ITO nanoparticles reused from ITO scraps and their applications to sputtering target for transparent conductive electrode layer
© Korea Nano Technology Research Society 2017
Received: 18 July 2017
Accepted: 16 August 2017
Published: 6 September 2017
In this study, ITO nanoparticles (ITO-NPs) were reused from ITO target scraps to synthesize low cost ITO-NPs and to apply to make sputtering target for transparent conductive electrodes (TCEs). By controlling heat-treatment temperature as 980 °C, we achieved reused ITO-NPs having Brunauer, Emmett and Teller specific surface area (BET SSA) and average particle size 8.05 m2/g and 103.8 nm, respectively. The BET SSA decreases along with increasing heat-treatment temperature. The ITO-NPs were grown as round mound shape, and highly crystallized to (222) preferred orientations. Also, applying the reused ITO-NPs, we achieved an ITO target of which density was 99.6%. Using the ITO target, we achieved high quality TCE layer of which sheet resistance and optical transmittance at 550 nm were 29.5 Ω/sq. and 82.3%. Thus, it was confirmed that the reused ITO-NPs was feasible to sputtering target for TCEs layer.
In recent, interests on fourth generation industrial revolution have been arisen, and it is strongly related to artificial intelligence (AI), internet of thing (IoT), advanced reality (AR), and virtual reality (VR), etc. It is heavily dictated by customer demand, and the customers intensively demand smaller and lighter electronics devices with high cost-effectiveness . To fulfill the demand, materials for the devices have to be supplied easily to lower cost manufacturing. One of factors determining the cost of the devices is transparent conductive electrodes (TCEs), and indium tin oxide (ITO) is mainly used as the TCEs. ITO is consistent of tin (Sn) doped indium oxide (In2O3). The In is an important material for the ITO owing to its unique characteristics, high optical transparency and electrical conductivity. However, the In is being exhausted in the earth in proportion to growth of the related market (display, photovoltaics, lightings, touch sensors, etc.) [2, 3]. In general, In is recycled from redundant ITO target scraps that is emitted from the factories using them . Because the In is recycled while being separated from Sn as indivisual , current process to manufacture ITO target increases processing steps that leads to higher manufacturing cost; i.e., In and Sn is indivisually oxidized to make In2O3 nanoparticles (NPs) and SnO2 NPs, and the two oxides are mixed together to dope Sn into In2O3 while sintering ITO target. Thus, improvement of recycling process to decrease steps and manufacturing cost is demanded for cost-effective devices . One of improvement is to make targets with ITO-NPs instead of indivisual In2O3 NPs and SnO2 NPs and, therefore, ITO has to be reused from redundunt ITO sputtering target to synthesize in its form. In this study, an attempt to reuse ITO-NPs from redandant ITO target scraps is introduced to synthesize low cost ITO-NPs and to apply to make sputtering target for transparent conductive electrodes (TCEs).
Firstly, ITO scraps were dissolved in a HCl with concentration of 100 g/L. As well, 0.1 wt% polyvinylpyrrolidones (PVPs), as a dispersing agent, were dissolved into NH4OH solution, as a reducing agent. Then, the HCl solution was put into the NH4OH solution to precipitate In-Sn hydroxide particles. The precipitates were washed several times until residuals such as NH4+ and Cl− were erased out. The precipitate particles were heat-treated at 400, 600, 800 and 980 °C for 2.5 h to crystallize ITO-NPs, respectively. The specific surface area (SSA), crystal structure, particle size, composition ratio of the ITO-NPs were analyzed by means of a Brunauer, Emmett & Teller specific surface area (BET SSA) analyzer, X-ray diffractometer (XRD, Rigaku Rotaflex D/MAX System) with monochromatic Cu target (λ = 0.1541 nm), field emission scanning transmission electron microscope (FESEM, MZ-15/EC/JSM-7000F), high resolution transmission electron microscope (HRTEM, JEOL, JEM-3010) and Inductively coupled plasma (ICP) spectrometer (Thermo, iCAP 7000).
Then, a 2-in. sized ITO target was made using the ITO-NPs followed by sintering at 1580 °C for 15 h and ITO thin films were coated on a 3 × 3 cm2 sized glass substrate by sputtering. Microstructures of ITO target and ITO thin films were observed with FESEM and, to determine feasibilty of application of ITO-NPs to ITO target, their characteristics were evaluated by measuring electrical sheet resistance and optical transmittance with a four -point probe electrical measurement system (Mitsubishi Chemical Analytech, MCP-T610) and a UV–VIS Spectrophotometer (JASCO, V-560).
3 Results and discussion
We controlled BET SSA and particle size of ITO-NPs by changing heat-treatment temperature, and ITO-NPs heat-treated at 980 °C, of which BET SSA and particle size were 8.05 m2/g and 103.8 nm, was chosen to improve target density. The target density is an important factor that affects electrical and optical properties of ITO coating layer by sputtering method . Lower resistivity and higher transparency can be obtained from the sputtering target with higher density and, in general, the higher density can be obtained with larger sized nanoparticles. The reason was mentioned at the later part of this section.
Nevertheless, smaller particle size shown in the HRTEM and FWHM is concerned with Derjaguin, Landau, Verwey and Overbeek (DVLO) theory . In DLVO theory, the potential energy of van der Waals attraction and the potential energy of the electrical double layer interaction  are summed to provide a total interaction potential energy between colloidal particles. Due to differences in the surface chemistries of dispersing agent and ITO-NPs, obtaining a stable dispersion of polymeric dispersing agent and ITO-NPs can be challenging. The importance of the colloidal stability of starting dispersions on the final properties of ink pastes has been demonstrated. Zhao et al.  found that the presence of aggregated titanium dioxide particles in dispersions deleteriously affects the optical and mechanical properties. Researchers have found that the colloidal stability of dispersions can be disrupted by changes in surface potential of dispersing agent , ionic strength , concentration of dispersing agent  and particle size . One way to address the questions about clustering and stability in ink paste is through predictions using DLVO theory . Under drying above the latex glass transition temperature, particles consolidated and compacted, forcing the ITO-NPs to segregate into the boundary regions between dispersing agent, PVP.
In this study, ITO-NPs were reused from redundant ITO target scraps to synthesize low cost ITO-NPs and to apply to make sputtering target for TCEs. By controlling heat-treatment temperature as 980 °C, we achieved reused ITO-NPs having BET SSA and average particle size 8.05 m2/g and 103.8 nm, respectively. The BET SSA decreases with raised heat-treatment temperature. The ITO-NPs were grown to round mound shape, and highly crystallized to (222) preferred orientations. Also, applying the reused ITO-NPs, ITO target, of which high density of 99.6%, was achieved. Using the ITO target, we achieved high quality TCE layer having sheet resistance and optical transmittance at 550 nm of 29.5 Ω/sq. and 82.3%. Thus, it was confirmed that the reused ITO-NPs was feasible to sputtering target for TCEs layer.
SH planned and executed this study. SS and BK fabricated ITO nanoparticles and target from ITO scraps. JL optimized to fabricate an ITO target sample. YK evaluated the ITO target by sputtering and measuring its characteristics. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
Ethics approval was obtained from Korea Electronics Technology Institute, Hanchung RF Co., Ltd., Seoul National University of Science & Technology.
This study was supported by the Industrial Original Technology Development Program of the Korea Evaluation Institute of Industrial Technology (KEIT) grant funded by the Ministry of Knowledge Economy (No. 10048248).
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