Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
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Controlled Microwave-Hydrolyzed Starch as a Stabilizer for Green Formulation of Aqueous Gold Nanoparticle Ink for Flexible Printed Electronics Nikita P. Bacalzo, Jr.,† Lance P. Go,† Christine Joy Querebillo,‡,§ Peter Hildebrandt,‡ F. T. Limpoco,∥ and Erwin P. Enriquez*,† †
Department of Chemistry, Ateneo de Manila University, 1108 Quezon City, Philippines Institut für Chemie, Technische Universität Berlin, Sekr. PC 14, Straße des 17. Juni 135, 10623 Berlin, Germany § School of Analytical Sciences Adlershof, Humboldt Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany ∥ Asylum Research, 6310 Hollister Avenue, Santa Barbara, California 93117, United States ‡
S Supporting Information *
ABSTRACT: Gold electrodes are important in some devices and certain applications where an inert, highly conductive feature is required. An aqueous gold nanoparticle (AuNP) ink suitable for inkjet printing was synthesized and formulated using starch and microwave-assisted heating. By varying the hydrolysis conditions of starch, the size, yield, and stability of the AuNP suspension can be controlled and optimized to achieve a jettable ink. The optimized formulation has a very low starch loading of only 1.75 wt % relative to gold, forming a highly stable AuNP ink, which upon drying already forms a very conductive film and sinters at low temperature. The overall synthesis protocol thus provides a greener and cheaper alternative to other AuNP synthesis methods. The sintering behavior of the film was monitored, wherein, upon heating, starch is degraded, crystallite growth increased, and the morphology changed from individual nanoparticles to a network of fused particles. The film sheet resistance decreased concomitant with these physical changes. By heating the film to at least 200 °C, a sheet resistance of 30 mV is generally desirable. Higher |ζ| values were measured for AuNP samples synthesized using starch hydrolyzed at higher temperature, signifying a more stable AuNP sample. Our optimized ink formulation remains stable for several months (we have not observed flocculation yet), keeping it only in the refrigerator (ca. 4 °C) for the usual storage. Because smaller starch coils are formed at elevated temperatures, more can bind with the AuNPs during the reduction of Au3+. These starch molecules are oxidized to form carboxylate or OH− end groups that contribute to the surface charge such as that reported for the case of the β-D-glucose reduction of Au3+.38 It should be noted that the hydrolysis time (for the range tested here) had no significant effect on the ζ potential, just like for the case of the starch MW and size. We note also that the starch/Au ratio is
AuNPs, the ratio must be optimized. In our case, the ratio was controlled by varying the extent of starch degradation. AuNPs synthesized using starch hydrolyzed at 100 °C was also done, but its PS was not measured because it generated large black particles that precipitated out of the mixture. The trend in the PS observed in the UV−vis spectra was also confirmed with the Z-average PS measurement using DLS, where there was a decrease in the PS of the AuNP−starch nanoparticles from about 175 nm to about 50 nm with increased heating temperature from 70 to 90 °C (Figure 2C). Also, the number density of the AuNP particles was estimated from the UV−vis spectra using a formula (eq 2) reported by Haiss et al.56 based on the theoretical treatment of the scattering of metal nanoparticles: N=
A450 × 1014
d 2⎡⎣ −0.295 + 1.36e−(d − 96.8/78.2) ⎤⎦ 2
(2)
where N is the number density, A450 is the absorbance at 450 nm, and d is the mean particle diameter in nanometers. It was calculated that the higher number density of AuNP was achieved at elevated hydrolysis temperature, as expected with the observed trend for the AuNP size. Because the initial concentration of Au3+ is equal in all samples, a higher AuNP concentration should be expected for a sample with a smaller AuNP size than with a larger one. A similar observation was noted by Polte et al.,59 wherein the AuNP PS increased during the synthesis, along with a decrease in the particle number density for the case of the classical citrate reduction method. Another value derived from the cumulant analysis of the DLS data is the polydispersity index (PdI), which is a dimensionless number indicating the broadness of the size distribution around the Z-average size. It ranges from 0.0 to 1.0, with 1.0 being the most polydisperse.52 In the case of the synthesized AuNP samples in this study, the PdI values range from 0.07 to 0.25, D
DOI: 10.1021/acsanm.7b00379 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX
Article
ACS Applied Nano Materials
reduced AuNP solution, which yielded 28% RSD [mean size of 8.2 nm based on transmission electron microscopy (TEM)],38 we have produced a more monodisperse AuNP dispersion at 12% RSD albeit at a larger mean size. Sintering Behavior of the AuNP Ink as a Function of the Temperature. The thermal properties of the Au film and the hydrolyzed starch were profiled using TGA (Figure 4). The
quite low, at less than 2 wt % [see the thermogravimetric analysis (TGA) data below]. So far, we have found only one prior literature on an aqueous Au ink formulation, synthesized using poly(N-vinylpyrrolidone) (PVP) and acrylic resin as stabilizers. This had as much as 57 wt % polymer in the final printed material based on their TGA data, and it needed to be burned off at 500 °C to achieve a highly conductive print.14 As shown and discussed below, our ink formulation with low starch loading already forms a highly conductive ink even by just drying at 50 °C, and it has an advantage because it can sinter at a much lower temperature of about 200 °C to burn off the starch in the printed material, in turn resulting in increased conductivity. Jettability of Optimized AuNP−Starch Ink. The synthesized AuNP ink was printed using a DoD inkjet printer. As previously mentioned, the jettability of an ink can be predicted by gauging its fluid properties such as the viscosity, surface tension, and density.22 For the ink used in this study, the viscosity is approximately measured to be 1.20 cP, the surface tension is 69.7 mN/m, and the density is 1.0246 g/mL; using eq 1, the Z number for this ink is 54.6. This is beyond the range set by most studies on inkjet printing; however, as discussed and demonstrated by Subramanian et al., there are inconsistencies regarding jettability criteria using the Z number.22 In any case, this specific ink was successfully jetted to single drops without satellite droplets, as shown in Figure 3A. A bipolar voltage waveform (Figure S2-A) was applied to a 60-μm-diameter inkjet nozzle, and the substrate (cleaned glass slides were used for most sintering characterizations) was heated at 50 °C during the printing. In the printing of isolated single lines, the coffee-ring effect (Figure S2-B) was observed in the as-synthesized AuNP ink, but it was suppressed by adding a minimal amount of surfactant (∼0.3% w/w Triton X-100) because it could not be suppressed by changing the drop spacing and printing speed alone. Adding surfactant changed the Z number of the ink to around 37.2 because the surface tension of the ink significantly decreased and it was still jetted to single drops. A sample grid pattern was printed on poly(ethylene naphthalate) (PEN) substrate, and a zoomed-in image of the printed lines is shown in Figure 3B. For the printing of solid square patterns (Figure S2-C), the assynthesized ink was used without the added surfactant because the close spacing of the lines will make up for the hollow space left by the coffee-ring effect. The square patterns were used to measure the sheet resistance values. The thicknesses of the printed Au films were approximated by extracting line profiles from atomic force micrscopy (AFM) images (Figure S3), generally showing ∼100 nm for a single layer print. Small-angle X-ray scattering (SAXS) in transmission mode was done to measure the PS in the ink (Figure 3C). The difference between this method and the DLS technique is the concentration of AuNP in the sample. In the SAXS measurement, the actual ink concentration was measured, while the DLS requires dilute sample. A Gaussian PS distribution was fitted to the SAXS data, which gave an average size of 21.8 nm ± 12%. This tolerance value is similar to some commercially available standards of AuNP suspension, indicating the advantage of using microwave heating in nanoparticle synthesis. The average PS from SAXS is smaller from the one measured in DLS because, as pointed out before, DLS considers the hydrodynamic size of the particle. This PS was verified from the AFM and scanning electron microscopy (SEM) images as discussed below. Furthermore, compared with the β-D-glucose-
Figure 4. Thermograms of dried AuNP ink (black) and starch (blue) at 10 °C/min rate. Onset degradation temperatures are approximated from the graph as indicated.
solid content of the AuNP ink was determined to be approximately 10.8% (w/w), while the TGA profile of the dried Au film sample indicated ∼0.25% moisture and ∼1.75% starch content. For the Au film, there are two major features in the thermogram. The first mass loss event at
