Template-Free Synthesis of One-Dimensional SnO₂ Nanostructures Using Highly Efficient Hydrothermal Method

Abstract

One-dimensional (1D) SnO₂ nanostructures, as n-type semiconductors with a wide band gap, exhibit unique photoelectric properties that offer potential applications in electronic, photoelectric, gas sensing, and energy generation devices. A detailed study of template-free reaction systems is essential to regulate and efficiently synthesise 1DSnO₂ nanostructures. This study employed the hydrothermal method to prepare 1DSnO₂ nanostructures, with SnCl₄·5H₂O as the tin source. The impact of various experimental conditions on SnO₂ morphology is analysed. Here, 1DSnO₂ nanostructures were characterised by analytical methods such as X-ray powder diffraction, transmission electron microscopy, scanning electron microscopy, and field emission double-beam electron microscopy. The results confirmed the formation of 1DSnO₂ nanostructures with a mixed morphology of nanorods and nanowires. The nanorods exhibited a length of 422.87 ± 110.15 nm, a width of 81.4 ± 16.75 nm, and an aspect ratio of 5:1, whereas the nanowires displayed a length of 200 ± 45.24 nm, a width of 15 ± 5.62 nm, and an aspect ratio of 13:1. With the addition of 50 mg of polyvinylpyrrolidone and seed crystal, the acquisition time of the 1DSnO₂ nanostructures decreased from 48 to 12 h. The 1DSnO₂ nanostructures were efficiently obtained without a template, laying the foundation for large-scale production and application.

1. Introduction

Metal oxides are ionic compounds composed of positive metal and negative oxygen ions. The completely filled s-shell of the metal oxides provides it with adequate thermal and chemical stability, whereas the d-shells may not be fully filled, which renders them with diverse unique properties. Owing to the quantification of state density, the electrical, optical, and chemical properties of high-aspect-ratio nanostructures and various material-dependent work functions can be modulated [1,2].

Nanostructure-based one-dimensional (1D) metal oxides have gained significant attention in recent years because of their unique electronic, optical, and thermal properties [3,4]. Moreover, because 1DSnO₂ nanostructures are n-type semiconductors with a wide band gap (e.g., 3.6 eV at 300 K), they exhibit unique photoelectric properties that offer potential applications of SnO₂ in electronic, photoelectric, gas sensing, and energy generation devices. The hydrothermal method is a bottom–up synthetic method that has become the most widely adopted method for preparing nanomaterials because of its facilely applicable procedure, the high crystallinity of the product, convenient control of morphology, and high sintering activity. Xi and Ye [5] adopted the hydrothermal method without exploiting any template or surfactant. They used SnCl₄∙5H₂O as the tin source and added CO(NH₂)₂ and HCl to ultrapure water, which was insulated at 90 °C for 24 h in a stainless-steel autoclave lined by polytetrafluoroethylene (PTFE). Ultrathin SnO₂ nanorods (SnO₂(NR)) with a diameter 2.0 ± 0.5 nm, a length 12 ± 3 nm, and a growth orientation in the [001] direction were obtained. Wang et al. [6] adopted the hydrothermal method to enable hydrothermal growth in a sealed PTFE-lined stainless-steel autoclave for 24 h at 200 °C. They obtained a rutile-structured SnO₂(NR) of thickness 100 nm, length 1 mm, and a growth orientation in the [001] direction. In addition, Chen et al. [7] implemented the same experimental conditions and obtained rutile-structured SnO₂(NR) with 4–15 nm diameter, 100–200 nm length, and (110)-directed growth orientation. Cheng et al. [8] implemented the hydrothermal method at 150 °C to induce hydrothermal growth for 24 h in a sealed PTFE-lined stainless-steel autoclave. They obtained SnO₂(NR) with 2.5–5 nm diameter, 15–20 nm length, and [001]-directed growth orientation. Thereafter, Chen et al. [9] adopted the sodium dodecyl sulfate-assisted hydrothermal method to prepare nano-polyhedrons that were self-assembled from ultrathin nanowires in a sealed PTFE-lined stainless-steel autoclave at 180 °C after hydrothermal growth for 24 h. Qin et al. [10] applied the hydrothermal method at 285 °C to prompt hydrothermal growth for 24 h in a sealed PTFE-lined stainless-steel autoclave; they obtained rutile-structured SnO₂(NR) with 80 ± 5 nm diameter, 2.5 ± 0.1 μm length, and [001]-directed growth orientation. Wang et al. [11] applied the hydrothermal method at 200 °C for 24 h in a sealed PTFE-lined stainless-steel autoclave to obtain a rutile-structured SnO₂(NR) with 15 nm diameter and 50 nm length. Jeong et al. [12] placed a precursor solution with four pieces of SnO₂ seed (SnO₂ nanoparticles deposited using a spin-coating technique) layer-coated F-doped tin oxide in an autoclave to conduct hydrothermal synthesis at 200 °C for 18 h. They obtained rutile-structured SnO₂(NR) with a diameter 85 nm and growth orientation in the [001] direction. Parveen et al. [13] implemented the hydrothermal method in an autoclave at 180 °C for 12 h, after which the SnO₂ precipitate was calcined at 350 °C for 2 h. They obtained a rutile-structured SnO₂(NR) with a diameter 250 nm and length 1 μm. Yadav et al. [14] applied the hydrothermal method at 180 °C for 24 h to obtain a rutile-structured SnO₂(NR) with a diameter of 32.83 nm.

However, these methods require long processing times, which is not ideal for controlling 1DSnO₂ structures and morphology. Thus, further investigations on such template-free reaction systems are vital for regulating the nanostructures of 1DSnO₂. Although certain scholars [15,16,17] adopted template methods to prepare 1DSnO₂, the high cost of the templates is not conducive for commercial production. Therefore, a highly efficient template-free hydrothermal synthesis of 1DSnO₂ is reported here. The morphology and composition evolution of nanomaterials were investigated by adjusting the experimental parameters such as reaction period, temperature, and type of surfactant (polyvinylpyrrolidone; PVP) and seed crystals.

2. Materials and Methods

The materials used in the study are listed in

Table 1. Raw materials and reagents.

Name	Grade	Manufacturer
Stannic chloride pentahydrate (SnCl4˙5H2O)	Analytically pure	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)
Sodium hydroxide (NaOH)	Analytically pure	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)
Absolute alcohol (C2H5OH)	Analytically pure	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)
Deionised water (H2O)		Self-manufactured
Polyvinylpyrrolidone (PVP-k30)	Analytically pure	Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China)

.

In the experimental process, 1.05 g of stannic chloride pentahydrate and 1.3 g of sodium hydroxide were added into a sealed PTFE-lined stainless-steel autoclave along with a certain volume of absolute alcohol and deionised water (total volume: 80 mL). The solution reacted at a specified temperature for a reaction period. Here, SnO₂ with varying morphologies was obtained after seal cooling, vacuum filtration, ethanol rinsing five times, and drying at 60 °C for 24 h. Subsequently, we investigated the structure and morphology evolution of the synthesised 1DSnO₂ nanomaterials. For parameter optimisation in the reaction, the material preparation was conducted at 150, 160, 170, 180, 190, and 200 °C with adjustments in the volume ratios of the absolute alcohol and deionised water along with modifying the reaction period from 12–48 h.

The synthesised material was characterised by X-ray powder diffraction (XRD; Rigaku multi-bending X-ray diffractometer), transmission electron microscopy (TEM; JEOL 2100F; accelerating voltage: 200 kV), scanning electron microscopy (SEM; Phenom XL), and field emission double-beam electron microscopy (FESEM; FEI-Versa3D).

3. Results and Discussion

Experimental principles:

SnCl₄ + 6NaOH = Na₂Sn(OH)₆ + 4NaCl

(1)

Na₂Sn(OH)₆ = SnO₂ + 2NaOH + 2H₂O

(2)

SnCl₄∙5H₂O dissolves rapidly when added to an aqueous solution of NaOH, which forms Na₂Sn(OH)₆ because of the quick hydrolysis reaction of Sn⁴⁺. When NaOH is excessive, Na₂Sn(OH) exists as colloids in the aqueous solution of NaOH. Upon exposure to absolute alcohol, the colloidal particles of Na₂Sn(OH)₆ agglomerate to form pre-condensation nuclei in the hydrothermal process, which evolve into condensation nuclei for the growth of SnO₂ crystals at high temperature and pressure.

3.1. Effects of Varying Reaction Times in Absolute Alcohol on SnO₂ Morphology

When absolute alcohol was used as the solvent and PVP was not added, the reaction temperature was 200 °C, and the reaction periods were 8, 12, and 48 h.

As observed in Figure 1, the morphology evolution of the products indicated that small particles with smooth edges aggregate into large particle clusters after reacting for 8 h. Furthermore, irregular flake morphology appears in the vicinity of aggregated particle clusters after reacting for 12 h, whereas the product morphology is dominated by lamelliform structures without any 1D structures after reacting for 48 h. Because of the absence of templates, surfactant, and aqueous solution under this condition, the chemical potential in the solution was insufficient to trigger the nucleation and growth of 1D structures.

3.2. Effects of Varying Volume Ratios between Absolute Alcohol and Deionised Water on SnO₂ Morphology

The effects of varying the volume ratios (V) between absolute alcohol and deionised water on SnO₂ morphology were investigated at a reaction temperature of 200 °C for 48 h without adding PVP.

As observed in Figure 2, with a decreased content of absolute alcohol and an increased content of deionised water, the SnO₂ morphology evolves from a lamelliform into a microsphere structure via self-assembled 1DSnO₂. Eventually, these microspheres aggregate into a quasi-microsphere structure. Note that 1D structures appear in final products only if the value of V approaches one. Based on the aggregation degree of 1D structures self-assembled into quasi-microspheres, the edges of the quasi-microspheres in Figure 2e are more prominent at V = 1, which is distinct from the quasi-microsphere aggregation indicated in Figure 2d,f. The formed microstructure is relatively clear in Figure 2e for V = 1:1.

3.3. Effects of Reaction Time on SnO₂ Morphology

The effects of the reaction time on SnO₂ morphology were investigated by adding 1DSnO₂, which was acquired by the same method as the seed crystal in Figure 2e: 200 °C, 50 mg PVP, and V = 1:1.

The PEG [18] and CTAB [19] were used as surfactants, the hydrothermal time was too long for 24h. However, the addition of PVP can inhibit the precipitation of solute on the particles, thereby inhibiting the growth of crystals and yielding smaller particles. So, we chose to use PVP as the surfactant. When the amount of PVP added was too low, the stability performance of small particles was weak [20]. However, the addition of too much PVP [21] (100 mg) had little effect on the morphology of 1DSnO₂. Therefore, we chose to add 50mg of PVP. As the reaction time decreased, the product morphology displayed microsphere structures caused by 1DSnO₂ self-assembly. The results indicated that the mutual adhesion between the particles absorbed on the surfaces of tin-oxide nanocrystals can be prevented by the effects of the surfactant PVP. Moreover, the presence of 1DSnO₂ seed crystal favours the acceleration of 1DSnO₂ nucleation and growth. Therefore, the required reaction time reduced from 48 h in Figure 3e to 12 h in Figure 3f.

3.4. Effects of Reaction Temperature on SnO₂ Morphology

The effects of the reaction temperature on SnO₂ morphology were investigated with 50 mg of PVP, a reaction time of 12 h, and V = 1:1.

As observed in Figure 4b–f, SnO₂ morphology is dominated by lamelliform structures at high reaction temperatures. Meanwhile, microelements of 1DSnO₂ nucleate at multiple sites on the flakes and grow via aggregation induced by the reaction conditions, thereby forming the microsphere structures of 1DSnO₂ self-assembly. More such microspheres are formed as the reaction temperature increases. At 200 °C, the lamelliform morphology disappears and is replaced by the microspheres of 1DSnO₂ self-assembly. Based on the morphology and structures, 1DSnO₂ is formed at the reaction temperature of 200 °C.

Only 1DSnO₂ were observed in Figure 4a. So, the optimum experimental conditions were at the reaction temperature of 200 °C, reaction time of 12 h, and V = 1:1. With the addition of 50 mg PVP, the product morphology exhibited microsphere structures of 1DSnO₂ self-assembly. The composition and microstructure of 1DSnO₂ nanocrystals are significantly affected by the reaction temperature. Below 200 °C, the condensation degree among Na₂Sn(OH)₆ mesophases is low, which may significantly decelerate the growth of 1DSnO₂. However, by raising the temperature to 200 °C and reacting for 12 h, 100% 1DSnO₂ can be formed. The conversion of Na₂Sn(OH)₆ into tin oxide is more apparent at a higher temperature, and the grain size of samples increases slightly upon increasing the hydrothermal temperature. This is because the pressure inside the reactor increases with the hydrothermal temperature and provides sufficient energy for grain growth. During the process, small grains are dissolved to increase the solute concentration, thereby facilitating the growth of large grains, which conforms to the mechanism of Ostwald ripening [22]. Ostwald ripening occurs via the dissolution and re-precipitation of matter at regions with small and large radii of curvatures, respectively. Ostwald ripening results in the dissolution of smaller solid grains, the diffusion of the solute throughout the liquid, and the re-precipitation of the solid into large grains. The net result is grain growth.

The XRD, TEM, and SEM results of 1DSnO₂, prepared at the condition, are stated as follows. The diffraction peaks of the products appear sharp and free of impurity peaks, indicating superior crystallinity and high purity of the final products. Here, (110), (101), and (211) are the three relatively strong peaks. Compared to the standard card, the results are consistent with those of the tetragonal rutile SnO₂ (JCPDS No. 41-1445, S.G.:P42/nm, a = b = 0.4738 nm, c = 0.3187 nm). Contrary to the standard card, the peak intensities of (101), (211), and (002) in the sample are significantly enhanced. According to Wang et al. [6], peak (002) is the characteristic peak of SnO₂(NR) array, and its enhancement suggests an improvement in the degree of order in the sample. The crystal structure is demonstrated in Figure 5a. Each Sn coordinate contains six oxygen atoms at the vertices of the twisted octahedron. Among these, four oxygen atoms are in the same plane, and the Sn–O bonds are shorter than those of the other two oxygen atoms.

TEM analysis is conducted on the products, as illustrated in Figure 6.

As verified from the TEM and SEM images, the products displayed a morphology of 100% 1D nanostructures with a length of 422.87 ± 110.15 nm, width of 81.4 ± 16.75 nm, and aspect ratio of 5:1, indicating the prevalence of strictly anisotropic growth during the entire process. The SEM result in Figure 6a indicates that the front end of a single nanorod resembles a pyramid, which is consistent with the FESEM result illustrated in Figure 6c. The inset portrays that the spacing between adjacent lattice planes corresponding to the [110] plane of rutile tin oxides is 0.34 nm, which indicates that the preferential growth direction is [001]. Projecting outward from the centre of the nanolayers, Figure 6b,c demonstrate the quasi-microspheres with a diameter of 2 μm, which are formed by the self-assembly of 1D nanorods. The highly magnified SEM image further indicate that all nanorods resemble a square (Figure 6c). The growth model with thermodynamic stability of square tin-oxide nanowires (SnO₂(NW)) in the solution is demonstrated in Figure 6f. The square tin-oxide nanorods grow from the centre of the quasi-microspheres, then elongate and protrude along the c-axis, thereby forming layered nanostructures. Figure 6 and Figure 7 show the different morphological types at different sites of same product. Figure 7 shows the morphologies of SnO₂ nanowires. These nanowires are estimated to be about 200 ± 45.24 nm in length and 15 ± 5.62 nm in width, and the aspect ratio of the nanowires is up to 1:13.

The specific surface energy of crystalline SnO₂ has been calculated earlier [23,24]. The same tendency is demonstrated in the results, i.e., the crystal surface energy increased in the order of (110) < (100) < (101) < (001) with incremental energy. As the lowest and highest surface energies appear on (110) and (001) surfaces, the growth in the [001] orientation yielded a higher aspect ratio of the particles than that in other orientations. The experimental results of SnO₂(NR) obtained in this study are consistent with the calculation results, as single crystal nanorods grow along the [001] orientation of the long axis.

Thus, 1DSnO₂ was prepared, adopting the hydrothermal method using SnCl₄∙5H₂O as the tin source, absolute alcohol and deionised water as the solvent, and PVP as the surfactant. Our hypothesis of the formation process is stated as follows. At the initial stage of the hydrothermal reaction, Na₂Sn(OH)₆ colloids are formed as condensation nuclei. As the temperature and pressure in the reaction system increases, small particles aggregate into large particles according to the mechanism of Ostwald ripening. The particle morphology evolves into lamelliform structures with temperature, pressure, and prolonged reaction period. As observed in the highly magnified SEM, small protrusions appear on the lamelliform morphology, which grow gradually during the process of hydrothermal crystallisation. The crystals will grow preferentially along the crystal plane with low nucleation energy, i.e., the growth rate of crystals is higher along the direction with lower nucleation energy on the corresponding crystal plane. Because the (110) plane of SnO₂ crystals exhibits the lowest nucleation energy, the crystals will grow preferentially along the [001] orientation, thus forming nanorods and nanowires. Because of various interactions between nanocrystals [25] (such as van der Waals force between coated molecules, dipole interaction between nanocrystals, and hydrophilic/hydrophobic interaction), quasi-microsphere structures are formed via the self-assembly of nanorods. The high surface to volume ratio of nanomaterials can provide more positions for absorbing gas molecules. The 1DSnO₂ nanostructures is used for preparing gas-sensitive components, which can improve the sensitivity and accuracy of gas detection. They are promising for further applications.

4. Conclusions

With absolute alcohol, without the addition of PVP, and after reaction for 8–48 h at a temperature of 200 °C, lamelliform SnO₂ was observed in the products, but 1DSnO₂ was not detected. Upon varying V from 5:3 to 9:7, the product morphology transformed from lamelliform to quasi-microsphere structures, a process facilitated by the 1DSnO₂ self-assembly. Notably, at V = 3:5, the agglomeration of quasi-microspheres became severe. In the absence of templates, the time required to form 1DSnO₂ decreased from 48 to 12 h with the addition of PVP and 1DSnO₂ seed crystals, thereby enhancing the synthesis efficiency. The products are SnO₂(NR) and SnO₂(NW), with tetragonal rutile structures. The spacing between the adjacent lattice planes was 0.34 nm. This corresponds to the (110) plane of the rutile tin oxide, indicating that [001] is the preferential growth direction. Nanorods of length 422.87 ± 110.15 nm, width 81.4 ± 16.75 nm, and an aspect ratio of 5:1 were observed, in addition to nanowires of length ~200 ± 45.24 nm, width 15 ± 5.62 nm, and an aspect ratio of 13:1. Owing to the aggregation initiated by various interactions between nanocrystals, quasi-microsphere structures were formed by the self-assembly of 1DSnO₂. The obtained 1DSnO₂ nanostructures can be used to prepare gas-sensitive components.