Ultrasonic-induced nanocomposites with anatase@amorphous TiO₂ core–shell structure and their photocatalytic activity

Abstract

Disorder engineering on TiO₂ nanocrystals (NCs) generates a nanocomposite with core–shell structure, which has been proven to achieve higher photocatalytic activity than the original TiO₂ NCs. In this contribution, we engineered disorder layers around hydrothermally synthesized anatase NCs using the method of ultrasonic irradiation, obtaining anatase@amorphous TiO₂ core–shell structure nanocomposites. The proportion of two phases (amorphism/anatase) in the nanocomposites was regulated by controlling the average diameter of the anatase NCs and the properties of the nanocomposites would change accordingly. The structure and photocatalytic activity of each sample were carefully compared to find out the appropriate synthesis conditions to obtain the optimal nanocomposite.

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Introduction

In order to break the limitation that TiO₂ NCs could only respond to UV light illumination¹ due to their wide band-gap, Chen et al. developed an approach of hydrogenation to improve the visible and infrared optical absorption by engineering disorder layers around TiO₂ NCs, through which band tails emerged, creating much narrower band-gap and providing trapping sites for photo-generated carriers to prevent them from rapid recombination, thus promoting electron transfer and photocatalytic reactions.² The obtained disordered TiO₂ was actually a kind of nanocomposite with anatase@amorphous TiO₂ core–shell structure, which opened a new view in the modification of TiO₂ NCs. Many following studies concluded that hydrogenation effectively yields electronic structure changes in the nanocomposite, causing extended optical absorption, band-gap narrowing and spatial separation of photo-excited electrons and holes, which induces enhanced photocatalytic activity as well as a black color in appearance.^3–7 However, some researchers didn't find advantages of such kinds of nanocomposites. For example, Leshuk et al. claimed that hydrogenated TiO₂ could significantly degrade the photocatalytic activity, through which they proposed that alternative synthesis and hydrogenation strategies must be explored.⁸ Yan et al. found that slightly hydrogenated TiO₂ NCs with the original white color exhibited enhanced photocatalytic activity, while the gray or black nanocomposites with higher hydrogenation degrees displayed much worse photocatalytic performances, which is probably because highly concentrated bulk defects in the black nanocomposites act as charge annihilation centers, so most of the photo-generated holes are consumed through significantly enhanced non-radiative recombination.⁹

Besides hydrogenation, methods such as localized surface plasmon resonance,¹⁰ Al reduction¹¹ and two step synthesis of Al reduction following by nonmetal incorporation¹² could also achieve the core–shell structure nanocomposites with modified properties. The complicated experimental facts indicates that characters of nanocomposites obtained from disorder engineering on TiO₂ NCs seem to be not simple and straightforward, and the properties of resultant products probably depend on the synthesis conditions and the proportion of two phases, which still needs to clarify.

In our former works, we used heating treatment to regulate the dehydration rate of amorphous TiO₂, which generated nanocomposites with anatase NCs separated out from amorphous phase.¹³ And the introduce of hydroxyls on amorphous TiO₂ could also be achieved by ultrasonic irradiation in water phase, through which the band-gap structure and photocatalytic activity were improved.¹⁴ In this contribution, we synthesized a group of varisized anatase NCs with hydrothermal reaction at different temperatures as the precursors, following by the disorder engineering through ultrasonic irradiation to create anatase@amorphous TiO₂ core–shell structure nanocomposites with various proportions of two phases. Then the changes in structures and properties among each sample of nanocomposite were investigated to demonstrate the appropriate synthesis conditions and phase proportion to achieve the most outstanding properties.

Experimental section

Preparation of samples

24 g of Ti(SO₄)₂ were slowly dissolved into 100 mL of deionized water to obtain a clear solution by magnetic stirring. Then 4 g of NaOH were added into the solution to form homogeneous suspension after magnetic stirring for 30 min. One of the suspensions was directly dried out at 80 °C. And the other three were transferred to Teflon-lined stainless steel autoclaves with an inner volume of 100 mL (filling ratio of 75%) for hydrothermal treatment at 100 °C/160 °C/200 °C for 4 h respectively. The resulting precipitates were collected and thoroughly rinsed by centrifugation (4500 rpm, 8 min) and ultrasonic washing (100 W, 20 min) for two times with deionized water. Then the products were dried out at 80 °C for the subsequent ultrasonic irradiation.

Hydrothermally synthesized precursors were added into deionized water to obtain the suspension of TiO₂ with concentration of 0.5 g/100 mL, which would be sent into an XH-300UL ultrasonic synthesis machine (Xianghu Science and Technology Development Limited Company, Beijing) to prepare the ultrasonic treated samples. The ultrasonic process was conducted with an ultrasonic probe and a thermocouple inserting into the solution. During the process of ultrasonic treatment, the reaction mode was set as constant temperature at 80 °C with an output power density of 1500 W/100 mL, and the duration of ultrasonic irradiation last for 8 h. The suspensions after ultrasonic irradiation were dried out at 80 °C and the solid samples of nanocomposites were collected and grinded into powders.

X-ray diffraction (XRD)

XRD measurement was performed on all the samples using an X'Pert PRO diffractometer operating at 3 kW and a Cu K_α radiation source. The scan range was 10–80° and the step size was 0.02 deg min⁻¹.

High resolution transmission electron microscopy (HRTEM)

The solid samples of nanocomposites were finely ground using an agate mortar and then dispersed in ethanol at an ultrasonic bath respectively. A drop of each suspension was deposited on a holey-carbon film supported on a copper 300 mesh grid. The specimens were taken micrographs by a Hitachi H-9500 HRTEM operating at 300 kV.

X-ray photoelectron spectroscopy (XPS)

All Ti 2p, O 1s and VB XPS spectra were measured by an Escalab 250Xi spectrometer operating at an Al K_α radiation source. The binding energy was corrected for specimen charging by referencing the C 1s peak to 284.6 eV. And the accuracy of the binding energy was 0.02 eV.

Diffuse reflectance UV-vis absorbance

The powders of each sample were pressed in a round glass model and a BaSO₄ disk was used as reference material for background measurement. All samples were measured by a Shimazu UV-4100 spectrophotometer, scanned from 300 nm to 1000 nm and the scanning speed was 300 nm s⁻¹.

Photocatalysis

The photocatalytic activity of each sample was measured by monitoring the change in optical absorption of acid fuchsin (AF) solution during the process of its degradation under illumination of a xenon lamp (the illumination current was 20 A), which was used to simulate the solar irradiation. The original concentration of the AF dyestuff solution was 0.0134 g L⁻¹, and each photocatalytic system contained 150 mL of the AF solution and 0.05 g powders of TiO₂ as photocatalyst. We first kept each system under magnetic stirring in dark for 30 min to reach the equilibrium of physical adsorption, then another magnetic stirring for 80 min under simulated solar-illumination of the xenon lamp. For each photocatalytic system, we took sample of degradative AF solution for every 10 min, using centrifugation to get rid of the photocatalyst powders inside, and measured the UV-vis absorbance of AF solution by a Shimazu UV-4100 spectrophotometer (scanned from 175 nm to 800 nm; scanning speed was 300 nm s⁻¹) to calculate the concentration decrease rate of AF solution.

Results and discussion

We define the obtained sample without hydrothermal treatment as C1, and the other three samples that have gone through hydrothermal treatment at 100 °C/160 °C/200 °C as C2, C3 and C4 respectively. The XRD patterns in Fig. 1 show the identical phase of anatase for all the samples, as well as the obvious variation in their crystallization. The calculated results through Scherrer equation demonstrated the average NC diameters of 6.75 nm, 10.61 nm, 16.67 nm and 19.64 nm from C1 to C4 respectively. Further investigation by HRTEM (Fig. 2) prove the NC size change and clarify the disorder layers around anatase NCs induced by ultrasonic irradiation on each sample (marked by white dash lines), which matched well with the XRD results, indicating samples of C1–C4 were all nanocomposites with anatase@amorphous TiO₂ core–shell structure.

Fig. 1 XRD patterns of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

Fig. 2 (a)–(d) HRTEM images of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions. The white dash lines mark the boundaries of crystal cores and amorphous layers.

XPS was used to reveal the chemical composition of each sample. The Ti 2p XPS spectra (Fig. S1†) display the identical peak locations and shapes for all samples, which shows the similar symmetric Ti 2p_3/2 peaks at 458.6 eV and the Ti 2p_1/2 peaks at 464.2 eV that attribute to Ti⁴⁺ in Ti–O bonds,¹⁵ as well as the absence of the shoulder peaks that associate with Ti³⁺ at 456.8 eV and 462.5 eV respectively.¹⁶ These results indicate that self-doped Ti³⁺ was not responsible for the structure changes in the nanocomposites. As the location of bridge oxygen in Ti–O bonds presents at 530 eV, all the samples' O 1s spectra could actually be divided into two Gauss peaks, including the one locates at 530 eV that represents for bridge oxygen (red peaks in Fig. 3), and another peak between 530.9 eV and 532 eV that assigns to binding oxygen in surface Ti–OH bonds (blue peaks in Fig. 3).¹⁷ According to the calculating results from the peak areas, we found out the decreasing trend of Ti–OH/Ti–O ratio with the average NC diameter growing (Fig. S2†). As is known to all, ultrasonic irradiation would cause the effect of acoustic cavitation, which triggered the process of rapid formation, growth, and collapse of bubbles in water. Once the bubbles collapsed for the perturbing by surface of anatase NCs, energy in sound field dramatically concentrated, generating the hot-spots with extreme environment such as highly elevated local temperature and pressure, which could greatly accelerate the surface renewal of anatase NCs. On one hand, the high-speed water jets that derived from bubble collapsing struck toward the surface of anatase NCs, causing the erosion with bonds breaking. On the other hand, the concentrated energy significantly improved the water activation, creating large amount of –OH and –H radicals, which combined with the surface of anatase NCs, leading to the hydroxylation and appeared as defects of disorder phase.

Fig. 3 O 1s XPS spectra of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

The mechanism of ultrasonic irradiation demonstrates the hydroxylation essential of the disorder layers in the ultrasonic-induced anatase@amorphous TiO₂ core–shell structure nanocomposites (Fig. 4). For the ultrasonic irradiation was conducted with identical conditions, the smaller of the hydrothermally synthesized anatase NCs, the higher degree of hydroxylation in the corresponding ultrasonic treated nanocomposites, as well as the larger portion of disorder layers. The quantitative analysis results of XRD patterns and XPS spectra confirm the above conclusions, which also indicate the value of Ti–OH/Ti–O ratio could be treated as the quantitative standard of the proportion of two phases (amorphism/anatase) in the nanocomposites.

Fig. 4 Schematic constructions of the anatase@amorphous TiO₂ core–shell structure nanocomposites. The empty circles represent Ti–O bonds and the filled dots represent Ti–OH bonds.

Compared to the white color of hydrothermally synthesized anatase NCs, the ultrasonic treated nanocomposites gradually turned to gray in appearance (Fig. 5). Their UV-vis spectral absorbencies (Fig. 6) demonstrate the entire enhancement of optical absorption in the whole visible and near-infrared regions with the growing degree of disorder in the core–shell structure nanocomposites, which explains the corresponding changes in grayness from shallow to deep. Based on the locations of absorption edges in Fig. 6, the value of intrinsic band-gap of each sample was calculated through the equation: αhν = A(hν − E_g)^p, where α is the absorption coefficient, hν is the photon energy, E_g is the optical band-gap, p is assumed to be 0.5 for the direct transition and A is a constant concerning the transition probability. The calculated results display a gradually narrowing in the intrinsic band-gap of the nanocomposites with the growing degree of disorder (marked by black arrows in Fig. 7). Further investigation on the energy band structure depended on valence band (VB) XPS spectra (Fig. S3†) to display the locations of VB of each sample. With the growing degree of disorder in the nanocomposites, the VB locations show a blue-shift from 2.74 eV to 2.37 eV reference to the Fermi level, which causes their gradually narrowed intrinsic band-gap. Moreover, VB XPS spectra also demonstrate the extended band tails in each sample, resulting in the blue-shifted valance band maximum (VBM) towards the Fermi level, which induces further band-gap narrowing (marked by blue arrows in Fig. 7) and the enhanced optical absorption in visible and near-infrared regions. Combined with the results of UV-vis spectral absorbencies and VB XPS spectra, we could construct the schematic illustrations of the density of states (DOS) of each sample (Fig. 7), which describe the number of states per interval of energy at each energy level that are available to be occupied in solid-state and condensed matter physics. It visually presents the larger extended band tail and narrower band-gap for the core–shell structure nanocomposite with larger portion of disorder phase. It had been worked out that the VB of TiO₂ is mainly composed of O 2p states, and the conduction band (CB) is mainly formed by Ti 3d states.¹⁸ The extended VB tail and the blue-shifted VBM are attributed to the overlap of O 2p and Ti 3d orbitals,^5–7 while the CB tail states have been already predicted arising from disorder.^2,3 Those electronic structure changes are the fundamental reasons for the modified energy band structure, which leads to the enhanced optical absorption in Vis-NIR regions and deeper grayness in appearance, resulting in the higher utilization of light energy, and may also in favour of the enhancement of photocatalytic activity.

Fig. 5 Photos of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

Fig. 6 UV-vis spectral absorbencies of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

Fig. 7 The schematic illustrations of DOS of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

To evaluate the efficiency of photocatalysis of each sample, we investigated the changes in optical absorption of acid fuchsin (AF) solution during the process of its catalytic degradation under illumination. The experimental results of solar-driven photocatalytic activity are displayed in Fig. S4–S6,† which demonstrate the quite similar photocatalytic activity among hydrothermally synthesized anatase NCs at different temperature and the non-monotonic change in photocatalytic activity of the core–shell structure nanocomposites with the growing portion of disorder phase. The results in Fig. S6† prove that change in average NC diameter has little effect on the photocatalytic activity. In addition, the photocatalytic activity could be described by the degradation rate of AF solution with Langmuir–Hinshelwood model: ln(c₀/c) = k_at, where c₀ corresponds to the concentration of AF solution after dark reaction, and k_a is the apparent first-order rate constant (Fig. S7†). The fitting values of k_a (Fig. 8) indicate the enhanced solar-driven photocatalytic activity from C4 to C2, and an abrupt drop to C1. Apparently, the fitting results of photocatalytic activity not fully matched with the modified energy band structure and optical absorption as we discussed above. This is probably because the photocatalytic activity not only depends on the utilization of light energy, but also depends on the separation of photo-excited electrons and holes. On one hand, higher degree of disorder in the nanocomposites induces narrower band-gap, expanding the photo-response range to long-wavelength and making the easier electronic transitions from tailed VB to CB, which is beneficial to photocatalytic activity enhancement. On the other hand, solar illumination excites the separation of electrons and holes in VB and surface hydroxyls respectively,¹⁹ the disorder layers induced by ultrasonic hydroxylation act as the surface defects for the hole trapping to forbid the recombination of photo-generated electrons and holes, which could also improve the photocatalytic activity. However, when the disorder phase takes excess portion in the nanocomposites, it would be transformed to concentrated bulk defects that act as photo-excited charge annihilation centers, through which most photo-generated holes are consumed and the corresponding photocatalytic activity is inhibited. In general, the anatase@amorphous TiO₂ core–shell structure nanocomposites have an optimal proportion of two phases to offer the best photocatalytic activity, where the disorder modification contributes to the separation of photo-generated electrons and holes as well as the band-gap narrowing with enhanced light harvesting.

Fig. 8 Photocatalytic activity comparisons of anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions.

Conclusions

In summary, we hydrothermally synthesized varisized anatase NCs at different temperatures, followed by ultrasonic irradiation to induce disorder layers around NCs, forming anatase@amorphous TiO₂ core–shell structure nanocomposites with various phase proportions. The ultrasonic-induced disorder modification in the nanocomposites derives from hydroxylation, which yields the electronic structure changes that will result in more extended band tails and blue-shifted VBM. The growing degree of disorder in nanocomposites induces narrower band-gap, easier electronic transition, as well as stronger and extended optical absorption, which leads to the gradually deeper grayness in appearance. The effects of phase proportion on the changes of photocatalytic activity are not monotonic, but with an optimal value existing. These results provide a kind of nanocomposite with special structure that synthesized through a practical and facile way, which could have beneficial effects on the photocatalysis.

Acknowledgements

The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 51229201, 51272231).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Ti 2p XPS spectra, VB XPS spectra, evaluations of solar-driven photocatalytic activity and kinetic plots of photocatalysis of each sample. See DOI: 10.1039/c6ra14007f

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