S. Fang, L.F. Hu, C.H. Ye, L.D. Zhang, One-dimensional inorganic semiconductor nanostructures: a new carrier for nanosensors, Pure Appl. Chem. 82 (2010) 2185–2198. [4] W.S. Jang, S.Y. Bae, J. Park, J.P. Ahn, Thorn-like BN nanostructures, Solid State Commun. 133 (2005) 139–143. [5] J.J. Niu, J. Sha, D.R. Yang, Silicon nanowires fabricated by thermal evaporation of silicon monoxide, Physica E 23 (2004) 131–134.

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