Matteo
Porta
a,
Mai Thanh
Nguyen
a,
Tetsu
Yonezawa
*a,
Tomoharu
Tokunaga
b,
Yohei
Ishida
a,
Hiroki
Tsukamoto
a,
Yuichi
Shishino
c and
Yoshikiyo
Hatakeyama‡
d
aDivision of Materials Science and Engineering, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan. E-mail: tetsu@eng.hokudai.ac.jp
bDepartment of Quantum Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8603, Japan
cTokai Optical, Co., Ltd., 5-26 Shimoda, Eta-cho, Okazaki, Aichi 444-2192, Japan
dDivision of Nanoscience, Graduate School of Advanced Integration Science, Chiba University, 1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
First published on 16th September 2016
A transparent resin containing titanium oxide nanoparticles (NPs) was prepared using a molten matrix sputtering (MMS) technique. The low vapour pressure of the liquid, pentaerythritol ethoxylate (PEEL) substrate permits the use of this vacuum technique directly with liquid PEEL under stirring conditions in order to obtain uniform dispersions of NPs. We found that it is possible to synthesize titanium oxide, TiOx, NPs with diameters of less than 5 nm with a controlled composition by simply adjusting the sputtering atmosphere. Furthermore, as the electronic structure of the TiOx NPs changes depending on the particle size, crystallinity and degree of oxidation, we were able to modify the optical properties of PEEL and the resin by embedding TiOx NPs in the matrix. The enhancement of the refractive index of a resin containing TiO2 NPs was also demonstrated. This synthetic method is promising for the advanced preparation of high purity TiOx NPs without using a reducing agent and leaving by-products for various applications in optical devices, energy conversion, and light harvesting in the UV visible region.
Titanium dioxide, TiO2, NPs are important wide-bandgap materials used as UV-active photo-catalysts and various optical applications such as a UV block material in sunscreens, or to increase the refractive index of thin films7,26 and polymers.7,9 Modification of the electronic structure of TiO2via doping or introducing oxygen defects can narrow the bandgap and extend its energy harvesting applications towards the visible region.28 In addition, one stoichiometric oxide of titanium, TiO, with strong visible-light absorption can also be used for this purpose.28 The preparation of titanium oxides with control of its oxidation state offers a facile way to modify the optical properties of hybrid materials that contain titanium oxide NPs. Despite the fact that conventional chemical synthesis can produce TiO2 NPs, to the best of our knowledge very small (1–10 nm in diameter) and clean TiO2 NPs dispersed in polymers for use in advanced optical applications have not yet been reported. It is still a challenge to use chemical methods for synthesizing titanium oxides with controlled composition since the decomposition of titanium complexes can result in various undesired TiO2 phases. Finally, previously reported fabrications of hybrid films or polymers containing titanium or titanium oxide NPs featured multi-step processes that are complicated and often produce products with a greater presence of contaminations.4,7–9,27,29 With its many advantages over more traditional chemical synthesis in terms of purity and composition control with regards to the oxidation state, we posit that direct sputtering of Ti under controlled atmospheric conditions could be a valuable technique for the synthesis of titanium oxides. The composition of the sputtered material is strongly related to the atmosphere in the sputtering chamber, as reported by Dreesen et al.30 By controlling the oxygen flow rate into the sputtering chamber, they were able to sputter a film of metallic titanium and its oxides with low oxidation states when the O2 flow was below a certain threshold, but they otherwise sputtered titanium dioxide. This threshold was indicated by a change in the cathode voltage, which was constant at a high oxygen flow rate but diminished as the oxygen flow rate was decreased. This result suggests that the cathode voltage can be used as an indirect but a quantitative indicator for the atmosphere inside the sputtering chamber: a high oxygen atmosphere corresponds to the formation of TiO2, and the low oxygen one corresponds to the formation of titanium and titanium oxides with a lower oxidation state. In the present study, we introduced only Ar into the sputtering chamber after pump-purging, leading to a reduced oxygen content in the atmosphere inside the chamber with prolonged sputtering time. Based on monitoring the cathode voltage, we sputtered titanium in a high or low oxygen atmosphere and investigate the structural and optical properties of the obtained titanium oxide NPs dispersed in PEEL. We demonstrate the capability of our method to tailor the composition of titanium oxide NPs for the preparation of TiO NPs in PEEL with strong visible light absorption and nanocomposite resins containing monodispersed TiO2 NPs with an enhanced refractive index and high transparency.
During sputtering, Ar gas was introduced into the chamber in order to change the sputtering atmosphere. With sputtering time, the sputtering atmosphere varied as indicated by a change in the cathode voltage (Fig. 1) from 310 to 265 V, which corresponds to the colour of the plasma changing from blue to green. Hereafter, we use the colour (“blue” and “green”) of plasma to refer to the sputtering mode.
Fig. 1 Typical cathode voltage curve that accompanies the change between the two sputtering modes: blue-plasma sputtering on the left, and green-plasma sputtering on the right. |
The total sputtering time was modified for each sample from 1 to 3 h (divided in intervals in order to avoid extreme overheating of the sputtering target and to select the sputtering plasma conditions), with a sputtering current of 400 mA and the atmosphere inside the sputtering chamber was varied in order to control the oxidation states of the synthesized titanium oxide NPs. More detailed reaction conditions are summarized in Table 1.
The formation of TiO2 and TiO under different sputtering atmospheres was confirmed by XRD measurements for the NP thin film produced under green and blue-plasma sputtering conditions (Fig. 2). The background subtraction with the XRD pattern of the glass substrate was performed for blue-sputtering samples to aid the analysis, and the raw patterns before subtraction are given in Fig. S2 (ESI†). Green-plasma sputtering produced a neat TiO film (the (111), (200), (220) and (222) crystal planes of TiO in the order from left to right, as shown in Fig. 2a), whereas under blue-plasma sputtering, while peaks corresponding to the TiO phase and possibly some rutile TiO2 were detected (Fig. 2b), they have significantly weaker intensities to the high background (Fig. S2, ESI†) with large XRD line broadening. This indicates that the TiO2 NPs generated via blue-plasma sputtering are either too small or amorphous in nature, as often reported for room temperature TiO2 formation.30,32,33 The blue-plasma sputtering rate (16 ± 10 nm min−1) was lower than the green-plasma sputtering rate (70 ± 10 nm min−1) as measured from the thickness of the oxide films formed under corresponding conditions (Fig. 3). In fact, after annealing at 500 °C for 2 h under N2, in addition to the XRD peaks of TiO, signals related to TiO2 are evident at 25.2° and 54.8° for the anatase phase and peaks at 27.4°, 36.1°, and 54.3° in 2θ for the rutile phase (Fig. 2c), as a result of either the growth of small crystalline NPs or the crystallization of the amorphous NPs. As a gradual change from blue to green-plasma sputtering occurred, some intermediate TiOx (1 < x < 2) NPs could be expected in samples that underwent blue-plasma sputtering. In order to simplify our analysis, we assume two distinct sputtering modes corresponding to blue- and green-plasma sputtering.
Fig. 2 XRD patterns of samples obtained via (a) green-plasma sputtering; (b) blue-plasma sputtering; (c) corresponds to sample (b) after annealing at 500 °C for 2 h under N2. TiO (JCPDS: 00-008-0117) is denoted by solid diamond (◆). Tetragonal anatase TiO2 (JCPDS: 01-071-1168) and rutile TiO2 (JCPDS: 00-021-1276) are noted with open circles (○) and triangles (△), respectively. (b) and (c) were obtained after background subtraction with the XRD pattern of the glass substrate (see Fig. S2 for the raw patterns, ESI†). |
Fig. 3 SEM images of the cross-section of the thin films obtained via (a) blue-, and (b) green-plasma sputtering for 5 min. |
Blue-plasma sputtered samples demonstrate high transparency with respect to visible light, and strong absorbance in the UV region when compared to pure PEEL, which does not show high absorbance in that region (Fig. 4a and b). Absorption in the UV region is strong in all the blue-plasma sputtering related samples, an expected behaviour for composites containing titanium oxide NPs.34 In the case of samples that underwent some green-plasma sputtering, a broad absorption peak centred at 520 nm appears in the visible region (e.g. samples 3 and 4 as shown in Fig. 3a). A similar phenomenon is also observed for samples 5 and 6 that underwent 30 min blue-plasma sputtering and different green-plasma sputtering times (Fig. S3, ESI†). It is noted that as the green-plasma sputtering time increases, the absorbance intensity centred at 520 nm increases and the samples become noticeably darker (Fig. 4c and d). The absorption band centred at 520 nm is assigned to TiO NPs, as reported elsewhere.28 Furthermore, the red shift in UV-Vis absorption (Fig. 4a and Fig. S3, ESI†) as a function of green-plasma sputtering time (for samples obtained in mixed blue- and green-plasma sputtering) is an indication that these samples are dispersions of mixed TiO2, TiO and TiOx (1 < x < 2). These results can be explained due to the gradual lack of oxygen in the chamber as the plasma atmosphere changes, resulting in the sputtering mode shift from blue to green. As a result, green-plasma sputtering did not allow for the complete oxidation of metallic titanium into its most stable oxide (TiO2), instead forming monoxide or TiOx. This finding is consistent with the XRD results discussed above. The visible absorption band obtained from the sample containing TiO NPs under green-plasma sputtering conditions allows the use of titanium oxides for energy conversion and harvesting in not only UV light, but also in the visible spectrum.
In order to obtain a more precise size measurement of the NPs dispersed in liquid PEEL and a rough estimation of the particle size evolution for long time sputtering, we performed measurements using SAXS. The results shown in Fig. 6 were obtained from liquid samples 7 and 8 produced by sputtering of titanium for 90 and 180 min under green-plasma conditions. Long sputtering time and green-plasma sputtering were chosen, as they are known to have a higher sputtering rate than blue-plasma sputtering.35,36 This experiment allowed us to get information about the maximal particle size in our samples. It can be noted that as the sputtering time is increased, the particle size increased slightly from 3.8 ± 0.6 nm (sample 7) to 4.4 ± 0.6 nm (sample 8), close to the size of the observed particles sputtered directly onto a carbon-coated TEM grid (4.6 ± 0.7 nm). This result strongly indicates that the titanium oxide NPs dispersed in PEEL were less than 5 nm in diameter, without aggregation during synthesis.
In summary, controlling the atmosphere of the sputtering chamber via monitoring the cathode voltage and plasma colour allows the control of the degree of oxidation for the obtained TiOx NPs dispersed in PEEL. In all samples, uniform and well dispersed TiOx NPs of less than 5 nm were achieved. The main product under blue-plasma sputtering conditions was TiO2 (some oxygen deficient TiO2 particles were also obtained), which does not affect the transparency of the NP-containing PEEL, while the green-plasma sputtering conditions yielded primarily TiO particles, which cause the NP dispersions to become coloured with high visible absorption.
Fig. 7 (a) Photograph of the resins, with green sputtering time decreasing from left (120 minutes) to right (0 min), and pictures of resins made from (b) pure PEEL and from (c) sample 1. |
Because the addition of high refractive index particles, e.g. TiO2, into low refractive index polymers can improve the refractive index of the composite (eqn (SE2), ESI†), we investigate the refractive index of various resins made using titanium oxide NPs dispersed in PEEL. The results (Fig. 8) show the effect of titanium oxide NPs on the refractive index of the resin. Each dot represents the difference between the refractive indices of a NP-embedded urethane resin sample and of the pure resin. A positive value indicates an increase of the refractive index. The resin made from pure PEEL has a refractive index of 1.5586 ± 0.0005, which increased to 1.5708 ± 0.0005 after 60 min of blue-plasma sputtering under optimal conditions (sample 1), corresponding to an increase in the refractive index of 0.0122 ± 0.0005. Small increases on this scale were observed in all samples in relation to blue-plasma sputtering time. The increase in the refractive index is, at a constant sputtering rate, proportional to the blue-plasma sputtering time, while inversely proportional to the green-plasma sputtering time. Furthermore, this increment-to-TiO2 ratio was in agreement with previously published results pertaining to other hybrid materials containing titanium dioxide.9,27
As an extreme case, a sample that underwent only green-plasma sputtering (120 min, sample 4) showed a negative change in the refractive index compared to that of the pure resin. The results for mixed sputtering modes show how the blue and green-plasma sputtering compete against each other in terms of increasing the refractive index of the composite materials. In particular, the refractive index measurements clearly demonstrate the importance of the TiO2 formation process, as only blue-plasma sputtering can produce high index NPs. This demonstrates that the conditions for increasing the refractive index of low vapour pressure compounds involve TiO2 formation during blue-plasma sputtering.
Under blue-plasma sputtering conditions, the NPs are formed by etching the TiO2 thin film that continuously regenerates on the titanium plate surface. On the other hand, when there is little oxygen in the atmosphere, atomic clusters of titanium and some titanium monoxide NPs get detached, which are subsequently oxidized to form a uniform TiO film. For each sample the Abbe number was above 40; this value corresponds to materials with low light dispersion characteristics (eqn (SE3), ESI†), making the samples suitable for optical applications.
As the degree of interaction between the PEEL and the NPs is low, this technique can theoretically be applied to any low vapour pressure monomer and obtain similar results. The dispersion of NPs in PEEL is enough stable (for several days) before polymerization to form resins (Fig. S5, ESI†). The effect of the kind of monomers on the dispersion and stability of NPs can be considered more in depth in future studies. The amount of titanium oxide inserted into the matrix was calculated to be approximately 1.4 vol% of the nanocomposite. Considering the low loading amount of titanium oxide NPs, the photodegradation of organic moieties is seemingly negligible.27
The low sputtering rate of our device affected the amount of titanium that could be sputtered, therefore, this limited the refractive index increase. By using higher sputtering rate devices this deficiency can be overcome, allowing the production of transparent resins featuring higher concentrations of TiO2 NPs.
Footnotes |
† Electronic supplementary information (ESI) available: Schematic illustration of the magnetron sputtering device; XRD patterns of the glass substrate and blue-plasma sputtered thin films before and after annealing, UV-Vis spectra of PEEL and the 30 minute blue-plasma sputtered samples; the TEM image of a sample sputtered under blue-plasma conditions; UV-Vis spectra for the as-synthesized blue-plasma sputtered sample and for the same sample after 3 days. Equations for the approximation of the refractive index of composite compounds, scattering probability of a photon passing through a nanoparticle, and the Abbe number. See DOI: 10.1039/c6nj01624c |
‡ Present address: Graduate School of Science and Technology, Gunma University, Tenjin-cho, Kiryu, Gunma 376-8516, Japan. |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2016 |