Extended View on the Vapor–Liquid–Solid Mechanism for Oxide

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Extended View on the Vapor−Liquid−Solid Mechanism for Oxide Compound Nanowires: The Role of Oxygen, Solubility, and Carbothermal Reaction Jasmin-Clara Bürger, Sebastian Gutsch,* and Margit Zacharias

J. Phys. Chem. C Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/12/18. For personal use only.

Department of Microsystems EngineeringIMTEK, Laboratory for Nanotechnology, University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany ABSTRACT: The Si nanowire growth can be well explained by the classical vapor− liquid−solid (VLS) process taking into account the respective Au−Si phase diagram. For oxide-based compound materials, no phase diagram with gold exists because of the insolubility of these materials into the Au catalyst material. Hence, it is not correct to claim a simple VLS mechanism for the respective growth. In this study, a more complex model for the growth of oxide nanowires (NWs) is proposed by analyzing the influence of oxygen concentration and timing of oxygen inflow into the furnace while growing SnO2 NWs by a carbothermal chemical vapor deposition process. It is shown that a controlled amount of oxygen is mandatory to grow the SnO2 NWs. However, either too low or too high oxygen concentration strongly suppresses the nanowire growth. On the basis of the here-presented experiments, we propose the formation of solid oxide flakes on the catalyst surface and their respective concurrence as guided by the Sn/O balance feeding the liquid catalyst surface. A new model is discussed, taking into account the effect of surface transport and the respective transport of SnO2 solid flakes, the effect of the Sn gradient in the catalyst droplet, and a possible viscosity gradient at the droplet−solid nanowire interface.

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INTRODUCTION Since the first description of the “vapor−liquid−solid” (VLS) process by Wagner and Ellis in the 1960s, the growth of nanowires (NWs) based on different materials such as insulators, metals, and semiconductors has been demonstrated.1,2 However, especially in the case of NWs based on oxide compound materials, for example, zinc oxide (ZnO) or tin oxide (SnO2), the “VLS growth model” and the role of the respective oxygen source are not well understood and analyzed in detail. Solubility of the material into the liquid seed droplet is a key point mandatory for the VLS process. For the respective systems such as Au−ZnO and Au−SnO2, no phase diagram exists, that is, there is also no solubility of ZnO or SnO2 into the liquid gold droplet, a fact that has not been analyzed systematically. Only the Au−Zn and Au−Sn systems establish phase diagrams, and hence, solubility and liquid phases are found.3,4 However, tin oxide NWs (SnO2 NWs) are reported to be grown by a “VLS process” based on carbothermal reduction of SnO2 using a mixture of graphite, respective active carbon, and SnO2 powder.5−9 As observed already for ZnO NWs, the growth of SnO2 NWs is expected to depend on a large amount of variables.10 Therefore, the purposeful growth of NWs requires a precise control, and understanding the influencing parameters was graphically summarized before.10 A conventional one-zone furnace does not allow to adjust the substrate and powder temperatures separately which often restricts the active nanowire growth zone to a narrow regionoften located within the steep temperature profile at the end of the tube.5,7−9 © XXXX American Chemical Society

To overcome these restrictions, in this study, a two-zone furnace has been used that allows for a nearly individual control of powder and substrate temperatures.11 Furthermore, other process gas parameters may have a crucial influence on the growth of NWs: on the one hand, the gas composition and pressure are controlling the carbothermal reaction at the source and thus determine the amount of active gaseous metal species evaporated.10 On the other hand, the gas pressure and flux will control the gas-phase diffusion, mass transport, and active species delivery to the substrate, that is, liquid catalyst droplet.10 Depending on the relative strengths of diffusive and convective transport, the optimal growth position can be shifted from downstream to upstream with respect to the carbothermal powder source.10 Therefore, the distance between powder and growth regions in the tube is also determined by the gas parameters.10,11 Essentially, the gas flow rates and partial pressures of all process gases must be precisely controlled to achieve a reproducible growth process to be able to analyze the growth mechanism of oxide-based compound NWs.10 For the growth of metal oxide NWs by carbothermal processes, the supply of oxygen and the role of oxygen are rarely considered, even though it is absolutely crucial for the growth of the metal oxide NWs.5−9,12 To summarize the above introduction: the use of a tube furnace under continuous flow conditions has advantages and Received: July 30, 2018 Revised: September 26, 2018

A

DOI: 10.1021/acs.jpcc.8b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

placed at specific positions and under defined gas conditions. With the here-used two-zone furnace, the powder temperature responsible for the source evaporation and the second zone used to place the substrates for the nanowire growth are spatially separated and hence can be stabilized at different temperatures. This allows growing the SnO2 NWs by a carbothermal process. We always load the catalyst-decorated substrates in the middle of the second heating zone at the downstream position to have the best control of the transient gas flow containing transport gas as well as evaporated source material. For the latter, an alumina boat with 0.3 g of the respective powder mixture was positioned in the first temperature zone. The powder mixture contained tin oxide and graphite in a mixing ratio of 1:1 wt %. Afterward, the tube was tightly connected to the vacuum system and pumped down first to reduce the possible influences of rest gases on the process. After reaching a pressure below 5 × 10−5 mbar, the tube was flooded with inert argon gas (25 sccm Ar, 99,9999%) until the working pressure of 200 mbar was reached. Afterward, the heating up process of the furnace is started. During the whole process, the total gas flow rate and process pressure were kept constant. During the up-heating time, the tube furnace was under the above-mentioned constant argon gas flow of 25 sccm. When the process temperature was reached, the Ar flow was substituted with a constant gas flow of 5% oxygen diluted in argon. The process temperature (850 °C as Tsubstrate and 950 °C as Tpowder) was kept for 8 min in all experiments, if not stated otherwise. After finishing the process, the oxygen flow was stopped and the samples were cooled down passively again in inert gas (Ar, 25 sccm). This process is called standard procedure: under the chosen standard conditions for flow, working pressure and temperature, a homogeneous growth zone of up to 17 cm was established which corresponds to nearly the whole second temperature zone of the here-used furnace. The role and timing of the oxygen can be studied in detail in a reliable way only by minimizing the influence of other parameters such as powder loading, temperature, pressure, and gas flow parameters. Hence, with a first set of screening experiments, we therefore focused on developing an optimized process in a way that the growth can be performed over a homogeneous elongated region in the downstream position in a reliable way resulting in the above-stated standard procedure. Before going into the actual experiments, some basics for VLS experiments and the respective phase diagram have to be discussed. In an VLS process, the liquid alloy droplet is continuously fed by the source gas to realize a supersaturation resulting in a pushing-out of the material layer by layer contributing to the nanowire growth as was shown for instance in the in situ growth video clips by the Ross group.13 In the video clips, one also sees the establishment of a triangular material tailback at the outer edges of the droplet contacting the solid nanowire which vanishes periodically, with the material obviously contributing to the new atomic layer grown below the liquid droplet. The so-called carbothermal reaction of an oxide powder is an elegant way to get a good vapor pressure from a source material having a high melting point and hence a low evaporation rate at lower temperature.5,8,14,15 In the herediscussed case, we are using a SnO2 powder source. Graphite powder is mixed with the oxide powder to generate the Sn gas. Because SnO2 is not soluble into Au,16 only Sn can contribute to the VLS process and understanding should come from the

disadvantages. As an advantage, it offers a broad variety of growth experiments and an easy control of the flow and temperature conditions.10 Process gases and the mixture ratio of the process gases are controlling the stoichiometry of the grown materials. What is the standard procedure for Si device technologythe vacuum tight equipmentis often neglected for the nanowire growth. However, leakage of atmospheric gases in particular oxygen into the tube will definitely influence the growth process. This serves as the starting point for purposefully designed experiments using a vacuum tight tube furnace equipment for the SnO2 NW growth. To contribute to a better understanding of the growth mechanism, we will address in this work in particular the role of oxygen for the growth of SnO2 NWs using experiments offering the oxygen separately and at defined process times. On the basis of our experiments, we will propose a new model for the growth of oxide NWs involving solid metal-oxide flakes on the surface of the catalyst droplet and a viscosity gradient in the catalyst droplet.

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EXPERIMENTAL SECTION For all experiments, a-plane sapphire substrates (5 × 5 mm2) were used. The Au catalyst was deposited in two different ways: (1) Thin Au films were thermally evaporated. The Au film thickness monitored by a quartz crystal microbalance was either 3 nm or 5 nm. (2) Size-controlled Au nanoparticles were deposited directly on the substrate. This was done in the following way: a 1:1 vol % solution of a citrate-stabilized 20 nm Au nanoparticle solution (20 nm Citrate NanoXact Gold ECP1159 0.05 mg/mL, nanoComposix) and methanol was prepared. Ten microliters of this solution was pipetted onto the substrates and left for 10 min. After rinsing thoroughly with deionized water, the samples were dried with nitrogen. Prior to insertion in the growth furnace, all samples were loaded into a UV− ozone cleaning system (UV−ozone cleaning system UVOH 150 LAB, FHR Anlagenbau GmbH) for 90 min to remove organic residues. For structural analysis and growth evaluation, imaging was carried out for all samples by scanning electron microscopy (SEM) with a FEI Nova NanoSEM 430. Figure 1 represents a schematic view of the tube furnace. The oven is accomplished by a quartz tube (inner diameter 5 cm) where the source powder and the substrate pieces can be

Figure 1. Schematic diagram of the used two-zone furnace setup: please note, there is a thermally insulated zone reducing the thermal overlap of the two zones. B

DOI: 10.1021/acs.jpcc.8b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Au−Sn phase diagram which is shown in Figure 2.3 The powder temperature was set to 950 °C offering enough carbon-

SnO2 + 2CO → Sn(s, l) + 2CO2

(3)

CO2 + C → 2CO

(4)

In a temperature range below 600 °C, that is, during heating up the furnace, gaseous Sn is only provided by a reduction of SnO2 by CO according to eq 3 as analyzed in ref 14. Already at a temperature of 600 °C, SnO2 reacts completely to Sn and CO/CO2, provided that the ratio of the SnO2/C powders are high enough.14 For the here-used 1:1 mass percentage ratio, there is enough carbon to do so. In literatures, the growth of SnO2 NWs by a carbothermal process has been observed for temperatures above 700 °C.5,8,9 As stated in ref 14, an equal probability of reactions 1−3 appears at 700 °C because of the similar Gibbs free enthalpy. At temperatures above 700 °C, Sn is generated by the direct reaction of the SnO2 source with the carbon powder according to reactions 1 and 2 and by reaction 3 due to the start of the so-called Boudouard reaction 4, too.14 That also rules out the creation of atomic or molecular oxygen that is available at the substrate position for the growth of SnO2 NWs in a vacuum tight system. Hence, we conclude that the mixture of Sn (gas) and CO/CO2 (gas) arriving at the substrate position cannot provide oxygen for the SnO2 NW growth as suggested for instance by Li et al.9 but has to be provided separately

Figure 2. Phase diagram of the binary system Au−Sn as developed by Okamoto and Massalski.3 The gray line added in the diagram corresponds to the substrate temperature used in our growth experiments which is 850 ± 10 °C. Reprinted with permission from ref 3. Copyright 2007 Springer Nature.

Sn + O2 → SnO2

cracked evaporated source gas to be transported from the source to the substrate. The gray horizontal line in Figure 2 represents the chosen substrate temperature of 850 ± 10 °C used in our experiments for the nanowire growth. The carbothermal reaction of SnO2 with different carbon sources was recently analyzed in another context in ref 14, and the thermochemistry was discussed in detail taking into account the Gibbs free energy of the possible reactions at different temperatures.14 The following possible reactions had been elaborated by Levêque et al. (2014)14 SnO2 + 2C → Sn(s, l) + 2CO

(1)

SnO2 + C → Sn(s, l) + CO2

(2)

(5)

In the following, the phase diagram of the Au−Sn system as presented in Figure 2 has to be considered.3 At a substrate temperature of 850 °C, crossing the solvus line at approx. 2 at. % of Sn dissolved into the Au where the system starts to become liquid is required in order to have a VLS process with the typical liquid alloy droplet at the tip. For higher Sn content, the system represents a mixture of solid and liquid which transfers completely into a liquid at the liquidus line at around 12 at. %. The advantage of the existence of a liquid surface is the strongly increased sticking coefficient of the incoming gas molecules, hence a faster contribution of the alloy feeding. For the growth of SnO2 NWs, in addition to the Sn gas atoms, O or O2 is also needed. At such high temperatures as

Figure 3. Growth experiments under variation of the oxygen concentration in process atmosphere: growth experiments with defined timing of the additional oxygen gas flow using 3 nm (a−c) and 5 nm (d−f) Au films. After reaching the process temperatures under pure Ar gas flow, the process atmosphere was switched to 0% (a,d), to 2.5% (b,e), and to 5% of oxygen in Ar gas (c,f). C

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The Journal of Physical Chemistry C 850 °C, the instability of any gold oxide results in a preferential bonding of oxygen to Sn at the liquid surface.17 However, the melting point of SnO2 is very high (1630 °C);18 hence, solid SnO2 flakes will immediately float on the surface of the liquid Au−Sn alloy if O gas is added at the here-used temperatures. For this phenomenon, the timing and amount of oxygen will play an important role for the growth kinetics of the SnO2 NWs. Please note, this is contrary to the supersaturation phenomena discussed normally for instance for the Au−Si system, where for a given temperature, the oversaturation of the liquid Au−Si alloy results in the forcing out of the Si nanowire.19 With these basic discussions, we will now present the results of our experiments.

analyze the structures and to detect gold and tin. It was found that the layer of clusters represents either Au or possibly a polycrystalline Au−Sn alloy. Figure 3b,e depicts the second experiment, which represents the standard procedure but with a reduced amount of oxygen: after reaching the process temperatures under pure Ar gas flow, the process atmosphere was switched to 2.5% of oxygen in Ar gas. The growth of NWs with Au tips on top can be observed. The grown NWs on the sample with the 5 nm gold film layer (cf. Figure 3e) show larger NW diameters in contrast to the corresponding sample with a gold film layer of 3 nm (cf. Figure 3b). In the third experiment presented in Figure 3c,f, the standard procedure was used, that is, shifting from pure Ar gas flow to the mixture containing 5% oxygen in process atmosphere, hence initiating the gas exchange to a reactive mixture, after reaching the desired process temperatures. Please note, the oxygen flow is transient in the beginning. This means that it continuously rises since opening the valve. An equilibrium concentration of gases is reached after a time of approx. 16 min at the here-used pressure of 200 mbar based on the geometry of the tube. On account of this, the equilibrium concentration of 5% in this experiment and of 2.5% in the previous experiment cannot be reached within the dwell time of only 8 min for the experiments presented in Figure 3. This transient increase of oxygen might even help to start the freestanding SnO2 NW growth. The continuous Sn-fed Au−Sn liquid alloy droplet gets more oxygen step by step, but not enough to transfer the surface-attached Sn atoms into a continuous solid SnO2 shell. After the dwell time of 8 min, the gas was shifted back to Ar for the cooling period, during which still some residual oxygen is available to influence the result seen in the respective SEM images after cooling down. Congruently with the first experiment (cf. Figure 3a,d), the respective SnO2 NWs are larger for the thicker gold film layer (cf. Figure 3b,c with Figure 3e,f), but not as large as the seen cluster in the growth without the oxygen feeding. These results also imply that the optimum concentration of oxygen differs for various gold film layer thicknesses, that is, the size of the liquid seed droplets. In the fourth experiment (Figure 4), 5% oxygen/argon mixture had been initiated already while heating up the tube furnace. In contrast to Figures 3a,d, 4a,b shows clusters, too, but with much smaller size and a smooth but path-covered background. The rough signature of the polycrystalline layer background seen in Figure 3a,d is missing. Similar clusters have been already observed by Bazargan et al. for the formation of the gold droplets by tempering at 500 °C before growing NWs.22 Here, contributing oxygen during the heating-up period hinders the fast alloying, and hence, the Au−Sn clusters would not transfer to a liquid state. As seen in Figures 3 and 4, the results of the four growth experiments are very different. Only the experiments adding additional oxygen after reaching the process temperatures result in SnO2 NWs with the typical metal cluster at the tip (VLS-related process, please see the enlarged image in the top right corner of Figure 3c). Thus, as our experiments clearly demonstrate: adding or not adding oxygen and its timing clearly change the grown nanostructures. Klamchuen et al. have analyzed the influence of partial pressure of oxygen on the growth mode of SnO2 NWs grown by pulsed laser ablation.23 They have observed a competition between VLS and VS mode.23 In contrast, in our work, we have never obtained VS growth of SnO2 NWs which is likely due to our different growth techniques, where parameters can

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RESULTS AND DISCUSSION Figures 3 and 4 represent a set of experiments studying the concentration and timing of the initiated oxygen into the tube

Figure 4. Influence of oxygen timing on SnO2−NW growth: 5% oxygen has been initiated already while heating up. The thickness of the gold layer amounts to 3 nm (a) and 5 nm (b).

furnace using a-plane sapphire substrates covered by 3 nm or 5 nm Au films, respectively. Using a-plane sapphire as a substrate guarantees that Au is not diffusing into the substrate during the high-temperature process which would be the case for the Si substrate.20 In addition, the ultrathin film is expected to separate into Au clusters during heating up by dewetting at the a-plane sapphire surface. In the first experiment represented in Figure 3a,d, the standard heating-up conditions were used. However, no reactive oxygen was added to the gas flow at any time, hence forcing a growth based on a pure carbothermal contribution of the SnO2 powder material cracked by the graphite powder. The structures in Figure 3a,d seem to be similar, but different in size because of the different thicknesses of the used gold film layer. The gold film separates into randomly distributed droplets while heating up. On account of surface tension, a thicker gold film layer forms larger droplets. A cluster-like accumulation of the material can be seen for these samples representing large disordered clusters but not wires. Some of the big clusters seem to have facets, and a kind of a jagged, grain-like structure is seen below, which can also be demonstrated for ZnO NWs.21 The below structure is irregular and could consist of Au, an Au−Sn alloy, or an oxide. The clustered film is mainly the accumulation of the transported Sn to the gold-decorated surface, where the Au surface might be a preferred adsorption site. This would lead to fast alloying of the Au cluster and after enough alloying to liquid droplets. Liquid droplets have a higher sticking coefficient, faster material feeding, and hence a faster growth.10 Energydispersive X-ray spectroscopy measurements were done to D

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phase as seen from the above phase diagram.3 In addition as stated before, above 700 °C, the Boudouard reaction 4 (backreducing CO2 into CO) will be speeding up the supply of Sn gas from the SnO2/C source.14 More Sn is attached to the solid/liquid interface. Diffusion of Sn into the solid part of the mixture for further alloying is slower (solid diffusion) than a diffusion along the liquid covered droplet surface. Dewetting will depend on the alloy concentration, changing the surface tension, that is, curvature, of the liquid alloy.26,27 The faster supply of Sn on the surface is not immediately transformed into a homogeneous Au−Sn alloy, but instead some liquid Sn will reach the wetting root of the droplet to the sapphire surface and will be solidified by the incoming oxygen into SnO2 pushing the droplet randomly away and eventually allowing a coalescence of nearby droplets. The available oxygen during the heating-up process is oxidizing preferentially the Sn layer during the dewetting because of the low solubility28 of oxygen in gold. Please also note that by initiating oxygen already while heating up, the gas equilibrium will be reached before reaching the process temperatures. As a result, the effective oxygen supply is much higher than the one in the standard experiment with a defined dwell time. In addition, the particle size might also play a role as a consequence of the surface energy. Small droplets of a liquid (i.e., particle with a high surface curvature) have an effective higher vapor pressure as a planar phase transition plane (liquid−gaseous).29 Because of the increased inner partial pressure (due to the curved phase transition plane), a decrease of the melting point temperature for smaller particles is often observed29 which could be the origin of the dewetting of the original pure Au film. This is often referenced as the Gibbs− Thomson effect and is responsible for the melting and dewetting of the ultrathin Au films at much lower temperature.29 Such smaller particles are faster covered by Sn completely and could be buried under a solid SnO2 shell depending on the incoming Sn/O ratio, and this solid SnO2 is stopping further alloying of the buried droplet which solidifies. The effect is clearly visible in Figure 5. To determine whether the process represented in Figure 4a is preventing a NW growth event at growth conditions, we did a further control experiment which is represented in Figure 6. Here, we used the samples in the final state of Figure 4a as substrates for our standard experiment; hence, a regrowth

be controlled individually. Therefore, we are only considering a VLS-related growth mode in the following. Although Figure 3a−f is easy to explain, there still remain questions regarding Figure 4. The experiment clearly shows the importance of the timing of the added oxygen. One still sees the Au cluster as darker Z-contrast in the SEM images, but it would be unclear with the rather short time used in the first set of experiments if maybe the time was not long enough for a wire growth based on a VS mechanism. If for instance the amount of Sn alloying the Au cluster is too low, the alloy stoichiometry needed for crossing the solvus line of the phase diagram might never be reached, and hence, only the 10−100 times slower VS nanowire process could contribute.24 To validate this, Figure 5 compares additional experiments of

Figure 5. Insertion of oxygen already while heating-up process with the increased growth period: increasing the dwell time from 8 min (inset image) to 30 min (large image) does not result in a VS growth of NWs. 5% oxygen has been initiated already while heating up. The thickness of the gold layer amounts to 5 nm. Please note that the inset image corresponds to the samples used in Figure 4b but showing an enlarged view of the respective sample.

growth using the established conditions corresponding to samples with 8 and 30 min dwell times, therefore excluding a VS process. Both samples were previously prepared with a Au film layer of 5 nm. However, the increase of the dwell time from 8 to 30 min did not result in a noticeable growth of NWs (cf. Figure 5), hence excluding a VS growth process at least for the growth times used here. Spherical particles are visible for both dwell times on top of a rather smooth crawling background. The structures in Figure 5 are larger and less dense than in Figure 4b, although the thickness of the Au layer was the same. A reason for this could be that droplets have coalesced during the longer high-temperature phase.25 The crawling paths visible in the image seem to track the Au nanoparticle movement on the surface (cf. Figure 5) and have a lighter contrast which point to an oxidized stoichiometry (lower Z-contrast). During the heating-up process, more and more Sn is transported from the source to the substrate. The rather spherical dark (Au) particles are a hint to a dewetting process under the influence of a liquid phase. As we learn from the phase diagram in Figure 2, already above 532 °C, a partly liquid Au−Sn phase could be established if at least 7.3 at. % Sn is solved into the Au.3 However, before any Sn is coming from the carbothermal reaction of the SnO2 source, a minimum temperature (i.e., Gibbs free energy) is needed to start the carbothermal reaction at all, which will be below 700 °C only based on eq 3, hence reduces the amount of Sn produced.14 With increasing temperature, less Sn is needed for maintaining a partly liquid

Figure 6. Two-step growth while initiating oxygen already while heating-up process in the first process and growth by the standard process in the second step: nanowire growth can be seen if the clusterlike structure established by contributing 5% oxygen in Ar while heating up (sample Figure 4a) is reused for a second growth process with an in-flow of 5% oxygen in Ar after heating up again to 850 °C. E

DOI: 10.1021/acs.jpcc.8b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C process with the oxygen supply after the final temperature of 850 °C is reached and using a dwell time of 8 min. As can be seen, the cluster-like structure in Figure 4a is again transformed into free-standing wires, proving again the importance of Sn− O balance (cf. Figure 6). The usage of Au clusters allows a precise definition of the diameter and a low density of NWs at the substrate.30,31 A low density of particles minimizes the competition for gaseous Sn and O atoms as well as avoids the above-discussed merging of Au during film dewetting. It allows seeing the wires as well as the nanowire root at the substrate. Hence, in a final experiment, we repeat the standard growth of Figure 3c but now using a-plane sapphire substrates decorated with wellseparated 20 nm sized Au particles. The growth results of the corresponding experiment is presented in Figure 7a.

we will not see any nanowire growth again. We still observe the nanoparticles on the substrates (not shown here). However, we also do not see crawling paths of solidified SnO2 in the vicinity of the droplets. Using again these already preprocessed particle-decorated samples now for a second growth under standard conditions with a dwell time of 8 min, we observe again NWs as in the experiment in Figure 6. These NWs have a metal tip corresponding to a VLS process, too. If the NWs of Figure 7a are reused in the standard growth process, the already grown wires elongate and develop a kinked structure (see Figure 7b), which can be understood by cooling down and reheating of the VLS tip droplet which forces a staking fault and a change in the growth direction. The here-selected controlled growth experiments are discussed in the following to develop a much deeper understanding of the processes going on during the carbothermal growth of metal oxide NWs. First, we develop models which represent the different states of the standard process (Figure 8) and the process with the oxygen feeding during heating up (Figure 9). The standard process can be separated into three different stages, and we suggest here a vapor-liquid-solid flake model (VLSF) as presented in Figure 8: • Liquidizing and dewetting of the Au film due to a lower melting point of ultrathin films and the correctly chosen substrate, that is, sapphire (cf. Figure 8a), • Sn feeding, alloy building, supersaturation of the Au−Sn alloy during heating up, and establishment of SnO2 surface flakes after reaching the substrate temperature Ts and exchange of the Ar to diluted O2/Ar (cf. Figure 8b). At Ts, the continuous Sn feeding under oxygen results not only in maintaining the supersaturation of the droplet but also in O−Sn−O flakes which move down to the sapphire surface below the droplet (cf. Figure 8b). The SnO2 flakes are insoluble in the Au or the Au−Sn alloy. Hence, the SnO2 flakes are floating and diffusing on the outer surface curvature of the droplet. • Reaching the droplet−nanowire interface, the prearranged molecular SnO2 units might be sucked into the interface contributing to the NW growth. The driving force of this process is unclear; different effects could contribute such as viscosity gradients based on alloy gradients and temperature/energy optimized driven attachment to the solid crystalline interface. Please note that atomic arrangement in the presented monolayer of the NW in Figure 8c is a (100)-layer referred to ref 32.

Figure 7. Grown SnO2 NWs out of Au nanoparticles: comparing the NWs grown out of (a) separated 20 nm Au particles and (b) reuse of NWs grown by the procedure of (a) in a second standard process. All NWs are grown by the standard procedure (dwell time 8 min, and 5% oxygen in process atmosphere after reaching the desired process temperatures) revealing a VLS process, respectively.

As can be seen here, it is possible to grow SnO2 NWs with the same parameters but different catalyst modes and their respective thickness (cf. Figures 3c,f and 7a). Using 20 nm of Au particles, the average diameter of the NWs was found to be approx. 30 nm, hence larger than the former Au cluster. Please take into consideration that the here-shown SEM pictures represent the sample after cooling down and under ambient conditions. As a result of the low catalyst density and size of the gold droplets, the substrate surface of Figure 7a is smooth and rather clean compared with Figure 3c,f. That also hints to a preaccumulation of Sn during the dewetting of the Au film with increasing temperature. In contrast, if we use such particle covered samples for a growth while contributing oxygen already during heating-up,

Figure 8. Vapor−liquid−solid flake-model (VLSF) for the different states of the growth of oxide compound (SnO2) NWs: (a) dewetting of the gold film, (b) formation of a Sn−Au alloy droplet with first molecular SnO2 flakes after reaching process temperature Ts and gas exchange to diluted O2/Ar, and (c) growth of the NW by integration of the SnO2 flakes passing from the surface of the liquid droplet into the interface to the solid crystalline NW. F

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Figure 9. Process under initiating gaseous oxygen feeding during heating-up time: (a) dewetting of the gold film, (b) intermediate state during heating-up characterized by droplet movement and SnO2 film delivery and (c) step by step solidification of the completely covered droplet with increasing temperatures due to the Sn/O ratio shift.

Meng et al. have already observed flake-like structures covering the catalyst tip in their work about indium−tin oxide (ITO) NWs.33 In contrast to our results, their work was based on the gold catalyst-assisted pulsed laser deposition of ITO templates made of different SnO2/In2O3 ratios.33 They have argued with different growth rates and varying adhesion rates of In2O3 and SnO2 NWs.33 In the case of SnO2 NWs without indium oxide, a different material system has to be considered than for ITO NWs. However, if the growth process is guided with oxygen from the very beginning, then also some of the surface-attached Sn atoms could form “solid” SnO2 molecule sheets at lower temperatures and the results will actually hinder the nanowire growth based on the following steps as presented in Figure 9:

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speeds up the carbothermal source evaporation (temperatures above 700 °C). • Too high oxygen concentrations during heating-up the tube furnace suppresses the nanowire growth of such oxide NWs because of fast solidification of surface metal atoms at the root of Au droplet to the dewetting substrate. This results in the development of crawling metal oxide paths. • The growth of the layer-by-layer metal oxide NWs at the liquid−solid interface is guided by viscosity gradients (concentration gradients at the inner substrate−droplet surface) and faster surface transport effects of materials along the droplet surface. This results in a contribution of the molecular metal oxide flakes which immediately is built-in at the crystal planes of the nanowire surface. • At growth temperatures, an optimized Sn/O ratio avoiding a complete coverage of the liquid drops with solid metal oxide flakes or even a monolayered oxide cap is mandatory for the growth of metal oxide NWs.

• Au diffusion and dewetting, no oxidation of the Au, very limited Sn (cf. Figure 9a) due to T dependence of carbothermal reaction of the source material. • Some Sn atoms attach to the surface of the droplet, diffuse faster on the droplet surface to the droplet− substrate surface than through the droplet and might even form SnO2 molecular units. A cavity effect delivers Sn and the previously formed SnO2 as a thin film, which is immediately transferred into an oxide and solidifies (cf. Figure 9b). The delivered solid SnO2 film forces the liquid droplet to move over the sapphire surface and collects further small Au drops. • With increasing process temperature, the larger amount of Sn and incoming O results in a complete droplet coverage with solid oxide. Therefore, any further flake movement and precipitation are impeded (cf. Figure 9c).

The phase diagrams of the alloy systems Au−In, Au−Zn, and Au−Sn show many similarities.3,4,34 We have observed similar phenomena for ZnO NWs in the past.10 Hence, we propose that the growth of In2O3 NWs might also be following the VLSF model suggested here, too. A validation for such other metal oxide systems using carbothermal chemical vapor deposition could be a part of future research.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 761 203-7258 (S.G.).

CONCLUSIONS We expect that the above-discussed growth modes can be generalized for the case of metal oxide materials having no solubility into the liquid metal and which are solid at the substrate temperatures Ts used in the experiments. Hence to generalize the fact and observations of the above experiments, we conclude:

ORCID

Sebastian Gutsch: 0000-0003-2468-7495 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the DFG for the support of this work under ZA 191/33-1. J.-C.B. thanks the “Studienstiftung des deutschen Volkes” for funding her study by a scholarship.

• For metal oxide NWs, understanding of the respective Au-metal phase diagram is crucial (i.e., Au−Sn, Au−Zn, ...). • The carbothermal reaction does not supply the oxygen needed for the growth of the oxide NWs (here SnO2). Additional oxygen in ratios depending on process details is mandatory. The oxygen gas should be added after the liquid droplets are established and a reasonable metal feeding of the droplets is guaranteed, that is, taking into account the Boudouard reaction of CO2 to CO which

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DOI: 10.1021/acs.jpcc.8b07332 J. Phys. Chem. C XXXX, XXX, XXX−XXX