Controlled synthesis of nanotubes and nanowires decorated with TiO₂ nanocuboids with exposed highly reactive (111) facets to produce enhanced photoelectrochemical properties

Abstract

The ability to control the exposure of facets of a crystal has been garnering considerable attention due to the fascinating dependence of the physical properties of crystals on their shapes. Herein, a TiO₂WT photoelectrode decorated with anatase TiO₂ nanocuboids (TiO₂C₁₁₁WTs) displaying high-energy exposed (111) facets and quantum dot dimensions was prepared for the first time by using a TiCl₄ treatment process. F⁻ and NH₄⁺ were introduced together to limit the randomness of the TiCl₄ hydrolysates. We propose that both F⁻ and NH₄⁺ were necessary for controlling the exposure of the (111) facets. In comparison with TiO₂WT photoelectrodes decorated with either randomly deposited TiO₂ nanoparticles (non-facet) or with TiO₂ nanofilms or with TiO₂ nanocuboids displaying exposed {001} facets (denoted as TiO₂PWT, TiO₂FWT and TiO₂C₀₀₁WT photoelectrodes, respectively), the hybrid-structured TiO₂C₁₁₁WT photoelectrode achieved superior PEC performances, with J_sc and photoconversion efficiency values of 0.97 mA cm⁻² and 0.49%. This enhancement was attributed to the exposure of the highly active (111) facets and the increased specific surface area of TiO₂C₁₁₁WTs. Meanwhile, the photoactive (111) facets offered oxygen vacancies, resulting in an increase of donor density and the enhancement of PEC performances. These facets also promoted the adsorption of CdS QDs and hence further sensitization. The CdS/TiO₂C₁₁₁WTs achieved J_sc and photoconversion efficiency values of 3.95 mA cm⁻² and 2.05%, which were about twice those of CdS/TiO₂WTs.

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1. Introduction

Anatase titanium dioxide (TiO₂) photoelectrodes constitute one of the most important semiconductors, playing an important role in solar energy conversion applications, and tremendous attention has been paid to improving its photoelectrochemical (PEC) properties.^1–10 One-dimensional nanostructured TiO₂ forms, such as nanowires and nanotubes, have been considered as candidates with enhanced efficiencies due to their direct pathways, which facilitate electron transport.^11,12 Moreover, these morphologies display large surface-to-volume ratios, and such morphologies can thus load more quantum dots (QDs) and provide more effective contact areas with the electrolyte.¹³ Thus, the hybrid structure of nanowire-decorated nanotubes (denoted here as TiO₂WTs) combines the advantages of both the nanotube and nanowire in charge separation, electron transport and light harvesting,^14–16 which broaden its prospects for many solar energy conversion applications.¹⁷

In recent years, considerable efforts have been devoted to improving the PEC properties of TiO₂ photoelectrodes.^14,18,19 TiCl₄ treatment is a simple and effective method,^20,21 which could decorate the structure with an extra layer of TiO₂ nanoparticles. These nanoparticles effectively enhance PEC performance by increasing the contact areas of TiO₂ photoelectrodes with electrolyte and reducing the recombination rate at the TiO₂/electrolyte interface by passivating the surface defects.²² However, those nanoparticles were found to be attached to TiO₂WTs in random orientations (non-facet), which restricts the further improvement of its PEC properties.

PEC properties differ for different TiO₂ facets, which has been attributed to their different bandgaps and surface electronic band structures.^23,24 The use of uniformly exposed high-energy facets of TiO₂ crystals could provide an approach for achieving a superior PEC performance,^25–27 by improving the utilization rate of the surface. Theoretical results have indicated the order of the average surface energies of anatase TiO₂ to be {111} facets (1.61 J m⁻²) > {001} facets (0.90 J m⁻²) > {010} facets (0.53 J m⁻²) > {101} facets (0.44 J m⁻²).^28–31 Moreover, the {111} facet, much of which is formed by undercoordinated Ti (Ti_5c and Ti_3c) atoms and O (O_2c) atoms (as shown in Fig. S1 in ESI†). These undercoordinated atoms affect the electronic properties by forming oxygen vacancies (donor densities) and hence acting as active sites in the photoreaction.^28,32,33 Consequently, the {111} facets of anatase TiO₂ have been considered as the most active facets in solar applications. Nevertheless, the energy values of facets with initially high surface energies diminish rapidly during the crystal growth process to minimize the total crystal energy.³⁴ Hence, it is important to explore an efficient method to accurately control the exposure of the {111} facets of TiO₂ nanoparticles that attach to TiO₂WTs.

In the present work, we successfully controlled the uniform exposed facets of TiO₂ nanoparticles attached to TiO₂WTs, by introducing F⁻/NH₄⁺ ions during the TiCl₄ treatment process. We synthesized TiO₂WTs photoelectrodes with randomly deposited TiO₂ nanoparticles (non-facet), TiO₂ nanofilms, TiO₂ nanocuboids with exposed {001} facets, and TiO₂ nanocuboids with exposed (111) facets, denoted as TiO₂ PWTs, TiO₂ FWTs, TiO₂C₀₀₁WTs and TiO₂C₁₁₁WTs, respectively, as illustrated in Scheme 1. Then we demonstrated that the TiO₂C₁₁₁WT photoelectrode displayed significantly greater PEC performances than did the other TiO₂WT photoelectrodes, owing to its increased surface-to-volume ratio and highly reactive (111) exposed facets, which effectively increased the utilization rate of the photoelectrode surface. Afterwards, we deposited CdS QDs on the photoelectrodes using the successive ionic layer adsorption and reaction (SILAR) method. The CdS/TiO₂C₁₁₁WT photoelectrode presents superior PEC performance, owing to the exposed high-energy (111) facets, which are the predominant sources of active sites and which favor CdS adsorption. This photoelectrode could be an excellent candidate for use in photovoltaics, gas sensors, photo-catalysis, Li-ion batteries and water splitting.

Scheme 1 Schematic illustration of the primary TiCl₄ treatment process from which TiO₂ PWTs were obtained (A). Introducing F⁻/NH₄⁺ ions during the TiCl₄ treatment process resulted in TiO₂ FWTs, TiO₂C₀₀₁WTs and TiO₂C₁₁₁WTs being obtained (B).

2. Experimental

2.1 Synthesis

All chemicals used were of the highest purity available and used as received without further purification. Deionized water was used in all cases. The TiO₂WT photoelectrode was prepared by carrying out an electrochemical anodization of a Ti foil as in our previous work (with details described in ESI†).¹⁴

To prepare the TiO₂C₁₁₁WT photoelectrode, a TiO₂WT photoelectrode was immersed into an aqueous solution containing 25 mM TiCl₄ and 2.6 mM (NH₄)₂TiF₆ at 70 °C for 20 min. Then, the treated photoelectrode was washed with DI water and annealed in the air at 450 °C for 1 h. In order to obtain a fair comparison, before characterization, the original TiO₂WT photoelectrode was annealed for a second time at 450 °C in air for 1 h.

The procedures used to prepare the TiO₂ PWT, TiO₂ FWT, and TiO₂C₀₀₁WT photoelectrodes were similar to the preparation process described above, the only difference was the addition amount of F⁻ and NH₄⁺ ions in solution containing TiCl₄. The details of these procedures are shown in ESI.†

2.1.1 Preparation of CdS QD-sensitized TiO₂ photoelectrodes. CdS QDs were assembled onto photoelectrodes by using the SILAR technique. Specifically, as-prepared TiO₂ photoelectrodes were successively immersed into three different solutions for 10 min each: first in 1 M mercaptoacetic acid in ethanol, then in 0.5 M Cd(NO₃)₂·4H₂O in ethanol, and then in 0.5 M Na₂S in deionized water. Following each immersion, the photoelectrodes were respectively rinsed with pure ethanol and deionized water to remove excess precursors and dried in air before the next dipping. This process was repeated six times. Eventually, the samples were annealed at 300 °C for 1 h.

2.2 Characterization of the materials

Scanning electron microscopy (SEM) images and EDS patterns were acquired by using a field-emission SEM (JEOL JSM-6700F). Transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images were acquired with a JEM-2100F high-resolution transmission microscope operating at 200 kV. X-ray diffraction (XRD) spectra of samples were obtained with a Rigaku D/max-2500 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). Raman spectra were acquired by using a Renishaw spectrometer (Renishaw in Via) equipped with a microscope (50× objective) with a spot size of 5–10 μm. Optical characterization (UV-Vis absorption spectroscopy) of the films was performed using a UV-3150 double-beam spectro-photometer (Shimadzu UV-3150).

2.3 Determination of photoelectrochemical (PEC) properties

The PEC properties of TiO₂ and CdS/TiO₂ photoelectrodes were determined by using a three-electrode electrochemical system made of a quartz cell and linked with an electrochemical workstation (CH Instruments, model CHI601C), and using an SCE reference electrode, a Pt wire as the counter electrode, and a 500 W Xe lamp (Spectra Physics) with a monochromator for simulated sunlight. Before the tests, a laser power meter (BG26M92C, Midwest Group) was used to adjust the light intensity, as effective as AM 1.5 light at 100 mW cm⁻². A mask with a 0.25 cm² window was clipped on the TiO₂ side to define the working area. The electrolyte solution was an aqueous solution composed of 0.25 M Na₂S·9H₂O, 0.35 M Na₂SO₃ and 0.1 M KCl (pH = 12). Transient photocurrent response (I–T) measurements were taken at a potential of 0 V versus SCE. Mott–Schottky plots were collected at a frequency of 5 kHz in a three-electrode electrochemical system made of a quartz cell and linked with an electrochemical workstation (CH Instruments, model CHI660E), and using an Ag/AgCl reference electrode and a Pt wire as the counter electrode.

3. Results and discussion

3.1 Morphological characterization of the TiO₂ photoelectrodes

SEM and TEM images of as-prepared TiO₂PWTs, TiO₂FWTs, TiO₂C₀₀₁WTs, and TiO₂C₁₁₁WTs are shown in Fig. 1. After TiCl₄ treatment, TiO₂WTs were decorated with TiO₂ nanoparticles/film (Fig. 1A, D, G and J), and they retained the original morphology (WT hybrid structure), as shown in Fig. S3.†

Fig. 1 SEM and TEM images of TiO₂ PWTs (A–C), TiO₂ FWTs (D–F), TiO₂C₀₀₁WTs (G–I) and TiO₂C₁₁₁WTs (J–L).

For the TiO₂PWT photoelectrode, nanoparticles attached to the TiO₂ tubes were randomly shaped (Fig. 1B). The HR-TEM result (Fig. 1C) further indicated that the outer-layer crystallites next to the exposed facets of the TiO₂ tube were also random (non-facet). As shown in Fig. 1C, the corresponding lattice fringes were found to be d_{004} = 0.237 nm and d_{103} = 0.244 nm, which could be indexed to the anatase phase of TiO₂ [JCPDS no. 71-1166]. When only introducing NH₄⁺ ions, as shown in Fig. 1E, the nanoparticles ultimately merged into the film (14 nm), and the HR-TEM results (Fig. 1F) showed a d_{204} crystalline spacing of 0.148 nm, corresponding to the anatase phase of TiO₂. When only introducing F⁻ ions, as shown in Fig. 1H, the nanoparticles adopted nanocuboid shapes (with dimensions of 12 nm). Their exposed facets were {001}, as indicated by the HR-TEM results in Fig. 1I, which also showed a lattice spacing of 0.189 nm for both d_{020} and d_{200}, which were observed to be related by an angle of 90°, indicating that the {001} facets constituted the basal plane of the cuboid, perpendicular to the [001] direction. The Fast-Fourier Transform (FFT) pattern (insert of Fig. 1I) provided further evidence that the {001} facets were the exposed facets. When introducing both NH₄⁺ and F⁻ ions, cuboid-shaped nanoparticles with dimensions of 12 nm were observed in their SEM image (Fig. 1K), and the corresponding HR-TEM image (Fig. 1L) showed lattice spacings of 0.352 nm and 0.350 nm related by an angle of 82° and matching the spacings between the {101} and {011} planes of anatase TiO₂ with the [111] zone axis. These observations confirmed the top and bottom faces of the nanocuboid to be the high-energy {111} facets.

In addition, we could see that the nanoparticle, nanofilm and nanocuboid lattices were stacked on the nanotube lattices with a certain angle at their interface, revealing that all of the nanoparticles and nanocuboids as well as the nanofilm grew on the nanotube. The interfaces between nanocuboids and nanotubes resulted in more charge transfer channels, which could enhance the injection of electrons.

We noticed that the inclusion of both F⁻ and NH₄⁺ was necessary to control the exposure of the {111} facets, and similar results were shown in the investigations by Sinha et al.³⁵ They pointed out that the formation of ammonium titanium oxide fluoride (NH₄)₂(TiF₄O) was important for leaving, the TiO₂ crystals with exposed {111} facets after the annealing process. When only NH₄⁺ was introduced into the system, the boundaries between nanoparticles vanished. Although we were not able to observe the exact process by which the nanoparticles aggregated, we would like to propose that NH₄⁺ accelerated the deposition of TiO₂ nanoparticles. When only F⁻ was introduced in the system, F⁻ selectively adsorbed on {001} facets to reduce the surface energy during the TiCl₄ hydrolyzation process, and then left TiO₂ with exposed {001} facets.³⁶

3.2 Performances of the TiO₂ PWT, TiO₂ FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes

Fig. 2A shows XRD patterns for the TiO₂ WT, TiO₂ PWT, TiO₂ FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes. All of the diffraction peaks in these patterns were found to correspond to anatase TiO₂ [JCPDS no. 71-1166] and Ti base, indicating that these TiO₂ WT photoelectrodes were pure anatase TiO₂. The strongest diffraction peak centered at 2-theta of 25° was attributed to the crystal face {101}. These results were also indicated by the corresponding Raman spectra (Fig. 2B). For the original and treated TiO₂WT photoelectrodes, five major Raman bands were observed at frequencies of 144, 196, 395, 516, and 637 cm⁻¹, which were assigned as E_g(1), E_g(2), B_1g(1), B_1g(2) + A_1g and E_g(3) vibration modes of anatase TiO₂, respectively.³⁷

Fig. 2 XRD patterns (A) and Raman spectra (B) for the TiO₂ WT, TiO₂ PWT, TiO₂ FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes.

Fig. 3A shows the UV-Vis absorption spectra of the photoelectrodes. These samples absorbed mainly UV light with wavelengths of about 370–385 nm, corresponding to 3.2–3.3 eV band gaps, and no significant absorbance was observed in the visible region. The TiO₂C₁₁₁WT photoelectrode showed the strongest absorbance, with the order of the absorbance intensities being TiO₂C₁₁₁WTs > TiO₂C₀₀₁WTs > TiO₂FWTs > TiO₂PWTs > TiO₂WTs. The superior absorbance of the TiO₂C₁₁₁WT photoelectrode than other photoelectrodes ​could be attributed to the exposure of highly active facets. Moreover, the {111} facets may have many oxygen vacancies, which would have improved the light utilization, and thus enhanced the absorbance intensity.

Fig. 3 UV-Vis absorption spectra (A), J–V curves (B), photoconversion efficiencies (C) and I–T measurements (D) of TiO₂WT, TiO₂PWT, TiO₂FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes.

The PEC performances of the various photoelectrode samples were evaluated by measuring their photocurrent densities under simulated sun light illumination (100 mW cm⁻², AM 1.5 G). Fig. 3B and C show linear sweep voltammetry (J–V) curves and chemical energy conversion (photoconversion) efficiencies recorded from these photoelectrodes. The TiO₂C₁₁₁WT photoelectrode achieved a short-circuit current density (J_sc) and photoconversion efficiency of 0.97 mA cm⁻² and 0.49%, respectively, which were much higher than those of the other electrodes, i.e., the TiO₂C₀₀₁WT (0.76 mA cm⁻² and 0.46%), TiO₂FWT (0.66 mA cm⁻² and 0.41%), TiO₂PWT (0.56 mA cm⁻² and 0.36%), and TiO₂WT (0.52 mA cm⁻² and 0.31%) photoelectrodes. The results indicated that the energy conversion efficiency of the photoelectrode increased sharply when modified with QD-sized TiO₂ nanocuboids with exposed (111) facets. Fig. 3D shows the transient photocurrent response (I–T) measurement of the aforementioned photoelectrodes. All of the photoelectrodes showed relatively good reproducibility and stability when illumination was turned on and off. The observed dark current densities (light off) for all photoelectrodes were negligible. A remarkable enhancement of the photocurrent response performance of the TiO₂C₁₁₁WT photoelectrode was observed, which could be attributed to the exposure of the highly active (111) facets. These facets increased the contact areas of the photoelectrode with electrolyte. Meanwhile, the photoactive (111) facets may have increased the number of oxygen vacancies, resulting in an increase of donor density. Furthermore, empirical evidence of such an increase was given by Mott–Schottky plots (Fig. S5†). The charge carrier density of the TiO₂C₁₁₁WTs was calculated to be 1.042 × 10²¹ cm⁻³, larger than the 0.431 × 10²¹ cm⁻³ value for TiO₂WTs.

3.3 PEC performances of CdS-sensitized TiO₂ WT, TiO₂ PWT, TiO₂ FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes

In order to broaden the photoresponses of these samples to the visible light region, we sensitized the TiO₂ photoelectrodes with CdS QDs and evaluated their PEC performances. Fig. 4A and B show the J–V curves and photoconversion efficiencies of TiO₂WT, TiO₂PWT, TiO₂FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes sensitized with CdS QDs. The CdS/TiO₂C₁₁₁WT photoelectrode exhibited J_sc and photoconversion efficiency values (3.95 mA cm⁻² and 2.05%) superior to those of the CdS/TiO₂C₀₀₁WT (2.74 mA cm⁻² and 1.72%), CdS/TiO₂FWT (2.47 mA cm⁻² and 1.68%), CdS/TiO₂PWT (2.08 mA cm⁻² and 1.66%) and CdS/TiO₂WT (1.49 mA cm⁻² and 1.17%) photoelectrodes. The superior photoconversion efficiency of the CdS/TiO₂C₁₁₁WT photoelectrode was attributed to the contribution of the highly active exposed (111) facets and oxygen vacancies of the TiO₂C₁₁₁WTs, which would allow more CdS QDs to be loaded on the TiO₂C₁₁₁WTs surface and hence produce more photoelectrons. This hypothesis was supported by the ICP-AES results shown in Table 1 in ESI:† the amount of CdS per unit area was calculated to be greater for the CdS/TiO₂C₁₁₁WTs than for the others. Moreover, the increased surface area would also contribute to the enhancement of the PEC performance by providing more electrochemical reaction locations at the interface between CdS/TiO₂C₁₁₁WTs and the electrolyte.

Fig. 4 J–V curves (A) and photoconversion efficiencies (B) of CdS-sensitized TiO₂ WT, TiO₂ PWT, TiO₂ FWT, TiO₂C₀₀₁WT, and TiO₂C₁₁₁WT photoelectrodes.

4. Conclusions

In summary, this paper reports an effective way to obtain (111)-facet-exposed quantum-dot-sized TiO₂ nanocuboids. These nanocuboids were then attached to the surfaces of TiO₂WT photoelectrodes. We propose that both F⁻ and NH₄⁺ together played important roles in exposing the (111) facets. This hybrid-structured TiO₂C₁₁₁WT photoelectrode exhibited improved surfactivity and superior PEC performances. The TiO₂C₁₁₁WT photoelectrode achieved J_sc and photoconversion efficiency values of 0.97 mA cm⁻² and 0.49%, much higher than those achieved by the other photoelectrodes tested. This considerably enhanced PEC performance of the TiO₂C₁₁₁WT photoelectrode can be attributed to its larger surface-to-volume ratio — and to this photoelectrode simultaneously having exposed highly reactive (111) facets and oxygen vacancies, which in turn was due to its superior surface atomic structure with a large percentage of undercoordinated atoms for (111) facets. Furthermore, we broadened the photoresponse regions of these samples in the visible light range by sensitizing the TiO₂ photoelectrodes with CdS QDs. The CdS/TiO₂C₁₁₁WT photoelectrode showed PEC performances, specifically J_sc and maximum photoconversion efficiency values of 3.95 mA cm⁻² and 2.05%, respectively, that were better than those of the other hybrid-structured photoelectrodes. This enhancement was attributed to the TiO₂C₁₁₁WT photoelectrode being able to adsorb many CdS QDs owing to the exposed highly active (111) facets. The exposure of these facets was confirmed by EDS, UV-Vis absorption spectra, XRD, Raman and ICP-AES results.

Importantly, this method, which helped control the exposure of facets of nanocuboids in TiO₂WT hybrid-structured photoelectrodes, could be further applied to other TiO₂ photoelectrode shapes such as nanorods and nanofilms. Moreover, the ability to improve the surface utilization rate of the TiO₂ photoelectrode opens up new opportunities in various areas, including PEC water splitting, dye-sensitized solar cells and photocatalysis. By replacing the original TiO₂ with hybrid-structured ones, the efficiencies of their devices are expected to be enhanced.

Acknowledgements

This work was financially supported by the Technology Development Program of Jilin Province (Grant no. 20130206078GX).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra20338h

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