tailoring 1d zno nanostructure using engineered catalyst enabled by poly(4-vinylpyridine)

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Article

Tailoring 1D ZnO Nanostructure Using EngineeredCatalyst Enabled by Poly(4-vinylpyridine)

Yang Liu, Jose Fernando Flores, and Jennifer LuJ. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp504783q • Publication Date (Web): 29 Jul 2014

Downloaded from http://pubs.acs.org on August 3, 2014

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Tailoring 1D ZnO Nanostructure Using Engineered

Catalyst Enabled by Poly(4-vinylpyridine)

Yang Liu,† Jose F Flores,† and Jennifer Lu*

School of Engineering, University of California, Merced, Merced, CA 95343, USA

Email: [email protected]

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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. 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 well-

defined 3D architectures.

Keywords: poly(4-vinylpyridine), polymer template, ZnO nanostructures, branched structure,

catalyst composition, 1D nanomaterial growth

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1. INTRODUCTION

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 carefully monitoring of

deposition parameters thus costly. Furthermore, it is difficult to deposit a conformal layer

uniformly on non-flat 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

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the form of colloids.14

The solution-based approach offers greatly improved control in catalyst

composition at low cost. Still, both catalyst film thickness as well as 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.38

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 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 vapor transport 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 as grown 1D

ZnO nanostructures, allow 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

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3D will enable a multitude of applications, such as photocatalysis, solar cells, water splitting, and

etc. Due to 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 respectively 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 manuscript 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

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substrates with a thin SiO2 layer, at 3000 rpm for 30 sec. 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 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 hours.

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 gram

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.

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The vapor transport-based method was employed to grow horizontal and branched structures

on etched Si wafers and as-grown 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 mins 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 Perkin-

Elmer) 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.

3. RESULTS AND DISCUSSION

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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 respectively. The new band at 1620 cm-1

indicates that Zn(II)-pyridine and Sn (II)-pyridine complexes have been formed. Comparing the

complexation tendency 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 4th

electron shell of Sn has

been partially filled with 2 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.

Figure 1. FTIR spectra of P4VP, P4VP/Sn, P4VP/Zn, P4VP/(Sn/Zn)

respectively. Insert is the chemical structure depicting the metal

sequestration route.

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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 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.

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 respectively (scan area: 1 x

1 µm, height scale: nm); (b) Corresponding XPS spectroscopy analysis.

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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 3(a) 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 respectively. The root mean square values of

surface roughness are around 0.7 nm, 2.2 nm, and 3.7 nm for 1 layer, 3 layers, and 5 layers

coating correspondingly. Using the same growth condition, increasing film thickness leads to

increased 1D ZnO nanostructure’s diameter as shown by the SEM images in Figure 3(b). The

diameters of 1D ZnO grown using 1 layer, 3 layers, and 5 layers are approximately 45 nm, 70

nm, 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.

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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 x 1 µm, height scale: nm); (b)

corresponding 1D ZnO growth results.

3.4. Effect of Catalyst Composition on 1D ZnO Nanomaterials

3.4.1. Vapor Transport-based Growth. 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 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.

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

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morphology.13,65,66

Analogous to the effect of Ag,66

Al2O3 and Ga2O3 in promoting asymmetric

growth and formation of nanobelts as a result, 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 4(a)-(d). The addition of ZnO into metal oxide catalysts results in more vertically oriented

ZnO 1D nanostructures as shown in Figure 4(e)-(h). These results further support the contention

that ZnO can promote the epitaxial growth, corroborating the results of other groups.13,65

Figure 4. SEM images of 1D ZnO nanostructures grown using single metal oxide

catalysts and cocatalysts of ZnO with other metal oxides. (a)-(d) are ZnO nanowires

grown by Ga2O3, Al2O3, Y2O3, and Fe2O3 respectively; (e)-(h) are ZnO nanowires grown

by those metal oxide catalysts with ZnO doping respectively.

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 1D ZnO nanomaterials at 550 °C or lower

temperature in our CVD system, as shown in Figure S2. However, all catalyst-cocatalyst systems

tested so far allow the growth 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

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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 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

Figure 5. SEM images of ZnO nanowires grown by different cocatalysts. (a)-(d) are the

nanowires grown by (Zn/Fe)Ox, (Zn/Ga)Ox, (Zn/Y)Ox, and (Zn/Cu)Ox at 525 ⁰C

respectively; (e)-(h) are the nanowires grown at 550 °C using same catalysts respectively.

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

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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 S1(a) 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 °C

and 600 °C proves this contention. 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 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,

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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 6(a) 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 stable in the alkaline growth medium (pH=8). This instability leads to very few and

large ZnO nanostructures.

Figure 6(b) 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.

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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) respectively; (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) respectively. The scan area of all AFM

images is 600 x 600 nm and the height scale is in nm.

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3.4.3. Comparison between Vapor Transport and Solution-based Approaches. Figure

S1(b) 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 6(a). 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 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.

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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) are ZnO nanowires grown by vapor-based method at 525 ⁰C with

Ga to Zn ratio of 1:3, 1:1, 3:1 respectively; (d)-(g) are ZnO nanowires grown by

hydrothermal method at 95 ⁰C using ZnO seeds, Ga-Zn 1:3, Ga-Zn 1:1, Ga-Zn 3:1

respectively.

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 enable 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 8(a) is a

representative SEM image of ZnO nanowire growth result on a sidewall of a trench. Uniform and

small nanowires that are aligned horizontally have been successfully synthesized.

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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.

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 8(b) 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 well-defined 3D structures over a large surface area. Such a structure is

highly desirable for energy related applications.

4. CONCLUSION

Among all the catalyst formation and deposition techniques, vacuum-based thin film

deposition is expensive and time consuming, 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

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20

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 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 enable 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.

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ASSOCIATED CONTENT

Supporting Information. 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.

AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]

Notes

The authors declare no competing financial interest.

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. †These authors

contributed equally.

ACKNOWLEDGMENTS

This work was supported by The Defense Advanced Research Projects Agency (DARPA) and

the National Science Foundation NSF-CBET. Special thanks to CC Wang at Fudan University in

China for the XPS analysis.

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Table of Contents (TOC)

Tailoring 1D ZnO Nanostructure Using Engineered

Catalyst Enabled by Poly(4-vinylpyridine)

Yang Liu, Jose F Flores, and Jennifer Lu*

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tailoring 1d zno nanostructure using engineered catalyst enabled by poly(4-vinylpyridine)

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