Trans. Nonferrous Met. Soc. China 25(2015) 3974-3979

Preparation and photoelectric properties of Ho³⁺-doped titanium dioxide nanowire downconversion photoanode

Yue-ying LI^1,2,3, Hong-shun HAO^1,2,3, Li-jun WANG^1,2,3, Wei-hua GUO^1,2,3, Qing SU^1,2,3, Lei QIN³, Wen-yuan GAO^1,2,3, Gui-shan LIU^1,2,3, Zhi-qiang HU^1,2,3

1. Department of Inorganic Nonmetallic Materials Engineering, Dalian Polytechnic University, Dalian 116034, China;

2. Liaoning Provincial College Key Laboratory of New Materials and Material Modification, Dalian Polytechnic University, Dalian 116034, China;

3. National Engineering Research Center of Seafood, Dalian Polytechnic University, Dalian 116034, China

Received 22 December 2014; accepted 17 June 2015

Abstract: Ho³⁺-doped titanium dioxide (TiO₂:Ho³⁺) downconversion (DC) nanowires were synthesized through a simple hydrothermal method followed by a subsequent calcination process after being immersed in Ho(NO₃)₃ aqueous solution. Moreover, TiO₂:Ho³⁺ nanowires (HTNWs) were used as the photoanode in dye-sensitized solar cells (DSSCs) to investigate their photoelectric properties. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the morphology and structure of the material, respectively. The photofluorescence and ultraviolet-visible absorption spectra of HTNWs reveal a DC from the near and middle ultraviolet light to visible light which matches the strong absorbed region of the N719 dye. Compared with the pure TNW photoanode, HTNWs DC photoanodes show greater photovoltaic efficiency. The photovoltaic conversion efficiency (η) of the DSSCs with HTNWs photoanode doped with 4% Ho₂O₃ (mass fraction) is two times that with pure TNW photoanode. This enhancement could be attributed to HTNWs which could extend the spectral response range of DSSCs to the near and middle ultraviolet region and increase the short-circuit current density (J_sc) of DSSCs, thus leading to the enhancement of photovoltaic conversion efficiency.

Key words: Ho³⁺-doped titanium dioxide nanowire; downconversion fluorescence; dye-sensitized solar cells; photovoltaic performance

1 Introduction

Because of its low cost and simple fabrication method, clean renewable energy utilization has become one of the primary underlying solutions for global warming [1], and improving the dye-sensitized solar cells (DSSC) efficiency has been of great significance for many research groups. To date, many investigators have pursed novel methods to enhance the battery performance based on the development of better elements, such as sensitizing dyes [2,3], transparent conducting oxides (TCOs) [4], redox electrolytes [5], nanocrystalline TiO₂ layers [6,7], and Pt counter electrodes [8]. Particularly, on account of the direct influence of nanocrystalline TiO₂ layer on the light harvesting, charge recombination and interfacial contact of the electrodes/electrolytes, it is a vital component in DSSCs. Nevertheless, the electrons generated by disordered TiO₂ films may suffer a series of trapping events between trap states on neighboring particles or recombine with oxidizing species in the electrolyte during random walking through the film, leading to the limitation of the charge-collection efficiency [9,10]. Hence, many attempts have been made to study TiO₂ nanofibers or nanowires for their promising microstructure and satisfied photoelectric property in energy utilization [11,12].

In addition, DSSCs have the maximum absorption up to 800 nm of the total incident solar irradiation [13]. This brings out a major issue for this technology because the unutilized ultraviolet and infrared occupy 50% of solar irradiation, and limits the solar energy conversion efficiency for DSSCs. Therefore, extending the spectral response range of DSSCs to the ultraviolet region has become a vital approach to increase the DSSCs efficiency. Many researchers used rare earth metal ions as the catalysts for their incompletely occupied 4f and empty 5d orbits [14]. Doped rare-earth compound can not only increase the photocurrent via conversion luminescence, but also improve the photovoltage by p-type doping effect. So far, rare-earth doped TiO₂ has been widely studied [15,16]. Compared with pure TiO₂- DSSC, all other lanthanide metal ions such as Nd, Eu, Gd and Sm doped TiO₂-DSSCs show an improvement in the performance of DSSCs [17,18]. In the previous work [19], SrTiO₃:Ho³⁺ nanoparticles were prepared and it was proved that the introduction of Ho³⁺ could increase the efficiency by 59%. But to our best knowledge, the investigation on the application of TiO₂:Ho³⁺ nanowire on DSSCs has rarely been reported.

In the present study, TiO₂:Ho³⁺ nanowires (HTNWs) were prepared through hydrothermal process in alkali liquid-based system followed by a subsequent calcination process after being immersed in Ho(NO₃)₃ aqueous solutions overnight. The Ho³⁺-doped content that crucially affects the microstructure and optical properties of the synthesized nanowires was optimized. Then, the morphology and crystal structure of the samples were studied. Finally, the effect of produced nanowires on the performance of DSSCs was investigated.

2 Experimental

2.1 Synthesis of HTNWs

2.1.1 Synthesis of HTNWs precursor

Appropriate TiO₂ (Degussa P25) was added to NaOH solution and then transfered to teflon autoclaves which was kept in an electric oven at 190 °C for 24 h. After cooling down to room temperature, the sample was washed with distilled water several times until the pH value of the washing solution was about 7.0 and then dried at 80 °C overnight in air. After being immersed in 0.1 mol/L HCl solution overnight, the sample was washed and dried again using the above method, and then TiO₂ nanowire precursor was obtained.

To prepare HTNWs precursor, the as-prepared TiO₂ nanowires precursor was immersed in Ho(NO₃)₃ aqueous solution at different concentrations of Ho₂O₃ overnight. After being filtered and washed with distilled water, different HTNWs precursors were obtained.

2.1.2 Synthesis of HTNWs

The HTNWs with different Ho₂O₃-doped contents were obtained by heating the corresponding precursor at 650 °C for 2 h in air. For the convenience of description, the HTNWs with Ho₂O₃-doped contents of 0, 2%, 4% and 6% (mass fraction) were labeled as S0, S1, S2 and S3, respectively.

2.2 Fabrication of DSSCs based on HTNWs photoanodes

The TiO₂ blocking layer for DSSCs was prepared according to the following strategy: tetrabutyl titanate was mixed with diethanolamine and absolute ethyl alcohol, and appropriate distilled water was added dropwise, then, the obtained colloidal sol was coated on FTO by spin-coating method, and the blocking layer film formed after being sintered at 500 °C for 30 min. In the next step, mesoporous HTNWs films were prepared through screen printing method as follows: firstly, appropriate HTNWs was added into the agate mortar in the presence of distilled water, acetic acid and triton X-100 to obtain the screen printing slurry. Secondly, the obtained slurry was screen printed on the blocking layer film and annealed at 500 °C for 30 min to remove the binders in the slurry. To prepare dye-sensitized electrodes, the mesoporous HTNWs film electrodes was immersed in 0.5 mmol/L N719 ethanol solution overnight at room temperature in the dark. After being removed from the dye solution, the electrodes were rinsed and dried. Finally, the unsealed sandwich type solar cells, assembled with dye sensitized HTNWs electrodes with Ho₂O₃ contents of 0, 2%, 4% and 6% (mass fraction) as the photoanodes, Pt coated FTO electrode as the counter electrode, and standard iodide electrolyte solution as a mediator, were labeled as W₀, W₁, W₂ and W₃, respectively. The area of working electrode was 0.25 cm².

2.3 Characterization

X-ray diffraction (XRD) experiments were carried out at room temperature using a D/max 3B diffractometer operated at 30 kV and 100 mA with Cu K_α radiation (λ=0.154 nm) to identify the crystal phase of samples. The surface morphology was examined using a scanning electron microscope (SEM, JSM-6460LV). Absorbance spectroscopy of samples was tested in an ultraviolet-visible absorbance spectrometer (UV-Vis, Lambda35). The photoluminescence (PL) spectra were analyzed using a fluorescence spectrometer (LS-55). A solar simulator (SS50) providing an illumination of AM1.5G simulated sunlight with 100 mW/cm² irradiation and a Keithley 2400 source meter were employed to measure the photovoltaic properties of the assembled DSSCs. The monochromatic incident photon- electron conversion efficiency (IPCE) of the DSSCs was also measured using a light meter (SM-25, Japan).

3 Results and discussion

3.1 Phase analysis

The XRD pattern of S2 with 4% Ho₂O₃ is shown in Fig. 1. The peaks at 2θ values of 25.26°, 36.88°, 47.78°, 62.74° and 68.99° correspond to the (101), (004), (200), (204) and (116) crystal planes of anatase TiO₂, respectively. All the peaks can be perfectly indexed as a pure anatase phase of TiO₂ (JCPDS card No. 21-1272) with lattice constants a=3.7852  and c=9.5139 . Moreover, no other peaks are detected, which indicates that the crystal structure of TiO₂ is not changed by doping with a small amount of Ho³⁺ ions. Furthermore, in the case of raw TiO₂ samples, the transition of anatase to rutile phase started at around 600 °C, and the two phases co-existed in a wide temperature range (from 600 to 900 °C) [20,21]. While, all peaks of the as-prepared samples can be assigned to the anatase phase calcined at 650 °C. As suggested by other authors, the rare earth element dopant influences the transition rates of anatase to rutile TiO₂ phase due to perturbation in the crystal nucleation process for either substitutional or interstitial dopant ion incorporation within the host lattice [22,23]. Indeed, the rare earth ion incorporation within the titania structural framework causes lattice site distortions due to the large difference of ionic radii between Ti⁴⁺(0.68 ) and Ho³⁺ (90.1 ).

Fig. 1 XRD pattern of S2 with 4% Ho₂O₃

3.2 Morphology analysis

Figures 2(a) and (b) show the SEM images of S0 and S2, respectively. As shown in Fig. 2(a), after hydrothermal reaction for 24 h, the high purity TNWs with large aspect ratio are obtained. After hydrothermal reaction, P25 nanoparticles were transformed into non-hollow nanowires with small size distributions both in length and in diameter. Approximately, the lengths are 2-20 μm and the diameters are 100-200 nm. Figure 2(b) shows the SEM image of the HTNWs with 4% Ho₂O₃ (S2). Compared with the pure TNWs, the HTNWs keep the original appearance and display highly porous network morphology. The magnified image shows that the network consists of ultrafine HTNWs with an average diameter of 150 nm.

Fig. 2 SEM images of S0 (a) and S2 (b)

3.3 Downconversion (DC) luminescence analysis

The excitation (emission at 587 nm) and emission spectra (excited at 412 nm) of S1-S3 with Ho₂O₃ of 2%, 4% and 6% in sequence are shown in Fig. 3. It can be seen that HTNWs possess an obvious luminescence function. A significant excitation peak at 412 nm and an emission peak at 587 nm are observed. The excitation peak at 412 nm and the emission peak at 587 nm corresponded to the Ho³⁺:⁵I₈→⁵G₅, ⁵S₂→⁵I₈ transitions, respectively. The luminescence at 587 nm is just within the absorption wavelength range of sensitizing dye N719. Combining the excitation and emission spectra, the ultraviolet irradiation can be absorbed by the N719 dye in the DSSCs via the conversion luminescence of HTNWs and the solar light harvest of DSSCs may thus increase. Figure 4 shows the PL intensity of HTNWs as a function of doping content of Ho₂O₃. According to the emission spectra, the relative PL intensity of HTNWs decreases slightly with the increase of Ho₂O₃ content from 2% to 4%. But larger amount of Ho₂O₃ (6%) because of filling the quenching sites decreases the PL intensity obviously. Figure 5 shows the energy-level diagram of Ho³⁺ and DC mechanisms. According to the energy band and energy matching between the excitation and emission wavelength, the possible DC energy transfer mechanism is suggested that ultraviolet photon excitation (at 412 nm) makes the electron of Ho³⁺ excited to a higher energy state (⁵G₅), where radiationless transition occurs until the electron resides at the ⁵S₂ energy level in 4f shell. Then, radiative transition occurs to the ⁵I₈ ground state (⁵S₂→⁵I₈) and generates green light emission around 587 nm.

Fig. 3 Excitation (a) and emission (b) spectra of S1-S3 with Ho₂O₃ of 2%, 4% and 6% in sequence

Fig. 4 PL intensity of HTNWs as function of doping content of Ho₂O₃ at 587 nm

Fig. 5 Schematic energy level diagram of Ho³⁺ in TiO₂and DC mechanisms on excitation by 412 nm radiation

3.4 Analysis of UV-Vis absorption spectra

The optical properties of TNWs with different Ho³⁺-doped contents in the range of 300-700 nm were studied by the UV-Vis absorption spectra and the results are shown in Fig. 6. All the samples have absorbance in the UV-Vis region. In addition, in the wavelength around 330 nm, the photoanodes with HTNWs (S2, S3) show more absorption. The possible reason is that the HTNWs could absorb and convert near and middle ultraviolet light to visible light which the N719 could absorb easily. Furthermore, the photoanodes with strong absorbance effects would improve the light-harvesting efficiency resulting in a higher current density, and the highest absorption spectrum in the case of HTNWs doped with 4% Ho₂O₃ is consistent with its highest short-circuit current density (J_sc) of the relevant DSSCs. In all, the highest efficiency of 1.03% is obtained for the DSSCs with HTNWs doped with 4% Ho₂O₃.

Fig. 6 UV-Vis absorbance spectra of different photoanodes sensitized with N719

3.5 Photoelectric properties of DSSCs

Figure 7 shows the current density-voltage curves of W0-W3 DSSCs based on the films of DC HTNWs with different doping contents of Ho₂O₃. The open circuit voltage (φ), short-circuit current density (J_sc), fill factor (F_f) and photoelectric conversion efficiency (η) of these DSSCs are summarized in Table 1, while Fig. 8 shows the dependence of J_sc and η on the Ho₂O₃ doping content in TNWs. As shown in Fig. 8, the photocurrent density and conversion efficiency first increase and then decrease with increasing the doping content of Ho₂O₃, and the open circuit voltage is similar. When the doping content of Ho₂O₃ reaches 4%, the photocurrent density and the conversion efficiency of the cell peak reach the optimum values of 5.6 mA/cm² and 1.93%, respectively. As the doping content increases beyond 4%, both the photocurrent density and conversion efficiency decrease. The increase of J_sc when the doping amount is less than 4% can be attributed to the DC luminescence of Ho³⁺ from ultraviolet light to visible light. Moreover, the changes of TiO₂ energy level by doping rare-earth ions improve the carrier transport at the interface of TiO₂/Dye, which further improves the J_sc. However, when the amount of Ho₂O₃ is beyond 4%, more grains, phases and domain interfaces are produced in the doping layer. These interfaces can capture photo-generated electrons and holes, hindering the charge carrier transportation, leading to a decrease of photocurrent [24]. This is the reason why J_sc increases first and then decreases with the increase of Ho₂O₃ content in the TiO₂ layer of the DSSCs.

Fig. 7 J_sc-φ curves of W0-W3 DSSCs

Table 1 Property parameters of W0-W3 DSSCs

Fig. 8 Dependence of J_sc and η on doping content of Ho₂O₃

In addition, the J_sc is also reflected in the IPCE performance (Fig. 9). The cell with 4% Ho₂O₃ shows the maximum IPCE of 33.25% at 520 nm, while the IPCE value of the cell without Ho₂O₃ is only 7.8%. This indicates that the significant improvement of J_sc is due to the enhancement of the photo-to-current efficiency.

Fig. 9 IPCE curves of W0-W3 DSSCs

4 Conclusions

1) As a conversion luminescence medium, the HTNWs broaden the near and middle ultraviolet light harvest via DC luminescence and increase the photocurrent.

2) When the doping content of Ho₂O₃ is 4%, the photoelectric conversion efficiency of the DSSC is two-fold increase compared with that of the DSSC without Ho³⁺ ions.

3) The research results reveal that the DC material HTNW is helpful to improve the performance of DSSCs.

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TiO₂:Ho³⁺纳米线下转换光阳极的制备及其光电性能

李月英^1,2,3，郝洪顺^1,2,3，王丽君^1,2,3，郭伟华^1,2,3，苏 青^1,2,3，秦 磊³，高文元^1,2,3，刘贵山^1,2,3，胡志强^1,2,3

1. 大连工业大学 无机非金属材料工程系，大连 116034；

2. 大连工业大学 新材料与材料改性省高校重点实验室，大连 116034；

3. 大连工业大学 国家海洋食品工程技术研究中心，大连 116034

摘  要：采用水热法制备TiO₂纳米线前驱体，将其浸入Ho(NO₃)₃水溶液并取出煅烧，得到TiO₂:Ho³⁺下转换纳米线，用其制备染料敏化太阳能电池的光阳极，探讨其光电性能。用扫描电镜和X射线衍射分别表征TiO₂:Ho³⁺纳米线的形貌和结构。荧光光谱和紫外-可见吸收光谱显示TiO₂:Ho³⁺下转换纳米线可以将近紫外光和中紫外光转化为可以被N719染料吸收的可见光。与纯TiO₂纳米线作为光阳极的电池相比，以TiO₂:Ho³⁺下转换纳米线作为光阳极的电池有较好的光电转换效率，且当Ho³⁺掺杂量为4%时(质量分数)，电池效率提高了一倍。TiO₂:Ho³⁺下转换纳米线拓宽光谱响应范围至近紫外光和中紫外光区，提高了电池的短路电流密度，从而电池效率提高。

关键词：TiO₂:Ho³⁺纳米线；下转换荧光；染料敏化太阳能电池；光伏性能

(Edited by Mu-lan QIN)

Foundation item: Project (2012FU125X03) supported by Open Research Fund Project of National Engineering Research Center of Seafood, China; Project (2011–191) supported by the Key Science and Technology Platform of Liaoning Provincial Education Department, China; Project (2010–354) supported by the Science and Technology Platform of Dalian, China

Corresponding author: Hong-shun HAO; Tel: +86-411-86323708; E-mail: beike1952@163.com

DOI: 10.1016/S1003-6326(15)64046-8