Synthesis of Au/SnO2 nanostructures allowing process variable control

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.

be realised. Herein, we introduce new processing advantages to easily fabricate a heterogeneous structure by attaching Au particles to pre-formed SnO 2 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. Figure 1 shows Au particle-decorated SnO 2 nanostructures prepared under various experimental conditions. As is well known 17,18 , smooth and long SnO 2 nanowires can be easily synthesised by thermal evaporation of Sn powder in an oxygen-containing atmosphere. Herein, the thickness of SnO 2 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 SnO 2 nanowires that had previously been exposed to HAuCl 4 ⋅4H 2 O/(CH 3 ) 2 CHOH, the originally smooth surface of SnO 2 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 SnO 2 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 SnO 2 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 SnO 2 nanostructures www.nature.com/scientificreports www.nature.com/scientificreports/ 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 SnO 2 nanowires using thermal activation and employed the large cross-sectional area of these nanoparticles to detect noxious gases 22 . Wu et al. suggested that hydrothermally prepared hollow hybrid Au-SnO 2 nanostructures can be utilised in photocatalysis application 23 , 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 SnO 2 hollow multi-layered nanosheets, composed of numerous nanoparticles as structural subunits 24 . 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 temperatures 27 . 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 SnO 2 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.

Result and Discussion
As shown in Fig. 1d-i, even though rugged Au particles were commonly observed on the originally smooth SnO 2 nanowires, bigger Au spheres were sometimes formed on the surface of certain SnO 2 nanowires through interfacially controlled spherical growth 30 . 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 diffusion 31 .
The above finding indicates that the increase of supplied energy amount with increasing heating time is related to the reduction of SnO 2 accompanied by the formation of Au (Fig. 2). Thus, even for Au-decorated SnO 2 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 SnO 2 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 SnO 2 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 SnO 2 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.
For each sample, the crystal phase composition and microstructure were verified by XRD (Fig. 3a) and TEM ( Fig. 3b-g), respectively. Bare SnO 2 nanowires were shown to have a tetragonal structure 32 34 (Fig. 3a). On the other hand, for samples prepared by applying thermal energy to bare SnO 2 nanowires, we observed a change of SnO 2 peak intensity and the appearance of new peaks. As described above, a reduction of SnO 2 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 SnO 2 to the (101) peak of Sn. At this time, a SnO 2 layer was detected on the surface of SnO 2 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 SnO 2 and Au were observed by XRD, although HRTEM line profiling indicated that the surface profiles of SnO 2 or Au could change. This means that the energy supplied to the existing SnO 2 nanowires was sufficient for SnO x to form on the Au surface ( Fig. 3d-g).
The efficiency of making Au-SnO 2 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  . 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 (2020) 10:346 | https://doi.org/10.1038/s41598-019-57222-z www.nature.com/scientificreports www.nature.com/scientificreports/ 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.
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 SnO 2 and converting it to a different standard has been proposed. This method allows one to relatively easily control the parameters of Au-decorated SnO 2 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 SnO 2 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 O 2 -Ar mixture (97:3) was flown at a pressure of 2 Torr for 1 h at 900 °C.  Table 2. Comparison of our process with previously reported ones. (f) interplanar spacing confirming the reduction of SnO 2 to Sn, measured on the other surface of the SnO 2 nanowire; (g) interplanar spacing confirming the reduction of SnO 2 to Sn, measured inside the SnO 2 nanowire.