The glancing angle deposition (GLAD) method in physical vapor deposition is proved to be a versatile tool to fabricate nanostructured TiO2 as the photocatalyst on specific substrates to form self-standing structures, which are much easier to be recycled. And novel designs of doping, decorating photocatalytic active substance, are brought in to make TiO2 respond to visible light. In this chapter, we introduce our previous works such as TiO2 nanorods with CdS quantum dots, noble metallic nanoparticles, coating TiO2 via atomic layer deposition (ALD), and so on.
- visible light
As we all know, population growth and rapid industrial development lead to global energy and environment crisis. However, sufficient utilization of solar energy could alleviate these issues, as the solar energy irradiating the surface of the Earth exceeds the current global human energy consumption by nearly four orders of magnitude. Thus, there has been keen interests in developing photocatalysts like semiconductors that can produce chemical energy from light. Semiconductor photocatalysis, which is regarded as a good candidate to convert renewable photon energy into chemical energy and decompose organic pollutants, has drawn much attention of researchers.
Since the first report of photocatalytic splitting over water taking use of TiO2 photoanode by Fujisima and Honda in 1972 , TiO2 has been widely studied and considered as one of the superior candidates to settle the environmental concerns owing to its chemical inertness, low cost, photostability, nontoxicity and strong photocatalytic activity . However, the industrial applications of it are also limited due to its large band gap (
From previous studies, there are many methods to fabricate nanostructured materials. Among them, the glancing angle deposition (GLAD) technique is regarded as one simple but highly repeatable physical vapor deposition technology to prepare 1-dimensional and 2-dimensional nanomaterials, such as slanted posts, spirals, zigzag columns, nanopillars, etc. . This method takes advantage of atomic shadowing and adatom diffusion to grow nanostructure materials. And these specific shaped nanostructures can be applied in many areas such as catalyst, optical element and magnetic storage media, etc. For instance, HfO2 nanocolumn arrays can be applied as antireflection coating . And altering GLAD fabrication conditions could also adjust relating the property parameter. Ag nanorod arrays (NRAs) could serve as surface-enhanced Raman scattering substrates. The limit Rhodamine 6G concentration of 10−14 mol/L can even be perceived using such kind of array structure. 
Traditional strategies in previous researches to broaden the solar spectral response of TiO2 include surface sensitization by organic dyes and doping with transition metals. However, the commonly used dyes are in high price and the long-term stability of many dyes is also uncertain. Doping with transition metals makes the absorption shift to the visible region, even including all the solar spectra. While the metal ions could form recombination centers in the TiO2 , which will reduce the efficiency of its photocatalytic activity.
Doping non-metal ion into TiO2 may be a promising way to avoid the deterioration of the thermal stability of oxide lattice . Many researchers reported doping experiments with various substances, such as nitrogen , sulfur  and fluoride  elements. Asahi et al.  reported that the band gap narrowed by N-doping, which improves the photocatalytic activity of TiO2 under visible light irradiation. Since this work, such type of doping has been considered as one of the most effective approaches to shift the optical response of TiO2 from the UV to the visible spectral range. There are mainly two major ways to prepare the N-doped TiO2. One of them is using incorporation of N into TiO2 lattice by ion-implantation technique, or other techniques, such as magnetron sputtering, hydrothermal method, etc. Another way, described in this work, relies upon the oxidation process of TiNx, like powders and films prepared by these techniques.
2.1. Fabrication of N-doped TiO2 NRAs
The TiN NRAs were deposited on F-doped SnO2 (FTO) and quartz substrates through GLAD method. The substrate temperature was controlled at
Figure 1 is the scanning electron microscope (SEM) images of samples with various annealing time. Top-view image from Figure 2(a) shows that the as-prepared TiN NRAs exhibits porous structure. These nanorods (NRs) are found to be quite uniform with the length of ~600 nm, which are separated by voids as exhibited specially in the inset of Figure 1(a). Moreover, it can be seen that the NRs are tilted with an angle of ~30° in the case of the substrate normal because of high angle of the incident adatom plume related to the substrate (α = 85°) . The porous structure was also formed during the deposition process due to the self-shadowing effects and the limited mobility of the deposited atoms. The morphologies of the TiN NRAs annealed at 330°C for 5, 15, 30, 60 and 120 min do not change mainly and is corresponding with that of the as-deposited NRAs as exhibited in Figures 1(b)–(f).
To study the microstructures of the as-prepared TiN NRAs and annealed samples furthermore, transmission electron microscope (TEM) is performed. Figure 2(a) shows the low-resolution TEM image of the TiN NRAs. It can be seen that the NR is of length ~600 nm and diameter of ~80 nm, which is in agreement with the SEM results. These pine needle structure NRs lead to an important enhancement in the total surface area, which is much higher than that of NRs with smooth surface. This microstructure can promote the performance of photoelectrochemistry because of the 1D structure with high specific surface area. Figure 2(b) exhibits the high-resolution TEM (HRTEM) image of the as-prepared TiN NRAs. The TiN crystalline grains can be observed clearly with interplanar lattice spacing of 0.212 nm, corresponding to (200) plane. TiN can be converted into TiO2 by a complete oxidation at medium temperature in air condition. The oxidation occurs from the surface to the inner of NRs with diffusion of O2. And the annealing time is also the critical parameter to decide the final oxidation degree. 15N-TiO2 NRAs that annealed for 15 min at 330°C have huge amount of amorphous state, and only trace amount of TiO2 crystalline grains with lattice spacing of 0.354 nm existed as shown in Figure 2(c). Prolonging the annealing time to 120 min, huge amount of TiO2 crystalline grains appeared and no TiN crystalline is found as shown in Figure 2(d).
Figure 3(a) exhibits the UV-visible light transmittance spectra of the TiN NRAs sample annealed during different time at 330°C. The as-prepared TiN films were assumed to be opaque with thickness of several tens of nanometers in previous report ; however, the transmittance of the as-prepared TiN NRAs actually in this study is ~25% under visible light test. With the increase in annealing time, the transmittance increases step by step at the wavelength of 300 to 600 nm, may due to the discrepancy in the degree of oxidation of the films from TiN to TiO2. The spectra are studied by a high regularity of the interference fringes and a systematic increase in wavelength in which the film practically is no longer transparent in the visible range. This behavior has been previously found in TiO2 doped with metals . The optical gap (
3. Coupling with other semiconductors
PbS , CdSe , ZnS  and CdS [29, 30] are semiconductors which own narrow band gap. They can be used as a sensitizer combined with TiO2 to enhance the photocatalytic property. Especially, the band gap of CdS is about 2.4 eV which is of great advantage in photoelectrochemisty and photocatalysis . Researchers have reported many works on growing CdS nanoparticles (NPs) on TiO2 directly by chemical bath deposition (CBD) , electrodeposition  and successive ion layer adsorption and reaction (SILAR) [35, 36]. Improving electron-hole separation efficiency is beneficial for the photocatalytic performance . Nano-sized TiO2 is also good for reduction on recombining with photo-generated electrons . What is more, nanostructured TiO2 always shows various characteristics on property enhancement according to huge surface areas, high optical absorption degree and low reflectivity .
3.1. Decoration CdS NPs on TiO2 NRAs
3.1.1. Fabrication of TiO2 NRAs
Choosing FTO and Si as substrates to deposit Ti NRAs using GLAD technique, in which the morphology of nanostructure was investigated by SEM. Firstly, clean the substrates sequentially in acetone, alcohol and deionized water by ultrasonic, respectively, and each for 5 min. Then, the as-prepared samples were oxidized from Ti NRAs to TiO2 NRAs using tube furnace of 450°C for 2 hrs with a rate of 5°C min−1 in air.
3.1.2. CdS NPs deposition on TiO2 substrates
CdS NPs deposited TiO2 NRAs by SILAR method have been reported previously [30, 46]. TiO2 NRAs substrates were alternatively dipped in Cd(Ac)2 and Na2S solutions each for 30 s to react with CdS nanostructure for each time. They were also rinsed in deionized water between every solutions for 30 s. The SILAR process was repeated until it reached the satisfied cycle numbers.
Top-view SEM image of Ti NRAs is showed in Figure 4(a) and the insert picture exhibits tilted angle-view SEM image of the samples. And the Ti NRs are uniformed with ~70 nm in diameter, ~220 nm in length. The top-view SEM image of oxide sample was exhibited in Figure 4(b) which kept the similar dimensions and sample compared to Ti NRAs. Figure 4(c–f) are the top-view SEM images of CdS deposition samples with 5, 10, 15 and 20 SILAR cycles. SILAR method has been used in previous research and it is thought as an effective way to synthesis of CdS . Large CdS particles were formed after 15 cycles, and after 20 cycles CdS particles even aggregated with each other.
Different cycle numbers of CdS SILAR coating on TiO2 NRs were observed by HRTEM technology. The morphology of TiO2 NRs are exhibited in Figure 5(a–b). It can be seen that TiO2 NRs are polycrystalline which contain two phases (anatase and the rutile). And the heterojunction of these two phases could block recombination of electrons and holes . HRTEM images of 15 cycles CdS NPs decoration are shown in Figure 5(c–d). And we can see that the average diameter of CdS NPs is ~5 nm, which attach to TiO2 NRs forming heterojunctions of TiO2 and CdS. And large amount of CdS wraps up TiO2 NRs when cycle number reaches to 25.
Discoloration of methyl orange (MO) under visible light of different SILAR cycle numbers is measured to test its photocatalytic performance. As we all know, the absorbance peak intensity of the
Discoloration rate of prepared samples with the irradiation time is exhibited in Figure 6. It can be seen that discoloration rate of pure TiO2 NRAs was only 3.2% just due to photobleaching process . Then with the increase in SILAR cycle number, the discoloration rate improved first and then reduced. Specially, when cycle number is 20, it showed the best photocatalytic performance which reached to 42.0% after 150 min irradiation.
When visible light irradiate on materials, the CdS reacted due to the narrow band gap and produce electrons and holes. What is more, CdS decorated with TiO2 NRAs also reduced the happening of recombination because of band gap matching, thus improving the photocatalytic performance. The electrons accumulated at the conduction band (CB) of semiconductors change to active oxygen species (
3.2. TiO2 NRAs/CdS quantum dots by coating TiO2
via atomic layer deposition (ALD)
One big disadvantage of CdS QDs is that the excited electrons and holes of CdS QDs will be trapped by the surface states in ambient conditions. This problem causes the degradation of photostability and photocatalytic efficiency. Hence, it is crucial to reduce the recombination of electrons and holes, in order to improve the photostability of CdS and enhance the photocatalytic efficiency . One effective way to decrease the surface recombination velocity is to fabricate a surface coating on CdS QDs. ALD is a kind of coating technique to deposit a very thin film that could passivate the surface states and then reduce the surface recombination velocity. The layer-by-layer deposition allows highly conformal coating even on the dense and rough surface of certain nanostructures. In previous study, ALD technique has had success in preventing anodic corrosion on other nanostructured materials .
Here, a TiO2 NRAs/CdS QDs/ALD-TiO2 composite material is investigated. The TiO2 NRAs were also made by GLAD method, while the CdS QDs and the ultra-thin TiO2 film were coated alternatively by SILAR and ALD technique, respectively. Outstanding photocatalytic property and stability were achieved in this kind of structure, which predicts huge potency on solar energy conversion.
3.2.1. Fabrication ALD on TiO2 NRAs
The substrate temperature of ALD was controlled at 150°C during the coating process. Four dimethylamino titanium, the precursor, was maintained at 110°C and water at 40°C. N2 was assumed as purge and carrier gas. To guarantee enough penetration of the precursors into the whole NRs, a soak step was taken, which is very similar to that adopted in previous study [55, 56]. First, four dimethylamino titanium was pulsed for 250 ms and kept for additional 5 s, and then the chamber was evacuated. After that H2O was pulsed in for 5 ms and allowed to soak for another 3 s, followed by a 20 s purge step. This process was repeated until the demanded deposition of TiO2 was complicated.
The typical top-view SEM image of 5 SILAR cycles CdS QDs decoration was showed in Figure 8(a), and the inset image of the NRs with a tilt angle of 45°. The uniform NRAs are about ~70 nm in diameter, and ~220 nm in length, while the CdS QDs deposited on TiO2 NRAs are rather small with size below 20 nm. Figure 8(b–f) show the top-view SEM images of the TiO2 NRAs/CdS QDs coated with TiO2 layers by 20, 40, 60, 80 and 160 ALD cycles, respectively. Specially, there is no obvious change in morphology when the ALD cycles are less than 80.
The TiO2 layers deposited on TiO2 NRAs/CdS QDs by ALD were studied in detail by HRTEM technique. Figure 9(a–b) are the images of TiO2 NRAs/CdS QDs before coating with TiO2 layer. The TiO2 NRs are in polycrystalline state with the length of ~220 nm and diameter of ~50 nm, which is corresponding with SEM results. Ultrafine CdS QDs with diameter of ~3 nm are coated on TiO2 NRs, making up the CdS/TiO2 heterojunction. Figure 9(c–d) are the HRTEM images of TiO2 NRAs/CdS QDs decorated with 60 ALD cycles TiO2 layer (i.e., TiO2 NRAs/CdS QDs/60c-ALD-TiO2). It is obvious that an ultrathin amorphous TiO2 layer (~2 nm thick) was deposited completely and uniformly on the TiO2 NRAs/CdS QDs, forming another CdS/TiO2 heterojunction. Furthermore, Figure 9(e–f) exhibit the TEM images of TiO2 NRAs/CdS QDs/80c-ALD-TiO2, and the thickness of the amorphous TiO2 layer is ~2.8 nm which can be seen clearly in the HRTEM image corresponding to Figure 9(f).
We also test the photocatalytic performance of composite materials by the photodegradation experiments of MO under visible light irradiation. It is also following the Beer’s law as introduced previously . Figure 10 shows the degradation rate of MO after 1 h irradiation under visible light range, choosing TiO2 NRAs/CdS QDs/ALD-TiO2 as the photocatalyst. The degradation rate is only 2.92% when choosing TiO2 NRAs/CdS QDs without coating ALD TiO2. And enhancement of degradation rate occurred when taking TiO2 NRAs/CdS QDs/ALD-TiO2 as the photocatalyst.
For instance, the degradation rate was 4.16% for TiO2 NRAs/CdS QDs/20c-ALD-TiO2, and it promoted 7.47% for TiO2 NRAs/CdS QDs/60c-ALD-TiO2, in which the rate improved 156% compared to that of TiO2 NRAs/CdS QDs in the absence of coating. However, it is also obvious that when taking the TiO2 NRAs/CdS QDs/80c-ALD-TiO2 and TiO2 NRAs/CdS QDs/160c-ALD-TiO2 as photocatalysts, the degradation rate of both decreases greatly. The annealed samples exhibit similar catalytic performance without obvious variation of degradation between the annealed and the original samples.
4. Coupling with noble metal
Combining with metal is a promising method to develop highly efficient visible light photocatalyst. On one hand, the deposition of the metal on TiO2 can greatly improve its photoefficiency through the Schottky barrier CB electron trapping and consequent longer electron-hole pair lifetime . Hu et al.  reported a highly efficient Pt-doped TiO2 which have enhanced photocatalytic activity for NOx oxidation both under UV and visible light irradiation. Ingram et al. managed to reduce the high rate of charge-carrier recombination by combining a semiconductor photocatalyst with tailored plasmonic-metal nanostructures . The presence of Pt deposited on TiO2 is believed to retard the rapid charge-pair recombination by serving as an electron sink and facilitating interfacial electron transfer to dioxygen or other electron acceptors. Pt can also trap electrons on the CB, which are subsequently transferred to electron acceptors . On the other hand, some noble metal NPs, such as Ag and Au, exhibit strong UV-vis absorption due to their plasmon resonance, produced by the collective oscillations of surface electrons. Pu et al. demonstrated those Au NPs, Au NRs and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays could be prepared for effective photocatalysis and the activities were enhanced in both the UV and the visible regions .
Despite these promising studies, combining the plasmonic effect of Au and electron sink effect of Pt has not been reported so far to our knowledge. Furthermore, photocatalysts in previous reports were usually in the form of powders and in an amorphous state, which was hard to handle and restricted its practical applications. In the present work, we designed a plasmonic photocatalyst consisting of bimetallic Au-Pt/TiO2 supported on specific SiO2 substrates. Firstly, the vertically aligned TiO2 NRAs were fixed on specific SiO2 substrates by GLAD technique. Then, the Au and Pt NPs were deposited on TiO2 by using SILAR method. Excellent photocatalytic property and stability were achieved and the fabrication of TiO2 self-standing structures, which will be easier for recycling and thus facilitating their potential applications in solar energy-driven photocatalysis.
4.1. Metallic NPs deposition on TiO2 NRAs
Au/Pt NPs were deposited on TiO2 NRs through SILAR method with slight modification as previously reported . Briefly, the TiO2 NRAs substrates were successively exposed to HAuCl4 (or HPt2Cl6) and NaBH4 solutions to deposit nanocrystallites. The TiO2 NRAs were immersed in 0.1 mg/mL HAuCl4 (or 0.1 mg/mL HPt2Cl6) solution for 60 s, followed by rinsing with DI water and then immersed in NaBH4 solution (1 mg/mL) for another 60 s, after which the resultant was rinsed with DI water for several times. This SILAR process was repeated for several cycles until the desired quantity of metallic nanocrystallites was achieved. Here, Au/Pt-TiO2 sample was alternately coated with Au and Pt NPs, respectively, for 5 times.
Figure 11 presents the X-ray diffraction (XRD) patterns of different samples. All samples exhibited diffraction peaks at 25.2° and 27.3° corresponding to the (101) crystal planes of the anatase phase (JCPDS No. 21–1272) and (110) crystal planes of the rutile phase (JPCDS No. 21–1276). Besides this, the diffraction peaks (38.2°, 41.4°) assigned to Au (JCPDS No. 04–0784) and the peak at 39.7° assigned to Pt (JCPDS No. 04–0802) were displayed in Au-TiO2 and Pt-TiO2, respectively. These three peaks were also observed in Au/Pt-TiO2 sample.
Figure 12 shows the SEM images of the pure TiO2 film and those coated with Au, Pt and Au/Pt NPs with 10 cycles, respectively. As-annealed film consists of vertically aligned TiO2 NRs with a diameter of ~50 nm and a length of ~200 nm (Figure 12(a)). Au and Pt NPs distribute uniformly on the TiO2 NRAs surface (Figure 12(d) and 12(c)).
Furthermore, TEM images in Figure 13 show that Au and Pt were uniformly dispersed on the surface of TiO2. Their average sizes were about ~4 nm, and in a regular cubic shape. According to the measurement of lattice fringes,
To evaluate the effect of bimetal Au-Pt on the photocatalytic activity of TiO2, the photodegradation of MO was carried out under visible irradiation. As a comparison, MO degradation were also performed in Pt/TiO2, Au/TiO2 and TiO2. As shown in Figure 14, neither TiO2 nor Pt-TiO2 showed any activity for the MO degradation, while 20% MO was degraded by Au-TiO2 after 120 illumination under otherwise condition. In the range of wavelength
Surface plasmon resonance (SPR) peak wavelength of Au NPs in visible light region is ~520 nm. And the incident photons are absorbed by Au NPs through SPR excitation . Soon afterwards, hot electrons move from the plasmon-excited Au to the CB of TiO2 . And then electrons transfer from CB to Pt due to the work function of Pt larger than Au. Here, Pt NPs take part in the process as cocatalyst at which electrons could react with electron acceptors (O2 absorbed on the surface of Ti3+ or dissolved in water) to create superoxide radicals (O2•−). At the meantime, the resultant electron-deficient Au particles can oxidize the organic molecule or react with OH− to form hydroxyl radicals, OH•, which are highly oxidizing species. The process is shown in Figure 15. And co-decoration of Au/Pt not only improves the efficiency of charge separation, improving its photocatalytic efficiency, but also expands the active range of TiO2 to visible light region. What is more, it was also verified that active radicals produced from UV-photoexcited TiO2 create electron-hole pairs reacting with adsorbed oxygen/H2O . As a result, the highly efficient degradation of dyes came from both photoexcited TiO2 and plasmon-excited Au NPs under UV irradiation.
In this chapter is introduced three main methods of coupling with different semiconductors, combining with noble metals and introducing dopants to make TiO2 be active in visible light region. Our studies demonstrate that through rational design of composite nanostructures, one can achieve on utilizing a high-energy photon under sunlight. And combining different materials together, which matches their bands with each other and with various nanostructures, could both eventually realize higher efficiency of solar spectrum.
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