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Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell
Advanced Powder Technology xxx (2017) xxx–xxx
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Advanced Powder Technology journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell Wei-Hua Lu a, Chuen-Shii Chou b,c,⇑, Chung-Yung Chen c, Ping Wu d,⇑ a
Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Research Center of Solar Photo-Electricity Applications, National Pingtung University of Science and Technology, Pingtung 912, Taiwan c Powder Technology R&D Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan d Entropic Interface Group, Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Rd, Singapore 487372, Singapore b
a r t i c l e
i n f o
Article history: Received 3 February 2017 Received in revised form 18 May 2017 Accepted 25 May 2017 Available online xxxx Keywords: Dye-sensitized solar cell Mesoporous Zr-doping TiO2 particles Multiple light-scattering layer Oxygen vacancies Electron-hole recombination
a b s t r a c t This paper presents an implementation of our recent theory on the suspension of electron-hole recombination via electronic- and micro-structure optimization to study the influence of Zr-doping on the efficiency (g) of TiO2-based dye-sensitized solar cells (DSSCs). We developed a four-layered working electrode, in which the size of particles increased from the bottom layer of TiO2 (P-25) through three successive layers of Zr-doped TiO2, which were calcined at 450, 600, and 850 °C respectively. The enhancement in open-circuit photovoltage (Voc) and short-circuit photocurrent density (Jsc) can be attributed to the electronic- and micro-structures in the working electrode. The former is related to band bending, whereas the latter is related to light-scattering within multiple layers. Simulation results (FactSage) demonstrate that Zr doping in TiO2 can suspend or delay the formation of oxygen vacancies and thereby reduce the number of electron scattering centers, which helps to suspend electron-hole recombination by strengthening Ti-O bonds. The proposed four-layered working electrode produced an 80.2% increase in g, compared with DSSCs using a TiO2 (P-25) electrode. This study demonstrated a novel metal doping strategy for the manipulation of electronic structure and photoelectron conversion efficiency. The proposed methodology could also be used to guide the design of photo-catalysts in general. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are photo-electric chemical solar cells that can be fabricated on a variety of substrates at low cost with little environmental impact [1,2]. The power conversion efficiency (g) of DSSCs is determined by short-circuit photocurrent density (Jsc) and open-circuit photovoltage (Voc). The former is related to the light absorption characteristics, whereas the latter is related to electron-hole recombination [3]. Attaining high Jsc values depends on effective light absorption Thus, numerous methods, including the formation of a lightscattering layer in the working electrode, have been developed to facilitate the capture of light [4–13]. However, increasing the pho⇑ Corresponding authors at: Powder Technology R&D Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Fax: +886 8 7740142 (C.-S. Chou) and Entropic Interface Group, Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Rd, Singapore 487372, Singapore (P. Wu). E-mail addresses: [email protected] (C.-S. Chou), [email protected] (P. Wu).
tocurrent by extending the retention period of light in a DSSC can subsequently increase the likelihood of electron recombination in the dye (or electrolyte). Yang et al. reported that adding a lightscattering layer of carbon spheres/TiO2 composite nanoparticles reduced the measured dye absorption by 10% [7]. Researchers have recently reported that Zr-doped TiO2 can enhance photocatalytic efficiency [14]. Kim et al. used Zr oxidepost-treated TiO2 photoelectrodes to increase dye loading [15] and Hore et al. introduced a light-scattering layer comprising TiO2-Rutile and ZrO2 (with a ratio of 1:3) to ensure adequate light trapping [4]. Tong et al. reported the use of mesoporous TiO2 (MPTiO2) spheres with a high specific surface area to increase the dye adsorption capability of working electrodes [16]. However, few researchers have reported that Zr-doped MP-TiO2 particles are capable of inheriting the advantages of Zr-doped TiO2 as well as MPTiO2 particles. To enhance the Jsc and Voc of DSSCs simultaneously, we developed a novel working electrode (Fig. 1) comprising an active layer of TiO2 (P-25) particles and a multiple light-scattering layer comprising mesoporous ZrO2-TiO2 (sol–gel) [MP-ZrO2-TiO2 (sol–gel)]
http://dx.doi.org/10.1016/j.apt.2017.05.026 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: W.-H. Lu et al., Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.026
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
Fig. 1. Schematic of dye-sensitized solar cell with a light-scattering layer of mesoporous ZrO2-TiO2 (Sol-gel) particles.
particles prepared using a sol–gel method. The proposed electrode greatly increases the absorption of light through the use of multiple light-scattering layers and an increase in the dye absorption capacity through the inclusion of MP-ZrO2-TiO2 (sol–gel)] particles. We then investigated the degree to which Zr-doping and the calcination temperature of MP-ZrO2-TiO2 (sol–gel) particles influence the g of DSSCs. We also conducted a performance comparison between conventional DSSCs and those fabricated using the proposed working electrode.
2. Experimental methods and materials 2.1. Mesoporous ZrO2-TiO2 (sol–gel) particles We extended the method reported by Chou et al. [9] to obtain MP-ZrO2-TiO2 (sol–gel) particles of different sizes using the sol– gel method as follows. (1) A precursor of titanium tetrachloride (TiCl4) with a purity of 99.9% was agitated while DI water was slowly added in a dropwise manner over the period of 0.5 h. (2) A preset quantity of zirconyl (di)chloride (ZrOCl28H2O) was added to the TiCl4 solution to form TiCl4/ZrOCl2 solution. (3) 10 ml of C2H6O with a purity of 99.5% was mixed with 1 g PEO-PPO-PEO block copolymer F-127 and then agitated until it became a transparent colloid. (4) The solution of TiCl4/ZrOCl2 was added slowly in a dropwise manner to the transparent colloid before heating the mixture in an oven at 70 °C for 2–3 days to precipitate agglomerated MP-ZrO2-TiO2. (5) The precipitate was ground into the powder, followed by calcination at preset temperatures (450, 600, or 850 °C) for 2 h in a high-temperature furnace to produce microcrystalline MP-ZrO2-TiO2 (sol–gel) particles. Table 1 lists conditions used in the preparation of MP-ZrO2-TiO2 (sol–gel) particles with Zr-doping of various quantities from 0 to 35 mol% and calcination temperatures. A digital camera (Panasonic DMC-LZ2) and a scanning electron microscope (SEM) (HITACHI, 600-S) were respectively used to obtain photographs and SEM micrographs of MP-ZrO2-TiO2
(sol–gel) particles. A powder X-ray diffractometer (Shimadzu, XRD-6000) was used to characterize the MP-ZrO2-TiO2 (sol–gel) particles and a dynamic light-scattering particle size analyzer (HORIBA, LB-550) was used to determine the particle size distribution of MP-ZrO2-TiO2 (sol–gel) particles. An UV–VIS-NIR spectrophotometer (Jasco, V-600) with an integrating sphere was used to obtain the absorbance (Ab) and the transmission of a layer of MP-ZrO2-TiO2 (sol–gel) particles. The absorption coefficient (a) of a layer of the MP-ZrO2-TiO2 (sol–gel) particles with a thickness of 10 lm was determined using a = 2.303 103 Abq, in which q represents the density of the particles. The Tauc-Sunds equation [17] in conjunction with the value of a was then used to calculate the band gap of MP-ZrO2-TiO2 (sol– gel) particles. The potentials of the conduction band (ECB) and valence band (EVB) were then calculated respectively using the following empirical formulae [18]:
ECB ¼ X EC 1=2Eg
ð1Þ
EVB ¼ ECB þ Eg
ð2Þ
where X is the absolute electronegativity of the atom semiconductor, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. EC and Eg represent the energy potential vs. free electrons of hydrogen (4.5 eV) and the band gap of the semiconductor, respectively. 2.2. DSSC with a working electrode of MP-ZrO2-TiO2 (sol–gel) particles The working electrode in this study is meant to promote lightscattering and dye absorption as well as to suppress electron-hole recombination. Fabrication involved the following two stages. First, a transparent layer (active layer) was produced by depositing a colloid of TiO2 (P-25) particles (Uniregion Biotech P-25) of 20% rutile and 80% anatase on an FTO glass substrate treated with TiCl4. This process is detailed in our previous research [9]. Table 2 lists the conditions used in the preparation of various colloids employed
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx Table 1 Test conditions of preparing MP-TiO2 (sol–gel) and MP-ZrO2-TiO2 (sol–gel) particles. Precursor solution
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15
Polymerization and hydrolysis
Calcination
TiCl4 (g)
ZrOCl2 (g)
DI water (g)
Ethanol (mL)
F-127 (g)
Time (day)
Temp. (°C)
Time (h)
Temp. (°C)
1.897
0
25
10
1
2–3
70
2
1.859
0.644
1.802
0.161
1.701
0.322
1.233
1.128
450 600 850 450 600 850 450 600 850 450 600 850 450 600 850
Table 2 Test conditions of preparing colloids. Solute
Solution
Binder
Particle
Mass (g)
Ethanol (mL)
Acetylacetone (mL)
Triton X-100 (mL)
Terpineol (ml)
Ethyl cellulose (g)
B0
P-25
2
8
0.8
0.1
–
–
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 P-25: A10: A11: A12 (1:1:1:1)
1
10
1
0.2
2.5
0.5
Table 3 Test conditions of preparing an active layer of working electrode. First layer
Second layer
Soaking
D1 D2
Solution
Time (h)
Temp. (°C)
0.05 M TiCl4
0.5
70
in this study, and Table 3 lists the conditions used in the preparation of the active layer of the working electrode. The second stage involved the application of ultrasonically homogenized colloids of microcrystalline MP-ZrO2-TiO2 (sol–gel) particles to produce a light-scattering layer. This process is detailed in the following paragraph. Fabrication of the working electrode with a multi-lightscattering layer was conducted as follows: (1) preparation of colloid by mixing MP-ZrO2-TiO2 (sol–gel) particles (1 g) with ethanol (10 mL), acetylacetone (1 mL), terpineol (2.5 ml), ethyl cellulose (0.5 g), and Triton X-100 (0.2 mL), as shown in Table 2; (2) deposition of colloid atop active layer of TiO2 (P-25) particles according to
Colloid
No. of layer daubed by spin coater
B0
5 2
the sequence of daubing colloids outlined in Table 4; (3) sintering of this substrate at 450 °C for 1 h in a high-temperature furnace; and (4) immersion of the substrate with TiO2 (P-25)/MP-ZrO2TiO2 (sol–gel) thin film in the solution of N-719 dye and ethyl alcohol at 70 °C for 6 h. All samples were 0.25 cm2 in area. A working electrode was fabricated with a single light-scattering layer (Test E7) comprising TiO2 (P-25) particles with the three types of MP-ZrO2-TiO2 (sol–gel) particles prepared in Tests A10 to A12 at a mass ratio of 1:1:1:1. Before immersing the TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) working electrode in the dye solution, a UV–VIS-NIR spectrophotometer (Jasco, V-600) was used to measure the transmittance of
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
Table 4 Test conditions of preparing a light-scattering layer of working electrode. Substrate
E1 E2
E3
E4
E5
E6
E7
D1 D2
First layer Spin coating
Second layer Soaking
Sintering
Colloid
No. of layer daubed by spin coating
Solution
Time (h)
Temp. (°C)
Time (h)
Temp. (°C)
– B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16
– 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3
0.05 M TiCl4
0.5
70
1
450
the working electrode and calculate its band gap, as outlined in our previous works [19,20]. After soaking the TiO2 (P-25)/MP-TiO2 (sol–gel) working electrodes in the dye solution, an energy dispersive spectrometer (EDS) (Horiba EX-200) was used to obtain the weight percentage of Ru on the surface. This was done with the aim of determining whether the absorption of dye was enhanced by the addition of the MP-TiO2 (sol–gel) particles in the electrode. A scanning electron microscope (SEM) (HITACHI, 600-S) and a-step (Dekeak 6 M) surface profiler were respectively used to obtain SEM micrographs and the average thickness of the TiO2 (P-25)/MPZrO2-TiO2 (sol–gel) working electrodes. The dye-covered working electrode was attached to a counter electrode covered with a platinum (Pt) film to form an open cell. We then injected a liquid electrolyte into the space between two electrodes [9]. An AM 1.5 solar simulator (San-Ei Pioneer of Light Technology, XES-310S) was used to illuminate the DSSC and a digital source meter (Keithley 2400) was used to measure Jsc and Voc. Details pertaining to the calculations used to determine the power conversion efficiency (g) of the DSSC are also outlined in our previous study [19,20].
Test A3 (calcined at 850 °C), as well as MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping in Test A10 (calcined at 450 °C), Test A11 (calcined at 600 °C), and Test A12 (calcined at 850 °C). Fig. 4 illustrates the particle size distributions of MP-TiO2 (sol–gel) particles in Test A3, as well as MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping in Tests A10 to A12. The results revealed the following insights. (1) With fixed Zr-doping (10 mol%), increasing the calcination temperature from 450 to 850 °C led to an increase in the average particle size of MP-ZrO2-TiO2 (sol–gel) from 318.4 nm (Test A10) to 1697.7 nm (Test A12), as well as a shift in the particle size distribution toward particles with a larger diameter. This can be attributed to the fact that a higher calcination temperature corresponds to a higher energy input, which facilitates the growth of TiO2 grains [9,22]. (2) With a fixed calcination temperature (850 °C), the average particle size of MP-TiO2 (sol–gel) in Test A3 (2105.3 nm) substantially exceeded that of MP-ZrO2-TiO2 (sol– gel) in Test A12 (1697.7 nm) because the addition of ZrO2 in TiO2 via the sol–gel method can suppress the growth of grains during calcination. This supposition is supported by the SEM micrographs in Test A3 (Fig. 3(c)) and Test A12 (Fig. 3(f)).
3. Results and discussion
3.1.2. XRD patterns Fig. 5(a) presents XRD patterns of MP-TiO2 (sol–gel) particles calcined at 450, 600, and 850 °C. At a fixed temperature of 450 °C, only the peaks of anatase phase and three major diffraction peaks from the (1 1 0), (0 0 4), and (2 0 0) planes were observed in the diffraction patterns (JCPDS 84-1285). At a fixed temperature of 850 °C, we also observed the peaks of rutile phase and three major peaks from (1 1 0), (1 0 1), and (2 1 1) planes (JCPDS 820514). This can be attributed to the fact that the addition of PEOPPO-PEO block copolymer F-127 to the TiCl4 precursor can delay the evolution of TiO2 phase from anatase to rutile, even at 850 °C. Burns et al. observed that rutile-related peaks began appearing between 700 and 750 °C, with the sample undergoing complete conversion to rutile phase between 850 and 900 °C [23]. Fig. 5(b) presents XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping calcined at 450, 600, and 850 °C. At 850 °C, peaks of anatase phase TiO2 (A-TiO2) appeared with the two peaks of tetragonal phase ZrO2 (T-ZrO2) diffracted from the (1 1 1) and (4 0 0) planes (JCPDS 14-0534), as shown in Fig. 5(b). Fig. 5(c) presents XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles with 35 mol% Zr-doping calcined at 450, 600, and 850 °C. At 850 °C, the peaks of A-TiO2 and T-ZrO2 appeared with three peaks of monoclinic phase of ZrO2 (M-ZrO2) diffracted from the (1 1 1), (0 2 2),
3.1. MP-ZrO2-TiO2 (sol–gel) particles 3.1.1. Morphology and size of particles Fig. 2 presents photographs of MP-ZrO2-TiO2 (sol–gel) particles calcinated at 600 °C in Test A5 (2 mol% Zr-doping), Test A8 (5 mol% Zr-doping), Test A11 (10 mol% Zr-doping), and Test A14 (35 mol% Zr-doping). At a fixed calcination temperature (600 °C), the color of MP-ZrO2-TiO2 (sol–gel) particles evolved from milky light yellow to pure white, as the amount of Zr-doping increased from 2 mol% to 35 mol%. This can be attributed to the fact that a higher proportion of ZrO2 (with a refractive index of 2.1) [4] may enhance the refractive capability of MP-ZrO2-TiO2 (sol–gel) particles. Furthermore, as discussed in our previous paper on Ru-doped TiO2, this green-like color may find its origin in the defect energy levels, which are approximately 1 eV below the conduction band minimum (CBM) of TiO2, as reported by Zhu et al. [21]. Increasing the proportion of ZrO2 from 2 mol% to 35 mol% caused the green-like color of the powder to disappear, which may be an indication of oxygen vacancy inactivation induced by Zr-doping. Fig. 3 presents SEM micrographs of MP-TiO2 (sol–gel) particles in Test A1 (calcined at 450 °C), Test A2 (calcined at 600 °C), and
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
(a) Test A5 (2 mol% Zr-doping)
(b) Test A8 (5 mol% Zr-doping)
(c) Test A11 (10 mol% Zr-doping)
(d) Test A14 (35 mol% Zr-doping)
5
Fig. 2. Photographs of MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C: (a) Test A5, (b) Test A8, (c) Test A11, (d) Test A14.
and (2 2 0) planes (JCPDS 37-1484), as shown in Fig. 5(c). Interestingly, even at 850 °C, peaks of rutile phase of TiO2 were not detected in the XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles in Fig. 5(b) and (c), due to the fact that the addition of PEO-PPOPEO block copolymer F-127 and ZrO2 can delay the evolution of TiO2 phase from anatase to rutile. Kokporka et al. reported that the incorporation of ZrO2 retards the anatase-to-rutile phase transformation of TiO2 at 800 °C [24]. According to the phase diagram of ZrO2-TiO2 [25], the solubility of Zr in TiO2 is below 2 mol% at temperatures lower than 900 °C. None of the Zr-doped samples in this study were in chemical equilibrium, due to production of a metastable mixture of ZrO2 (either T-ZrO2 or M-ZrO2) and Zr-doped TiO2 solids, rather than a mixture of ZrTiO4 and Zr-doped TiO2, which would have been produced under chemical equilibrium. Judging from the increased width of the (1 0 1) peak of anatase in the 10 mol% and 35 mol% Zr-doped samples in Fig. 5(b) and (c), it is reasonable to conclude that a greater proportion of Ti4+ were substituted by Zr4+ ions in the 35 mol% doped TiO2 samples than in the 10 mol% doped TiO2 samples. Fig. 6 presents peaks of A-TiO2 in the diffraction patterns of the (1 0 1) plane of MP-TiO2 (sol–gel) particles in Test A3 as well as MPZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15. At 850 °C, an increase in the quantity of ZrO2 from 0 mol% to 35 mol % led to a gradual decrease in the 2h of diffraction peak from the (1 0 1) plane from 25.28° to 25.13°. This can be attributed the following: (1) According to Bragg’s law, an increase in the distance between crystal planes leads to a decrease in 2h value; and (2) the radius of Zr4+ (0.072 nm) exceeds that of Ti4+ (0.065 nm) [14]. In addition, at a fixed calcination temperature, the grain size of MP-ZrO2-TiO2 (sol–gel) particles decreased with an increase in the quantity of ZrO2. For example, at 850 °C, the grain size of MP-ZrO2-TiO2 (sol–gel) particles was as follows: 38.8 nm in Test A3 (0 mol% Zr-doping), 32.4 nm in Test A6 (2 mol% Zr-doping), 25.6 nm in Test A9 (5 mol% Zr-doping), 20.1 nm in Test A12 (10 mol% Zr-doping), and 16.9 nm in Test A15 (35 mol% Zrdoping). This decrease in grain size was unexpected due to the fact
that the radius of Zr4+ is larger than that of Ti4+. This can be partially explained as follows: The addition of Zr to TiO2 leads to a reduction in the concentration of oxygen vacancies, and metastable ZrO2 particles can block or slow down the growth of TiO2. This issue is examined in greater detail in the following section. 3.1.3. Band gap Fig. 7 presents variations in absorbance with the wavelength of light (ranging from 250 to 800 nm) from the MP-TiO2 (sol–gel) particles in Test A3 and the MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15, which underwent calcination at 850 °C. The wavelength of light was less than 325 nm; therefore, the absorbance values of MP-ZrO2-TiO2 (sol–gel) particles slightly exceeded those of MP-TiO2 (sol–gel) particles; however, the absorbance values of MP-ZrO2-TiO2 (sol–gel) particles were substantially smaller than those of MP-TiO2 (sol–gel) particles, when the wavelength of light exceeded 325 nm. This difference can be attributed to the fact that the band absorptions of the MP-ZrO2-TiO2 (sol–gel) particles gradually shifted to shorter wavelengths (i.e., blue-shift) with an increase in the content of ZrO2 [14]. The results obtained using the Tauc-Sunds equation [17] in conjunction with Eqs. (1) and (2) reveal that the Eg of MP-ZrO2-TiO2 (sol–gel) particles increased with the amount of ZrO2. The Eg values of MP-TiO2 (sol–gel) particles and MP-ZrO2-TiO2 (sol–gel) particles calcined at 850 °C were calculated as follows: 3.05 eV in Test A3 (0 mol% Zr-doping), 3.14 eV in Test A6 (2 mol% Zr-doping), 3.23 eV in Test A9 (5 mol% Zr-doping), 3.26 eV in Test A12 (10 mol% Zr-doping), and 3.36 eV in Test A15 (35 mol% Zrdoping), as shown in Fig. 8. In addition, the ECB edge/EVB edge of MP-TiO2 (sol–gel) particles and MP-ZrO2-TiO2 (sol–gel) particles were as follows: 0.215 eV/2.835 eV (Test A3), 0.220 eV/2.920 eV (Test A6), 0.265 eV/2.965 eV (Test A9), 0.280 eV/2.980 eV (Test A12), and 0.330 eV/3.030 eV (Test A15). Gao et al. claimed that the enlarged Eg of the Zr-doped TiO2 materials can be ascribed to the elevation of the ECB [14]. However, we determined that the enlarged Eg of MP-ZrO2-TiO2 (sol–gel) particles may be the result of an increase in ECB and EVB due to Zr-doping, compared with
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
(d)
(a)
80000
100 nm
(b)
80000
100 nm
(e)
100 nm
80000
100 nm
(f)
(c)
80000
80000
100 nm
80000
100 nm
Fig. 3. SEM micrographs: (a) Test A1 [MP-TiO2 (sol–gel) particles calcined at 450 °C]; (b) Test A2 [MP-TiO2 (sol–gel) particles calcined at 600 °C]; (c) Test A3 [MP-TiO2 (sol– gel) particles calcined at 850 °C]; (d) Test A10 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 450 °C]; (e) Test A11 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C]; (f) Test A12 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 850 °C].
Fig. 4. Particle size distributions in Test A3 [MP-TiO2 (sol–gel) particles calcined at 850 °C], Test A10 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 450 °C], Test A11 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C], Test A12 [MP-ZrO2-TiO2 (sol– gel) particles calcined at 850 °C].
MP-TiO2 (sol–gel) particles (Test A3). It may be helpful to determine the influence Zr4+ doping on the intrinsic TiO2 defect energy levels within the band gap. To evaluate the influence of Zr-doping on TiAO bonding strength, we calculated the oxide formation energy (DGT) at the temperature T for the reaction: M + O2 = MO2 (M = Ti or Zr) using FactSage [26]. The calculated results are presented in Table 5. Doping TiO2 with Zr leads to an increase in average bonding energy of Ti-O, which results in a lower concentration of oxygen vacancies and the associated defects. The effect of Zr-doping can be used to partially explain the reduction in the size of Zr-doped TiO2 particles mentioned above. Furthermore, the sequential reduction in the absorption of visible light in the 2, 5 and 10 mol % Zr-doped samples (Fig. 7) is in agreement with the theory governing the reduction in defects related to oxygen vacancies. As discussed earlier, stronger (Ti, Zr)-O bonds lead to fewer defects related to oxygen vacancies and a reduction in the absorption of visible light, which originates in such defects. However, the absorption of visible light in the 35 mol% ZrO2 sample (Fig. 7) does not follow the trend observed in the samples with 2, 5, 10 mol% ZrO2. This difference may due to the fact that it has deviated too far from equilibrium, such that new metastable phases formed.
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Fig. 6. Peaks of anatase diffracted from (1 0 1) plane for MP-TiO2 (sol–gel) particles in Test A3 as well as MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15.
Fig. 7. Variations in absorbance with the wavelength of light for MP-TiO2 (sol–gel) particles in Test A3 as well as MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15.
3.2. Working electrodes
Fig. 5. XRD patterns: (a) MP-TiO2 (sol–gel) particles; (b) MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping; (c) MP-ZrO2-TiO2 (sol–gel) particles with 35 mol % Zr-doping.
Fig. 9(a) and (b) presents SEM micrographs (5 k) and EDS analysis of two dye-covered electrodes, i.e., the MP-TiO2 (sol–gel)/N719 electrode and TiO2 (sol–gel)/N719 electrode, respectively. These results demonstrate that the mesoporous structure of MP-TiO2 (sol–gel) particles can enhance the dye absorption capacity of the electrode, based on the fact that the weight percentage of ruthenium (Ru) in the MP-TiO2 (sol–gel)/N719 electrode (0.80%) substantially exceeded that of TiO2 (sol–gel)/N719 electrode (0.27%). Furthermore, the average thickness of the active layer was 4.39 lm, and the average film thickness of electrode (comprising an active layer and a multiple light-scattering layer) was 13.02 lm. Furthermore, the thickness of film deposited on the working electrode in Tests E1 to E7 is listed in Table 6. Fig. 10 presents variations in transmittance with the wavelength of light (ranging from 400 to 800 nm) from the TiO2 (P-25) electrode in Test E1, the TiO2 (P-25)/MP-TiO2 (sol–gel) electrode in Test E2, and the TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel)
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Fig. 8. The values of band gap: (a) MP-TiO2 (sol–gel) particles in Test A3; (b) MP-ZrO2-TiO2 (sol–gel) particles in Tests A6 (2 mol% Zr-doping), A9 (5 mol% Zr-doping), A12 (10 mol% Zr-doping), and A15 (35 mol% Zr-doping).
Table 5 The oxide formation energy of TiO2 and ZrO2. Oxide formation energy DGT (kJ/mol)
TiO2 ZrO2
400 °C
600 °C
850 °C
871 1020
834 982
789 945
electrode in Tests E3 to E6. At a wavelength of 800 nm, the transmittance of the TiO2 (P-25) electrode was the highest (7.9%), whereas the transmittance of the TiO2 (P-25)/MP-TiO2 (sol–gel) electrode was the lowest (0.44%). The transmittance of the TiO2
(P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes in Tests E3 to E6 were between 7.9% and 0.44%. These results can be attributed to the following. (1) The addition of ZrO2 to the TiO2 via the sol–gel method suppressed the growth of grains during calcination. As a result, the particle size of MP-ZrO2-TiO2 (sol–gel) is substantially smaller than that of MP-TiO2 (sol–gel). (2) The average size of TiO2 (P-25) particles in this study (20% rutile and 80% anatase) was 21 nm. Using the method reported in our previous research [19,20], our results revealed that the Eg of the TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode increased slightly with an increase in the amount of ZrO2. The Eg values of the TiO2 (P-25)/MP-TiO2 (sol–gel) and TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes were as follows:
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(a) MP-TiO2(Sol-gel)/N719 Element
Weight (%)
Atomic (%)
O
49.51
74.75
Ti
49.69
25.06
Ru
0.80
0.19
Total:
100.00
100.00
(b) TiO2(Sol-gel)/N719 Element
Weight (%)
Atomic (%)
O
47.18
72.84
Ti
52.55
27.10
Ru
0.27
0.07
Total:
100.00
100.00
Fig. 9. SEM micrographs and EDS analyses of dye-covered electrodes: (a) MP-TiO2 (sol–gel)/N719 electrode; (b) TiO2 (sol–gel)/N719 electrode.
Table 6 Voc, Jsc, FF, and g of DSSC.
G1 G2 G3 G4 G5 G6 G7
Counter electrode
Working electrode/film thickness (lm)
Voc (V)
Jsc (mA/ cm2)
FF (%)
g
FTO glass substrate with a Pt film
E1/11.2 E2/13.6 E3/13.3 E4/13.7 E5/12.3 E6/12.2 E7/12.5
0.63 0.67 0.67 0.67 0.67 0.67 0.66
12.05 17.66 18.03 18.13 18.81 16.74 16.26
44.9 46.2 47.8 48.0 49.0 48.6 49.1
3.44 5.45 5.74 5.81 6.20 5.49 5.28
(%)
2.97 eV in Test E2 (0 mol% Zr-doping), 3.04 eV in Test E3 (2 mol% Zr-doping), 3.06 eV in Test E4 (5 mol% Zr-doping), 3.07 eV in Test E5 (10 mol% Zr-doping), and 3.10 eV in Test E6 (35 mol% Zrdoping), as shown in Fig. 11. This can be explained by the fact that the Eg of MP-ZrO2-TiO2 (sol–gel) particles increased with the amount of ZrO2. 3.3. Voc, Jsc, g, and J-V curves of DSSCs Table 6 presents the Voc, short-circuit photocurrent per unit area (Jsc), fill factor (FF), and g in Tests G1 to G7. The highest g (6.20%) was obtained from a DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode (Test G5). Variations in Jsc with Voc are presented in Fig. 12(a) (Tests G1 to G3) and Fig. 12(b) (Tests G1, G5, and G7). Three DSSCs were prepared for each test condition. We then calcu-
Fig. 10. Variations in transmittance with the wavelength of light for working electrodes in Test E1 [TiO2 (P-25) electrode], Test E2 [TiO2 (P-25)/MP-TiO2 (sol– gel)], and Tests E3 to E6 [TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel)].
lated the standard deviation of short-circuit photocurrent density (r) under each test condition at every open-circuit voltage using rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! P3 P3 3 Jsci ðJsci Þ2 i¼1 i¼1 , the results of the following equation r ¼ 9 which are plotted using error bars in Fig. 12.
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Fig. 11. The values of band gap: (a) TiO2 (P-25)/MP-TiO2 (sol–gel) electrode in Test E2; (b) TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes in Test E3 (2 mol% Zr-doping), Test E4 (5 mol% Zr-doping), Test E5 (10 mol% Zr-doping), and Test E6 (35 mol% Zr-doping).
3.3.1. Effects of mesoporous multi-light-scattering layer Our results demonstrate that an electrode with a multiple lightscattering layer of MP-TiO2 (sol–gel) particles can substantially enhance the Jsc and g of DSSCs. For example, the g in Test G2 [DSSC with a TiO2 (P-25)/MP-TiO2 (sol–gel) electrode] (5.45%) exceeded that of conventional DSSCs with a TiO2 (P-25) in Test G1 (3.44%). The Jsc values in Tests G1 and G2 were respectively 12.05 and 17.66 mA/cm2, and the Voc value in Test G2 (0.67 V) slightly exceeded that in Test G1 (0.63 V). This can be explained as follows. (1) The use of a multiple lightscattering layer of MP-TiO2 (sol–gel) particles enhanced the Jsc by extending the retention time of light and enhancing dye absorption capacity. (2) The weight percentage of ruthenium (Ru) in the
dye-covered MP-TiO2 (sol–gel)/N719 electrode (0.80%) substantially exceeded that of the TiO2 (sol–gel)/N719 electrode (0.27%), as shown in Fig. 9. 3.3.2. Effects of ZrO2 content Increasing the amount of ZrO2 added to the MP-ZrO2-TiO2 (sol– gel) particles led to an increase in the g of DSSCs fabricated with a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles. However, g decreased when the content of ZrO2 exceeded a given threshold. For example, when the Zr-doping was increased from 2 mol% to 10 mol%, g increased from 5.74% (Test G3) to 6.20% (Test G5), but decreased to 5.49% (Test G6) when the Zr-doping was increased to 35 mol%, as shown in Table 6. In addition, the Jsc val-
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ues were as follows: 18.03 (Test G3), 18.13 (Test G4), 18.81 (Test G5), and 16.74 (Test G6) mA/cm2. The Voc value remained at 0.67 V in Tests G3 to G6 (Table 6). This can be explained as follows. (1) Our results reveal that the additions of ZrO2 can delay the formation of rutile phase TiO2 during calcination, such that the MP-ZrO2-TiO2 (sol–gel) particles with anatase phase TiO2 inherit superior electrical conductivity. (2) As shown in Fig. 7, even in cases of unfavorable visible light absorption by individual particles (like MP-ZrO2-TiO2 (sol–gel) particles), appropriate Zr doping can suppress the formation of oxygen vacancies, which would otherwise result in a longer electron free path, as well as reduce electron scattering and recombination with holes [27,28]. However, increasing the quantity of ZrO2 beyond a given threshold can lead to the formation of other phases, such as ZrTiO4, which could upset the entire design. Troitzsch and Ellis revealed that a higher Zr concentration in the Zr doped-TiO2 compounds can result in the formation of ZrTiO4 or (Zr,Ti)2O4, in accordance with the ZrO2-TiO2 phase diagram [29]. (3) The slight increase in band gap induced by Zr-doping is also beneficial for band bending, which is more evident when observing particles with different concentrations of Zr. This kind of band bending can help to increase Voc by suspending electron-hole recombination. Interestingly, we observed an 80.2% increase in g in Test G5 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrode], compared with g in Test G1 [DSSC with a TiO2 (P-25) electrode]. This was likely due to the fact that the DSSC in Test G5 included a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles capable of enhancing the capture of light and dye absorption while suppressing electron-hole recombination. Furthermore, Jsc and Voc values in Test G1 were lower than those in Test G5. These results demonstrate that a working electrode composed of MP-ZrO2-TiO2 (sol–gel) particles can enhance the Voc as well as the Jsc of DSSCs.
Fig. 12. Variations in photocurrent density with photovoltage: (a) Test G1 [DSSC with a TiO2 (P-25) electrode], Test G2 [DSSC with a TiO2 (P-25)/MP-TiO2 (sol–gel) electrode], and Test G3 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrode]; (b) Test G1, as well as Tests G5 and G7 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode].
3.3.3. Multiple light-scattering layer vs. single light-scattering layer In Test G5 [DSSC with a multiple light-scattering layer of MPZrO2-TiO2 (sol–gel) particles], g was 17.4% higher and Jsc 15.7% higher than the values obtained in Test G7 [DSSC with a single light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles]. Furthermore, the Jsc and Voc values in Test G7 were lower than those in Test G5, as shown in Table 6. This difference can be explained as follows. At wavelengths of 400–800 nm, the transmittance values in Test E5 [working
7.9%
1.13% 0.62%
Fig. 13. Variations in transmittance with the wavelength of light in Test E1 [TiO2 (P-25) electrode], Test E5 [working electrode with a multi-light-scattering layer of MP-ZrO2TiO2 (sol–gel) particles], and Test E7 [working electrode with a single-light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles].
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electrode with a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles] were smaller than those in Test E7 [working electrode with a single light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles], as shown in Fig. 13. For example, at a wavelength of 800 nm, the transmittance in Test E7 (working electrode from DSSC in Test G7) was 82.3% higher than the transmittance in Test E5 (working electrode from DSSC in Test G5). 4. Conclusions This paper presents a novel electrode design involving the application multi-layer light scattering to overcome many of the shortcomings in the light absorption of individual particles associated with doping (unavoidable or intended). More importantly, we present a novel strategy in which the formation of oxygen vacancies is intentionally reduced using simple chemical thermodynamics. The objective is to suspend or delay electron scattering and electron-hole recombination. In the case of TiO2 electrodes, the proposed strategy achieves the following: (1) doping metals (like Ru) with standard electrode potentials (SEP) higher than that of Ti will increase the formation of oxygen vacancies and the absorption of visible light [30]; (2) doping metals (like Zr) with SEP lower than that of Ti will reduce the formation of oxygen vacancies and the absorption of visible light while increasing carrier mobility. These simple design rules may be used in the development of photo-catalysts in general. Acknowledgements Chuen-Shii Chou would like to thank Ministry of Science and Technology, R.O.C. for financially supporting this research under Contract No. MOST 104-2221-E-020-036. Wu’s work is partially funded by the Changi-SUTD research program for photocatalytic research. References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–739. [2] S. Caramori, V. Cristino, R. Boaretto, R. Argazzi, C.A. Bignozzi, A.D. Carlo, New components for dye-sensitized solar cells, Int. J. Photo. 2010 (2010) 16. [3] Z.S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Significant influence of TiO2 photo electrode morphology on the energy conversion efficiency of N719 dyesensitized solar cell, Coord. Chem. Rev. 248 (2004) 1381–1389. [4] S. Hore, C. Vetter, R. Kern, H. Smit, A. Hinsch, Influence of scattering layers on efficiency of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 90 (2006) 1176–1188. [5] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel, Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%, Thin Solid Films 516 (2008) 4613–4619. [6] J.K. Lee, B.H. Jeong, S.I. Jang, Y.G. Kim, Y.W. Jang, S.B. Lee, M.R. Kim, Preparations of TiO2 pastes and its application to light-scattering layer for dye-sensitized solar cells, J. Ind. Eng. Chem. 15 (2009) 724–729. [7] G. Yang, J. Zhang, P. Wang, Q. Sun, J. Zheng, Y. Zhu, Light scattering enhanced photoanodes for dye-sensitized solar cells prepared by carbon spheres/TiO2 nanoparticle composites, Curr. Appl. Phys. 11 (2011) 376–381.
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Original Research Paper
Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell Wei-Hua Lu a, Chuen-Shii Chou b,c,⇑, Chung-Yung Chen c, Ping Wu d,⇑ a
Graduate Institute of Materials Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Research Center of Solar Photo-Electricity Applications, National Pingtung University of Science and Technology, Pingtung 912, Taiwan c Powder Technology R&D Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan d Entropic Interface Group, Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Rd, Singapore 487372, Singapore b
a r t i c l e
i n f o
Article history: Received 3 February 2017 Received in revised form 18 May 2017 Accepted 25 May 2017 Available online xxxx Keywords: Dye-sensitized solar cell Mesoporous Zr-doping TiO2 particles Multiple light-scattering layer Oxygen vacancies Electron-hole recombination
a b s t r a c t This paper presents an implementation of our recent theory on the suspension of electron-hole recombination via electronic- and micro-structure optimization to study the influence of Zr-doping on the efficiency (g) of TiO2-based dye-sensitized solar cells (DSSCs). We developed a four-layered working electrode, in which the size of particles increased from the bottom layer of TiO2 (P-25) through three successive layers of Zr-doped TiO2, which were calcined at 450, 600, and 850 °C respectively. The enhancement in open-circuit photovoltage (Voc) and short-circuit photocurrent density (Jsc) can be attributed to the electronic- and micro-structures in the working electrode. The former is related to band bending, whereas the latter is related to light-scattering within multiple layers. Simulation results (FactSage) demonstrate that Zr doping in TiO2 can suspend or delay the formation of oxygen vacancies and thereby reduce the number of electron scattering centers, which helps to suspend electron-hole recombination by strengthening Ti-O bonds. The proposed four-layered working electrode produced an 80.2% increase in g, compared with DSSCs using a TiO2 (P-25) electrode. This study demonstrated a novel metal doping strategy for the manipulation of electronic structure and photoelectron conversion efficiency. The proposed methodology could also be used to guide the design of photo-catalysts in general. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) are photo-electric chemical solar cells that can be fabricated on a variety of substrates at low cost with little environmental impact [1,2]. The power conversion efficiency (g) of DSSCs is determined by short-circuit photocurrent density (Jsc) and open-circuit photovoltage (Voc). The former is related to the light absorption characteristics, whereas the latter is related to electron-hole recombination [3]. Attaining high Jsc values depends on effective light absorption Thus, numerous methods, including the formation of a lightscattering layer in the working electrode, have been developed to facilitate the capture of light [4–13]. However, increasing the pho⇑ Corresponding authors at: Powder Technology R&D Laboratory, Department of Mechanical Engineering, National Pingtung University of Science and Technology, Pingtung 912, Taiwan Fax: +886 8 7740142 (C.-S. Chou) and Entropic Interface Group, Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Rd, Singapore 487372, Singapore (P. Wu). E-mail addresses: [email protected] (C.-S. Chou), [email protected] (P. Wu).
tocurrent by extending the retention period of light in a DSSC can subsequently increase the likelihood of electron recombination in the dye (or electrolyte). Yang et al. reported that adding a lightscattering layer of carbon spheres/TiO2 composite nanoparticles reduced the measured dye absorption by 10% [7]. Researchers have recently reported that Zr-doped TiO2 can enhance photocatalytic efficiency [14]. Kim et al. used Zr oxidepost-treated TiO2 photoelectrodes to increase dye loading [15] and Hore et al. introduced a light-scattering layer comprising TiO2-Rutile and ZrO2 (with a ratio of 1:3) to ensure adequate light trapping [4]. Tong et al. reported the use of mesoporous TiO2 (MPTiO2) spheres with a high specific surface area to increase the dye adsorption capability of working electrodes [16]. However, few researchers have reported that Zr-doped MP-TiO2 particles are capable of inheriting the advantages of Zr-doped TiO2 as well as MPTiO2 particles. To enhance the Jsc and Voc of DSSCs simultaneously, we developed a novel working electrode (Fig. 1) comprising an active layer of TiO2 (P-25) particles and a multiple light-scattering layer comprising mesoporous ZrO2-TiO2 (sol–gel) [MP-ZrO2-TiO2 (sol–gel)]
http://dx.doi.org/10.1016/j.apt.2017.05.026 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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Fig. 1. Schematic of dye-sensitized solar cell with a light-scattering layer of mesoporous ZrO2-TiO2 (Sol-gel) particles.
particles prepared using a sol–gel method. The proposed electrode greatly increases the absorption of light through the use of multiple light-scattering layers and an increase in the dye absorption capacity through the inclusion of MP-ZrO2-TiO2 (sol–gel)] particles. We then investigated the degree to which Zr-doping and the calcination temperature of MP-ZrO2-TiO2 (sol–gel) particles influence the g of DSSCs. We also conducted a performance comparison between conventional DSSCs and those fabricated using the proposed working electrode.
2. Experimental methods and materials 2.1. Mesoporous ZrO2-TiO2 (sol–gel) particles We extended the method reported by Chou et al. [9] to obtain MP-ZrO2-TiO2 (sol–gel) particles of different sizes using the sol– gel method as follows. (1) A precursor of titanium tetrachloride (TiCl4) with a purity of 99.9% was agitated while DI water was slowly added in a dropwise manner over the period of 0.5 h. (2) A preset quantity of zirconyl (di)chloride (ZrOCl28H2O) was added to the TiCl4 solution to form TiCl4/ZrOCl2 solution. (3) 10 ml of C2H6O with a purity of 99.5% was mixed with 1 g PEO-PPO-PEO block copolymer F-127 and then agitated until it became a transparent colloid. (4) The solution of TiCl4/ZrOCl2 was added slowly in a dropwise manner to the transparent colloid before heating the mixture in an oven at 70 °C for 2–3 days to precipitate agglomerated MP-ZrO2-TiO2. (5) The precipitate was ground into the powder, followed by calcination at preset temperatures (450, 600, or 850 °C) for 2 h in a high-temperature furnace to produce microcrystalline MP-ZrO2-TiO2 (sol–gel) particles. Table 1 lists conditions used in the preparation of MP-ZrO2-TiO2 (sol–gel) particles with Zr-doping of various quantities from 0 to 35 mol% and calcination temperatures. A digital camera (Panasonic DMC-LZ2) and a scanning electron microscope (SEM) (HITACHI, 600-S) were respectively used to obtain photographs and SEM micrographs of MP-ZrO2-TiO2
(sol–gel) particles. A powder X-ray diffractometer (Shimadzu, XRD-6000) was used to characterize the MP-ZrO2-TiO2 (sol–gel) particles and a dynamic light-scattering particle size analyzer (HORIBA, LB-550) was used to determine the particle size distribution of MP-ZrO2-TiO2 (sol–gel) particles. An UV–VIS-NIR spectrophotometer (Jasco, V-600) with an integrating sphere was used to obtain the absorbance (Ab) and the transmission of a layer of MP-ZrO2-TiO2 (sol–gel) particles. The absorption coefficient (a) of a layer of the MP-ZrO2-TiO2 (sol–gel) particles with a thickness of 10 lm was determined using a = 2.303 103 Abq, in which q represents the density of the particles. The Tauc-Sunds equation [17] in conjunction with the value of a was then used to calculate the band gap of MP-ZrO2-TiO2 (sol– gel) particles. The potentials of the conduction band (ECB) and valence band (EVB) were then calculated respectively using the following empirical formulae [18]:
ECB ¼ X EC 1=2Eg
ð1Þ
EVB ¼ ECB þ Eg
ð2Þ
where X is the absolute electronegativity of the atom semiconductor, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. EC and Eg represent the energy potential vs. free electrons of hydrogen (4.5 eV) and the band gap of the semiconductor, respectively. 2.2. DSSC with a working electrode of MP-ZrO2-TiO2 (sol–gel) particles The working electrode in this study is meant to promote lightscattering and dye absorption as well as to suppress electron-hole recombination. Fabrication involved the following two stages. First, a transparent layer (active layer) was produced by depositing a colloid of TiO2 (P-25) particles (Uniregion Biotech P-25) of 20% rutile and 80% anatase on an FTO glass substrate treated with TiCl4. This process is detailed in our previous research [9]. Table 2 lists the conditions used in the preparation of various colloids employed
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx Table 1 Test conditions of preparing MP-TiO2 (sol–gel) and MP-ZrO2-TiO2 (sol–gel) particles. Precursor solution
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15
Polymerization and hydrolysis
Calcination
TiCl4 (g)
ZrOCl2 (g)
DI water (g)
Ethanol (mL)
F-127 (g)
Time (day)
Temp. (°C)
Time (h)
Temp. (°C)
1.897
0
25
10
1
2–3
70
2
1.859
0.644
1.802
0.161
1.701
0.322
1.233
1.128
450 600 850 450 600 850 450 600 850 450 600 850 450 600 850
Table 2 Test conditions of preparing colloids. Solute
Solution
Binder
Particle
Mass (g)
Ethanol (mL)
Acetylacetone (mL)
Triton X-100 (mL)
Terpineol (ml)
Ethyl cellulose (g)
B0
P-25
2
8
0.8
0.1
–
–
B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11 A12 A13 A14 A15 P-25: A10: A11: A12 (1:1:1:1)
1
10
1
0.2
2.5
0.5
Table 3 Test conditions of preparing an active layer of working electrode. First layer
Second layer
Soaking
D1 D2
Solution
Time (h)
Temp. (°C)
0.05 M TiCl4
0.5
70
in this study, and Table 3 lists the conditions used in the preparation of the active layer of the working electrode. The second stage involved the application of ultrasonically homogenized colloids of microcrystalline MP-ZrO2-TiO2 (sol–gel) particles to produce a light-scattering layer. This process is detailed in the following paragraph. Fabrication of the working electrode with a multi-lightscattering layer was conducted as follows: (1) preparation of colloid by mixing MP-ZrO2-TiO2 (sol–gel) particles (1 g) with ethanol (10 mL), acetylacetone (1 mL), terpineol (2.5 ml), ethyl cellulose (0.5 g), and Triton X-100 (0.2 mL), as shown in Table 2; (2) deposition of colloid atop active layer of TiO2 (P-25) particles according to
Colloid
No. of layer daubed by spin coater
B0
5 2
the sequence of daubing colloids outlined in Table 4; (3) sintering of this substrate at 450 °C for 1 h in a high-temperature furnace; and (4) immersion of the substrate with TiO2 (P-25)/MP-ZrO2TiO2 (sol–gel) thin film in the solution of N-719 dye and ethyl alcohol at 70 °C for 6 h. All samples were 0.25 cm2 in area. A working electrode was fabricated with a single light-scattering layer (Test E7) comprising TiO2 (P-25) particles with the three types of MP-ZrO2-TiO2 (sol–gel) particles prepared in Tests A10 to A12 at a mass ratio of 1:1:1:1. Before immersing the TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) working electrode in the dye solution, a UV–VIS-NIR spectrophotometer (Jasco, V-600) was used to measure the transmittance of
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
Table 4 Test conditions of preparing a light-scattering layer of working electrode. Substrate
E1 E2
E3
E4
E5
E6
E7
D1 D2
First layer Spin coating
Second layer Soaking
Sintering
Colloid
No. of layer daubed by spin coating
Solution
Time (h)
Temp. (°C)
Time (h)
Temp. (°C)
– B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12 B13 B14 B15 B16
– 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3
0.05 M TiCl4
0.5
70
1
450
the working electrode and calculate its band gap, as outlined in our previous works [19,20]. After soaking the TiO2 (P-25)/MP-TiO2 (sol–gel) working electrodes in the dye solution, an energy dispersive spectrometer (EDS) (Horiba EX-200) was used to obtain the weight percentage of Ru on the surface. This was done with the aim of determining whether the absorption of dye was enhanced by the addition of the MP-TiO2 (sol–gel) particles in the electrode. A scanning electron microscope (SEM) (HITACHI, 600-S) and a-step (Dekeak 6 M) surface profiler were respectively used to obtain SEM micrographs and the average thickness of the TiO2 (P-25)/MPZrO2-TiO2 (sol–gel) working electrodes. The dye-covered working electrode was attached to a counter electrode covered with a platinum (Pt) film to form an open cell. We then injected a liquid electrolyte into the space between two electrodes [9]. An AM 1.5 solar simulator (San-Ei Pioneer of Light Technology, XES-310S) was used to illuminate the DSSC and a digital source meter (Keithley 2400) was used to measure Jsc and Voc. Details pertaining to the calculations used to determine the power conversion efficiency (g) of the DSSC are also outlined in our previous study [19,20].
Test A3 (calcined at 850 °C), as well as MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping in Test A10 (calcined at 450 °C), Test A11 (calcined at 600 °C), and Test A12 (calcined at 850 °C). Fig. 4 illustrates the particle size distributions of MP-TiO2 (sol–gel) particles in Test A3, as well as MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping in Tests A10 to A12. The results revealed the following insights. (1) With fixed Zr-doping (10 mol%), increasing the calcination temperature from 450 to 850 °C led to an increase in the average particle size of MP-ZrO2-TiO2 (sol–gel) from 318.4 nm (Test A10) to 1697.7 nm (Test A12), as well as a shift in the particle size distribution toward particles with a larger diameter. This can be attributed to the fact that a higher calcination temperature corresponds to a higher energy input, which facilitates the growth of TiO2 grains [9,22]. (2) With a fixed calcination temperature (850 °C), the average particle size of MP-TiO2 (sol–gel) in Test A3 (2105.3 nm) substantially exceeded that of MP-ZrO2-TiO2 (sol– gel) in Test A12 (1697.7 nm) because the addition of ZrO2 in TiO2 via the sol–gel method can suppress the growth of grains during calcination. This supposition is supported by the SEM micrographs in Test A3 (Fig. 3(c)) and Test A12 (Fig. 3(f)).
3. Results and discussion
3.1.2. XRD patterns Fig. 5(a) presents XRD patterns of MP-TiO2 (sol–gel) particles calcined at 450, 600, and 850 °C. At a fixed temperature of 450 °C, only the peaks of anatase phase and three major diffraction peaks from the (1 1 0), (0 0 4), and (2 0 0) planes were observed in the diffraction patterns (JCPDS 84-1285). At a fixed temperature of 850 °C, we also observed the peaks of rutile phase and three major peaks from (1 1 0), (1 0 1), and (2 1 1) planes (JCPDS 820514). This can be attributed to the fact that the addition of PEOPPO-PEO block copolymer F-127 to the TiCl4 precursor can delay the evolution of TiO2 phase from anatase to rutile, even at 850 °C. Burns et al. observed that rutile-related peaks began appearing between 700 and 750 °C, with the sample undergoing complete conversion to rutile phase between 850 and 900 °C [23]. Fig. 5(b) presents XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping calcined at 450, 600, and 850 °C. At 850 °C, peaks of anatase phase TiO2 (A-TiO2) appeared with the two peaks of tetragonal phase ZrO2 (T-ZrO2) diffracted from the (1 1 1) and (4 0 0) planes (JCPDS 14-0534), as shown in Fig. 5(b). Fig. 5(c) presents XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles with 35 mol% Zr-doping calcined at 450, 600, and 850 °C. At 850 °C, the peaks of A-TiO2 and T-ZrO2 appeared with three peaks of monoclinic phase of ZrO2 (M-ZrO2) diffracted from the (1 1 1), (0 2 2),
3.1. MP-ZrO2-TiO2 (sol–gel) particles 3.1.1. Morphology and size of particles Fig. 2 presents photographs of MP-ZrO2-TiO2 (sol–gel) particles calcinated at 600 °C in Test A5 (2 mol% Zr-doping), Test A8 (5 mol% Zr-doping), Test A11 (10 mol% Zr-doping), and Test A14 (35 mol% Zr-doping). At a fixed calcination temperature (600 °C), the color of MP-ZrO2-TiO2 (sol–gel) particles evolved from milky light yellow to pure white, as the amount of Zr-doping increased from 2 mol% to 35 mol%. This can be attributed to the fact that a higher proportion of ZrO2 (with a refractive index of 2.1) [4] may enhance the refractive capability of MP-ZrO2-TiO2 (sol–gel) particles. Furthermore, as discussed in our previous paper on Ru-doped TiO2, this green-like color may find its origin in the defect energy levels, which are approximately 1 eV below the conduction band minimum (CBM) of TiO2, as reported by Zhu et al. [21]. Increasing the proportion of ZrO2 from 2 mol% to 35 mol% caused the green-like color of the powder to disappear, which may be an indication of oxygen vacancy inactivation induced by Zr-doping. Fig. 3 presents SEM micrographs of MP-TiO2 (sol–gel) particles in Test A1 (calcined at 450 °C), Test A2 (calcined at 600 °C), and
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
(a) Test A5 (2 mol% Zr-doping)
(b) Test A8 (5 mol% Zr-doping)
(c) Test A11 (10 mol% Zr-doping)
(d) Test A14 (35 mol% Zr-doping)
5
Fig. 2. Photographs of MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C: (a) Test A5, (b) Test A8, (c) Test A11, (d) Test A14.
and (2 2 0) planes (JCPDS 37-1484), as shown in Fig. 5(c). Interestingly, even at 850 °C, peaks of rutile phase of TiO2 were not detected in the XRD patterns of MP-ZrO2-TiO2 (sol–gel) particles in Fig. 5(b) and (c), due to the fact that the addition of PEO-PPOPEO block copolymer F-127 and ZrO2 can delay the evolution of TiO2 phase from anatase to rutile. Kokporka et al. reported that the incorporation of ZrO2 retards the anatase-to-rutile phase transformation of TiO2 at 800 °C [24]. According to the phase diagram of ZrO2-TiO2 [25], the solubility of Zr in TiO2 is below 2 mol% at temperatures lower than 900 °C. None of the Zr-doped samples in this study were in chemical equilibrium, due to production of a metastable mixture of ZrO2 (either T-ZrO2 or M-ZrO2) and Zr-doped TiO2 solids, rather than a mixture of ZrTiO4 and Zr-doped TiO2, which would have been produced under chemical equilibrium. Judging from the increased width of the (1 0 1) peak of anatase in the 10 mol% and 35 mol% Zr-doped samples in Fig. 5(b) and (c), it is reasonable to conclude that a greater proportion of Ti4+ were substituted by Zr4+ ions in the 35 mol% doped TiO2 samples than in the 10 mol% doped TiO2 samples. Fig. 6 presents peaks of A-TiO2 in the diffraction patterns of the (1 0 1) plane of MP-TiO2 (sol–gel) particles in Test A3 as well as MPZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15. At 850 °C, an increase in the quantity of ZrO2 from 0 mol% to 35 mol % led to a gradual decrease in the 2h of diffraction peak from the (1 0 1) plane from 25.28° to 25.13°. This can be attributed the following: (1) According to Bragg’s law, an increase in the distance between crystal planes leads to a decrease in 2h value; and (2) the radius of Zr4+ (0.072 nm) exceeds that of Ti4+ (0.065 nm) [14]. In addition, at a fixed calcination temperature, the grain size of MP-ZrO2-TiO2 (sol–gel) particles decreased with an increase in the quantity of ZrO2. For example, at 850 °C, the grain size of MP-ZrO2-TiO2 (sol–gel) particles was as follows: 38.8 nm in Test A3 (0 mol% Zr-doping), 32.4 nm in Test A6 (2 mol% Zr-doping), 25.6 nm in Test A9 (5 mol% Zr-doping), 20.1 nm in Test A12 (10 mol% Zr-doping), and 16.9 nm in Test A15 (35 mol% Zrdoping). This decrease in grain size was unexpected due to the fact
that the radius of Zr4+ is larger than that of Ti4+. This can be partially explained as follows: The addition of Zr to TiO2 leads to a reduction in the concentration of oxygen vacancies, and metastable ZrO2 particles can block or slow down the growth of TiO2. This issue is examined in greater detail in the following section. 3.1.3. Band gap Fig. 7 presents variations in absorbance with the wavelength of light (ranging from 250 to 800 nm) from the MP-TiO2 (sol–gel) particles in Test A3 and the MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15, which underwent calcination at 850 °C. The wavelength of light was less than 325 nm; therefore, the absorbance values of MP-ZrO2-TiO2 (sol–gel) particles slightly exceeded those of MP-TiO2 (sol–gel) particles; however, the absorbance values of MP-ZrO2-TiO2 (sol–gel) particles were substantially smaller than those of MP-TiO2 (sol–gel) particles, when the wavelength of light exceeded 325 nm. This difference can be attributed to the fact that the band absorptions of the MP-ZrO2-TiO2 (sol–gel) particles gradually shifted to shorter wavelengths (i.e., blue-shift) with an increase in the content of ZrO2 [14]. The results obtained using the Tauc-Sunds equation [17] in conjunction with Eqs. (1) and (2) reveal that the Eg of MP-ZrO2-TiO2 (sol–gel) particles increased with the amount of ZrO2. The Eg values of MP-TiO2 (sol–gel) particles and MP-ZrO2-TiO2 (sol–gel) particles calcined at 850 °C were calculated as follows: 3.05 eV in Test A3 (0 mol% Zr-doping), 3.14 eV in Test A6 (2 mol% Zr-doping), 3.23 eV in Test A9 (5 mol% Zr-doping), 3.26 eV in Test A12 (10 mol% Zr-doping), and 3.36 eV in Test A15 (35 mol% Zrdoping), as shown in Fig. 8. In addition, the ECB edge/EVB edge of MP-TiO2 (sol–gel) particles and MP-ZrO2-TiO2 (sol–gel) particles were as follows: 0.215 eV/2.835 eV (Test A3), 0.220 eV/2.920 eV (Test A6), 0.265 eV/2.965 eV (Test A9), 0.280 eV/2.980 eV (Test A12), and 0.330 eV/3.030 eV (Test A15). Gao et al. claimed that the enlarged Eg of the Zr-doped TiO2 materials can be ascribed to the elevation of the ECB [14]. However, we determined that the enlarged Eg of MP-ZrO2-TiO2 (sol–gel) particles may be the result of an increase in ECB and EVB due to Zr-doping, compared with
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
(d)
(a)
80000
100 nm
(b)
80000
100 nm
(e)
100 nm
80000
100 nm
(f)
(c)
80000
80000
100 nm
80000
100 nm
Fig. 3. SEM micrographs: (a) Test A1 [MP-TiO2 (sol–gel) particles calcined at 450 °C]; (b) Test A2 [MP-TiO2 (sol–gel) particles calcined at 600 °C]; (c) Test A3 [MP-TiO2 (sol– gel) particles calcined at 850 °C]; (d) Test A10 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 450 °C]; (e) Test A11 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C]; (f) Test A12 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 850 °C].
Fig. 4. Particle size distributions in Test A3 [MP-TiO2 (sol–gel) particles calcined at 850 °C], Test A10 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 450 °C], Test A11 [MP-ZrO2-TiO2 (sol–gel) particles calcined at 600 °C], Test A12 [MP-ZrO2-TiO2 (sol– gel) particles calcined at 850 °C].
MP-TiO2 (sol–gel) particles (Test A3). It may be helpful to determine the influence Zr4+ doping on the intrinsic TiO2 defect energy levels within the band gap. To evaluate the influence of Zr-doping on TiAO bonding strength, we calculated the oxide formation energy (DGT) at the temperature T for the reaction: M + O2 = MO2 (M = Ti or Zr) using FactSage [26]. The calculated results are presented in Table 5. Doping TiO2 with Zr leads to an increase in average bonding energy of Ti-O, which results in a lower concentration of oxygen vacancies and the associated defects. The effect of Zr-doping can be used to partially explain the reduction in the size of Zr-doped TiO2 particles mentioned above. Furthermore, the sequential reduction in the absorption of visible light in the 2, 5 and 10 mol % Zr-doped samples (Fig. 7) is in agreement with the theory governing the reduction in defects related to oxygen vacancies. As discussed earlier, stronger (Ti, Zr)-O bonds lead to fewer defects related to oxygen vacancies and a reduction in the absorption of visible light, which originates in such defects. However, the absorption of visible light in the 35 mol% ZrO2 sample (Fig. 7) does not follow the trend observed in the samples with 2, 5, 10 mol% ZrO2. This difference may due to the fact that it has deviated too far from equilibrium, such that new metastable phases formed.
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
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Fig. 6. Peaks of anatase diffracted from (1 0 1) plane for MP-TiO2 (sol–gel) particles in Test A3 as well as MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15.
Fig. 7. Variations in absorbance with the wavelength of light for MP-TiO2 (sol–gel) particles in Test A3 as well as MP-ZrO2-TiO2 (sol–gel) particles in Tests A6, A9, A12, and A15.
3.2. Working electrodes
Fig. 5. XRD patterns: (a) MP-TiO2 (sol–gel) particles; (b) MP-ZrO2-TiO2 (sol–gel) particles with 10 mol% Zr-doping; (c) MP-ZrO2-TiO2 (sol–gel) particles with 35 mol % Zr-doping.
Fig. 9(a) and (b) presents SEM micrographs (5 k) and EDS analysis of two dye-covered electrodes, i.e., the MP-TiO2 (sol–gel)/N719 electrode and TiO2 (sol–gel)/N719 electrode, respectively. These results demonstrate that the mesoporous structure of MP-TiO2 (sol–gel) particles can enhance the dye absorption capacity of the electrode, based on the fact that the weight percentage of ruthenium (Ru) in the MP-TiO2 (sol–gel)/N719 electrode (0.80%) substantially exceeded that of TiO2 (sol–gel)/N719 electrode (0.27%). Furthermore, the average thickness of the active layer was 4.39 lm, and the average film thickness of electrode (comprising an active layer and a multiple light-scattering layer) was 13.02 lm. Furthermore, the thickness of film deposited on the working electrode in Tests E1 to E7 is listed in Table 6. Fig. 10 presents variations in transmittance with the wavelength of light (ranging from 400 to 800 nm) from the TiO2 (P-25) electrode in Test E1, the TiO2 (P-25)/MP-TiO2 (sol–gel) electrode in Test E2, and the TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel)
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
Fig. 8. The values of band gap: (a) MP-TiO2 (sol–gel) particles in Test A3; (b) MP-ZrO2-TiO2 (sol–gel) particles in Tests A6 (2 mol% Zr-doping), A9 (5 mol% Zr-doping), A12 (10 mol% Zr-doping), and A15 (35 mol% Zr-doping).
Table 5 The oxide formation energy of TiO2 and ZrO2. Oxide formation energy DGT (kJ/mol)
TiO2 ZrO2
400 °C
600 °C
850 °C
871 1020
834 982
789 945
electrode in Tests E3 to E6. At a wavelength of 800 nm, the transmittance of the TiO2 (P-25) electrode was the highest (7.9%), whereas the transmittance of the TiO2 (P-25)/MP-TiO2 (sol–gel) electrode was the lowest (0.44%). The transmittance of the TiO2
(P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes in Tests E3 to E6 were between 7.9% and 0.44%. These results can be attributed to the following. (1) The addition of ZrO2 to the TiO2 via the sol–gel method suppressed the growth of grains during calcination. As a result, the particle size of MP-ZrO2-TiO2 (sol–gel) is substantially smaller than that of MP-TiO2 (sol–gel). (2) The average size of TiO2 (P-25) particles in this study (20% rutile and 80% anatase) was 21 nm. Using the method reported in our previous research [19,20], our results revealed that the Eg of the TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode increased slightly with an increase in the amount of ZrO2. The Eg values of the TiO2 (P-25)/MP-TiO2 (sol–gel) and TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes were as follows:
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W.-H. Lu et al. / Advanced Powder Technology xxx (2017) xxx–xxx
(a) MP-TiO2(Sol-gel)/N719 Element
Weight (%)
Atomic (%)
O
49.51
74.75
Ti
49.69
25.06
Ru
0.80
0.19
Total:
100.00
100.00
(b) TiO2(Sol-gel)/N719 Element
Weight (%)
Atomic (%)
O
47.18
72.84
Ti
52.55
27.10
Ru
0.27
0.07
Total:
100.00
100.00
Fig. 9. SEM micrographs and EDS analyses of dye-covered electrodes: (a) MP-TiO2 (sol–gel)/N719 electrode; (b) TiO2 (sol–gel)/N719 electrode.
Table 6 Voc, Jsc, FF, and g of DSSC.
G1 G2 G3 G4 G5 G6 G7
Counter electrode
Working electrode/film thickness (lm)
Voc (V)
Jsc (mA/ cm2)
FF (%)
g
FTO glass substrate with a Pt film
E1/11.2 E2/13.6 E3/13.3 E4/13.7 E5/12.3 E6/12.2 E7/12.5
0.63 0.67 0.67 0.67 0.67 0.67 0.66
12.05 17.66 18.03 18.13 18.81 16.74 16.26
44.9 46.2 47.8 48.0 49.0 48.6 49.1
3.44 5.45 5.74 5.81 6.20 5.49 5.28
(%)
2.97 eV in Test E2 (0 mol% Zr-doping), 3.04 eV in Test E3 (2 mol% Zr-doping), 3.06 eV in Test E4 (5 mol% Zr-doping), 3.07 eV in Test E5 (10 mol% Zr-doping), and 3.10 eV in Test E6 (35 mol% Zrdoping), as shown in Fig. 11. This can be explained by the fact that the Eg of MP-ZrO2-TiO2 (sol–gel) particles increased with the amount of ZrO2. 3.3. Voc, Jsc, g, and J-V curves of DSSCs Table 6 presents the Voc, short-circuit photocurrent per unit area (Jsc), fill factor (FF), and g in Tests G1 to G7. The highest g (6.20%) was obtained from a DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode (Test G5). Variations in Jsc with Voc are presented in Fig. 12(a) (Tests G1 to G3) and Fig. 12(b) (Tests G1, G5, and G7). Three DSSCs were prepared for each test condition. We then calcu-
Fig. 10. Variations in transmittance with the wavelength of light for working electrodes in Test E1 [TiO2 (P-25) electrode], Test E2 [TiO2 (P-25)/MP-TiO2 (sol– gel)], and Tests E3 to E6 [TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel)].
lated the standard deviation of short-circuit photocurrent density (r) under each test condition at every open-circuit voltage using rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ! P3 P3 3 Jsci ðJsci Þ2 i¼1 i¼1 , the results of the following equation r ¼ 9 which are plotted using error bars in Fig. 12.
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Fig. 11. The values of band gap: (a) TiO2 (P-25)/MP-TiO2 (sol–gel) electrode in Test E2; (b) TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrodes in Test E3 (2 mol% Zr-doping), Test E4 (5 mol% Zr-doping), Test E5 (10 mol% Zr-doping), and Test E6 (35 mol% Zr-doping).
3.3.1. Effects of mesoporous multi-light-scattering layer Our results demonstrate that an electrode with a multiple lightscattering layer of MP-TiO2 (sol–gel) particles can substantially enhance the Jsc and g of DSSCs. For example, the g in Test G2 [DSSC with a TiO2 (P-25)/MP-TiO2 (sol–gel) electrode] (5.45%) exceeded that of conventional DSSCs with a TiO2 (P-25) in Test G1 (3.44%). The Jsc values in Tests G1 and G2 were respectively 12.05 and 17.66 mA/cm2, and the Voc value in Test G2 (0.67 V) slightly exceeded that in Test G1 (0.63 V). This can be explained as follows. (1) The use of a multiple lightscattering layer of MP-TiO2 (sol–gel) particles enhanced the Jsc by extending the retention time of light and enhancing dye absorption capacity. (2) The weight percentage of ruthenium (Ru) in the
dye-covered MP-TiO2 (sol–gel)/N719 electrode (0.80%) substantially exceeded that of the TiO2 (sol–gel)/N719 electrode (0.27%), as shown in Fig. 9. 3.3.2. Effects of ZrO2 content Increasing the amount of ZrO2 added to the MP-ZrO2-TiO2 (sol– gel) particles led to an increase in the g of DSSCs fabricated with a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles. However, g decreased when the content of ZrO2 exceeded a given threshold. For example, when the Zr-doping was increased from 2 mol% to 10 mol%, g increased from 5.74% (Test G3) to 6.20% (Test G5), but decreased to 5.49% (Test G6) when the Zr-doping was increased to 35 mol%, as shown in Table 6. In addition, the Jsc val-
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ues were as follows: 18.03 (Test G3), 18.13 (Test G4), 18.81 (Test G5), and 16.74 (Test G6) mA/cm2. The Voc value remained at 0.67 V in Tests G3 to G6 (Table 6). This can be explained as follows. (1) Our results reveal that the additions of ZrO2 can delay the formation of rutile phase TiO2 during calcination, such that the MP-ZrO2-TiO2 (sol–gel) particles with anatase phase TiO2 inherit superior electrical conductivity. (2) As shown in Fig. 7, even in cases of unfavorable visible light absorption by individual particles (like MP-ZrO2-TiO2 (sol–gel) particles), appropriate Zr doping can suppress the formation of oxygen vacancies, which would otherwise result in a longer electron free path, as well as reduce electron scattering and recombination with holes [27,28]. However, increasing the quantity of ZrO2 beyond a given threshold can lead to the formation of other phases, such as ZrTiO4, which could upset the entire design. Troitzsch and Ellis revealed that a higher Zr concentration in the Zr doped-TiO2 compounds can result in the formation of ZrTiO4 or (Zr,Ti)2O4, in accordance with the ZrO2-TiO2 phase diagram [29]. (3) The slight increase in band gap induced by Zr-doping is also beneficial for band bending, which is more evident when observing particles with different concentrations of Zr. This kind of band bending can help to increase Voc by suspending electron-hole recombination. Interestingly, we observed an 80.2% increase in g in Test G5 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrode], compared with g in Test G1 [DSSC with a TiO2 (P-25) electrode]. This was likely due to the fact that the DSSC in Test G5 included a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles capable of enhancing the capture of light and dye absorption while suppressing electron-hole recombination. Furthermore, Jsc and Voc values in Test G1 were lower than those in Test G5. These results demonstrate that a working electrode composed of MP-ZrO2-TiO2 (sol–gel) particles can enhance the Voc as well as the Jsc of DSSCs.
Fig. 12. Variations in photocurrent density with photovoltage: (a) Test G1 [DSSC with a TiO2 (P-25) electrode], Test G2 [DSSC with a TiO2 (P-25)/MP-TiO2 (sol–gel) electrode], and Test G3 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol–gel) electrode]; (b) Test G1, as well as Tests G5 and G7 [DSSC with a TiO2 (P-25)/MP-ZrO2-TiO2 (sol– gel) electrode].
3.3.3. Multiple light-scattering layer vs. single light-scattering layer In Test G5 [DSSC with a multiple light-scattering layer of MPZrO2-TiO2 (sol–gel) particles], g was 17.4% higher and Jsc 15.7% higher than the values obtained in Test G7 [DSSC with a single light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles]. Furthermore, the Jsc and Voc values in Test G7 were lower than those in Test G5, as shown in Table 6. This difference can be explained as follows. At wavelengths of 400–800 nm, the transmittance values in Test E5 [working
7.9%
1.13% 0.62%
Fig. 13. Variations in transmittance with the wavelength of light in Test E1 [TiO2 (P-25) electrode], Test E5 [working electrode with a multi-light-scattering layer of MP-ZrO2TiO2 (sol–gel) particles], and Test E7 [working electrode with a single-light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles].
Please cite this article in press as: W.-H. Lu et al., Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.026
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electrode with a multiple light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles] were smaller than those in Test E7 [working electrode with a single light-scattering layer of MP-ZrO2-TiO2 (sol–gel) particles and TiO2 (P-25) particles], as shown in Fig. 13. For example, at a wavelength of 800 nm, the transmittance in Test E7 (working electrode from DSSC in Test G7) was 82.3% higher than the transmittance in Test E5 (working electrode from DSSC in Test G5). 4. Conclusions This paper presents a novel electrode design involving the application multi-layer light scattering to overcome many of the shortcomings in the light absorption of individual particles associated with doping (unavoidable or intended). More importantly, we present a novel strategy in which the formation of oxygen vacancies is intentionally reduced using simple chemical thermodynamics. The objective is to suspend or delay electron scattering and electron-hole recombination. In the case of TiO2 electrodes, the proposed strategy achieves the following: (1) doping metals (like Ru) with standard electrode potentials (SEP) higher than that of Ti will increase the formation of oxygen vacancies and the absorption of visible light [30]; (2) doping metals (like Zr) with SEP lower than that of Ti will reduce the formation of oxygen vacancies and the absorption of visible light while increasing carrier mobility. These simple design rules may be used in the development of photo-catalysts in general. Acknowledgements Chuen-Shii Chou would like to thank Ministry of Science and Technology, R.O.C. for financially supporting this research under Contract No. MOST 104-2221-E-020-036. Wu’s work is partially funded by the Changi-SUTD research program for photocatalytic research. References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737–739. [2] S. Caramori, V. Cristino, R. Boaretto, R. Argazzi, C.A. Bignozzi, A.D. Carlo, New components for dye-sensitized solar cells, Int. J. Photo. 2010 (2010) 16. [3] Z.S. Wang, H. Kawauchi, T. Kashima, H. Arakawa, Significant influence of TiO2 photo electrode morphology on the energy conversion efficiency of N719 dyesensitized solar cell, Coord. Chem. Rev. 248 (2004) 1381–1389. [4] S. Hore, C. Vetter, R. Kern, H. Smit, A. Hinsch, Influence of scattering layers on efficiency of dye-sensitized solar cells, Sol. Energy Mater. Sol. Cells 90 (2006) 1176–1188. [5] S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel, Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%, Thin Solid Films 516 (2008) 4613–4619. [6] J.K. Lee, B.H. Jeong, S.I. Jang, Y.G. Kim, Y.W. Jang, S.B. Lee, M.R. Kim, Preparations of TiO2 pastes and its application to light-scattering layer for dye-sensitized solar cells, J. Ind. Eng. Chem. 15 (2009) 724–729. [7] G. Yang, J. Zhang, P. Wang, Q. Sun, J. Zheng, Y. Zhu, Light scattering enhanced photoanodes for dye-sensitized solar cells prepared by carbon spheres/TiO2 nanoparticle composites, Curr. Appl. Phys. 11 (2011) 376–381.
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Please cite this article in press as: W.-H. Lu et al., Preparation of Zr-doped mesoporous TiO2 particles and their applications in the novel working electrode of a dye-sensitized solar cell, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.05.026