Synthesis of Au/SnO₂ nanostructures allowing process variable control

Abstract

Theoretical advances in science are inherently time-consuming to realise in engineering, since their practical application is hindered by the inability to follow the theoretical essence. Herein, we propose a new method to freely control the time, cost, and process variables in the fabrication of a hybrid featuring Au nanoparticles on a pre-formed SnO₂ nanostructure. The above advantages, which were divided into six categories, are proven to be superior to those achieved elsewhere, and the obtained results are found to be applicable to the synthesis and functionalisation of other nanostructures. Furthermore, the reduction of the time-gap between science and engineering is expected to promote the practical applications of numerous scientific theories.

Introduction

Currently, although the boundaries of the academic area do not seem to be important, a clear-cut borderline separates pure science^1,2, which explores the principles of nature, from applied engineering^3,4, which deals with real-life processes. This separation largely reflects the corresponding difference in the utilised approaches, highlighting the fact that the application of new theories to real-world problems is difficult and time-consuming. For this reason, scientific heritage newly published every day is often discarded without actually being phenomenologically expressed. On the contrary, our daily life presents numerous strange phenomena that cannot be scientifically explained because of the lack of a proper academic background. That is, there may be cases of a theory not backed by experimental results or results not explained by any theory. As mentioned above, science and engineering can be viewed to be in a state of temporal hysteresis, and the search for ways of narrowing the corresponding time gap should therefore be regarded as a task of high significance. For example, Shi et al. reported the hetero-structured AgBr/ZnO photocatalyst, but their synthesis requires long reaction times and complex multi-step processes⁵. Ellis et al. also proposed the morphology control of hydrothermally prepared LiFePO4 with long reaction times and post heat treatment processes⁶. In other words, engineering techniques that can easily and economically confirm competitive scientific theories are shortcuts that can reduce the time cost of the practical application of science and achieve unique and meaningful results. Unluckily, because of the atmosphere that emphasises originality in research, one tends to think that only complicated and difficult-to-perform experiments can produce unique results. However, the reason why we cannot conclude that it is preconceived is that many of the results have received good evaluation in the meantime. For example, when studies on various nanostructures^7,8,9,10 performed so far are divided into those dealing with morphology^11,12, crystallography^13,14, and elemental composition^15,16 control, one can recognise that these investigations have a certain research value when the desired shape, microstructure, or function has been fully achieved. In this case, the employed raw materials and equipment are costly, the use of in-house-made equipment precludes verification in other laboratories, the experiment condition that was different from the existing experiment was exactly met in the repeated experiment, and technological differences related to the use of high-end analytical equipment are rarely considered. In other words, the outcomes of such experiments emphasise specificity rather than generality, and consequently require much time to be verified by engineering in real life, i.e., in such cases, one can only imply that a new theory can be realised. Herein, we introduce new processing advantages to easily fabricate a heterogeneous structure by attaching Au particles to pre-formed SnO₂ nanostructures and compare the advantages of our work with the disadvantages of existing works in six representative categories used in science/engineering fields. The proposed method allows one to induce nucleation and growth in nanostructures in a shorter time than in the case of other synthesis/deposition techniques. Moreover, thermal energy injection allows the phase change and composition to be relatively easily altered, and the developed technique also allows one to easily change the shape and microstructure of pre-formed nanostructures, which is attributed to energy injection variation with temperature and holding time. Thus, in contrast to the existing principle of one-to-one matching, which assumes that one factor depends on one process variable, the described technique utilises a new one-to-many matching concept, allowing one to simultaneously control multiple factors with one process variable.

Result and Discussion

Figure 1 shows Au particle–decorated SnO₂ nanostructures prepared under various experimental conditions. As is well known^17,18, smooth and long SnO₂ nanowires can be easily synthesised by thermal evaporation of Sn powder in an oxygen-containing atmosphere. Herein, the thickness of SnO₂ nanowires ranged from 20 to 120 nm, and their length ranged from several tens to several hundred μm (Fig. 1a–c). However, when a 5-s energy pulse was applied to SnO₂ nanowires that had previously been exposed to HAuCl₄⋅4H₂O/(CH₃)₂CHOH, the originally smooth surface of SnO₂ nanowires got covered by Au particles and therefore became rough. The spherical Au particles attached to nanowires had a size of roughly 100–300 nm and were in a discrete state (Fig. 1d–i). Specifically, these particles did not aggregate to lower their surface energies and existed independently at regular intervals, which was ascribed to the fact that the thermal energy applied to SnO₂ nanowires was not concentrated in a narrow region adjacent to nanowires but was uniformly dispersed in space. In other words, it was concluded that the applied thermal energy allowed the rate of nucleation to be held constant at all points of SnO₂ nanowires. Thus, it could be said that this simultaneous energy injection was different from the general mechanism of nucleation and growth in the local region of interest. Only morphologically, Au-decorated SnO₂ nanostructures can be prepared in any number of ways. However, most of these methods refrain from instantaneous processing to increase the cross-sectional area and require various pre- and post-processing techniques^19,20,21. For example, Kim et al. synthesised noble metal–decorated SnO₂ nanowires using thermal activation and employed the large cross-sectional area of these nanoparticles to detect noxious gases²². Wu et al. suggested that hydrothermally prepared hollow hybrid Au-SnO₂ nanostructures can be utilised in photocatalysis application²³, with advantages such as environmentally friendly solution-basis, low-cost, and surfactant-free. Bing et al. used a rational combinational multi-step synthetic route to prepare Au-loaded SnO₂ hollow multi-layered nanosheets, composed of numerous nanoparticles as structural subunits²⁴. Furthermore, although previous reports could realise nanocomposite structures of the abovementioned morphology^25,26, they remain inferior in terms of speed, accuracy, yield, and economy, i.e., accessibility. For example, Lai et al. realised heterogeneous nucleation sites using a template with a low vaporisation point and fabricated nanobeads by subsequent template removal at high temperatures²⁷. Some studies suggested that Pd-functionalised nanostructures can be formed by post-heat treatment and/or electron beam irradiation, without the involvement of extra precursors, because extra energy itself facilitates nucleation at an energy lower than that of homogeneous nucleation^28,29. However, in the two cases mentioned above, there is no way to control nanowire parameters until the end stage, since metal particle nuclei are formed on the metal oxide before or during synthesis. That is, if the desired metal-decorated nanowires are not obtained, the experiment needs to be re-started from the very beginning. In contrast, our method relies on simultaneous annealing, allowing one to rapidly functionalise existing SnO₂ nanowires in the desired way. This advantage cannot be found in any other post-processing technique, and one can therefore say that in addition to the abovementioned five advantages (speed, accuracy, yield, economy, and accessibility), our method also guarantees stability.

Figure 1

Typical SEM and TEM images of SnO₂ nanowires formed by conventional thermal evaporation and of Au particles formed on SnO₂ nanowires by flame chemical vapour deposition. (a) SEM image of bare SnO₂ nanowires with a smooth surface; (b) low-magnification and (c) high-magnification SEM images of Au-SnO₂ hybrid nanostructures; (d–i) variable-magnification TEM images of Au-SnO₂ nanostructures of different shapes.

As shown in Fig. 1d–i, even though rugged Au particles were commonly observed on the originally smooth SnO₂ nanowires, bigger Au spheres were sometimes formed on the surface of certain SnO₂ nanowires through interfacially controlled spherical growth³⁰. The above figures indicate no change in the morphology of Au-decorated nanowires; however, even in a localised region, the size of Au particles produced on nanowires tended to decrease with increasing time of thermal energy injection in that region. This behaviour was ascribed to the role of injected heat energy in inducing simultaneous nucleation over a large area, so that the effect of growing the nucleated first in a momentary difference is relatively insufficient. Therefore, as the retention time of energy injection at each point increased, the distance between Au particles generated in the nanowire decreased, while the density of Au particles produced at a fixed length increased. These results are in stark contrast with the fact that after nucleation, the rapid growth of nanostructures is commonly controlled by the rate of constituent atom diffusion³¹.

The above finding indicates that the increase of supplied energy amount with increasing heating time is related to the reduction of SnO₂ accompanied by the formation of Au (Fig. 2). Thus, even for Au-decorated SnO₂ of the same type, the reduction gradient is determined by the energy injection time (heating time). To investigate the degree of reduction, all samples were subjected to elemental mapping (Fig. 2a–d) and EDX (Fig. 2e–h) measurements, which revealed that the Sn:O ratio was different in nanowires and Au particles, as described above. In SnO₂ nanowires, Au was not observed at all (Fig. 2e,f), but Sn and O were detected together with Au in the particle region (Fig. 2g,h). That is, the precipitation of Au and the reduction of SnO₂ occurred simultaneously. This behaviour probably reflects the fact that when the Au solution was pushed to one side to become a particle as a result of energy supply, a part of the SnO₂ nanowire surface was exposed and lost O because of the effect of direct energy injection. Thus, one has only started exploring the applications of these two effects, which are believed to have much scientific and engineering potential.

Figure 2

Zone composition of Au-SnO₂ hybrid nanostructures identified by elemental mapping and EDX. (a–d) Distributions of Sn, O, and Au in a typical Au-decorated SnO₂ nanowire; (e,f) contents of Sn and O in a SnO₂ nanowire determined excluding Au particles; (g,h) contents of Sn, O, and Au in a Au particle excluding the SnO₂ nanowire.

For each sample, the crystal phase composition and microstructure were verified by XRD (Fig. 3a) and TEM (Fig. 3b–g), respectively. Bare SnO₂ nanowires were shown to have a tetragonal structure³², and peaks at 2θ = 26.61°, 33.89°, 37.95°, 38.97°, and 42.63° were in good agreement with reflections from (110), (101), (200), (111), and (210) planes of tetragonal SnO₂, respectively (JCPDS No. 41–1445) (Fig. 3a)³³. Peaks at 2θ = 38.18° and 44.39° were ascribed to reflections from the (111) and (200) planes of Au (JCPDS No. 04–0784), respectively³⁴ (Fig. 3a). On the other hand, for samples prepared by applying thermal energy to bare SnO₂ nanowires, we observed a change of SnO₂ peak intensity and the appearance of new peaks. As described above, a reduction of SnO₂ to Sn may occur under the employed conditions, which may result in the formation of non-equilibrium SnO_x (0 < x < 2) phases (Fig. 3b–g). Although these peaks did not exactly match those in JCPDS cards because of the non-equilibrium nature of the former, such chemical changes could be sufficiently inferred from the shift of the (101) peak of pre-formed SnO₂ to the (101) peak of Sn. At this time, a SnO₂ layer was detected on the surface of SnO₂ nanowires (Fig. 3d,f) and on the surface of Au particles (Fig. 3e). However, interplanar spacings of unbalanced compositions could be observed with proceeding partial reduction. In other words, no crystal phases except for those of SnO₂ and Au were observed by XRD, although HRTEM line profiling indicated that the surface profiles of SnO₂ or Au could change. This means that the energy supplied to the existing SnO₂ nanowires was sufficient for SnO_x to form on the Au surface (Fig. 3d–g).

Figure 3

Crystallinity and microstructure of several parts of Au-decorated SnO₂ nanostructures. (a) XRD spectrum of Au-SnO₂ mixture; (b,c) HRTEM images acquired at the interface between a Au particle (dark region) and a SnO₂ nanowire (white region); (d) interplanar spacing showing the reduction of SnO₂ to Sn on the surface of SnO₂ nanowire; (e) interplanar spacing of SnO₂-based layers formed on the Au particle surface; (f) interplanar spacing confirming the reduction of SnO₂ to Sn, measured on the other surface of the SnO₂ nanowire; (g) interplanar spacing confirming the reduction of SnO₂ to Sn, measured inside the SnO₂ nanowire.

The efficiency of making Au-SnO₂ should be further objectified to allow the clear application of the competitiveness of the new method and its difference from the existing techniques. Consequently, the process methods were evaluated using six parameters (Tables 1, 2, S1 and S2), namely the employed precursor, equipment, pre- and post-treatment, temperature, time, and vacuum^{35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66}. First, the new processing technique was found to be applicable to all materials regardless of their type. Second, while the previously used equipment occupies much space, requiring additional equipment to achieve the special purpose, our processing technology is not affected by location and does not involve the utilisation of useless accessories. Third, researches conducted so far, especially those requiring pre-processing such as templating and post-processing such as heat treatment, have often involved supplementary procedures to address the difficulty of direct synthesis and deposition, whereas our high-efficiency method does not require any pre- or post-processing. Fourth, in previous methods, the temperature had to be maintained within the range of at least 500 to 1000 °C for a long time to adjust the synthesis temperature of heterogeneous materials, whereas our method instantaneously provides a temperature of 1300 °C. Fifth, our process is operated on a timescale of seconds and is clearly different from other processes operated on the time scales of minutes or even hours, allowing one to control instantaneous processing conditions on the spot to match material properties. Sixth, conventional synthesis and deposition equipment requires the use of variable (low to ultra-high) vacuum depending on the specific case, whereas our technique does not require additional vacuum conditions, since it can be operated under atmospheric pressure. There may be many other classification criteria, but it seems clear that the above six advantages provide overwhelming evidence of the superiority of our method.

Table 1 Comparison of our process with previously reported ones.

Table 2 Comparison of our process with previously reported ones.

As mentioned above, differences between science and engineering inevitably result in the need for a certain time period to achieve coincidence. In most cases, a theory is first established, and the corresponding time-saving potential is evaluated in the next step. Therefore, it is meaningful to find a new method allowing one to control several variables through a simple experiment, which can simplify the whole process but produce various results. In the meantime, we have invested a lot of time and money in the synthesis and functionalisation of new materials with novel properties. Thus, the know-how to produce the desired results with this simple method can be applied to other materials in the same way, and the search for even simpler and more powerful derivation methods should not be stopped.

Conclusion

A new method of synthesising SnO₂ and converting it to a different standard has been proposed. This method allows one to relatively easily control the parameters of Au-decorated SnO₂ nanowires using thermal energy, i.e., the growth factors for each sample can be freely controlled depending on the given materials and processing time. Specifically, the reaction proceeds from a homogeneous structure to a heterogeneous structure, or from a stoichiometric structure to a non-stoichiometric structure, depending on the amount of applied energy. The trend of this transition is also consistent with the results of SEM, XRD, and TEM analyses. This seemingly ordinary process technology has proven to be overwhelmingly superior in terms of precursor, equipment, pre- and post-treatment, temperature, time, and vacuum. From an energy point of view, all experimentation with science/engineering bases relies on the idea of making a difference in energy or eliminating the energy difference. Thus, the ability to achieve a variety of effects by reducing the number of process variables and simply adjusting them should substantially contribute to reducing the congenital time gap between science and engineering.

Methods

To prepare SnO₂ nanostructures, Sn powder (1 g; Daejung Co., 99.9%) was placed in an alumina boat of a thermal evaporation furnace. The silicon substrate with 3-nm Au was placed upside down on the alumina boat to create conditions facilitating the adsorption of Au onto the substrate upon vaporisation. The temperature was raised to 900 °C at a rate of 10 °C/min, and an O₂-Ar mixture (97:3) was flown at a pressure of 2 Torr for 1 h at 900 °C.

Gold Chloride hydrate 99.995% (HAuCl₄·4H₂O (0.23 g, 99.995%) and 2-propanol (10 g, 99.5%) were well mixed, and 3 mL of the mixture was dropped on the substrate part where the SnO₂ nanowires were to be synthesised. Thereafter, a flame with a temperature of 1300 °C was applied for 5 s in the standby state using a special flame chemical vapour deposition (FCVD) technique.

Morphology was probed by scanning electron microscopy (SEM; Hitachi S-4200, Hitachi) and transmission electron microscopy (TEM; JEM-2100F, JEOL), crystallinity was probed by X-ray diffraction (XRD; Philips X’pert diffractometer, Philips) and high-resolution transmission electron microscopy (HRTEM), while elemental composition was probed by XRD, elemental mapping, and energy-dispersive X-ray spectroscopy (EDX).