David
Zopes
,
Robin
von Hagen
,
Ralf
Müller
,
Raquel
Fiz
and
Sanjay
Mathur
*
University of Cologne, Division of Inorganic and Materials Chemistry, Greinstrasse 6, D-50939, Köln, Germany. E-mail: sanjay.mathur@uni-koeln.de; Fax: (+49) 221 470 4899; Tel: (+49) 221 470 5627
First published on 4th August 2010
Nanosized (20–30 nm) colloidal gold, silver and their alloys were obtained by reductive transformation of corresponding metal salts. Dispersions of metal nanoparticles (σ < 4%) in aqueous solutions were obtained by appropriate surface functionalization which led to inorganic inks with solid fraction ranging from 0.01–4%. Judicious choice of a polymer additive (polyethylene glycol or carboxymethyl cellulose) was found to be crucial to avoid the agglomeration of nanocrystals in the ink-jetted structures upon solvent evaporation. The versatility of the nanoparticle-based printing technology was demonstrated by fabrication of dot-matrices and circuitry patterns on different substrates. Characterization of printed structures showed a homogeneous topography (AFM) and uniform distribution of metallic nanoparticles (SEM/TEM) within the ink-jetted microdrops. The site-specific patterning on silicon (001) substrates with nanoparticle (mono)layers could also be achieved by printing the linker molecule, aminopropyltriethoxysilane, followed by selective attachment of gold nanoparticles. Positionally ordered and chemically bonded gold catalyst patterns were used for the chemical vapour deposition (CVD) of nanowires, which led to site-specific growth of nanowires via the vapour-liquid-solid (VLS) mechanism and unlike in the case of spin-coated metal colloids no significant lateral diffusion of metal nanoparticles was observed, in chemically anchored Au nanoparticles. Nanoparticle containing inks allow a user-defined dilution to vary the density of CVD grown nanowires, which was utilized to show the differences in catalytic activities of silver and gold catalysts in the VLS growth.
Examples of inkjet printing in the fabrication of functional microstructures include printed electronics,1 lubricants for micromechanical parts1 or defined polymer deposition for manufacturing multicoloured displays.2 The dispersions of inorganic colloids and organic dyes can be dispensed in a continuous or on-demand mode. However, the printing configurations3 in general consists of a dispensing-head connected to an ink-reservoir that is filled and maintained with the ink through pressure difference. The core of the dispensing-head is a glass capillary surrounded by a piezo actuator. A voltage pulse contracts the piezo actuator thereby creating a pressure wave which is amplified to dispense a small column of the liquid.4 The strength of the pulse, the used solvent and the diameter of the nozzle diversifies thereby drop size and printing speed.5
In this study, colloidal dispersions of Ag, Au and Ag–Au alloy nanoparticles were used as inorganic inks to fabricate positionally defined microstructures for catalytic applications such as decomposition of metal–organic precursors on metal nanocrystals to impart preferential (anisotropic) growth of nanostructures. Noble and transition metal particles are extensively used as growth templates for synthesizing nanowires and nanotubes of different materials.6 However, their deposition is generally achieved by physical methods (e.g., sputtering of metallic targets, spin-coating of colloids), which leads to a random distribution of catalyst particles on the substrate. In this context, inkjet printing of catalysts allows fabrication of periodic arrays and site-specific deposition of functional nanostructures7,8 what plays an important role in controlled positional assembly for device applications. The recent developments include methods involving the chemical patterning, whereby a bi-functional linker molecule is immobilized on the substrate in the regions of interest by site-selective printing of the linker such as aminopropyltriethoxysilane (APTES), which binds the gold colloids on the surface by strong chemical interaction.9 Self assembled nanoparticle monolayers on silicon10,11 as well as inkjet printed self assembled alkanethiols12 on noble metal substrates are well-known. Herein a combination of inkjet printed self assembled monolayers on silicon was shown to obtain regioselective dense packed layers of gold colloids useful for a wide range of possible applications reaching from chemical sensing to novel electrical devices.13
Semiconducting materials in nanoscopic forms exhibit unique electronic, optical and transport properties which scale-up with the degree of dimensional confinement.14 SnO2 is a wide bandgap semiconductor (Eg = 3.6 eV) and therefore transparent in visible light making it a useful material for conductive electrodes and anti-reflective coatings. Furthermore SnO2-based materials are very suitable for gas sensing devices due to their high sensitivity towards reducing gases (such as carbon monoxide) which results from a strong influence of the adsorbed gases on the intrinsic conductivity of the material.15 This transduction (charge transfer) phenomenon is a function of surface-to-volume16 ratio that makes tin oxide nanostructures preferred candidates for gas sensors.
The synthesis for SnO2 nanowires by chemical vapour deposition (CVD) on gold catalysts follows the vapour–liquid–solid (VLS) mechanism, originally postulated by Wagner and Ellis for the growth of silicon whiskers,6 had been reported.17 At CVD growth temperatures (550–800 °C) the sputtered thin gold layer disintegrates, due to intrinsic stresses and differential thermal expansion coefficients, into particles in the range of 50–1000 nm, whereby the particle size is a function of film thickness and is governed by the surface tension in spherical clusters.17 This method however, offers limited control on the distribution of metal clusters; also the high polydispersity in obtained nanoparticles limits the control over the axial and radial dimensions in as-grown nanowires and whiskers. Furthermore, it is cumbersome to coat different types of metal on the same substrate (achievable through masking and etching procedures) and there is also a low-concentration-limit for the particles which can not be under-run. All these aspects restrict the practicality of the CVD method in the controlled and selective synthesis of SnO2 (or any other metal oxide) nanowires.
In this report, we describe deposition of metal catalysts on various substrates using nanoparticle dispersions as inks for inkjet printing with narrow size-dispersion and controllable particle concentration. Inkjet printing with nano-inks permitted a flexible and fast printing of different metal catalysts on a single substrate that enabled regio-selective growth of nanowires by CVD process. In addition, printing of aminopropyltriethoxysilane monolayer as primer allowed a chemical fixation and site-specific dense arrays of metal catalysts for CVD process. These new options substantially enhance the potential of CVD process and may have technological implications for the rapid and reproducible processing of CVD-grown nanostructures.
The hydrophilic Si(001) substrates (contact angle, 3°) used to obtain self-assembled monolayers were prepared by treating the substrates in oxygen plasma (Plasma Electronic GmbH, Germany). The functional ink contained a mixture of H2O and ethylene glycol (3:1) as solvent for the APTES (20 mM). The remaining steps were similar to those reported by Liu et al.11
The aqueous dispersion of metal nanoparticles and the APTES-ink were printed using a commercial printer (Model MD-P-802 of Microdrop Technologies) equipped with a dispensing-head having a 50 μm diameter nozzle. The printed patterns were analysed using an optical microscope (Axiom Imager.M1m Zeiss). The alkoxide precursor Sn(OtBu)4 used in the CVD of SnO2 nanowires was synthesized following the reported procedure.21 Tin oxide nanowires were grown in a horizontal CVD reactor in which a high-frequency field was used to inductively heat the substrates (Si(001)) by placing them on a graphite susceptor (750 °C). The results of the CVD process were imaged by scanning electron microscopy (Zeiss, Germany).
Fig. 1 UV/Vis spectra and TEM pictures of as-obtained (a) gold and (b) silver nanoparticles. |
This strategy was effective in producing stable inks, however the high amount of polymer leads to NP-polymer composites in which the metal NPs are masked by the polymers and being less accessible for the catalytic applications. The dispersion in the particle size was monitored by UV/Vis spectroscopy of various NPs. An adequate stability and reproducibility was achieved for NPs with an average size of 30 nm (λmax = 403 nm) (Fig. 1(b)).22
The dispersion of the alloy-particles was green in colour, which was corroborative of the spectral maximum (λmax = 485 nm) observed in the UV/Vis data (Fig. 2(a)) and the formation of alloy particles.22 The absence of any other pronounced UV/Vis signature confirmed the formation of a homogenous alloy composition. Furthermore the measured maximum lies between the maxima observed for gold (λmax = 519 nm) and silver (λmax = 403 nm) particles. The TEM measurement showed spherical particles with sizes between 20–30 nm (Fig. 2(a)). The EDX analysis showed a higher content of gold (66.4%) than silver (33.6%) in the Au–Ag particles (Fig. 2(b)).
Fig. 2 (a) TEM image, UV/Vis spectrum and (b) EDX analysis of Ag/Au alloy particles. |
Fig. 3 Procedure for inkjet printed substrates and following regioselective CVD experiments. |
Fig. 4(a) shows a SEM image of printed Si(001) substrate, which was decorated with rows of Au nanoparticles to perform CVD of SnO2 nanowires.17 Also, compound substrates containing printed structures of Ag, Au and Ag–Au NPs were fabricated in order to evaluate the differential catalytic activities towards NW growth. The SEM micrograph of the Au decorated substrate revealed that the nanowire growth occurred preferentially and site-selective on the Au catalyst nanoparticles. A slight migration of gold clusters was observed which can be explained by the higher diffusion rate and the liquefied state of gold at the growth temperature (∼750 °C). Apparently, the presence of CMC as a steric stabilizer decreases the lateral mobility of Au NPs. On the other hand, the high degree of entanglement of Au NPs with the polymer leads to residual carbon that hampers the catalytic activity (Fig. 4(a)). The printing experiments with less viscous polyethylene glycol (PEG) instead of CMC led to a much faster evaporation process, which reduced the carbon contamination (Fig. 4(b)). Fig. 4(b) shows the SEM images of printed catalysts and the as-grown nanowires, which supports the concept of local and confined growth of nanowires by pre-positioning the metal catalyst particles. The coffee-drop effect caused by concentration gradient in aqueous micro-droplets of metal NPs could be overcome by mixing small amounts of polymeric additives.
Fig. 4 Inkjet printed dots after CVD experiment with Sn(OtBu)4 of (a) Ag, Au and alloy dispersions with added CMC and (b) Au dispersion with added PEG. |
Further, it was found that Ag and Au NPs show different catalytic activity due to intrinsic reactivity patterns as well as due to the dispersions in the particle sizes, which are more pronounced for the Ag species. This was verified by printing Ag, Au and Ag–Au alloy NPs on Si(001) substrates and by performing CVD under similar conditions. As expected, the density of as-grown nanowires followed the trend Au > Ag–Au > Ag. Further experiments to elucidate this aspect are currently under way.
The inkjet printing of metal NP dispersions is an interesting technique for rapid patterning of growth templates (catalysts), which can be applied to various substrates and allows to fabricate different patterns for the structural manipulation of CVD-grown nanostructures. Fig. 5 shows the scanning electron and optical micrograph of patterns obtained from Au@PEG inks. The distance between drops in a dot-matrix pattern could be varied from 80–500 μm, whereas the diameter of the drop ranged from 30 to 180 μm due to different surface energies of the substrates interacting with the ink.
Fig. 5 (a) Scanning electron and optical (b, c) micrographs of patterns printed by Au@PEG inks. |
Fig. 6 Schematic images showing the fabrication of the gold monolayer beginning from the silicon (001) substrate. |
Fig. 7 SEM images of a line pattern of nanoparticles monolayer achieved by inkjet printing of APTES linker. |
Fig. 8 (a) SEM images of a line pattern of nanoparticles monolayer after CVD experiment with Sn(OtBu)4 with focusing on the border (b) to demonstrate the regioselective growing of SnO2 nanowire. |
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