Synthesis and characterization of UV upconversion material Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ with enhanced the photocatalytic properties under a xenon lamp

Abstract

The upconversion luminescence agents Y₂SiO₅:Pr³⁺, Li⁺, that can be effectively excited by the blue light from a xenon lamp 150 W with grating as its light splitter, are fabricated by a hydrothermal method with mesoporous molecular sieves of MCM-48 as the suitable silica source. The obtained emission spectra of Y₂SiO₅:Pr³⁺, Li⁺ and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ excited by a xenon lamp indicate that the UV upconversion luminescence of Pr³⁺ can be effectively transferred to TiO₂. The photocatalytic performances of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ are evaluated by the degradation of methylene blue upon xenon lamp irradiation. The degradation rate of the methylene blue with Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ is 14 times as high as that with only TiO₂.

---

1. Introduction

With the growth of worldwide industry, severe environmental contaminations have become a major concern of our society. Photocatalysis, as an environmentally friendly technique in utilizing clean, safe, and renewable solar energy to eliminate toxic organic substances in air and water, has attracted considerable attention.^1–5 TiO₂ is known as the most widely investigated photocatalyst due to its high oxidative efficiency, high chemical stability, non-toxicity and low-cost.^6–8 Presently, the photocatalytic degradation activity of TiO₂ is still low, mainly due to the poor solar energy utilization. As is well known, TiO₂ with a bandgap of 3.2 eV can be only excited by photons with light wavelengths shorter than about 400 nm in the UV wavelength range, which accounted for about 4% of the solar radiation energy.⁹ It is of interest to find a TiO₂-based photocatalyst which is sensitive to visible light (∼48%) and near infrared-light (∼44%) in order to make more efficient use of solar energy in practical applications. Numerous methods had been adopted, such as surface modification,^10,11 metal or nonmetal elements doping,^12–15 and the combination with a visible light excited semiconductor (narrow band-gap semiconductor such as CdS, CdSe, CdSSe) and so on.^16–20

Recently, the upconversion luminescence agent that could transform the visible-light and near infrared-light into the ultraviolet light to satisfy the genuine requirement of TiO₂ catalysts became the investigated subject.^21–23 It is generally known that the upconversion emission power p is proportional to ϕⁿ(n ≥ 2) (ϕ for the excitation density). So the upconversion emission power p would quickly decreased in magnitude as the excitation density ϕ decreased. When the excitation density ϕ decreased to a certain threshold value, the upconversion emission power p is close to zero. This threshold value is very high for general materials, about 1 mW cm⁻².^24,25 So almost all of the upconversion luminescence emissions can only be achieved under the laser excitation. Although laser can be used as the driving source for photocatalysis, it may not be practical in view of its small excitation area.^26–29 In this scenario, only the point light source such as fluorescent lamp or xenon lamp, and also the area light source such as sunlight irradiation are meaningful for application. But those sources are polychromatic light and the excitation density of the effective light is relatively lower compared with laser, it is difficult to achieve upconversion luminescence. Now let's compare the excitation density of fluorescent lamp and fiber laser. The illumination area (light spot) is fixed at 1 cm², the power of fiber laser and fluorescent lamp is 1 W and the distance from light sources to samples is fixed at 10 cm. So, the energy density of excitation light of fiber laser is 1/π(10 × 0.22)² = 65.7 mW cm⁻² (0.22 is the divergence angle provided by manufacturers) and the energy density of excitation light of fluorescent lamp is 1/4π(10)²/20 = 0.04 mW cm⁻² (20 is the effective wavelength width of the visible-light). The excitation density of the fiber laser is 1642 times as high as the fluorescent lamp.

To make upconversion materials used for photocatalysis, it is necessary to reduce the threshold of the energy density of excitation light. Simultaneously, the excitation range of the activation center should be as wide as possible so as to improve the utilization rate of the excitation light. Rare earth ions have been commonly considered as the luminescence center in upconversion process, for example Er³⁺, Ho³⁺ and Tm³⁺. Er³⁺ ions can produce efficient ultraviolet (UV) emissions under the excitation of green or blue lasers.^30,31 Recently, some reports demonstrated that TiO₂ photocatalysis behavior was extremely enhanced by doping Er³⁺ ions under visible light irradiation.^32–39 However, it still raises the question of whether UV upconversion emissions result in the enhanced photoactivities because no emission spectra have been given under a xenon lamp excitation. If upconversion emission cannot be achieved under 150 W xenon lamp excitation (the minimum power excitation source for spectrometers), it is hardly realistic for upconversion materials to be used for photocatalysis. Our previous works have reported UV upconversion emission of Er³⁺ upon 1 W 532 nm LED and 300 mW 460 nm LED irradiation.⁴⁰ So, it is possible for Er³⁺ doped upconversion material to be used in many fields, such as biology application and photocatalysis. However, the narrow absorption range in visible light region prevents its practical use in the fields of biology and photocatalysis. Y₂SiO₅:Pr³⁺, Li⁺ can emit a wide UV radiation of 260–360 nm under blue, green even red light excitation.⁴¹ E. L. Cates et al. have systemically investigated this material and successfully used it in microbial inactivation with two “cool white” 13 W compact fluorescent bulbs radiation.⁴² All of those provide a possible for Y₂SiO₅:Pr³⁺, Li⁺ to be used for photocatalysis.

In this work, in order to make TiO₂ fully absorbed, Y₂SiO₅:Pr³⁺, Li⁺ was synthesized using mesoporous silica (MCM-48) as the silica source material. The obtained data showed that Y₂SiO₅:Pr³⁺, Li⁺ can be effectively excited by 496 nm light obtained by a 150 W xenon lamp spectrometer (Hitachi F-4600). Afterwards, Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites were prepared through the ultrasonic dispersion and liquid boiling method. The photocatalytic property of the sample was examined in detail by decomposing the methylene blue. The results demonstrated that the visible-light photocatalytic activity of TiO₂ could be significantly enhanced in the presence of Y₂SiO₅:Pr³⁺, Li⁺.

2. Experimental section

2.1 Synthesis of MCM-48 powders

The homogeneous precipitation method was used for the preparation of mesoporous silica MCM-48. At first, cetyltrimethylammonium bromide (C₁₆H₃₃(CH₃)₃NBr, 0.0143 mol), ammonia solution (NH₃·H₂O, 24 mL), ethanol (CH₃CH₂OH, 100 mL) as well as deionized water (H₂O, 240 mL) were mixed by stirring magnetically until the homogenous solution was obtained. Then, tetraethylorthosilicate (TEOS, 7.8 mL) was added dropwise into the above mixed solution and further stirred until the transparent sol was successfully formed. After filtration and lavation, the separated deposit was calcined at 550 °C for 5 h, and then cooled down to room temperature naturally.

2.2 Synthesis of Y₂SiO₅:Pr³⁺, Li⁺ microcrystals

The various concentrations of Y₂SiO₅:Pr³⁺, Li⁺ microcrystals were synthesized via a hydrothermal method. Briefly, 0.03592 mol Y(NO₃)₃·6H₂O, 0.00048 mol Pr(NO₃)₃·6H₂O, 0.0018 mol Li₂CO₃ and 0.02 mol MCM-48 were dissolved into deionized water by the ultrasonic dispersion with 100 W output power ultrasound. Then, 0.12 mol NaOH solution which should control the mole ratio of n(OH⁻) : n(Y³⁺ + Pr³⁺ + Li⁺) for 3 : 1 was added dropwise into the above suspension. After thorough stirring, the mixture was transferred into a 100 mL teflon lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 110 °C for 20 h, and then cooled down slowly to room temperature. The product was centrifuged and washed by deionized water for several times. After drying in air at 60 °C for 20 h, the particles were heated in a muffle furnace at 700–1300 °C for 4 h. At last, the desired white Y₂SiO₅:Pr³⁺, Li⁺ particles were obtained.

2.3 Preparation of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites

Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ catalysts were prepared through the ultrasonic dispersion and liquid boiling methods in terms of the literature.⁴³ In a typical procedure, Y₂SiO₅:Pr³⁺, Li⁺ and TiO₂ powders (mole ratio for 5 : 2) were added into a 200 mL beaker filled with 30 mL deionized water, adequately dispersed by ultrasound for 30 min. The suspended liquid was heated up to the boiling point and kept for 30 min. After filtration and lavation, the separated deposit was put into a crucible and heated in a muffle furnace. The temperature was controlled at 550 °C for 90 min.

2.4 The evaluation of the photocatalytic performance

The photocatalytic performances of the obtained products were evaluated by the degradation of methylene blue (MB) in aqueous solution. In the experiment, 1.15 g of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composite was dispersed into a teflon lined stainless steel reactor containing 80 mL 10 mg L⁻¹ MB aqueous solution. Prior to irradiation, the reactor was kept in dark for 12 h for establishing an adsorption–desorption equilibrium of MB on the surface of photocatalysts. Then the suspensions were irradiated using a 500 W xenon lamp. A cut off filter (λ > 400 nm) was fitted to the aforementioned light source to get the applicable light irradiation. After each irradiation, 6.0 mL of the MB aqueous solution was taken out for the absorbance measurements. The dye concentration was calibrated using the absorption peak at 664 nm of MB.

2.5 Characterizations

Phase identification was performed at ambient temperature by X-ray diffraction (XRD, Bruker Optics, Ettlingen, Germany) with Cu Kα radiation (40 kV, 40 mA). The emission (PL) spectra were measured by a spectrophotometer (Hitachi F-4600) equipped with a 150 W Xenon lamp source and double excitation monochromators. The morphology was characterized by SEM (JSM-7500FJEOL, Japan) with the operating voltage of 30.0 kV. Energy dispersive spectroscopy (EDS) was obtained on a JSM-7500F microscope coupled with an EDS spectrometer. The absorption spectra were recorded with a UV-NIR spectrophotometer (Hitachi U-4100). Fourier transformation infrared spectroscopy (FTIR, Tensor 27, Bruker, Germany) was employed to check the crystal and groups of samples. Similar photocatalysis experiments were carried out under the 500 W xenon lamp (CEL-LAX500) of sunlight to check the photocatalytic activity of the Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ catalysts.

3. Results and discussion

3.1 Structural and morphological characterizations

3.1.1 XRD, SEM and FTIR of MCM-48 powder. Fig. 1 showed the XRD pattern and SEM image of the as-prepared sample. The XRD of the as-prepared sample reflected the intense diffraction peak of (211) plane approximately appeared at 2θ = 2.6° that matched with the standard reference (PDF#50-0511), indicating the formation of MCM-48 mesoporous silica. According to Bragg equation (2d sin θ = nλ, λ = 0.154 nm, n = 1), the mesoporous channels calculated value of diameter d was 3.31 nm. The scanning electron image indicated that the average particle size of the as-prepared sample was about 400 nm.

Fig. 1 XRD pattern and SEM image of the as-prepared MCM-48 powder and the XRD pattern of the standard data of MCM-48 powder.

Fourier transformation infrared spectroscopy (FTIR) was carried out to investigate the composition of the sample, as shown in Fig. 2. The absorption band at 462.9, 811.9, 960.5 cm⁻¹ could be attributed to the bending vibration, the stretching vibration as well as the asymmetric vibration of Si–O bond, respectively. Similarly, the Si–O–Si bond appeared at 1091 cm⁻¹. The absorption bands at 1260–1620 cm⁻¹ were attributed to the bending vibration of O–H bond of H₂O, which was adsorbed on the internal holes from the surface of the mesoporous silica (MCM-48) in some ways. The other absorption bands, such as 1468, 1491, 2855, 2924 cm⁻¹ and 3458 cm⁻¹ were mainly derived from the vibration absorption of the quaternary ammonium surfactant (CTAB). It was shown that some organic residues still existed in the material.

Fig. 2 FTIR spectra of the MCM-48 powder.

3.1.2 The crystal structure, surface profile, chemical character of Y₂SiO₅:Pr³⁺, Li⁺ microcrystal. The phase is related to the calcining temperature, the amount and type of dopants.⁴⁴ In this paper, we investigated the effect of sintering temperature and Li⁺ doping concentration on the material structure. Table 1 showed the effects of sintering temperature as well as Li⁺ concentration on the phase of Y₂SiO₅. The corresponding XRD pattern of Y₂SiO₅:Pr³⁺ was shown in Fig. S1 and S2, ESI.†

From the obtained data, it can be concluded that the calcinations temperature at 1100 °C with the concentration of Li⁺ above 7% allowed obtaining more suitable β phase structure.

Table 1 (a) The changing phase of Y₂SiO₅:Pr³⁺ doped with 0% and 9% Li⁺, respectively, calcined at different temperatures from 700 °C to 1300 °C. (b) Effect of various concentrations of Li⁺ co-doped on the crystal structure of Y₂SiO₅:Pr³⁺ annealed at 1100 °C

(a)
Temperature °C	Phase(0% Li^+ co-doped)	Phase(9% Li^+ co-doped)
700	Un-crystallization	Un-crystallization
800	Un-crystallization	Un-crystallization
850	Un-crystallization	α Phase
900	α Phase	α Phase
950	α Phase	α Phase
1000	α Phase	β Phase
1100	α Phase	β Phase
1200	β Phase	β Phase
1300	β Phase	β Phase

---

(b)
Li^+ concentration%	0	5	7	8	9	10	11	15
Phase 1100 °C	α	α	β	β	β	β	β	β

---

3.1.3 The combination of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂. Fig. 3(a) and (b) showed the morphologies of the as-prepared Y₂SiO₅:Pr³⁺, Li⁺ microcrystals and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites. The obvious agglomerates phenomenon could be observed in Fig. 3(b), which was resulted from the post-processing. The EDS spectra of synthesized Y₂SiO₅:Pr³⁺, Li⁺ and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ were shown in Fig. 3(c) and (d). Fig. 3(c) showed Pr particles percent content was 1.2%, which was approximate with the theoretical atomic ratio. Fig. 3(d) showed the ratio of Ti element was also similar with the precursor. All the data indicated that the TiO₂ nanocrystallites were well-mixed with Y₂SiO₅:Pr³⁺, Li⁺.

Fig. 3 SEM images of Y₂SiO₅:Pr³⁺, Li⁺ microcrystals (a) and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites. (b) The corresponding EDS pattern (c) and (d).

3.2 Optical properties

3.2.1 The optical properties of Y₂SiO₅:Pr³⁺, Li⁺. Upconversion emission spectra of Y₂SiO₅:Pr³⁺, Li⁺ with different dopant concentration were obtained by a xenon lamp 150 W with grating as its light splitter, as shown in Fig. 4. Upon the 496 nm excitation, the emission spectra in a broad range from 260 nm to 400 nm were obtained. With Li⁺ concentration increasing, the upconversion emission intensity increased firstly and then remarkably decreased as shown in Fig. 4(a). The similar result can be obtained by doping Pr³⁺ ions. So, the strongest upconversion emission came from Y₂SiO₅: 1.2% Pr³⁺, 9% Li⁺ calcined at 1100 °C.

Fig. 4 Upconversion emission spectra of Y₂SiO₅: x% Pr³⁺, y% Li⁺ excited by a xenon lamp 150 W with grating as its light splitter, calcined at 1100 °C (a) co-doped with different Li⁺ concentration (x = 1.2; y = 0, 5, 7, 8, 9, 10, 11, 15); (b) co-doped with different Pr³⁺ concentration (x = 0.9, 1.0, 1.1, 1.2, 1.3; y = 9).

3.2.2 The optical properties of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂. Fig. 5 showed the emission spectra of Y₂SiO₅:Pr³⁺, Li⁺ and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites under 496 nm excitation by a xenon lamp 150 W with grating as its light splitter. The emission spectrum of Y₂SiO₅:Pr³⁺, Li⁺ microcrystals contained two emission peaks located at 288 nm (4.306 eV) and 320 nm (3.875 eV), respectively. It's worth noting that the emission spectrum of Y₂SiO₅:Pr³⁺, Li⁺ microcrystals disappeared while TiO₂ was adsorbed on their surface. It indicated that UV upconversion emission of Y₂SiO₅:Pr³⁺, Li⁺ can be efficiently transferred to TiO₂. As seen in the inset of Fig. 5 Y₂SiO₅: 7.2% Pr³⁺, 9% Li⁺ can be efficiently excited by blue green-light (460–520 nm) in visible-light region. So, Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites can be efficiently excited by photons with light wavelengths longer than about 400 nm in the visible-light range, which enables more efficient use of solar energy.

Fig. 5 Upconversion emission spectra of Y₂SiO₅:Pr³⁺, Li⁺ phosphor powders and Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites under 496 nm excitation by a xenon lamp 150 W with grating as its light splitter, with the inset of excitation spectra (λ_em = 283 nm) of Y₂SiO₅:Pr³⁺, Li⁺ in visible-light region.

3.3 Photocatalytic activity and mechanism

From the previous section, it can be seen that Y₂SiO₅:Pr³⁺, Li⁺ can be efficiently excited by a xenon lamp 150 W and UV upconversion emission can be efficiently transferred to TiO₂. In this section, the decolorization of MB was carried out to verify the photocatalytic activity of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ catalysts under visible-light by the 500 W xenon lamp. Fig. 6(a) showed the absorption spectra of MB catalyzed by the Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites as a function of irradiation time. With the increase of irradiation time, the strong absorption band at 664 nm decreased steadily. This result clearly demonstrated that the obtained composites could be used to decompose MB by visible light, as predicted. The changes in concentration of C/C₀ with different exposure times were plotted in Fig. 6(b), where C₀ was the original concentration of MB and C was the concentration of MB irradiated with a 500 W xenon lamp for time t. The degradation efficiency could be evaluated through the change of the MB concentrations before and after irradiation. Fig. 6(b) indicated that 80% of MB was decomposed after 80 h irradiation by Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites, which was far higher than that by pure TiO₂. First-order fitting of the ln(C/C₀) versus time for the photodegradation was usually used to study the reaction kinetics. Based on the photodegradation rate of MB with time, the photodegradation reaction kinetics lines were drawn out, as shown in Fig. 6(c). According to the reaction kinetics linear equation and correlation indexes: −ln(C/C₀) = 0.01747t − 0.11227, R = 0.97520, −ln(C/C₀) = 0.00119t − 0.00555, R = 0.96991, for Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites and pure TiO₂, respectively, it clearly presented that the rate constant for Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites (0.01747) was about 14 times as high as that for pure TiO₂ (0.00119). All of these results implied that the degradation reaction of MB in the visible light follows the first-order kinetic law.

Fig. 6 (a) Variation in absorbance spectra of MB photocatalyzed by Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ as a function of the irradiation time under a 500 W xenon lamp irradiation. (b) Comparison of the normalized concentration of MB decomposed by Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ and TiO₂. (c) The photodegradation reaction kinetics data of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites and TiO₂.

To explain the above phenomenon, the photocatalysis mechanism of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ catalyst was discussed. In Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composite, Pr³⁺ ion only exists in Y₂SiO₅ host. Fig. 7 showed the energy transfer upconversion (ETU) mechanism between neighboring Pr³⁺ ions and TiO₂. The population of ³P_J states via blue photon absorption and the subsequent energy transfer between ions could promote an electron to the 4f5d energy band, resulting in the emission of UVB- and UVC-range photons which corresponding to 4f5d → ³H_J/³F_J.^45,46 Then TiO₂ was excited by the ultraviolet-light photon. The activated TiO₂ produced reductive electrons (e⁻) and oxidative holes (h⁺) in the conduction band (CB) and the valence band (VB), respectively, and these electron–hole pairs migrated from the inner region to the surfaces to take part in surface reactions. Eventually, hydroxyl radical (˙OH), O₂²⁻ and H⁺ were formed.⁴⁷ The product can mineralize the organic chemicals and it could also destroy the chromophore position. Obviously, from the above photocatalysis mechanism, the photocatalytic activity of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composite depended strongly on the contact area between Y₂SiO₅:Pr³⁺, Li⁺ and TiO₂ particles. To enhance the TiO₂ photocatalysis behavior, it is necessary to enlarge the contact area between Y₂SiO₅:Pr³⁺, Li⁺ and TiO₂ particles.

Fig. 7 Photocatalysis mechanism of Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ catalyst (ET: energy transfer).

4. Conclusions

The upconversion luminescence agents that can be effectively excited by the ordinary illumination sources (fluorescent lamp or xenon lamp) have broad applying prospect in biology, medicine, environment and soon. In this paper, Pr³⁺ doped Y₂SiO₅ upconversion nanomaterials can be effectively excited by the blue light from a xenon lamp 150 W with grating as its light splitter. The obtained data indicated that UV upconversion emission can be effectively transferred to TiO₂. All of these performances ensure the Y₂SiO₅:Pr³⁺, Li⁺/TiO₂ composites to be a good visible light catalyst.

Acknowledgements

This work was supported by National Science Foundation of China (no. 11474083), Hebei Province Department of Education Fund (ZD2014069).

Notes and references

1. S. Shen, J. Chen, X. Wang, L. Zhao and L. Guo, J. Power Sources, 2011, 196, 10112–10119.
2. G. Kim, S. H. Lee and W. Choi, Appl. Catal., B, 2015, 162, 463–469.
3. D. X. Xu, Z. W. Lian, M. L. Fu, B. Yuan, J. W. Shi and H. J. Cui, Appl. Catal., B, 2013, 142–143, 377–386.
4. Z. Zhang and W. Wang, Dalton Trans., 2013, 12072–12074.
5. Y. He, Y. Wang, L. Zhang, B. Teng and M. Fan, Appl. Catal., B, 2015, 168–169, 1–8.
6. Z. Gao, Z. Cui, S. Zhu, Y. Liang, Z. Li and X. Yang, J. Power Sources, 2015, 283, 397–407.
7. K. Nakata and A. Fujishima, J. Photochem. Photobiol., C, 2012, 13, 169–189.
8. A. Fujishima and X. Zhang, C. R. Chim., 2006, 9, 750–760.
9. Y. Chen, S. Mishra, G. Ledoux, E. Jeanneau, M. Daniel, J. Zhang and S. Daniele, Chem.–Asian J., 2014, 9, 2415–2421.
10. Q. Yin, R. Qiao, L. Zhu, Z. Li, M. Li and W. Wu, Mater. Lett., 2014, 135, 135–138.
11. L. W. Zhang, H. B. Fu and Y. F. Zhu, Adv. Funct. Mater., 2008, 18, 2180–2189.
12. Y. H. Ng, S. Ikeda, M. Matsumura and R. Amal, Energy Environ. Sci., 2012, 5, 9307–9318.
13. J. Reszczyńska, T. Grzyb, J. W. Sobczak, W. Lisowski, M. Gazda, B. Ohtani and A. Zaleska, Appl. Catal., B, 2015, 163, 40–49.
14. D. Hou, R. Goei, X. Wang, P. Wang and T. T. Lim, Appl. Catal., B, 2012, 126, 121–133.
15. A. Zaleska, P. Górska, J. W. Sobczak and J. Hupka, Appl. Catal., B, 2007, 76, 1–8.
16. X. Guo, C. Chen, W. Song, X. Wang, W. Di and W. Qin, J. Mol. Catal. A: Chem., 2014, 387, 1–6.
17. S. Guo, D. Li, Y. Zhang, Y. Zhang and X. Zhou, Electrochim. Acta, 2014, 121, 352–360.
18. C. Li, F. Wang, J. Zhu and J. C. Yu, Appl. Catal., B, 2010, 100, 433–439.
19. H. Wang, G. Wang, Y. Ling, M. Lepert, C. Wang, J. Z. Zhang and Y. Li, Nanoscale, 2012, 4, 1463–1466.
20. Y. Zhu, Z. Chen, T. Gao, Q. Huang, F. Niu, L. Qin, P. Tang, Y. Huang, Z. Sha and Y. Wang, Appl. Catal., B, 2015, 163, 16–22.
21. Y. Li, K. Pan, G. Wang, B. Jiang, C. Tian, W. Zhou, Y. Qu, S. Liu, L. Feng and H. Fu, Dalton Trans., 2013, 7971–7979.
22. X. Guo, W. Song, C. Chen, W. Di and W. Qin, Phys. Chem. Chem. Phys., 2013, 15, 14681–14688.
23. J. Méndez-Ramos, P. Acosta-Mora, J. C. Ruiz-Morales, T. Hernández, M. E. Borges and P. Esparza, RSC Adv., 2013, 3, 23028.
24. Y. Yang, C. Mi, F. Jiao, X. Su, X. Li, L. Liu, J. Zhang, F. Yu, Y. Liu, Y. Mai and J. Ballato, J. Am. Ceram. Soc., 2014, 97, 1769–1775.
25. Y. Yang, C. Mi, F. Yu, X. Su, C. Guo, G. Li, J. Zhang, L. Liu, Y. Liu and X. Li, Ceram. Int., 2014, 40, 9875–9880.
26. S. Huang, Z. Lou, Z. Qi, N. Zhu and H. Yuan, Appl. Catal., B, 2015, 168–169, 313–321.
27. X. Guo, W. Di, C. Chen, C. Liu, X. Wang and W. Qin, Dalton Trans., 2014, 1048–1054.
28. W. Qin, D. Zhang, D. Zhao, L. Wang and K. Zheng, Chem. Commun., 2010, 46, 2304–2306.
29. Y. Tang, W. Di, X. Zhai, R. Yang and W. Qin, ACS Catal., 2013, 3, 405–412.
30. F. Qin, Y. Zheng, Y. Yu, C. Zheng, H. Liang, Z. Zhang and L. Xu, J. Lumin., 2009, 129, 1137–1139.
31. H. Guo, Y. Li, D. Wang, W. Zhang, M. Yin, L. Lou and S. Xia, J. Alloys Compd., 2004, 376, 23–27.
32. J. Wang, R. Li, Z. Zhang, W. Sun, R. Xu, Y. Xie, Z. Xing and X. Zhang, Appl. Catal., A, 2008, 334, 227–233.
33. S. Obregon and G. Colon, Chem. Commun., 2012, 48, 7865–7867.
34. L. Yin, Y. Li, J. Wang, Y. Zhai, J. Wang, Y. Kong, B. Wang and X. Zhang, Environ. Prog. Sustainable Energy, 2013, 32, 697–704.
35. S. L. Guangjian Feng, Z. Xiu, Y. Zhang, J. Yu, Y. Chen, P. Wang and X. Yu, J. Phys. Chem. C, 2008, 112, 13692–13699.
36. S. Li, Y. Guo, L. Zhang, J. Wang, Y. Li, Y. Li and B. Wang, J. Power Sources, 2014, 252, 21–27.
37. Y. Yang, C. Zhang, Y. Xu, H. Wang, X. Li and C. Wang, Mater. Lett., 2010, 64, 147–150.
38. Y. Zheng and W. Wang, J. Solid State Chem., 2014, 210, 206–212.
39. S. Obregón, A. Kubacka, M. Fernández-García and G. Colón, J. Catal., 2013, 299, 298–306.
40. Y. Yang, C. Mi, X. Su, F. Jiao, L. Liu, J. Zhang, F. Yu, X. Li, Y. Liu and Y. Mai, Opt. Lett., 2014, 39, 2000.
41. E. L. Cates and J.-H. Kim, Opt. Mater., 2013, 35, 2347–2351.
42. E. L. Cates, M. Cho and J.-H. Kim, Environ. Sci. Technol., 2011, 45, 3680–3686.
43. B. Mitu, S. Vizireanu, R. Birjega, M. Dinescu, S. Somacescu, P. Osiceanu, V. Pârvulescu and G. Dinescu, Thin Solid Films, 2007, 515, 6484–6488.
44. E. L. Cates, A. P. Wilkinson and J.-H. Kim, J. Phys. Chem. C, 2012, 116, 12772–12778.
45. S. L. Cates, E. L. Cates, M. Cho and J. H. Kim, Environ. Sci. Technol., 2014, 48, 2290–2297.
46. C. L. Sun, J. F. Li, C. H. Hu, H. M. Jiang and Z. K. Jiang, Eur. Phys. J. D, 2006, 39, 303–306.
47. Q. Kang, Q. Z. Lu, S. H. Liu, L. X. Yang, L. F. Wen, S. L. Luo and Q. Y. Cai, Biomaterials, 2010, 31, 3317–3326.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra06416c

‡ These authors contribute equally to this work.

---