Switching of the products by changing the size and shape of catalytic nanoparticles during CVD growth of MoS2 nanotubes

Weng Mengting *, Takashi Yanase *, Fumiya Uehara , Sho Watanabe , Takuya Miura , Taro Nagahama and Toshihiro Shimada *
Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo 060-8628, Japan. E-mail: wengmengting1@gmail.com; yanase42@eng.hokudai.ac.jp; shimadat@eng.hokudai.ac.jp

Received 30th March 2017 , Accepted 5th June 2017

First published on 7th June 2017


Nanowires of layered materials are important because they exhibit the highest sensitivity as electrically-detecting chemical sensors. MoS2 nanowires have been synthesized by a catalytic chemical vapor deposition method on silicon substrates drop-coated with FeO nanoparticles of different shapes. Switching of the products (MoS2 nanowires to SiO2 nanowires) has been observed when the shapes and sizes of the FeO nanoparticles changed. MoS2 nanowires were grown in the presence of six-horned octahedral nanoparticles, whereas SiO2 nanowires were formed in the presence of spherical nanoparticles. Their morphology, crystal structure and elemental composition have been fully investigated to elucidate the growth mechanism of the nanowires. The kinetics of the grown SiO2 and MoS2 nanowires are competing, giving rise to the observed switching.


Introduction

Semiconductor nanowires have been a hot topic over the past decades. One of their major applications is in nanoscale sensor arrays, which are important, for example, as interfaces between bio-systems and information technologies.1,2 The sensitivity of the nano-sensors depends on the capping layer on the surfaces, which is primarily introduced for the protection of the semiconductor from the surrounding solution or atmosphere. The capping layer can also be used to provide selectivity to the sensing species. For conventional semiconductors, such as Si or GaAs, the surface is chemically active and has to be capped by oxides or organic species, which sometimes reduces their sensitivity, and the long-term stability is still problematic. The highest sensitivity and stability are expected from layered materials such as graphene and transition metal dichalcogenides (TMDs). Graphene can be made into nanowires, i.e., carbon nanotubes, and its application in nanoscale sensors is promising.3–5 However, the physical properties of carbon nanotubes are strongly dependent on their chirality which is difficult to control.6,7 Therefore, it is worth studying the nanotubes of TMDs, in which no strong chirality effects have been reported.

Molybdenum dioxide (MoS2), a representative TMD material, has a broad range of applications including field effect transistors (FET),8–10 gas sensors,11–13 lithium batteries14–16 and solar cells.17–20 Several methods have been used to grow MoS2 nanowires, such as the vapor transport reaction with MoS2 powders,21,22 H2S reduction of molybdenum oxide mixtures,23 thermal decomposition of the precursor (NH4)2MoS4 using the template method,24 hydrothermal treatment of LiMoS2 lamellae,25 and sulfurization of the precursor Mo6S4I6 nanowires.26 Nevertheless, progress towards the control of the nanowires growth has still been limited. It is necessary to investigate the mechanism of their growth process to control the size of the nanowires and optimize their properties.

In this paper, we focus on the chemical vapor deposition (CVD) with solid catalyst nanoparticles of MoS2 nanowires. This approach is successful for the growth of carbon nanotubes, Si, GaAs and ZnO. In our previous study, it was found that FeO and other metal oxides can function as catalysts for the growth of MoS2 nanotubes.27 In this study, we focus on the impact of the size and shape of the chemically synthesized FeO nanoparticles on nanowire growth.

FeO is one of the common iron oxides, which consist of goethite (α-FeOOH), hematite (α-Fe2O3), magnetite (Fe3O4), maghemite (γ-Fe2O3), etc. Among them, FeO is the most chemically reactive, as a result of its defective NaCl structure which is non-stoichiometric with an O-vacancy.28 As reported by Sun's group, they have synthesized FeO nanoparticles with two shapes, i.e., octahedral and spherical.29 It is of great interest to investigate whether the shape has an effect on the catalytic reactivity of the FeO nanoparticles. Actually, we found switching of the products between MoS2 and SiO2 as described in the following sections.

Results and discussion

Fig. 1 shows the typical TEM images of the synthesized FeO nanoparticles. As already reported,29 the shapes and sizes of the particles are controlled by changing the reaction conditions. In this study, we synthesized 9 nm spherical nanoparticles (Fig. 1(a)) by heating a mixture of iron(III) acetylacetonate [Fe(acac)3], oleic acid (OA) (4 ml) and oleylamine (OAm) (6 ml) at 220 °C, then at 300 °C, each for 30 min. Under the same conditions, 30 nm six-horned octahedral nanoparticles (Fig. 1(c)) were obtained by the decomposition of Fe(acac)3 mixed with OA (5.5 ml) and OAm (5 ml). Additionally, we found that stirring is another essential factor for controlling the shape of the particles during decomposition. For example, heating a mixture of Fe(acac)3, OA (5.5 ml) and OAm (5 ml) without stirring at 220 °C for 1 h, then at 300 °C for 30 min, formed 50 nm six-horned octahedral nanoparticles (Fig. S1), whereas 24 nm spherical nanoparticles (Fig. 1(b)) were obtained by continuous stirring. The flow chart of the synthesis process is shown in Fig. S2.
image file: c7ce00608j-f1.tif
Fig. 1 TEM images of FeO nanoparticles: (a) 9 nm, (b) 24 nm spherical and (c) 30 nm six-horned octahedral nanoparticles.

The crystal structures of the 9 nm and 24 nm spherical particles and 30 nm six-horned octahedral particles were characterized by XRD, as shown in Fig. 2. It is clear that all the samples exhibit the 111, 200, 220, 311 and 222 characteristic peaks of the FeO structure, indicating that the synthesized nanoparticles are almost pure FeO. The Raman spectra were also taken, as shown in Fig. S3. The morphologies of the nanoparticles heated to 1000 °C on SiO2/Si substrates are shown in Fig. S4, and their corresponding Raman spectra and XRD patterns are shown in Fig. S5 and S6. Even though some morphology change has occurred in the nanoparticles due to their agglomeration at high temperatures, it is obvious that the size and shape of the nanoparticles are distinct from each other. We admit that very thin residual carbon might exist on the surface of catalysts, but the coverage of the carbon is the same for all the catalysts.


image file: c7ce00608j-f2.tif
Fig. 2 XRD patterns of FeO nanoparticles: (a) 9 nm, (b) 24 nm and (c) 30 nm nanoparticles.

The nanowires were grown on 285 nm SiO2/Si substrates in a CVD reactor made of a quartz tube (Fig. S7)30,31 at 1000 °C with MoO3 and S using the 9 nm, 24 nm spherical, 30 nm and 50 nm six-horned octahedral particles as catalysts. Powders of 0.15 g MoO3 and 1.5 g S were evaporated at 650 °C and 280 °C, respectively. The gas flow of the MoO3 and S lines was started when the temperature reached 1000 °C. The pressure in the growth tube was 1 atm and the growth time was 1 hour.

The SEM images of the products after CVD using the 9 nm, 24 nm, 30 nm and 50 nm FeO are shown in Fig. 3(a)–(c) and S8 respectively. TEM equipped with EDS was employed to investigate their crystal structures and morphologies, as shown in Fig. 4. In the case of the 9 nm spherical catalyst particles, high-aspect ratio nanowires with a diameter of 40 nm and length of 10 μm, were found to grow with a high density. The EDS and diffraction results show that they are amorphous SiO2 nanowires with a particle cap. EDS analysis revealed that the particles consist of Mo, Fe and S.


image file: c7ce00608j-f3.tif
Fig. 3 SEM images after CVD with (a) 9 nm, (b) 24 nm and (c) 30 nm catalyst nanoparticles.

image file: c7ce00608j-f4.tif
Fig. 4 TEM images of (a) SiO2 and (c) MoS2 nanowires. (b) and (d) are the EDS results corresponding to (a) and (c), respectively.

When the diameter of the spherical particles increased to 24 nm, as shown in Fig. 3(b), low-density nanowires and particles covered by MoS2 films were observed. Similar to the ones grown with the 9 nm particles, a particle can be found at the tip of a nanowire (Fig. 3b inset). From the Raman spectra and EDS results, the nanowires were SiO2. Compared to Fig. 3(a), the diameter of the nanowire increased and the length became shorter.

When the shape of the catalyst nanoparticles changed from spherical to 30 nm six-horned octahedra, nanowires with no particle caps were found (Fig. S9). The nanowires are typically a few micrometers in length with diameters from 40 to 80 nm. Based on the EDS, they were composed of Mo and S. The peaks in the TEM diffraction (Fig. 4(c) inset) and Raman spectra (Fig. S10) of the nanowires correspond to those of MoS2. The two peaks in the Raman spectra, centered at 380 and 406 cm−1, were assigned to the E2g and A1g modes, respectively.32–34 In the case of 50 nm, low-density MoS2 nanowires were obtained (Fig. S8), which were evidenced by TEM-EDS (Fig. S11).

We have now observed drastic switching of the CVD product depending on the size and shape of the FeO nanoparticle catalysts. This result is summarized in the first row of Table 1. In order to study the growth mechanism, CVD experiments with only flowing MoO3 (no S) were conducted. The SEM images are shown in Fig. S12, and no nanowires were observed in them. Instead, the particles randomly dispersed on the substrates.

Table 1 Summary of the CVD products
Catalysts (FeO particles) 9 nm sphere 24 nm sphere 30 nm six-horned octahedra
Growth of nanowires (MoO3 and S flow) SiO2 nanowires Few SiO2 nanowires MoO3 nanowires
MoO2 Raman peaks (only MoO3 flow) Weak None Strong


These products were characterized by Raman spectroscopy. Fig. 5(a)–(c) are the results of using the 9 nm, and 24 nm spherical and 30 six-horned octahedral catalysts, respectively. In the 9 nm case, peaks of both MoO2 and FeOx (x = 1, 3/2, 4/3) were found in contrast to that of the 24 nm spherical nanoparticles, in which no MoO2 peaks could be found, indicating that MoO3 could be deposited on the smaller spherical particles. When the shape of the particles changed to six-horned octahedra, significant peaks of MoO2 could be observed, as shown in Fig. 5(c). It is proposed that the sharp edges and acute angles in the six-horned octahedra contribute to the deposition of MoO3.


image file: c7ce00608j-f5.tif
Fig. 5 Raman spectra after CVD with (a) 9 nm spherical FeO + MoO3, (b) 24 nm spherical FeO + MoO3 and (c) 30 nm six-horned octahedral FeO + MoO3. Raman peaks were identified using ref. 35 and 36.

Based on the results of the above experiments summarized in Table 1, the growth mechanism can be described as follows.

Although SiO2 has a very low vapour pressure at 1000 °C, SiO can be vaporized under reducing conditions from the substrate surface and furnace wall. We suggest that based on the reaction SiO2 + S → SiO + SO2, SiO (∼1.33 Pa, 1000 °C)37 is the source of the SiO2 nanowires. The MoO3 vapor pressure in the furnace is estimated to be 4 Pa, which is described in the ESI. Based on this value, the competition between the SiO and MoO3 vapor gives rise to the formation of two different nanowires with the assistance of the different shapes of the catalyst nanoparticles, as illustrated in Fig. 6.


image file: c7ce00608j-f6.tif
Fig. 6 Mechanism of switching between SiO2 and MoS2.

By using 9 nm particles, the particles consisting of Mo, Fe and S were observed at the top of the SiO2 nanowires, indicating that the SiO2 nanowire growth proceeded through a vapor–liquid–solid (VLS) or vapor–solid–solid (VSS) mechanism.38–42 In the VSS mechanism, the growth temperature is lower than the eutectic temperature (50–90% of the reported eutectic temperature) and more facets of the catalysts at the top would be observed due to the lower solubility of the precursor atoms.41,42 The round particle shape of the catalyst existing at the top of the nanowire in Fig. 4(a) illustrates that the growth of the SiO2 nanowire is not explained by the VSS mechanism. The reaction temperature at 1000 °C, which is higher than the eutectic point (≤940 °C, because of the size effect) in the FeO–FeS system,43,44 makes explanation by the VLS mechanism possible. At 1000 °C, Ar gas carrying MoO3 and sulfur from the low-temperature region starts to flow, depositing Mo, O and S atoms onto the particles. Some sulfur atoms react with FeO, leading to the formation of the FeO–FeS system. When the FeS content in the particle reaches a certain value, a liquid alloy drop will be formed. Meanwhile, MoO3 tends to quickly evaporate rather than diffuse into the particles. Additionally, an experiment without MoO3 (only S) was also conducted, as shown in Fig. S13. SiO2 nanowires did not grow in this case, which demonstrates that the Mo absorbed into the drop played an important role in its catalyzing effect.

With the increasing size of the nanoparticles, the growth of the SiO2 nanowires seems to become more difficult. As shown in Fig. 3(b), lower-density nanowires were found compared with Fig. 3(a). With the increasing size of the nanoparticles, the surface to volume ratio decreased, leading to fewer active sites for interaction with the chemical adsorbate.45 Thus, the absorption of S, MoO3 and SiO becomes lower than that in the smaller particles. For example, as shown in Fig. 5(b) and Table 1, compared with the weak peaks in the 9 nm particles, no MoOx peaks were found in the case of the larger particles. However, due to the increased volume of the particles, more atoms need to be absorbed to grow SiO2 nanowires by the VLS mechanism. In other words, it required a longer time to form the supersaturated alloy drop, whereas at the same time, MoS2 thin films are formed in the presence of the MoO3 and S vapors, which is not affected by the nanoparticles. Thus, less SiO2 nanowires were formed as the size of the nanoparticles increased. Instead, the particles tended to be covered by MoS2 thin films.

When the shape of the catalyst nanoparticles changed from spherical to six-horned octahedra, MoS2 nanowires were formed. Unlike the SiO2 nanowires, no particle cap at the tip of the MoS2 nanowires was ever found, indicating that the mechanism of MoS2 cannot be the VLS mechanism. As the temperature increased to 1000 °C, six-horned octahedral FeO nanoparticles started forming frustums, as shown in Fig. S4(c). Their sharp edges and acute angles contribute to the deposition of MoO3 (Fig. 5(c) and Table 1). At high temperatures, an oxygen deficiency in the deposited MoO3 would occur,46,47 which can be incorporation sites for sulfur atoms, leading to the nucleation of MoS2. With the orientation effect of the sharp edges of the catalyst, MoS2 nanowires were then formed. Thus, we suggest that the mechanism of the MoS2 nanowires is the VSS mechanism. Compared with the VLS mechanism of SiO2 nanowires, the MoS2 nanowires with the VSS mechanism can be understood as a simple surface reaction system, while the mechanism of the SiO2 nanowire growth is more complex because it involves diffusion and precipitation of silicon atoms into the alloy drops. Additionally, when the nanoparticle size increased to 50 nm, lower-density MoS2 nanowires were grown, indicating that the increase in the size of nanoparticles slowed down the growth of MoS2 nanowires.

Conclusions

In this study, high-aspect-ratio MoS2 and SiO2 nanowires have been successfully fabricated by catalytic CVD, in which FeO nanoparticles with different and well-defined shapes were used as catalysts. It was found that switching of the composition of the nanowires occurred when the shapes of the nanoparticle catalysts changed. In the case of spherical nanoparticles, high-density SiO2 nanowires were fabricated, whereas in the case of six-horned octahedral nanoparticles, MoS2 nanowires were formed. This switching can be explained by the competition between the growth of the SiO2 nanowires by the VLS mechanism and MoS2 nanowires grown by the VS mechanism. The deposition of MoO2 species on the catalyst seems critical in controlling the process.

Experimental

Synthesis

Catalysts. FeO nanoparticles were synthesized by thermal decomposition of iron(III) acetylacetonate [Fe(acac)3] (99.9%, Sigma-Aldrich), according to a previous report.29 Briefly, 0.7 g Fe(acac)3 was mixed with either 4 ml oleic acid (OA), 6 ml oleylamine (OAm) or 5 ml OA and 5 ml OAm. The solutions were heated at 220 °C and 300 °C under Ar (99.9999%) for 1–2 h to yield FeO nanoparticle solutions with different sizes and shapes. The flow chart is shown in the ESI, Fig. S2.
Nanowires. MoS2 nanowires were grown using a CVD apparatus, as shown in the ESI. The furnace tube had a 28 mm diameter and was separated into three zones: zones A, B and C. MoO3 (99.5%) and S powders were purchased from Kanto Chemistry. MoO3 (0.15 g) was placed into a quartz tube in zone A, and the S powders (1.5 g) were placed in an alumina boat in another heating apparatus.

Si wafers with 500 nm SiO2 layers were drop-coated with the FeO catalyst solution (0.1 mmol L−1) after RCA cleaning. The substrates were then transferred to zones B and C of the tube furnace. In typical experiments, after the furnace and S apparatus were heated to the set temperature, Ar gas carrying MoO3 (40 sccm) and S (800 sccm) were flowed into the furnace. The temperatures of the zones were 280 °C for the sulfur source, 650 °C in zone A, and 1000 °C in zones B and C. After maintaining the temperature for 1 h, the furnace was naturally cooled to room temperature. The experiment was conducted under atmospheric pressure.

Characterization

The FeO catalyst nanoparticles and MoS2 nanowires have been characterized using a 200 kV JEOL JEM-2010 transmission electron microscope (TEM) equipped with an energy-dispersive (EDS) analyzer. The crystallinity of the nanoparticles was examined using a Rigaku Rint Ultima 2000 X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.1541 nm). The morphology of the nanowires was characterized using a JSV-6510LA scanning electron microscope (SEM) and a JSM-6500F field emission scanning electron microscope (FE-SEM). Raman spectroscopy (Renishaw Invia) with 532 nm excitation was used.

Acknowledgements

The present work is partly supported by the CREST-JST grant. Instrumental analysis was supported by the Nanotechnology platform at Hokkaido University by METI, Japan.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7ce00608j

This journal is © The Royal Society of Chemistry 2017