Metal-Seeded Growth Mechanism of ZnO Nanowires - Crystal Growth

Article pubs.acs.org/crystal

Metal-Seeded Growth Mechanism of ZnO Nanowires Heike Simon, Tobias Krekeler, Gunnar Schaan, and Werner Mader* Institute of Inorganic Chemistry, University of Bonn, Roemerstrasse 164, D-53117 Bonn, Germany S Supporting Information *

ABSTRACT: The widely applied metal-catalyzed growth mechanism of ZnO nanowires (NWs) is investigated by advanced methods of transmission electron microscopy and is discussed with respect to thermodynamic growth conditions. Au catalyst particles do not contain a substantial amount of Zn proving a solid Au catalyst at 1173 K growth temperature. This result is owed to the high equilibrium Zn partial pressure over Au−Zn alloys which in turn leads to a very low sticking coefficient of Zn from vapor and prevents alloying. Growth rates of ZnO NWs were measured between 5.5 nm s−1 and 36 nm s−1 as a function of oxygen partial pressure. The enhanced growth rate at higher oxygen partial pressures is explained by an increased sticking coefficient of Zn atoms at the Au catalyst. A growth mechanism is proposed which is quite different from the classic vapor−liquid−solid (VLS) mechanism: Zn alloys only in a thin surface layer at the catalyst and diffuses to the vapor− catalyst−NW triple phase line. There, together with oxygen, ZnO ledges nucleate which grow laterally to inner regions of the ZnO−Au heterointerface where Zn and oxygen can diffuse and finally promote NW growth in a rather kinetically controlled process. The geometry of the ZnO−Au interface  planar or stepped  and the associated diffusional transport properties are shown to be determined by the orientation relationship between Au and ZnO and hence by the atomic structure of the interface.

1. INTRODUCTION One-dimensional nanostructures exhibit distinctive and novel properties that may differ from those of bulk and thin-film materials.1,2 Hence, semiconductor nanowires (NWs) have been extensively studied for fundamental science as well as for potential applications in electronics, photonics, and as sensors.3 One of the most widely used growth methods of NWs is the socalled vapor−liquid−solid (VLS) process, because it offers excellent control over size, shape, and location of growth,4−7 which are most important issues for the integration of onedimensional nanostructures into functional devices.8,9 In the VLS process, first proposed by Wagner and Ellis,10 atoms or precursor molecules are supplied by the gas phase (vapor). The vapor species decompose and stick at a metallic seed particle acting as a catalyst. Soluble atoms alloy with the particle, and finally a supersaturated eutectic liquid is formed. At the contact face with a substrate a solid NW nucleates and grows as long as vapor species are supplied. The VLS process has become a widely used method for fabricating one-dimensional nanostructures from a rich variety of inorganic materials that include elemental semiconductors, compound semiconductors, sulfides, and oxides.11−14 Owing to unique semiconducting and piezoelectric properties of ZnO15,16 as well as due to simplicity of fabrication, a wealth of work has been published on the preparation, structural characterization, and chemical and physical properties of ZnO nanostructures grown by the VLS process in the past decade.4,5,17−23 In nearly all of these studies, zinc vapor is generated by the reduction of ZnO with carbon, and oxygen is © 2012 American Chemical Society

supplied with the inert carrier gas. This is a very efficient and fairly simple method to apply. Only a few reports exist which systematically focus on the growth conditions of such as oxygen partial pressure,24,25 growth temperature,26,27 the influence of the substrate,28 and details of the nucleation.29 In these studies it is assumed that the growth mechanism follows the VLS concept where a liquid seed particle is involved. However, growth of ZnO NWs should be different from VLS growth because oxygen has virtually no solubility in gold, neither in the solid nor in the liquid state. Hence, oxygen cannot reach the catalyst−ZnO interface (growth interface) via diffusion through the catalyst. On the other hand, mechanisms of nucleation and growth have been intensively studied for NWs of elemental and III−V semiconductors including thermodynamic and kinetic aspects at the seed particle.11−14,30−35 In studies of catalyzed GaAs and InAs NW growth it was observed that the solubility and content of arsenic in the Au catalyst is very low, and it was concluded that the NWs grow via a solid-phase diffusion mechanism, and hence with growth different from VLS.32,35 These results indicate that NW growth of compounds can be quite different from standard VLS growth of elemental semiconductors. Thermodynamic and kinetic aspects may lead to alternative models of catalyzed NW growth including Received: August 10, 2012 Revised: December 12, 2012 Published: December 19, 2012 572

dx.doi.org/10.1021/cg301640v | Cryst. Growth Des. 2013, 13, 572−580

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growth experiments. For very low partial pressures of oxygen (pO2 < 10−10 Pa), realized by setting the electrolysis cell voltage to zero, growth of structures of any type was not observed. Nanostructures grown with oxygen contents lower than 20 ppm oxygen have a low density on the substrate, exhibit lessdefined shapes, and partial formation of ZnO films is observed. Oxygen contents between 40 ppm (pO2 = 4 Pa) and 100 ppm (pO2 = 10 Pa) in the carrier gas resulted in well-defined nanorods as shown in Figure 1. Oxygen contents higher than

surface diffusion models and have been discussed and proposed.36 In the literature, growth of ZnO NWs via VLS appears to be generally accepted. However, there is no proven evidence that catalyzed growth of ZnO NWs has to follow the known mechanisms of VLS. Hence, a better understanding of the growth mechanisms is certainly desirable and may be helpful for catalyzed growth of ZnO NWs together with dopants. In this contribution we present a detailed and systematic study of Au-catalyzed growth of ZnO NWs. The carbothermal method enables a precise monitoring of the supply of zinc and oxygen in the vapor. Methods of transmission electron microscopy (TEM) are applied to clarify the growth situation at a microscopic scale focusing (i) on structural details at the catalyst surface and the catalyst−NW interface, and (ii) on the chemical composition of the catalyst particle. Experimental findings as well as thermodynamic considerations confirm that alloying of the Au catalyst with Zn is not possible and that the catalyst interior is solid. A kinetic transport growth model is developed which is based on surface and interface diffusion and hence is quite different from classical VLS growth.

2. EXPERIMENTAL SECTION ZnO nanowires were grown in a furnace with two heating zones by a vapor phase transport process. As source material, an equimolar mixture of high purity ZnO powder (Sigma Aldrich 99.999%) and graphite (Alfa Aesar 99.9995%) was placed in an alumina boat located at heating zone I held at 1000 °C. Fused silica substrates coated with Au were placed vertically at heating zone II at variable temperatures. The sputter-deposited Au particles, originally having diameters of 5− 10 nm, increased in size to several 10 nm by heating in zone II. The vapor produced in zone I was transported to the substrate by an Ar gas flow of 500 sccm at ambient pressure. The dried 5 N Ar gas is purified with a titanium furnace built into the Ar feeding pipe. Oxygen is generated by electrolysis of concentrated H2SO4 where the amount of oxygen and hence the partial pressure can be precisely adjusted with the current through the cell. The complete setup of the thermal CVD growth system is shown in Figure S1 (Supporting Information). For rapid cooling in some of the growth experiments a wire was fixed to the substrate to quickly (ca. 0.5 s) pull it out of the furnace into a cup with petroleum. Nanowire structures on the substrate were examined by field emission scanning electron microscopy (FESEM, JEOL JSM 6400F and LEO Supra 55). Transmission electron microscopy (TEM) and electron diffraction studies were conducted with a Philips CM30ST transmission electron microscope at 300 kV, and for chemical microanalysis by energy-dispersive X-ray (EDX) spectroscopy a Si(Li) X-ray detector fitted to the electron microscope was used. High resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) were performed in a Philips CM300UT field-emission electron microscope (FEG-TEM) with postcolumn imaging filter (Gatan Imaging Filter, GIF) operated at 297 kV. For TEM studies the NWs were separated from the substrate by ultrasonic treatment in methanol. A drop of the suspension on a holey carbon film supported by a copper grid was dried in air and used for the TEM studies. To study isolated Au particles by TEM and EDX spectroscopy the ZnO NWs were dissolved in dilute (7%) HCl solution. The remaining Au particles were dispersed in ethanol, and the suspension was dropped and dried onto a carbon film supported by an aluminum TEM grid.

Figure 1. SEM images of ZnO nanowires grown at different oxygen partial pressures: 15 ppm (a), 40 ppm (b), and 80 ppm (c, d).

ca. 2000 ppm resulted in oxidation of the graphite in the source mixture, and growth of nanostructures was not observed. Earlier reports on ZnO NW growth with “pure” argon as carrier gas can be concluded to be the result of oxygen impurities in the argon gas bottle. Comparison of the lengths of NWs shown in Figure S2 (Supporting Information) proves the growth rate to clearly depend on the supply of oxygen in the carrier gas. After reaction time of 20 min and at oxygen contents of 40 and 80 ppm, the NWs are ca. 6.5 and 42 μm in length, respectively. For length determination ca. 10 of the longest NWs were measured, because they can be presumed to have nucleated at the very beginning of the growth experiment. This assumption is supported by the result of length measurements for 5, 10, and 20 min reaction time t shown in Figure 2. The lengths l as

3. RESULTS To investigate the optimum growth conditions of ZnO NWs, the oxygen content of the Ar carrier gas and the temperature of deposition were varied. Best conditions for ZnO NW growth were observed between deposition temperatures TII of 870 and 940 °C. In the following, TII = 900 °C was chosen for all

Figure 2. Lengths of longest ZnO NWs measured after growth for 5, 10, and 20 min. Linear fit results in a growth rate of 5.6 nm s−1 for 40 ppm O2 and 36 nm s−1 for 80 ppm O2. 573



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signal at E = 8.49 keV interferes with the Zn line. Hence, the lines of the Zn_L emission (Zn_Lα1: E = 1.01 keV) are more appropriate for the detection of Zn. Neither by visual inspection nor by quantification of the spectra with the EDX analysis software an X-ray signal of Zn could be detected, and the detection limit is ca. 0.3 atom %. This means that Zn is virtually not present in the volume of the catalyst during growth of the NW. Imaging of ZnO NWs in the electron microscope yields further information on the processes at the Au catalyst particles. Electron microdiffraction was used to determine the absolute growth direction which is [0001] of the polar ZnO NWs (see Figure S3, Supporting Information).40 The NWs grow in the positive direction of the ZnO c axis, and hence the Au catalyst particles are on top of the zinc-terminated face of ZnO NWs.15 Au particles of moderately cooled samples are clearly faceted, whereas the particles of quenched samples are either roundish in shape or they exhibit small facets and round edges as shown in Figure 4. At many NWs a specific geometry at the interface between the growing ZnO NW and the Au catalyst is observed. Such configurations are shown in Figure 5, where the ZnO

function of time are on a straight line with slopes equal to the growth rate which meet at l = 0, t = 0 proving that the longest NWs started growing at the beginning of the growth experiment. With 40 ppm oxygen content NWs grow with a rate of ca. 5.5 nm s−1, with 80 ppm oxygen content the growth rate is ca. 36 nm s−1. A volume of 1 nm3 ZnO contains 42 atoms of Zn and O. Hence, at a growth rate of 5.5 nm s−1 (36 nms−1) 231 atoms (1.512 atoms) of Zn and O are deposited and attached to a 1 nm2 area of a growing ZnO NW. Furthermore, it is worth mentioning that the growth rate is virtually independent of the NW diameter and that all nanostructures are grown via a VLS mechanism proven by Au particles on top of the wires. An important issue is the yield of Zn used for the growth of NWs as part of the amount of Zn vapor delivered with the carrier gas. For that purpose the mass loss of the source mixture is determined, which is ca. 25 mg/h. The products of the carbothermal reaction of the source mixture, ZnO(s) + C(s) → Zn(g) + CO(g)

(1)

are essentially Zn and CO in equal molar quantity, and the mass loss of ZnO is calculated to be 17.5 mg/h = 7.5 × 10−8 mol/s. The amount of Ar fed into the tube is 3.7 × 10−4 mol/s. From that, the partial pressure of Zn and CO is determined to pZn = pCO = 20 Pa. Kinetic gas theory gives the number z of atoms impacting per area and time as z = n/4v,̅ where n is the number of atoms/molecules in a standard volume and v ̅ is the mean velocity of the gas species. In a gas at 1 bar the number n of gas species producing 20 Pa partial pressure corresponds to n = 5 × 10−6 nm−3, and Zn vapor at 900 °C yields v ̅ = 616 m/s = 6.16 × 1011 nm/s. As a result, z = 770.000 Zn atoms are hitting a surface area of 1 nm2 in one second. The process of VLS growth of semiconductor nanostructures was originally explained by supersaturation of the liquid catalyst with the growth material.10,37−39 The growth material is alloyed and has to be present in the catalyst particle also after cooling. Hence, the chemical composition of the catalyst particle gives valuable information on the processes associated with the catalyst function. We have probed the content of Zn by EDX analysis in the transmission electron microscope in ca. 50 Au catalyst particles. A particle isolated by dissolving the entire ZnO NW is shown in Figure 3 together with the corresponding EDX spectrum. The Zn_Kα line at E = 8.64 keV would be most appropriate for EDX analysis; however, a nearby Au_Lλ

Figure 3. EDX spectrum and TEM image of an isolated Au catalyst particle. The Zn content of the catalyst particle is below the detection limit of ca. 0.3 atom % as shown by the missing Zn_L line at ionization energy E = 1.01 keV (see inset).

Figure 4. TEM images of ZnO NWs with faceted Au catalyst particles of moderately cooled (a) and nonfaceted particles of quenched samples (b). 574



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Figure 5. TEM image of frequently observed interface geometry between ZnO NW and Au catalyst where outer regions have grown further than inner regions of the NW (a, b). Steps at the interface are observed in the highly magnified image (c) of the boxed area in (b). Note that ZnO is sensitive to electron radiation visible at the NW close to the interface.

Figure 6. HRTEM image of interface region between ZnO NW and Au catalyst quenched to RT (a). Fourier transforms of ZnO NW (b), interface region (c), and Au catalyst (d) indicate crystal overlap in projection as illustrated schematically in (e).

NWs have grown further at outer regions of the reaction face than in the center of the wire, and the catalyst appears to be sunk into the NW. This is also shown and illustrated in Figure 6 where epitaxially grown ZnO can be detected in lattice images around the Au particle even some 10 nm away from the bottom face of the Au catalyst. The high-resolution TEM image, considered as a projection of the crystal structures, shows overlapping images of Au and ZnO which confirms the geometry shown in Figure 6e. A further observation at Au particles  rapidly quenched or moderately cooled to RT  is a noncrystalline surface layer with thickness in the range of 1−2 nm (Figure 7). The layer thickness does not depend on the volume of the Au particle shown at catalyst particles with different sizes. The chemical composition of the surface layer was determined by EELS using an electron probe of ca. 5 nm diameter at 20 catalyst particles. The accumulated EEL spectrum of seven single spectra is shown in Figure 8. The quantification of the intensities of the Zn_L2,3 edge vs that of the O_K edge yields the composition of ZnO. Hence, the ZnO surface layer is likely to be formed by oxidation of some Zn content at or near the surface of the Au catalysts upon removal of the substrate out of the furnace into the air.

Figure 7. (a−c) Gold catalyst particles of different sizes and volume exhibit a surface layer of virtually the same thickness (1−2 nm) proving the layer to be generated by a surface process.

4. DISCUSSION The conditions for the growth of ZnO NWs in our experiments, i.e., growth temperature, oxygen content of the carrier gas, and Zn vapor supply by carbothermal reduction of ZnO, are very similar to those of many previous studies in the

literature.4−6,17,20 Note that in earlier studies oxygen was not added to the carrier gas; there it was just an impurity in commercially supplied argon gas. With equilibrium thermody575



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concentration xZn = 12.5 atom % (Figure S4, Supporting Information). The equilibrium Zn partial pressure pZn(T) of that liquid is given by 0 pZn (T ) = a Zn(T ) ·pZn (T )

(2)

p0Zn(T)

where is the partial pressure of pure Zn and aZn(T) is the activity of Zn in the liquid alloy. Since NW growth is close to the boiling temperature 1184 K of Zn, the equilibrium pressure over liquid Zn is high, i.e., p0Zn(1173 K) = 8.7 × 104 Pa.43 In Figure S5 (Supporting Information) the Zn activity is extrapolated for x = 12.5 atom % from the activities given by Ipser et al. resulting in aZn(1173 K) = 2.4 × 10−4.44 The equilibrium Zn pressure is determined from eq 2 as pZn(1173 K) = 21 Pa. It is worth noting that the activity and hence the Zn pressure of a Au−Zn liquid with xZn = 18 atom % is already 60 Pa, and Zn would even evaporate from that liquid alloy to constitute its equilibrium partial pressure. In thermodynamics of crystal growth, the Gibbs energy of formation ΔGf is the driving force for the reaction of the constituents, which is for ZnO NWs Zn(Au, l) + 1/2 O2 (g) → ZnO(s), T = 1173 K

(3)

The standard Gibbs energy of formation of ZnO(s) at 1173 K is ΔG0f = −217 kJ/mol, calculated for liquid Zn.45 For a Au−Zn liquid alloy with xZn = 12.5 atom % this value has to be corrected for the Gibbs energy of mixing, ΔGm = −12 kJ/mol, which has to be taken as a positive value for Zn exsolution.44,46,47 Hence, the Gibbs energy of formation of the reaction 3 is ΔGf = −205 kJ/mol, which proves a very high driving force for ZnO NW formation. A comparison with III−V semiconductors can be made with the standard enthalpy of formation ΔH0f (ZnO) = −350.5 kJ/mol versus ΔH0f (GaN) = −110.5 kJ/mol and ΔH0f (GaP) = −88 kJ/mol.45 The energies associated with the precipitation of Si (and Ge) wires from a eutectic melt with a metal in the VLS process are even lower.48 Hence, the resulting growth rates are much lower than for ZnO NW growth. The oxygen partial pressure was shown to strongly influence the ZnO NW growth rate. The increase of pO2 from 4 to 8 Pa results in an increase of the growth rate by a factor of ca. 7, whereas the Zn vapor supplied by the carrier gas is kept virtually constant. In fact, the higher growth rate of the ZnO NW requires more of the impacting Zn atoms to stick at the surface of the Au catalyst which, at first sight, seems contradictory in view of a higher pO2 as that might suppress the deposition rate or the surface diffusion.49 The effect of the oxygen content on the growth rate can, however, be understood having a view on the associated kinetics and thermodynamics at the Au catalyst: The Au−ZnO interface is an effective sink for the Zn delivered via the Au catalyst and the Zn is finally consumed for ZnO NW growth. (i) A higher growth rate implies an enhanced consumption of Zn from the Au catalyst which is very likely to reduce the Zn concentration there. (ii) With the same thermodynamic arguments as used above, the equilibrium Zn partial pressure over the Au−Zn catalyst is lowered. (iii) As a result, the Zn vapor supplied by the gas phase exceeds the equilibrium pressure over the catalyst, and the system will react to increasing the Zn content which is realized by a higher number of Zn atoms sticking to the Au catalyst. (iv) The amount of Zn transferred by the Au catalyst to the Au−ZnO heterointerface will increase until steady state growth conditions, i.e., equal rates of supply of Zn and

Figure 8. EEL spectrum accumulated from seven individual spectra at Au catalyst surfaces proving that the surface layers consist of ZnO (a). TEM image of one catalyst with electron probe position indicated at surface region (b).

namic calculations using the code CVTRANS we can show the products of the carbothermal reaction (eq 1) to be essentially Zn vapor and CO over solid ZnO and carbon.41 This is in full agreement with the well-known Boudouard equilibrium of C, CO, and CO2 at high temperatures. The content of CO2 is smaller than that of CO by a factor of 1.000 and hence is a minor product not essential for the growth process. Thermodynamic Considerations of ZnO NW Growth. In our study, the Zn content in the carrier gas could be well estimated from the mass loss of the source material. The high number of impacting Zn atoms (770.000 nm−2 s−1) onto any surface in the reaction tube and hence on the Au catalyst seems to be in contradiction to the relatively small number of Zn atoms (231 and 1.512 atoms nm−2 s−1 at 40 and 80 ppm O2, respectively) used for ZnO NW growth. Here the larger surface of the Au catalyst compared to the growth interface is not even considered. Additionally, it is known that the sticking coefficient, i.e., the number ratio between impacting atoms from the gas phase and atoms attached to the surface, is close to 1 on a liquid surface. The small number of Zn atoms effectively sticking and being chemisorbed at the surface of the Au catalyst can be explained well by thermodynamic considerations: the equilibrium Zn partial pressure over a Au−Zn melt has to be compared with the Zn partial pressure of ca. 20 Pa supplied by the carbothermal reaction. The Au−Zn liquid with lowest Zn content is at the concentration of the liquidus boundary in the Au−Zn phase diagram.42 At 1173 K this is a liquid with Zn 576



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state conditions must exist at the catalysts surface demanding equal rates of Zn incorporated from vapor and Zn transported away to be consumed for ZnO crystallization and NW growth. If the rate of Zn uptake were higher than that of Zn consumption, excess Zn would alloy with the catalyst. It seems that the surface layer takes up a limited amount of Zn and readily compensates for consumption losses by a higher incorporation rate of Zn from vapor. On the basis of the experimental results we may discuss the transport kinetics of the Zn atoms to the reaction front as well as the growth kinetics of the ZnO NWs on a microscopic scale. During growth, the Zn concentration gradient or difference between the Au catalyst surface and the reaction interface is the driving force for diffusional transport of Zn atoms. They may be transported (i) via volume diffusion through the solid catalyst or (ii) along the catalysts surface along a thin liquid alloy film and by surface diffusion. The difference in diffusion coefficients in the two regions will be decisive for the kind of path the Zn atoms prefer. For Zn diffusion in solid Au the diffusion energy Q = 1.64 eV and the pre-exponential factor D0 = 0.082 × 10−4 m2 s−1 are known which yields a diffusion coefficient of D(1173 K) = 7.4 × 10−13 m2 s−1.55,56 There are no diffusion data available for Zn in a liquid Au−Zn alloy or for Zn surface diffusion on a Au surface. However, typical diffusion coefficients in liquids are in the range of 10−8 m2 s−1.57 Apart of the diffusion constant, the transport rate depends on the length and the cross section of the diffusion path. The paths through the interior and around the catalyst particle are similar, independent of the starting point of Zn atoms: If we consider the Au particle to be a cube with edge length a having a surface film with thickness b, the cross section for diffusion through the interior is a2 and the one along the surface film is 4 ab, which results in a/4b as the ratio of the two cross sections. For a = 50 nm and b = 2 nm the ratio is ca. 6, and even for a large particle with a = 100 nm and a thin film with b = 1 nm the cross-section ratio is only 25. The large difference in diffusion constants indicates that the preferred path of Zn atoms is along a liquid alloy film at the catalyst particles surface which is consistent with the observation of the catalyst interior being virtually free of Zn. The length increase of zinc oxide nanowires should be explained by nucleation and growth on a molecular or atomic scale where the reacting constituents or monomers are Zn atoms delivered by catalyst surface regions and oxygen in the vapor. The activities of the monomers are constant everywhere; however, the existing ZnO nanowire provides appropriate sites for ZnO nucleation. It was reported that ZnO nanostructures which were grown via VLS along [110̅ 0] thicken during growth and finally grow to nanosails.28 At ZnO NWs grown in [0001], the prism side faces, however, do not thicken at all. The fast growing zinc-terminated [0001] face in contact with the Au catalyst offers favorable nucleation sites, and it is there, at the junction of the three phases  the vapor, the liquid (catalyst surface) and the solid (ZnO)  where nucleation of new layers starts: at this triple phase junction the activities of Zn and oxygen are higher than at regions further inside the Au−ZnO interface. The function of the gold may be 2-fold: besides trapping Zn from the vapor, it may also act as a catalyst weakening the double bond of oxygen molecules and hence to facilitate the dissociation of O2. Then, activated oxygen molecules or atoms together with Zn atoms may diffuse along the Au−ZnO interface to react and nucleate to ZnO at appropriate sites. These sites will be steps at the interface or even kinks where two steps meet, similar as at free surfaces

consumption for ZnO formation, are reached. Thereby, the amount of Zn at the Au catalyst is regulated by the oxygen content in the carrier gas, and the mechanism is kinetically controlled over a wide range of pO2. State of Catalyst Particles. In this context, the state of the Au catalyst during growth is of general interest for the path of Zn atoms migrating to the growing Au−ZnO interface. According to the Au−Zn phase diagram,42 the Zn content has to be at least 12.5 atom % for a completely liquid catalyst droplet. For a solid catalyst with a liquid surface layer it would be sufficient that the overall composition is in the two-phase field between solidus and liquidus, i.e., between xZn = 8.5 atom % and xZn = 12.5 atom %, respectively. The catalyst surface where the Zn is supplied will exhibit the liquidus composition and the interior has the solidus composition (Figure S4, Supporting Information). However, as proven by experiment, the Au particles essentially do not contain Zn which has two consequences: (i) the interior of the catalyst particles has to be solid during growth at 1173 K, and (ii) there is no thermal equilibrium established at the catalyst particles. This leads to the conclusion that the solid Au particles feature a thin liquid alloy film at the surface by impacting Zn atoms supplied by the gas phase. This is also reasonable since a liquid alloy film produces isotropic surface tensions and will not lead to faceting which was observed at quenched catalyst particles. However, annealing of previously grown ZnO NWs at 1173 K on air and subsequent quenching to RT also did not produce faceting of Au catalyst particles. Obviously, the surface energy anisotropy at that temperature is substantially decreased and is not sufficient to prove a liquid film at the catalyst surface.50 A further reason for the existence of a liquid film on Au catalyst particles may be premelting or surface melting. This phenomenon, known since the 1950s and observed at nanoparticles of various metals, may occur well below the bulk melting temperature as a result of extreme surface curvatures.51−54 Premelting of entire Au particles with diameters of 10 nm at 1225 K, more than 100 K below Tm, Au = 1337.6 K, was observed by Buffat and Borel.52 Frenken et al. observed surface melting of ca. 5 monolayers even on a flat [110] surface of a Pb crystal 150 K below Tm.53 Qualitatively, premelting of an alloy may be understood as temperature reduction of solidus and liquidus lines which is displayed schematically for the Au−Zn system in Figure S4 (Supporting Information). Assuming premelting at 60 K below Tm,Au, a Au alloy particle containing somewhat more than 5 atom % Zn may exhibit a molten surface layer at 1173 K. Therefore, in addition to alloying with Zn, surface melting may further enhance the formation of a liquid layer of Au(Zn) on the catalyst particles. It is worth noting that NWs of other metal oxides may well grow via VLS with a liquid alloy catalyst particle. We have produced In2O3 NWs which grow with a Au− In catalyst with In contents up to 30 atom %, and which are definitely liquid at 1173 K according to the phase diagram. This observation can be explained by the equilibrium partial pressure of In over liquid In which is lower by ca. 105 than the Zn partial pressure at the same temperature (1100 K).43 Hence, the In content supplied by the gas phase is much higher than the equilibrium pressure whereby the In is alloyed in the Au catalyst particles. Growth Mechanism. It has been shown that Zn does not alloy in the catalyst interior and is present only at surface regions of the catalyst particles. As mentioned above, steady 577



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as zinc and oxygen. (ii) Some of the ZnO NWs have an interface shaped as a bowl with the catalyst as counterpart (c.f. Figure 5). This geometry is generated when lateral growth of ledges is not yet completed and new ledges already nucleate on top of the existing ones at the triple phase junction. The reason for this may be a hindered diffusional transport of species at the interface. At conditions where nucleation only occurs at the outer rim of the catalyst particle it is possible to even grow ZnO nanotubes.19 The formation of the bowl-shaped and faceted interface geometry is likely to be caused by the special crystallographic orientation relationship between ZnO NW and Au catalyst resulting in interfaces having different transport properties than general interfaces. A high resolution TEM micrograph of the NW in Figure 5b is shown in Figure 10,

where ad-atoms preferentially attach. Scenarios of the ZnO NW growth mechanism are illustrated in Figure 9, starting with the

Figure 9. Schematic of zinc and oxygen paths at Au catalyst (a). Nucleation at triple phase junction and lateral growth of ZnO ledges by diffusion of species along the Au−ZnO interface and attachment to steps. Schematics of the diffusion path of oxygen and Zn species along the Au−ZnO interface (b).

nucleation of a new ZnO ledge around the edge of the NW. The minimum ledge height may have the size of a ZnO4 tetrahedron equivalent to c/2 of the ZnO unit cell. Essentially, two different shapes of the catalyst−ZnO interface were observed, and hence different growth modes may be assumed. (i) Many of the ZnO NWs exhibit virtually flat interfaces with the Au catalyst particle. Formation of this geometry requires that the time for completion of a ZnO layer by lateral ledge growth is shorter than the time scale for nucleation of a new ledge at the edge. Rapid ledge growth requires fast diffusional transport of species along the Au−ZnO interface. Similar as in polycrystalline materials where grain boundary diffusion is much faster than diffusion in the lattice, diffusion of Zn and O atoms is preferred at the Au−ZnO interface for the following reasons: interfaces between noble metals and oxides exhibit weak chemical bonding, characterized by high interfacial energies and small work of adhesion known from contact angle measurements.58,59 Furthermore, interfacial energy is a function of the interface geometry or orientation relation and hence of the atomic structure.58,60 General heterophase interfaces are created when the Au crystals are not in a special orientation with the underlying ZnO crystal which is the statistically most probable situation. Owing to this, high-indexed planes of the metal with only a small number of atoms are in contact with the oxide crystal as schematically shown in Figure S6a (Supporting Information). However, general interfaces provide good diffusion paths for species such

Figure 10. High-resolution TEM micrograph of region between Au catalyst and ZnO NW exhibiting steps and ledges at the interface. The special orientation relationship of crystals is proven by Fourier transforms of regions 1 and 2.

where the lattices of Au in ⟨110⟩ and ZnO in a axis orientation ⟨211̅ 0̅ ⟩ are imaged. The orientation relationship is special, given by {111}Au (0001)ZnO and ⟨110⟩Au ⟨2 1̅ 1̅ 0⟩ZnO

The interface is formed by a {111} plane of Au and the (0001) face of the ZnO crystal, which both are closed-packed planes. Despite a lattice mismatch of 12%, the interface exhibits a high density of coincidence sites and appears dense as illustrated in Figure S6b. As a consequence, diffusional transport along this interface can be imagined to be much more difficult as along a general interface which is more “open”. This observation may 578



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serve as proof for the effect of interface geometry and atomic structure to transport properties. A similar observation of step formation and ledge growth was made by in situ TEM experiments at Si NWs growing by a vapor−solid−solid (VSS) mechanism where the catalyst particles are solid.30,31 Different from the ZnO−Au system the supply of Si atoms occurs over the complete heterointerface by supersaturation of the catalyst. In this context, the absolute values of growth rates of Si and Ge NWs provide a further argument for a liquid phase being involved in catalyzed growth of ZnO NWs. While Ge NWs grow from a liquid AuGe droplet (VLS) with a rate of 0.11 nm s−1, the rate drops to 0.013 nm s−1 when the catalyst solidifies (VSS) at virtually the same temperature.12,31 The authors suggest that the reduced transport kinetics as well as the reduced sticking probability at the catalyst surface is responsible for slowing down growth in the VSS process. If we consider the high growth rates of ZnO NWs ranging between 5 to 36 nm s−1, it is difficult to imagine that there should be no liquid phase involved. A further issue is the formation of a reduced diameter or “neck” of the NW at the triple phase junction often observed at III−V semiconductor systems.32 The neck appears to be important in forming geometry with contact angles where the three energies γSL of the solid−liquid interface, γLV of the liquid−vapor interface, and γSV of the solid−vapor interface are close to equilibrium. Whereas VLS growth rates of III−V semiconductor NWs are as low as of Si and Ge NWs and obviously allow formation of equilibrium geometries at the triple phase junction, the situation will be certainly different in ZnO NW growth. The high growth rates prove a rapid delivery of Zn and oxygen right to the triple phase junction, and such kinetically dominated growth conditions will prevent formation of equilibrium geometries at most of the NW−catalyst interfaces. Cheyssac et al. discussed different growth mechanisms of nanowires together with related issues where one is a surface diffusion model inspired by the observations of Persson et al. where solid Au catalyst particles on GaAs NWs were grown by chemical vapor deposition.32,36 The As concentration in the catalyst particle was shown to be very low, and transport of As was suggested to proceed along the catalyst surface and through the interface between catalyst and GaAs NW.32 This scenario resembles very much the situation of Zn and O in ZnO NW growth. Furthermore, in a recent study by Bao et al.29 with a focus on nucleation and kinking mechanisms of ZnO NWs, elemental mapping using EDX shows a very low signal of Zn inside the Au catalyst which confirms our findings of catalysts consisting of virtually pure Au. The Zn signal as well as the O signal29 is likely to originate from the ZnO scale which we observe on all of the Au catalyst particles.

(3) The growth rate of ZnO NWs is hardly influenced by the amount of Zn vapor since it usually does not exceed the partial pressure over Au−Zn alloy catalysts. However, the oxygen content regulates the consumption of the Zn for ZnO NW growth: increase of oxygen results in a higher consumption of Zn which decreases the Zn concentration at the catalyst surface and leads to a higher uptake of Zn from the vapor and finally to a higher growth rate of the NWs. (4) The transport of Zn from the vapor to the reaction zone at the catalyst−NW heterointerface is concluded to take place at the Au catalyst surface via a liquid Au−Zn surface layer followed by diffusion along the Au−ZnO interface. Obviously, oxygen does not react with Zn at the catalyst surface. It may be concluded that O2 entering the Au−ZnO interface dissociates to oxygen atoms which then facilitates the diffusional transport. The preferred nucleation site of new ZnO unit cells is the outer periphery of the catalyst−NW heterointerface where the oxygen activity is highest, and, as a start, ZnO ledges are formed there. Growth of the ledges toward inner interface regions proceeds by diffusion of oxygen and zinc along the heterointerface. The interface geometry, planar or bowl-shaped and stepped, is the result of the crystallographic orientation relationship between ZnO NW and Au catalyst resulting in differing atomic structures of the interfaces, which then possess different transport properties. (5) Catalyzed growth of NWs of any metal oxide must be quite different from classical VLS growth where the components are soluble in the liquid catalyst. Since oxygen is not soluble in Au or is soluble in very small amounts in other metals, it can be supplied only at the triple phase line where new metal oxide units nucleate. Growth proceeds by a ledge mechanism, whereas the constituents of the oxide have to diffuse along the catalyst−NW interface. In the case of NWs where the constituent metal is soluble in the catalyst, the particle may be liquid at growth temperature. Nevertheless, NW growth will not be homogeneous because of the gradient of oxygen activity along the catalyst−NW interface. This conclusion may be generalized to any system where one component of the NW is not soluble in the catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1. Schematic of the thermal CVD growth system. Figure S2. SEM images of ZnO nanowires grown for different times. Figure S3. TEM image, electron diffraction pattern, selected area electron diffraction (SAED) pattern, microdiffraction pattern. Figure S4. Au-Zn phase diagram. Figure S5. Activity of Zn in the liquid Au-Zn phase. Figure S6. Atomic configurations of Au crystals on a (0001) face of ZnO, unrelaxed models. Figure S7. Examples of planar interfaces generated by general orientations between Au catalyst and ZnO NW. This material is available free of charge via the Internet at http://pubs.acs.org.

5. CONCLUSIONS (1) The carbothermal reduction of ZnO essentially produces CO and Zn vapor. The formation of ZnO NWs by the oxidation of Zn vapor requires a supply of additional oxygen which can be precisely added to the carrier gas by electrolysis. (2) The catalyzed growth of ZnO NWs is characterized by the high partial pressure of Zn over Au−Zn alloys at typical growth temperatures which prevents alloying of the Au catalyst. Therefore, the catalyst particles do not melt at the growth temperature; only thin surface regions may become a liquid alloy, shown by a zinc free catalyst and by a uniformly thin ZnO scale after removal of the furnace.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 579



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