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Article

Electrochemical Anodization-Induced {001} Facet Exposure in A-TiO2 for Improved DSSC Efficiency

by
Jolly Mathew
1,
Shyju Thankaraj Salammal
1,2,3,*,
Anandhi Sivaramalingam
4 and
Paulraj Manidurai
3,*
1
Centre for Nanoscience and Nanotechnology, Sathyabama Institute of Science and Technology, Chennai 600119, India
2
Centre of Excellence for Energy Research, Sathyabama Institute of Science and Technology, Chennai 600119, India
3
Department of Physics, Faculty of Physics and Mathematical Sciences, University of Conception, Concepcion 4070386, Chile
4
Department of Physics, Sathyabama Institute of Science and Technology, Chennai 600119, India
*
Authors to whom correspondence should be addressed.
J. Compos. Sci. 2025, 9(9), 462; https://doi.org/10.3390/jcs9090462
Submission received: 19 July 2025 / Revised: 20 August 2025 / Accepted: 27 August 2025 / Published: 1 September 2025
(This article belongs to the Topic Multiscale Characterization, Mechanical Behavior and Computational Simulation of Bulk Materials, Metallic Powders and/or Nanoparticles)
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Abstract

We developed dye-sensitized solar cells based on anatase–titanium dioxide (A-TiO2) nanotubes (TiNTs) and nanocubes (TiNcs) with {001} crystal facets generated using simple and facile electrochemical anodization. We also demonstrated a simple way of developing one-dimensional, two-dimensional, and three-dimensional self-assembled TiO2 nanostructures via electrochemical anodization, using them as an electron-transporting layer in DSSCs. TiNTs maintain tubular arrays for a limited time before becoming nanocrystals with {001} facets. Using FESEM and TEM, we observed that the TiO2 nanobundles were transformed into nanocubes with {001} facets and lower fluorine concentrations. Optimizing the reaction approach resulted in better-ordered, crystalline anatase TiNTs/Ncs being formed on the Ti metal foil. The anatase phase of as-grown TiO2 was confirmed by XRD, with (101) being the predominant intensity and preferred orientation. The nanostructured TiO2 had lattice values of a = 3.77–3.82 and c = 9.42–9.58. The structure and morphology of these as-grown materials were studied to understand the growth process. The photoconversion efficiency and impedance spectra were explored to analyze the performance of the designed DSSCs, employing N719 dye as a sensitizer and the I−/I3− redox pair as electrolytes, sandwiched with a Pt counter-electrode. As a result, we found that self-assembled TiNTs/Ncs presented a more effective photoanode in DSSCs than standard TiO2 (P25). TiNcs (0.5 and 0.25 NH4F) and P25 achieved the highest power conversion efficiencies of 3.47, 3.41, and 3.25%, respectively. TiNcs photoanodes have lower charge recombination capability and longer electron lifetimes, leading to higher voltage, photocurrent, and photovoltaic performance. These findings show that electrochemical anodization is an effective method for preparing TiNTs/Ncs and developing low-cost, highly efficient DSSCs by fine-tuning photoanode structures and components.

1. Introduction

Self-assembled crystal facet-engineered semiconductors are a way to improve the morphology and physicochemical properties of semiconductors, which are becoming more and more important in current research [1]. At the atomic level, particles are self-assembled under thermodynamic rules [2]. Furthermore, self-assembly is a spontaneous process that preserves structures by achieving particle equilibrium. As a result, experts are now very interested in adjusting their morphology and structure. Zhang et al. [3] proposed a bottom-up approach to the synthesis of nanoparticles, showing that different tethers can interact with nano-building blocks to produce a variety of particle patches and well-organized nanostructures. One of the most widely used metal oxide semiconductors, A-TiO2 is employed in many applications, including solar cells [4,5], gas sensors [6,7], electrochromic devices [8], lithium-ion batteries [9,10], photoelectrochemical cells, photo-catalysts [11], and water-splitting technology [12], due to its non-toxic nature, high porosity, low cost, high surface area, and particle size, as well as its superior thermal stability. The most stable surface for A-TiO2 among these facets is the (101) plane, which is frequently seen in particles with different or spherical shapes.
Advanced one-dimensional A-TiO2 nanostructures, including nanowires, nanorods, and nanotubes, are employed in energy storage and conversion devices. O’Regan and Grätzel (1991) [13] detailed the application of TiO2 as a photoanode in DSSCs. Since that time, there has been considerable interest in the design and manufacturing of dye-sensitized solar cells (DSSCs) and the enhancement of power conversion efficiency. In the work of Michael Gratzel [14], energy conversion devices were created based on mesoscopic oxide semiconductor films. Recently, the usage of one-dimensional A-TiO2 as a photoanode in DSSCs has been tested [15]. Additionally, vertically oriented TiO2 photoanode arrays made by anodizing Ti foils are becoming more and more significant for raising the photoconversion efficiency of perovskite solar cells and DSSCs [16,17]. To understand the different roles of A-TiO2, various researchers have reported the performance of DSSCs by introducing different TiO2 nanostructures as a photoanode. Fengxian Xie et al. demonstrated a very simple approach to developing self-assembled compact 4 nm TiO2 nanocrystals that can improve electrical characteristics and device performance [18]. According to research published by Jongmin Choi et al., vertically oriented TiNTs were used as an electron-transporting layer in the solar cell, effectively completing electron extraction.
Researchers have worked hard to design novel sensitizers that boost photovoltaic and photocurrent efficiency [19], improving the light-scattering properties [20], suppressing charge recombination [21], improving the interfacial energetics [22], and varying the particle morphology and size distribution [23,24]. According to Nwanya et al., the high surface area available for dye adsorption in highly porous films may be enormous, leading to the near-extinction of incident light [25]. Filippo De Angelis et al. theoretically studied the dye adsorption of {001} and (101) surfaces. Interestingly, they concluded that the {001} surface anchors the dye molecules, mostly due to the two dye carboxylic groups’ improved structural matching performance with the five coordinated Ti surface-atom arrangement. Chu et al. observed that A-TiO2 single crystals with exposed {001} facets have high surface energy, making them useful for dye adsorption and charge transfer [5]. Gordon et al. used a co-surfactant and multiple titanium precursors (TiF4, TiCl4) to study truncated tetragonal bipyramidal anatase nanocrystals with discrete facets ({001} and {101}) in a nitrogen atmosphere, utilizing Schlenk line techniques [26].
Electrochemical anodization is a preferred approach for producing TiNTs for energy harvesting because of its ability to control pore width and tube length by modifying the duration, voltage, and electrolyte concentration. Nitish Roy et al. [27] revealed that regulated crystal formation influences the appearance, size, and visible facets of a crystal, which normally has a variety of surface physicochemical characteristics. Recently, Vipin Amoli et al. [28] successfully generated anatase TiO2 with extremely reactive {111} facets via the hydrothermal method. Research has shown that TiO2 may be generated in various nanostructures, such as tubes [17,29], rods [30], and mesoporous [31] and hierarchical microspheres [32]. Additionally, the facets of TiO2 NCs were regulated by altering the reaction period (reaction temperature and the molar concentrations of the precursor), solvents, and capping agents.
Yahya Alivov and Z.Y. Fan (2009) discovered that truncated pyramid-like nanoparticles, made by thermally annealing TiO2 nanotubes in a fluorine environment, contain a high percentage of active {001} facets [33]. When the F percentage was reduced from 1% to 0.1% throughout the annealing process, they observed that the nanoparticle sizes ranged from 20 to 500 nm. The equilibrium shape of anatase TiO2, which has eight isosceles trapezoidal {101} facets and two top square {001} facets, is a slightly truncated tetragonal bipyramidal, as established using a Wulff construction with surface energy considerations [34]. The surface energy of A-TiO2 in the (101) plane is at its lowest at 0.43 J m−2, whereas the {001} facet is the highest at 0.90 J m−2. Michele Lazzeri et al and Dudziak, S et al. [35,36] used the Wulff architecture and first-principles calculations to predict that anatase TiO2 crystals had two exposed surfaces, {101} and {001}. The most stable surface is the exposed {101} surface, which makes up nearly 94% of the crystal’s surface. Based on the local density approximation (LDA) of surface energies, three different surfaces are revealed for a rutile crystal structure: {110}, {101}, and {001}. To complete the above results, Yang et al. [24] revealed that concluding with F atoms (capping agent stabilization) resulted in {001} surfaces with lower surface energy and greater stability than {101} surfaces, using hydrochloric acid as a morphological control agent. Researchers have employed fluorine sources such as HF, NH4F, NH4HF2, NaF, TiF4, and TiOF2 to stabilize the structure and morphology of crystals. Under certain conditions, A-TiO2 single crystals exist in octahedral bipyramidal morphology. To our knowledge, only a few papers have focused on the construction of DSSCs employing {001} facets with TiO2.
The present study included anodizing Ti metal foil for varying durations at two distinct electrolyte concentrations to produce the {001} facets. We found that the concentration of fluorine has a major impact on the growth of crystal facets. We investigated the structure and morphology of anodized TiNTs/Ncs and the photoelectric conversion efficiencies of DSSCs that were fabricated using anodized TiNTs/Ncs as a photoanode. Attempts have also been made to prepare phase pure A-TiO2 with an observed mixed-crystal morphology since it takes more time to self-assemble the nanoparticles from nanotubes to {001} facets via anodization. Thus, self-assembled A-TiO2 is very important to develop efficient solar devices, particularly DSSCs. For this study, we created more exposed {001} facets at different fluorine concentrations and studied their device performance.

2. Experimental Procedure

2.1. Synthesis of TiNTs/Ncs

TiNcs {001} facets were created using two electrode systems on Ti foil (Sigma-Aldrich, Saint Louis, MO, USA, 99.7% pure, 0.127 mm thick) with a graphite (20 × 20 mm) counter-electrode. An electrochemical cell that had been internally designed was used to achieve this. Figure 1a illustrates how a spring-loaded spacer was used to keep the electrodes separated by 2.5 cm. A water, acetone, and ethanol ultrasonic bath were used to clean the metal sheet. Freshly prepared solutions with different weight percentages of NH4F (0.25 and 0.5) and a 3 vol.% H2O combination in ethylene glycol (Merck 98%) were used as the electrolyte. The anodization time and ammonium fluoride (NH4F) concentration (Sigma-Aldrich; 98%) were adjusted to produce TiNTs with varying tube lengths and diameters. A direct current (DC) source with a constant potential of 60 V was used for the electrochemical anodization process, which was conducted for 6 h at room temperature throughout different periods (6–24 h). Anodization time (Ta) increased the nanotube growth rate at a constant anodization voltage (Va). TiNTs/Ncs were seen on both sides of the Ti sheet. Extending the growth period to 18 h caused the anodized layer to separate from the titanium foil surface. We also discovered that this separation is dependent on the concentration of NH4F and the area of the Ti foil. The anodized layer was sintered for three hours at 500 °C (1 °C/min) in a tube furnace to enhance its crystalline nature. As seen in Figure 1b, the nanotubes were initially grey, but annealing (Figure 1c) caused them to become white.

2.2. Fabrication of DSSC

FTO substrates were ultrasonically cleaned and then dried in a nitrogen environment. The photoanode was created using as-prepared TiNcs (0.25% and 0.5% NH4F @ 24 h anodized), based on the process described by Alagar Ramar et al. [37]. The procedure involved dissolving 0.06 g of PEG20,000 in 1 mL of deionized water, 0.5 g of TiNcs (as manufactured), 20 μL of Merck acetylacetone, and 20 μL of Triton X-100 in an agate mortar. The mixture was then put into a vial and continuously stirred for up to 24 h. The same technique was followed to create the TiO2 paste as for the conventional TiO2 (Sigma Aldrich). The colloidal paste was sintered at 450 °C after being applied to a compact TiO2 layer via a doctored blade technique. The photoanode (TiO2 electrode) was treated with a 40 mM TiCl4 solution, then sintered at 450 °C. The compact layers made greater contact with the FTO substrate and filled the gaps/voids between the FTO and the porous anatase TiO2. The finished electrode had a total thickness of 10 µm. Based on our previous report [38], the prepared photoanodes were sensitized using an N719 dye (Ruthenizer 535-bisTBA) solution for 24 h. After 24 h of dye adsorption, the anodes were taken out of the solution, rinsed with anhydrous ethanol, and dried in an air-conditioned oven at 80 °C. The counter-electrodes were formed on an FTO glass substrate using a diluted H2PtCl6 solution, drop cast, and annealed at 400 °C. A thin coating of electrolyte, comprising 0.05M I2, 0.5M LiI, and 0.5M tert-butyl pyridine in 5 mL of methoxy propionitrile, was applied to the photoelectrode. A Pt counter-electrode was placed between the photoelectrodes to create an open cell and clipped in place. Polyimide tape (50 µm) was used between electrodes to prevent short-circuiting and to construct a full DSSC; the active area was 0.64 cm2.
The photovoltaic measurements were performed using a Photo Emission Tech device (PET Model: CT80AAA, Camarillo, CA, USA). It irradiates the entire surface of the cells and provides 1 sun AM 1.5G, calibrated with a standard Si photodiode (2 × 2 cm). Both the test and calibration cells were positioned in the same location. A Keithley 2400 digital source meter was used to capture current density–voltage (J–V) graphs. Three different cells were assembled and labeled as a, b, and c, respectively, where ‘a’ represents standard TiO2 (P25). The b and c cells were developed using prepared TiNcs with fluorine concentrations (0.25 and 0.5% at 24 h anodized materials). All the photovoltaic devices (DSSCs) were fabricated and measured under an ambient atmosphere. The electrochemical impedance study of the developed DSSC was carried out utilizing a potentiostat/galvanostat electrochemical workstation (CH Instruments-CHI6044E, Austin, TX, USA).

3. Results and Discussion

Titanium plates are prone to oxidation in electrochemical environments, making the formation of titanium nanotubes (TiNTs) readily achievable using this method. The titanium foil is oxidized throughout the development process by O2− ions. In an electrochemical setup (shown in Figure 1a), metals are exposed to any high enough voltage to cause an oxidation reaction, according to P. Schmuki et al. [15]. The rate at which the related ionic species (Ti4+, O2−) travel will determine whether a new oxide layer forms at the interface between the metal and oxide or between the oxide and electrolyte. Moreover, self-organization occurs during the formation of the porous oxide layer. Furthermore, under certain conditions, substantial self-organized mesoporous layers and the chaotic, fast proliferation of TiO2 nanotube bundles are observable. In our given electrochemical growth conditions, we observed that these TiO2 nanobundles were transformed into nanocubes with {001} facets when increasing the growth time. Both theoretical and experimental studies have indicated that the minority {001} facets in the anatase TiO2 equilibrium state are highly reactive. Producing high-quality anatase single crystals with a sizable fraction of {001} facets is challenging. Anhedral-shaped uneven aggregates of polymorphic TiO2 have been reported in some studies; it remains challenging to control the crystallographic facets. Anatase TiO2 exhibits the most stable {100} facets on oxygenated surfaces and {101} facets on clean and hydrogenated surfaces. Electrochemical anodization was used to create distinct crystal facets. However, the high surface energy of both H-terminated and O-terminated anatase surfaces prevents the development of large anatase single crystals [24].

3.1. Self-Assembly: Morphological Transformation

In a self-assembling process, to create a regular pattern, the particles need to be able to move and interact with one another. The development of crystal assembly may be influenced by several forces, such as interatomic interactions and van der Waals forces. Atoms can self-assemble into perfect crystals under specific circumstances without the help of a human. Furthermore, particle–particle and particle–environment forces, such as particle–fluid contact forces, which reduce the surface energy, have a significant impact on self-assembling particles. Nanoparticles are more challenging to control in terms of size and shape, which makes them less able to self-assemble. Despite multiple experiments, it has proven impossible to make monodispersed nanoparticles with size fluctuations of less than 5%. This is because atomic stacking for nanocrystals cannot be properly regulated [2]. Scientists are now developing nanoparticles of various sizes and shapes using the hydrothermal technique [39,40]. The schematic representation shows the morphological transformation of TiO2 nanostructures shown in Figure 2. However, we suggest that their effectiveness stems from one fundamental mechanism, the “bottom-up” approach. Atomic locations can be adjusted, and interactions between nanoparticles can be either attractive or repulsive. Under certain conditions, nanoparticles can alter their locations to form self-assembled structures. Zhang et al. [3] have reported these interactions, demonstrating that topological constraints can self-assemble into various configurations.
In our study, we can clearly state that a one-dimensional nanotube undergoes dissolution on the surfaces and subsequent recrystallization on the mother particles on the surface. These mother particles, which are in a disoriented/disordered state, are transformed into unique structures such as two-dimensional nano-sheets and three-dimensional nanocubes or truncated octahedra. In this case, the adsorbed fluorine atom is a crucial parameter that affects the surface energy of the TiO2 facets. It will also serve as a morphology-controlling agent to produce crystals with the facets that are required. Numerous investigations have reported that fluorine is responsible for the largest percentage of {001} facets. Furthermore, knowledge of the interactions that exist between atoms and nanoparticles is significant. The particle form and the lack of a controlled environment make it challenging to precisely estimate interparticle forces. Previous researchers employed ab initio density function theory to examine these molecular dynamics [35,41]. Also, Xue-Qing Gong and Annabella Selloni determined that minority surfaces, edges, and possibly corners have an important influence on nanoparticle reactivity. They demonstrated that altering anodization time can tune {001} facets, owing to the participation of fluorine species.

3.2. Structural Properties

To establish the impact of film-growing conditions on the crystalline properties of TiNTs/Ncs, the structural characteristics of the anodized TiO2 layer were investigated using X-ray diffraction (Bruker AXS D4 Endeavor) and are shown in Figure 3A,B. It is implied by the intense diffraction peaks that the anodized materials are highly crystalline. The crystal structure of anatase TiO2 (JCPDS No. 21-1272) was compatible with all the diffraction peaks. By employing X-ray diffraction, it was discovered that growing TiNTs/Ncs had a tetragonal structure. The XRD patterns that were utilized to compute structural properties such as crystallite size, dislocation density, lattice parameters, and lattice strain are displayed in Table 1. The crystallite sizes ranged from 14 to 16 nm. The high-intensity peak at 25.28° corresponds to the (101) plane for A-TiO2. However, in our experiment settings, we observed (101) high-intensity orientation for 12-, 18-, and 24-hour anodized films at 25.31. However, after 6 h of anodization at low NH4F concentrations (0.25), we found a peak at 37.8 that corresponded to the (004) plane of A-TiO2, which is more intense than the (101) orientation. It suggests that A-TiO2 may preferentially align along certain crystallographic orientations, according to the growing circumstances. Similar behavior was noted by Acevedo-Pena et al. [42] for TiO2 nanotubes in electrolytes, based on ethylene glycol with different proportions of NH4F and H2O. It was also discovered that the increased roughness of the nanotubes allows for the piling up of multiple crystallites in a direction perpendicular to the substrate. The process was employed to transfer electrons produced by light from the photo absorber to the conductive substrate, as we can observe from the scanning electron microscope image.
Raman scattering is an effective technique for determining the crystal structure of manufactured materials [43,44]. Horiba Jobin Yvon system Raman spectroscopy was used to study the Raman scattering of the materials with a 633 nm laser line at different potentials. Raman spectroscopy revealed optical modes (Figure 4A,B) and vibrational modes of anatase TiO2 at 145, 198, 397, 514, and 634 cm−1. As previously mentioned, these vibrations are linked to anatase TiO2 in one A1g, two B1g, and three Eg modes. The low-frequency O-Ti-O bending vibration associated with the Eg vibrational mode of TiO2 is characterized by a high peak at around 145 cm−1. The vibrational modes of the Ti-O stretching type were Eg (634 cm−1), A1g (514 cm−1), and B1g (514 cm−1). In addition to the XRD measurements, the absence of Raman vibrational modes associated with the rutile phase confirmed the creation of pure A-TiO2. Figure 5 depicts the influence of fluorine on Raman vibrations. Figure 5 shows that high fluorine-concentrated TiO2 nanoparticles exhibit a higher frequency Eg(ν1) vibration, consistent with the findings of Jian Pan et al. [39].

3.3. Surface Morphology

The surface morphology of anodized TiO2 was investigated using FESEM; it exhibited well-ordered honeycomb-shaped TiO2 arrays for A-TiO2 layers anodized for 6 h in both NH4F concentrations (Figure 6). After six hours of anodization, the film had an average tube diameter of roughly 100 nm and a wall thickness of 10–20 nm. Figure 6a–d,i–l) depicts a FESEM picture of anodized TiNTs/Ncs grown for 6–24 h. We concluded from these microscopic photos that the morphology of these particles is changing (self-assembling) from one-dimensional nanotubes into three-dimensional nanocubes as the growth time and concentrations increase. Figure 6a–d displays FESEM images of A-TiO2 anodized at 0.25% NH4F for 6–24 h, revealing TiO2 self-assembly. In Figure 6b, the surface clearly shows that the layers of films were disoriented into two-dimensional nanoparticles. Furthermore, increasing the NH4F concentration (0.5%) resulted in tightly packed TiO2 nanocubes with varied size distributions. The 6- and 12-hour anodized foils were mechanically stable on Ti foil. However, we noticed morphological alterations from nanotubes to nanocubes on the surface, as depicted in the schematic view. The anodized layer had a thickness of around 15–30 µm. Figure 7 shows the surface properties of the prepared photoanode. Importantly, the as-prepared TiO2 films exhibited improved surface shape, homogeneity, and contact with compact TiO2 electrodes. Also, we concluded that the prepared TiO2 had higher surface aggregation than the TiO2 (P25) photoelectrode (Prepared), as shown in Figure 7a–c. Figure 7b–d depicts the cross-sectional image of an as-prepared photoelectrode.
Figure 6f–h,n–p shows transmission emission microscopy (TEM) images of electrochemically anodized TiO2 nanostructures. From this image, we inferred the formation of discrete TiNTs/Ncs at different NH4F concentrations. Nanocube is the term used to describe a nanostructure with two neighboring edges of equal length at a 90° angle. Figure 6f–h,n–p show magnified TEM images, which reveal that the grown nanoparticles are in the form of single nanocubes and nanotubes without any agglomerations, and the images confirm the formation of nanoparticles. The SAED pattern indicates that the nanoparticles are crystalline; the different planes of A-TiO2 depicted in Figure 6e,m are responsible for the diffraction spots, which are represented by circular lines. Therefore, we show that as-prepared TiO2 nanocrystals are an efficient electron transport layer for DSSCs. As shown in Figure 7, the thickness of the generated compact TiO2 layer was examined using a cross-section scanning electron microscope and was found to be between 80 and 100 nm.

3.4. Electrochemical Analysis

Electrochemical impedance spectroscopy (EIS) is a frequency-based technique that has been widely utilized to study the kinetics, charge transport characteristics, and recombination that occur inside DSSCs. The Nyquist plots of the fabricated DSSC TiNcs [0.25 and 0.5% at 24 h of anodization] and TiO2 (P25) layers are shown in Figure 8, concerning Z” as a function of Z’. The Bode graphs can be used to calculate the electron lifetime values for various photoanodes in DSSCs. The Nyquist plot shows two semicircles in the frequency range of 1 Hz to 80 kHz under investigation. Overseeing the intermediate frequency response is the charge-transfer resistance at the photoanode/electrolyte interface (Rct2), while the charge-transfer resistance at the counter-electrode (Rct1) is represented by a semicircle in the high-frequency region [37]. Figure 8, the inset image, shows the built DSSCs’ charge-transfer resistance (Z1) at high frequency. The charge-transfer resistance (Z2) at the interface between the photoanode and electrolyte is represented by the intermediate frequency. Additionally, the Nyquist spectra revealed an additional resistance (R0), which is the distance between the axis origin and the start of the high frequency. Therefore, its physical meaning lies in describing the inherent ohmic resistance within the device component. The performance of the device will be lowered by some carriers rapidly recombining around the FTO contact, as implied by the conventional anatase TiO2/FTO contact. At the FTO/TiO2 interface, the compact layer will function as a potential barrier, preventing electron return travel and improving the energy conversion efficiency of the cell. Fengxian Xie et al. [18] observed that a compact layer improves the interfacial properties and improves the DSSCs’ performance because it reduces recombination. Moreover, it improves the electron collection of the FTO electrode. Therefore, in our work, we used a treated compact layer. From the results in Figure 8, we inferred that using an as-prepared TiNTs/Ncs photoelectrode reduces the charge transfer resistance. However, this increases the charge transfer at the photoanode/electrolyte interface. This may be due to the surface agglomeration of the as-prepared photoanode.

3.5. I-V Characteristics

The photovoltaic investigation was carried out by assembling three identical cells for each TiNTs/Ncs combination with varying fluorine concentrations and TiO2 (P25). The photoanodes were built on TiCl4-treated FTO to improve the DSSC charge transport efficiency (as described in the experimental technique). The photovoltaic characteristics derived from the top cells are displayed in Figure 9. The J–V curves were utilized to evaluate the DSSC short-circuit current (Jsc), open circuit voltage (Voc), fill factor (FF), and efficiency (η); the outcomes are shown in Table 2. The results show that the three cells have similar VOC and efficiency (η) values, but their fill factors differ. Compared to the as-prepared anatase TiNTs/Ncs used in the DSSC, the ultrafine TiO2 nanocrystals (standard) yielded improved film quality, which raised the fill factor. However, there was a minor difference in the fill factor value for the {001} faceted photoanode. The maximum efficiency and current density (3.47%) were found in TiO2 Ncs cells. The following factors may be responsible for this: (i) excellent crystallization, with no extra crystalline phases; (ii) theoretical and experimental research suggesting that exposed {001} facets can improve cell efficiency and that the {001} surface of anatase TiO2 adsorbs more dye molecules, increasing the photoelectrode’s capacity to harvest light.

4. Conclusions

In this study, we have presented a novel and efficient approach for electrochemical anodization using two electrode systems to create self-assembled (1D, 2D, and 3D) TiO2 nanocrystals. As shown in our results, as the anodization time increased, a change from nanotube to nanocube was seen. NH4F concentrations may significantly influence the generation of TiNcs during anodization and annealing. Our findings showed that the surface engineering of Ti metal foils, crystallographic control of the facets, and tuning of the characteristics of TiO2 crystalline materials into truncated octahedrons can all be accomplished easily using electrochemical anodization. Our study shows that well-defined, anatase single crystals of considerable purity with a high proportion of reactive {001} facets are reproducible and have the potential for use in solar cells. The self-assembled TiNTs/Ncs have been successfully applied as an electron transporting layer in DSSCs and show comparable efficiency with standard TiO2 powder. Because of their high activity and improved light scattering, the electrochemically anodized {001} facets increase the photoelectric conversion efficiency of DSSCs.

Author Contributions

Conceptualization, S.T.S.; methodology, A.S.; software, A.S.; validation, J.M. and S.T.S.; formal analysis, J.M. and A.S.; investigation, S.T.S.; data curation, J.M.; writing—review and editing, S.T.S. and P.M.; visualization, P.M.; supervision, P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors express their gratitude to CONICYT, the Government of Chile, for financial support through the FONDECYT Postdoctoral Program No. 3160445, which facilitated the completion of this work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Jcs 09 00462 g001
Figure 1. (a) Image illustrating the schematic of the anodization setup and the corresponding morphological evolution; (b) shows the oxidized Ti layer at different anodization durations, and (c) presents the annealed layer.
Figure 1. (a) Image illustrating the schematic of the anodization setup and the corresponding morphological evolution; (b) shows the oxidized Ti layer at different anodization durations, and (c) presents the annealed layer.
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Figure 2. Schematic illustration of the morphological evolution of TiO2 during anodization.
Figure 2. Schematic illustration of the morphological evolution of TiO2 during anodization.
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Figure 3. The XRD results for TiO2 NTs/NCs formed at different NH4F concentrations: (A) 0.25%; (B) 0.5%.
Figure 3. The XRD results for TiO2 NTs/NCs formed at different NH4F concentrations: (A) 0.25%; (B) 0.5%.
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Figure 4. The Raman spectra of TiO2NTs/NCs synthesized with varying NH4F concentrations: (A) 0.25%; (B) 0.5%.
Figure 4. The Raman spectra of TiO2NTs/NCs synthesized with varying NH4F concentrations: (A) 0.25%; (B) 0.5%.
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Figure 5. The Raman shift results of anodized TiO2 nanoparticles synthesized using 0.25 and 0.5% NH4F for different anodization times.
Figure 5. The Raman shift results of anodized TiO2 nanoparticles synthesized using 0.25 and 0.5% NH4F for different anodization times.
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Figure 6. (a–p) The surface morphology, SAED of anodized TiO2 nanoparticles prepared with 0.25 and 0.5% NH4F at various anodization times.
Figure 6. (a–p) The surface morphology, SAED of anodized TiO2 nanoparticles prepared with 0.25 and 0.5% NH4F at various anodization times.
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Figure 7. The SEM of the as-prepared photoanode (a) TiO2 (P25) (c) TiNTs/Ncs. (0.5%) (b,d) cross-section.
Figure 7. The SEM of the as-prepared photoanode (a) TiO2 (P25) (c) TiNTs/Ncs. (0.5%) (b,d) cross-section.
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Figure 8. The Nyquist plots of fabricated DSSCs: (a) TiO2 (P25); (b,c) anodized TiO2 synthesized at different NH4F concentrations [0.25 and 0.5% @ 24 h of anodization].
Figure 8. The Nyquist plots of fabricated DSSCs: (a) TiO2 (P25); (b,c) anodized TiO2 synthesized at different NH4F concentrations [0.25 and 0.5% @ 24 h of anodization].
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Figure 9. The J–V curves of the TiO2 (NTs/NCs) and anodized TiO2 used in DSSCs.
Figure 9. The J–V curves of the TiO2 (NTs/NCs) and anodized TiO2 used in DSSCs.
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Table 1. Structural parameters of the annealed TiO2 nanotubes.
Table 1. Structural parameters of the annealed TiO2 nanotubes.
Sl.NoNH4F
(%)		Time
(h)		2θ(hkl)Crystalline Size
(10−9 m)		d-Spacing (A°)Dislocation Density
(1015 lines/m2)		Lattice Parameter (A°)Strain
ExpCal
ExpCalExpCalacac
10.25625.3125.3210114.53.5183.5174.73.789.553.789.510.043
21225.425.3210114.63.4884.693.799.560.043
31825.3525.3210114.73.5134.63.779.420.043
42425.3325.3210115.13.5164.363.779.420.042
50.5625.1825.3210113.63.5123.5175.33.759.423.789.510.047
61225.525.3210113.83.5065.23.819.450.045
71825.425.3210114.33.4924.83.829.580.044
82425.3625.3210114.53.5364.63.779.420.043
Table 2. Summary of the overall efficiency of the fabricated devices.
Table 2. Summary of the overall efficiency of the fabricated devices.
DetailsJsc (mA/cm2)VOC/VFill Factor (FF)η (%)
a8.350.7370.5273.251
b9.550.7770.4593.413
c13.110.7370.3593.473
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Mathew, J.; Thankaraj Salammal, S.; Sivaramalingam, A.; Manidurai, P. Electrochemical Anodization-Induced {001} Facet Exposure in A-TiO2 for Improved DSSC Efficiency. J. Compos. Sci. 2025, 9, 462. https://doi.org/10.3390/jcs9090462

AMA Style

Mathew J, Thankaraj Salammal S, Sivaramalingam A, Manidurai P. Electrochemical Anodization-Induced {001} Facet Exposure in A-TiO2 for Improved DSSC Efficiency. Journal of Composites Science. 2025; 9(9):462. https://doi.org/10.3390/jcs9090462

Chicago/Turabian Style

Mathew, Jolly, Shyju Thankaraj Salammal, Anandhi Sivaramalingam, and Paulraj Manidurai. 2025. "Electrochemical Anodization-Induced {001} Facet Exposure in A-TiO2 for Improved DSSC Efficiency" Journal of Composites Science 9, no. 9: 462. https://doi.org/10.3390/jcs9090462

APA Style

Mathew, J., Thankaraj Salammal, S., Sivaramalingam, A., & Manidurai, P. (2025). Electrochemical Anodization-Induced {001} Facet Exposure in A-TiO2 for Improved DSSC Efficiency. Journal of Composites Science, 9(9), 462. https://doi.org/10.3390/jcs9090462

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