Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi

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Oct 6, 2018 - Generally, TiO2 in particulate form possesses ... photocatalyst of TiO2 powder called P-25 (Degussa) has been ..... Corning E2000 glass sheet with dimension of 3 × 2.5 cm2 was used as the substrate for the .... Wei, N.; Cui, H.Z.; Song, Q.; Zhang, L.Q.; Song, X.J.; Wang, K.; Zhang, Y.F.; Li, J.; Wen, J.; Tian, J.
catalysts Article

Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique Hsyi-En Cheng 1 , Chi-Hsiu Hung 2 , Ing-Song Yu 3 and Zu-Po Yang 2, * 1 2 3

*

Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan 710, Taiwan; [email protected] Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan; [email protected] Department of Materials Science and Engineering, National Dong Hwa University, Hualien 97401, Taiwan; [email protected] Correspondence: [email protected]; Tel.: +886-6-3032121 (ext. 57762)

Received: 13 September 2018; Accepted: 3 October 2018; Published: 6 October 2018

 

Abstract: The photocatalysts of immobilized TiO2 film suffer from high carrier recombination loss when compared to its powder form. Although the TiO2 with rutile-anatase mixed phases has higher carrier separation efficiency than those with pure anatase or rutile phase, the single junction of anatase/rutile cannot avoid the recombination of separated carriers at the interface. In this study, we propose a TiO2 /SnO2 /Ni multi-heterojunction structure which incorporates both Schottky contact and staggered band alignment to reduce the carrier recombination loss. The low carrier recombination rate of TiO2 film in TiO2 /SnO2 /Ni multi-heterojunction structure was verified by its low photoluminescence intensity. The faster degradation of methylene blue for TiO2 /SnO2 /Ni multi-junctions than for the other fabricated structures, which means that the TiO2 films grown on the SnO2 /Ni/Ti coated glass have a much higher photocatalytic activity than those grown on the blank glass, SnO2 -coated and Ni/Ti-coated glasses, demonstrated its higher performance of photogenerated carrier separation. Keywords: multijunction; titanium dioxide; thin film; photocatalysis; functional coatings

1. Introduction Titanium dioxide, TiO2 , is an inexpensive, non-toxic and chemically stable material. Its high refractive index and transparency for visible light have made it a widely used painting material, and its biological properties have made it a promising material for biomedical applications [1–3]. Recently, its strong oxidizing power under ultraviolet (UV) irradiation attracts great attention due to the applications of antibacterial, deodorizing, and remediation of environmental pollutions [4–10]. TiO2 has a conduction band edge potential lower than that of hydrogen revolution and valence band edge potential higher than that of oxygen revolution. Therefore, in addition to decomposing the organic pollutions, the photogenerated electrons and holes in TiO2 can be used to split water into hydrogen and oxygen directly [11]. A lot of studies on the properties of TiO2 photocatalysts for water treatment or water splitting have been conducted [12–18]. Generally, TiO2 in particulate form possesses higher photocatalytic performance due to its large surface area. It was found that the TiO2 particulates with mixed-phase structure of anatase and rutile exhibit the best photocatalytic activity, followed by with the pure anatase phase, and then with the pure rutile phase. A high photocatalytic efficiency photocatalyst of TiO2 powder called P-25 (Degussa) has been developed based on the mixture of anatase and rutile phases [19]. Unfortunately, precipitating and recovering the TiO2 particulates from water limits its widespread use. In contrast, immobilized TiO2 film is more practical because of its

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controllability. Therefore, the way to improve the photocatalytic performance of immobilized TiO2 films is urgently pursued. The problem with the immobilized TiO2 films is the high carrier recombination rate of TiO2, which results in a low effective thickness for converting photon energy into chemical energy. Luttrell et al. [20] pointed out that the bulk transport ability of excitons to the surface dominates the photocatalytic activity of TiO2 films. Their studies on the photodegradation of methyl orange demonstrated that the photocatalytic activity of TiO2 films increases with the film thickness but reaches a maximum at ~2.5 nm for rutile and ~5 nm for anatase. This means that the carriers generated deeper than 2.5 nm and 5 nm for rutile and anatase, respectively, contribute little to the photodegradation of methyl orange. However, the thickness of 5 nm is too small when absorbing the incident light. The optical absorption coefficient of anatase TiO2 in the near-UV region is rather low due to its indirect bandgap nature. The thickness required for anatase film to absorb 50% of the incident light with a wavelength of 340 nm has been reported to be around 500 nm [21]. Therefore, most of the incident lights pass through the 5 nm anatase film. Even if we thicken the film to absorb more incident photons, the carriers generated deeper than 5 nm from the surface cannot migrate to the surface for utilization. Therefore, the immobilized TiO2 films suffer from either low photon absorption efficiency or high carrier recombination loss. A variety of strategies have been adopted to enhance the photocatalytic activity of immobilized TiO2 films. The most frequently investigated strategy involves growing a porous structure like a nanotube, nanopillar, nanorod, nanosheet, nanoflake or nanobelt arrays [22–30]. Although a porous structure can provide more reaction surface area for photocatalysis, it still cannot overcome the issue of low carrier transport ability. An alternative method which involves loading metal nanoparticles, e.g., platinum, gold or silver [31–37], on TiO2 can create an electric field across the interface to facilitate the separation of photogenerated electron-hole pairs. Thus, the carrier transport in TiO2 is improved by the prolonged carrier lifetime. As a consequence, the photocatalytic activity of TiO2 films is enhanced. In our previous study [38], we grew TiO2 films on Ni, Ta, and Ti coated glass substrates and found that the TiO2 films on the Ni-coated substrate performs the highest photocatalytic activity, followed by on the Ti-coated substrate, and then on the Ta-coated substrate, the same as the sequence of their electron work function of Ni ~5.04–5.35 eV, Ti 4.33 eV, and Ta 4.00–4.15 eV [39]. This is because the high work function metals like platinum, gold, silver or nickel can attract the photogenerated electrons from TiO2 as they come into contact with TiO2 , leading to a decrease of carrier recombination loss. Nevertheless, the photogenerated holes in TiO2 adjacent to the filled states of metal can also possibly cross into the metal to recombine with the electrons. Hence, introducing a semiconductor layer with an appropriate energy band structure in between the metal and TiO2 is considered to be a feasible method for alleviating the recombination issue because the built energy barrier from the heterojunction between semiconductor and TiO2 can block the holes in TiO2 from entering the semiconductor and the metal. A similar concept of multi-heterojuction has been applied on the photo-induced hydrophilic conversion for TiO2 /WO3 systems by Miyauchi et al [40]. The rutile-anatase mixed-phase TiO2 with higher photocatalytic activity than the pure anatase or pure rutile phase TiO2 has been attributed to the staggered energy band alignment at the anatase/rutile interface [41]. Similarly, SnO2 with suitable conduction and valence band edge potentials can form a staggered energy band alignment with anatase or rutile TiO2 . It is expected that placing a thin SnO2 layer in between the TiO2 and high work function metal can effectively improve the separation of the photogenerated electron-hole pairs. In this study, a thin SnO2 layer was placed in between the TiO2 and Ni metal films to alleviate the recombination issue. The results show that with the appropriate band alignment of the heterojunction TiO2 /SnO2 /Ni, the photocatalytic activity of TiO2 films has been highly improved. 2. Results and Discussion Besides the heterojunction, the structure and surface roughness also affect the photocatalytic activity of TiO2 films. Therefore, we first describe the structure and surface roughness of TiO2 films grown on the chosen substrates of blank glass, Ni/Ti coated glass, SnO2 coated glass and SnO2 /Ni/Ti

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coated glass. Then, the measured photocatalytic activity of TiO2 films is rendered and the junctions of TiO2 /Ni, TiO2 /SnO2 and TiO2 /SnO2 /Ni on the photocatalytic activity of TiO2 films are discussed. Figure 1a,b show the XRD patterns of TiO2 films grown on the blank glass, Ni/Ti coated glass, SnO2 coated glass and the SnO2 /Ni/Ti coated glass at 250 and 350 ◦ C, respectively. For the deposition temperature of 250 ◦ C the films grown on the blank glass, Ni/Ti coated glass and SnO2 coated glass are all crystallized in anatase form, but those grown on SnO2 /Ni/Ti coated glass are a mixture of anatase and rutile. As the deposition temperature is raised to 350 ◦ C, the structure of TiO2 films grown on the blank glass and SnO2 coated glass become a mixture of anatase and rutile, but the structure of TiO2 films grown on the Ni/Ti coated glass become an almost pure rutile. The TiO2 films grown on the SnO2 /Ni/Ti coated glass, however, still maintain the anatase-rutile mixed structure. Apparently, the structure of TiO2 films is related to both the deposition temperature and the substrate material. The SnO2 /Ni/Ti composite layer tends to enhance the formation of the rutile phase at a low temperature, but the Ni/Ti composite layer facilitates to the formation of pure rutile film at a relatively higher deposition temperature. The values of the surface roughness of these TiO2 films are summarized in Table 1. For a given deposition temperature, the films grown on SnO2 /Ni/Ti coated glass have the roughest surface, followed by the films grown on blank glass, then the films grown on SnO2 coated glass, and then the films grown on Ni/Ti coated glass. For a selected substrate, however, the films grown at 350 ◦ C are rougher than the films grown at 250 ◦ C. Table 1. Surface roughness of TiO2 films for the same samples in Figure 1. Underlying Materials TiO2 deposition temperature (◦ C) Ra of deposited TiO2 film (nm)

Blank Glass

Ni/Ti Coated Glass

SnO2 Coated Glass

SnO2 /Ni/Ti Coated Glass

250

350

250

350

250

350

250

350

5.1

8.2

2.4

3.0

3.4

5.2

6.5

8.9

The photocatalytic activities of the TiO2 films grown at 250 ◦ C are shown in Figure 2. As expected, the SnO2 /Ni underlying layer performs far better than the SnO2 and Ni underlying layers for improving the photocatalytic activity of TiO2 films. However, it is interesting that the SnO2 underlying layer has similar ability to the Ni underlying layer for improving the photocatalytic activity of TiO2 films. Because the TiO2 films on the Ni/Ti and SnO2 coated glass have the same structure of anatase form as those on the blank glass but with a lower surface roughness, the mechanism of improvement in the photocatalytic activity of TiO2 films by the Ni and SnO2 underlying layers can be concluded to be the heterojunction of TiO2 /Ni and TiO2 /SnO2 . Without the effect of heterojunction, the TiO2 films on the Ni or SnO2 should have lower photocatalytic activity than on blank glass because of their lower surface roughness. The improvement in photocatalytic activity of TiO2 films by the Ni underlying layer can be described by the mechanism of Schottky-contact assisted carrier separation. The Ni has a work function of ~5.04-5.35 eV, and the TiO2 has a conduction band minimum of ~−4.21 eV [42]. The junction of Ni with TiO2 will exist a Schottky barrier. Figure 3 shows the I-V characteristics of the ALD TiO2 on Ni. The diode behavior verifies the Schottky junction of Ni with TiO2 . As the band diagram of TiO2 /Ni shown in Figure 4, when the TiO2 is irradiated with an intense UV light, a large amount of electron-hole pairs will be created and then the electron carriers in TiO2 will flow to the Ni layer as depicted in Figure 4. As the photogenerated electron-hole pairs are separated, the Schottky barrier at the interface will block the photogenerated electrons from backing to the TiO2 , leaving the photogenerated holes, which have been considered to be the rate limiting carrier for methylene blue photooxidation in TiO2 . It is why the TiO2 /Ni heterojunction can improve the photocatalytic activity of TiO2 films.

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Figure 1. XRD patterns of TiO2 films grown at (a) 250 ◦ C and (b) 350 ◦ C on various indicated underlying Figure 1.The XRD patterns TiO2 films grown at rutile (a) 250 and (b) 350 ˚C on various indicated materials. peak A andofR indicate anatase and TiO˚C 2 , respectively. underlying materials. The peak A and R indicate anatase and rutile TiO 2, respectively. 1.0

C/C0

glass The photocatalytic activities of the TiO2 films grown at 250 ˚C are shown in Figure 2. As expected, Ni/Ti/glass 0.8 performs far better than the the SnO2/Ni underlying layer 2 and Ni underlying layers for SnOSnO /glass 2 improving the photocatalytic activity of TiO2 films. However, it is interesting that the SnO2 SnO2/Ni/Ti/glass underlying layer has similar 0.6ability to the Ni underlying layer for improving the photocatalytic activity of TiO2 films. Because the TiO2 films on the Ni/Ti and SnO2 coated glass have the same structure of anatase form as those on the blank glass but with a lower surface roughness, the 0.4 mechanism of improvement in the photocatalytic activity of TiO2 films by the Ni and SnO2 underlying layers can be concluded to be the heterojunction of TiO2/Ni and TiO2/SnO2. Without the effect of 0.2 heterojunction, the TiO2 films on the Ni or SnO2 should have lower photocatalytic activity than on blank glass because of their lower surface roughness. 0.0 0

3

6

9

12

15

18

21

24

UV irradiation time (h) Figure 2. Residual MB concentration (C/C0 ) versus UV irradiation time for characterizing photocatalysis ofFigure 250 ◦ C-deposited on various indicated underlying materials. time for characterizing 2 films 2. ResidualTiO MB concentration (C/C0) versus UV irradiation photocatalysis of 250 °C-deposited TiO2 films on various indicated underlying materials.

The improvement in photocatalytic activity of TiO2 films by the Ni underlying layer can be described by the mechanism of Schottky-contact assisted carrier separation. The Ni has a work function of 5.04‒5.35 eV, and the TiO2 has a conduction band minimum of −4.21 eV [42]. The

barrier at the interface will block the photogenerated electrons from backing to the TiO2, leaving the photogenerated holes, which have been considered to be the rate limiting carrier for methylene blue photooxidation in TiO2. It is why the TiO2/Ni heterojunction can improve the photocatalytic activity of TiO2 films. Catalysts 2018, 8, 440

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0.003

Current (mA)

0.002 0.001 0.000 -0.001 -0.002 -0.003 -1.0

-0.5

0.0

0.5

1.0

Voltage (V)

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Figure 3. Current-voltage characteristic curve of TiO2 -Ni junction.

Figure 3. Current-voltageNi characteristic TiO curve of TiO2-Ni junction. 2

~4.21 eV ~5 eV

Figure 4. Schematic energy band diagrams of TiO2 /Ni heterojunction with electron-hole pairs separation mechanism after UV light irradiation. Figure 4. Schematic energy band diagrams of TiO2/Ni heterojunction with electron-hole pairs The mechanism of TiO separation mechanism after UVSchottky-contact light irradiation. assisted carrier separation was further verified by 2 /Ni

the dependence of the photocatalytic activity of TiO2 films on the thickness of Ni underlying layer. The mechanism of TiO2/Ni Schottky-contact separation was with further by Figure 5 shows the photocatalytic activities of TiO2 assisted films on carrier the Ni/Ti coated glass Ni verified thicknesses the dependence of the photocatalytic activity of TiO 2 films on the thickness of Ni underlying layer. of 25, 50 and 100 nm, and Figure 6 is the extracted MB decay constants from the curves in Figure 5. Figure 5 shows the photocatalytic of TiOwith 2 films onincrease the Ni/Ti glass with Nibut thicknesses The photocatalytic activity of TiO2activities film increases the ofcoated Ni layer thickness becomes of 25, 50 and nm,thickness. and Figure 6 isresult the extracted constants from the curves Figure 5. saturated at a 100 certain This coincides MB withdecay the mechanism described above.in The thicker The photocatalytic activity of TiO2 states film increases the increase of Ni layer thickness but becomes the Ni layer, the more low energy in Ni forwith receiving the photogenerated electrons from TiO2 , saturated a certain result However, coincides as with mechanism described above. most The leading to at higher carrierthickness. separationThis efficiency. the the Ni layer is thick enough to receive thicker the Ni layer, theelectrons more lowinenergy in Ni for receiving theofphotogenerated electrons of the photogenerated TiO2 , states the photocatalytic activity TiO2 film would becomefrom less TiO 2, leading carrier separation efficiency. However, as the Ni layer is thick enough to dependent on to thehigher thickness of Ni layer. receive most of the photogenerated electrons in TiO2, the photocatalytic activity of TiO2 film would become less dependent on the thickness of Ni layer.

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1.0 blank glass Ni(25nm)/Ti/glass blank glass Ni(50nm)/Ti/glass Ni(25nm)/Ti/glass Ni(100nm)/Ti/glass Ni(50nm)/Ti/glass Ni(100nm)/Ti/glass

C/C0C/C

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1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0

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UV irridiation time (h) 8

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UV irridiation time (h) Figure 5. Residual MB concentration (C/C0) versus UV irradiation time for characterizing Figure 5. Residual MB concentration (C/C0 ) versus UV irradiation time for characterizing photocatalysis of 250◦ °C-deposited TiO2 films on blank glass and Ni/Ti coated glass with indicated Ni photocatalysis of 250 C-deposited TiO2 films on 0blank glassUV and irradiation Ni/Ti coatedtime glass for withcharacterizing indicated Ni Figure 5. Residual MB concentration (C/C ) versus layer thickness. layer thickness. of 250 °C-deposited TiO2 films on blank glass and Ni/Ti coated glass with indicated Ni photocatalysis layer thickness.

Figure 6. Dependence of MB decay constant on the thickness of Ni underlying layer for anatase TiO2 Figure 6. Dependence of MB decay constant on the thickness of Ni underlying layer for anatase TiO 2 films grown on Ni/Ti coated glass. The data are extracted from curves in Figure 4. films grown on Ni/Ti coated glass. The data are extracted from curves in Figure 4. Figure 6. Dependence of MB decay constant on the thickness of Ni underlying layer for anatase TiO 2 The improvement the photocatalytic of TiO SnO4.2 underlying layer is, 2 films films grown on Ni/Tiinin coated glass. The data activity are extracted from curvesby inthe Figure The improvement the photocatalytic activity of TiO 2 films by the SnO2 underlying layer is,

however, due to the carrier separation assisted by staggered band alignment. SnO2 is also an inherent however, due to the carrier separation assisted by staggered band alignment. SnO2 is also an inherent n-typeThe semiconductor with conduction band minimum ~−24.50 eVby and of ~3.67 layer eV [43]. improvement in athe photocatalytic activity ofofTiO films thea band SnO2 gap underlying is, n-type semiconductor with a conduction band minimum of -4.50 eV and a band gap of 21 3.67−eV 3 In this work thetocarrier concentration inassisted the SnOby measured by 4-point probe is as high as ~10 cm however, due the carrier separation staggered band alignment. SnO 2 is also an inherent 2 [43]. In this work the carrier concentration in the SnO2 measured by 4-point probe is as high as ~1021 which makes the Fermiwith levela close to the conduction bandofminimum. Likeathe TiOgap 2 /Ni n-type semiconductor conduction band minimum -4.50 eV and band of junction, 3.67 eV cm−3 which makes the Fermi level close to the conduction band minimum. Like the TiO 2/Ni junction, the TiO /SnO is also favorable for separation of photogenerated electron-hole [43]. In2this work the carrier concentration in the SnO 2 measured by 4-point probe is as high aspairs ~1021 2 junction the TiO2/SnO2 junction is also favorable for the separation of photogenerated electron-hole pairs as −3 as shown in Figure 7. Despite the same features of TiO /Ni and TiO /SnO junctions for carrier 2 2 the TiO 2/Ni junction, cm which makes the Fermi level close to the conduction2 band minimum. Like shown in Figure 7. Despite the same features of TiO2/Ni and TiO2/SnO2 junctions for carrier separation, two2 junction differences existfavorable between them. is that the low energy states in SnO the TiO2/SnO is also for theOne separation ofavailable photogenerated electron-hole pairs as 2 are separation, two differences exist between them. One is that the available low energy states in SnO 2 less than in in Ni for receiving the photogenerated electrons from the 2Nijunctions underlying is 2 ; that shown Figure 7. Despite the same features of TiO 2/NiTiO and TiOis, 2/SnO forlayer carrier are less than in Ni for receiving the photogenerated electrons from TiO 2; that is, the Ni underlying more conductive than the SnO layer to theisseparation of photogenerated carriers separation, two differences exist between them. One that the available low energy states in in TiO SnO 2 underlying 2 .2 layer is more conductive than the SnO2 underlying layer to the separation of photogenerated carriers The is that /SnO2 junction has a higher barrier to hinder photogenerated are other less than in the Ni TiO for 2receiving the photogenerated electrons from the TiOdiffusion 2; that is, of the Ni underlying in TiO2. The other is that the TiO2/SnO2 junction has a higher barrier to hinder the diffusion of holes from TiO into SnO ; that is, the SnO underlying layer is more able than the Ni underlying layer layer is more conductive layer to the separation of photogenerated carriers 2 2 than the SnO2 underlying 2 photogenerated holes from TiO2 into SnO2; that is, the SnO2 underlying layer is more able than the to avoid recombination of the separated carriers. The combination of the two differences may be in TiO2. The other is that the TiO2/SnO2 junction has a higher barrier to hinder the diffusionthe of photogenerated holes from TiO2 into SnO2; that is, the SnO2 underlying layer is more able than the

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Ni underlying layer to avoid recombination of the separated carriers. The combination of the two differences may be the reason for the similar ability to improve the photocatalytic activity of TiO2 reason for the similar ability to improve the photocatalytic activity of TiO2 films for both SnO2 and Ni films for both SnO2 and Ni underlying layers. underlying layers.

SnO2

TiO2 ~4.21 eV

~4.50 eV

3.20 eV 3.67 eV

Figure 7. heterojunction showing showing the the electron-hole Figure 7. Schematic Schematic energy energy band band diagrams diagrams of ofTiO TiO2 2/SnO /SnO22 heterojunction electron-hole separation mechanism after UV light irradiation. separation mechanism after UV light irradiation.

The advantages of the high Schottky barrier of the TiO /Ni junction and the high hole diffusion The advantages of the high Schottky barrier of the TiO22/Ni junction and the high hole diffusion barrier of TiO2 /SnO2 junction can be incorporated together by inserting a thin SnO2 layer in between barrier of TiO2/SnO2 junction can be incorporated together by inserting a thin SnO2 layer in between TiO2 and Ni. Figure 8 shows that the energy band diagram of TiO2 /SnO2 /Ni multi-junctions. TiO2 and Ni. Figure 8 shows that the energy band diagram of TiO2/SnO2/Ni multi-junctions. The The multi-junctions result in a staggered valence band alignment to block the holes from diffusing multi-junctions result in a staggered valence band alignment to block the holes from diffusing into into the Ni layer and simultaneously keep the band bending structure for driving the electrons to the the Ni layer and simultaneously keep the band bending structure for driving the electrons to the Ni Ni layer. Thus the photogenerated electron-hole pairs are efficiently separated, and the separated layer. Thus the photogenerated electron-hole pairs are efficiently separated, and the separated carriers are isolated by a distance of SnO layer from recombination at the interface. The low carrier carriers are isolated by a distance of SnO22 layer from recombination at the interface. The low carrier recombination rate for TiO /SnO /Ni multi-heterojunctions is verified by the PL spectra shown in recombination rate for TiO22/SnO22/Ni multi-heterojunctions is verified by the PL spectra shown in Figure 9. The PL intensity of TiO film on SnO /Ni is less than one-fifteenth of that on Ni, indicating Figure 9. The PL intensity of TiO22 film on SnO22/Ni is less than one-fifteenth of that on Ni, indicating that placing the SnO layer in between TiO and Ni layers highly reduces the recombination of that placing the SnO22 layer in between TiO22 and Ni layers highly reduces the recombination of photogenerated carriers in TiO2 . It is considered to be the reason for the TiO2 films grown on the photogenerated carriers in TiO2. It is considered to be the reason for the TiO2 films grown on the SnO /Ni/Ti underlying layer with the best photocatalytic activity. However, the photocatalytic activity SnO22/Ni/Ti underlying layer with the best photocatalytic activity. However, the photocatalytic is also a function of the crystalline structure and surface roughness. The TiO2 films on the SnO2 /Ni/Ti activity is also a function of the crystalline structure and surface roughness. The TiO2 films on the coated glass have a mixture of anatase and rutile structures, which is different from the pure anatase SnO2/Ni/Ti coated glass have a mixture of anatase and rutile structures, which is different from the structure of TiO2 films on SnO2 or Ni/Ti coated glass. Moreover, the TiO2 films on the SnO2 /Ni/Ti pure anatase structure of TiO2 films on SnO2 or Ni/Ti coated glass. Moreover, the TiO2 films on the coated glass have a higher surface roughness than those on the SnO2 or Ni/Ti coated glass. Both factors SnO2/Ni/Ti coated glass have a higher surface roughness than those on the SnO2 or Ni/Ti coated glass. beneficial to the photocatalytic activity of TiO2 films have been reported. Therefore, to identify this Both factors beneficial to the photocatalytic activity of TiO2 films have been reported. Therefore, to conclusion, it is necessary to further investigate the photocatalytic activity of TiO films with the same identify this conclusion, it is necessary to further investigate the photocatalytic 2activity of TiO2 films structure and the same surface roughness on these underlying layers. Unfortunately, it is difficult to with the same structure and the same surface roughness on these underlying layers. Unfortunately, get TiO2 films with the same structure and the same surface roughness on the SnO2 /Ni/Ti coated it is difficult to get TiO2 films with the same structure and the same surface roughness on the glass and on the SnO2 or Ni/Ti coated glass. Out of compromise, the films grown at 350 ◦ C were SnO2/Ni/Ti coated glass and on the SnO2 or Ni/Ti coated glass. Out of compromise, the films grown investigated to further understand the effects of mixed structure and roughness on the photocatalytic at 350 ˚C were investigated to further understand the effects of mixed structure and roughness on the activity of TiO films. photocatalytic2activity of TiO2 films.

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SnO2

Ni

SnO2

Ni

TiO2 TiO2 ~4.21 eV

~4.50 eV

~5 eV ~5 eV

~4.21 eV

~4.50 eV

Figure 8. Schematic energy band diagrams of TiO2/SnO2/Ni multi-junctions showing the mechanism of efficient carrier separation. Figure 8. Schematic energy band diagrams of TiO2/SnO2/Ni multi-junctions showing the mechanism Figure 8. Schematic energy band diagrams of TiO2 /SnO2 /Ni multi-junctions showing the mechanism of efficient carrier separation. of efficient carrier separation. 4000

PL Intensity (a.u.) PL Intensity (a.u.)

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Wavelength (nm) coated glass and on SnO2 /Ni/Ti coated Figure 9. Photoluminescence spectra of TiO 2 films on Ni/Ti glass under 325 nm excitation. spectra of TiO2 films on Ni/Ti coated glass and on SnO2/Ni/Ti coated Figure 9. Photoluminescence glass under 325 nm excitation. Figure 9. Photoluminescence spectra of TiO2 films on Ni/Ti coated glass and on SnO2/Ni/Ti coated Figure 10 shows the photocatalytic activities of TiO2 films grown on those underlying layers glass ◦ under 325 nm excitation. ◦

at 350 Figure C. Interestingly, compared with 250 C theofphotocatalytic activity of the TiO2 films grown 10 shows the photocatalytic activities TiO2 films grown on those underlying layers at on350 the˚C. SnO /Ni/Ti coated glass increased slightly, but the photocatalytic activity of the TiO films Interestingly, compared with 250 ˚C the photocatalytic activity of the TiO 2 films grown on the 2 shows the photocatalytic activities of TiO2 films grown on those underlying layers2 at Figure 10 SnO2/Ni/Ti coated the photocatalytic activity ofactivity the TiO2offilms grown on grown on the blank glass glass increased decreasedslightly, slightly.but Moreover, the photocatalytic the TiO 2 films 350 ˚C. Interestingly, compared with 250 ˚C the photocatalytic activity of the TiO2 films grown on the the blank glass decreased slightly. Moreover, thecoated photocatalytic activity of the TiO2From films the grown on grown on the SnO glassslightly, and thebut Ni/Ti glass drops results 2 coated SnO 2/Ni/Ti coated glass increased the photocatalytic activitydramatically. of the TiO2 films grown on the SnO 2 Figure coated 1, glass and the Ni/Ti coated glass drops dramatically. From the results of XRD of XRD in the structure of TiO films grown on the blank glass and SnO coated glass 2 the photocatalytic activity of the TiO2 films2 grown on in the blank glass decreased slightly. Moreover, Figure 1, the structure of TiO 2 films grown on theand blank glass anddeposition SnO2 coated glass changes from changes anatase to aNi/Ti mixture of anatase rutile as the increases the SnO2 from coatedpure glass and the coated glass drops dramatically. From the temperature results of XRD in ◦ pure anatase to a mixture of anatase and rutile as the deposition temperature increases from 250 to from 250 to structure 350 C. The results in Figure 10 seem contradictory to the report that the TiO2 with Figure 1, the of TiO 2 films grown on the blank glass and SnO 2 coated glass changes from 350 ˚C The results in Figure seem photocatalytic contradictory to the report that the TiOpure 2 with rutile-anatase rutile-anatase phases has10higher activity than those with or to rutile pure anatase tomixed a mixture of anatase and rutile as the deposition temperature increasesanatase from 250 mixed phases has higher photocatalytic activity than those with pure anatase or rutile phase. The phase. Theresults contradiction was in our previous studies of TiO films grown on Ni and Ta 350 ˚C The in Figure 10 also seemfound contradictory to the report that the 2TiO 2 with rutile-anatase contradiction was also found in our previous studies of TiO 2 films grown on Ni and Ta underlying underlying layers that the photocatalytic activity of TiO films decreases when the film structure mixed phases has higher photocatalytic activity than those 2with pure anatase or rutile phase. The changes fromwas pure anatase mixture of anatase and rutile [38]. Although the fundamentals contradiction also found to in aour previous studies of TiO 2 films grown on Ni and Ta underlying are still under investigation, this phenomenon indicates a high carrier recombination occurring in our

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layers that the photocatalytic activity of TiO2 films decreases when the film structure changes from pure anatase to a mixture of anatase and rutile [38]. Although the fundamentals are still under Catalysts 2018, 8, 440 9 of 12 investigation, this phenomenon indicates a high carrier recombination occurring in our rutile-anatase mixed-phase films. The most plausible reason for the decrease is that the separated carriers recombine at the interfaces of rutile/anatase, rutile/SnO2 and rutile/Ni. Despite the fact that these junctions can rutile-anatase mixed-phase films. The most plausible reason for the decrease is that the separated separate the photogenerated electron-hole pairs, the separated electrons and holes are able to carriers recombine at the interfaces of rutile/anatase, rutile/SnO2 and rutile/Ni. Despite the fact that recombine through the defects at the interface. In addition, the lower oxidizing power of rutile TiO2 these junctions can separate the photogenerated electron-hole pairs, the separated electrons and holes compared to anatase TiO2 may result in a pileup of hole carriers in rutile and thus a more serious are able torecombination. recombine through the defects at the interface. In addition, the lower oxidizing power of interface rutile TiO2 compared to anatase TiO2 may result in a pileup of hole carriers in rutile and thus a more serious interface recombination. 1.0

glass Ni/Ti/glass SnO2/glass

0.8

SnO2/Ni/Ti/glass

C/C0

0.6

0.4

0.2

0.0 0

4

8

12

16

20

24

UV irradiation time (h) Figure 10. Residual MB concentration (C/C0 ) versus UV irradiation time for characterizing photocatalysis of 350 ◦ C-deposited TiO2 films on0) various underlying Figure 10. Residual MB concentration (C/C versus indicated UV irradiation timematerials. for characterizing photocatalysis of 350 °C-deposited TiO2 films on various indicated underlying materials.

In contrast, the SnO2 interlayer between Ni and TiO2 can separate the electrons from holes by a distance of SnOthe avoid thebetween recombination the rutile-SnO interface. Therefore, theby TiO 2 layer In contrast, SnOto 2 interlayer Ni and at TiO 2 can separate2 the electrons from holes a2 films grown on SnO /Ni/Ti coated glass possess high photocatalytic activity even if they have the 2 distance of SnO2 layer to avoid the recombination at the rutile-SnO2 interface. Therefore, the TiO2 anatase-rutile mixed-phase Furthermore, the 350 ◦ C-deposited film has ifa they slightly higher films grown on SnO2/Ni/Ti structure. coated glass possess high photocatalytic activity even have the ◦ surface roughness than the 250 C-deposited film, resulting a slightly higher activity. anatase-rutile mixed-phase structure. Furthermore, the 350in˚C-deposited film photocatalytic has a slightly higher The results in Figurethan 10 verify the conclusion thatfilm, the high photocatalytic activity of TiO surface roughness the 250 ˚C-deposited resulting in a slightly higher photocatalytic 2 films grown on SnO2 /Ni/Ti coated is due to the the TiOconclusion multi-junctions rather than the anatase-rutile activity. The results inglass Figure 10 verify the high photocatalytic activity of TiO2 2 /SnO2 /Nithat mixed-phase structure or thecoated surface roughness. is worth nothing that besides application forthe the films grown on SnO2/Ni/Ti glass is due toItthe TiO2/SnO 2/Ni multi-junctions rather than photocatalysis, the high carrier separation ability of the multi-junctions can also be applied to other anatase-rutile mixed-phase structure or the surface roughness. It is worth nothing that besides devices suchfor as solar cells. application the photocatalysis, the high carrier separation ability of the multi-junctions can also be applied to other devices such as solar cells. 3. Experimental Section 3. Experimental Section Corning E2000 glass sheet with dimension of 3 × 2.5 cm2 was used as the substrate for the deposition of E2000 various thinsheet films. Four kinds of configurations, Corning glass with dimension of film 3 × 2.5 cm2 was usednamely as the TiO substrate the2 , 2 , TiOfor 2 /SnO TiO /Ni/Ti and TiO /SnO /Ni/Ti, were adopted in this study. The use of the Ti layer with a fixed deposition of various thin films. Four kinds of film configurations, namely TiO 2 , TiO 2 /SnO 2, 2 2 2 thickness of ~30 nm was to improve the adhesion of the Ni layer to the glass substrate. The adoption TiO2/Ni/Ti and TiO2/SnO2/Ni/Ti, were adopted in this study. The use of the Ti layer with a fixed of Ni layerofis~30 duenm to was its high work function and low cost the noble metals as Ag, thickness to improve the adhesion of the Nicompared layer to thetoglass substrate. Thesuch adoption Au andlayer Pt. The thickness of Niwork layerfunction was fixed at low ~50 cost nm except for the samples evaluate of Ni is due to its high and compared to the nobleto metals suchthe aseffect Ag, Au and Pt. The thickness ofon Nithe layer was fixedofatphotogenerated 50 nm except for the samples evaluate the effect of TiO heterojunction separation carriers in TiO2to . The adoption of the 2 /Ni of TiO 2/Ni was heterojunction the separation of photogenerated in TiO 2. The adoption of edge the SnO because of on its appropriate energy band structurecarriers which has higher valence band 2 layer SnO2 layer wasTiO because of its appropriate energy band structure which has higher valence band edge potential than for blocking the holes from entering into the Ni layer and higher electron affinity 2 potential than TiOfrom 2 for blocking holesthe from entering into the layer and higher electron affinity to catch electrons TiO2 [41].the Before film deposition, theNi glass substrates were ultrasonically to catchby electrons TiO2 [41]. the for film5 deposition, glass were ultrasonically cleaned acetone,from methanol andBefore DI water min in eachthe step, andsubstrates then dried by nitrogen purge cleaned methanol and DI water for 5 min in each step, and then dried by nitrogen purge gas. Theby Ti acetone, and Ni layers were grown by E-beam evaporator, and the SnO and TiO layers were 2 2 grown by atomic layer deposition (ALD). SnCl4 and TiCl4 were used as the precursors of Sn and Ti for the ALD SnO2 and ALD TiO2 , respectively, H2 O was used as the oxygen source and Ar gas as the purge gas. Each cycle of ALD SnO2 (TiO2 ) includes four steps of SnCl4 (TiCl4 ) pulse with 1 s, Ar purge

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with 2 s, H2 O pulse with 1 s and Ar purge with 2 s. The SnO2 films were deposited at 300 ◦ C, and the TiO2 films were grown at 250 and 350 ◦ C. The deposition cycle for both the ALD SnO2 and ALD TiO2 was 1000, yielding a film thickness of ~52 nm for SnO2 and ~55 nm for TiO2 . The conductivity of Ni/Ti and SnO2 layers was characterized by 4-point probe. The thickness of Ni/Ti layers was determined by profilometer, and the thickness of SnO2 and TiO2 layers was determined by ellipsometer. The surface roughness of films was measured by atomic force microscope. The crystalline structures of deposited films were identified by a grazing incident X-ray diffractometer with a voltage of 40 kV and a current of 40 mA at a wavelength of 1.5418 Å. The photoluminescence (PL) of TiO2 on Ni and SnO2 /Ni coated glass was recorded using a He-Cd laser of 325 nm wavelength as the excitation source at room temperature. The photocatalytic activity of TiO2 films was evaluated by measuring the degradation of methylene blue (MB) under UV-light irradiation at room temperature. Three 10 W of Sankyo Denki blacklight lamps with a center wavelength at 352 nm in parallel were used as the UV light sources. The samples were placed at the bottom of the glass cells (50 × 40 × 50 mm3 internal dimensions) filled with the MB solution of concentrations of 10−5 mol L−1 with the height of 10 mm. The measured irradiation intensity at the film surface was 0.59 mW cm−2 , which is relatively low compared with others [44,45] in order to prevent the MB diffusion in the solution from becoming a limitation factor for the photocatalysis experiment. According to the Beer-Lambert law, the absorbance peak intensity of MB solution at 668 nm is proportional to the MB concentration, so that can be used to monitor the degradation of MB solution. The decrease of the absorbance of MB solutions was measured by a spectrometer at fixed intervals, and the residual MB concentration (C/C0 ) was extracted by the change of absorbance at 668 nm. The photocatalytic degradation of MB can be described by an exponential decay function C (t) = C0 e−kt , where C0 and C(t) is the MB concentration of initial and after exposure time t, and k is the exponential decay constant or photocatalytic activity. 4. Conclusions TiO2 films with thickness of ~55 nm were grown on blank glass, Ni/Ti coated glass, SnO2 coated glass and SnO2 /Ni/Ti coated glass. The photocatalytic activity of these TiO2 films was evaluated by measuring the MB degradation rate under irradiation of 352 nm UV light at room temperature. The results demonstrate that all the underlying layers of Ni, SnO2 and SnO2 /Ni can improve the photocatalytic activity of the deposited TiO2 films. Among them, the SnO2 /Ni underlying layer performs the best. The photocatalytic activity of TiO2 films improved by Ni underlying layer and SnO2 underlying layer is due to the Schottky barrier and staggered band alignment at the TiO2 /Ni and TiO2 /SnO2 interfaces, respectively. However, the single junction of TiO2 /Ni or TiO2 /SnO2 cannot avoid the carrier recombination at the interface. The multi-junctions of TiO2 /SnO2 /Ni can further separate the photogenerated electrons from holes by a distance of SnO2 layer to avoid the recombination of separated carriers, and thus the photocatalytic activity of TiO2 films is highly improved. Author Contributions: H.-E.C. conceived and designed the experiments; C.-H.H. performed the experiments; H.-E.C., I.-S.Y. and Z.-P.Y. analyzed the data; H.-E.C. and Z.-P.Y. wrote the paper. Acknowledgments: The authors would like to thanks the financial support from Ministry of Science and Technology (contract no. MOST 105-2221-E-218-001 and MOST 106-2221-E-009-122-MY3). Conflicts of Interest: The authors declare no conflict of interest.

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