Laser-induced fabrication of highly branched Au@TiO₂ nano-dendrites with excellent near-infrared absorption properties

Abstract

In this report, highly branched Au@TiO₂ nano-dendrites with a tailored surface topography have been conveniently fabricated via a novel and effective laser-induced strategy. The TiO₂ nanospheres fabricated by laser ablation of the Ti target in H₂O₂ solution are characterized by abundant oxygen vacancies (OVs) on the (110) surface. Then, the hydroxyl radical (OH⁻) groups will be easily formed on the OVs in terms of water dissociation by UV laser irradiation of the TiO₂ nano-spheres at the liquid interface. With the aid of OH⁻ groups, the formation of Au@TiO₂ nano-dendrites is based on the overgrowth of gold on the TiO₂ nano-spheres through the reduction of HAuCl₄. The absorption spectra of Au@TiO₂ nano-dendrites show that localized surface plasmon resonance (LSPR) peaks can be effectively modulated from the visible region (∼545 nm) to the near-infrared region (NIR ∼ 1090 nm). The as-prepared Au@TiO₂ nano-dendrites with excellent NIR absorption properties have a promising potential for developing important novel sensors and super photo-catalysts.

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Introduction

Hybrid nano-composites constructed by noble metal elements (Au, Ag, Pt, Pd, etc.) supported on TiO₂ semiconductors are emerging as attractive multifunctional materials that integrate the unique optical and electronic properties of a metal with the super photochemical/physical-activity of the TiO₂ nano-structure.^1–6 Among these noble metals, Au nano-crystals are the most promising material owing to their high electro-catalytic activity and tailored localized surface plasmon resonance (LSPR) property, which can separate efficiently photo-excited electron–hole pairs of the TiO₂ semiconductor. In particular, because of the synergistic effects between the TiO₂ and Au metal structures, the Au@TiO₂ nano-composites with tailored surface topography can also show strong LSPR absorption in the near-infrared (NIR) region. The fascinating Au@TiO₂ hybrid structures with enhanced reaction kinetics have attracted tremendous attention, owing to their intriguing applications for super-active catalysts, biomedical sensors, novel solar cells, photo-degradation, etc. For example, Razzari and Claverie group illustrated that an enhanced local electric field can be established at the Au–TiO₂ interface, and the plasmonic absorption can also be extended over the visible range.⁷ Most recently, other researchers demonstrated that the novel nano-composites of TiO₂@TiO₂–Au incorporated copolymer show a higher rate of adsorption of toxic cationic methyl violet (MV)/anionic acid orange dyes.⁸ Increasing evidence has shown that the excellent properties of Au@TiO₂ nano-composites are highly dependent on their compositions and hybrid nano-structures.

Most recently, noble metal bimetallic nano-dendrite/hyper-branch structures with controllable branch lengths, such as Au@Pd,^9–11 Au@Au,¹² Au@Pt,¹³ etc., exhibited good plasmonic properties, high-performance H₂O₂ sensing and excellent catalytic activities, because their LSPR peaks can be modulated from the visible to the NIR region. It is noted that previous reports mainly focused on the synthesis of isolated Au island-like structures on the TiO₂ precursors.^5–8 If the Au nano-branches can be linked on the TiO₂ nano-structures, the novel hybrid nano-composites should possess superior catalytic performances than the Au island-structure, due to the longitudinal plasmonic properties. Up to now, there are very few reports on the controllable synthesis of Au@TiO₂ nano-dendrites. The main reason is related to the fact that the inevitable large mismatch of crystal lattices between Au and TiO₂ composites, which will limit the overgrowth of Au branch on the TiO₂ surface.^2–4

It is well known that the oxygen vacancy (OV) will be easily generated on the TiO₂(110) surface by UV light irradiation, resulting in abundant hydroxyl radical (OH⁻) groups formed and embedded on the surface (110).^14–20 Taking advantage of the enough OH⁻ groups embedded on the TiO₂(110) nano-structures, we deduce that the oriented crystal growth of Au composite on TiO₂ surface will occur by the reduction of HAuCl₄ with surface OH⁻ groups. Herein, for the first time, we demonstrate an innovative synthetic approach for the formation of novel Au@TiO₂ nano-dendrites with controllable surface structures. The adopted strategy is based on laser-induced fabrication of TiO₂(110) nano-spheres in H₂O₂ solution, and then the UV-laser irradiation of TiO₂. The H₂O₂ provides the oxygen sources, which is environmentally friendly and has negligible toxicity. Laser fabrication in liquid is a new green technology for the synthesis of novel metal stable phase of materials, which is characterized by unique high non-equilibrium processing with high temperature and pressure.^21–27 This is a breakthrough in the toxicity problems that arise from standard chemical fabrication. In this way, the TiO₂(110) nano-spheres are helpful for the generation of abundant OH⁻ groups via UV-laser irradiation in liquid. It is therefore possible to tailor the morphology of the Au@TiO₂ branches over a broad range by changing irradiation time. Moreover, the LSPR peaks of the as-prepared Au@TiO₂ nano-dendrites can be modulated from the visible (∼545 nm) to the near-infrared region (NIR ∼ 1090 nm). There is no doubt that the present results in this paper will be a breakthrough in the fabrication of noble metal–metal oxide semiconductor with hyper-branched structures.

Experimental section

In a typical experiment of fabrication TiO₂ nano-spheres, a well-polished Ti metal (99.99%) has been used as a target placed on the bottom of a rotating glass dish (X–Y stage) with speed of ∼1000 rpm filled with 3 mm depth of H₂O₂ (30%) water solution (100 mL). A Q-switched Nd-YAG (yttrium aluminum garnet) laser (Quanta Ray, Spectra Physics) beam operating at a wavelength of 1064 nm with a pulse duration of about 6 ns and 10 Hz repetition was focused onto the Ti target by a quartz lens with 60 mm focal length. The power density of laser beam was about 3.2 GW cm⁻², and the average spot size of the laser beam at the target surface was measured to be 300 μm. The ablation time lasted about 40 minutes. After laser fragmentation of Ti metal, the colloidal suspensions were divided into ten beakers. The series of TiO₂ colloidal suspensions were separately irradiated under different time region (0–120 min) by a 365 nm cw semiconductor laser with power of 10 mW. Then, the Au@TiO₂ nano-dendrites were synthesized by co-reduction of HAuCl₄ with the OH⁻ groups on the surface of TiO₂ nano-particles. Typically, 20–100 μL of aqueous solution containing about 3.5 μmol of HAuCl₄ were added into 10 mL TiO₂ solution. The mixture solutions were sonicated for 1 min. Then the products were separated by centrifugation at 18 000 rpm for 10 min in an ultracentrifuge, and the precipitates were carefully washed in distilled water three times. The sediments were dropped on a copper mesh and dried in an oven at 60 °C for observation by transmission electron microscopy (JEOL-JEM-2100F). Morphological investigations and chemical composition measurements were performed by field emission scanning electron microscope (SEM, Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDS). And the crystallographic investigation of the TiO₂ fabricated by laser ablation was analyzed by X-ray diffraction (XRD) patterns (Rigaku, RINT-2500HF) using Cu Kα radiation (λ = 0.15406 nm). The Fourier transforms infrared spectra (FTIR) of the TiO₂–OH groups dried in vacuum at 60 °C were measured by a UV-vis-NIR spectrometer (Shimadzu, UV-1700). Finally, the UV-visible-NIR absorption spectra of Au@TiO₂ nano-dendrites were measured with UV-vis-IR spectrometer (Shimadzu, UV-1800).

Results and discussion

After pulse laser ablation of pure Ti target in H₂O₂ solution, the low-magnification TEM image of the TiO₂ nano-structures is shown in Fig. 1(a). The morphology of the products clearly shows numerous liquid-dispersed nano-particles with mean diameter of about 110 nm are well-defined spherical-like structures. The nano-spheres with solid interiors and smooth surfaces are not hinge joined, but are dispersed individually. In addition, the crystallographic investigation of the product was carried out by XRD in Fig. 1(b). The XRD (20–60°) pattern reveals that a series of (110), (101), (111) and (211) TiO₂ diffraction peaks centered at 27.4°, 36.1°, 41.2° and 54.3° were detected, which proves to be the rutile phase of TiO₂. The relatively strong peak centered at 36.1° also implies that the TiO₂(110) should be preferentially formed in the nano-spheres. After UV-laser irradiation for 120 min, the SEM image in Fig. 1(c) illustrates that the smooth surfaces of the TiO₂ changed to slightly rough-like structures. Moreover, the inset result shows that the ratio of O to Ti is about 4 : 1 according to the EDS result, which is obviously different from a common TiO₂ material. Because some light elements, such as hydrogen, cannot be detected in EDS pattern, the existence of extra oxygen element originated from OH⁻ groups was evidenced by the FTIR spectra. FTIR spectrum of the same product in Fig. 1(d) shows that this spectrum (1000–4000 cm⁻¹) is dominated by the broad band with peak at 3436 cm⁻¹ of the strongly adsorbing OH stretching vibrations and relatively sharp peaks at 1636 and 1437 cm⁻¹ of OH bending mode. In addition to the TiO₂ lattice vibrations at about 670 cm⁻¹, the several improved FTIR spectra from OH⁻ groups combined with the obvious higher O element in the products confirm that abundant OH⁻ groups can be indeed embedded on the TiO₂ nano-spheres by UV-light irradiation. Increasing the irradiation time should result in more OH⁻ groups formed on the TiO₂ structures, which can be confirmed by measuring the PH value of the TiO₂ water solution. It is well known that the hydroxyl radicals can be weakly ionized in the water solution, resulting in some hydrogen ions formed in the liquid. As shown in Fig. S1,† the PH value of the TiO₂ solution linearly decreases from about 6.76 to 5.38 with an increase of irradiation time (0–120 min), implying more hydrogen ions formed in the solution. In this way, it is therefore to deduce that a longer irradiation time can lead to more OH⁻ groups formed on the TiO₂ structures.

Fig. 1 (a) The typical low-magnification TEM image of the TiO₂ nano-spheres by laser ablation of Ti target in H₂O₂ liquid. (b) XRD pattern of the products (c) the representative SEM image of the TiO₂ nano-spheres after UV-laser irradiation for 120 min. (d) FTIR spectrum of the products.

The evolution and modification of the TiO₂ nano-structures by using UV-laser irradiation are illustrated by the high-resolution transmission electron microscopy (HRTEM) of the products before and after irradiation for 120 min. The HRTEM images in Fig. 2(a and b) clearly reveal that the well crystalline of the TiO₂(110) structure with a periodicity corresponding to a d-spacing of 0.325 nm can be de-crystallized after UV-laser irradiation, resulting in some amorphous or distortion structures. The distortion structures should be attributed to the formation of OH⁻ groups on the TiO₂ nano-spheres. The possible TiO₂–OH structure growth processes have been proposed in a schematic growth diagram, as shown in Fig. 2(c). At the moment of pulse laser arriving at Ti target, rapid boiling and vaporization of Ti element will occur, resulting in the nucleation of explosive Ti plasma with O element in H₂O₂ solution. After laser ablation (∼6 ns), the rapid quenching process enables the disorder degrees of Ti and O species to become more and more intensive, which makes O species at a highly excited state. Under the UV-laser irradiation of the as-prepared TiO₂(110) nano-spheres, oxygen vacancy (OV) sites can thus be expected to occur on the rutile surface owing to the photo-induced hole–electron pairs, which is a ubiquitous feature in nature.^12–17 Compared with commercial TiO₂ nano-particles, the degree of OV sites will be significantly enhanced due to the unique active O species generated in TiO₂ nano-spheres during laser ablation, leading abundant/enough OV sites formed on the surface. The abundant OVs will then act as adsorption sites for surrounding H₂O molecules, and then the hydrophilic nature of the TiO₂ surface increases remarkably.^12–20 Meanwhile, the photo-induced holes enable the water dissociation to dominate the following process. The longer UV-irradiation time can lead to an increase in the number of surface OH⁻ groups. The inset in Fig. 2(c) shows that the UV-laser irradiation for 120 min leads to an obvious change in the colloidal suspension color from light yellow to dark yellow, which is a reasonable evidence for the formation of abundant OH⁻ groups embedded on the TiO₂ nano-spheres. The unique TiO₂ nano-sphere with enough OH⁻ groups embedded on the surface is highly dependent on the enhanced active O species and subsequent UV-laser irradiation. Finally, interesting chemistry can thus be expected to occur at the abundant surface OH⁻ groups on the TiO₂ nano-spheres.

Fig. 2 (a and b) The HRTEM images of the TiO₂ nano-spheres before and after UV-laser irradiation for 120 min, respectively. (c) The schematic growth of OH⁻ groups on the TiO₂ nano-spheres by using UV-laser irradiation.

The abundant/enough OH⁻ groups formed on the surface of TiO₂ nano-spheres play a crucial role in the production of Au@TiO₂ nano-dendrites. To verify it, the as-prepared TiO₂ nano-spheres without any UV-laser irradiation were selected as the precursor for the fabrication of Au@TiO₂ nano-composites through the reduction of HAuCl₄. As shown in Fig. 3(a and b), isolated island-shaped Au nano-structures were formed on the TiO₂ nano-spheres, which is similar to the previous reports.^1–7 The typical region in Fig. 3(c) with a d-spacing of 0.235 nm could be indexed as the Au(111) plane structure. The large mismatch of crystal lattices between Au and TiO₂ is considered to be the main reason for the formation of island-shaped Au structures. The LSPR spectra of the TiO₂, two Au@TiO₂ nano-composites by using 20 and 70 μL HAuCl₄ are shown in Fig. 3(c). The increasing HAuCl₄ concentration improved the peak intensity of Au particle-structure at about 530 nm. There are no red-shifted spectra in the different nano-composites. The Au@TiO₂ nano-dendrites with obvious hierarchical branches cannot be generated owing to the inadequate OH⁻ groups in the precursors.

Fig. 3 (a and b) The representative low-magnification and enlarged TEM images of the Au@TiO₂ hetero-structures fabricated by overgrowth of Au on the TiO₂ nano-spheres in the presence of 20 μL HAuCl₄. (c) The UV-visible absorption spectra of the as-prepared TiO₂ nano-spheres and Au@TiO₂ hetero-structures generated by 20 and 70 μL HAuCl₄.

Then, based on UV-laser irradiation for 60 min, the TiO₂ nano-spheres with OH⁻ groups embedded on the surfaces were adopted as the precursors for the fabrication of Au@TiO₂ hybrid nano-materials. The obtained novel nano-structures by using 20 μL and 100 μL HAuCl₄ are shown in Fig. 4(a and b). The low-magnification TEM image in Fig. 4(a) displays that the hybrid nano-materials appeared to be complex nano-structures with dense core and hierarchical branches. The inset shows that the typical small branch with a d-spacing of 0.235 nm can be indexed to be the (111) plane of the Au structure. Moreover, enlarged and high-magnification TEM images in Fig. 4(b and c) illustrate that the higher HAuCl₄ concentration (100 μL) enables the Au@TiO₂ nano-dendrites to possess more clearly hyper-branched surface structures. Compared with the nano-dendrites in Fig. 4(a), the branch length significantly increases from about 35 nm to approximately 75 nm by measuring the size of more than 200 particles in sight of the TEM images. The Au(111) structure is also illustrated in Fig. 4(d). The unique optical properties of the as-prepared Au@TiO₂ nano-dendrites with controllable branch lengths are illustrated by the UV-visible absorption spectra in Fig. 4(e). The direct photographs of the TiO₂ nano-spheres and the Au@TiO₂ nano-dendrites (inset in Fig. 4(e)) show that Au branch-structures lead to a significant change in the colloidal solution color from dark yellow to grey brown. The absorption spectra of the Au@TiO₂ nano-dendrites illustrate that the LSPR peaks distinctly red shifted from the visible ∼545 nm to about 806 nm with an increase of HAuCl₄ content during the reduction process.

Fig. 4 (a and b) The typical low-magnification TEM images of the Au@TiO₂ nano-dendrites by using 20 μL and 100 μL HAuCl₄, respectively. (c and d) The corresponding enlarged and high-magnification TEM images of Au@TiO₂ nano-dendrites by using 100 μL HAuCl₄, respectively. The precursors were modified by UV-laser irradiation for 60 min. (e) The UV-visible absorption spectra of the as-prepared Au@TiO₂ nano-dendrites by using different amounts of HAuCl₄.

Finally, we further increase the UV-laser irradiation time to 120 min in order to get the TiO₂ nano-spheres with abundant/enough OH⁻ groups. By using 100 μL HAuCl₄ in the overgrowth process, the low- and high-magnification TEM images in Fig. 5(a–d) clearly show that highly branched Au@TiO₂ nano-dendrites can be obtained by using TiO₂ nano-spheres with abundant OH⁻ groups. Compared with the nano-dendrites in Fig. 4, the branch is highly elongated by using the co-reduction of HAuCl₄ with enough surface OH⁻ groups in the precursors, as shown in Fig. 5(b and c). Meanwhile, Fig. 5(d) reveals that the Au(111) structures can also be formed on the Au branches. Moreover, the elemental mapping images of the typical Au@TiO₂ nano-branched structure in Fig. S2† show that the hybrid nano-dendrite is indeed composed of Au, Ti and O elements, and the corresponding relative ratio is about 75 : 8 : 17, respectively. Then, the Au content in the Au@TiO₂ nano-branched structure is about 75%. The elemental mapping images can also provide detailed information of the elements distributions in the branched nano-structures. We found that the Ti and O elements can be detected in core region, and Au element was formed on the outside. The detailed structure-evolutions of Au@TiO₂ nano-dendrites obtained from a same amount (100 μL) of HAuCl₄ in each experiment but different UV-laser irradiation times (0–120 min) were shown in Fig. S3.† The experimental results are obtained by measuring the Au contents and the Au branch lengths of more than 200 particles in sight on the elemental mapping images and TEM images, respectively. As illustrated in Fig. S3(a and b†), the results clearly reveal that both the average Au content and Au branch length in the hybrid Au@TiO₂ nano-structures obtained from a some amount Au source increase linearly with an increase of irradiation time. Increasing irradiation time (0–120 min) result in the more OH⁻ groups formed on the surface of TiO₂ structures. Therefore, the results in Fig. S3(a and b†) show that the mean Au content and Au branch length separately increases from about 11% and 24 nm to 75% and 167 nm with an increase of irradiation time (0–120 min), respectively. The above results show that the amount of OH⁻ groups plays an important role in the formation of higher branched Au@TiO₂ nano-dendrites. Then, the UV-visible absorption spectra of the hyper-branched Au@TiO₂ nano-dendrites are illustrated in Fig. 5(e). It is interesting to note that the LSPR peaks can be modulated from the visible ∼545 nm to the NIR region ∼1090 nm with the increase of the HAuCl₄ content (20–100 μL) during the overgrowth process. The enhanced NIR absorption property should be attributed to longitudinal plasmon wavelength of the Au elongated branches on the TiO₂ nano-dendrites. It is reasonable to deduce that the highly branched Au@TiO₂ nano-dendrites with excellent NIR absorption properties will be very suitable for developing super-active photo-catalysts, novel solar cells, and biomedical sensor, etc.

Fig. 5 (a–d) The typical low- and high-magnification TEM images of the Au@TiO₂ nano-dendrites by using 100 μL HAuCl₄, respectively. The precursors were modified by UV-laser irradiation for 120 min. (e) The UV-visible absorption spectra of the as-prepared Au@TiO₂ nano-dendrites by using different amount HAuCl₄.

Conclusions

In summary, hybrid Au@TiO₂ nano-dendrites with tailored branched structures have been carefully devised by using the reduction of HAuCl₄ with abundant OH⁻ groups embedded on the TiO₂ nano-spheres. To reveal the formation mechanism of abundant OH⁻ groups on the TiO₂ surfaces, the unique active O species generated in TiO₂ nano-spheres during pulse laser fabrication and subsequent UV-laser irradiation have been proposed. The absorption spectra of the as-prepared Au@TiO₂ nano-dendrites demonstrate that the LSPR peaks can be effectively modulated from the visible ∼545 nm to NIR ∼ 1090 nm. The obtained results in this paper will be a breakthrough in the fabrication of noble metal@metal oxide semiconductor hyper-branched nano-structures. The obtained materials may have significant applications in super-active photo-catalysts, novel solar cells, and biomedical sensors, etc.

Acknowledgements

This work was supported by the Natural Science Foundation of China under Grant No. 11575102, 11105085, 11275116 and 11375108, the Fundamental Research Funds of Shandong University under Grant No. 2015JC007.

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Footnote

† Electronic supplementary information (ESI) available: The pH value of the TiO₂ solution versus the UV-laser irradiation time. The typical TEM images and corresponding elemental mapping images of the Au@TiO₂ nano-dendrites obtained by UV-laser irradiation for 120 min and using 100 μL HAuCl₄. The structure-evolutions of Au@TiO₂ nan-dendrites obtained from a same amount (100 μL) of Au sours in each experiment versus UV-laser irradiation time. See DOI: 10.1039/c6ra15398d

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