Enhanced Photoelectrochemical Properties of Ti³⁺ Self-Doped Branched TiO₂ Nanorod Arrays with Visible Light Absorption

Abstract

A novel Ti³⁺ self-doped branched rutile TiO₂ nanorod arrays (NRAs) was successfully grown on an F-doped tin oxide (FTO) transparent conductive glass by a combined hydrothermal and magnetron sputtering method. Surface morphology, structure, optical properties, and photoelectrochemical behavior of the branched TiO₂ NRAs are determined. Using TiO₂ nanoparticles (NPs) deposited on the top of the nanorods as seeds, TiO₂ nanobranches can easily grow on the top of the nanorods. Moreover, the Ti³⁺ defects in the TiO₂ NPs and associated oxygen vacancies, and the nanobranches expend the optical absorption edge of the TiO₂ NRAs from 400 nm to 510 nm. Branched TiO₂ NRAs exhibit excellent photoelectrochemical properties compared to the pure TiO₂ NRAs, as revealed by photoelectrochemical measurements. This enhanced photoelectrochemical properties is induced by the increased surface area and expanded optical absorption range. Due to their favorable characteristics, these novel branched TiO₂ NRAs will provide a new path to the fabrication of hierarchical nanostructured materials.

1. Introduction

In the past few years, titanium oxide (TiO₂) nanoarrays (i.e., nanotube, nanorod, and nanowire (NW) arrays) have attracted considerable attention as photoelectrodes in various photoelectrochemical (PEC) applications [1,2,3]. Compared with conventional TiO₂ nanoparticle (NP)-film photoelectrodes, TiO₂ nanoarray photoelectrodes have direct and ordered carrier transport channels, which can decouple a minor amount of charge diffusion paths into different directions to improve charge collection efficiency [4,5]. Moreover, with vertically aligned one-dimensional (1D) structures, light scattering and absorption can be improved greatly [6].

Among these 1D TiO₂ nanoarrays, TiO₂ nanorod arrays (NRAs) have been recognized as one of the most anticipated TiO₂ nanoarrays due to unique physical and chemical properties and excellent stability [7,8]. Nevertheless, TiO₂ NRAs are limited to a small specific surface area and a wide band gap (3.2 eV). Many efforts have been made to overcome these limitations of TiO₂ NRAs. The growth of branched TiO₂ NRAs has been proven to be an effective way to increase the specific surface area [9,10,11,12]. Wang et al. [9] prepared branched rutile TiO₂ NRAs via a two-step wet chemical synthesis process, and Cho and co-workers [10] also prepared a kind of branched TiO₂ NRAs by a two-step hydrothermal process by using a TiCl₃ aqueous solution as a precursor for the growth of branches. Similarly, flower-like branched TiO₂ NRAs have been prepared by Liu and co-workers with a modified two-step hydrothermal method [12]. It is found that the anatase/rutile junctions on the surface of TiO₂ nanorod are favorable to the photoelectric properties of NRAs. Nevertheless, these TiO₂ branches are still relatively short. Besides, many attempts such as element doping [13] and sensitization with dyes or narrow band-gap semiconductors [14,15] have been made to extend optical absorption ranges of TiO₂ NRAs. Unfortunately, the stability of these dyes and semiconductors is not satisfactory. Thus, at the present, it is still attractive to develop novel branched TiO₂ NRAs that exhibit a larger surface area and a wider absorption range at the same time. To the best of our knowledge, such attempts have been rarely reported.

In this paper, following our previous work on synthesis of branched TiO₂ NRAs and TiO₂ NP/NRA composites [16,17], a combined magnetron sputtering and hydrothermal method has been developed to grow Ti³⁺ self-doped branched TiO₂ NRAs. With the larger surface area and improved optical absorption, the PEC properties of branched TiO₂ NRAs are significantly improved compared with those of pure TiO₂ NRAs.

2. Materials and Methods

First, TiO₂ NRAs were prepared on TiO₂-seeded FTO transparent conductive glass using the typical hydrothermal method [17]. Half a milliliter titanium butoxide was added to a 24-mL de-ionized (DI) water and hydrochloric acid (mass fraction: 36.5–38%)-mixed solution (a volume ratio of DI water and hydrochloric acid is 1:1). The mixture was stirred for 10 minutes and transferred to a 50-mL Teflon lined stainless steel autoclave. A TiO₂-seeded FTO transparent conductive glass was put in the Teflon liner and heated to 150 °C for 5 h. After the growth of TiO₂ NRAs, Ti NPs were deposited on the top of the TiO₂ NRAs by direct current (DC) magnetron sputtering in a physical vapor deposition system (PVD75, Kurt J. Lesker Company, Jefferson Hills, PA, USA). A high-purity titanium wafer (99.995%, ZhongNuo Advanced Material Technology CO., LTD, Beijing, China) was used as a sputtering target. The base vacuum of the sputtering chamber was 1.0 × 10⁻⁶ Torr and the deposition pressure was carried out at 8 m Torr by using Ar gas (99.999%) as the working gas. The source-to-sample distance and the sample rotation speed were 150 mm and 6 rev·min⁻¹, respectively. The sputtering power was 100 W and maintained for 60 min. The substrate temperature was kept at room temperature. The prepared products were annealed in air at 450 °C for 1 h to form TiO₂ NP/NRA composites.

For the formation of branched TiO₂ NRAs, the prepared TiO₂ NP/NRA composites were subjected to a second hydrothermal treatment. DI water (12.5 mL), HCl (12.5 mL), and titanium butoxide (0.15 mL) were used as the precursor. The mixture was added into the autoclave, to which the TiO₂ NP/NRAs composite was placed in. The autoclave temperature was increased to 160 °C for 3 h. After the synthesis, the branched TiO₂ NRAs were rinsed with DI water and ethanol. The final annealing was performed at 450 °C for 30 min.

The phase structures of as-prepared products were identified by X-ray diffraction (XRD, D8 Advance, Bruker, Madison, WI, USA) with Cu-Kα radiation (λ = 1.54060 Å), and the 2θ scanning speed was 5°/min. The morphologies and microstructure were studied on a field-emission scanning electron microscope (FESEM, Hitachi, S-4800 and acceleration voltage was 10 kV, Tokyo, Japan). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained on an FEI Tecnai G2 F30 microscope operating at 200 KV, Hillsboro, OR, USA. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi (Thermo, Waltham, MA, USA) system with an Al-Kα X-ray source. The spot size was 500 μm and the energy step size was 0.1 eV. Diffuse reflectance and absorption spectra were measured using a UV-Vis spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan) equipped with integrating spheres with a scanning range of 300 nm to 700 nm by scanning at a high scan speed. The sample interval and slit width were 0.5 nm and 20 nm, respectively. PEC measurements and electrochemical impedance spectroscopy (EIS) were determined on an electrochemical workstation (Autolab/PGSTAT302N, Metrohm Autolab, Herisau; Switzerland) with a standard three-electrode electrochemical cell in a 0.5-M Na₂SO₄ solution. TiO₂ NRAs, or branched TiO₂ NRAs were used as working electrodes; a platinum plate electrode (dimension: 15 mm × 15 mm) was used as the counter electrode and Ag/AgCl in saturated KCl as the reference electrode. A Xe lamp with an intensity of 100 W/cm² was used as the illumination source. The active area of the working electrode was 1.5 cm². The frequency range of EIS measurements was from 0.01 Hz to 100 kHz and the ac amplitude was set at 10 mV.

3. Results and Discussion

The morphologies of TiO₂ NRAs, TiO₂ NP/NRA composites, and branched TiO₂ NRAs are shown in Figure 1a–f. Figure 1a displays the surface SEM image of TiO₂ NRAs, exhibiting a unified rod-like structure. Figure 1b presents the side-view SEM image of the same sample, and its inset shows higher-magnification image of the arrays, exhibiting that the nanorods grew nearly vertically on the substrate with a length of about 2.5 μm. When Ti NPs were deposited on the top of TiO₂ NRAs and annealed to form TiO₂ NPs/NRAs composites, the initial square morphology of the nanorods changed to sphere morphology, as shown in Figure 1c,d. The match-like TiO₂ NP/NRA composites were then subjected to the second hydrothermal growth, and tree-like branched TiO₂ NRAs were formed successfully (see Figure 1e,f). It was noticed that the nanobranches densely and uniformly covered TiO₂ nanorods on the top. The length of nanobranches was much longer than those previously reported, which are grown on the side surface of TiO₂ nanorods [10,11,12]. Close observation (the inset in Figure 1f) shows that the branches mainly grew on the top of the nanorods and diverged in all directions to form a spherical shape. Obviously, these nanobranches significantly increased the surface area of the TiO₂ NRAs.

Figure 2a shows the XRD patterns of TiO₂ NRAs, TiO₂ NP/NRA composites, and branched TiO₂ NRAs. The XRD patterns showed that all the crystal structures of these three samples could be classified as the tetragonal rutile phase of TiO₂. The peak intensities of the (101), (110), and (002) planes of branched TiO₂ NRAs were stronger than those of pure NRAs. This indicated that the branches were well crystallized, and the growth mechanism is the same as for the TiO₂ nanorod trunk. The growth rate on the (101) plane of rutile TiO₂ nanorods is faster than that on the (110) plane [7,18], explaining the greatly enhanced intensity of the diffraction peak of the (101) plane with respect to the other diffraction peaks. Figure 2b shows the formation process of the branched TiO₂ NRA structure. The deposited TiO₂ NPs at the top of the nanorods served as crystal seeds for subsequent branching growth at energetically favorable sites on the top of nanorods. As shown in Figure 1f, TiO₂ seeds were grown into dendritic branches, while the nanorod trunks did not grow further.

Figure 3a displays the TEM image of branched TiO₂ NRAs. It can be seen that nanobranches with about 200 nm in length and 40 nm in diameter uniformly covered nanorods on the top. The HRTEM image of a single branch is shown in Figure 3b, which exhibited clear and discernible lattice fringes, indicating good crystallinity of TiO₂ nanobranches. The lattice constant with an interplanar spacing of 0.32 nm in the parallel direction to the length suggested the nanobranches were also crystallized to tetragonal rutile phase and had the same [001] growth direction as nanorods.

XPS was exploited to characterize the chemical valence state and composition of branched TiO₂ NRAs. As shown in Figure 4a, the observed two peaks at 458.6 and 464.4 eV corresponded to Ti 2p_3/2 and Ti 2p_1/2 of the branched TiO₂ NRAs, respectively. Two Ti 2p peaks can be deconvoluted into four peaks, including the peaks of Ti³⁺ 2p_3/2 at 457.7 eV and Ti³⁺ 2p_1/2 at 463.7 eV, which indicates the existence of Ti³⁺ species [19]. These Ti³⁺ species are introduced by TiO₂ NPs at the top of the nanorods [20,21,22,23]. In this case, the Ti NPs were deposited on the top of the nanorods by magnetron sputtering, and then annealed in air to form TiO₂ NPs, which can also form a certain amount of reduced TiO₂ (TiO_2−x) and result in the formation of Ti³⁺ species. On the other hand, in order to maintain the charge equilibrium, oxygen vacancies were formed around Ti³⁺ defects. The O 1s spectrum is shown in Figure 4b. The main peak at 529.9 eV can be assigned to the O lattice of TiO₂ and the binding energy of 531.3 eV can be ascribed to lattice oxygen (Ti–O) and oxygen in surface –OH groups [24]. Ti³⁺ defects and oxygen vacancies existing in the branched TiO₂ NRAs can cause the formation of TiO₂-localized states, thus promoting the separation of photoinduced electrons and holes [25,26].

The light absorption properties of branched TiO₂ NRAs and pure TiO₂ NRAs were studied by diffuse reflection absorption spectroscopy. The rutile TiO₂ NRAs exhibited a single absorption edge at 400 nm, consisting with the rutile TiO₂ band gap of 3.0 eV. Distinct from the TiO₂ NRAs, the spectrum of the branched TiO₂ NRAs (Figure 5a) exhibited a structure of multiple band gaps, and a new absorption edge appeared around the 510 nm, which was a strong indicator for the unique geometric structure. The inset in Figure 5a shows the photographic images of TiO₂ NRAs and branched TiO₂ NRAs. The pure TiO₂ NRAs showed gray white color, while the branched NRAs changed to light yellow. Furthermore, a plot of the modified Kubelka–Munk function [F(R∞)E]^1/2 vs. the energy of absorbed light E was used to calculate values of E_gap1 and E_gap2 to be 3.0 eV and 2.43 eV for these two band gaps, respectively, as shown in Figure 5b. The multiple band gaps presented in the branched NRAs should result from two reasons: (i) defect energy levels introduced by Ti³⁺ species in the reduced TiO₂ NPs at the top of the nanorods, which was confirmed by the XPS results. It has been demonstrated that reduced TiO₂ (TiO_2−x), which contains the Ti³⁺ or oxygen vacancy, exhibit visible light absorption [23,24,25]; (ii) the quantum confinement of the electrons in the TiO₂ nanobranches. Previous studies have demonstrated that, when the diameter of anatase TiO₂ NWs reduces to 40 nm, the multiple band-edge absorptions could occur, which can be induced by quantum confinement [27,28]. In this work, the morphology of TiO₂ nanobranches was similar to that of TiO₂ NWs, and the diameter of nanobranch was about 40 nm. Therefore, the absorption step could be also attributed to the quantum confinement in rutile TiO₂ nanobranches. Figure 5c presents the reflectance spectrum of the TiO₂ NRAs and branched NRAs. Obviously, branched TiO₂ NRAs exhibited lower reflectance as compared to the pure TiO₂ NRAs. The branched nanorod structure with higher surface roughness can increase the incident light scattering path, and result in the reflectivity reduction of branched NRAs [29,30]. However, it was also noticed that the branched TiO₂ NRAs had lower absorption than that of TiO₂ NRAs at the wavelength range from 510 nm to 700 nm even though the surface area increased, which can be ascribed to the light scattering effect from the increased surface roughness of the branched geometric structure [12,31,32].

Figure 6a shows the linear sweep voltammpgrams curves of pure TiO₂ NRAs and branched TiO₂ NRAs under AM1.5G simulated sunlight. These results clearly showed that the photocurrent of branched TiO₂ NRAs was much higher than that of the pure TiO₂ NRAs film under visible-light illumination. The higher photocurrent indicated a higher efficiency in the separation of photon-generated electrons and holes, which resulted in a better PEC activity. Figure 6b shows the photocurrent response of TiO₂ NRAs and branched NRAs under pulsed visible-light irradiation at zero bias. TiO₂ NRAs and branched NRAs both exhibited the quick response to the switching of incident light, indicating a quick transfer of photogenerated electrons from the nanorod to the substrate [33]. This showed that the branched TiO₂ NRAs had the same high electron transport efficiency as pure TiO₂ NRAs. This conclusion was further confirmed by EIS spectroscopy (Figure 6c), as the exhibited large semicircle corresponds to the resistances of the TiO₂/FTO and TiO₂/electrolyte interfaces [34]. The diameter of the large semicircle measured for the cell using branched TiO₂ NRAs as the photoanode was only slightly larger than that for the cell using pure TiO₂ NRAs as the photoanode, suggesting that branched TiO₂ NRAs still exhibit better electron transport properties and lower series resistances [35].

4. Conclusions

In summary, Ti³⁺ self-doped branched TiO₂ NRAs with visible light absorption were successfully prepared by combining a hydrothermal method with magnetron sputtering technology. Using TiO₂ NPs on the nanorods as seeds, the tree-like branched TiO₂ NRAs can be easily formed. The Ti³⁺ defects and oxygen vacancies in TiO₂ NPs and nanobranches expanded the absorption range of the TiO₂ NRAs to visible light region. Based on the larger surface area, the expanded optical absorption range, and the better carrier transport properties, branched TiO₂ NRAs exhibit better PEC activity than pure TiO₂ NRAs, which makes them promising candidates for applications in PEC, photovoltaic, and photocatalytic devices.