Engineering disorder into exotic electronic 2D TiO₂ nanosheets for enhanced photocatalytic performance

Abstract

We successfully prepared exotic electronic 2D TiO₂ nanosheets and then engineered disorder into them to obtain black TiO₂ nanosheets. The structural changes induced by this engineering were carefully investigated. The salient properties of the black TiO₂ nanosheets were made evident by their enhanced photocurrent and H₂ evolution properties.

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As a cheap, efficient, and stable photocatalyst, titanium dioxide (TiO₂) has drawn considerable interest owing to its wide applications in solar cells, solar-driven hydrogen production, photocatalytic degradation of dyes and so forth.^1–3 Because the band gap energy of anatase is 3.2 eV, which limits its photocatalytic activity in the ultraviolet range, great efforts have been dedicated to extending the light absorption of TiO₂ into the visible range of the spectrum and hence allowing more efficient use of solar light.⁴ Recently, the fabrication of “black” TiO₂ reported by Chen et al. has attracted a lot of attention and has been considered a breakthrough approach owing to the enhanced photocatalytic properties of this type of TiO₂ including its high photocatalytic efficiency.⁵ It has been suggested that hydrogenation of TiO₂ impressively boosts its harvesting of solar light by introducing disorder on its surface.^6,7

In contrast to bulk TiO₂ materials, the vast majority (nearly 100%) of ultrathin 2D TiO₂ nanosheet material is located at the surface, resulting in exotic electronic properties and a significant quantum confinement effect in the thinnest dimension.⁸ It is therefore necessary to hydrogenate these nanosheets. Ultrathin TiO₂ nanosheets are in theory relatively prone to being hydrogenated, because Ti atoms are on the exposed surface and have higher electron densities.^9–11 From the point view of applications, the quantum confinement effects and the large specific surface areas of ultrathin nanosheets are expected to improve their photocatalytic properties.¹² Therefore, exploring the hydrogenation of 2D TiO₂ nanosheets to obtain a strong and stable photocatalyst, as well as investigating the synergistic effects of the structure of the nanosheets and their electronic properties, constitute an important and attractive research topic.¹³

Fig. 1 shows a schematic diagram of our fabrication of ultrathin black TiO₂ nanosheets. First, raw TiO₂ nanosheets were prepared by using P123 (polyethylene oxide-polypropylene oxide-polyethylene oxide (PEO20-PPO70-PEO20, Pluronic P123)) as a surfactant together with ethylene glycol (EG) as a co-surfactant in ethanol solvent. The surfactant and co-surfactant formed inverse lamellar micelles, inside which the hydrated inorganic TiO₂ precursor oligomers were confined. Then, a solvothermal treatment was carried out to form crystallized 2D nanosheets.⁹ After being washed and dried, the raw TiO₂ nanosheets were annealed at 400 °C for 1 h to improve their crystallinity. Second, the annealed TiO₂ nanosheets were mixed with NaBH₄ powder by grinding them together thoroughly and heating them up to 220–300 °C for twenty minutes under a N₂ atmosphere. This process produced the hydrogenated TiO₂ nanosheets. As shown in Fig. S1,† the color of the TiO₂ nanosheets changed from light yellow to black as the temperature was increased, which implies an enhancement of light adsorption and a potential high-efficiency utilization of sunlight. Note that, compared with previously reported hydrogenation temperatures,^13,14 the hydrogenation temperatures in this work were much lower, indicative of the relative ease of hydrogenating TiO₂ nanosheets rather than bulk TiO₂ materials.

Fig. 1 Schematic of the synthesis of the ultrathin black TiO₂ nanosheets.

The highly exposed surfaces of the TiO₂ nanosheets are shown in Fig. S2.† The average thickness of the ultrathin TiO₂ nanosheets was determined to be less than 2 nm based on observing the edge configuration. The transformation of the crystal structure during the hydrogenation can be seen in Fig. 2. Fig. 2a and b show images of the white TiO₂ nanosheet; the clear lattice fringes demonstrate its high degree of crystallization. Fig. 2c and d show images of samples hydrogenated at 220 °C. Disordered regions appeared in this image, as delineated by the dotted line. When the hydrogenation temperature was increased to 260 °C, the surfaces of the TiO₂ nanosheets became more disordered, and the disordered outer layer around the crystalline region was about 0.5 nm in thickness as shown in Fig. 2e and f, and even reached 1.0 nm as shown in Fig. S3f.† When the hydrogenation temperature was further raised up to 300 °C, the crystal structures were observed to be totally disrupted and their characterizations are presented in Fig. S3g–i.† Based on these observations, we suggest that the hydrogenation first occurred on the edges of the nanosheets because of the high chemical activity of these edges,¹⁵ and then occurred on the inner surface at higher hydrogenation temperatures. After hydrogenation, the TiO₂ nanosheets maintained their large average specific surface area (black-260 °C: 179.6 m² g⁻¹, Fig. S4†), which was also indicative of the highly exposed surface and of the ultrathin morphology of the nanosheets.

Fig. 2 HRTEM images of the white TiO₂ nanosheet (a and b) and the black TiO₂ nanosheets resulting from hydrogenation at 220 °C (c and d) and at 260 °C (e and f); b, d and f are magnified images of the regions in the red boxes in a, c, and e, respectively.

To confirm this transformation of the crystal structure, the as-prepared TiO₂ nanosheets were characterized by X-ray diffraction (XRD). XRD of the raw TiO₂ nanosheets indicated that they formed a TiO₂-B phase (Fig. S5†).^16a After being annealed, most of the TiO₂ nanosheets adopted the anatase form (Fig. 3). The strong XRD diffraction peaks indicated the highly crystalline nature of the TiO₂ nanosheets. As the hydrogenation temperature was increased to 220 and 260 °C, the XRD peaks of the samples became weaker, yet remained significant (Fig. 3). The deformation and disorder of the crystalline lattice at these temperatures caused the decreased intensities of the XRD peaks.^16b The disorder appearing at these relatively low hydrogenation temperatures also indicated the 2D TiO₂ nanosheets to be much more easily reduced than the bulk materials.

Fig. 3 XRD patterns of the white TiO₂ nanosheets, which were annealed in air, and of the black ones, which were hydrogenated at 220 and 260 °C.

Fourier transform infrared (FTIR) spectra, shown in Fig. 4A, were acquired to study the chemical structures of the samples. All of the samples show strong absorption at about 3400 cm⁻¹, 1630 cm⁻¹, and 500 cm⁻¹, which were assigned to O–H stretching vibrations of surface hydroxyl groups, H–O–H bending vibrations of physically adsorbed water and Ti–O–Ti stretching vibration of the interconnected octahedral [TiO₆], respectively.¹⁷ Raman spectroscopy, a technique for measuring molecular vibrations, was also used to examine the chemical structures of the samples. Before hydrogenation, the ultrathin TiO₂ nanosheets not only exhibited E_g modes (640.0 cm⁻¹), a B_1g mode (398.6 cm⁻¹) and an A_1g mode (518.0 cm⁻¹) (assigned to the anatase), but also yielded a new peak between 450–500 cm⁻¹ (indicated by the blue arrow). This peak could not be ascribed to the other phases (rutile and brookite) (Fig. 4B),¹⁸ and thus confirmed the transformed surface chemical states of the ultrathin TiO₂ nanosheets. After hydrogenation, this new peak shifted to a lower wavenumber. The typical Raman peaks of TiO₂ underwent red shifts of about 8 cm⁻¹ (to E_g = 631.5 cm⁻¹, A_1g = 510.1 cm⁻¹, B_1g = 389.8 cm⁻¹), and became both broader and lower. Besides these modes, an additional active mode ranging from 400 cm⁻¹ to 500 cm⁻¹ was observed for the black TiO₂ nanosheets (as shown in the dotted box), which could not be attributed to any phases of TiO₂. Perhaps hydrogenation of TiO₂ nanosheets broke down the Raman selection rules and further generated a new active mode by lowering the geometric symmetries of TiO₂.^7b,19

Fig. 4 (A) FTIR spectra of the various TiO₂ nanosheet samples. (B) Raman spectra of white and black (260 °C) TiO₂ nanosheets. (C) UV-vis diffuse reflectance absorption (DRS) spectra of the samples. (D) A plot transformed according to the Kubelka–Munk function versus energy of light.

Compared with the white nanosheets, the hydrogenated samples significantly enhanced the absorption of visible light. As seen in Fig. 4C, the absorption of visible light was promoted as the hydrogenated temperature was increased. Moreover, the absorption of ultraviolet light by the black TiO₂ nanosheets was reduced slightly, which can be explained by the structural changes. Considering that TiO₂ is an indirect semiconductor, a plot of (F(R)E)^1/2 versus the energy of absorbed light (Fig. 4D) can be derived according the Kubelka–Munk function from the UV-vis DRS spectrum.²⁰ The band gap energy of white ultrathin TiO₂ nanosheets estimated from the intercept of the tangent to the plot was found to be larger than 3.3 eV (3.2 eV for bulk TiO₂), which showed a blue shift compared with the bulk materials, and demonstrated the quantum confinement effect in the thinnest dimension. After hydrogenation, the band gaps of the nanosheets decreased to about 2.9 and 3.1 eV. The reduction of the band gaps and the dramatic changes of the absorption of visible light were ascribed to the increase of the number of oxygen vacancies and of the Ti³⁺ concentration caused by the hydrogenation.⁵

X-ray photoelectron spectroscopy (XPS) studies were performed to investigate the chemical binding and valence band positions of the samples.¹⁷ Compared to other bulk TiO₂ materials that are not hydrogenated,^21,22 the white ultrathin TiO₂ nanosheets were found to have a shift to lower binding energy (by more than 1 eV, Table S1†) of the core level of Ti 2p, suggestive of its exotic electronic properties. The lower binding energy for surface atoms was previously suggested to be the result of Ti atoms gaining electrons (i.e., undergoing reduction) from surrounding oxygen atoms,⁹ which would be indicative of the different bonding environments and the easier hydrogenation of the TiO₂ nanosheets. This explanation is consistent with the low hydrogenation temperature of the ultrathin TiO₂ nanosheets.

Fig. 5A shows the normalized Ti 2p core level XPS spectra of TiO₂ nanosheets before and after hydrogenation. The characteristic Ti 2p_1/2 and Ti 2p_3/2 peaks of the Ti⁴⁺ center were observed at ∼464.1 and ∼458.3 eV for both samples.^5,23 The peaks of the black sample showed a slight negative shift in the binding energy compared to those of the white sample. By subtracting the normalized Ti 2p spectrum of the white TiO₂ nanosheet from that of the black nanosheet, two extra peaks centered at ca. 462.9 and 457.7 eV were discerned (Fig. 5B).²⁴ These two peaks are in line with the characteristic Ti 2p_1/2 and Ti 2p_3/2 peaks of Ti³⁺,²⁵ which confirmed the presence of Ti³⁺ in the hydrogenated TiO₂ nanosheets. Fig. 5C depicts the O 1s core level XPS spectra of the different samples. After hydrogenation, a broader O 1s peak with a strong shoulder was observed at high binding energy. The shoulder at 531.9 eV arose from the Ti–OH species, implying an increase of number of hydroxyl groups on the surface during hydrogenation.⁵

Fig. 5 (A) Ti 2p XPS spectra of the samples. (B) A difference spectrum obtained by subtracting the normalized Ti 2p spectrum of the white TiO₂ nanosheet from that of the black nanosheet. (C) O 1s XPS spectra of the white and black samples. (D) XPS valence band spectra of the samples. Blue lines highlight the linear extrapolations of the curves. (E) Schematic of band structures based on the experimental data from UV-vis spectroscopy and XPS analysis: (a) white TiO₂ nanosheets and (b) black TiO₂ nanosheets. (F) Photoluminescence of samples using 325 nm excitation.

In addition, the valence bands of the different samples displayed different characteristics, as shown in Fig. 5D. The blue lines show the linear extrapolation of the curves used for deriving the band edge position. The main absorption onset was found to be shifted toward the vacuum level and a small tail appeared at the maximum valence band energy. All of the binding energy changes during hydrogenation can be ascribed to the formation of oxygen vacancies or Ti³⁺ centers.⁵ The energy band diagram was schematized (Fig. 5E) to help us understand the behavior of the black TiO₂ nanosheets. The relatively low energy values of the CB and of the tails of the VB in the black nanosheets, compared to those of the white nanosheets, which occurred because of the increased number of O vacancies or Ti–H bonds in the black nanosheets, contributed to their narrower band gaps.⁵

In this study, photoluminescence (PL) spectra were used to characterize the oxygen vacancies and other defects of the samples.²⁶ The normalized PL spectra of different TiO₂ nanosheets at an excitation of 320 nm are shown in Fig. 5F. In all cases, a broad PL peak corresponding to the typical emission for TiO₂ can be found at about 350–500 nm. After hydrogenation, the overall maxima of the emission peaks were shifted to longer wavelengths, and new peaks at 360 nm appeared and became stronger as the hydrogenation temperature was increased. Self-trapped exciton recombination localized at neighboring Ti³⁺–O⁻ sites is considered to have led to the appearance of the significant new peaks.⁶

To investigate the improved photocatalytic performance, we explored the photoelectrochemical (PEC) behaviors of the samples. The chronoamperometry responses of the black samples under illumination were clearly higher than that of the white nanosheets, as shown in Fig. 6A. And the photocurrent of the sample hydrogenated at 260 °C reached a maximum of 80 μA cm⁻², nearly four times that of the white one. To further study the photocatalytic performance, the different TiO₂ nanosheets were evaluated for their ability to photocatalyze the production of H₂ under a 300 W xenon lamp. The black TiO₂ showed a much higher photocatalytic activity, with a reaction rate is 400 μmol h⁻¹, than did the white one, whose reaction rate was only 263.3 μmol h⁻¹ (Fig. 6B). The photodegradation of MO was chosen to test the photocatalytic activity of the as-prepared samples (AM1.5, 90 mW cm⁻²). Fig. 6C shows UV-vis absorption spectra of MO during degradation. At first, the suspension was stirred in the dark for 30 min to reach the adsorption equilibrium. In the dark state, the samples showed an obvious adsorption of dyes because of the large specific surface area. After irradiation for 90 min, less than 6% of the MO remained in the suspension for the black TiO₂ nanosheets, while for the white sample, more than 20% remained (Fig. 6D). The higher photocurrent responses and H₂ evolution and MO degradation rates of the black TiO₂ nanosheets were all results of the narrow band gaps of these nanosheets and their enhanced adsorption of visible light.

Fig. 6 (A) The chronoamperometry responses of the samples (0.6 V bias). (B) The H₂ evolution of the nanosheets. (C) The UV-vis absorption spectra of MO with black TiO₂ nanosheets (hydrogenated at 260 °C). (D) The photocatalytic degradation of an MO solution in the presence of white and black TiO₂ nanosheets.

In summary, we have demonstrated that ultrathin 2D black TiO₂ nanosheets can be successfully fabricated by engineering disorder on white nanosheets. TEM and XRD revealed the differences between crystal structures of the samples during the hydrogenation process. And Raman spectra indicated that a new activate mode formed as well. The changed chemical structure of the TiO₂ nanosheets from white to black was also confirmed by XPS and PL. All of the characterizations indicated the unique properties of 2D TiO₂ nanosheets. Furthermore, due to the salient properties of the black 2D TiO₂ nanosheet, its photocurrent (80 μA cm⁻²) was found to be four times greater than that of the white one, and it displayed a higher solar-driven hydrogen production rate of 400 μmol h⁻¹. Therefore, the advantageous functionality of the fabricated black TiO₂ nanosheets can be expected to have wide applications from nanostructured photoelectronic devices to energy generation and storage systems.²⁷

Acknowledgements

We are grateful for the National Natural Science Foundation of China (No. 51173170, 21101141), the financial support from the Innovation Talents Award of Henan Province (114200510019) and the Key program of Science and Technology (121PZDGG213) from Zhengzhou Bureau of Science and Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experiments and characterization, TEM, XRD and etc. See DOI: 10.1039/c5ra24126j

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