Tailoring 1D ZnO Nanostructure Using Engineered Catalyst Enabled

Article pubs.acs.org/JPCC

Tailoring 1D ZnO Nanostructure Using Engineered Catalyst Enabled by Poly(4-vinylpyridine) Yang Liu, Jose F. Flores, and Jennifer Q. Lu* School of Engineering, University of California, Merced, Merced, California 95343, United States S Supporting Information *

ABSTRACT: We have demonstrated a cost-effective homopolymer, poly(4-vinylpyridine), to prepare engineered catalyst systems. Using the polymer template, the composition of a catalyst system can be adjusted by metal incorporation while the size can be controlled by the catalyst layer thickness via the number of polymer coatings. We have found that the morphology of 1D ZnO nanostructures can be rationally tailored by catalyst composition in both vapor and solution-based approaches. For the vapor-based growth, incorporating a non-Zn cocatalyst reduces the onset growth temperature and affords higher growth rate. In the solution-based growth, the stability of catalyst in alkaline medium becomes critical. The presence of cocatalyst will lead to 1D ZnO nanostructures with larger diameter and lower density. Using catalyst which is identical to 1D nanostructure composition promotes epitaxial growth. A cocatalyst can be selected to tailor interactions with the precursors of 1D nanostructures for tunable morphology. These findings offer a new basis for controllable synthesis. Furthermore, harnessing the conformal coating ability of a polymeric material, catalyst nanoparticles can be uniformly deposited on the sidewalls of trenches or using selective interaction on surfaces of as-grown 1D ZnO nanostructures for sequential growth. This offers a new way to fabricate consistent and welldefined 3D architectures.

1. INTRODUCTION

parameters is thus costly. Furthermore, it is difficult to deposit a conformal layer uniformly on nonflat surfaces. The solution-based synthesis is more cost-effective compared to the vacuum-based approach.13,17,30−33 Catalyst coated substrates can be prepared by pyrolysis of catalyst precursors such as salts or organometallic compounds.13,17 Catalyst nanoparticles can also be synthesized in the form of colloids.14 The solution-based approach offers greatly improved control in catalyst composition at low cost. Still, both catalyst film thickness and coating uniformity over a large surface area cannot be well controlled by the aforementioned solution approaches. Another method that has been extensively investigated is to use block copolymer micelles as nanoreactors to generate well-dispersed catalyst nanoparticles with uniform size and periodicity.20,30−36 We previously reported that catalyst-containing micelles can be deposited not only uniformly but also conformally on uneven surfaces. Horizontally aligned and suspended CNTs with similar diameter have been generated.37 Nevertheless, block copolymer templates are expensive. Harnessing the ability of pyridine-based polymers to sequester metal species, we have employed poly(4-vinylpyridine) (P4VP) homopolymer as a low cost alternative to block copolymers to derive catalysis nanoparticles. In this

One-dimensional (1D) nanomaterials such as nanotubes and nanowires offer exciting properties resulting from radial confinement. The unconfined dimension along the length of 1D nanomaterials provides a path to “communicate/manifest” these properties. Their technological significances have been increasingly demonstrated.1−11 Both vapor transport-based and solution-based deposition techniques have been widely exploited for the growth of a variety of 1D nanomaterials.2,6,12−16 It has been suggested that catalysts play a critical role for enabling 1D nanomaterial formation for both techniques especially for the vapor transport-based approach. This role is exemplified by the fact that single-walled carbon nanotubes (CNTs) cannot be readily produced without using catalyst nanoparticles.17 Catalysts also provide an essential means to control diameter and crystal orientation.14,18−22 Over the years, two major categories have been established to generate catalysts nanoparticles. One is the vacuum-based thin film deposition technique that includes ion sputtering deposition, e-beam or thermal evaporation, laser ablation, and molecular beam epitaxy. All these methods have been proven to be effective for 1D nanomaterial growth.16,23−25 The film thickness has been adjusted to tailor the average diameter and density of 1D nanostructures.25−29 However, catalyst composition cannot be readily adjusted by these physical methods. In addition, deposition of nanometer thick films requires an ultrahigh-vacuum system, and careful monitoring of deposition © 2014 American Chemical Society

Received: May 15, 2014 Revised: July 17, 2014 Published: July 29, 2014 19387

dx.doi.org/10.1021/jp504783q | J. Phys. Chem. C 2014, 118, 19387−19395

The Journal of Physical Chemistry C

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and other metal oxides such as Ga2O3 or CuO are one of the cocatalyst species. 2.3. Growth of ZnO Nanostructures. 1D ZnO nanostructures were grown using both vapor transport and solution-based methods. The solution-based hydrothermal synthesis was carried out in a Teflon-lined stainless steel autoclave. A solution of 20 mM hexamethylenetetramine (HMTA, VWR, 99+%) and zinc nitrate hexahydrate (VWR, 99.998%) was used for the synthesis. The substrates were placed on the surface of solution with catalysts side facing down. An oven (Yamato ADP 21 vacuum oven) was preheated to 95 °C, and then the sealed autoclave was transferred into the oven and maintained at this temperature for 6 h. Vapor transport-based growth was carried out in a three-zone horizontal furnace. A mixture of ZnO powder (99.99%, Alfa Aesar) and graphite powder (99%, 300 mesh, Alfa Aesar) with a ratio of 3:4 by mass was used as Zn precursor for 1D ZnO nanomaterial growth. A 0.8 g precursor was loaded into an alumina boat, and the boat was placed in the upstream side of a quartz tube inside the furnace. A catalyst-coated substrate was placed in the downstream side of the quartz tube. The distance between substrate and the Zn precursor was 13 cm. To grow 1D ZnO nanostructures, ZnO/graphite precursor was heated to 950 °C to reduce ZnO to Zn. Ar with 2% O2 at a flow rate of 50 sccm was used as carrier gas to bring Zn vapor onto the substrate which was heated to 500−650 °C. The growth time was 20 min. The vapor transport-based method was employed to grow horizontal and branched structures on etched Si wafers and asgrown 1D ZnO nanowires, respectively. Zn(II) acetylacetonate/P4VP solution with a polymer Mw of 36 300 g/mol was first used to deposit ZnO catalyst nanoparticles on sidewalls of trenches. After annealed in air at 350 °C for 30 min to remove the polymer template, vapor-based growth was conducted to generate horizontally aligned nanowires in the trenches. For sequential growth, ZnO nanostructures were first synthesized using Fe2O3 as catalysts via the solution-based method. Then a thin layer of polymer containing Ga and Zn for the formation of (Zn/Ga)Ox was coated on the surfaces of ZnO 1D nanostructures. After annealing in air, the vapor-based method was used to produce branched structures. 2.4. Characterization. After deposition of a polymer and organometallic solution mixture on Si substrates followed by solvent removal, Fourier transform infrared spectra (Nicolet 380 FT-IR) were collected to verify the formation of coordination bonds between metal species and N of pyridine rings. Atomic force microscopy (AFM, XE-70, Park System) was employed to characterize catalyst size and to monitor the change of surface morphology during the solution-based hydrothermal synthesis. X-ray photoelectron spectroscopy (XPS, PHI-5000C ESCA system from PerkinElmer) was used to study catalyst composition. Monochromatic Mg Kα with photon energy of 1253.6 eV was selected as the X-ray source. Scanning electron microscopy (SEM, FEI XL30) was used to examine ZnO morphology.

paper, we have demonstrated the capability of P4VP template to synthesize a catalyst−cocatalyst system with controlled size and composition, thus enabling the first systematic study of the role of catalyst composition in both vapor- and solution-based 1D ZnO synthesis. We have observed that catalyst composition can be used to adjust 1D ZnO morphology in both cases. However, how the catalyst affects growth is different. In the vapor transport-based approach, the tendency of Zn vapor incorporation modulated by catalyst composition is a determining factor. In the solution-based hydrothermal synthesis, the catalyst stability in alkaline growth media have a profound impact on 1D nanomaterial growth. Furthermore, the conformal coating ability of polymeric materials in which catalyst species are incorporated, together with the selective interaction with the Zn rich surfaces of asgrown 1D ZnO nanostructures, allows even deposition of catalysts on a surface with topography. Therefore, 3D structures that are composed of high-density nanobrushes emanating from these intricate surfaces can be created. The ability of generating semiconducting 1D nanostructures arranged in 3D will enable a multitude of applications, such as photocatalysis, solar cells, water splitting, etc. Because of its unique properties and rich morphologies, ZnO has been employed as a model system to examine the role of catalyst in 1D nanomaterial synthesis.2,10,38−47 Nevertheless, these new findings are applicable to 1D nanomaterial syntheses in general.

2. EXPERIMENTAL SECTION 2.1. Catalyst-Containing Solution Preparation. To prepare catalyst-containing polymer precursor solutions, first P4VP with different molecular weights (purchased from Polymer Source Inc. without further purification) were dissolved in butanol. Zn(II) acetylacetonate, Ga(III) acetylacetonate, Sn(II) acetylacetonate, Fe(II) acetylacetonate, Al(III) acetylacetonate, and Cu(II) acetylacetonate were also dissolved in butanol to form organometallic solutions (remaining reagents purchased from Sigma). Then an appropriate amount of an organometallic solution was added into a polymer solution to form a catalyst metal-containing polymer solution with a polymer concentration of 0.275 wt % and the molar ratio between metal species and pyridine groups of 0.8. The molecular weights (Mw) of P4VP used in this investigation were 19 000 and 36 300 g/mol. To study the influence of catalyst stoichiometry on growth, Ga(III) acetylacetonate and Zn(II) acetylacetonate were used as precursors to generate the (Zn/Ga)Ox catalyst nanoparticles. The ratio between Ga and Zn in the catalyst was adjusted by varying the relative amount of Ga(III) acetylacetonate and Zn(II) acetylacetonate. Note that all notations used in both the text and Supporting Information indicate the ratio of loaded metal precursors. 2.2. Catalyst Coated Substrate Preparation. To form a thin layer of catalysts, first catalyst-containing films were generated by spin-coating of catalyst-containing solutions on flat Si substrates with a thin SiO2 layer, at 3000 rpm for 30 s. The film thickness was controlled by the number of coatings. After each coating, a 2 min baking at 100 °C was used to remove residual solvent and also avoid intermixing. Then the substrates were annealed at 350 °C for 30 min in air to remove polymer template and promote nanoparticle formation. A catalyst system contains single metal oxide such as ZnO or Ga2O3. For a catalyst−cocatalyst system, ZnO is the catalyst

3. RESULTS AND DISCUSSION 3.1. Catalyst Sequestration by P4VP. Figure 1 is the FTIR analysis of solids prepared from P4VP, P4VP/Sn, P4VP/ Zn, and P4VP/(Sn/Zn) solutions. The new band at 1620 cm−1 indicates that Zn(II)−pyridine and Sn(II)−pyridine complexes have been formed. Comparing the complexation tendency 19388



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located at 485.6, 933.6, 1022, and 1117.4 eV indicate the formation of SnO, CuO, ZnO, and Ga2O3 respectively.49 Together with AFM height images, it can be concluded that nanoscale particles, (Zn/Sn)Ox, (Zn/Cu)Ox, and (Zn/Ga)Ox have been formed. It has been reported that a catalyst−cocatalyst system offers synergistic or complementary effects to promote the controllable growth of 1D nanostructures, especially 1D carbon nanomaterials.50−62 Using the polymer template approach to incorporate a variety of metal species, a catalyst−cocatalyst system in which ZnO is catalyst and metal oxides such as SnO, Ga2O3, CuO, Y2O3, and Fe2O3 are cocatalysts can be prepared. 3.3. Diameter Control via Polymer Template Approach. It is known that diameter and crystal orientation of 1D nanomaterials can be tailored by catalyst particle size14,18 or the film thickness.29,45,63,64 Using the polymer template-based approach, thickness as well as the surface roughness, which are related to catalyst size, can be easily adjusted by the number of coatings. Figure 3a is a set of AFM height images that show the ZnO film prepared by 1, 3, and 5 coatings of Zn(II) acetylacetonate-containing P4VP. The root-mean-square values of surface roughness are around 0.7, 2.2, and 3.7 nm for 1 layer, 3 layers, and 5 layers coating, respectively. Using the same growth condition, increasing film thickness leads to increased 1D ZnO nanostructure’s diameter as shown by the SEM images in Figure 3b. The diameters of 1D ZnO grown using 1 layer, 3 layers, and 5 layers are approximately 45, 70, and 100 nm, respectively, with narrow size distribution. Therefore, the homopolymer template-based approach is capable of producing 1D ZnO nanomaterials with narrow diameter distribution. Besides the expensive ultrahigh-vacuum methods, this homopolymer template-based approach provides a straightforward and cost-effective method to adjust catalyst layer thickness and consequently enables diameter control of 1D nanostructures. 3.4. Effect of Catalyst Composition on 1D ZnO Nanomaterials. 3.4.1. Vapor Transport-Based Growth.

Figure 1. FTIR spectra of P4VP, P4VP/Sn, P4VP/Zn, and P4VP/ (Sn/Zn). Inset is the chemical structure depicting the metal sequestration route.

between Zn(II) and Sn(II) in the form of acetylacetone, the greater change of the band associated with pyridinic ring indicates that Zn has a stronger tendency to complex with pyridine group. This can be postulated on the basis that Zn(II) has partially unfilled d orbitals while Sn(II) has filled 3d orbitals. However, the fourth electron shell of Sn has been partially filled with two electrons and thus can act as a Lewis acid to form a complex with the pyridine ligand,48 as supported by the FTIR spectrum of Sn(II) pyridine. As expected, the FTIR spectrum of P4VP/(Sn/Zn) shows the combination of both effects. This result further supports that P4VP is an effective metal sequestration agent and thus can be used as a template for the formation of metal nanoparticles. 3.2. AFM and XPS Analysis of As-Synthesized Nanoparticles. Figure 2 contains a set of AFM images and corresponding XPS elemental analysis of nanoparticles prepared using selected solutions. The binding energy peaks

Figure 2. Catalyst nanoparticles derived from P4VP (Mw = 19 000 g/mol) template. (a) AFM height images of (Zn/Sn)Ox, (Zn/Cu)Ox, and (Zn/ Ga)Ox (scan area: 1 × 1 μm, height scale: nm). (b) Corresponding XPS spectroscopy analysis. 19389



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Figure 3. ZnO nanoparticles derived from the P4VP (Mw =19 000 g/mol) polymer template: (a) AFM height images of ZnO catalysts with 1, 3, and 5 layers of coating respectively (scan area: 1 × 1 μm, height scale: nm); (b) corresponding 1D ZnO growth results.

Figure 4. SEM images of 1D ZnO nanostructures grown using single metal oxide catalysts and cocatalysts of ZnO with other metal oxides: (a−d) ZnO nanowires grown by Ga2O3, Al2O3, Y2O3, and Fe2O3, respectively; (e−h) ZnO nanowires grown by those metal oxide catalysts with ZnO doping, respectively.

Figure 5. SEM images of ZnO nanowires grown by different cocatalysts: (a−d) nanowires grown by (Zn/Fe)Ox, (Zn/Ga)Ox, (Zn/Y)Ox, and (Zn/ Cu)Ox at 525 °C, respectively; (e−h) nanowires grown at 550 °C using same catalysts, respectively.

been prepared to investigate the role of catalyst composition on growth in both vapor-based and solution-based approaches. For the purpose of comparison, single metal oxide catalysts were used as references.

Using the polymer template approach to incorporate a variety of metal species, catalyst and catalyst−cocatalyst can be readily synthesized. The ZnO catalyst system doped with other metal oxides (SnO, Ga2O3, CuO, Y2O3, and Fe2O3) as cocatalyst have 19390



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Figure 6. (a) SEM images of ZnO nanowires grown by CuO, Ga2O3, and Fe2O3, and AFM images before (left) and after 20 min growth (right). (b) SEM images of ZnO nanowires grown by (Zn/Cu)Ox, (Zn/Ga)Ox, and (Zn/Fe)Ox and AFM images before (left) and after 30 min growth (right). The scan area of all AFM images is 600 × 600 nm, and the height scale is in nm.

1D ZnO nanomaterials at 550 °C or lower temperature in our chemical vapor deposition (CVD) system, as shown in Figure S2. However, all catalyst−cocatalyst systems tested so far allow the growth of 1D ZnO nanomaterials at lower temperature, i.e., 525 °C, as demonstrated by the SEM images in Figure 5. Comparing growth results at different temperatures, 550 °C vs 525 °C, higher growth temperature gives smaller diameter nanostructures in general except for CuO. This result can be attributed to different melting temperatures of cocatalyst species and their ability to incorporate Zn vapor. Y2O3 has the highest melting temperature, and it cannot be reduced by Zn vapor because of the low electronegativity of Y. Therefore, (Zn/Y)Ox catalyst is most likely to promote a vapor−solid− solid (VSS) process, thus giving smaller diameter and less vertically aligned 1D nanostructures. In contrast, Fe2O3, CuO, and Ga2O3 can be reduced to metallic species due to their higher electronegativity. Therefore, the growth of 1D ZnO nanostructures is greatly affected by the melting temperature of

Figure 4 is a set of SEM images of 1D ZnO nanostructures grown using single metal oxides catalysts and their corresponding catalyst−cocatalyst system, respectively. Ample experimental results indicate that catalyst composition play an important role in determining 1D nanostructure morphology.13,65,66 Analogous to the effect of Ag,66 Al2O3, and Ga2O3 promotes asymmetric growth and formation of nanobelts. In general, the use of metal oxide nanoparticles in the absence of ZnO, gives rise to random or less vertically aligned nanostructures as shown in Figure 4a−d. The addition of ZnO into metal oxide catalysts results in more vertically oriented ZnO 1D nanostructures as shown in Figure 4e−h. These results further support the contention that ZnO can promote the epitaxial growth, corroborating the results of other groups.13,65 We further investigated the formation of 1D ZnO nanostructure using catalyst−cocatalyst systems where catalyst is ZnO and cocatalysts are metal oxides such as Fe2O3, Ga2O3, Y2O3, and CuO. Pure ZnO catalysts were not be able to grow 19391



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Figure 7. SEM images of ZnO nanowires grown by (Zn/Ga)Ox cocatalysts with different Ga to Zn ratios: (a−c) ZnO nanowires grown by vaporbased method at 525 °C with Ga to Zn ratio of 1:3, 1:1, and 3:1, respectively; (d−g) ZnO nanowires grown by hydrothermal method at 95 °C using ZnO seeds, Ga to Zn ratio of 1:3, 1:1, and 3:1, respectively.

study of the vapor-based growth. According to the SEM images in Figure 6, single non-ZnO metal oxide catalysts generate sparsely populated 1D ZnO nanostructures whereas their counterparts, catalyst−cocatalyst systems, can produce much higher density and smaller sized 1D ZnO nanostructures. A systematic AFM study has been used to monitor the change of CuO, Fe2O3, Ga2O3, and corresponding catalyst− cocatalyst systems during the initial growth stage. Figure 6a contains a set of AFM images of pure metal oxide before growth and after 20 min growth and associated AFM height traces. CuO and Ga2O3 nanoparticles are mostly dissolved after 20 min in the growth media at 95 °C, whereas ZnO nanostructures appear on Fe2O3 coated substrates. This observation is corroborated with SEM images that only Fe2O3 can produce 1D ZnO nanostructures. According to Pourbaix diagrams,71 singular metal oxides such as CuO and Ga2O3 might not be stable in the alkaline growth medium (pH = 8). This instability leads to very few and large ZnO nanostructures. Figure 6b contains a set of AFM images of (Zn/Cu)Ox, (Zn/ Fe)Ox, and (Zn/Ga)Ox nanoparticles before and after 30 min growth. The nanoparticle size does not decrease after 10 min as shown in Figure S3, indicating that adding Zn can stabilize these metal oxides nanoparticles. After 30 min, the appearance of features that are significantly greater than catalyst nanoparticles is indicative of the formation of higher density 1D ZnO nanostructures. 3.4.3. Comparison between Vapor Transport and Solution-Based Approaches. Figure S1b shows the XPS data of (Zn/Ga)Ox catalysts with tuned atomic ratios of Zn to Ga. The growth results using those different (Zn/Ga)Ox catalysts are shown in Figure 7. The result of increasing the amount of Ga is that the diameter of ZnO nanostructures increases in both vapor deposition and hydrothermal process, but the density of nanowires decreases significantly in the hydrothermal process. In the vapor-based process, pure ZnO seeds cannot generate 1D ZnO nanostructures at 525 °C. Adding Ga into the ZnO seeds promotes vapor-based growth as aforementioned. Furthermore, higher Ga content increases the ability to incorporate Zn vapor, enhancing VLS growth. We have demonstrated that Ga2O3 nanoparticles are not stable in the hydrothermal synthesis as shown in Figure 6a. As a result, during the hydrothermal synthesis, the higher the percentage of Ga is, the less stable the (Zn/Ga)Ox system will be. Consequently, the resulting lower density of catalysts with predominately larger diameter caused by their instability in the growth media lead to sparsely populated microscopic sized 1D

Fe, Cu, and Ga. Compared to Fe and Cu, Ga possesses a very low melting temperature. Thus, partial evaporation of Ga might be responsible for the formation of small diameter nanostructures. The epitaxial relationship between ZnO and (Zn/Ga)Ox also promotes the formation of vertically aligned 1D ZnO nanostructures.67−69 Both (Zn/Fe)Ox and (Zn/Cu)Ox produce large diameter rods at 525 °C. Nevertheless, at 550 °C, (Zn/Fe)Ox produces smaller diameter nanorods whereas (Zn/Cu)Ox induces the formation of larger diameter nanorods. Both Fe2O3 and CuO can be reduced by Zn vapor during the initial stage. Yet, due to the different melting temperature between Fe and Cu (1538 °C for Fe vs 1085 °C for Cu), the growth at 550 °C is most likely a VSS process for (Zn/Fe)Ox and a vapor−liquid−solid (VLS) process for (Zn/Cu)Ox. In conclusion, the cocatalyst property can play an influential role in ZnO nanowire growth. These new findings provide a way to adjust 1D nanostructure morphology. Figure S2 is a set of SEM images of 1D ZnO nanostructures using SnO, (Zn/Sn)Ox, and ZnO grown at various temperatures. The chemical composition of (Zn/Sn)Ox has been analyzed by XPS in Figure S1a showing the control of catalyst− cocatalyst composition. Comparing the growth results using (Zn/Sn)Ox and ZnO, adding Sn leads to the growth of longer 1D ZnO nanostructures at lower temperature. The possible mechanism is that Sn has higher electronegativity than Zn, so during the initial stage of the growth, it is expected that SnO can be reduced by Zn vapor. VLS would most likely take place due to the low melting temperature of Sn and the high solubility of Zn according to the phase diagram.70 Therefore, the growth rate of 1D ZnO nanostructures will be enhanced. Indeed, the fact that the length of 1D ZnO nanostructures synthesized using (Zn/Sn)Ox is greater than those using ZnO catalyst at 550 and 600 °C proves this contention. A catalyst− cocatalyst advantage can be observed by the growth at 525 °C where (Zn/Sn)Ox promotes the growth of about 0.5 μm tall vertically aligned ZnO nanowires. SnO-catalyzed ZnO nanowires show disordered orientation demonstrating the need of catalyst−cocatalyst system. No growth is observed using SnO at 600 °C, which is plausibly due to the loss of low melting temperature Sn at higher growth temperature. This finding further supports that 1D nanomaterial morphology, length, diameter, and even the required growth temperature can be adjusted by tuning a catalyst−cocatalyst composition. 3.4.2. Solution-Based Growth. To understand the role of catalyst properties in solution-based 1D nanostructure growth, we have used the same set of catalyst-coated substrates from the 19392



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affect growth substantially. In the vapor-based approach, the tendency of catalyst to incorporate Zn is critical whereas in the solution-based approach, the stability of catalyst in the growth media becomes a predominating factor to affect the spatial density and morphology of 1D nanostructures. Tailoring the catalyst−cocatalyst ratio enables us to define 1D ZnO morphology. Furthermore, we have exploited the conformal coating nature of P4VP homopolymer to deposit a catalyst thin layer on a sloped surface and on the surfaces of as-grown 1D nanostructures. Horizontally aligned 1D ZnO nanostructures and highly branched ZnO nanostructures can be synthesized reproducibly by this polymer template approach. It is expected that tuning catalyst composition to investigate 1D growth and forming 3D nanostructures, enabled by this polymer template approach, can be applicable to a broad field of 1D nanomaterial synthesis.

structures. Therefore, the length and diameter of 1D ZnO nanostructures can be adjusted in both vapor transport-based and hydrothermal approaches using catalyst composition. Static and dynamic photoluminescence will be conducted to examine the effect of cocatalyst on optical properties of grown ZnO nanostructures in the future work. 3.4.4. 3D Growth in Trench and As-Grown 1D Nanostructures. In addition to the ability to adjust the catalyst composition, P4VP homopolymer can be employed to uniformly distribute catalyst payload onto topographic surfaces. The conformal coating ability of high molecular weight polymer enables the deposition of the catalyst-containing polymer thin layer on a sloped surface such as sidewalls of trenches. Owing to the selective interaction of pyridyl groups with Zn-rich surface, a catalyst-containing polymer thin layer can also be formed on the surfaces of as-grown 1D nanostructures. The Zn(II) acetylacetonate−P4VP solution with a polymer Mw of 36 300 g/mol was used to deposit ZnO catalyst nanoparticles on sidewalls. Figure 8a is a representative SEM image of ZnO

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ASSOCIATED CONTENT

* Supporting Information S

XPS compositional analysis of (Zn/Sn)Ox and (Zn/Ga)Ox with adjusted catalyst−cocatalyst ratios; cross-section SEM images of 1D ZnO nanostructures grown by (Zn/Sn)Ox catalyst−cocatalyst system with molar ratios adjusted by concentration of Zn(II) acetylacetonate and Sn(II) acetylacetonate; AFM height images and height profile of Si substrates coated with (Zn/Cu)Ox, (Zn/Fe)Ox, and (Zn/Ga)Ox nanoparticles before and after 10 min solution-based growth. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Figure 8. 3D growth using the polymer template approach: (a) horizontally aligned nanostructures on the side wall of a trench; (b) 3D branched growth. Scale bar in the inset is 200 nm.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.L.).

nanowire growth result on a sidewall of a trench. Uniform and small nanowires that are aligned horizontally have been successfully synthesized. The homopolymer template approach can also be used to fabricate highly branched 3D structures. After the growth of 1D ZnO nanostructures followed by deposition of catalyst species on surfaces of the as-grown ZnO nanostructures, a sequential growth was carried out using the vapor-based approach. Figure 8b contains the SEM image after the second growth showing that highly branched structures have been formed. This method to generate 3D nanostructures can afford uniform and welldefined 3D structures over a large surface area. Such a structure is highly desirable for energy-related applications.

Author Contributions

The manuscript was written mainly by Jennifer Lu with the help of Yang Liu and Jose F. Flores. All authors have given approval to the final version of the manuscript. Y.L. and J.F.F. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by The Defense Advanced Research Projects Agency (DARPA) and the National Science Foundation NSF-CBET. Special thanks to C. C. Wang at Fudan University in China for the XPS analysis.

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4. CONCLUSION Among all the catalyst formation and deposition techniques, vacuum-based thin film deposition is expensive and timeconsuming and cannot coat catalyst uniformly on sidewalls readily due to shadowing effect. Stoichiometry control is also a challenge. Conventional solution-based deposition methods cannot effectively deposit nanocatalysts uniformly over a large surface area nor conformally on surfaces with significant topography. In contrast, this low-cost homopolymer template approach has demonstrated the formation of catalysts with not only engineered composition and size but also uniform coating across a large surface area and on the sidewalls of trenches and on surfaces of nanostructures as well. Thus, this approach has enabled the first systematic study of the role of catalyst composition for both vapor- and solution-based approaches. We have revealed that the ratio of catalyst to cocatalyst will

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