One‐Step SnO2 Nanotree Growth

Abstract A comparison between Au, TiO2 and self‐catalysed growth of SnO2 nanostructures using chemical vapour deposition is reported. TiO2 enables growth of a nanonetwork of SnO2, whereas self‐catalysed growth results in nanoclusters. Using Au catalyst, single‐crystalline SnO2 nanowire trees can be grown in a one‐step process. Two types of trees are identified that differ in size, presence of a catalytic tip, and degree of branching. The growth mechanism of these nanotrees is based on branch‐splitting and self‐seeding by the catalytic tip, facilitating at least three levels of branching, namely trunk, branch and leaf.

Metal-oxide semiconductors have served as functionalm aterials for gas sensors for decades. [1] The sensitivity,r esponse speed and powere fficiency scale with the size of the sensing element. Nanostructures of SnO 2 are ideal building blocks for realising improved sensors, for example, for inflammable gases such as CO. [2][3][4] Branched nanowires are particularlys uited to provide high sensitivity at low cost without the need of spatial resolution, that is, it is of minor importance where as ingle gas molecule is detected on the sensora sl ong as it can be detected. The most economical methodt of abricate such structures is chemicalv apour transport. [5] This method employs ac atalysed growth process in which nanowires are seeded by Au nanoparticles. [6] Branches are typicallya dded by multi-step catalyst seeding. [7] Here, we demonstrate the efficient one-step growth of SnO 2 nanotrees using Au catalyst nanoparticlesa nd explain the growth mechanism.

Experimental Methods
Samples were synthesised in ah orizontal tube furnace( Nabertherm B180, 25 mm-diameter,h eatedl ength 600 mm) from Sn granules placed in aq uartz boat. N 2 at atmosphericp ressure was used as ac arrier gas, with at ypical flow rate of 300-900 sccm. The boats were physically connected to magnetic trans-fer arms. The furnace was heatedu pt ogrowth temperature (typically8 00-1000 8Cf or nanowireg rowth) over ar amp time of one hour.S i(100) substrates were cleaved into pieces measuring 10 10 mm 2 and then cleaned using trichloroethylene, 2propanol, methanola nd DI water.S omeo ft he substrates were then further functionalised with 0.1 %p oly-l-lysine solution and coatedw ith either a5nm-diameter Au nanoparticle solution or aw ater-based TiO 2 solution, flushedw ithD Iw ater,a nd then blow-dried with nitrogen. [8] They werei nserted through al oadlock with the precursor retracted to the cold zone under maximum N 2 flow.T his procedure minimises the exposuret oa ir. The furnacew as then pumped (using am embrane pump) and flushedt wice for seven minutes. The precursor was subsequently movedt othe centre of the furnacea nd the substrates 12 cm downstream. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM) were used to characterise the samples. Typical growth parameters for SnO 2 nanotrees are aN 2 flow rate of 700 sccm and af urnace temperature of 950 8C.

Results
The parameter space of SnO 2 nanowireg rowth was studied systematically using af ast-load-lock CVD system. Figure 1 showst he results of three sets of growth experimentsv arying the catalyst, furnace temperature and flux (F igure 1a-c). Three samples were prepared for the catalystc omparison shown in Figure 1a:withoutcatalyst, with 20 nm-diameter TiO 2 nanoparticles (rutile and anatase mixture P-25) [8] and with 5nm-diameter Au nanoparticles. Recently,T iO 2 nanoparticles were demonstratedt oo utperform Au as ac atalyst for Bi 2 Se 3 nanowire growth. [8] Only plates grow from the substrate without catalyst, whereas both TiO 2 and Au enable nanowire growth. TiO 2grown nanowires form an intertwined, free-standing network. Au leads to ah omogeneous coverage with straight nanowires (diameter :50 nm) and nanostars, as shown in Figure 1a.T herefore, we used Au nanoparticles in all subsequentg rowth experiments. The temperature window forn anowireg rowth between 800 and1 000 8Ci sd epicted in Figure 1b.I nprinciple, Sn vapour transport is possible at lower temperatures because we found occasional 20 mm-sized clusters down to 600 8C. The highest yield of nanowires was achieved at temperatures around8 00 8C. At 1000 8C, the nanowires have ap oor morphology, as determined by SEM. At around 950 8C, the formation of branches from very long nanowires dominates the growth (c.f. Figure 1b,c entre). These nanotreesl ie flat on the substrate or growf ree-standing from the sides of the substrate. Af lux of 700 sccm was used fort he temperature series. The high flux is crucial for SnO 2 nanowire growth, as can be seen in Figure 1c.T he catalyst particles only accumulate material and grow into spheres at too low flux but nucleation is not initiated owing to limited precursor supply.There is athreshold between 300 and 500 sccm at 800 8Cf urnace temperature where nucleations tarts to occur.T he large diameter of the nanowires grown at 500 sccm meanst hat the axial growth rate is very slow and that radial growth dominates. It is the supersaturation of the catalytic tip that drives the growth at high enough flux (900 sccm). The resulting nanowires are thin and long. Many catalysts ites can be activated such that the substrate coverage, that is, the yield, is very high.
All nanowires that wereg rown using the Au catalyst have ac atalytic tipl arger than the 5nmd iametero ft he initial Au nanoparticle. This is evidenceo ft he fact that the catalyst forms an alloy with incoming precursor vapour and grows in size until supersaturationi sr eached, consistent with the vapour-liquid-solid (VLS) model. TiO 2 ,h owever,i sn ot liquid during the growth because of its higher meltingp oint, so the VLS mechanism can be ruled out. It either formsasolid solution with SnO 2 or facilitates the formation of Sn droplets that allow for self-catalysed growth. TiO 2 catalystp articless hould be found at the tip if as olid solution was formed to lead to tip-catalysed growth. TEM EDSd oes not detect any TiO 2 in the tip of the nanowire. Therefore, it can be concluded that the TiO 2 nanoparticles are located at the base of the wire. Growth is initiatedb yt he formationo fS nd roplets on their surface. Figure1as hows such droplets as dark spots on TiO 2 patches. The nanowires are amorphous.
Au-catalysedn anowires grow longer and straight compared to the TiO 2 catalysed nanowires (c.f. Figure 2b)a nd branch oc-casionally,a sa lso reported by Jin et al. for catalyst-free growth. [5] In the centreo ft he catalytic tip is ad ark spot, which is roughlyt he size of as ingle Au nanoparticle or ac luster of several nanoparticles, indicative of VLS growth.W hereas most of the nanostructures are composed of SnO 2 ,asmall fraction of pure Sn is also found (see high resolution image of the tip area in Figure 2c). The growth directiono ft his short wire is [200],c onsistent with Sn nanowire growth. [9] The SnO 2 nanowires have at etragonal crystal structure and grow along the [101]-direction, as shown in Figure 2d.
Growth at 950 8Ca nd af lux of 700 sccm lead to trees with and without catalytic tips. Ananowiretree without catalytic tip is tapered, as shown in Figure 3a and b. It consists of three components:t runk, branchesa nd leaves. The components grow perpendicular to each other and become gradually smaller.B ranches at the bottom are longer than branches at the top. The nanotree in Figure3ci sl arger and consists of at runk with 50 nm diameter and fine branches with 30 nm diameter. The branchesa re curled up and have grown denser than the tree in Figure 3a.
The four sections of the trunk are described in the following. The tip is formed by ad ropleta su sually observed for VLSgrown nanowires.T he next section is marked by the appearance of short branches. The number and length of branches increases towards the main section. Branches are connected to the tree by extruding bases (Figure 3c(II), inset). The fourth section near the base is completely free of branches. An amorphous structure on the root indicatest he first tens of nanometres where the growth started.  What is the mechanism for the formation of branches?A low magnification TEM image in Figure4ag ives an overview over the density and length of the branches. The density is especially increased by separation. In Figure 4b an anobranch splits in the centre ands eparatesi nto two branches. This can be induced by ac rystal defect or,m ore likely,asplit catalyst particler esulting in an additional growth direction. High magnificationi magesr eveal 5nm-sized particles on the walls of the trunk, as shown in Figure 4c.T he size is identical to the particles ize of our commercial Au nanoparticles. There are three scenarios that can explain this feature:1 )Aun anoparticles migrate over the SnO 2 surface and deposito nto the trunk. The 5nmparticle shown in Figure 4c could be such ananoparticle. However,t his would require an exceptional mobility of Au on SnO 2 at this temperature;2 )Snp recursor atomss pontaneously form droplets on the SnO 2 trunk andb ranchesg row self-catalysed following the VLS mechanism. This also explains the observation in Figure 4c and the formation of leaves grown from smaller droplets on the branches;3 )the main catalyst particle at the tip gives off smaller particles if the size exceeds ac ertain threshold, [10] as depicted in Figure 4c.T hese particles can even migrate several micrometers along the trunk. [11] Duringt he first few micrometers of growth the size does not exceed the threshold yet. Hence, fewer branches grow at the bottom. Branch length is af unctiono fh eight, measured from the trunk, with the longest branches towards the root and short branches, or even seeds, at the top. Leaves grow also mediated by catalystm igration on branches. Some of the nanotreesd on ot have ac atalyst particle attached to the tip, and it can be assumed that it has been consumed during growth, yet they still grow branches.
In conclusion, Au and TiO 2 nanoparticles are efficient catalysts for SnO 2 nanowire growth. Networks of polycrystalline and bent or single-crystalline, straight and branched nanowires can be grown by tuning the growth parameters. TiO 2 catalyst particles facilitate the self-catalysed growth by enablingS n dropletf ormation on the catalyst surface. At 950 8CA un anoparticles diffuse over the substrate and the grown structures. This leads to ap referentialg rowth on nanowires forming trunks, branches and leaves.T hese nanotrees are excellent buildingb locks for highly-sensitive gas detectors and other nanoelectronic devices.
Keywords: crystal growth · nanocatalysis · nanogrowth · nanostructures · physicalv apor deposition  The insetshowsatrunk (brown arrow)w ith branches (yellow arrow) and leaves (green arrow). b) Sketch of an anowire tree. The tip is tapered with no catalyst particle visible at the top (upperi nset). Both trunk and branches, as well as branches and leaves are perpendicular to each other (lower inset). c) Horizontally oriented nanotree on the surface. Therea re four sections( I-IV)w ith distinctive features:t ip (I), littleb ranch growth with extrusion in the inset (II), middle of the trunk (III) and root (IV).