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Conducting interface in oxide homojunction: understanding of superior properties in black TiO2

Black TiO2 nanoparticles with a crystalline-core and amorphous-shell structure exhibit superior optoelectronic properties in comparison with pristine TiO2. The fundamental mechanisms underlying these enhancements, however, remain unclear, largely due to the inherent complexities and limitations of powder materials. Here, we fabricate TiO2 homojunction films consisting of an oxygen-deficient amorphous layer on top of a highly crystalline layer, to simulate the structural/functional configuration of black TiO2 nanoparticles. Metallic conduction is achieved at the crystalline-amorphous homointerface via electronic interface reconstruction, which we show to be the main reason for the superior properties of black TiO2. This work not only achieves an unprecedented understanding of black TiO2, but also provides a new perspective for investigating carrier generation and transport behavior at oxide interfaces, which are of tremendous fundamental and technological interest.

As one of the most attractive functional materials, titanium dioxide (TiO2) has been investigated extensively in advanced energy conversion and storage fields, [1][2][3][4][5] where TiO2 is required to possess outstanding functionalities of light absorption, charge separation and electron transport. The wide band gap (> 3.0 eV) and relatively low conductivity, however, limit its technological applications. Numerous efforts have been made to improve these properties, including ion-doping, semiconductor-compositing, and nanofabrication. [6][7][8] Recently, an appealing approach based on disorder engineering was proposed, resulting in a new form of TiO2 nanoparticles with black color 9 . Such black TiO2 exceeds the limitations of pristine TiO2 by showing much improved performance in important applications, such as photocatalytic hydrogen generation/water decontamination, 9 rechargeable batteries, 10 and supercapacitors. 11 These improved properties have been demonstrated to largely benefit from the enhanced electron transport of black TiO2. [12][13][14] Therefore, understanding the origin of enhanced electron transport would hold the key to the mechanism of the superior properties.
Black TiO2 nanomaterials (e.g. nanoparticles, nanotubes) have been synthesized by various methods, including high-pressure hydrogenation, hydrogen-plasma assisted reaction and aluminum reduction. 9,15,16 Although these materials have demonstrated improved performance, the explanations are largely diverse. 9,[15][16][17][18][19][20] Some studies attribute to the existence of hydrogen, 9,15,19 while others reported that the samples prepared by non-hydrogen methods also exhibit substantially enhanced properties. 16,18,20 Interestingly, the black TiO2 materials prepared by different synthetic methods share a unique core-shell structure with an oxygen-deficient disordered shell and a crystalline core, which is believed to be a key factor. 12,17,18 However, owing to the complicated effects from a number of factors in powder materials, such as size distributions, grain boundaries and hydrogen incorporation, the role of the core-shell structure in the improved properties of black TiO2 remain unclear.
Here, we fabricate the black TiO2 structure into a thin-film form in a controlled manner, which can exclude the aforementioned influencing factors and thus brings a simpler system with excellent reproducibility, to facilitate understanding the origin of the enhanced properties.
Specifically, TiO2 bilayer thin films with a crystalline layer (core) covered by an oxygendeficient amorphous layer (shell) have been designed to simulate the structural/functional configuration of black TiO2 using pulsed laser deposition (PLD), as illustrated in Figure 1a. 3 Strikingly, metallic conduction is achieved at the crystalline-amorphous interface in the bilayer films, which is responsible for the enhanced electron transport of black TiO2. The mechanism of the interfacial metallic conduction has been revealed from systematic experiments, including Hall measurements, terahertz (THz) spectroscopy, scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS), that demonstrate the dominant role of an electronically reconstructed homointerface.
Bilayer TiO2 films were constructed by depositing a crystalline layer at 600 o C, followed by growing an amorphous layer at 100 o C. Single-layer crystalline and amorphous films were also deposited for comparison. Experimental details can be found in the Supporting Information. Figure 1b shows the X-ray diffraction (XRD) patterns of these TiO2 films on LaAlO3 (LAO) substrates. The anatase TiO2 (004) peak appears in both bilayer and single-layer crystalline films, indicating their oriented growth. By contrast, the single-layer film grown at 100 o C does not show any TiO2 diffraction peak, indicating its amorphous nature. In-situ reflection high-energy electron diffraction (RHEED) reveals the high crystallinity of the crystalline layer ( Figure 1c).   of the TiO2 films. As is seen, the three types of films show dramatically different transport properties. At room temperature (RT), the resistivity of the bilayer TiO2 film is 5.9  10 −3  cm, which is more than five (two) orders of magnitude smaller than that of the crystalline (amorphous) film. More importantly, the resistivity of bilayer TiO2 decreases upon cooling from RT, a characteristic of metallic behavior. In contrast, both the crystalline and amorphous films exhibit insulating behavior, with ρxx(T) increasing exponentially with decreasing temperature.
This implies a metallic conducting interface between the crystalline and amorphous layers, which will be systemically elaborated on later. Note that ρxx(T) of bilayer TiO2 increases when T

< 110 K. Such a behavior cannot be interpreted by the Arrhenius relation [ρxxexp(Ea/kBT),
where Ea is the activation energy and kB is Boltzmann constant], but well conforms to a -log T dependence (cf. the inset of Figure 2a), suggesting that the increase in resistivity at low temperatures is probably caused by electron localization due to disorder potentials. 21

5
In the bilayer film, two possible reasons for its high conductivity could be (i) oxygen vacancies in the amorphous layer, and (ii) electron transport at the crystalline-TiO2/LAO interface. The former possibility is ruled out by comparing its temperature-dependent resistivities with that of the single-layer amorphous TiO2 film (Figure 2a), which has two orders of magnitude higher resistivity at RT and shows a typical insulating behavior. The latter one can easily be eliminated due to the high resistivity (410  cm at RT) and insulating R-T behavior of the single-layer crystalline film. A simple parallel-circuit contribution also is unlikely, for which we calculated  concentration of bilayer TiO2 is much higher than that of both the crystalline (~10 9 cm −2 ) and amorphous (~10 13 cm −2 ) films. Its mobility at RT is ten times higher than that of the amorphous film (~2 cm 2 V −1 s −1 ), but is comparable to that of the crystalline film (22 cm 2 V −1 s −1 ). Generally, Nsheet decreases with increasing temperature, contrary to an insulator (or semiconductor) in which the carrier concentration increases exponentially as temperature rises. 21 This further demonstrates the metallic conduction of the bilayer TiO2 film. Note that the mobility of the bilayer film first increases with decreasing temperature, reaches a maximum value of ~30 cm 2 V −1 s −1 , and decreases thereafter. Such a µH(T) profile is not surprising considering the aforementioned disorder-induced electron localization. Significantly, by constructing such a homointerface, the electron concentration and mobility in the bilayer TiO2 films are comparable to values obtained in TiO2-based conducting oxides via transition-metal doping (Table S2).
These results suggest that high carrier concentration and mobility can be achieved concurrently at oxide homointerfaces.
Optical conductivity, with its zero-frequency extrapolation being the direct-current conductivity, is sensitive to the presence of metallic behavior, which usually gives rise to a Drude response in the low-frequency spectrum. 22 To further confirm metallicity of the bilayer film, we measured the optical conductivity in the THz range. As shown in Figure 3  These intriguing findings drove us to further investigate the origin of the interface-induced metallic behavior in the bilayer TiO2 film. The conducting crystalline-amorphous homointerface is responsible for the enhanced electron conduction of black TiO2 with crystalline-core and amorphous-shell structure. Emergent properties have been generated at interfaces between oxides, 23-27 such as interfacial superconductivity 25 and a high-mobility electron gas at LAO/SrTiO3 (STO) interface. 23 However, interface-induced emergent properties have only been investigated well in heterostructures, and highly depend on the surface termination. 23 For instance, in the LAO/STO system, TiO2-terminated STO induces a conducting interface while SrO termination brings an insulating interface. Our work thus provides a simpler system, a TiO2 homostructure, for investigating carrier generation and transport at oxide interfaces, and demonstrate that oxide homointerfaces can also induce emergent phenomena.
Having demonstrated interface-induced metallic conduction in the bilayer TiO2 film, we trace the underlying mechanisms. In oxides, compositional compensation is not the only option for charge rearrangement: mixed-valence charge compensation occurs if electrons can be redistributed at a lower energy cost than that for redistributing ions. 28 In the bilayer TiO2 system, oxygen vacancies exist in the amorphous layer because of the oxygen-deficient growth condition (10 −6 Torr); meanwhile, Ti has accessible mixed-valence character, allowing for reduction from Ti 4+ towards Ti 3+ . Thus, the amorphous layer is denoted as oxygen-deficient Ti (4−2x)+ O2−x. For the crystalline layer, its high crystallinity ensures low defect content, thus low carrier density but relatively high electron mobility (Figure 2). We have probed the microscopic distribution of and Ti 3+ states, and the O−K edge is sensitive to the presence of oxygen vacancies. 28,29 EELS spectra collected close to (0.75, 1.5, 2.25 and 3.0 nm) and far away (7.5 nm) from the interface are displayed in Figure 4b, which shows that the Ti−L2 peak shifts by 0.5 eV from the crystalline side to amorphous side. The change of the Ti−L energy demonstrates the existence of Ti 3+ in the amorphous layer, at the interface, and in the crystalline side close to the interface (within 3 nm).
In contrast, the O−K EELS spectrum collected in the crystalline side close to the interface (0.75 nm) is almost the same as that collected far away from the interface, suggesting that the interface at the crystalline layer side possesses few oxygen vacancies, as expected for the high-crystalline TiO2.  21 EF in the amorphous layer is closer to CB than it is in the crystalline layer.
When they contact, band bending occurs to balance their Fermi levels, which allows transfer of electrons from the amorphous layer to the crystalline layer, as illustrated in Figure 4c&d. This process renders the amorphous side positive and crystalline side negative, keeping an overall neutral charge. Such a situation is similar to a high-low junction (or n−n + homojunction), where the transferred electrons are confined around the interface. 30, 31 In other words, electrons accumulate at the interface on the crystalline side within ~3 nm (Figure 4b), resulting in a high interfacial carrier concentration. These interfacial charges are compensated by the reduced Ti 3+ that places extra electrons in the TiO2 CB to occupy Ti 3d states, undergoing an electronic interfacial reconstruction. 32 Experimentally, these interfacial electrons also possess a high mobility as in crystalline TiO2, which has been proven by transport measurements (Figure 2 and Table S2). In this case, the two functions of carrier supply and transport are spatially separated in the bilayer film: the amorphous layer provides electrons and the interfacial area of the crystalline layer acts as the electron transport channel. This design enables both high carrier concentration and high mobility, which are concurrently achieved at the homointerface to gain superior collective properties.
In summary, the findings presented here not only provide an unprecedented understanding of black TiO2 and the mechanism of its enhanced electron transport, but also open a new route for developing novel highly conducting oxides and their films by constructing a homointerface with mixed valences. By engineering the interface, high electron concentration and mobility have been concurrently achieved at the homointerface between crystalline and amorphous layers in a bilayer TiO2 thin film. The RT resistivity of the bilayer film is measured to be 5.9 × 10 -3  cm, which is several orders of magnitude lower than that of the single-layer crystalline and amorphous films. The remarkable electronic properties generated at the TiO2 homointerface provide a simpler and cleaner system for investigating the carrier generation and transport behavior at oxide interfaces. The interfacial engineered TiO2 thin films possessing metallic conduction are promising for various applications such as thin-film transistors, low-cost LEDs and high-performance photovoltaic devices. The exploration of unique interfaces in oxides is still in its infancy; more extraordinary phenomena, such as high-mobility 2D electron gases and interfacial superconductivity, could be generated in homointerfaces by selecting suitable systems, optimizing growth conditions, or making a gate electrode in electronic devices.

ASSOCIATED CONTENT
Supporting Information. Experimental details of preparation and characterization for TiO2 films, comparison of properties of various films, X-ray reflectivity pattern of the bilayer film, sheet resistance of TiO2 bilayer films film different thicknesses, THz optical conductivities of the bilayer film at various temperatures, 46 EELS spectra recorded for the bilayer film, electrical field transmitted through the TiO2 bilayer film as a function of gate delay, and detailed discussion of the -T curve fitting. This material is available free of charge via the Internet at http://pubs.acs.org.

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