Optical properties of composited TiO2-aluminium-doped ZnS photoanode

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ScienceDirect Materials Today: Proceedings 17 (2019) 1693–1701

www.materialstoday.com/proceedings

MRS-Thailand 2017

Optical properties of composited TiO2-aluminium-doped ZnS photoanode Yingyot Infahsaenga*, Sarute Ummartyotinb a

b

Physics, Faculty of Science and Technology, Thammasat University, Pathum Thani 12120, Thailand Materials and Textile Technology, Faculty of Science and Technology, Thammasat University, Pathum Thaini 12120, Thailand

Abstract A composited photoanode for optoelectronic devices have widely attracted much attention for many decades. In this work, wide band gap semiconductors, ZnS, and aluminium-doped ZnS were synthesized through simply chemical synthetic route with band gap energy of 3.67 – 3.69 eV. The thin films of TiO2/ZnS and TiO2/aluminium-doped ZnS composites with various concentration were prepared by doctor blade technique. By means of X-ray diffraction, the thin film structures of TiO2 is rarely dominated by ZnS or aluminium-doped ZnS. A ruthenium sensitizer (N719) was sensitized on the composited thin films. The absorption and photoluminescence of the composited photoanode were characterized, indicating the band gap energy of 3.41 eV for all thin films. It was observed that both ZnS and aluminium-doped Zns reduce the amount of dye uptake. Also, the photoluminescence of dye sensitized TiO2/ZnS and TiO2/aluminium-doped ZnS photoanode was rather blue-shifted and enhanced with the increasing of ZnS or aluminium-doped ZnS amount. Moreover, using Time Correlated Single Photon Counting technique (TCSPC), the emission lifetimes of dye sensitized TiO2, TiO2/ZnS and TiO2/aluminium-doped ZnS anodes are 3.32 ns, 8.40 ns and 6.42 ns, respectively which indicate the charge injection from excited dye to photoanode. © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: DSSCs; composited photoanode; ZnS; Al-doped ZnS

* Corresponding author. Tel: +66 2564 4529.; fax: +66 2986 9112-3 E-mail address: [email protected]

2214-7853 © 2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.

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1. Introduction Wide band gap semiconductors, especially Zinc Sulphide (ZnS), have been extensively interested due to its widely application such as light emitting diode [1], active sensor [2] and photovoltaic device [3]. It has been reviewed that many approaches of ZnS nanostructure based solar cell can be achieved [4]. This is due to the variation on structural and optical properties of ZnS, which can be simply controlled and modified. Up to the present time, the synthetic route of ZnS has been extensively investigated either physical or chemical techniques for various application and modifications [5]. One of the famous modification is metal doping ZnS, for example, Mn-doped, Cudoped, Al-doped ZnS, which effect on physical and optical properties [6–8]. In recent years, Al-doped ZnS particle was successfully synthesized via simple wet chemical synthetic route [9]. Moreover, Al-doped ZnS have been prepared by several methods such as close-spaced evaporation [10] and solution growth method [11]. Up to the present time, ZnS and metal-doped ZnS nanostructure have been intensively studied from the point of synthetic techniques to its application, especially solar cells. One of the prospective application of ZnS was employed as anode materials or buffer layer for solar cells. The ZnS have been attracting attention because the properties of ZnS and traditional semiconductor such as titanium dioxide (TiO2) or zinc oxide (ZnO) are rather similar. For the dye sensitized solar cells (DSSCs), wide band gap semiconductors were assigned to be the mesoporous anode, however it might be used as buffer layer in quantum dots sensitized solar cells [3]. Recently, the three monolayers of ZnS was overcoated between TiO2 and QD interface, this treatment can improve the solar cell performance [12]. Although the ZnS has been applied to many type of solar cells, but Al-doped ZnS has very few reports about its solar cells application. It was reported that Al-doped ZnS shell in the core-shell quantum dots solar cell can enhance the photo-stability [13,14]. From the fundamental point of view, the optical properties and its dynamics of ZnS and Al-doped ZnS are the important knowledge. By means of time correlated single photon counting (TCSPC) technique, the lifetime of photoluminescence on the nanosecond time scale could be investigated. Recently the ultrafast charge transport dynamics of core-shell system with ZnS were studied by TCSPC [15,16]. Typically, the lifetime of photo-excited Ruthenium dye on TiO2 photoanode is ca. 2.0 ns [17], which may be accelerated or suppressed by the intermolecular interaction. Lately, photoactive ZnS/TiO2 nano-composited has been studied [18,19], which reveal that the study of dye sensitized composited photoanode is possible. Although ZnS and Al-doped ZnS have been studied and applied to some solar cells, but there has been no detailed report on the optical properties and dynamics of dye sensitized composited photoanode. In this article, we wish to present on the preparation of ZnS and Al-doped ZnS from a simple synthetic route. Then the dye sensitized TiO2:ZnS and TiO2:Al-doped ZnS will be prepare by a simple method. The physical properties of powder and thin film were identified to confirm the synthesized materials. Finally, the optical properties and dynamics of composited photoanode will be investigated. 2. Experimental 2.1. Materials Zinc chloride (ZnCl2·7H2O, 99.99%) and Aluminium Hydroxide (Al(OH)3, >95%) were purchased from Sigma Aldrich. Sodium Sulphide (Na2S·9H2O, >95%) was purchased from Ajax Fine Chem. Titanium dioxide (TiO2) paste with ~18% wt nanoparticles and Ruthenium dye or N719 (cis-diisothiocyanato-bis(2,2’-bipyridyl-4,4’dicarboxylato) ruthenium(II) bis(tetrabutyl-ammonium), Ruthenizer 535-bisTBA) was purchased from Solaronix. Analytical grade of ethanol, methanol and polyvinyl alcohol (PVA) was purchased from RCI Labscan, Thailand. Distilled water and analytical grade of methanol were used as solvent. All the chemical reagents were used as received without any purification. 2.2. Sample synthesis At first the ZnS powder was prepared by one pot synthesis method. Aqueous solution of 0.2 mol Na2S and ZnCl2 was prepared in 200 ml of deionized (DI) water. At the same time, 3.0 mg of PVA was dispersed in 10 ml of DI

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water. All solutions were stirred for 30 min at 55 oC. PVA solution was mixed with zinc chloride solution. Subsequently, sodium sulphide solution was poured into the mixture and stirred for 4 hr at 55o. For Al-doped ZnS, 0.2 mol of ZnCl2 was blended with Al(OH)2 with Zn:Al molar ratio of 9:1, so that the doping concentration of aluminium is 10%. The blend was dissolve into 200 ml of DI water and stirred for 30 min at 55 oC. The ZnCl2:Al(OH)2 and Na2S solutions were mixed and stirred for 4 hr at 55 oC. Afterwards, the ZnS and Al-doped ZnS powder were obtained through the filter and vacuum pump. The impurities were removed using methanol and DI water. Finally, the ZnS and Al-doped ZnS powder were kept in oven at 80 oC overnight and later grinded through 75 µm filter. 2.3. Photoanode preparation Nanocrystalline TiO2 and its composited films were prepared as follow. Firstly, a composited TiO2:ZnS and TiO2:Al-doped ZnS gels were prepared in which the ZnS or Al-doped ZnS were added into 2 g TiO2 pastes with different TiO2:ZnS or TiO2:Al-doped ZnS ratio of 99:1, 98:2, 96:4, 94:6, 92:8, and 90:10. All gels were stirred for 5 min at 1000 rpm. Note that the amount of TiO2 is ca. ~18% of colloidal suspension paste. TiO2 and its composited paste were deposited on a clean grass substrate by using the doctor blade technique. All films were sintered in furnace at 475 oC for 30 min to yield a 5 µm thick films. To prepare the photoanode, TiO2 and its composited films were sensitized in a 0.1 mM N719 solution of ethanol at room temperature for 12 hr. After sensitization, the N719 sensitized TiO2 or its composited electrode was taken from the bath and washed off by rinsing the film with the same solvent. The photoanode was dried at room temperature for certain time. 2.4. Characterizations The structural properties of ZnS and Al-doped ZnS powder and TiO2 and its composited films were carried out by X-ray diffraction (XRD, model D8-discover, Bruker) system using CuKα radiation. The morphologies and compositions of sample powders were investigated by scanning electron microscopy, SEM (JOEL JSM-6301F) at an acceleration voltage of 20 keV and EDX, respectively. The absorption measurement of all samples were performed by UV-Vis Spectrophotometers (Shimadzu, UV-2600) in the wavelength range of 250 – 700 nm. Steadystate fluorescence spectra and time-resolved fluorescence measurement of photoanodes were carried out by spectrofluorometer and time correlated single photon counting technique, respectively (Edinburgh Instrument, FLS980) with an excitation wavelength of 560 nm which is close to the maximum absorption peak of N719. The emission photons for TCSPC were collected at wavelength of 720 – 735 nm. The fluorescence decay was analyzed thorough DecayFit software (FluorTools) using a double exponential model. 3. Results and discussion 3.1. Structural and optical properties of ZnS and Al-doped ZnS powders The XRD patterns of ZnS and Al-doped ZnS powders are shown in Fig. 1. All samples show three diffractions peak at 29o, 48o, and 57o, which assigned to the lattice planes of (111), (220), and (311), respectively. These peaks imply that the ZnS and Al-doped ZnS form the cubic zinc blended structure (JCPDS No. 05-0566). Note that the extra phase is not presented in the XRD patterns of Al-doped ZnS powders, which confirm the similar structure of both powders. The crystalline size is calculated by the Debye-Scherer formula, =

/

(1)

where D is the mean crystalline size, k is constant (shape factor ca. 0.9 for ZnS),  is the X-ray wavelength (1.5406 Å for Cu-Kα), β is the full width at half maximum of the diffraction peak and  is the Bragg angle. As

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shown in Table 1, the crystalline size and lattice parameter of Al-doped ZnS at the (111) peak are slightly less than the crystalline size of ZnS, which correspond to the previous reported [9,20]. Table 1. Crystalline size and lattice parameter of ZnS and Al-doped ZnS. Crystalline size Lattice parameter Samples (Å) (Å) ZnS 19.72 5.40 Al-doped ZnS 19.19 5.38

Intensity (a.u.)

Al-doped-ZnS (111)

(220)

(311)

ZnS 20

30

40

50

60

70

2 (Degree) Fig. 1. XRD pattern of ZnS and Al-doped ZnS powders.

The morphologies and compositions of ZnS and Al-doped ZnS powder were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX), respectively. Fig. 2 represent the SEM image of ZnS and Al-doped ZnS samples and the insets show EDX spectra of each samples. All samples are blocky particles shape with the formation of agglomeration, however, the small particle size of Al-doped ZnS can be more observed. The EDX spectra confirm that ZnS sample consist of mainly Zn and S, while Al-doped ZnS exhibit an additional Al atom. The qualitative elemental analysis of ZnS powder shown that the EDX atomic% of Zn2+ is 55.34% and decrease to 48.83% when Al substituted into ZnS. (a)

(b)

Fig. 2. SEM images of ZnS (a) and Al-doped ZnS (b). The insets show the EDX spectrum of each samples.

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The optical absorbance spectra of ZnS and Al-doped ZnS powders are shown in Fig. 3. All samples are transparent in the visible region and opaque in the ultra-violet region. The insets of Fig. 3 represent the Tauc’s plot that the optical band gap of semiconductor can be identified via the equation: ( ℎ )

/

= (ℎ −

)

(2)

where A is a constant related to the effective mass, α is the absorption coefficient, Eg is the band gap energy, ℎν is the photon energy, and n is ½ for direct band gap semiconductor. By using a linear fitting, the band gap energy of bulk ZnS is approximately at 3.69 eV which is close to the previous reported [9,11]. By doping ZnS with small amount of Al, the band gap is slightly decreased to ca. 3.67 eV due to the formation of shallow levels or impurity level in the band gap [20]. 1.6 (h) x10 (a.u.)

5

ZnS Al-doped ZnS

4 3

12

1.2

2

2

Absorbance (a.u.)

1.4

1.0

1 0

0.8

3.0

0.6

3.5

4.0

4.5

5.0

650

700

Energy (eV)

ZnS Al-doped ZnS

0.4 0.2 0.0 250

300

350

400

450

500

550

600

Wavelength (nm) Fig. 3. Absorption spectra for the ZnS and Al-doped ZnS samples. The inset shows the Tauc’s plot.

3.2. Structural and optical properties of thin films and photoanodes Figure 4 shows a comparison of the XRD spectra of different wt% of ZnS or Al-doped ZnS. All patterns exhibit the diffraction peaks approximately at 26o, 38.5o, 48.7o, 54.4 and 55.6 which is correspond to the anatase structure of TiO2 (JCPDS:21-1272). Note that the diffraction peaks of ZnS and Al-doped ZnS are not pronounced which may be caused by a deep embed ZnS or Al-doped ZnS particles in the samples. However, all XRD patterns indicate that the TiO2 structure is not dominated by ZnS and Al-doped ZnS particles. All the films have been sensitized with ruthenium dye that supposed to be attached on TiO2 surface. Thus, unaffected TiO2 structure can confirm that the dye will be adsorbed on the thin film samples. Optical properties of TiO2:ZnS and TiO2:Al-doped ZnS composited thin films were depicted in Fig. 5(a) and 5(c), respectively. Interestingly, the slightly increasing of absorption in the visible region can be observed when the amount of ZnS and Al-doped ZnS highly increase. However, the absorption of composited thin films does not significantly change at low amount of ZnS and Al-doped ZnS, approximately 1 – 2 wt%. Note that the effect does not dramatically difference for ZnS and Al-doped ZnS adding at all amount. The results indicate that the aluminum substitution in ZnS at 10% doping concentration does not significantly affect the optical properties of composited thin film, even though the band gap energies of ZnS and Al-doped ZnS are fairly difference. Using Tauc’s plot analysis as shown in the inset of Fig. 5(a) and 5(c), the band gap energy is ca. 3.41 eV for all TiO2, TiO2:ZnS and TiO2:Al-doped ZnS composited thin films which is close to the band gap energy of TiO2 thin film in previous reported [21].

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(a)

TiO2:ZnS (10%)

TiO2:Al-doped ZnS (8%)

Intensity (a.u.)

Intensity (a.u.)

TiO2:ZnS (8%) TiO2:ZnS (4%) TiO2:ZnS (2%) TiO2:ZnS (1%)

TiO2:Al-doped ZnS (4%) TiO2:Al-doped ZnS (2%) TiO2:Al-doped ZnS (1%) TiO2

TiO2

20

25

30

(b)

TiO2:Al-doped ZnS (10%)

35

40

45

50

55

60

65

70

20

25

30

35

2 (degree)

40

45

50

55

60

65

70

2 (degree)

Fig. 4. XRD patterns of TiO2:ZnS (a) and TiO2:Al-doped ZnS (b)

0.4

TiO2:ZnS Films

3.0

(a)

2.5

0.2

0.1

0.0 350

0% 1% 2% 4% 8% 10% 400

8 6 4

Absorbance (a.u.)

(h)2 (a.u.) x107

Absorbance (a.u.)

10

0.3

0% 1% 2% 4% 8% 10%

2 0 3.0

3.1

3.2

3.3

3.4

3.5

Energy (eV)

450

500

550

600

650

2.0

0% 1% 2% 4% 8% 10%

1.5 1.0 0.5 0.0 350

700

400

2.5

0.2

0.1

0.0 350

0% 1% 2% 4% 8% 10% 400

6 4 2 0 3.0

0% 1% 2% 4% 8% 10%

3.1

3.2

3.3

3.4

3.5

Energy (eV)

450

500

550

600

Wavelength (nm)

650

700

Absorbance (a.u.)

(h)2 (a.u.) x107

Absorbance (a.u.)

10

0.3

500

550

600

650

700

3.0

(c)

TiO2:Al-doped ZnS Films 8

450

Wavelength (nm)

Wavelength (nm) 0.4

(b)

TiO2:ZnS - Dye

(d)

TiO2:Al-doped ZnS - Dye

2.0

0% 1% 2% 4% 8% 10%

1.5 1.0 0.5 0.0 350

400

450

500

550

600

650

700

Wavelength (nm)

Fig. 5. Absorption spectra of TiO2:ZnS films (a) and photoanode (b) and TiO2:Al-doped ZnS films (c) and photoanode (d). The inset shows the Tauc’s plot.

In case of photoanode usage, the high amount of ZnS and Al-doped ZnS may cause a lower photon transmission which may lower the number of excited dye and the power conversion efficiency of dye sensitized solar cells. In addition, the absorption of dye sensitized composited photoanodes were carried out as shown in Fig. 5(b) and 5(d) for TiO2:ZnS and TiO2:Al-doped ZnS, respectively. As we expected, the absorption of dye sensitized TiO2

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photoanode present the highest absorption at all conditions with the absorption peak at wavelength of 535 nm, whereas the absorption of dye sensitized composited photoanode are significantly lower. Moreover, the absorption peak of all composited photoanode is at the same wavelength peak of dye sensitized TiO2 photoanode, indicating that ZnS and Al-doped ZnS do not affect the electronic structure of dye and the aggression effect does not exist. While the absorption of dye sensitized TiO2:ZnS photoanode are slightly increased when the amount of ZnS addition increase, the absorption of dye sensitized TiO2:Al-doped ZnS photoanode does not reveal any difference. Note that the background spectra due to the absorption of composited thin films were yet subtracted from the absorption spectra of dye sensitized photoanode. These results imply that the dye may be adsorbed on the TiO2:ZnS photoanode more appropriate than the TiO2:Al-doped ZnS photoanode. Hence, aluminum atom may limit the dye adsorption on TiO2 and Al-doped ZnS photoanode. The dye sensitized composited photoanodes were excited at wavelength of 535 nm which is the maximum peak of N719 dye to ensure that no excitation occurs on TiO2, ZnS, and Al-doped ZnS elements. Thus, the room temperature photoluminescence spectra are mainly due to the emission of photo-excited dye. Typically, the electron from photo-excited dye are injected into the conduction band of TiO2 semiconductors and thus, leave a less photoexcited dye to emit the light as shown in the dash line of Fig. 6.

4500

TiO2:ZnS - Dye

4000 3500 3000 2500 2000 1500

Photoluminescence (a.u.)

Photoluminescence (a.u.)

4500

(a)

0% 1% 2% 4% 8% 10%

1000 500 0 600

650

700

750

Wavelength (nm)

800

850

TiO2:Al-doped ZnS - Dye

4000 3500 3000 2500 2000

(b)

0% 1% 2% 4% 8% 10%

1500 1000 500 0 600

650

700

750

800

850

Wavelength (nm)

Fig. 6. Photoluminescence spectra of dye-sensitized TiO2:ZnS (a) and TiO2:Al-doped ZnS photoanodes at various amount of additive.

All photoluminescence intensities of dyes sensitized composited photoanode are higher than that of TiO2 photoanode. Note that the dye adsorption on TiO2:ZnS and TiO2:Al-doped ZnS is less than that on TiO2. Interestingly, the high photoluminescence intensity of dye sensitized TiO2:ZnS and TiO2:Al-doped ZnS photoanode can be observed due to less electron injection into the conduction band of TiO2 and ZnS or Al-doped ZnS. Moreover, the photoluminescence intensity increases as the amount of ZnS highly increase, which imply that the ZnS addition may cause the electron injection barrier. However, the intensities from dye sensitized TiO2:Al-doped ZnS photoanode is not dramatically changed when the amount of Al-doped ZnS increase. Also, the low intensity of dye sensitized TiO2:Al-doped ZnS photoanode exhibit an appropriate electron injection. All above photoluminescence results imply that there is more photo-excited dye left due to the electron injection blocking of ZnS and Al-doped ZnS. According to the previous reported [22], the conduction band of ZnS is moderately higher than the conduction band of TiO2, and thus the electron injection from photo-excited dye to semiconductors could be blocked. Moreover, the photoluminescence maximum peak of dye sensitized TiO2 photoanode is at 735 nm, but the maximum peak of dye sensitized composited photoanode is blue shifted to be 720 nm. Earlier studies shown that a small amount of Zn2+ ions can dissolve into the dye solution during sensitization process [23]. Subsequently, the surface aggregation of dye with ZnS or Al-doped ZnS may occur, leading to the blue shift in photoluminescence spectra.

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Transient fluorescence decay measurement was carried out using time-correlated single photon counting (TCSPC) technique as shown in Fig. 7. It is clearly seen that the emission decay dynamics is strongly accelerated by TiO2, while the decay dynamics is retard to longer life time for TiO2:ZnS and TiO2:Al-dopec ZnS photoanode. The decay dynamics was fitted with a double exponential model convoluted with the experimental IRF from the equation: /

( )=

+ (1 −

/

)

)

(3)

where t1 and t2 are electron lifetime, a1 is amplitude component. The fitting results are shown in Table 2. The emission lifetime of dye sensitized TiO2 photoanode is 3.32 ns, which is at the same order of previous reported [17]. The emission decay of dye sensitized composited photoanode take place comparatively slowly due to electron injection blocking of ZnS and Al-doped ZnS. Interestingly, Al-doped ZnS photoanode exhibited a faster emission decay compare to ZnS photoanode indicating that the electron transfer was faster from the photo-excited dye to Aldoped ZnS than to ZnS. It was reported that the aluminum may cause the effect on the valence band edge of Aldoped ZnS [24], imply that the faster electron transfer on TiO2:Al-doped ZnS photoanode is not due to the lower of conduction band. Instead, Al-doped ZnS may slightly interact with TiO2 nanoparticle and lead to an electron transfer channel. 1 TiO2

Intensity (Norm)

TiO2:ZnS TiO2:Al-doped ZnS Fitting lines 0.1

0.01

0

25

50

75

100

125

Time (ns)

150

175

200

Fig. 7. Emission decay dynamics of dye sensitized on TiO2, TiO2:ZnS, and TiO2:Al-doped ZnS photoanode.

Table 2. Relative amplitude and lifetime constant obtained from the emission decay fitting. Samples TiO2 TiO2:ZnS TiO2:Al-doped ZnS

a1 0.70 0.63 0.65

t1 (ns) 3.32 8.40 6.42

t2 (ns) 21.99 45.24 38.58

4. Conclusions ZnS and Al-doped ZnS were successfully synthesized by simply method. The crystal structure, morphologies, transmittance, and band gap energy of ZnS and Al-doped ZnS were rather similar. By simply mixing, dye sensitized composited photoanode were prepared with mainly structure of TiO2. Although, the absorption of composited thin films was slightly increased, but the absorption of dye sensitized composited photoanode was lower than that of TiO2 photoanode due to a lack of dye adsorption. The blue shift of photoluminescence is assigned to the aggregation

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of dye with Zn2+. It was found that the electron transfer from photo-excited dye to TiO2 was suppressed by ZnS and Al-doped ZnS. Moreover, the decay lifetimes of dye sensitized composited photoanode were extended to be longer lifetime. Acknowledgements The authors would like to thank Vidyasirimedhi Institute of Science and Technology for photoluminescence facility. The authors also thank Siripassorn Sukhkhawut and Kamonchanok Wangcharoen for their help. The authors gratefully acknowledge the financial support provided by the Thailand Toray Science Foundation (TTSF), Thailand Contract No. 19-Phy01-2015 and Faculty of Science and Technology, Thammasat University, Thailand Contract No. (2) 9/2559. References [1] X. Ma, J. Song, Z. Yu, Thin Solid Films 519 (2011) 5043–5045. [2] S. Park, S. An, Y. Mun, C. Lee, Curr. Appl. Phys. 14 (2014) S57–S62. [3] Y. Lin, Y. Lin, Y. Meng, Y. Wang, Ceram. Int. 40 (2014) 8157–8163. [4] S. Ummartyotin, Y. Infahsaeng, Renew. Sustain. Energy Rev. (2016). [5] U.T.D. Thuy, N.Q. Liem, C.M.A. Parlett, G.M. Lalev, K. Wilson, Catal. Commun. 44 (2014) 62–67. [6] X. Wang, Q. Zhang, B. Zou, A. Lei, P. Ren, Appl. Surf. Sci. 257 (2011) 10898–10902. [7] N.G. Imam, M. Bakr Mohamed, Superlattices Microstruct. 73 (2014) 203–213. [8] S. Ummartyotin, N. Bunnak, J. Juntaro, M. Sain, H. Manuspiya, Solid State Sci. 14 (2012) 299–304. [9] Y. Infahsaeng, S. Ummartyotin, Results Phys. 7 (2017) 1245–1251. [10] B. Sotillo, P. Fernández, J. Piqueras, J. Alloys Compd. 603 (2014) 57–64. [11] K. Nagamani, N. Revathi, P. Prathap, Y. Lingappa, K.T.R. Reddy, Curr. Appl. Phys. 12 (2012) 380–384. [12] Q. Shen, J. Kobayashi, L.J. Diguna, T. Toyoda, J. Appl. Phys. 103 (2008) 84304. [13] L. Yan, Z. Li, M. Sun, G. Shen, L. Li, ACS Appl. Mater. Interfaces 8 (2016) 20048–20056. [14] P. Rao, W. Yao, Z. Li, L. Kong, W. Zhang, L. Li, Chem. Commun. 51 (2015) 8757–8760. [15] B.C. Fitzmorris, Y.-C. Pu, J.K. Cooper, Y.-F. Lin, Y.-J. Hsu, Y. Li, J.Z. Zhang, ACS Appl. Mater. Interfaces 5 (2013) 2893–2900. [16] A. Makhal, H. Yan, P. Lemmens, S.K. Pal, J. Phys. Chem. C 114 (2010) 627–632. [17] L.J. Antila, P. Myllyperkio, S. Mustalahti, H. Lehtivuori, J. Korppi-Tommola, J. Phys. Chem. C 118 (2014) 7772–7780. [18] Y. Xiaodan, W. Qingyin, J. Shicheng, G. Yihang, Mater. Charact. 57 (2006) 333–341. [19] V. Štengl, S. Bakardjieva, N. Murafa, V. Houšková, K. Lang, Microporous Mesoporous Mater. 110 (2008) 370–378. [20] P. Prathap, N. Revathi, Y.P.V. Subbaiah, K.T. Ramakrishna Reddy, R.W. Miles, Solid State Sci. 11 (2009) 224–232. [21] T. Guang-Lei, H. Hong-Bo, S. Jian-Da, Chinese Phys. Lett. 22 (2005) 1787–1789. [22] S. Ananthakumar, J. Ramkumar, S.M. Babu, Renew. Sustain. Energy Rev. 57 (2016) 1307–1321. [23] R. Schölin, M. Quintana, E.M.J. Johansson, M. Hahlin, T. Marinado, A. Hagfeldt, H. Rensmo, J. Phys. Chem. C 115 (2011) 19274–19279. [24] Y. Imai, A. Watanabe, I. Shimono, J. Mater. Sci. Mater. Electron. 14 (2003) 149–156.