materials Article
Thin Film Coating with Highly Dispersible Barium Titanate-Polyvinylpyrrolidone Nanoparticles Jinhui Li 1,2 , Koji Inukai 3 , Yosuke Takahashi 3 , Akihiro Tsuruta 2 and Woosuck Shin 1,2, * 1 2 3
*
Department of Frontier Materials, Nagoya Institute of Technology, Nagoya 466-8555, Japan;
[email protected] Inorganic Functional Material Research Institute AIST, Nagoya 463-8560, Japan;
[email protected] R&D Center, Noritake Co., Ltd., Miyoshi 470-02, Japan;
[email protected] (K.I.);
[email protected] (Y.T.) Correspondence:
[email protected]; Tel.: +81-52-736-7107
Received: 4 April 2018; Accepted: 26 April 2018; Published: 1 May 2018
Abstract: Thin BaTiO3 (BT) coating layers are required in various multilayer ceramic technologies, and fine nanosized BT particles with good dispersion in solution are essential for this coating process. In this work, cubic and tetragonal phase monodispersed BT nanoparticles—which were referred to as LBT and HBT-PVP coated on their surface by polyvinylpyrrolidone (PVP) polymer—were prepared by low temperature synthesis (LTS) and hydrothermal method (HT) at 80 and 230 ◦ C, respectively. They were applied for the thin film coating on polyethylene terephthalate (PET) and Si wafer substrates by a simple bar coating. The thickness of BT, LBT-PVP, and HBT-PVP films prepared by their 5 wt % coating agent on Si are around 268, 308, and 263 nm, and their surface roughness are 104.6, 91.6, and 56.1 nm, respectively. The optical transmittance of BT, LBT-PVP, and HBT-PVP films on PET are 55, 66, and 73% at 550 nm wavelength and the haze values are 34.89, 24.70, and 20.53% respectively. The mechanism of dispersant adsorbed on the BT surface for densification of thin film during the drying process of the film was discussed. Keywords: BaTiO3 ; poly(vinylpyrrolidone); dispersion; bar coating; thin film; transmittance
1. Introduction BaTiO3 (BT) is widely used in multilayer ceramic capacitors, semiconductors, positive temperature coefficient thermistors, and piezoelectric devices [1,2] owing to its ferroelectric [3,4] and piezoelectric [5–7] properties. In recent years, its characteristic of wide band gap and high refractive index is also attractive [8–10]. High refractive index thin film can be used in waveguide sensors, optical filters, and anti-reflection coatings. Thin films of BT have a great potential application in erasable image storage devices and optical applications, such as non-linear optical systems, optical computing, and light guiding [11,12]. In recent years, with miniaturization of devices and thin films, the demand for nanosized BT has increased. Many processing methods for ferroelectric, high refractive index thin films have been developed for this purpose, such as chemical solution deposition by the sol–gel process [13] or physical depositions by sputtering [14], pulsed laser ablation [15], and chemical vapor deposition [16]. Each of these processes has advantages and disadvantages. Among these, the sol–gel coating method is the most popular process to obtain large-area thin films with good homogeneity and smooth surface at low temperatures. In the sol–gel process, films are deposited by dip coating [17,18] and spin coating [19,20], followed by high-temperature treatment. As the particle size of BT decreases, the aggregation of nanoparticles becomes a serious problem. This induces defects in the thin films and imparts an uneven surface, and the nanoparticles
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cannot exhibit their inherent characteristics, which reduces the performance of the electrical components. In order to solve this problem, highly dispersible BT nanoparticles were prepared with polyvinylpyrrolidone (PVP) by the low temperatures synthesis (LTS) and hydrothermal (HT) process, referred to as LBT-PVP and HBT-PVP respectively, as reported in our previous paper [21,22]. The nanoparticles prepared by these methods can be dispersed as suspension in a coating slurry, and with these particles thin film layers can be prepared easily and quickly by a bar coating process as Itoh et al. reported [23]. This process does not require high-temperature treatment and the performance of the film depends on the crystalline phase of the powder. The characteristic of this process is that there is no troublesome process and the performance of the thin-film can be easily controlled. It is expected that highly dispersible nanomaterials contribute to the densification of the film and improve the performance of the device by this method. In the present work, BT-PVP films of BT-PVP were prepared, and for comparison the film of commercial BT particle was also prepared. The characteristic of this process is that it neither involves complicated procedures nor high-temperature treatment, because the phase of the BT-PVP particles can be controlled by synthesis method. The performances of the thin film—such as morphology, thickness, and roughness—were evaluated in this study, which is expected to be related the dispersion of nanoparticle and surface of PVP, contributing to the dense and lower defect coating films and eventually improve the performance of the device. Optical transmittance in visible range of the thin films of different BT particles was also evaluated also. 2. Results and Discussion 2.1. Properties of BT-PVPs Prepared by Different Synthesis Method The crystalline phases of commercial BT and LBT-PVP prepared by LTS process are cubic, and that of HBT-PVP prepared by HT process is tetragonal as reported in our previous paper [21,22]. The tetragonality (c/a) of the particle is 1.0008, 1.0005, and 1.0058 for BT, LBT-PVP, and HBT-PVP respectively. The particle size and tetragonality (c/a) values of these particles in this study are list in Table 1 in detail. Table 1. Comparison of BT, LBT-PVP, and HBT-PVP particles size and tetragonality. Average particle size was measured by SEM and Dynamic light scattering (DLS) using ethanol (EtOH) suspensions with 5 wt % particles.
Particles BT LBT-PVP HBT-PVP
Particle Size (nm)
Tetragonality (c/a)
SEM
CV* (%)
DLS
CV* (%)
81.0 128 106
20.1 16.6 20.7
141 164 131
55.2 48.2 47.2
1.0008 1.0005 1.0058
CV*: coefficient of variation.
Figure 1 shows the FE-SEM images and dynamic light scattering of BT, LBT-PVP, and HBT-PVP, and their particle size measured by both SEM and DLS analysis. BT and LBT-PVP are spherical with a particle size of 81.0 nm (CV: 20.1%), and 128 nm (CV: 16.6%) as measured by FE-SEM. The particle sizes measured by DLS, are 141 nm (CV: 55.2%) and 164 nm (CV: 48.2%) respectively. The particle size of the dried powder evaluated by FE-SEM and the DLS particle size in EtOH slurry are so similar that BT and LBT-PVP can be considered monodisperse in EtOH. The cube-shaped HBT-PVP particles have a similar particle size of 106 nm (CV: 20.7%) and 131 nm (CV: 47.2%), as measured by FE-SEM and DLS, respectively, and demonstrate a monodisperse state in EtOH. Although the particle sizes of these three powders are not exactly the same, each of them is monodispered in EtOH solution.
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Figure 1. FE-SEM images (a) and dynamic light scattering (DLS) (b) of 5 wt % BT, LBT-PVP, and HBTFigure 1. FE-SEM images (a) and dynamic light scattering (DLS) (b) of 5 wt % BT, LBT-PVP, and PVP in EtOH solution. The measurement of DLS is sequenced five times. HBT-PVP in EtOH solution. The measurement of DLS is sequenced five times.
The surface of BT, LBT‒PVP, and HBT‒PVP were characterized by FT-IR as shown in Figure 2a. The surface LBT-PVP, and HBT-PVP were characterized byand FT-IR as cm shown in Figure 2a. −1 can −1 corresponds The peak of 1640 of cmBT, be attributed to symmetric C=O stretching, 1280 to −1 can be attributed to symmetric C=O stretching, and 1280 cm−1 corresponds to The peak of 1640 cm ring (CH2) wag +(CN) stretching of PVP rings [24]. It can be seen that the structure of dispersant of ring wag +(CN)on stretching of PVP rings It can be that the structure of dispersant of PVP (CH does2 )not change the particle surface of[24]. LBT-PVP, butseen decomposes on the HBT-PVP, where PVP doesof not change surface of LBT-PVP, but decomposes onon theparticle HBT-PVP, where BTthe the peak 1280 cm−1on is the notparticle observed. Although the dispersant adsorbed different, −1 is not observed. Although the dispersant adsorbed on particle different, BT-PVPs peak of 1280 cm PVPs demonstrate high dispersion. demonstrate high dispersion.
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Figure 2. (a) FT-IR spectra of BT, HBT-PVP LBT-PVP and PVP; (b) relationship of zeta potential and Figure 2. (a) FT-IR spectra of BT, HBT-PVP LBT-PVP and PVP; (b) relationship of zeta potential and pH of BT, LBT-PVP, and HBT-PVP in H2O. pH of BT, LBT-PVP, and HBT-PVP in H2 O.
It is well known that the zeta potential (ζ) depends on the pH value following Helmholtz– It is well known Smoluchowski formula:that the zeta potential (ζ) depends on the pH value following Helmholtz– Smoluchowski formula: η𝜂 𝑣v (1) ζζ== 𝜀 𝑉 (1) εV where where η, η, υ, υ, ε, ε, and and V V are are solution solution viscosity, viscosity, electrophoretic electrophoretic mobility, mobility, relative relative permittivity permittivity of of solution, solution, and voltage respectively. Because the pH effects the electrophoretic mobility of particle. and voltage respectively. Because the pH effects the electrophoretic mobility of particle. The The high high dispersion effect of of PVP for for the the (ζ) does not change withwith pH, and dispersion of ofLBT-PVP LBT-PVPdepends dependsononthe thesteric steric effect PVP (ζ) does not change pH, HBT-PVP depends on the electrostatic repulsion of decomposed PVP for the ζ changed with pH. The and HBT-PVP depends on the electrostatic repulsion of decomposed PVP for the ζ changed with pH. ζThe of HBT-PVP is asislow as −25 mV, as the is 9.3 the the pH pH is adjusted, as ζ of HBT-PVP as low as − 25 mV, as pH the of pHits ofsuspension its suspension is before 9.3 before is adjusted, shown in Figure 2b.2b. In In thethe other words, HBT-PVP as shown in Figure other words, HBT-PVPisiscoated coatedby bynegative-charged negative-chargedionic ionicdispersant dispersant as as our previous reported results indicate [21,22]. On the contrary, the ζ of BT particles depends on pH, our previous reported results indicate [21,22]. On the contrary, the ζ of BT particles depends on pH, and the value valueisisaround around 0–15 as is pH is 4–10. It indicates BT nanoparticles aggregate in and the 0–15 mVmV as pH 4–10. It indicates that BTthat nanoparticles aggregate in aqueous aqueous solution a wide range, these although these areinobtained aqueousthe solution, the solution in a widein pH range,pH although results areresults obtained aqueousinsolution, dispersion dispersion mechanism is not different in EtOH. The ζ of the particle in EtOH would be large because mechanism is not different in EtOH. The ζ of the particle in EtOH would be large because the relative the relative permittivity of is EtOH (25.3) is water smaller than water (80). used The particles usedhave in this study permittivity of EtOH (25.3) smaller than (80). The particles in this study high zeta have high zeta potential in EtOH, leading them to demonstrate high dispersion in EtOH as shown potential in EtOH, leading them to demonstrate high dispersion in EtOH as shown in Figure 1b. in Figure 1b. 2.2. Properties of the BT-PVP Thin Film 2.2. Properties of the BT-PVP Thin Film The high dispersion characteristics of BT, LBT-PVP, and HBT-PVP were used to prepare BT films The high BT,LBT-PVP, LBT-PVP,and andHBT-PVP HBT-PVPinwere used todifferent prepare BT films on PET and Sidispersion substrates.characteristics Suspensions ofofBT, EtOH with contents on PET andas Sithe substrates. of BT, LBT-PVP, and HBT-PVP EtOH with different were used coating Suspensions agents. The film morphologies observed by in FE-SEM, are shown in contents Figure 3. were as the agents. Thefilm filmfilled morphologies FE-SEM, are shown Figure 3. It canused be seen thatcoating the density of the by powderobserved increases by with the particle contentinincreasing It can1be of the filmcoated filled by increases particle content from to seen 10 wtthat %. the Thedensity role of dispersant on powder the particle surfacewith can the be also observed in increasing from 1 to 10 wt %.of The rolehas of dispersant coatedinon the particle surface can be 1 wt % film. Although each them high dispersion EtOH, but morphologies of also filmsobserved are very in 1 wt % film. of theminhas in EtOH, butwithout morphologies of films are different. ThereAlthough are large each aggregates BT high filmsdispersion here and there for BT dispersant coating, very different. There aggregates in BT are films here and there for BT without dispersant coating, as shown in Figure 4.are Thelarge particles of LBT-PVP evenly dispersed on the PET substrate. It is thought as shown in FigurePVP 4. The particles LBT-PVP are evenlythe dispersed on the PET substrate. It is that the dispersant coated on theofnanoparticle prevents aggregates of nanoparticles during thought that the dispersant PVP coated on the nanoparticle prevents the aggregates of nanoparticles EtOH evaporation. during EtOH evaporation.
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Figure 3. SEM images of the thin films prepared using different content of BT, LBT-PVP, and HBTFigure 3. SEM images of the thin films prepared using different content of BT, LBT-PVP, and HBT-PVP PVP in EtOH coated on PET. The contents of particles in EtOH are 1, 5, 10 wt %. in EtOH coated onimages PET. The contents of particles EtOH are 1, 5,content 10 wt %. Figure 3. SEM of the thin films preparedinusing different of BT, LBT-PVP, and HBTPVP in EtOH coated on PET. The contents of particles in EtOH are 1, 5, 10 wt %. The HBT-PVP film looks relatively denser than LBT-PVP film, because uncoated PET areas of The HBT-PVP filmfrequent looks relatively than because PET of areas LBT-PVP film is more as showndenser in Figure 4. LBT-PVP This is duefilm, to the particleuncoated aggregations BT of The HBT-PVP film looks relatively denser than LBT-PVP film, because uncoated PET areas of LBT-PVP film is more frequent as shown in Figure 4. This is due to the particle aggregations of BT films films and LBT-PVP films as shown in Figure 3, and this low surface coverage is more serious in the LBT-PVP film is more frequent as shown in Figure 4. This is due to the particle aggregations of BT and LBT-PVP films as shown in Figure 3, and this low surface coverage is more serious in the case case of BT film. Although all the DLS results of three different particles in EtOH are very good, their films and LBT-PVP films as shown in Figure 3, and this low surface coverage is more serious in the offilm BT film. Although all theassembly DLS results of three different particles in EtOH very good, their formation or particle during the drying process are different. It are is speculated that bothfilm case of BT film. Although all the DLS results of three different particles in EtOH are very good, their formation particle assembly during the dryingare process arewith different. It is speculated both the the steric or effect of PVP and electrostatic repulsion assisted the capillary force whichthat assemble film formation or particle assembly during the drying process are different. It is speculated that both the dispersed particles during the drying process, a short-range assembly of nanoparticles is built steric PVP repulsion are and assisted with thethe capillary force theeffect stericof effect ofand PVPelectrostatic and electrostatic repulsion are assisted with capillary forcewhich whichassemble assemble the inthe thedispersed case of HBT-PVP. Inthe other words, the dispersant decomposed PVP of on BT-PVP is andis dispersed particles during drying process, andand a short-range assembly iseffect built in particles during the drying process, aof short-range assembly ofnanoparticles nanoparticles built only for the dispersion of nanoparticles but also for close packing particle assembly, leading tonot thenot case of HBT-PVP. In other words, the dispersant of decomposed PVP on BT-PVP is and effect in the case of HBT-PVP. In other words, the dispersant of decomposed PVP on BT-PVP is and effect dense and flat films. onlynot foronly the dispersion of nanoparticles but also close packing particle assembly, leading to dense for the dispersion of nanoparticles butfor also for close packing particle assembly, leading to anddense flat films. and flat films.
Figure 4. Enlarged SEM images of the thin films prepared 1 wt % BT, LBT-PVP, and HBT-PVP in EtOH coated on PET. SEM images of the thin films prepared 1 wt % BT, LBT-PVP, and HBT-PVP in Figure 4. Enlarged Figure 4. Enlarged SEM images of the thin films prepared 1 wt % BT, LBT-PVP, and HBT-PVP in EtOH EtOH coated on PET. coated on PET. The films of LBT-PVP and HBT-PVP made by 5 wt % are very dense and smooth, but the film
made by have holes and large particles aggregate. Thevery roledense of dispersant, which TheBT films ofmany LBT-PVP and HBT-PVP made by 5 wt % are and smooth, butprevents the film The films LBT-PVP and HBT-PVP madeofby 5 wtin%the are very and smooth, but film made by BTof have many holes andevaporation large particles aggregate. The roledense of dispersant, which prevents the aggregation of particles during EtOH drying process of BT-PVP film, isthe also theby aggregation of particles evaporation ofaggregate. EtOH in theThe drying of BT-PVP film, is also made BT(Figure have many holesduring and large particles roleprocess of dispersant, which prevents observed 3). observed 3).the magnified Figure (Figure 5 shows SEM images thin in surface and their cross the aggregation of particles during evaporation of of EtOH the drying process of section BT-PVPthat film,were is also Figure shows the magnified their cross sectionare that were prepared by 55wt agents onSEM PET.images It can of be thin seensurface that theand particles of BT-PVP densely observed (Figure 3).% coating prepared 5 wt % coating agentsSEM on PET. It can seen that particles BT-PVP are BT-PVP densely arranged same height, but unevenness can be observed for thethe film of BT. Itofindicates that Figurein5by shows the magnified images ofbethin surface and their cross section that were arranged in same height, but unevenness can be observed for the film of BT. It indicates that BT-PVP is easy to prepare smooth and dense thin film. The BT film has a very relatively rough surface, prepared by 5 wt % coating agents on PET. It can be seen that the particles of BT-PVP are while densely is easy to prepare smooth and dense thin film. The BT film has a very relatively rough surface, while arranged in same height, but unevenness can be observed for the film of BT. It indicates that BT-PVP is easy to prepare smooth and dense thin film. The BT film has a very relatively rough surface, while the
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BT-PVP film exhibited a relatively uniformuniform surface,surface, with a thickness of around nm measured the BT-PVP film exhibited a relatively with a thickness of 200–300 around 200–300 nm from the cross-section images. the BT-PVP filmthe exhibited a relatively measured from cross-section images.uniform surface, with a thickness of around 200–300 nm measured from the cross-section images.
Figure SEMimages imagesofofthe thethin thinfilms films surface surface (top), (top), and cross films were Figure 5. 5.SEM cross section section(down). (down).These These films were Figure 5. images of the thin films surfacein(top), and crosson section prepared by 5 wt BT, LBT-PVP, and HBT-PVP inEtOH EtOH coated on PET. (down). These films were prepared bySEM 5 wt %% BT, LBT-PVP, and HBT-PVP coated PET. prepared by 5 wt % BT, LBT-PVP, and HBT-PVP in EtOH coated on PET.
The coating of PVP on the surface of BT, which plays an important role in particle dispersion The coating of PVP on the surface of BT, which plays an important role in particle dispersion and coating of coating, PVP on the surface ofaBT, which plays an important in particle dispersion and The uniform thin is definitely significant advantage in therole industrial application of uniform thin ceramics. coating, isTransparent definitely aoxide significant advantage in the industrial application of multilayer and uniform thin coating, is definitely a significant industrial application of multilayer coating can beadvantage applicablein to the optoelectronics or UV-shield ceramics. oxide coating be applicable optoelectronics or UV-shield and the multilayer ceramics. Transparent oxide coating betoapplicable or film, UV-shield film, andTransparent the optical transmittance ofcan BT films for can UV–visible range istoanoptoelectronics important parameter. Figure optical transmittance of BT films for UV–visible range is anrange important parameter. Figure 6a shows film, and the transmittance ofofBT films forfilm UV–visible is an important parameter. Figure 6a shows theoptical transmittance spectra LBT-PVP prepared in different concentration suspension. the6a transmittance spectra of LBT-PVP film prepared in different concentration suspension. Overall showstransmittance the transmittance spectra ofisLBT-PVP film prepared in different Overall of LBT-PVP lower than PET substrate. The 1concentration wt % film suspension. shows high transmittance of for LBT-PVP lower than The 1 wtis% very film transmittance Overall transmittance ofisLBT-PVP is PET lower than substrate. Theshows 1 wt % film shows high transmittance the particle coverage on substrate. the PETPET substrate low as high shown in Figure 4.for thetransmittance particle coverage onparticle the PET substrate very lowparticle as shown Figure of 5be and for onis the substrate is in very low4.asTransmittance shown in Figure 4. Transmittance of 5the and 10 wt %coverage films decreases asPET the content increases. This decrease can 10 Transmittance wt % films decreases as the content increases. This decreaseincreases. can be understood by can the be light 5 and 10 wt particle % films decreases as of thefilm particle content This decrease understood byof the light scattering with thickness increase. understood the light scattering with thickness of film increase. scattering withbythickness of film increase.
(a) (a)
(b) (b)
Figure 6. (a) Transmittance spectra of LBT-PVP films prepared in different concentration suspension Figure (a) Transmittance of LBT-PVP films prepared different concentration suspension from 1 6. wt % in EtOH.spectra (b) Optical transmittance spectra ofinfilms prepared by different particles Figure 6. to (a)10 Transmittance spectra of LBT-PVP films prepared in different concentration suspension from to 10 wt %and in EtOH. (b) Optical spectraof ofcoating films prepared different particles of BT,1 LBT-PVP, HBT-PVP on PET.transmittance The particle content agent is 5bywt %. from 1 to 10 wt % in EtOH. (b) Optical transmittance spectra of films prepared by different particles of of BT, LBT-PVP, and HBT-PVP on PET. The particle content of coating agent is 5 wt %. BT, LBT-PVP, and HBT-PVP on PET. The particle content of coating agent is 5 wt %.
In visible region (380–750 nm), the transmittance, BT, LBT-PVP, and HBT-PVP film is 55, 65, and visible (380–750 nm), the transmittance, BT, LBT-PVP, and HBT-PVP film is 55, 65,films and 75% In at 550 nmregion wavelength, respectively as shown in Figure 6b. Overall transmittance of BT-PVP Inatvisible region (380–750 nm),and themaximum transmittance, BT, LBT-PVP, and HBT-PVP film is 55, 65, and 75% 550 nm respectively as shown transmittance in Figure 6b. Overall transmittance BT-PVP exceeds 60% inwavelength, the visible region, of HBT-PVP film hasof reached tofilms 80%. 75% at 55060% nm respectively in Figure 6b.HBT-PVP Overall transmittance exceeds inwavelength, the region, and of has reached 80%. It indicates that it isvisible advantageous themaximum useas of shown PVPtransmittance coated on particle surface film to prepare a filmofto ofBT-PVP high films exceedsthat 60%The visible and transmittance of HBT-PVP filmahas It indicates itinis the advantageous use ofmaximum PVP coated on particle and surface prepare film of high to transmittance. haze valuesregion, of the these films were also measured, are to 34.89, 24.70, andreached 20.53% transmittance. The haze values offilm these films were also measured, are 34.89, 24.70, and that 20.53% 80%. indicates that it is advantageous the use of (list PVPin coated particle surface to prepare athe film for ItBT, LBT-PVP, and HBT-PVP respectively Table on 2). and Itoh et al. reported [25] BT, LBT-PVP, andThe HBT-PVP film respectively (list invalue Table 2). Itoh with et al.and reported [25] that the transmittance of short wavelength visible haze decrease increasing nanoparticle of for high transmittance. haze values of light theseand films were also measured, are 34.89, 24.70, and transmittance of short wavelength visiblefilm light and hazethe value with size based on LBT-PVP, Rayleigh scattering theory. In this study, particle of BT increasing is small (81nanoparticle nm), that of 20.53% for BT, and HBT-PVP respectively (listdecrease in size Table 2). Itoh et al. reported [25] size based on Rayleigh scattering theory. In this study, the particle size of BT is small (81 nm), that of HBT-PVP (106 nm) is medium, and that of LBT-PVP (128 nm) is large. However, the trend of that the transmittance of short wavelength visible light and haze value decrease with increasing HBT-PVP (106 nm) is medium, and that of LBT-PVP (128 nm) is large. However, the trend of
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nanoparticle on Rayleigh theory. In this study, the obtained particle size of BT transmittancesize andbased haze value of their scattering thin film are different to the results by Itoh etisal.small This (81 nm), that of HBT-PVP (106 nm) is medium, and that of LBT-PVP (128 nm) is large. However, the leads us to the conclusion that BT nanoparticle have a strong aggregate and the aggregates as large trend of transmittance haze of their thin film are different to the results obtained by Itoh et al. particles scatter visibleand light, butvalue BT-PVP have less aggregates on PET substrate and the particle size This leads us to the conclusion that BT nanoparticle have a strong aggregate and the aggregates as is relatively small. Maneehyya et al. [26] reported the sharp fall in the absorption edge at 330 nm for large particles light, but is BT-PVP less on PET and the particle the BT film of scatter sol–gelvisible coating which due tohave make a aggregates smooth surface of substrate BT coating processed by size is relatively small. Maneehyya et al. [26] reported the sharp fall in the absorption edge at 330 nm liquid source. However, the broad absorption in this study is considered to be due to the surface for the BT film sol–gel coating, coating which is due make a smooth surface of BT coating processed by roughness of BTofparticle scattering theto light waves. liquid source. However, the broad absorption in this study is considered to be due to the surface roughness BT particleofcoating, scattering light waves. Table of 2. Properties BT, LBT-PVP, and the HBT-PVP thin films coated on PET substrates. Particle concentration in the EtOH coating slurry is 5 wt %. Table 2. Properties of BT, LBT-PVP, and HBT-PVP thin films coated on PET substrates. Particle concentration in the EtOH coating slurry is 5Thickness wt %. Particles (nm) Transmittance * (%) Haze (%)
BT Particles BT LBT-PVP
LBT-PVP HBT-PVP HBT-PVP
268.2 (nm) Thickness 268.2 307.9
307.9 266.8 266.8
55.91 * (%) Transmittance 55.91 66.33
66.33 73.31 73.31
34.89(%) Haze 34.89 24.70
24.70
20.53 20.53
* Transmittance is at 550 nm wavelength. * Transmittance is at 550 nm wavelength.
Presently, silicon extremely utilized substrates to build electronic devices, BT thin Presently, siliconwafers wafersare are extremely utilized substrates to build electronic devices, BTfilms thin on theon Si substrates is expected as microelectronic devices [27]. In this thestudy film isthe also coated on films the Si substrates is expected as microelectronic devices [27].study In this film is also Si substrate with 5 wt % coating agents. Figure 7 shows the surface and cross section morphology of coated on Si substrate with 5 wt % coating agents. Figure 7 shows the surface and cross section the films coated on Si wafers. The same trend of morphology can be observed as on PET (Figures morphology of the films coated on Si wafers. The same trend of morphology can be observed as on3 and The surface of The BT film is rougher compared to those of PVP-coated BTPVP-coated films. The BT particles PET 5). (Figures 3 and 5). surface of BT film is rougher compared to those of films. are densely arranged in the films prepared by high-dispersion BT-PVP, but the particles aggregate The particles are densely arranged in the films prepared by high-dispersion BT-PVP, but the particles in the filmin prepared BT as shown in shown the high-magnification SEM images. thickness of these aggregate the film by prepared by BT as in the high-magnification SEMThe images. The thickness films are estimated to be about 209, 292, and 293 nm from the cross section images for the BT, LBT-PVP, of these films are estimated to be about 209, 292, and 293 nm from the cross section images for the BT, and HBT-PVP, respectively. LBT-PVP, and HBT-PVP, respectively.
Figure 7. SEM images images of of film filmsurface surface(top) (top)and andcross crosssection section(down). (down).The Thefilm filmwere were prepared 5 wt prepared byby 5 wt % % LBT-PVP, and HBT-PVP EtOH coated substrate.The Theinsets insetsare arehigh-magnification high-magnification SEM BT,BT, LBT-PVP, and HBT-PVP in in EtOH coated ononSiSisubstrate. images of the films surface.
Another that HBT-PVP HBT-PVP films films is is aa short-range short-range face-to-face face-to-face particle particle ordering ordering of of Another unique unique feature feature is is that size 1 μm, regardless of the substrate (Figures 5 and 7). This block of ordering can be formed during size 1 µm, regardless of the substrate (Figures 5 and 7). This block of ordering can be formed during the evaporation a the evaporation of of EtOH EtOH solvent, solvent,and andconsidering consideringboth boththe thethickness thicknessand androughness, roughness,it itshould shouldbebe aplate-like plate-liketwo-dimensional two-dimensionalassembly. assembly. Recently, BT films films composed composed of of ordered ordered cubic-shaped cubic-shaped nanoparticles, nanoparticles, so so called called nanocube nanocube Recently, BT assemblies, have been reported [18,19]. These state-of-the-art nanocube assembly films are fabricated assemblies, have been reported [18,19]. These state-of-the-art nanocube assembly films are fabricated by an anevaporation-induced evaporation-induced self-assembling process, resulting in adegree high of degree of face-to-face by self-assembling process, resulting in a high face-to-face ordering. ordering. However, this process requires a long time for the ordering of nanoparticles. The PVP-
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However, this process requires a long the ordering of nanoparticles. particles coated particles reported herein weretime wellfordispersed in the coating slurry The andPVP-coated led to a short-range reported herein were well dispersed in the coating slurry and led to a short-range particle ordering particle ordering in the film by a similar mechanism, but with much faster evaporation conditions.in the film by a 8similar with much conditions. Figure showsmechanism, the surfacebut profiles of thefaster BT, evaporation LBT-PVP, and HBT-PVP films coated on Si Figure 8 shows the surface profiles of the BT, LBT-PVP, and HBT-PVP films coated on coated Si substrates. substrates. The thickness and roughness of the BT, LBT-PVP, and HBT-PVP films on Si The thickness and roughness of the BT, LBT-PVP, and HBT-PVP films coated on Si substrates are substrates are listed in Table 3. The average surface roughness of the BT, LBT-PVP, and HBT-PVP listed The average of the(on BT,SiLBT-PVP, and are 105, films in areTable 105, 3. 91.6, and 56 nm,surface while roughness the thicknesses substrate) areHBT-PVP 226, 308,films and 283 nm, 91.6, and 56 nm, while the thicknesses (on Si substrate) are 226, 308, and 283 nm, respectively respectively (list in Table 3), which are close to the values measured by FE-SEM (209, 292, and(list 293 in Table 3), which are close to the values measured by FE-SEM (209, 292, and 293 nm, respectively). nm, respectively). The roughness of LBT-PVP is higher than that of HBT-PVP. It demonstrates that The roughness that of LBT-PVP is higher that HBT-PVP. It demonstrates that theindispersant the dispersant decomposed PVP than coated onofHBT-PVP particle is more effective preventingthat the decomposed PVP coated on HBT-PVP particle is more effective in preventing the agglomeration agglomeration of particles during evaporating of EtOH than that PVP adsorbed on LBT-PVP on of Si particles evaporating EtOH thanrepulsion that PVP of adsorbed on LBT-PVP on Si effective substrate. other substrate.during In other words, theof electrostatic decomposed PVP is more toIn prepare words, the electrostatic of decomposed PVP is more effective to prepare the smooth film the smooth film than therepulsion steric effect of PVP. than the steric effect of PVP.
Figure 8. 8. Stylus Stylus profiles profiles of of BT, BT, LBT-PVP, LBT-PVP, and and HBT-PVP HBT-PVP films films coated coated on on Si Si substrates substrates measured measured by by Figure stylus profiler. stylus profiler. Table3.3.Properties Properties of LBT-PVP, BT, LBT-PVP, and HBT-PVP films coated on SiParticle substrates. Particle Table of BT, and HBT-PVP thin filmsthin coated on Si substrates. concentration concentration in the slurry EtOH was coating was 5 wt %roughness). (Ra*: average roughness). in the EtOH coating 5 wtslurry % (Ra*: average
Particles Particles BT BT LBT-PVP HBT-PVP
LBT-PVP
Film (nm) FilmThickness Thickness (nm) SEM SEM 209.4
209.4 292.1
293.2
292.1
Profiler Profiler 226.2
226.2 308.4 283.1
308.4
Ra*(nm) (nm) Ra* 104.6
104.6 91.6 56.1
91.6
293.2 283.1 It is extremely easy toHBT-PVP coat the particle film on the substrates 56.1 with the slurries prepared from BT-PVP particles in a uniform way. The thickness of the film can be controlled by choosing the wire diameter of the bar.toAs explained in the a single stroke ofslurries coatingprepared roughly from deposits It is extremely easy coat the particle filmmethod, on the substrates with the BTaPVP 0.025particles mm thick wet slurry film (from the diameter of winding of 0.5 mm and geometrical calculation). in a uniform way. The thickness of the film can be controlled by choosing the wire An extremely low particle concentration, as 1 awtsingle %, can lead to low surface coverage; however, diameter of the bar. As explained in the such method, stroke of acoating roughly deposits a 0.025 amm defect-free clean layer can(from be prepared from 5ofwt % particle content. Considering the calculation). particle content thick wet slurry film the diameter winding of 0.5 mm and geometrical An 3 , the volume ratio of ~1 vol % corresponds to a dried film thickness of 5 wt % and density of 6.02 g/cm extremely low particle concentration, such as 1 wt %, can lead to a low surface coverage; however, a of 250 nm. clean This estimation well to % theparticle thickness of the observed layer, which content is also defect-free layer can becorresponds prepared from 5 wt content. Considering the particle 3 confirmed from the cross-sectional morphology shown in Figure 7 (data listed in Table 3). of 5 wt % and density of 6.02 g/cm , the volume ratio of ~1 vol % corresponds to a dried film thickness Deposition smooth and defect-freewell thintoBTthe films by the sol–gel method has been previously of 250 nm. This of estimation corresponds thickness of the observed layer, which is also reported and with thicknesses less than 1 µm can be7 (data prepared However, it is confirmed[13], from thefilms cross-sectional morphology shown in Figure listedeasily. in Table 3). Deposition of smooth and defect-free thin BT films by the sol–gel method has been previously reported [13], and films with thicknesses less than 1 μm can be prepared easily. However, it is
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necessary to crystallize the film by heat treatment in high temperature. Nevertheless, BT film is limited to the cubic phase, which does not show dielectric properties and requires a thermal post-treatment. Depending on the process, the sol–gel-deposited films remain in the cubic phase even after heat treatment at 1100 ◦ C [20]. For thick films with above several microns in thickness, films are coated by the screen printing process using BT particles from solid-phase reaction; however, they yield much thicker and coarser films with many defects [28]. It is noteworthy that the coated films can be controlled in a narrow range of 0.1–0.3 µm thickness of the nanoparticles by a single stroke of bar coating in our study. Furthermore, the performance of the film can be controlled easily during the particle synthesis step. Considering industrial applications, the coating of the BT-PVP film on a PET substrate demonstrated in this report has the significant advantage of room-temperature coating without thermal treatment. 3. Materials and Methods Titanium tetrachloride (TiCl4 , 90+) and barium chloride dehydrate (BaCl2 ·2H2 O, 90%) precursors as well as potassium hydroxide (KOH, 85%) mineral agent were purchased from Wako Pure Chemical Industries Ltd., Tokyo, Japan. The dispersant, PVP, with a molecular weight of 10,000 g/mol was obtained from Sigma-Aldrich Corp. (St. Louis, MI, USA). BT particles coated by PVP were prepared following the procedure reported in our previous reports [21,22] by the LTS and HT methods. In the LTS process, the raw material of 0.2 M TiCl4 and BaCl2 , 1.8 M KOH, and 100 g/L PVP were mixed in a vessel, and reacted at 80 ◦ C for 1 h. In the HT process the precursors of 0.2 M TiCl4 and 0.32 M BaCl2 , 2.3 M KOH, and 100 g/L PVP were mixed in a 50-mL Teflon container mixed with 20 mL of distilled water (75 vol %) and ethanol (25 vol %), and heated in the autoclave to 230 ◦ C for 24 h. After completion of reaction, the solution was centrifuged to obtain the precipitates, which were washed and dried at 60 ◦ C for 24 h. For comparison, commercial BaTiO3 (BT) powder (Sigma-Aldrich Corp., St. Louis, MI, USA) was used in this study. BT and BT-PVPs EtOH suspensions were used as coating agents. The coatings were applied to polyethylene terephthalate (PET; Toray, Tokyo, Japan) and Si substrates using a bar coater (Matsuo Sangyo Co. LTD., Osaka, Japan) with a coating speed of about 1.3 m/min. The standard bar RSD20 which its diameter is 6 mm and diameter of winding is 0.5 mm was used to control the thickness of the films. After coating, the films were dried in ambient atmosphere for 5 min and then at 80 ◦ C for 8 min. The powder was characterized by X-ray diffraction (XRD; Smartlab, Rigaku Corp., Tokyo, Japan) using a standard diffractometer with CuKα radiation as an X-ray source in the 2θ-θ scan mode. In order to evaluate the tetragonality of the BT particles, the peak corresponding to the (200) plane was scanned at 0.01◦ /min in the range of 44–46◦ and fitted by the Gaussian function to deconvolute into two tetragonal peaks corresponding to (200) and (002) planes; the c and a lattice parameters were calculated as reported by Kwon et al. [29]. The morphologies of the films were observed by field-emission scanning electron microscopy (FE-SEM: JSM-6335FM, JEOL Ltd., Tokyo, Japan ) In addition, the size distribution of the particles was determined from the FE-SEM images, and the film thickness was measured from the cross-section FE-SEM images. The chemical structure of PVP adsorbed on the BT were evaluated by Fourier transform infrared spectroscopy (FT/IR-610, JASCO Corp., Tokyo, Japan). Dynamic light scattering (DLS; FPAR-10001, Otsuka Electronics Co., Ltd., Osaka, Japan) was used to evaluate the dispersibility and size distribution of the particles in an EtOH solution—a coating slurry containing 1, 5, and 10 wt % powder, respectively. Five sequential DLS measurements were made for each data point. The variations in particle size and distribution measured by SEM and DLS were evaluated by a coefficient variation (CV) method. The surface roughness of the film was measured using a surface profilometer (P-17 stylus profiler, KLA-Tencor Co., Milpitas, CA, USA), which includes the evaluation of film thickness. Transmittance of light by BT-PVP film on PET were characterized using JASCO V-670 ultraviolet–visible (UV–vis) spectrophotometer. The haze value was evaluated by a Nippon Denshoku Industries NDH 5000 haze meter, equipped with a white light emitting diode as the light source. Zeta potentials (ξ) of the synthesized BT–PVP and BT nanoparticles in aqueous
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solution were measured by electrophoretic light scattering (ELS–Z1/Z2, Ostuka Electronics Co. Ltd., Osaka, Japan). 4. Conclusions Monodispersed cubic and tetragonal BaTiO3 (BT) particles coated on its surface by PVP polymer was synthesized by low temperature directly synthesis and hydrothermal method respectively, and was applied for the thin film coating on PET and Si wafer substrates by a simple bar coating and drying process. While the surface of BT film of commercial nanoparticle is coarse and has many defects due to the particle aggregation, smooth and dense film deposition of cubic LBT-PVP and tetragonal HBT-PVP particle with c/a = 1.0005 and 1.0058. The crystalline phase can be simply controlled during the synthesis process. Heat treatment is unnecessary after the film prepared with BT-PVP. The thicknesses of LBT-PVP and HBT-PVP films on Si substrate coated by a single stroke of bar coating process were 308 and 283 nm, and their surface roughnesses were 91.6 and 56.1 nm respectively. The coating of PVP on the surface of BT particle facilitated the formation of ultra-thin film via two-dimensional assembly of particles with thickness corresponding to 2–3 particle stacking. The mechanism of dispersant adsorbed on BT surface for densification of the thin film during the drying process of film is discussed. The optical properties of BT thin film is expected to be transparent in the visible range for the application of optoelectronic devices. The optical transmittances of BT, LBT-PVP, and HBT-PVP films at 550 nm wavelength are 55, 66, and 73%, respectively, and the fall in the absorption edge of LBT-PVP and HB-PVP film is not sharp in our study. The thin coating film using BT-PVP particle in this work could be a potential promising candidate for the ultra-thin multilayer ceramic application, and would be advantageous in the powder electronics if the post-annealing temperature is limited in the process. The high transmittance of BT-PVP film is expected to be used in various electro-optics. Author Contributions: J.L., W.S., and K.I. conceived and designed the experiments; J.L. performed the experiments; all authors including Y.T. and A.T. analyzed and discussed the experimental results. Acknowledgments: We express our sincere thanks to Takamasa Isobe of Noritake Co., Ltd. for supporting this study. We thank Ayako Uozumi, Toshio Itoh, and Takafumi Akamatsu, for their comments and support with the particle preparation. Conflicts of Interest: The authors declare no conflict of interest.
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