The Facile Construction of Anatase Titanium Dioxide Single Crystal Sheet-Connected Film with Observable Strong White Photoluminescence

Abstract

Deposited by a reactive atmospheric pressure non-thermal TiCl₄/O₂/Ar plasma, anatase TiO₂ single crystal sheet-connected film exhibits two large exposed {001} facets and a high concentration of oxygen defects. Strong white photoluminescence centered at 542 nm has been observed with naked eyes, whose internal quantum efficiency is 0.62, and whose intensity is comparable to that of commercial fluorescent lamp interior coatings. Based on the simulation results of a hybrid global–analytical model developed on this atmospheric pressure non-equilibrium plasma system, the mechanism of a self-confined growth of single crystal sheets was proposed. A high concentration of oxygen defects is in situ incorporated into the anatase crystal lattice without damaging its crystallographic orientation. This method opens a new way to construct 3D porous metal-oxide single crystal sheet-connected films with two exposing high energy surfaces and a large concentration of oxygen defects.

1. Introduction

Appropriately designed anatase TiO₂ single crystal sheet-connected films with two exposed {001} reactive facets and highly reduced characteristics are highly desirable for scientific research and practical applications [1,2,3]. Desirable characteristics include directly connected boundaries, high carrier mobility, low incident light reflection loss, large specific surface area, and large reactive surfaces. Constructing such architecture films is expected to have interesting photo, electromagnetic, cytokine, chemical, and catalyst effects and applications to photocatalysts, photonics crystals, spintronic devices, anticancer or gene therapy modalities, photo/electrochromic devices, gas sensors, chemical degradation, water splitting, solar cells, etc. [1,2,4,5,6,7]. Recently, anatase micro/nano powders composed of single crystal sheets or flowerlike clusters with a large percentage of exposed {001} faces have been synthesized through a high concentration of hydrofluoric acid (HF) as a crystal shape controller [3]. Anatase TiO₂ architecture films were constructed from its powders through post-binding and annealing processes. This could have problems of nanoparticle coagulation, weak boundary connection, or crystal phase transformation. Post-vacuum annealing, ion implantation, plasma treatment, or electron bombardment are often applied to reduce TiO₂ surface via the introduction of oxygen defects, and to improve its surface reactivity [8,9,10]. Plasma sputtering and plasma-enhanced chemical vapor deposition (PECVD) have been applied to fabricate anatase crystalline films, but with no obvious exposed {001} facets [11]. The growth of anatase single crystal sheet-connected architecture film with two exposed {001} facets and a high surface reactivity is an area that has had little success until now, to the best of our knowledge. Two challenges have to be taken to obtain such structured anatase films in one deposition step. One is that the highly reactive surface diminishes rapidly during crystal growth; this is due to the minimization of surface energy [12]. The other is that the regular crystal lattice growth is usually interrupted by incorporating oxygen defects into the crystal facets and interconnecting omnidirectional crystal sheets into the film architecture. This suggests that a well-designed crystal growth process has to balance the opposite aspects of high surface reactivity, crystal lattice regularity, defect introduction, and crystal sheet interconnection.

Hydrothermal [13] and sol-gel [14] methods are widely employed in the preparation of {001} facets TiO₂ films. The processes are usually complicated and can last for several hours. Fluorine ions (F⁻) are always used as crystal shape controllers in the process [15]. Atmospheric pressure plasma deposition has been well studied in recent decades due to its low-cost equipment, simple steps, and its capability for continuous process [16,17]. Meanwhile, the plasma provides a unique (non-thermal equilibrium) deposition environment when compared to traditional chemical process.

In this paper, we provide a one-step, crystal-shape, controller-free method to deposit single crystal sheet-connected {001} facets TiO₂ film. The TiO₂ film exhibits unusually strong white photoluminescence due to the oxygen defects generated during the plasma deposition.

2. Experimental

The apparatus was a homemade atmospheric pressure plasma reactor, composed of two coaxial quartz glass tubes or parallel quartz plates with a gap of 1.5 mm (Figure 1a). A 13.56 MHz radio-frequency power supply (RF-10S/PWT, Advanced Energy Co., Ltd., Denver, CO, USA) was applied to ignite the plasma in the 5.4 × 2.0 × 0.15 cm³ space between the two electrodes. The power was set at 100 W. Argon (Ar, 99.99%, Shanghai Central Gases Co., Ltd., Shanghai, China) was used as the discharge gas, oxygen (O_2, 99.99%, Shanghai Central Gases Co., Ltd., China) was used as the oxidant, and titanium tetrachloride (TiCl₄, 98.0%, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) was selected as the titanium precursor. A bubbler containing TiCl₄ was immersed in a water bath which was heated and maintained at 40 °C. The connecting tube from the bubbler to the discharge chamber was kept at 70 °C using a heating band in ordwer to prevent the TiCl₄ vapor from condensing. TiCl₄/O₂/Ar (0.17/30/203 sccm) was fed into the reactor through mass flow controllers. The deposition time was 1 h.

The morphology and crystal structure of the deposited films were examined using a field emission scanning electron microscope (FESEM, Hitachi S-4800, Tokyo, Japan) and a transmission electron microscope (TEM, JEM-2100, JEOL Ltd., Tokyo, Japan). The chemical states of the samples were investigated using X-ray photoelectron spectroscopy (XPS, ThermoFisher Scientific ESCALAB 250Xi, Waltham, MA, USA), equipped with Al-Kα X-ray radiation (1486 eV). For XPS analysis, Avantage software (version 5.934, from Thermo Fisher) was employed to deconvolve and analyze the detailed binding energy and the ratios of species. The photoluminescence (PL) of all the samples was measured using a 325 nm He-Cd laser (Renishaw) with a pump power of 30 mW.

3. Results and Discussion

The FE-SEM images of the deposited films in the in-glow zone and in the after-glow zone are shown in Figure 1b,d, respectively. They show that both films are composed of single crystal sheets which connect with each other. According to the symmetries of anatase TiO₂, the two square surfaces and the eight isosceles trapezoidal surfaces in Figure 1c are, respectively, recognized to be {001} and {101} facets. Importantly, almost all the connected crystal sheets stand vertically on the substrate, especially in the after-glow zone, which are exposed high {001} facets, as shown in Figure 1e. Cl⁻ plays an important role in promoting the {001} facets of TiO₂ [18]. The plasma sheath will repel negatively charged ions or particles, causing them to be unable to reach the surface of the substrate and participate in chemical reactions [19]. In the “in-glow” region, the sheath is strong, but in the after-glow region, the sheath is weak. A weak plasma sheath leads to more Cl ions reaching the surface, which promotes the growth of the film 001 facets.

Figure 2a shows the XRD spectra of the films deposited in the in-glow and after-glow conditions. Both films exhibit the typical pattern for anatase TiO₂. Peaks are identified at 2θ values of 25.4°, 38.1°, 48.2°, 53.9°, 55.2°, 62.9°, 68.9°, 70.5°, and 75.5°, corresponding to the crystal planes of (101), (004), (200), (105) (211), (204), (116), (200), and (215), respectively, agreeing with the standard XRD pattern of anatase TiO₂ (JCPDS files No. 21-1272). Figure 2b shows the Raman spectra of the TiO₂ films. These are in agreement with the XRD results. The samples show the five Raman-active modes of E_g, E_g, A_1g, A_1g, and E_g, corresponding to 144, 197, 399, 515, and 639 cm⁻¹ [20].

Figure 3a–d show the single crystal growth process in the 120 min deposition in the after-glow condition. After 1 min (Figure 3a), the TiO₂ seeds are observed on the substrate, and, after 10 min (Figure 3b), the film shows then step-growth of the crystals. The crystals keep growing and then presented both {001} and {101} facets (Figure 3c) after 30 min. Figure 3d shows the crystals after deposition for 2 h. Huge TiO₂ crystals with exposed high {101} facets are presented. High resolution transmission electron microscopy (HR-TEM) images and selected-area electron diffraction (SAED) patterns are given in Figure 3e,f, which clearly show the single crystal characteristics. As shown in Figure 3e and in its inset, the (200) and (020) atomic planes have a lattice spacing of 0.189 nm, and the SAED spot pattern is well-indexed into the zone, ascribing the two flat square surfaces to the {001} facets of the anatase single crystal [21]. The rotational Moiré fringes and the corresponding SAED spot pattern in Figure 3f and in its inset confirm the existence of the dislocation structure of the crystal sheets [22], which corresponds to the dislocation steps in Figure 1. The results indicate that grain sizes and shapes grow with the deposited time, showing the 001 facet TiO₂ crystalline growth processes. The density of these films, alongside the presence of visible voids in the composition, has not yet been studied.

According to the Wulff construction, the lower reactive {101} facet of anatase is more thermodynamically stable (surface energy = 0.44 J m⁻²) than the higher reactive {001} facet (surface energy = 0.90 J m⁻²), and dominates more than 90% of the single-crystalline particles of anatase under equilibrium conditions [15]. In this atmospheric pressure non-equilibrium reactive plasma system, high densities of reactive species, such as Ar⁺, Ar₂⁺, e, TiOCl2, O, Cl⁻, Cl₂, ClO, etc., can be generated in the plasma gas phase [23]. It is demonstrated that a self-confined growth and the connections of the anatase TiO₂ single crystal sheets with the two exposed {001} reactive crystal facets, as well as Cl ions all play a role in the control of surface morphology in TiO₂ crystal growth [18]. Because the binding energy of Cl-Cl (D₀^Cl-Cl = 242.6 kJ mol⁻¹) is much lower than that of Cl-Ti (D₀^Cl-Ti = 494 kJ mol⁻¹), the bonding of Cl to the TiO₂ crystal surface will lower the surface energy of the (001) surface to almost the same extent as that of the (101) surface, and will also form anatase TiO₂ crystals with large {001} facets. Therefore, under the circumstances of a high density of Cl⁻ containing precursor species, the newly formed nanosized anatase TiO₂ nuclei come with exposed {001} facets. Subsequently, these loosely and randomly packed anatase TiO₂ nuclei redevelop into branched or cross-linked crystal sheets with large {001} facets. When the branched or cross-linked crystal sheets grow large enough via a self-assembly extension recrystallization process (a self-confined Ostwald process in the reactive plasma) and combine together, a porous 3D film architecture, composed of micro/nano anatase TiO₂ single crystal sheets with two exposed {001} facets, can be constructed directly on the substrate surface without any catalysts or templates after a certain time of deposition.

A high concentration of oxygen defects is in situ introduced into the anatase TiO₂ single crystal sheets. The O 1s band is obviously different from that of pure unreduced anatase TiO_2, and has a shoulder peak at about 531.5 eV, as shown in Figure 4a,b. The O 1s band can be resolved and assigned to three valence states at 530.1 ± 0.1 eV, 531.5 ± 0.1 eV, and 533.0 ± 0.1 eV. The peak with a binding energy at 531.6 ± 0.1 eV, which is 1.6 eV higher than the common Ti-O bond, is caused by the lower electron density of oxygen ions when compared to that of the O²⁻. This can be attributed to the oxygen ion O⁻ with specific coordination, particularly integrated into the bulk structure near the surface and associated with oxygen defects in the subsurface [24]. The resolved band at 531.5 ± 0.1 eV covers, respectively, 41.0% and 25.2% of the O 1s spectrum for the TiO₂ film deposited in the in-glow zone and in the after-glow zone. The peak of 533.1 ± 0.1 eV is related to the Ti-OH bond [25]. According to the references, O 532 eV is often related to defects or oxygen vacancy [26]. And the oxygen vacancy in TiO₂ has a great influence on photoluminescence, as they will induce a higher number of photoexcited carriers to recombine cause more photoexcited carriers to be recombined [27]. The thermal effect of plasma treatment can accelerate the formation of the above-mentioned defects in TiO₂. And, because of the destruction of O-Ti-O and Ti-O-Ti bonds via the plasma bombard, the residual electrons can migrate from the lattice Ti and O atom, and then create the defective lattice structure of TiO₂ [28]. The thin film in the “in-glow” period will be subjected to more plasma bombarding and heat than the “after-glow” film. The high-resolution XPS spectra of Ti 2p are shown in Figure 4c,d. The peaks with a binding energy at 458 eV and 464 eV are Ti⁴⁺. The thin film deposited in the in-glow period shows Ti³⁺ at the binding energy of 457.2 eV [29]. However, the thin film deposited in the after-glow period does not show any Ti³⁺ peak.

The large percentage of oxygen defects in the film is confirmed by the PL spectrum of the film. As seen from Figure 5, the deposited films exhibit light emissions from 390 nm to 700 nm, centered at 542 nm, which is strong enough to be viewed easily with the naked eye. The strong PL, centered at 542 nm, are attributed to oxygen vacancies (OVs) [30,31]. Furthermore, the PL intensity increases with the percentage of the oxygen defects, and is higher for the film deposited in the in-glow zone than in the after-glow zone. TiO₂ powder (Degussa, P25) and commercial fluorescent coating (get from Opple T8 fluorescent lamp) were also tested at the same condition for use as a contrast.

4. Conclusions

In summary, 3D porous anatase TiO₂ single crystal sheet-connected films with two large exposed {001} reactive facets and a high percentage of oxygen defects has been successfully deposited onto the substrate in one step, without any catalysts or templates. This provides a feasible way for the fast synthesis of 3D porous metal-oxide films that are composed of single crystal sheets connected with each other, and vertically aligned on the substrate. A self-confined crystal sheet growth mechanism has been proposed for the architecture formation, based on the hybrid analytical global model simulation and the experimental results. Unusually, strong white photoluminescence centered at 542 nm has been observed for the first time, and is mainly caused by the high percentage of oxygen defects introduced into the crystal in situ.