# Highly Porous and Novel 1D-TiO2 Nanoarchitecture with Light Harvesting Morphology for Photovoltaic Applications

K Pugazhendhi, W Jothi Jeyarani, Tenzin Tenkyong, P Naveen Kumar, B

Praveen, J Merline Shyla

► To cite this version:

K Pugazhendhi, W Jothi Jeyarani, Tenzin Tenkyong, P Naveen Kumar, B Praveen, et al. Highly Porous and Novel 1D-TiO2 Nanoarchitecture with Light Harvesting Morphology for Photovoltaic Applications. Mechanics, Materials Science & Engineering Journal, 2017, 9, 10.2412/mmse.3.32.722. [https://doi.org/10.2412/mmse.3.32.722](https://doi.org/10.2412/mmse.3.32.722).

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# Highly Porous and Novel 1D-TiO₂ Nanoarchitecture with Light Harvesting Morphology for Photovoltaic Applications

K. Pugazhendhi¹, W. Jothi Jeyarani¹, Tenzin Tenkyong¹, P. Naveen Kumar¹, B. Praveen¹,
J. Merline Shyla¹

1 - Department of Physics, Energy NanoTechnology Centre (ENTeC), Loyola Institute of Frontier Energy (LIFE), Loyola College, Chennai, India

<figure><img src="image_2.png" /></figure>

DOI 10.2412/mmse.3.32.722 provided by [Seo4U.link](https://link.springer.com/10.2412/mmse.3.32.722)

**Keywords:** photoanode, dandelion-like TiO₂ structures, nanowire, solvothermal, light harvesting.

**ABSTRACT.** A novel photoanode architecture revamped by dandelion-like TiO₂ (DS) structures as light harvesting ingredients over the vertically oriented TiO₂ nanowire (NW) bunches has been prepared on FTO glass without any seed layer through a single-step solvothermal process. High Resolution SEM observations showed that the synthesized photoanode consists of dandelion-like structures over vertically oriented nanowires with high surface porosity. Both the nanowire and dandelion structures possess rutile TiO₂ crystal phase as confirmed with XRD analysis which also revealed that the nanoarchitecture is single crystalline, an essential condition for faster electron transport at the interfaces. The enhanced light harvesting capabilities and bandgap were examined using UV-DRS. Field dependent dark and photo conductivity measurements were performed to observe the response of the material to the visible spectrum. The proposed photoanode incorporating 1D-TiO₂ electron transporters provides a novel configuration for improved light harvesting application.

**Introduction.** Demand for energy finds first place in the list of humanity's top ten problems to be faced in the next 50 years [1]. The biggest challenge of our modern society is to find and replace the gradually vanishing conventional energy resources by renewable energy resources and at the same time ensuring safety, cleanliness and eco-friendly approach. Among the various sources of renewable energy known so far, solar energy is considered to assume the role of sustainable future energy. Dye Sensitized Solar Cell (DSSC) is a low cost and ecofriendly technology for the conversion of sunlight into electricity [2]. Nanostructured Titanium dioxide (TiO₂) has attracted growing interest in DSSCs on account of its unique structure and properties [3]. The maximum power conversion efficiency in DSSCs reported so far 13% has been achieved by incorporating nanostructured Titanium dioxide (TiO₂) [4]. The performance of these cells depend much on the shape and morphology of the TiO₂ photoanode.

Usually, 3D-TiO₂ nanostructures such as mesoporous nanoparticles, nanoflowers, nanospheres, and hierarchical nanostructures possess high surface area but they exhibit weak light scattering ability and lesser electron transport mobility which hinder the efficiency [5-8]. To improve the electron transport properties the integration of 1D-TiO₂ nanostructures such as nanowires (NWs), nanotubes (NTs) which serve as direct pathways for electrons, provide enhanced transport rate and suppressed recombination rate have also been reported [9-11]. However 1D nanostructures suffer from low internal surface area, resulting in lower adsorption sites for sensitizer molecules and hence low light harvesting efficiency [12]. Recent discoveries showed that growing hierarchical structures on the top surfaces of 1D nanostructures improved light harvesting ability and internal surface area while keeping the direct and faster electron transport properties of 1D nanostructures [13, 14]. However the synthesis of hierarchical structures directly on 1D-TiO₂ nanostructures remains a challenge.

In this paper, we elucidate the synthesis and characterizations of highly porous and novel 1D-TiO₂ Photoanode Nanoarchitecture (1D-TPA) with dandelion like TiO₂ structures on the top surface as a
light scattering layer through single step solvothermal method for the first time without using any seed layer. The morphological and optical properties revealed that the proposed photoanode is a suitable candidate for achieving higher efficiencies in DSSCs.

**Preparation and Characterization.** The nanoarchitecture was prepared on a seedless FTO (Fluorine doped Tin Oxide) substrate through one step solvothermal reaction. A homogenous solution was obtained by mixing and stirring 3mL of Ti containing precursor with a mixture solution containing 30mL conc. HCl, 10mL deionized water and 20mL ethanol. An ultrasonically cleaned FTO substrate with its conducting side facing down was placed inside a 100mL autoclave reactor containing the obtained homogenous solution and then solvothermal reaction was carried out at 150°C for 12 hours. The sample was then annealed at 450°C for 30 minutes to obtain the desired TiO₂ morphology.

The crystallinity, crystal phases and crystallite size were determined using RICH SEIFERT X-ray Diffractometer equipped with CuKα irradiation (λ=0.154 nm). The morphology and elemental composition of the nanoarchitecture were observed using HITACHI S-4800 High Resolution Scanning Electron Microscope and Energy Dispersive X-ray Spectrometer respectively. The optical properties and band gap were determined using PERKIN ELMER (Lambda 35) UV/VIS Spectrometer. The electrical response of the photoanode to the visible spectrum of light was recorded using Kethley Picoammeter 6485-Photoconductivity studies.

**Results and Discussion.** To identify the crystal phases of the synthesized photoanode, XRD measurements were conducted. In the obtained XRD pattern (figure 1) the characteristic peaks at 36.16° and 62.92° are well indexed with standard rutile TiO₂ JCPDS card no.89-4920 while the peaks at 37.78° and 51.51° are due to the presence of SnO₂ in the conducting glass which matched well with standard FTO substrate JCPDS card no.41-1445 [15]. The above interpretation confirmed the presence of rutile TiO₂ in the photoanode.

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD pattern of rutile TiO₂ Nanoarchitecture.</figcaption></figure>

The maximum intensity of peak at the angle 62.92° (2θ) corresponding to the crystal plane (002) indicated that the 1D-TPA is well crystalized and oriented perpendicular to the FTO substrate. Since no other phases are identified, the XRD analysis also revealed that the nanoarchitecture is single
crystalline which is an essential condition for faster electron transport at the interfaces [16]. The average crystallite size is found to be approximately 19 nm using Scherrer's equation [17]. HRSEM observations showed that the synthesized 1D-TPA consists of DS over the vertically oriented NWs with high surface porosity where the length of the NWs array is about 2.5 µm. Figure 2(a) is the cross-sectional view of the photoanode showing the vertical growth of the NWs from the substrate and Figure 2(b) shows the existence of nanowire aggregates in the photoanode. Figure 2(c) is the top view of the photoanode revealing the formation of DS on the top surface of the NW aggregates. Figure 2(d) shows the cross sectional view of an individual dandelion structure. The aggregation of NWs is due to the excess content of ethanol because high ethanol content reduces the absorption of Cl⁻ ions on the (110) plane of the nanocrystals and promotes the increase in diameter of NWs [18]. Figure 3 is an EDAX spectrum of the photoanode which confirmed the presence of Ti and O whose weight and atomic percentages are on par with the standard TiO₂ molecular data tabulated as inset.

<figure><img src="image_4.png" /><figcaption>Fig. 2. (a) Cross sectional view of the photoanode, (b) Existence of nanowire aggregates in the photoanode, (c) Top view of the photoanode, (d) Cross sectional view of an individual dandelion structure.</figcaption></figure>

Figure 4 (a) is a graph between wavelength of the incident light and the absorbance of the 1D-TPA. It is observed that there is an absorption in the UV region whereas no absorption is found in the visible region indicating that the 1D-TPA is highly transparent to the visible spectrum due to its wide bandgap. Another graph as shown in figure 4 (b) is plotted between wavelength of the incident light and the diffused reflectance of the 1D-TPA and it can be clearly observed that the entire visible wavelengths were diffusively reflected by the anode confirming the light scattering ability of the 1D-TPA[19]. The enhanced light scattering ability is due to multiple back reflections of the incident light from the dandelion structures. This occurs when the wavelength of the incident radiation is comparable to the dimension of the dandelion structures. The energy of the incident light is plotted against the Kubelka-Munk equation [20] as shown in figure 4 (c) to find the energy band gap of the photoanode and the value is found to be 2.93 eV which is equal to the theoretically calculated energy bandgap value of 2.95 eV using the energy equation $E = hc/λ$.
<figure><img src="image_5.png" /><figcaption>Fig. 3. EDAX spectrum of the photoanode.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. (a) UV-VIS Absorption Spectrum, (b) UV-VIS Diffused reflectance Spectrum, (c) K-M analysis for finding bandgap.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. Field dependent dark and photocurrents of the photoanode respectively.</figcaption></figure>

Field dependent dark and photocurrent measurements were taken on the basis of an earlier report [21] and the behavior of the 1D-TPAis shown in figure 5. The graph indicates the variation of current in the dark (I_d) and visible light illuminated (I_ph) photoanode with increase in applied electric field. It is observed that the photocurrent is greater than the dark current by about 2 folds for an applied field of 690 V cm⁻¹. This is due to the remarkable liberation of electrons upon excitation by the visible light and efficient mobility of the generated carriers triggered by the applied field. Earlier reports showed that the dark and photocurrent values were obtained in microampere range for TiO₂ nanostructures [22] and the photocurrent value of the proposed photoanode is about 4 folds greater than that of the TiO₂ nanorods based photoanode for an applied field of 400 V cm⁻¹ [23]. The significant hike in the photocurrent values of the 1D-TPA when compared with the earlier reports is because of unidirectional and faster transportation of the electrons through the 1D-TiO₂ nanowire arrays.

**Summary.** Numerous dandelion-like TiO₂ scattering structures atop 1D-TiO₂ nanowire aggregates were successfully prepared on FTO substrate via single step reaction for the first time targeting improved performance of the TiO₂ based photoanodes for DSSCs. The incorporation of porous dandelion-like structures leads to increase in diffused reflectance which enhanced the light harvesting capability. The increase in the photocurrent shows that the proposed photoanode is conductive and photoactive as combining the advantages of nanowires and dandelion-like scattering structures. Therefore, the photoconductivity, surface porosity and light harvesting capacity of the photoanode are significantly improved by the new nanoarchitecture. Thus, the proposed photoanode provides a novel structure for improved light harvesting application incorporating 1D-TiO₂ electron transporters.

**Acknowledgements.** This research was supported in part by the Loyola College-Times of India Major Research Grants (6LCTOI14LIF002) and the authors wish to acknowledge the same.

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Cite the paper

K.Pugazhendhi, W. Jothi Jeyarani, Tenzin Tenkyong, P. Naveen Kumar, B. Praveen, J. Merline Shyla (2017). *Highly Porous and Novel 1D-TiO2 Nanoarchitecture with Light Harvesting Morphology for Photovoltaic Applications*. *Mechanics, Materials Science & Engineering*, Vol 9. doi:10.2412/mmse.3.32.722