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Article

Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique

by
Hsyi-En Cheng
1,
Chi-Hsiu Hung
2,
Ing-Song Yu
3 and
Zu-Po Yang
2,*
1
Department of Electro-Optical Engineering, Southern Taiwan University of Science and Technology, Tainan 710, Taiwan
2
Institute of Photonic System, National Chiao Tung University, Tainan 71150, Taiwan
3
Department of Materials Science and Engineering, National Dong Hwa University, Hualien 97401, Taiwan
*
Author to whom correspondence should be addressed.
Catalysts 2018, 8(10), 440; https://doi.org/10.3390/catal8100440
Submission received: 13 September 2018 / Revised: 1 October 2018 / Accepted: 3 October 2018 / Published: 6 October 2018
(This article belongs to the Section Nanostructured Catalysts)
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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.

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 [1,2,3]. Recently, its strong oxidizing power under ultraviolet (UV) irradiation attracts great attention due to the applications of antibacterial, deodorizing, and remediation of environmental pollutions [4,5,6,7,8,9,10]. 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 [12,13,14,15,16,17,18]. 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 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 [22,23,24,25,26,27,28,29,30]. 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 [31,32,33,34,35,36,37], 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 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.
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.
The mechanism of TiO2/Ni Schottky-contact assisted carrier separation was further verified by the dependence of the photocatalytic activity of TiO2 films on the thickness of Ni underlying layer. Figure 5 shows the photocatalytic activities of TiO2 films on the Ni/Ti coated glass with Ni thicknesses of 25, 50 and 100 nm, and Figure 6 is the extracted MB decay constants from the curves in Figure 5. The photocatalytic activity of TiO2 film increases with the increase of Ni layer thickness but becomes saturated at a certain thickness. This result coincides with the mechanism described above. The thicker the Ni layer, the more low energy states in Ni for receiving the photogenerated electrons from TiO2, leading to higher carrier separation efficiency. However, as the Ni layer is thick enough to receive most of the photogenerated electrons in TiO2, the photocatalytic activity of TiO2 film would become less dependent on the thickness of Ni layer.
The improvement in the photocatalytic activity of TiO2 films by the SnO2 underlying layer is, however, due to the carrier separation assisted by staggered band alignment. SnO2 is also an inherent n-type semiconductor with a conduction band minimum of ~−4.50 eV and a band gap of ~3.67 eV [43]. In this work the carrier concentration in the SnO2 measured by 4-point probe is as high as ~1021 cm−3 which makes the Fermi level close to the conduction band minimum. Like the TiO2/Ni junction, the TiO2/SnO2 junction is also favorable for the separation of photogenerated electron-hole pairs as shown in Figure 7. Despite the same features of TiO2/Ni and TiO2/SnO2 junctions for carrier separation, two differences exist between them. One is that the available low energy states in SnO2 are less than in Ni for receiving the photogenerated electrons from TiO2; that is, the Ni underlying layer is more conductive than the SnO2 underlying layer to the separation of photogenerated carriers in TiO2. The other is that the TiO2/SnO2 junction has a higher barrier to hinder the diffusion of photogenerated holes from TiO2 into SnO2; that is, the SnO2 underlying layer is more able than the 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 films for both SnO2 and Ni underlying layers.
The advantages of the high Schottky barrier of the TiO2/Ni junction and the high hole diffusion 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. The multi-junctions result in a staggered valence band alignment to block the holes from diffusing into the Ni layer and simultaneously keep the band bending structure for driving the electrons to the Ni layer. Thus the photogenerated electron-hole pairs are efficiently separated, and the separated carriers are isolated by a distance of SnO2 layer from recombination at the interface. The low carrier recombination rate for TiO2/SnO2/Ni multi-heterojunctions is verified by the PL spectra shown in Figure 9. The PL intensity of TiO2 film on SnO2/Ni is less than one-fifteenth of that on Ni, indicating that placing the SnO2 layer in between TiO2 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 SnO2/Ni/Ti underlying layer with the best photocatalytic activity. However, the photocatalytic activity is also a function of the crystalline structure and surface roughness. The TiO2 films on the SnO2/Ni/Ti coated glass have a mixture of anatase and rutile structures, which is different from the pure anatase structure of TiO2 films on SnO2 or Ni/Ti coated glass. Moreover, the TiO2 films on the SnO2/Ni/Ti coated glass have a higher surface roughness than those on the SnO2 or Ni/Ti coated glass. Both factors beneficial to the photocatalytic activity of TiO2 films have been reported. Therefore, to identify this conclusion, it is necessary to further investigate the photocatalytic activity of TiO2 films with the same structure and the same surface roughness on these underlying layers. Unfortunately, it is difficult to get TiO2 films with the same structure and the same surface roughness on the SnO2/Ni/Ti coated glass and on the SnO2 or Ni/Ti coated glass. Out of compromise, the films grown at 350 °C were investigated to further understand the effects of mixed structure and roughness on the photocatalytic activity of TiO2 films.
Figure 10 shows the photocatalytic activities of TiO2 films grown on those underlying layers at 350 °C. Interestingly, compared with 250 °C the photocatalytic activity of the TiO2 films grown on the SnO2/Ni/Ti coated glass increased slightly, but the photocatalytic activity of the TiO2 films grown on the blank glass decreased slightly. Moreover, the photocatalytic activity of the TiO2 films grown on the SnO2 coated glass and the Ni/Ti coated glass drops dramatically. From the results of XRD in Figure 1, the structure of TiO2 films grown on the blank glass and SnO2 coated glass changes from pure anatase to a mixture of anatase and rutile as the deposition temperature increases from 250 to 350 °C. The results in Figure 10 seem contradictory to the report that the TiO2 with rutile-anatase mixed phases has higher photocatalytic activity than those with pure anatase or rutile phase. The contradiction was also found in our previous studies of TiO2 films grown on Ni and Ta underlying 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 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 separate the photogenerated electron-hole pairs, the separated electrons and holes are able to recombine through the defects at the interface. In addition, the lower oxidizing power of rutile TiO2 compared to anatase TiO2 may result in a pileup of hole carriers in rutile and thus a more serious interface recombination.
In contrast, the SnO2 interlayer between Ni and TiO2 can separate the electrons from holes by a distance of SnO2 layer to avoid the recombination at the rutile-SnO2 interface. Therefore, the TiO2 films grown on SnO2/Ni/Ti coated glass possess high photocatalytic activity even if they have the anatase-rutile mixed-phase structure. Furthermore, the 350 °C-deposited film has a slightly higher surface roughness than the 250 °C-deposited film, resulting in a slightly higher photocatalytic activity. The results in Figure 10 verify the conclusion that the high photocatalytic activity of TiO2 films grown on SnO2/Ni/Ti coated glass is due to the TiO2/SnO2/Ni multi-junctions rather than the anatase-rutile mixed-phase structure or the surface roughness. It is worth nothing that besides application for 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

Corning E2000 glass sheet with dimension of 3 × 2.5 cm2 was used as the substrate for the deposition of various thin films. Four kinds of film configurations, namely TiO2, TiO2/SnO2, TiO2/Ni/Ti and TiO2/SnO2/Ni/Ti, were adopted in this study. The use of the Ti layer with a fixed thickness of ~30 nm was to improve the adhesion of the Ni layer to the glass substrate. The adoption of Ni layer is due to its high work function and low cost compared to the noble metals such as Ag, Au and Pt. The thickness of Ni layer was fixed at ~50 nm except for the samples to evaluate the effect of TiO2/Ni heterojunction on the separation of photogenerated carriers in TiO2. The adoption of the SnO2 layer was because of its appropriate energy band structure which has higher valence band edge potential than TiO2 for blocking the holes from entering into the Ni layer and higher electron affinity to catch electrons from TiO2 [41]. Before the film deposition, the glass substrates were ultrasonically cleaned by acetone, methanol and DI water for 5 min in each step, and then dried by nitrogen purge gas. The Ti and Ni layers were grown by E-beam evaporator, and the SnO2 and TiO2 layers were 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, H2O 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 with 2 s, H2O 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 ) = C 0 e − k t ,
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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Figure 1. XRD patterns of TiO2 films grown at (a) 250 °C and (b) 350 °C on various indicated underlying materials. The peak A and R indicate anatase and rutile TiO2, respectively.
Figure 1. XRD patterns of TiO2 films grown at (a) 250 °C and (b) 350 °C on various indicated underlying materials. The peak A and R indicate anatase and rutile TiO2, respectively.
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Figure 2. Residual MB concentration (C/C0) versus UV irradiation time for characterizing photocatalysis of 250 °C-deposited TiO2 films on various indicated underlying materials.
Figure 2. Residual MB concentration (C/C0) versus UV irradiation time for characterizing photocatalysis of 250 °C-deposited TiO2 films on various indicated underlying materials.
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Figure 3. Current-voltage characteristic curve of TiO2-Ni junction.
Figure 3. Current-voltage characteristic curve of TiO2-Ni junction.
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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 separation mechanism after UV light irradiation.
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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 layer thickness.
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 layer thickness.
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Figure 6. Dependence of MB decay constant on the thickness of Ni underlying layer for anatase TiO2 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 TiO2 films grown on Ni/Ti coated glass. The data are extracted from curves in Figure 4.
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Figure 7. Schematic energy band diagrams of TiO2/SnO2 heterojunction showing the electron-hole separation mechanism after UV light irradiation.
Figure 7. Schematic energy band diagrams of TiO2/SnO2 heterojunction showing the electron-hole separation mechanism after UV light irradiation.
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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 of efficient carrier separation.
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Figure 9. Photoluminescence spectra of TiO2 films on Ni/Ti coated glass and on SnO2/Ni/Ti coated glass under 325 nm excitation.
Figure 9. Photoluminescence spectra of TiO2 films on Ni/Ti coated glass and on SnO2/Ni/Ti coated glass under 325 nm excitation.
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Figure 10. Residual MB concentration (C/C0) versus UV irradiation time for characterizing photocatalysis of 350 °C-deposited TiO2 films on various indicated underlying materials.
Figure 10. Residual MB concentration (C/C0) versus UV irradiation time for characterizing photocatalysis of 350 °C-deposited TiO2 films on various indicated underlying materials.
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Table
Table 1. Surface roughness of TiO2 films for the same samples in Figure 1.
Table 1. Surface roughness of TiO2 films for the same samples in Figure 1.
Underlying MaterialsBlank GlassNi/Ti Coated GlassSnO2 Coated GlassSnO2/Ni/Ti Coated Glass
TiO2 deposition temperature (°C)250350250350250350250350
Ra of deposited TiO2 film (nm)5.18.22.43.03.45.26.58.9

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Cheng, H.-E.; Hung, C.-H.; Yu, I.-S.; Yang, Z.-P. Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique. Catalysts 2018, 8, 440. https://doi.org/10.3390/catal8100440

AMA Style

Cheng H-E, Hung C-H, Yu I-S, Yang Z-P. Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique. Catalysts. 2018; 8(10):440. https://doi.org/10.3390/catal8100440

Chicago/Turabian Style

Cheng, Hsyi-En, Chi-Hsiu Hung, Ing-Song Yu, and Zu-Po Yang. 2018. "Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique" Catalysts 8, no. 10: 440. https://doi.org/10.3390/catal8100440

APA Style

Cheng, H.-E., Hung, C.-H., Yu, I.-S., & Yang, Z.-P. (2018). Strongly Enhancing Photocatalytic Activity of TiO2 Thin Films by Multi-Heterojunction Technique. Catalysts, 8(10), 440. https://doi.org/10.3390/catal8100440

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