Skip to main content

Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell


Large-sized titanium dioxide (TiO2) nanotube arrays with an outer diameter of approximately 500 nm have been successfully synthesized by potentiostatic anodization at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. It is found that the synthesized large-diameter TiO2 nanotube array shows a superior light scattering ability, which can be used as a light scattering layer to significantly enhance the efficiency of TiO2 nanoparticle-based dye-sensitized solar cells from 5.18% to 6.15%. The remarkable light scattering ability makes the large-diameter TiO2 nanotube array a promising candidate for light management in dye-sensitized solar cells (DSSCs).


Dye-sensitized solar cells (DSSCs) have received considerable interest since 1991 [1] with the growing concern on sustainable and renewable energies. The highest power conversion efficiency (PCE) of DSSCs based on TiO2 nanoparticle mesoporous films has been reported [2], and to further improve the PCE, plenty of research has been carried out, such as the development of new dyes with broadband absorption [3, 4], the increase of the sensitized surface area of the TiO2 film [5, 6], and the use of a scattering layer for enhanced light harvesting [713]. Among them, the introduction of a scattering layer with different structures has been widely studied and proven to be effective in light harvesting enhancement. TiO2 nanorods with a length of 180 to 250 nm have been used as scattering centers in DSSCs by Yoon et al. [9]. Liu et al. had dispersed TiO2 nanospheres into nanocrystallites for increased light harvesting in DSSCs [10].

However, scattering centers of large-scale micrometer particles embedded in the absorbing layer of DSSCs would reduce the dye loading amounts. Hence, a bi-layer structure with the scattering layer beneath the absorbing layer to increase the optical path length is more favorable. Hierarchical TiO2 hollow spheres with an outer diameter of 300 to 700 nm [11] and size-tunable mesoporous spherical TiO2[12] have been tried as the scattering layer in bi-layer-structured DSSCs.

While the scattering of nanofibers and nanotubes was found to satisfy the Mie theory, which was originally proposed to describe the scattering of particles of a size similar to the wavelength [1315], there are only few relevant reports on applying TiO2 nanotubes with a subwavelength-sized diameter as the scattering layer. Herein, we succeeded in a straightforward approach to the fabrication of large-diameter (comparable to wavelength) TiO2 nanotubes and characterized the light scattering effect by transmittance spectra measurement and finite-element full wave simulation. The anodization was processed at 180 V in a used electrolyte with the addition of 1.5 M lactic acid. The transmittance spectra showed that the large-diameter nanotubes have a superior light scattering property. When the large-diameter TiO2 nanotube membrane was successfully peeled off and used as the scattering layer in DSSCs, the PCE was found to increase from 5.18% to 6.15% under 1 Sun (or 5.23% to 6.36% under 0.5 Sun) and increased by 19% (or 22%) due to the strong light scattering of large-diameter TiO2 nanotubes.


The large-diameter TiO2 nanotubes were fabricated through potentiostatic anodization in a conventional two-electrode electrochemical cell. Titanium sheets (0.125 mm in thickness, Strem Chemicals, Newburyport, MA, USA) were used as the working electrode while Pt foil was the counter electrode, with the distance between electrodes being 2 cm. The anodization process was divided into three steps: (1) The Ti foil was electrochemically pretreated for 0.5 h at 60 V in an ethylene glycol electrolyte containing 0.5 wt% NH4F and 3 vol% H2O (anodization electrolyte #1). After anodization, the anodized layer was peeled off by intense ultrasonication to expose the substrate. (2) The surface-exposed Ti was processed in another ethylene glycol electrolyte with 0.5 wt% NH4F and 10 vol% H2O, added with 1.5 M lactic acid (LA) (anodization electrolyte #2). Electrolyte #2 was aged by anodization reaction at 60 V for about 10 h before usage. To fabricate large-diameter nanotubes, the anodization voltage was fixed at 120 V for 10 min and then gradually increased to 180 V for 10 min at a rate of 0.1 V/s. (3) The as-grown large-diameter nanotubes were annealed at 450°C for 2 h and then detached from the Ti substrate by a third anodization at 60 V in electrolyte #1 to obtain the freestanding membranes [16]. For comparison, freestanding TiO2 nanotube membranes of the same thickness but with smaller diameters were also fabricated by anodization at 60 V for 10 min in electrolyte #1.

The resulting nanotube membrane was used as a scattering layer by adhering to the fluorine-doped tin oxide (FTO) substrate with TiO2 NP paste via a doctor blade method, followed by sintering at 450°C for 2 h. The sintered photoanodes were immersed in a dye-containing solvent (N719 dye, Dyesol, Queanbeyan, New South Wales, Australia) for 3 days. A 25-μm-thick hot-melt spacer was used to separate the sensitized photoanode and the counter electrode which was prepared by thermal decomposition of H2PtCl6 isopropanol solution on FTO glass at 380°C for 30 min. The interspace was filled with a liquid electrolyte of DMPII/LiI/I2/TBP/GuSCN in 3-methoxypropionitrile.

The structure and morphology of the TiO2 nanotubes were analyzed using field-emission scanning electron microscopy (FESEM; JEOL JSM-6335 F, JEOL Ltd., Tokyo, Japan). The current density-voltage (J-V) characteristics were measured by a sourcemeter (Model 2420, Keithley Instruments, Inc., Cleveland, OH, USA) under AM 1.5G illumination (100 mW cm−2) which was provided by a 300-W solar simulator (Model 91160, Newport Corporation-Oriel Instruments, Irvine, CA, USA). Transmittance spectra were acquired using a UV–Vis spectrophotometer (Model UV-2550, Shimadzu Corporation, Tokyo, Japan). The amount of dye was measured by desorbing the attached dye molecules in 0.1 M NaOH aqueous solution, with the concentration determined by a UV–Vis spectrophotometer. The normalized incident photon-to-current conversion efficiency (IPCE) values were measured with an IPCE system equipped with a xenon lamp (Oriel 66902, 300 W), a monochromator (Newport 66902), and a dual-channel power meter (Newport 2931_C) equipped with a Si detector (Oriel 76175_71580).

Results and discussion

Shown in Figure 1a,b are top and cross-sectional SEM images of the large-diameter TiO2 nanotube arrays (LTNAs). As reported before, the nanotube diameter is determined by the water content in the electrolyte and the anodization voltage, with a larger diameter obtained under more water content and higher voltage [17, 18]. Meanwhile, the addition of LA and the use of an aged electrolyte can prevent the anodic breakdown and the oxide burning under too large a current density at high anodization voltages [19, 20]. In the second step of the anodization process, prior to the anodization at 180 V, a pretreatment at 120 V for 10 min was adopted to maintain a flat anodic TiO2 film surface. With this pretreatment, the surface diameter was smaller than that at the bottom of the nanotubes. As can be seen from Figure 1a,b, the diameters of LTNA are approximately 500 nm at the bottom and approximately 300 nm at the surface. The nanotubes have a typical length of approximately 1.8 μm, with roughened tube walls. For comparison, we also fabricated small-diameter TiO2 nanotube arrays (STNAs) with a diameter of approximately 120 nm, which were anodized at 60 V.

Figure 1
figure 1

SEM images and schematic of the photoanode. (a) Top and (b) cross-sectional SEM images of LTNAs. (c) Cross-sectional SEM image of the LTNA as a scattering layer on top of TiO2 nanoparticles. (d) Schematic of the photoanode structure with scattered incident light.

The light scattering effect was characterized by measuring the transmittance spectra of three types of photoanodes adhered to FTO glass substrates (Figure 2a), namely, TiO2 particles (TP), TP + STNA, and TP + LTNA. It can be seen clearly that LTNA has a superior light scattering property than STNA, as the TP + LTNA sample is opaque and the TP + STNA sample is semitransparent. The TP sample is the most transparent, with the highest transmittance in the visible range. Finite-element full wave simulation (Additional file 1: Figure S1) was used to numerically calculate the transmittance spectra of the two different types of TNAs [21, 22], which revealed that light propagates through STNA without remarkable scattering, while pronounced scattering occurs in LTNA. The high anodization voltage also enables the formation of some randomly orientated nanotubes and defects [23], which further enhance the light scattering in LTNA. Hence, experimentally, the TP + LTNA sample shows extremely low transmittance in the whole visible range (Figure 2a).

Figure 2
figure 2

Optical and photovoltaic properties. (a) Transmittance of the three types of photoanodes adhered to the FTO glass substrates before the sensitization with N719. The insets from left to right show the photos of the photoanodes, TP (3 L), TP (3 L) + STNA, and TP (3 L) + LTNA, respectively. Here, 3 L stands for the optimized thickness of the TiO2 particle layer in a TP-based DSSC. (b) Photocurrent-voltage curves (1 Sun) of the TP (3 L)-based DSSCs coupled with different scattering layers, i.e., LTNA and STNA, with a thickness of 1.8 μm.

To study the effect of the scattering layer on the PCE of DSSC, the thickness of the TiO2 particle layer was first optimized by measuring the PCE of five TP-based DSSCs in different thicknesses (Additional file 1: Figure S2). The PCE was found to increase from 3.52% for TP (1L) to 5.18% for TP (3L) due to increased thickness (from 5 to 14 μm). It then starts to decrease when the TP layer thickness was further increased. The sample with the optimized thickness, TP (3L), was chosen to be attached to the STNA and LTNA scattering layers, with a thickness of around 1.8 μm as shown in Figure 1c,d. At least four cells were tested for each type of the solar cells, and their representative I-V curves are shown in Figure 2b and Table 1 with the photovoltaic properties. It is found that both η and JSC were enhanced due to the attachment of a scattering layer. The JSC is increased from 11.3 mA cm−2 for the TP (3L) cell to 13.9 mA cm−2 for the TP (3L) + LTNA cell. Due to the higher light scattering power of the LTNA than that of the STNA, the percentage increase in η is approximately 19% (from 5.18% to 6.15%) for the TP (3L) + LTNA cell, higher than the approximately 6.5% increase for the TP (3L) + STNA cell. It is also noted that due to the attachment of the scattering layer, the dye loading amount was increased. However, the increased dye loading contributes less to the increase of η than the enhanced light scattering does due to the fact that the TP layer thickness has already been optimized. Further increase in the thickness of the photoanode will result in a decrease in η, though the dye loading is increased. Indeed, although the TP (3L) + STNA cell has a higher dye loading than the TP (3L) + LTNA one, its η is much lower (Table 1). This further demonstrates the importance of light scattering.

Table 1 Photovoltaic properties of the DSSCs with and without the scattering layers

The I-V curves of the three types of DSSCs under lower irradiation (0.5 Sun) were also measured (Additional file 1: Figure S3). Owing to the excellent scattering property of the LTNA layer, an efficiency of 6.36% was achieved in the TP (3L) + LTNA cell, in comparison with the efficiencies of 5.23% and 5.64% achieved in the TP (3L) and TP (3L) + STNA cells, respectively. The angular response of the three types of DSSCs was also investigated and compared (Figure 3a). Due to the high scattering power of the LTNA layer for the different photon propagation directions, the enhanced light absorption effect is less sensitive to the tilting of the cells.

Figure 3
figure 3

DSSC angle performance and IPCE. (a) Variation of efficiency with the angle of incidence of incoming light with respect to the three types of cells. (b) IPCE of the TP (3 L)-based DSSCs coupled with different scattering layers, i.e., LTNA and STNA.

The incident photon-to-current conversion efficiency (IPCE) spectra are depicted in Figure 3b to provide detailed information on light harvesting. It is observed that the main light harvesting enhancement caused by the scattering layer occurs not only in the dye absorption range but also in the long wavelength side [24, 25], which is exactly the wavelength range for the small dye absorption. Consequently, the TP (3L) + LTNA cell is able to more efficiently recapture the unabsorbed light which resulted from the efficient light scattering capability of the LTNA layer. A further insight into the electrochemical behavior was provided by the EIS measurement in the dark at different applied bias voltages. The electron recombination time (τn) was calculated from the Bode phase plots by τn = 1/(2πfpeak), where fpeak is the characteristic peak frequency in the mid-frequency (1 to 100 Hz) region [5, 26]. As shown in Additional file 1: Figure S4, the use of the light scattering layer does not significantly influence the τn and hence does not affect the electron transport.


Large-diameter TiO2 nanotube arrays were successfully synthesized. The outstanding scattering power of the LTNA layer was demonstrated by the transmittance spectra and the optical simulation. The LTNA layer is superior to the STNA one in terms of light scattering. The use of the LTNA as the scattering layer in DSSCs enhances the PCE (from 5.18% to 6.15%) and the short-circuit current density much more than the STNA does. It is believed that the large-diameter nanotubes can be applied to other types of solar cells and higher conversion efficiency can be achieved by further optimization.


  1. O'Regan B, Grätzel M: A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 1991, 353: 737. 10.1038/353737a0

    Article  Google Scholar 

  2. Yella A, Lee H, Tsao H, Yi C, Chandiran A, Nazeeruddin M, Diau E, Yeh C, Zakeeruddin S, Grätzel M: Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science 2011, 334: 629–634. 10.1126/science.1209688

    Article  Google Scholar 

  3. Miao Q, Wu L, Cui J, Huang M, Ma T: A new type of dye-sensitized solar cell with a multilayered photoanode prepared by a film-transfer technique. Adv Mater 2011, 23: 2764. 10.1002/adma.201100820

    Article  Google Scholar 

  4. Kamat P: Quantum dot solar cells. Semiconductor nanocrystals as light harvesters. J Phys Chem C 2008, 112: 18737. 10.1021/jp806791s

    Article  Google Scholar 

  5. Lin J, Liu X, Guo M, Lu W, Zhang G, Zhou L, Chen X, Huang H: A facile route to fabricate an anodic TiO2 nanotube-nanoparticle hybrid structure for high efficiency dye-sensitized solar cells. Nanoscale 2012, 4: 5148–5153. 10.1039/c2nr31268a

    Article  Google Scholar 

  6. Liu X, Lin J, Chen X: Synthesis of long TiO2 nanotube arrays with a small diameter for efficient dye-sensitized solar cells. RSC Adv 2013, 3: 4885–4889. 10.1039/c3ra40221e

    Article  Google Scholar 

  7. Lin J, Guo M, Yip G, Lu W, Zhang G, Liu X, Zhou L, Chen X, Huang H: High temperature crystallization of free-standing anastase TiO2 nanotube membranes for high efficiency dye-sensitized solar cells. Adv Funct Mater 2013, 23: 5952. 10.1002/adfm.201301066

    Article  Google Scholar 

  8. Lu H, Deng K, Shi Z, Liu Q, Zhu G, Fan H, Li L: Novel ZnO microflowers on nanorod arrays: local dissolution-driven growth and enhanced light harvesting in dye-sensitized solar cells. Nanoscale Res Lett 2014, 9: 183. 10.1186/1556-276X-9-183

    Article  Google Scholar 

  9. Yoon J, Jang S, Vittal R, Lee J, Kim K: TiO2 nanorods as additive to TiO2 film for improvement in the performance of dye-sensitized solar cells. J Photoch Photobio A 2006, 180: 184–188. 10.1016/j.jphotochem.2005.10.013

    Article  Google Scholar 

  10. Liu Z, Su X, Hou G, Bi S, Xiao Z, Jia H: Mixed photoelectrode based on spherical TiO2 nanorod aggregates for dye-sensitized solar cells with high short-circuit photocurrent density. RSC Adv 2013, 3: 8474–8479. 10.1039/c3ra40371h

    Article  Google Scholar 

  11. Dadgostar S, Tagabadi F, Taghavinia N: Mesoporous submicrometer TiO2 hollow spheres as scatterers in dye-sensitized solar cells. ACS Appl Mater Interfaces 2012, 4: 2964–2968. 10.1021/am300329p

    Article  Google Scholar 

  12. Park Y, Chang Y, Kum B-G, Kong E, Son J, Kwon Y, Park T, Jang H: Size-tunable mesoporous spherical TiO2 as a scattering overlayer in high-performance dye-sensitized solar cells. J Mater Chem 2011, 21: 9582. 10.1039/c1jm11043h

    Article  Google Scholar 

  13. Hwang S, Kim C, Song H, Son S, Jang J: Designed architecture of multiscale porous TiO2 nanofibers for dye-sensitized solar cells photoanode. ACS Appl Mater Interfaces 2012, 4: 5287–5292. 10.1021/am301245s

    Article  Google Scholar 

  14. Raza S, Toscano G, Jauho A, Mortensen N, Wubs M: Refractive-index sensing with ultrathin plasmonic nanotubes. Plasmonics 2012, 8: 193–199.

    Article  Google Scholar 

  15. Chen Y, Chang Y, Huang J, Chen I, Kuo C: Light scattering and enhanced photoactivities of electrospun titania nanofibers. J Phys Chem C 2012, 116: 3857–3865. 10.1021/jp2117246

    Article  Google Scholar 

  16. Lin J, Chen J, Chen X: Facile fabrication of free-standing TiO2 nanotube membranes with both ends open via self-detaching anodization. Electrochem Commun 2010, 12: 1062–1065. 10.1016/j.elecom.2010.05.027

    Article  Google Scholar 

  17. Valota A, LeClere D, Schmuki P, Curioni M, Hashimoto T, Berger S, Kunze J, Schmuki P, Thompson G: Influence of water content on nanotubular anodic titania formed in fluoride/glycerol electrolytes. Electrochim Acta 2009, 54: 4321–4327. 10.1016/j.electacta.2009.02.098

    Article  Google Scholar 

  18. Sun L, Zhang S, Sun X, He X: Effect of the geometry of the anodized titania nanotube array on the performance of dye-sensitized solar cells. J Nanosci Nanotechnol 2010, 10: 4551–4561. 10.1166/jnn.2010.1695

    Article  Google Scholar 

  19. Ni J, Noh K, Frandsen C, Kong S, He G, Tang T, Jin S: Preparation of near micrometer-sized TiO2 nanotube arrays by high voltage anodization. Mater Sci Eng C 2013, 33: 259–264. 10.1016/j.msec.2012.08.038

    Article  Google Scholar 

  20. So S, Lee K, Schmuki P: Ultrafast growth of highly ordered anodic TiO2 nanotubes in lactic acid electrolytes. J Am Chem Soc 2012, 134: 11316–11318. 10.1021/ja301892g

    Article  Google Scholar 

  21. Guo M, Xie K, Lin J, Yong Z, Yip C, Zhou L, Wang Y, Huang H: Design and coupling of multifunctional TiO2 nanotube photonic crystal to nanocrystalline titania layer as semi-transparent photoanode for dye-sensitized solar cell. Energy Environ Sci 2012, 5: 9881–9888. 10.1039/c2ee22854h

    Article  Google Scholar 

  22. Yip CT, Huang H, Zhou L, Xie K, Wang Y, Feng T, Li J, Tam W: Direct and seamless coupling of TiO2 nanotube photonic crystal to dye-sensitized solar cell: a single-step approach. Adv Mater 2011, 23: 5624–5628. 10.1002/adma.201103591

    Article  Google Scholar 

  23. Zhang Q, Myers D, Lan J, Jenekhe S, Cao G: Applications of light scattering in dye-sensitized solar cells. Phys Chem Chem Phys 2012, 14: 14982–14998. 10.1039/c2cp43089d

    Article  Google Scholar 

  24. Huang F, Chen D, Zhang X, Caruso R, Cheng Y: Dual-function scattering layer of submicrometer-sized mesoporous TiO2 beads for high-efficiency dye-sensitized solar cells. Adv Funct Mater 2010, 20: 1301–1305. 10.1002/adfm.200902218

    Article  Google Scholar 

  25. Chang Y, Kong E, Park Y, Jang H: Broadband light confinement using a hierarchically structured TiO2 multi-layer for dye-sensitized solar cells. J Mater Chem A 2013, 1: 9707–9713. 10.1039/c3ta11527e

    Article  Google Scholar 

  26. Kern R, Sastrawan R, Ferber J, Stangl R, Luther J: Modeling and interpretation of electrical impedance spectra of dye solar cells operated under open-circuit conditions. Electrochim Acta 2002, 47: 4213. 10.1016/S0013-4686(02)00444-9

    Article  Google Scholar 

Download references


The work was supported by grants received from the Research Grants Council of the Hong Kong Special Administrative Region (Project Nos. PolyU5159/13E and PolyU5163/12E) and the Hong Kong Polytechnic University (Project No. G-YL06). The work was also supported by the National Natural Science Foundation of China (Grant No. 61125503) and the Foundation for Development of Science and Technology of Shanghai (Grant No. 11XD1402600).

Author information

Authors and Affiliations


Corresponding authors

Correspondence to Xianfeng Chen or Haitao Huang.

Additional information

Competing interests

The authors declare that they have no competing interests.

Authors’ contributions

XC and HH proposed the idea and presided over the study. XL, MG, JC, and YT conceived and designed the experiment. XL and JL wrote the paper. All authors read and approved the final manuscript.

Electronic supplementary material


Additional file 1: Supporting information. Figure S1 The normalized and simulated transmittance spectra of the three types of photoanodes adhered to the FTO glass substrates before the sensitization with N719. Figure S2 (a) Photocurrent-voltage curves and (b) photovoltaic properties of the TP based DSSCs with different thickness. Figure S3 (a) Photocurrent-voltage curves under 0.5 Sun and (b) photovoltaic properties of the TP(3 L) based DSSCs coupled with different scattering layers, i.e., LTNA and STNA with the same thickness of 1.8 μm. Figure S4 Electron lifetime of three types of DSSCs in the dark at different applied bias voltages. (DOC 212 KB)

Authors’ original submitted files for images

Below are the links to the authors’ original submitted files for images.

Authors’ original file for figure 1

Authors’ original file for figure 2

Authors’ original file for figure 3

Rights and permissions

Open Access  This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.

The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

To view a copy of this licence, visit

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Liu, X., Guo, M., Cao, J. et al. Large-diameter titanium dioxide nanotube arrays as a scattering layer for high-efficiency dye-sensitized solar cell. Nanoscale Res Lett 9, 362 (2014).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI:


  • Large-diameter titanium dioxide nanotubes
  • Scattering layer
  • Dye-sensitized solar cells
  • Electrochemical reactions