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A simple route to prepare anatase TiO2 film onto polyimide substrate by DC pulsed magnetron sputtering
Materials Chemistry and Physics 243 (2020) 122678
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A simple route to prepare anatase TiO2 film onto polyimide substrate by DC pulsed magnetron sputtering Jindong Liu a, *, Xin Zhang b, Yanfei He b, Qing Chen a, Aihua Tian a, Shukun Gan a, Haibo Wang a, Yuanbing Liu b, Wanyu Ding b a b
College of Mechanical and Electrical Engineering, Jilin Institute of Chemical Technology, Jilin, 132022, China College of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, China
H I G H L I G H T S
� The crystallinity of TiO2 film improved with increasing sputtering power density. � Auxiliary heating result in as-deposited TiO2 film with better anatase structure. � The reason for as-deposited TiO2 film crystallization was investigated. A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 film Polyimide Different sputtering power densities An auxiliary heating Crystallization
TiO2 film was deposited onto polyimide substrate by DC pulsed magnetron sputtering with different sputtering power densities. XRD results showed that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the crystallinity of as-deposited TiO2 film improved monotonously. And 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand. Then, XRD and TEM results showed that the as-deposited TiO2 film could have a better anatase structure by providing an auxiliary heating to the substrate at the highest temperature during TiO2 film deposition. Combining the sput tering parameters, as well as DC pulsed voltage waveform, the reason for as-deposited TiO2 film crystallization was investigated systematically.
1. Introduction It was well known that TiO2 film could be applied into the field of photocatalysis and solar cell [1–5]. However, as the small specific sur face area of the film would limit its application as photocatalyst, and the research on solar cell was the current hot spot. Therefore, TiO2 film should be more researched for application in solar cell. Nowadays, TiO2 film played a significant part in the application of electron transport layer (ETL) and hole blocking layer (HBL) for perovskite solar cell, as its advantages in many aspects [6–9]. TiO2 film could be prepared by various techniques, such as physical vapor deposition, chemical vapor deposition, and chemical liquid deposition [1–5,10–21]. Among them, TiO2 films deposited by chemical liquid deposition were mostly loose and porous, it would cause the recombination of photo-generated elec trons and holes in the perovskite solar cell, so that to reduce the pho toelectric conversion efficiency of the solar cell, which were ultimately
limited utilizing as HBL [10–15]. Most of the raw materials utilized by chemical vapor deposition are organic compounds, in addition to the high cost, it would also cause pollution and be harmful to the human body, which also would be limited utilizing in the perovskite solar cell [16,17]. However, TiO2 film deposited by magnetron sputtering tech nique could have the advantages of good adhension and densification, which could be better utilized as ETL and HBL [1–5,8,19]. Generally, as-deposited TiO2 film prepared by magnetron sputtering was basically amorphous structure, if anatase structure was wanted, TiO2 film might need to be annealed (>300 � C), as anatase structure had better electron mobility than other crystal and amorphous structure of TiO2 [1–5, 22–24]. However, this annealing treatment would limit the use of sub strate materials (such as polyimide flexible material-PI), thereby greatly limiting the application of TiO2 in many fields [25]. Therefore, in order to solve this problem, the authors designed the following experiment scheme. Firstly, TiO2 films were deposited by DC pulsed magnetron
* Corresponding author. College of Mechanical and Electrical Engineering, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Longtan District, Jilin, Jilin Province, 132022, China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.matchemphys.2020.122678 Received 5 October 2019; Received in revised form 13 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
J. Liu et al.
Materials Chemistry and Physics 243 (2020) 122678
plasma was carried out to deposit TiO2 films during the sputtering process, the detailed description of this technique had been discussed in our previous publications [26,27]. TiO2 films were deposited onto kapton type PI and Si (100) substrate. The thickness was about 300 � 20 nm for all films. The detailed deposition parameters were listed in Table 1. The crystal structure of TiO2 film was analyzed by a PANalytical Empyrean X-ray diffractometer (XRD) with Cu kα1 radiation (λ ¼ 1.54056 Å) in the mode of Bragg-Brentano film θ/2θ configuration, which the step size and the counting time was 0.02� and 0.5 s, respec tively. The microstructure of TiO2 film was observed by a JEM-2100F field emission transmission microscopy (TEM) system. During Ti target sputtering process, the output waveform of Pinnacle™ Plusþ 5 kW DC pulsed power supply unit was recorded by the home-made shunt circuit with non inductive resistances and Tektronix TDS 2022B oscilloscope.
Table 1 Deposition parameters. Parameter
Value
Ar O2 Sputtering power density Deposition pressure Pulsed frequency Reverse time
20 sccm 5 sccm 083–5.00 W/cm2 0.6 Pa 100 kHz 1.0 μs
3. Result and discussion In our experiment, PI was fixed onto the general glass, and TiO2 films were deposited onto PI substrate. Fig. 1 showed the substrate tempera ture varied with sputtering time in different sputtering power densities. From Fig. 1, it could be seen that when the sputtering time exceeded 20 min, all the substrate temperature tended to be stable. When the sput tering power density was 0.83 W/cm2, the substrate temperature was stabilized at 78 � C. With increasing the sputtering power density, the substrate temperature monotonously increased. When the sputtering power density was 5.00 W/cm2, the substrate temperature was stabi lized at 249 � C. The reason for the increase in substrate temperature could be explained as follows. Firstly, during the sputtering process, when the sputtering atoms/ions/groups bombarded the surface of the substrate to form TiO2 film, the Ti–O bond was an exothermic reaction, and the energy released would cause the substrate temperature to rise. Secondly, with increasing the sputtering power density from 0.83 W/ cm2 to 5.00 W/cm2, the quantity of the sputtering atoms/ions/groups increased by nearly 5.0 times, the energy of the sputtering atoms/ions/ groups increased by nearly 1.6 times, so more sputtering atoms/ions/ groups with higher energy would result in substrate temperature higher (more details in the below). For the substrate temperature measurement result in our experiment, it might have deviations with the actual temperature. For example, in our experiment 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand, the temperature was 249 � C. And in reports, the long-term
Fig. 1. The substrate temperature varied with sputtering time in different sputtering power densities.
sputtering with different sputtering power densities. Then, in order to improve the crystallinity of as-deposited TiO2 film, an auxiliary heating was provided to the substrate during TiO2 film deposition. The aim of our work was to prepare as-deposited TiO2 film with anatase structure onto PI substrate, and systematically investigate the crystallization reason of as-deposited TiO2 film. 2. Experimental In our experiment, high-purity Ar and O2 were used as sputtering gas and reactive gas, respectively. And a metal Ti target in Ar/O2 mixture
Fig. 2. XRD patterns of as-deposited TiO2 films on PI substrate. (a) Without an auxiliary heating. (b) With an auxiliary heating. 2
J. Liu et al.
Materials Chemistry and Physics 243 (2020) 122678
Fig. 3. TEM results of as-deposited TiO2 films. (a) 2.50 W/cm2 without an auxiliary heating. (b) 5.00 W/cm2 without an auxiliary heating. (c) 2.50 W/cm2 with an auxiliary heating. (d) 5.00 W/cm2 with an auxiliary heating. (a1), (a2) and (a3) were Cross-section TEM image, SAED and High-resolution TEM image, respectively. The consist of (b), (c) and (d) is the same to (a).
use temperature for PI was no more than 300 � C. The deviations might be related to the home-made substrate temperature measurement sys tem. But the substrate temperature measurement result was only a unified comparison within the same method, so the impact of the de viations could be ignored. (For the substrate temperature measurement methods, details in Supplementary Materials.) Fig. 2 showed the XRD patterns of all TiO2 films on PI substrate. From Fig. 1(a), it could be found that the XRD patterns of TiO2 film with sputtering power density � 2.50 W/cm2 display no obvious diffraction peak, which indicated the amorphous structure for them. On the con trary, the XRD patterns of TiO2 film with sputtering power density �3.33 W/cm2 exhibited clearly diffraction peaks, which corresponded well with those of standard polycrystal anatase TiO2, respectively [28]. With increasing the sputtering power density from 3.33 W/cm2 to 5.00 W/cm2, the relative intensity and integrality of XRD peaks increased monotonously, it indicated the better crystallinity of film. According to above results, it could be found that the crystallinity of as-deposited TiO2 film improves with the increase of sputtering power density, which corresponded well with our previous reports [27]. It meant that if the better crystallinity was wanted, higher sputtering power density was necessary. However, higher sputtering power density would result in higher substrate temperature, and too high temperature would destroy PI substrate. So 5.00 W/cm2 was determined as the critical sputtering power density in our experiment. In order to improve the crystallinity of TiO2 film, an auxiliary heating (the highest temperature that the substrate withstand in 5.00 W/cm2) was provided to the substrate during TiO2 film deposition. The auxiliary heating was to give the substrate a temperature at 249 � C from the beginning of the deposition, that was to change the normal substrate temperature curve, so that the substrate temperature curve was a straight line. From Fig. 2(b), it could be found that when sputtering power density was 2.50 W/cm2, there was diffraction peak come out, which indicated the better crystallinity comparing with TiO2 film deposited without an auxiliary heating. When sputtering power density was 5.00 W/cm2, the diffraction peak became sharper comparing with TiO2 film deposited without an auxiliary heating, which indicated the better crystallinity too [29].It meant this method can improve the crystallinity of as-deposited TiO2 film effectively, at the same time it will not destroy PI substrate as too high temperature. The similar result can
be obtained from TiO2 films prepared on Si (100) substrate (details in Supplementary Materials). In order to further confirm the above XRD results, TEM measurement was carried out for TiO2 films on Si (100) substrate, as shown in Fig. 3 (as the energy of ion beam milling would destroy PI substrate). From Fig. 3(a1), it could be seen clearly that some nanocrystalline appear in the TEM image, shown in the red circle. Of course, most parts of the TiO2 film were having an amorphous structure, which corresponded well with SAED result in Fig. 3(a2) and XRD result in Fig. 2(a). Then, the nanocrystalline was selected for the high-resolution TEM measurement, as shown in Fig. 3(a3). From Fig. 3(a3), the lattice fringes could be seen clearly and the spacing of lattice fringe was about 0.353 nm, which corresponded well with the interplanar spacing of anatase TiO2 (101) plane. When sputtering power density increased to be 5.00 W/cm2, it could be found that the nanocrystalline belonging to anatase TiO2 become larger and more, as shown in Fig. 3(b1) and (b3). And there were some diffraction spots could be seen in the SAED pattern, as shown in Fig. 3(b2). Combining Fig. 3(a) and (b), one conclusion could be deduced that with increasing the sputtering power density, the crystal linity of TiO2 film improved gradually, which corresponded well with Fig. 2(a). When sputtering power density was 2.50 W/cm2, an auxiliary heating was provided to the substrate during TiO2 film deposition. From Fig. 3(c1) and (c3), it could be found that the nanocrystalline belonging to anatase TiO2 become larger and more too, comparing with TiO2 film deposited (2.50 W/cm2) without an auxiliary heating. And some diffraction spots came out in Fig. 3(c2). Above these, it meant that TiO2 film deposited with an auxiliary heating could have a better crystallinity. When sputtering power density was 5.00 W/cm2, an auxiliary heating was provided to the substrate during TiO2 film deposition. From Fig. 3(d1) and (d3), it could be found that most parts of the film is crystallized, which belonged to the anatase structure. And from Fig. 3 (d2), it could be seen that there are obvious diffraction rings existed [30]. Above these, it meant that TiO2 film deposited (5.00 W/cm2) with an auxiliary heating have the best crystallinity comparing with other TiO2 films, which corresponded well with XRD result. Combining XRD and TEM results, the reason for as-deposited TiO2 film crystallization could be explained as follows. Firstly, for the detailed 3
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Materials Chemistry and Physics 243 (2020) 122678
Fig. 4. (a) DC pulsed voltage waveform with different sputtering power densities. (b) The enlarged image of part closed by violet dash rectangle in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
information about DC pulsed magnetron sputtering process, DC pulsed waveform was recorded, as shown in Fig. 4. From Fig. 4(a), it could be seen that one complete pulsed circle consisted of the positive voltage in 1 μs reverse time and negative voltage in 9 μs sputtering time, which the former had no contribute to the sputtering process. So for one complete pulsed circle, more attention should be focused on the negative voltage in sputtering time. And, the sputtering time could be divided into two parts. One was 3–10 μs stable sputtering period, which was similar to the ideal one. The other was 1–3 μs overload sputtering period, which had some differences from the ideal one, as shown as the violet dash rect angle in Fig. 4(a). From Fig. 4(b), it could be seen that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the Vlowest monotonously increased from 510 V to 702 V, and the mean value over the period monotonically increased from 293 V to 467 V (nearly 1.6 times). So this period was considered to be the voltage that affects the sputtering result [25]. Meanwhile, the sputtering current during the sputtering process was measured to monotonically increase from 0.51 A to 2.52 A (nearly 5.0 times) with the increase of sputtering power density. As we know, higher sputtering voltage can result in sputtering atoms/ions/groups with higher kinetic energy, higher sput tering current can increase the quantity of sputtering atoms/ions/groups [25]. So, higher sputtering power density could result in more sputtering atoms/ions/groups with higher kinetic energy. With other parameters constant, after the atoms/ions/groups bombarded the substrate, part of the energy would be converted into thermal energy to raise substrate temperature, the other part would crystallize TiO2 film. Therefore, TiO2 film deposited with higher sputtering power density had higher crys tallinity and substrate temperature, which had been proven in Figs. 2(a) and Fig. 3(a)–(b). Secondly, providing higher substrate temperature from the begin ning of the deposition could also improve the crystallinity of TiO2 film, which could make the particles on the substrate tend to be arranged in a long-range order, it was more conducive to film crystallization, which had been proven in Figs. 2(b) and Fig. 3(c)–(d). However, only higher substrate temperature was not enough, such as TiO2 film deposited (2.50 W/cm2) with an auxiliary heating, although higher substrate temperature made its crystallinity higher than that of TiO2 film depos ited (2.50 W/cm2) without an auxiliary heating, the crystallinity was still not as well as that of TiO2 film deposited (5.00 W/cm2) without an auxiliary heating, which was shown in Figs. 2 and 3. Therefore, combining the above two aspects, one conclusion could be deduced that as-deposited TiO2 film could have higher crystallinity under the action of more sputtering atoms/ions/groups with higher kinetic energy and higher substrate temperature. Then, TiO2 film with relatively better crystallinity could be prepared onto many flexible materials by our method, such as PI. Specifically, the highest sputtering condition that the substrate could withstand should be determined firstly, measured the substrate temperature at this time, and then providing an auxiliary
heating to substrate at this temperature during the deposition. 4. Conclusions In our experiment, TiO2 film was deposited onto PI substrate by DC pulsed magnetron sputtering with different sputtering power densities. With the increase of sputtering power density, the crystallinity of asdeposited TiO2 film improved. Then, in order to improve the crystal linity of as-deposited TiO2 film, during TiO2 film deposition an auxiliary heating was provided to the substrate at the temperature which the substrate could withstand under the highest sputtering condition. By this way, the as-deposited TiO2 film with anatase structure could be prepared onto many flexible materials, such as PI. The reason for asdeposited TiO2 film crystallization could be explained as the action of more sputtering atoms/ions/groups with higher kinetic energy and higher substrate temperature. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51772038), Major Scientific Research Project of Jilin Institute of Chemical Technology (No. 2018013). One of the authors (Dr. Jindong Liu) thanks Dalian Jiaotong University for experiment supporting. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122678. References � _ E. Navickas, D. Mil�cius, C. Laukaitis, Titanium oxide thin films [1] J. Cyvien e, synthesis by pulsed-DC magnetron sputtering, Vacuum 83 (2009) S91–S94. [2] Y. He, X. Zhang, Y. Liu, H. Liu, Y. Zhang, G. Li, Z. Wang, Y. Lin, H. Wang, H. Tao, C. Liu, W. Jiang, N. Wang, S. Liu, Y. Cui, W. Ding, J. Liu, The relationship between carrier mobility and bicrystalline grains boundary of rutile TiO2 film, Mod. Phys. Lett. B 197 (2019), 1950176. [3] S. Haque, P. Sagdeo, A. Sagdeo, S. Jha, D. Bhattacharyya, N. Sahoo, Effect of oxygen partial pressure on properties of asymmetric bipolar pulse dc magnetron sputtered TiO2 thin films, Appl. Opt. 54 (2015) 3817–3825. � [4] J. Sícha, D. He�rman, J. Musil, Z. Strýhal, J. Pavlík, High-rate low-temperature dc pulsed magnetron sputtering of photocatalytic TiO2 films: the effect of repetition frequency, Nanoscale. Res. Lett. 2 (2007) 123–129. [5] S. Takeda, S. Suzuki, H. Odaka, H. Hosono, Photocatalytic TiO2 thin film deposited onto glass by DC magnetron sputtering, Thin. Solid. Films 392 (2001) 338–344.
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5
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
A simple route to prepare anatase TiO2 film onto polyimide substrate by DC pulsed magnetron sputtering Jindong Liu a, *, Xin Zhang b, Yanfei He b, Qing Chen a, Aihua Tian a, Shukun Gan a, Haibo Wang a, Yuanbing Liu b, Wanyu Ding b a b
College of Mechanical and Electrical Engineering, Jilin Institute of Chemical Technology, Jilin, 132022, China College of Materials Science and Engineering, Dalian Jiaotong University, Dalian, 116028, China
H I G H L I G H T S
� The crystallinity of TiO2 film improved with increasing sputtering power density. � Auxiliary heating result in as-deposited TiO2 film with better anatase structure. � The reason for as-deposited TiO2 film crystallization was investigated. A R T I C L E I N F O
A B S T R A C T
Keywords: TiO2 film Polyimide Different sputtering power densities An auxiliary heating Crystallization
TiO2 film was deposited onto polyimide substrate by DC pulsed magnetron sputtering with different sputtering power densities. XRD results showed that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the crystallinity of as-deposited TiO2 film improved monotonously. And 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand. Then, XRD and TEM results showed that the as-deposited TiO2 film could have a better anatase structure by providing an auxiliary heating to the substrate at the highest temperature during TiO2 film deposition. Combining the sput tering parameters, as well as DC pulsed voltage waveform, the reason for as-deposited TiO2 film crystallization was investigated systematically.
1. Introduction It was well known that TiO2 film could be applied into the field of photocatalysis and solar cell [1–5]. However, as the small specific sur face area of the film would limit its application as photocatalyst, and the research on solar cell was the current hot spot. Therefore, TiO2 film should be more researched for application in solar cell. Nowadays, TiO2 film played a significant part in the application of electron transport layer (ETL) and hole blocking layer (HBL) for perovskite solar cell, as its advantages in many aspects [6–9]. TiO2 film could be prepared by various techniques, such as physical vapor deposition, chemical vapor deposition, and chemical liquid deposition [1–5,10–21]. Among them, TiO2 films deposited by chemical liquid deposition were mostly loose and porous, it would cause the recombination of photo-generated elec trons and holes in the perovskite solar cell, so that to reduce the pho toelectric conversion efficiency of the solar cell, which were ultimately
limited utilizing as HBL [10–15]. Most of the raw materials utilized by chemical vapor deposition are organic compounds, in addition to the high cost, it would also cause pollution and be harmful to the human body, which also would be limited utilizing in the perovskite solar cell [16,17]. However, TiO2 film deposited by magnetron sputtering tech nique could have the advantages of good adhension and densification, which could be better utilized as ETL and HBL [1–5,8,19]. Generally, as-deposited TiO2 film prepared by magnetron sputtering was basically amorphous structure, if anatase structure was wanted, TiO2 film might need to be annealed (>300 � C), as anatase structure had better electron mobility than other crystal and amorphous structure of TiO2 [1–5, 22–24]. However, this annealing treatment would limit the use of sub strate materials (such as polyimide flexible material-PI), thereby greatly limiting the application of TiO2 in many fields [25]. Therefore, in order to solve this problem, the authors designed the following experiment scheme. Firstly, TiO2 films were deposited by DC pulsed magnetron
* Corresponding author. College of Mechanical and Electrical Engineering, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Longtan District, Jilin, Jilin Province, 132022, China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.matchemphys.2020.122678 Received 5 October 2019; Received in revised form 13 January 2020; Accepted 17 January 2020 Available online 18 January 2020 0254-0584/© 2020 Elsevier B.V. All rights reserved.
J. Liu et al.
Materials Chemistry and Physics 243 (2020) 122678
plasma was carried out to deposit TiO2 films during the sputtering process, the detailed description of this technique had been discussed in our previous publications [26,27]. TiO2 films were deposited onto kapton type PI and Si (100) substrate. The thickness was about 300 � 20 nm for all films. The detailed deposition parameters were listed in Table 1. The crystal structure of TiO2 film was analyzed by a PANalytical Empyrean X-ray diffractometer (XRD) with Cu kα1 radiation (λ ¼ 1.54056 Å) in the mode of Bragg-Brentano film θ/2θ configuration, which the step size and the counting time was 0.02� and 0.5 s, respec tively. The microstructure of TiO2 film was observed by a JEM-2100F field emission transmission microscopy (TEM) system. During Ti target sputtering process, the output waveform of Pinnacle™ Plusþ 5 kW DC pulsed power supply unit was recorded by the home-made shunt circuit with non inductive resistances and Tektronix TDS 2022B oscilloscope.
Table 1 Deposition parameters. Parameter
Value
Ar O2 Sputtering power density Deposition pressure Pulsed frequency Reverse time
20 sccm 5 sccm 083–5.00 W/cm2 0.6 Pa 100 kHz 1.0 μs
3. Result and discussion In our experiment, PI was fixed onto the general glass, and TiO2 films were deposited onto PI substrate. Fig. 1 showed the substrate tempera ture varied with sputtering time in different sputtering power densities. From Fig. 1, it could be seen that when the sputtering time exceeded 20 min, all the substrate temperature tended to be stable. When the sput tering power density was 0.83 W/cm2, the substrate temperature was stabilized at 78 � C. With increasing the sputtering power density, the substrate temperature monotonously increased. When the sputtering power density was 5.00 W/cm2, the substrate temperature was stabi lized at 249 � C. The reason for the increase in substrate temperature could be explained as follows. Firstly, during the sputtering process, when the sputtering atoms/ions/groups bombarded the surface of the substrate to form TiO2 film, the Ti–O bond was an exothermic reaction, and the energy released would cause the substrate temperature to rise. Secondly, with increasing the sputtering power density from 0.83 W/ cm2 to 5.00 W/cm2, the quantity of the sputtering atoms/ions/groups increased by nearly 5.0 times, the energy of the sputtering atoms/ions/ groups increased by nearly 1.6 times, so more sputtering atoms/ions/ groups with higher energy would result in substrate temperature higher (more details in the below). For the substrate temperature measurement result in our experiment, it might have deviations with the actual temperature. For example, in our experiment 5.00 W/cm2 was determined as the critical sputtering power density in which the highest temperature the substrate could withstand, the temperature was 249 � C. And in reports, the long-term
Fig. 1. The substrate temperature varied with sputtering time in different sputtering power densities.
sputtering with different sputtering power densities. Then, in order to improve the crystallinity of as-deposited TiO2 film, an auxiliary heating was provided to the substrate during TiO2 film deposition. The aim of our work was to prepare as-deposited TiO2 film with anatase structure onto PI substrate, and systematically investigate the crystallization reason of as-deposited TiO2 film. 2. Experimental In our experiment, high-purity Ar and O2 were used as sputtering gas and reactive gas, respectively. And a metal Ti target in Ar/O2 mixture
Fig. 2. XRD patterns of as-deposited TiO2 films on PI substrate. (a) Without an auxiliary heating. (b) With an auxiliary heating. 2
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Fig. 3. TEM results of as-deposited TiO2 films. (a) 2.50 W/cm2 without an auxiliary heating. (b) 5.00 W/cm2 without an auxiliary heating. (c) 2.50 W/cm2 with an auxiliary heating. (d) 5.00 W/cm2 with an auxiliary heating. (a1), (a2) and (a3) were Cross-section TEM image, SAED and High-resolution TEM image, respectively. The consist of (b), (c) and (d) is the same to (a).
use temperature for PI was no more than 300 � C. The deviations might be related to the home-made substrate temperature measurement sys tem. But the substrate temperature measurement result was only a unified comparison within the same method, so the impact of the de viations could be ignored. (For the substrate temperature measurement methods, details in Supplementary Materials.) Fig. 2 showed the XRD patterns of all TiO2 films on PI substrate. From Fig. 1(a), it could be found that the XRD patterns of TiO2 film with sputtering power density � 2.50 W/cm2 display no obvious diffraction peak, which indicated the amorphous structure for them. On the con trary, the XRD patterns of TiO2 film with sputtering power density �3.33 W/cm2 exhibited clearly diffraction peaks, which corresponded well with those of standard polycrystal anatase TiO2, respectively [28]. With increasing the sputtering power density from 3.33 W/cm2 to 5.00 W/cm2, the relative intensity and integrality of XRD peaks increased monotonously, it indicated the better crystallinity of film. According to above results, it could be found that the crystallinity of as-deposited TiO2 film improves with the increase of sputtering power density, which corresponded well with our previous reports [27]. It meant that if the better crystallinity was wanted, higher sputtering power density was necessary. However, higher sputtering power density would result in higher substrate temperature, and too high temperature would destroy PI substrate. So 5.00 W/cm2 was determined as the critical sputtering power density in our experiment. In order to improve the crystallinity of TiO2 film, an auxiliary heating (the highest temperature that the substrate withstand in 5.00 W/cm2) was provided to the substrate during TiO2 film deposition. The auxiliary heating was to give the substrate a temperature at 249 � C from the beginning of the deposition, that was to change the normal substrate temperature curve, so that the substrate temperature curve was a straight line. From Fig. 2(b), it could be found that when sputtering power density was 2.50 W/cm2, there was diffraction peak come out, which indicated the better crystallinity comparing with TiO2 film deposited without an auxiliary heating. When sputtering power density was 5.00 W/cm2, the diffraction peak became sharper comparing with TiO2 film deposited without an auxiliary heating, which indicated the better crystallinity too [29].It meant this method can improve the crystallinity of as-deposited TiO2 film effectively, at the same time it will not destroy PI substrate as too high temperature. The similar result can
be obtained from TiO2 films prepared on Si (100) substrate (details in Supplementary Materials). In order to further confirm the above XRD results, TEM measurement was carried out for TiO2 films on Si (100) substrate, as shown in Fig. 3 (as the energy of ion beam milling would destroy PI substrate). From Fig. 3(a1), it could be seen clearly that some nanocrystalline appear in the TEM image, shown in the red circle. Of course, most parts of the TiO2 film were having an amorphous structure, which corresponded well with SAED result in Fig. 3(a2) and XRD result in Fig. 2(a). Then, the nanocrystalline was selected for the high-resolution TEM measurement, as shown in Fig. 3(a3). From Fig. 3(a3), the lattice fringes could be seen clearly and the spacing of lattice fringe was about 0.353 nm, which corresponded well with the interplanar spacing of anatase TiO2 (101) plane. When sputtering power density increased to be 5.00 W/cm2, it could be found that the nanocrystalline belonging to anatase TiO2 become larger and more, as shown in Fig. 3(b1) and (b3). And there were some diffraction spots could be seen in the SAED pattern, as shown in Fig. 3(b2). Combining Fig. 3(a) and (b), one conclusion could be deduced that with increasing the sputtering power density, the crystal linity of TiO2 film improved gradually, which corresponded well with Fig. 2(a). When sputtering power density was 2.50 W/cm2, an auxiliary heating was provided to the substrate during TiO2 film deposition. From Fig. 3(c1) and (c3), it could be found that the nanocrystalline belonging to anatase TiO2 become larger and more too, comparing with TiO2 film deposited (2.50 W/cm2) without an auxiliary heating. And some diffraction spots came out in Fig. 3(c2). Above these, it meant that TiO2 film deposited with an auxiliary heating could have a better crystallinity. When sputtering power density was 5.00 W/cm2, an auxiliary heating was provided to the substrate during TiO2 film deposition. From Fig. 3(d1) and (d3), it could be found that most parts of the film is crystallized, which belonged to the anatase structure. And from Fig. 3 (d2), it could be seen that there are obvious diffraction rings existed [30]. Above these, it meant that TiO2 film deposited (5.00 W/cm2) with an auxiliary heating have the best crystallinity comparing with other TiO2 films, which corresponded well with XRD result. Combining XRD and TEM results, the reason for as-deposited TiO2 film crystallization could be explained as follows. Firstly, for the detailed 3
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Fig. 4. (a) DC pulsed voltage waveform with different sputtering power densities. (b) The enlarged image of part closed by violet dash rectangle in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
information about DC pulsed magnetron sputtering process, DC pulsed waveform was recorded, as shown in Fig. 4. From Fig. 4(a), it could be seen that one complete pulsed circle consisted of the positive voltage in 1 μs reverse time and negative voltage in 9 μs sputtering time, which the former had no contribute to the sputtering process. So for one complete pulsed circle, more attention should be focused on the negative voltage in sputtering time. And, the sputtering time could be divided into two parts. One was 3–10 μs stable sputtering period, which was similar to the ideal one. The other was 1–3 μs overload sputtering period, which had some differences from the ideal one, as shown as the violet dash rect angle in Fig. 4(a). From Fig. 4(b), it could be seen that with increasing the sputtering power density from 0.83 W/cm2 to 5.00 W/cm2, the Vlowest monotonously increased from 510 V to 702 V, and the mean value over the period monotonically increased from 293 V to 467 V (nearly 1.6 times). So this period was considered to be the voltage that affects the sputtering result [25]. Meanwhile, the sputtering current during the sputtering process was measured to monotonically increase from 0.51 A to 2.52 A (nearly 5.0 times) with the increase of sputtering power density. As we know, higher sputtering voltage can result in sputtering atoms/ions/groups with higher kinetic energy, higher sput tering current can increase the quantity of sputtering atoms/ions/groups [25]. So, higher sputtering power density could result in more sputtering atoms/ions/groups with higher kinetic energy. With other parameters constant, after the atoms/ions/groups bombarded the substrate, part of the energy would be converted into thermal energy to raise substrate temperature, the other part would crystallize TiO2 film. Therefore, TiO2 film deposited with higher sputtering power density had higher crys tallinity and substrate temperature, which had been proven in Figs. 2(a) and Fig. 3(a)–(b). Secondly, providing higher substrate temperature from the begin ning of the deposition could also improve the crystallinity of TiO2 film, which could make the particles on the substrate tend to be arranged in a long-range order, it was more conducive to film crystallization, which had been proven in Figs. 2(b) and Fig. 3(c)–(d). However, only higher substrate temperature was not enough, such as TiO2 film deposited (2.50 W/cm2) with an auxiliary heating, although higher substrate temperature made its crystallinity higher than that of TiO2 film depos ited (2.50 W/cm2) without an auxiliary heating, the crystallinity was still not as well as that of TiO2 film deposited (5.00 W/cm2) without an auxiliary heating, which was shown in Figs. 2 and 3. Therefore, combining the above two aspects, one conclusion could be deduced that as-deposited TiO2 film could have higher crystallinity under the action of more sputtering atoms/ions/groups with higher kinetic energy and higher substrate temperature. Then, TiO2 film with relatively better crystallinity could be prepared onto many flexible materials by our method, such as PI. Specifically, the highest sputtering condition that the substrate could withstand should be determined firstly, measured the substrate temperature at this time, and then providing an auxiliary
heating to substrate at this temperature during the deposition. 4. Conclusions In our experiment, TiO2 film was deposited onto PI substrate by DC pulsed magnetron sputtering with different sputtering power densities. With the increase of sputtering power density, the crystallinity of asdeposited TiO2 film improved. Then, in order to improve the crystal linity of as-deposited TiO2 film, during TiO2 film deposition an auxiliary heating was provided to the substrate at the temperature which the substrate could withstand under the highest sputtering condition. By this way, the as-deposited TiO2 film with anatase structure could be prepared onto many flexible materials, such as PI. The reason for asdeposited TiO2 film crystallization could be explained as the action of more sputtering atoms/ions/groups with higher kinetic energy and higher substrate temperature. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51772038), Major Scientific Research Project of Jilin Institute of Chemical Technology (No. 2018013). One of the authors (Dr. Jindong Liu) thanks Dalian Jiaotong University for experiment supporting. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2020.122678. References � _ E. Navickas, D. Mil�cius, C. Laukaitis, Titanium oxide thin films [1] J. Cyvien e, synthesis by pulsed-DC magnetron sputtering, Vacuum 83 (2009) S91–S94. [2] Y. He, X. Zhang, Y. Liu, H. Liu, Y. Zhang, G. Li, Z. Wang, Y. Lin, H. Wang, H. Tao, C. Liu, W. Jiang, N. Wang, S. Liu, Y. Cui, W. Ding, J. Liu, The relationship between carrier mobility and bicrystalline grains boundary of rutile TiO2 film, Mod. Phys. Lett. B 197 (2019), 1950176. [3] S. Haque, P. Sagdeo, A. Sagdeo, S. Jha, D. Bhattacharyya, N. Sahoo, Effect of oxygen partial pressure on properties of asymmetric bipolar pulse dc magnetron sputtered TiO2 thin films, Appl. Opt. 54 (2015) 3817–3825. � [4] J. Sícha, D. He�rman, J. Musil, Z. Strýhal, J. Pavlík, High-rate low-temperature dc pulsed magnetron sputtering of photocatalytic TiO2 films: the effect of repetition frequency, Nanoscale. Res. Lett. 2 (2007) 123–129. [5] S. Takeda, S. Suzuki, H. Odaka, H. Hosono, Photocatalytic TiO2 thin film deposited onto glass by DC magnetron sputtering, Thin. Solid. Films 392 (2001) 338–344.
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