D. P.
Howard
,
P.
Marchand
,
I. D.
Johnson
,
C. J.
Carmalt
,
I. P.
Parkin
and
J. A.
Darr
*
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK. E-mail: j.a.darr@ucl.ac.uk
First published on 1st August 2016
A high-throughput synthesis, screening and subsequent scale-up approach was utilised for the optimisation of conductive aluminium and gallium-doped zinc oxide (AZO and GZO, respectively) nanoparticles. AZO and GZO nanoparticles with up to 6 at% dopant (with respect to Zn) were directly synthesised using a laboratory scale continuous hydrothermal process at a rate of 60 g per hour. The resistivities were determined by Hall effect measurements on pressed, heat-treated discs. Both Al- and Ga-doping yielded resistivities of the order of 1 × 10−2 Ω cm for most samples; the lowest resistivity of AZO was 7.0 × 10−3 Ω cm (at 2.5 at% Al doping), and the lowest resistivity of GZO was 9.1 × 10−3 Ω cm (at 3.5 at% Ga doping), which are considered exceptionally conductive for pressed nanopowders. Synthesis of the optimised lab-scale compositions was scaled-up using a pilot-scale continuous hydrothermal process at a production rate of 8 kg per day (by dry mass); results obtained from these nanopowders generally retained resistivity trends observed for the lab-scale analogues.
The synthesis of transparent conducting nanomaterials includes methods such as batch coprecipitation and batch solvothermal and hydrothermal methods.17,21–24 However, the use of organic solvents with high volatile organic compound (VOC) content is unsustainable and batch-to-batch variations can make these methods unsuitable for industrial scale-up.17,25,26
Continuous Hydrothermal Flow Synthesis (CHFS) processes offer the possibility of controlled and consistent syntheses of nanomaterials at large scale. In such processes, nanoparticles of metal oxides are typically formed upon mixing a feed of supercritical (sc) water with a stream of ambient-temperature aqueous metal salts. The precursors react with the sc-water resulting in simultaneous hydrolysis and dehydration of the metal salt to the corresponding metal oxide, typically in less than a second. In recent years, the use of a Confined Jet Mixer (CJM) has been reported by the authors for the continuous synthesis of various nanoparticles. In the CJM, a flow of aqueous precursors is entrained in a jet of supercritical water, affording rapid mixing, which is generally under turbulent conditions.27–29 The CJM is also highly scalable, such that similar particle properties have been observed for nanoparticles made on the lab and pilot plant scales.30,31
For the discovery and optimisation of TCO nanomaterials, the development of high-throughput synthesis and rapid screening methods for doped metal oxide systems is highly desirable. Synthesis of doped metal oxides can readily be accomplished by mixing the appropriate ratios of metal salts (e.g. nitrate salts of the dopant or most metals in the oxide) into a single precursor solution, which is then used in the CHFS process. One particular benefit of CHFS-type reactors is that many different compositions of such doped materials can be synthesised sequentially by simply adjusting the relative ratio of the various metal salt solutions in the precursor feed; great care must be taken to avoid cross-contamination between different samples.31 Variations of this high-throughput approach for CHFS have previously been demonstrated by some of the authors for the synthesis of an entire phase diagram of CexZryYzO2−δ,30–33 Eu-doped Y(OH)3 phosphor libraries,34 Fe-doped La4Ni3O10 mixed ion conductor libraries,35 and a rare earth-doped zinc oxide photocatalyst library.32 Application of this high-throughput approach to TCO systems would be expected to facilitate rapid optimisation of dopant concentrations for maximised conductivity.
The use of CHFS for the direct/indirect synthesis of TCO nanoparticles is limited in the literature; ITO has recently been synthesised via CHFS methods by some of the authors with pressed-disc resistivities as low as 6.0 × 10−3 Ω cm.31 The authors have also investigated the CHFS synthesis of co-doped zinc oxide (with both Al and Ga and known as AGZO) using formic acid as an in-process reducing agent, which gave a resistivity of 4.6 × 10−2 Ω cm.36
Herein, we report the first CHFS synthesis of highly conductive optimised AZO and GZO nanoparticles followed by heat-treatment in reducing conditions. Compositional space for each doped system was first explored (up to 6 at%) using a lab-scale CHFS reactor at a production rate of 60 g per hour. These materials were screened as heat-treated pellets, and then the composition with the highest conductivity was remade using a pilot-scale CHFS process at a production rate of 8 kg per day (by dry mass). The conductivity of this material was also evaluated as a heat-treated pellet.
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Transmission electron microscopy (TEM) was performed using a Jeol 200 kV transmission electron microscope in imaging mode. Samples were dispersed in MeOH and drop coated onto a carbon-coated copper TEM grid purchased from Agar Scientific. Image analysis and particle size measurements were carried out using ImageJ® software. Energy dispersive X-ray spectroscopy (EDS) analysis was carried out using an Oxford Instruments X-MaxN 80-T Silicon Drift Detector (SDD) fitted to the transmission electron microscope and processed using Aztec® software. X-ray photoelectron spectroscopy (XPS) was performed a Thermo Scientific K-alpha photoelectron spectrometer using monochromatic Al-Kα radiation. Survey scans were collected in the range 0–1100 eV (binding energy) at a pass energy of 160 eV. Higher resolution scans were recorded for the principal peaks of Zn (2p), Al (2p), Ga (3d), O (1s) and C (1s) at a pass energy of 50 eV. Peak positions were calibrated to carbon and plotted using the CasaXPS® software. Additional chemical analysis for Zn, Al and Ga was performed by ICP-AES using dilute solutions of the samples dissolved in 1% HNO3 (aq.). Analyses were carried out using a Varian 720 ICP-AES (axial configuration) equipped with an autosampler. Prior to sample analysis, calibrations were carried out using standards at concentrations of 2.5, 5.0, 7.5 and 10 ppm for Zn and 0.25, 0.5, 0.75 and 1.0 ppm for Al and Ga. To assess the conductivity of the materials, the powders were pressed into 16 mm diameter green-coloured compacts of thickness 1.0 mm under a force of 50 kN using a hydraulic press (Specac, Orpington, UK). The discs were then heat-treated under 5% hydrogen in nitrogen at 500 °C for 3 hours, after which a change in colour from green to blue was observed. Hall effect measurements were carried out using the Van der Pauw method to determine the bulk resistivity of the materials. In order to do this, four gold contacts were first sputtered onto the heat-treated discs, which were then subjected to an input current of 1 mA and a calibrated magnetic field of 0.58 T using the Van der Pauw probe. The transverse voltage was then measured. The measurement was repeated by reversing the direction of the magnetic field and the current. Resistivity measurements were made in triplicate for each pellet and the mean value and standard deviations were calculated.
Selected XRD patterns are shown in Fig. 2, with dopant levels in the precursor feeds of 1, 3, and 5 at% for each AZO and GZO series, as well as a reference pattern for ZnO.41 The broad XRD peaks in the as-prepared materials was indicative of nano-crystallites; Scherrer peak analysis was performed on the diffractograms for the (100), (002), and (101) XRD peaks, the averages for which are collated in Tables S1 and S2 in the ESI.† Estimated crystallite size was observed to decrease with increasing dopant concentration in the precursor for each series; the sizes were calculated as 35 and 31 nm for samples A1.0 and A5.0, respectively, and 28 and 21 nm for samples G1.0 and G5.0, respectively.
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Fig. 2 XRD patterns for samples of each AZO and GZO. (a) shows a full pattern typical of the zinc oxides, in this case sample A1.0. (b) shows a standard ZnO pattern.40 (c1) and (d1) show peaks in the range 13.5 to 17.5° 2θ for A2.5p and G2.5p, respectively (pilot scale materials), (c2–4) and (d2–4) show these same peaks for 5, 3, and 1 at% nominal dopant level for each AZO and GZO synthesised on the laboratory scale. |
TEM images revealed that the morphology of the aluminium-doped samples was a mixture of small (<10 nm) rounded particles and larger rod-like structures. On average these particles were 43 ± 14 nm wide by 132 ± 83 nm long (mean from 250 particles), an aspect ratio of 3.1 for sample A2.5 with 2.5 at% Al dopant (see Fig. 3a and b).
Increasing the aluminium content in the precursor to 5.0 at% (Fig. 3c and d), resulted in a decrease in average rod width to 38 ± 17 nm, but an increase in length to 187 ± 92 nm, corresponding to an increase in average aspect ratio to 4.9. This was consistent with the crystallite size as estimated by the Scherrer method; the crystallite size decreased with increasing Al at%, as does the width of the rods as determined from TEM images.
BET analyses were conducted in order to understand the changes in surface area across the compositions. BET data suggested an increase in surface area from 19 m2 g−1 at 1 at% Al up to 27 m2 g−1 for 5 at% Al that was also consistent with the trends from TEM and Scherrer analyses, though the BET values were lower than expected. This was due to the difficulty in fully removing moisture from very high surface area ZnO-based samples prior to BET analysis, despite extensive degassing for 18 hours at 150 °C prior to BET measurements. XPS analyses revealed that only one type of Al-species was present up to 5 at% Al (the highest nominal concentration tested), as shown in Fig. S1 in the ESI.† The binding energy peak at 73.2 eV corresponds to Al3+ in coordination with oxygen.42 A single environment was observed for Zn, with the 2p3/2 peak at 1021.5 eV characteristic of ZnO.43 ICP analyses suggested that the actual loadings of Al in the solids were 2.2 and 4.1 at% for samples A2.5 and A5.0, respectively, indicating a dopant uptake of >80% from the nominal compositions in the precursor solutions.
TEM images of the Ga-doped ZnO samples showed a rounded morphology with an aspect ratio close to 1 for the 2.5 at% (Fig. 4a and b) and for the 5.0 at% samples (Fig. 4c and d); the increase in [Ga] in the precursor resulted in a decrease of particle size from 44 (±8) to 36 (±9) nm, respectively. BET analyses of the Ga-doped ZnO samples demonstrated an increase in surface area with increasing Ga at%, from 18 m2 g−1 at 1 at% Ga up to 31 m2 g−1 for 5 at% Ga. Concurrently, as expected, the crystallite size as calculated using the Scherrer method, showed a general decrease in crystallite size with increasing dopant concentration.
TEM, XRD and BET data for the Ga-doped materials suggested that the effect of increasing [Ga] in the precursor solutions (and thus in the product) reached a plateau, such that the crystallite size and surface area did not dramatically change at >3.5 at% Ga. XPS data suggested a possible reason for this; in the 2.5 at% Ga sample, the 3d binding energy doublet revealed only a single gallium environment at 22.1 eV. This was consistent with the presence of Ga3+ and was attributed to GaZn, gallium substituted into the Zn sites in ZnO. For sample G5.0, two sets of doublets were present in the spectrum; one at the same binding energy, representing a slightly higher at% of Ga present in the surface structure, as well as a second peak at a lower binding energy of 19.7 eV. This second peak was representative of Ga2O3,44 a separate species that appeared to have formed once a saturation point of GaZn was reached in the ZnO structure at ca. 3.0 (or 3.5) at% gallium. ICP data indicated that the actual values of dopant Ga in the solid was 2.5 and 5.5 at% for samples G2.5 and G5.0, respectively, suggesting a slightly increased dopant uptake compared to the AZO samples.
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Fig. 5 Resistivity data for Al-doped ZnO with increasing dopant level. Error bars correspond to standard deviations in Hall probe data for 3 repeat measurements. |
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Fig. 6 Resistivity data for Ga-doped ZnO with increasing dopant level. Error bars correspond to standard deviations in Hall probe data for 3 repeat measurements. |
The lowest mean resistivity value for 3.5 at% GZO (sample G3.5), was 9.1 × 10−3 Ω cm. Both the optimum resistivity of AZO and that of GZO reported herein, are a significant improvement on the value of 4.6 × 10−2 Ω cm, as reported by the authors elsewhere for the corresponding co-doped system.36
Herein for AZO, as the optimal dopant level was observed to be in the range ca. 2.0 to 2.5 at% Al on the lab scale, dopant precursor solutions corresponding to 1.0, 2.0, 2.5, and 3.5 at% were selected for the pilot plant synthesis. For GZO, 2.0, 3.0, 3.5, and 4.5 at% Ga were selected for scale-up; in both cases, these dopant levels should have given an indication of any trends in the data, i.e. if trends on lab scale were transferable to the pilot plant scale.
The as-prepared materials made on the pilot scale CHFS were phase-pure wurtzite structure as determined by XRD (Fig. 2c1 and d1; full patterns are included in Fig. S3 in the ESI,† as well as representative TEM images). XPS analysis revealed that the uptake of aluminium in AZO was reduced on the pilot scale by almost half, whereas uptake of gallium in GZO was increased by a third compared to the lab scales. ICP analyses also indicated that the dopant uptake was 1.9 at% Al in sample A2.5p and 1.7 at% Ga in G2.5p. This showed quantitatively the decrease in dopant uptake when synthesising on the pilot scale, compared to 2.2 at% Al and 2.5 at% Ga in each A2.5 and G2.5, respectively, the lab-scale analogues.
The lower-than-expected dopant levels on the pilot plant can be understood when one considers the following: the residence time on the pilot plant (<4 s) is considerably less than the lab scale process under similar conditions (>12 s) and this is very important for precipitation of precursors that are highly soluble and that tend to nucleate and have a significant growth or maturation phase in flow. With less time to nucleate out from solution, the dopant elements were incorporated less effectively into the ZnO structure, and thus the materials made on the pilot scale demonstrated reduced uptake of the dopants compared to those synthesised on the lab scale. This could be rectified in the future by increasing the residence time on the pilot plant or by using more supersaturating conditions to drive the conversion of the Zn precursor.
To confirm the electrical performance of the pilot scale samples, compact pellets were again pressed and resistivity measurements taken. As detailed in Tables S1 and S2,† the trend of conductivity as seen on the lab scale CHFS for both materials was carried forward to the pilot scale. The optimal resistivity obtained for AZO at 2.0 at% on the pilot scale was 6.1 × 10−2 Ω cm, which is higher than the laboratory scale analogue of 1.6 × 10−2 Ω cm. This was believed to be due to increased grain boundary effects in the pellets due to reduced crystallite size of the pilot scale material, as well as differences in Al uptake between lab and pilot scale materials. The optimum GZO sample was at 4.5 at% Ga, and this had a resistivity of 1.3 × 10−2 Ω cm, comparable to the analogous lab-scale material.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ta03364d |
This journal is © The Royal Society of Chemistry 2016 |