Vapor-solid Research Articles

Growth of high-quality GaN nanowires on p-Si (1 1 1) and their performance in solid state heterojunction solar cells

We report on the optimized growth of catalyst-freeGaN nanowires (NWs)/p-Si by the vapor–solid (V-S) method using chemical vapor deposition (CVD). The effect of NH3 gas flow rate on the morphology and photovoltaic behavior of the material has been investigated. The length and the diameter of the NWs decrease as the NH3 flow rate increases. Raman and X-ray diffraction (XRD) analyses reveal lower internal stress in the prepared NWs. The photoluminescence (PL) spectra indicate strong near band-edge (NBE) peaks extending from 365 to 368 nm and their intensity varied significantly with the NH3 flow rate. The assembled n-GaN NWs/p-Si solar cell devices reveal a maximum conversion efficiency of ∼7.87% under AM 1.5G illumination. This study shows that the morphology, optical, and performance of the fabricated n-GaN NWs on p-Si are strongly affected by the gas flow rate.

The catalyst-free controllable growth behavior and mechanism of HfCnws on C/C composites

Using no catalyst, hafnium carbide nanowires (HfCnws) layer was in-situ synthesized and its controllable growth was realized on carbon/carbon (C/C) composites. Microstructural evolution from CNTs to HfCnws revealed that the growth of HfCnws was based on vapor-solid (VS) mechanism and root growth was the common growth type. Increased relative content of carbon source reduced the aspect ratio but promoted the transformation of HfCnws from single-prong growth to multiple-prong growth. After the introduction of HfCnws, the oxidation resistance of SiC coating was increased by 58.7%, which could be attributed to its improved toughness from the multiple-prong growth of HfCnws.

Graphene-layer-coated boron carbide nanosheets with efficient electromagnetic wave absorption

Boron carbide (B4C) is normally known as a structural ceramic with excellent mechanical properties. Herein, the application of graphene-layer-coated B4C nanosheets (B4C@GN NSs) as efficient electromagnetic (EM) wave absorption materials was reported for the first time. B4C@GN NSs grew via the vapor–solid (VS) mechanism and coated with an ultrathin graphene layer through annealing. In addition to its graphene shell, the B4C@GN NSs had a unique small-sized nanosheet structure with a single sheet thickness of less than 50 nm. Concurrently, B4C micro- and nanoparticles were purchased or prepared to compare the morphologies and performances of several kinds of different B4C materials via characterization and analyses. The comparison revealed that the novel B4C@GN NSs possessed a special structure and a unique graphene shell that led to the great improvement in EM wave absorption. With a thickness of 1.19 mm, the B4C@GN NSs exhibited a minimal reflection loss (RLmin) of − 24.6 dB and a broad effective absorption bandwidth of 3.9 GHz. The electronic structure was analyzed by XANES to clarify the mechanism. The above results proved B4C@GN NSs to be an expectant absorber. The development of B4C@GN NSs might lay a theoretical foundation for the functionalization of structural ceramic materials.

The growth behaviors and high controllability of GaN nanostructures on stripe-patterned sapphire substrates

The growth behaviors of multi-dimensional GaN nanostructures including simultaneous epitaxial growth of films and horizontal nanowires on the sapphire substrates were observed by making substrates per-patterned, leading to some interesting phenomena, thereby helping us understand the characteristics of GaN growth more comprehensively and clearly. The nonpolar a-plane GaN films grown on the Au-coated areas were formed by many triangular pyramids. And there are two kinds of GaN horizontal nanowires in the Au-free areas containing straight-growing nanowires and U-shaped nanowires. The detailed mechanism of those growth phenomena is proposed: when Au catalysts and Ga are sufficient, the growth of GaN is mainly controlled by vapor-solid (VS) mechanism, gradually growing into films. While when Au catalysts are insufficient, the growth of GaN mainly follows vapor-liquid-solid (VLS) mechanism. The difference between the growth of straight and U-shaped nanowires reflects that ±polarity has an important impact on the growth direction of nanowires, making the nanowires take an U-turn during the growth. In addition, the GaN nanostructures show a high degree of controllability. The width and length of nanowires, the continuity of films, even the presence of Au particles at the end of nanowires can be controlled, which provides a simple and convenient synthesis strategy.

Experimental and simulation study on high-pressure V-L-S cryogenic hybrid network for CO2 capture from highly sour natural gas

Cryogenic carbon dioxide (CO2) capture technologies showed promising results for the purification of highly sour natural gas reserves. However, quantitative experimental data for solid and liquid CO2 formation during cryogenic separation is not adequately examined in the previous studies. Moreover, an economical and efficient cryogenic CO2 capture technology with reduced energy and hydrocarbon losses is necessary to make it attractive for commercial use. A high-pressure cryogenic hybrid network comprised of the packed bed and the cryogenic separator was developed for the cryogenic experimental study on CO2 capture from binary CO2−CH4 mixture to focus these areas. Feed containing 30, 50 and 70 % CO2 content were used and the separation study was conducted in vapor-solid (V-S), vapor-liquid (V-L), and vapor-liquid-solid (V-L–S) regions of phase equilibria. The packed bed was used in the V-S operational domain to quantify CO2 solid up to 20 bar because of the operational limitations. Separation characteristics and V-L isothermal flash measurements at 20, 30, and 40 bar pressure and temperature ranges from -20 to −60 °C were carried out in the cryogenic separator. The operation in the setups was carried out at different compositions of the CH4−CO2 binary mixture to define the boundaries of the hybrid cryogenic network. Liquid formation at 40 bar for 70 % CO2 feed was 0.15 kg at -55 °C as compared to 0.03 kg for 30 % CO2 feed. The simulation study was carried out using Aspen Plus along with the Peng Robinson Equation of State (EoS), and the results were compared with the experimental data, which showed good agreement. The hybrid cryogenic network showed an energy reduction of 37 % compared to the conventional cryogenic distillation network with 90.6–97.3 % CH4 purity and 2.65–12.39 % methane losses with different arrangements of the hybrid cryogenic network.

One dimensional Au-ZnO hybrid nanostructures based CO2 detection: Growth mechanism and role of the seed layer on sensing performance

Abstract In the present research, hybrid Au-ZnO one-dimensional (1-D) nanostructures were grown on silicon substrates with an Al-doped ZnO (AZO) seed layer (Ultrasonic Spray Pyrolysis: USP grown) and no seed layer (NSL) using two different catalytic gold films of 2 nm and 4 nm, respectively. Consequently, such 1-D nanostructures growth was associated with the vapor-liquid-solid (VLS) and vapor-solid (VS) processes. Scanning electron microscopy (SEM) imaging analysis confirms that heat treatment triggered Au nanoparticles nucleation with varying diameters. The Au nanoparticles size and underneath seed layer texture strongly affect the morphology and aspect ratio of 1-D ZnO nanostructures. The seed layer (1-D USP) sample resulted in the growth of longer nanowires (NWs) with a high aspect ratio. The NSL sample showed the formation of nanorods (NRs) with a low aspect ratio mainly via VS growth process. X-ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), and photoluminescence (PL) analysis also revealed the differences in the NWs and NRs properties and confirmed VLS and VS growth mechanisms. CO2 gas sensing performance at different concentrations was demonstrated, and NWs with seed layer showed a relatively higher sensing response. In contrast, NSL samples (NRs) exhibited two times faster response. A detailed gas sensing mechanism with different CO2 adsorption modes based on properties of 1-D nanostructures has been discussed. Currently, CO2 sensing and capturing are critical topics in the green transition framework. The present work would be of high significance to the scientific field of NW growth and fulfill the urgent need for CO2 gas sensing.

Non-catalytic vapor synthesis of α-Si3N4 nanowhiskers via direct nitridation of silicon powders

Single crystalline α-Si3N4 nanowhiskers were synthesized by direct nitridation of the NH3-pretreated silicon powders under N2/H2 atmosphere without adding any catalysts. The phase composition and microstructure of the as-obtained products were investigated. It is shown that the NH3 pretreatment has a great influence on the phase and microstructure of the products. With NH3 pretreatment, single crystalline α-Si3N4 nanowhiskers were obtained. Whereas, the dominant β-Si3N4 particles were formed without NH3 pretreatment. The isolated α-Si3N4 crystal nuclei and the relatively thick oxide layer were formed on the surface of the silicon particles after pretreating by NH3. The growth of the α-Si3N4 nanowhiskers is obeyed the vapor-solid (VS) mechanism.

Mechanistic study on the formation of silicon carbide nanowhiskers from biomass cellulose char under microwave

The paper reports the synthesis of SiC nanowhiskers via a process that utilizes carbon and silica from the same renewable source without the addition of external Si precursors as a sustainable and cost-effective method. This study reports an improved and sustainable method of synthesizing silicon carbide (SiC) nanowhiskers using cornstalk cellulose char under microwave heating. The aim was to optimize the process and gain a deeper understanding of the formation and growth mechanism of SiC nanowhiskers from biomass under microwave treatment in a temperature range of 1200–1400 °C. The synthesized product was then characterized by various analytical techniques. The formation of SiC nanowhiskers (β-SiC) was promoted at higher reaction temperatures, increasing from 6.83 wt% at 1200 °C to 9.35 wt% at 1400 °C. The SiC nanowhiskers displayed straight rod and smooth cylindrical structures. The nanostructure of β-SiC was confirmed along the d-spacing (111) plane, with a characteristic lattice fringe spacing of 0.25 nm. The growth mechanism of the SiC nanowhiskers followed two reaction pathways of vapor-solid (VS) and the vapor-liquid-solid (VLS). Increased CO and CO2 concentrations due to the evolution of gaseous products (SiO and CO) from the reactive cellulose char led to the growth of SiC nanowhiskers under the solid-vapor mechanism. The presence of inherent metallic species, such as Fe in biochar was found to catalyze the formation and growth of SiC nanowhiskers.

Transformation of polysilicon cutting waste into SiC/α-Si3N4 composite powders via electromagnetic induction heating

In order to use polysilicon cutting waste more efficiently, SiC/α-Si3N4 composite powders were prepared via electromagnetic induction heating at ambient pressure. Factors affecting synthesis and reaction mechanisms of the SiC/α-Si3N4 composite powders were investigated. Results show that the morphology of obtained α-Si3N4 consists of particles equiaxed with diameter in the micron range and one-dimensional Si3N4 nanowires with diameter of tens to hundreds of nanometers and length of up to tens of microns. It was found that the addition of NH4Cl can promote nitridation by controlling reaction temperature and participating in the reaction. In addition, the existence of Fe2O3 impurities was found to be beneficial to the formation of α-Si3N4 in reaction process by removing the SiO2 film from silicon and providing FeSi2 liquid phase as effective pass for vapor phase. Mechanisms used to grow Si3N4-NWs were vapor-solid (VS) and vapor-liquid-solid (VLS) depositions.

Structure and Photoluminescence Properties of Thermally Synthesized V2O5 and Al-Doped V2O5 Nanostructures.

Al-free and Al-doped V2O5 nanostructures were synthesized by a thermal-chemical vapor deposition (CVD) process on Si(100) at 850 °C under 1.2 × 10−1 Torr via a vapor-solid (V-S) mechanism. X-ray diffraction (XRD), Raman, and high-resolution transmission electron microscopy (HRTEM) confirmed a typical orthorhombic V2O5 with the growth direction along [110]-direction of both nanostructures. Metallic Al, rather than Al3+-ion, was detected by X-ray photoelectron spectroscopy (XPS), affected the V2O5 crystallinity. The photoluminescence intensity of V2O5 nanostructure at 1.77 and 1.94 eV decreased with the increasing Al-dopant by about 61.6% and 59.9%, attributing to the metallic Al intercalated between the V2O5-layers and/or filled in the oxygen vacancies, which behaved as electron sinks. Thus the Al-doped V2O5 nanostructure shows the potential applications in smart windows and the electrodic material in a Li-ion battery.

An efficient method for large-scale preparation of high-purity α-Si3N4 nanowires and their electrochemical performance

An efficient and catalyst-free method for the large-scale preparation of high-purity α-Si3N4 nanowires was demonstrated by annealing absorbent cotton and silicic acid in a flowing nitrogen atmosphere. The pyrolysis of absorbent cotton generated a fluffy structure constructed by carbon microfibers, which could promote the growth of α-Si3N4 nanowires. The nanowires were characterized by XRD and XRD Rietveld refinement, SEM and TEM. The results indicated that the nanowires have diameters of 50–100 nm and lengths of tens to hundreds of microns. Vapor-solid (VS) mechanism was proposed to be responsible for the formation of nanowires. The α-Si3N4 nanowires were then applied as anode materials for lithium-ion batteries (LIBs), exhibiting good rate capability and cycling property.

Synthesis of 3C-silicon carbide 1D structures by carbothermal reduction process

In this work, cubic 3C-silicon carbide (beta-SiC) 1D-structures were successfully synthesized at various temperatures (1600 °C, 1650 °C and 1700 °C) by using graphite flakes and microfine silica through the carbothermal reduction process. The grown SiC structures were observed with two distinct morphologies viz. ribbon-type (diameter 2–5 µm) and rod-type (diameter ≤ 2 µm). The ribbon-type morphology was formed on the surface of the graphite flakes at relatively low temperature (1600 °C) whereas, the rod-type morphology was formed at a higher temperature (≥1650 °C) between the interlamellar spacing of graphite flakes. The TEM analysis confirms the growth direction of [111] for the as-grown 3C-SiC structures. The growth process of these two distinct morphologies is discussed in terms of nucleation and precipitation associated with vapour-solid (VS) and vapour-liquid-solid (VLS) mechanisms. Moreover, Raman spectroscopy indicated that the corresponding peaks of the as-grown SiC structures had red shifted as compared to the bulk SiC and that could be attributed to the confinement effect, internal stresses in the structure and stacking faults.

Synthesis of boron carbonitride nanosheets using for delivering paclitaxel and their antitumor activity

As a structural analog of graphene and boron nitride, hexagonal boron carbonitride nanosheets (BCNNSs) are supposed to be a potential drug deliverer. In the present work, an improved solid-state reaction method combined with ultrasonic exfoliating was reported for preparing BCNNSs. Vapor-solid (VS) mechanism was proposed to be responsible for the formation of BCNNSs. The BCNNSs were further modified by DSPE-mPEG-5000 to improve their dispersion in aqueous solution. It was found that the BCNNSs-PEG nanocomplex could be efficiently taken in by MDA-MB-231 breast cancer cells evidenced by inverted fluorescence microscopy. The PEGylated BCNNSs showed an outstanding ability to load paclitaxel through π-π interaction and hydrophobic interaction, and BCNNSs-PEG-loaded paclitaxel presented higher cytotoxicity in comparison with free paclitaxel. BCNNSs may become a promising candidate for delivering paclitaxel and other hydrophobic drugs.

In-situ growth and relevant mechanisms of thick SiC nanowhiskers from hybrid silicon sources

SiC nanowhiskers were prepared from photovoltaic silicon waste and quartz sand (as a hybrid silicon source) and coconut fiber (as a carbon source). The phase evolution of the reaction was investigated using X-ray diffraction (XRD) and the microstructure of the product powders was observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range of 1300–1500 °C, SiC whiskers were grown on the surfaces of the carbonized coconut fibers through vapor-solid (VS) mechanism. Above 1100 °C, straight SiC whiskers were also generated in the residual powders by vapor-liquid-solid (VLS) mechanism. The number of SiC whiskers increased significantly with increased reaction temperature. At the optimum reaction temperature of 1500 °C, thick carbon-rich SiC whiskers with straight and dendrite shape covered the whole surface of the carbonized coconut fiber. We have developed a novel, clean, and sustainable method of preparing SiC nanowhiskers. It provides a new way to recycle valuable resources from photovoltaic waste and enables the high-value utilization of agricultural wastes.

Spontaneous CVD growth of InxGa1-xN/GaN core/shell nanowires for photocatalytic hydrogen generation

InxGa1-xN nanowires (NWs) have drawn great attentions as a promising photocatalyst for H2 generation via solar water splitting. In this article, m-axial NWs with InxGa1-xN/GaN core/shell structure can be formed spontaneously on the n-type Si (111) substrate via Ni-assisted chemical vapor deposition (CVD) method. CVD growth conditions were adjusted by tuning the duration and pressure. The microstructures, surface chemistry, and optical properties of the obtained NWs were characterized using various techniques. The growth mechanism is identified as a two-step process. The InxGa1-xN cores with a high density of basal stacking faults were grown by a vapour-liquid-solid (VLS) mode, while the GaN shells were grown on the sidewalls of InxGa1-xN core through a vapor-solid (VS) mode. The photocatalytic activities of the NWs were evaluated by the amount of H2 generated from water splitting. A thicker GaN shell was revealed to decrease the photoactivity by hindering the charge separation and transfer of electron-hole pairs generated inside the InxGa1-xN cores. A strategy can be proposed to improve the photocatalytic performance of NWs by terminating the VS growth of the GaN shells during the CVD process.

Tailoring zinc oxide nanowire architectures collectively by catalytic vapor-liquid-solid growth, catalyst-free vapor-solid growth, and low-temperature hydrothermal growth

We present a comprehensive study of the fabrication of zinc oxide (ZnO) nanowires (NWs) with tailored structural morphologies and functional features by utilizing three major growth methods based on vapor-liquid-solid (VLS), vapor-solid (VS), and hydrothermal growth mechanisms. The vertically aligned ZnO NW (ZNW) array can be constructed with tailored length, diameter, and density by controlled catalytic epitaxial VLS growth; the catalyst-free VS growth can create a hierarchical ZNW assembly onto complementary topological frameworks, such as transparent conducting oxides and carbon nanotubes; and the hydrothermal growth practically enables low-temperature growth of conformal ZNWs on large-area and flexible substrates. The growth mechanism and tailoring of the ZNW architectures for each growth strategy were investigated in detail, combined with rational analyses and parametric experiments. This study provides a critical route to the purposeful tailoring of ZNW architectures toward targeted ZnO-driven applications that require specific ZNW morphologies, assembly configurations, and substrate materials for many diverse fields including energy conversion and harvesting, electronics, photonics, and smart and wearable sensors.

Catalyst‐free in situ synthesis of ZrC nanowires with excellent thermal stability

AbstractNo catalyst‐assisted zirconium carbide nanowires (ZrCNWs) were successfully synthesized by thermal evaporation and in situ reaction using carbon nanotubes (CNTs) and ZrCl4 as carbon and zirconium sources. Vapor‐Solid (VS) mechanism was proposed to explain the growth process of ZrCNWs. The ZrCNWs prepared at 1400°C have a single crystal structure with the diameters of 130‐290 nm. The axial growth direction of ZrCNWs is [111] and the yield is 76.5 wt%. After heat treatment at 1700°C, 2100°C, and 2450°C, the diameter of ZrCNWs increased and their aspect ratio decreased. By simulating the thermal stress of a single ZrCNW, it is speculated that the coarsening and fracture of ZrCNWs are caused by the overall slip of the atomic layers and the stress concentration at the dislocations.

Ultrafast nucleation and growth of high-quality monolayer MoSe2 crystals via vapor-liquid-solid mechanism

The controlled production of two-dimensional atomically thin transition metal dichalcogenides (TMDs) is fundamentally important for their device applications. However, the synthesis of large-area and high-quality TMD monolayers remains a challenge due to the lack of sufficient understanding of growth mechanisms, especially for the chemical vapor deposition (CVD). Here we report molten-salt assisted CVD growth of highly crystalline MoSe2 monolayers via a novel vapor-liquid-solid (VLS) mechanism. Our results show that the growth rate of the VLS-grown monolayer MoSe2 is about 40 times faster than that of MoSe2 grown via the vapor-solid (VS) mechanism, which makes the fabrication of 100 μm domains for ∼2 min and a uniform monolayer film within 5 min. The ultrafast growth of monolayer MoSe2 crystals benefits from the synergic effect of one-dimensional VLS growth and two-dimensional VS edge expansion. Moreover, these MoSe2 monolayers exhibit high crystal quality and enhanced photoluminescence due to efficient Se-vacancy repairing by the doping of halogen atoms. These findings provide a new understanding of MoSe2 growth and open up an opportunity for the rapid synthesis of high-quality TMD monolayers and heterostructures.

Formation mechanism of whiskers in Al–MgAl2O4–MgO refractories at 1400 °C under N2 atmosphere

Al–MgAl2O4–MgO refractories were prepared using fused magnesia, metal Al, fused spinel and sintered high purity magnesia as raw materials. The phase composition and microstructure of Al–MgAl2O4–MgO refractories treated at 1400 °C under N2 atmosphere were investigated by means of XRD, SEM and EDS. The results showed that magnesia (MgO) whiskers and MgAlON whiskers were formed on the surface and in the inner area of the Al–MgAl2O4–MgO refractories, respectively. The MgO whiskers grew preferentially along the axial direction, forming cylindrical shape MgO whiskers. Then the cylindrical MgO whiskers further absorbed Mg(g) and O2(g), and grew along the radial direction to form the square columnar shape MgO whiskers. The MgAlON whiskers firstly grew in one-dimensional direction, forming whisker shape MgAlON, then some whisker shape MgAlON gradually developed and grew into two-dimensional flake shape MgAlON. The sintering and thermal shock resistance was significantly improved by the whiskers. The growth process of magnesia whiskers and MgAlON whiskers were dominated by a vapor-solid (VS) mechanism.

Self‐coiling silicon nitride nanobelts into microrings

AbstractIn the present work, we reported the growth of silicon nitride (Si3N4) microrings formed by self‐coiling of nanobelts with a typical saw‐like structure on one side, which were synthesized via catalyst‐assisted pyrolysis of organic precursors. The as‐grown microrings were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X‐ray diffraction (XRD). The pyrolysis temperature played a crucial role in the self‐coiling of Si3N4 microrings, enabling the tailored growth of Si3N4 microrings. The mechanism for the growth of Si3N4 microrings was discussed, which mainly involved the growth of Si3N4 nanobelts via a vapor‐solid (VS) process, followed by bending into a ring‐like shape driven by the surface stress.