A facile strategy for the synthesis of monodispersed W₁₇O₄₇ nanoneedles

Abstract

We present a facile one-pot wet chemical strategy for the synthesis of monodispersed W₁₇O₄₇ nanoneedles, of which the synthetic conditions and photothermal properties are systematically investigated. By modulating experimental conditions such as the ratio of solvent, the dose of sulfur powder and the reaction time, uniform W₁₇O₄₇ nanoneedles with an aspect ratio larger than 10 are obtained. These hydrophobic W₁₇O₄₇ nanoneedles are subsequently transferred into aqueous solution by coating with amphiphilic polyvinylpyrrolidone (PVP) for photothermal imaging applications. The results suggest that these monodispersed nanoneedles have potential for photothermal detection and photothermal therapy.

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Introduction

Recently, nanostructured tungsten oxide has attracted extensive attention due to its unique physical/chemical properties such as high surface areas, light absorption, electrical conduction, thereby being explored for use in photocatalysis,^1–3 electrochemistry,⁴ photoelectricity,^5–7 gas sensing,⁸ and phototherapy.⁹ To date, a lot of effort has been devoted to preparing nanostructured tungsten oxide materials^10–13 with various morphologies and phases such as nanosheets,^14–16 nanowires,^17–20 nanorods,²¹ nanoparticles,²² and hollow nanospheres²³ using either physical or chemical routes. As a typical solution-phase synthesis method, Xi and co-workers^17,24 synthesized W₁₈O₄₉ nanostrucutures by solvothermal treatment of anhydrous WCl₆ in ethanol. This work presents not only a possibility for the use of ultrathin W₁₈O₄₉ nanowires as a functional material in the conversion of carbon dioxide but also a new strategy to design oxygen-vacancy-rich nonstoichiometric simple oxides with high photochemical activity. Wang and co-workers²⁵ utilized vapor-phase deposition method to prepare three-dimensional (3D) WO₃ nanostructures on carbon cloths with tungsten powder as the precursor by adjusting the growth temperature and the rate of O₂ flow. The afforded 3D WO₃ nanostructures displayed strong photocurrent responses under visible light irradiation, demonstrating its potential applicability for organic compounds decomposition and wastewater treatment. Kalantar-zadeh and co-workers²⁶ employed another important method to prepare highly ordered 3D tungsten oxide with thickness up to 2 μm on a fluoride doped tin oxide (FTO) conductive glass by using an electrochemical anodization under mild chemical dissolution conditions at a low voltage of 10 V. These highly ordered 3D WO₃ nanoporous networks have excellent coloration efficiencies that greatly exceeded the general range of the crystalline WO₃ films.

On the other hand, nanostructures with near-infrared (NIR) light absorption as well as high photothermal performance^27,28 are highly expected in view of photothermal applications including photothermal ablation,^29–33 photothermal controlled drug delivery³⁴ and photothermal imaging³⁵ detection. Due to the unique defect structure and LSPR effect oxygen-deficient tungsten oxides (WO_3−x) display strong absorption in the NIR region, which open the possibility of using them as photothermal agents. Such properties also promote great efforts in exploring novel synthetic strategies of tungsten oxides. Recent studies indicated that high photothermal conversion efficiency could be rationally improved by modulating the composition, morphology, surface functionalization, and assembly of the nanostructure.^36–38 For example, WO_3−x nanowires with strong NIR photo absorption have been proved to be a promising candidate as a photothermal agent due to their excellent biocompatibility and high photothermal effect.¹⁹ Hu and co-workers³⁸ prepared WO_3−x hierarchical nanostructures, which could serve as excellent laser-cavity mirrors, and be used to promote the photothermal conversion efficiency. Notwithstanding great success in the fabrication of WO_3−x nanomaterials, it is still highly desirable and challenging to prepare the monodispersed WO_3−x nanocrystals with a uniform shape and small size. Herein, we explored a facile strategy to prepare the monodispersed tungsten oxide (W₁₇O₄₇) nanoneedles with average size of ∼90 nm and broad NIR absorption (700–3300 nm). The uniform needle morphology could be obtained by tuning the solvent ratio, the dosage of sulfur powder, and the reaction time. After being encapsulated with polyvinylpyrrolidone (PVP), these W₁₇O₄₇ nanoneedles were further utilized for the photothermal imaging applications.

Experimental

Materials and methods

Oleylamine (80–90%, Acros Organics), 1-octadecene (90%, Sigma-Aldrich), oleic acid (90%, Alfa Aesar) and tungsten(VI) chloride (99%, Alfa Aesar) were used as obtained. Sulfur powder (CP), absolute ethyl alcohol (GR), cyclohexane (AR), tri-chloromethane (AR), n-hexane and polyvinylpyrrolidone (PVP, MW 10 000–70 000 g mol⁻¹) were obtained from Beijing Chemical Reagent Company and were used as received without further purification.

Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were recorded on a Hitachi H-800 transmission electron microscope operating at 100 kV and JEOL JEM-2100F transmission electron microscope operating at 200 kV, respectively. Bruker AXS D8-Advanced X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) was employed to test powder X-ray diffraction (XRD) patterns with operation current and voltage at 40 mA and 40 kV (2θ ranging from 5° to 80°), respectively. The standard XRD pattern of W₁₇O₄₇ (JCPDS card no. 79-0171) was employed as a reference. UV-3600 UV-vis-NIR spectrophotometer (Shimadzu) equipped with a plotter unit was employed to measure absorption spectra. The photothermal performance was carried out with an FLIR camera.

Synthetic procedures

Synthesis of W₁₇O₄₇ nanoneedles. In a 100 mL three-neck flask, 2 mL of oleic acid and 8 mL of 1-octadecene were degassed for 15 min by nitrogen at room temperature. The solvent mixture was then heated up to 310 °C under nitrogen atmosphere. Meanwhile, 0.1 mmol of tungsten chloride, 0.2 mmol of sulfur powder and 2 mL of oleylamine were mixed in a centrifuge tube and subjected to sonication until full dissolution, which was then injected into the hot mixture solution quickly. Afterward, the temperature was kept at 310 °C for 1 h under nitrogen atmosphere. Then the mixture was left to cool down to room temperature naturally, followed by precipitation with ethanol. Finally, the powder nanomaterials were obtained by centrifuging and washing with ethanol and cyclohexane.

Preparation of hydrophilic W₁₇O₄₇@PVP nanoneedles. In a typical procedure,³⁹ 33 mg of hydrophobic W₁₇O₄₇ nanoneedles were dissolved in chloroform (20.0 mL) and then mixed with 125 mg of PVP (dissolved in 10.0 mL chloroform). The mixture was then sealed in beaker and stirred at room temperature for 24 h. The hydrophilic W₁₇O₄₇@PVP nanoneedles were collected by centrifugation (12 000 rpm, 10 min) with adding n-hexane, washed with chloroform and cyclohexane, and then redispersed in deionized water (0.1 mM) and stored for later use.

Results and discussion

As shown in the TEM images (Fig. 1a and b), the obtained needle-like nanostructure has a length around 90 nm and width about 9 nm with an aspect ratio of ∼10. The HRTEM image (Fig. 1c) showed a clear lattice fringes of 0.378 nm, consistent with the (010) lattice spacing of W₁₇O₄₇. It's notable that after coating with PVP, the afforded W₁₇O₄₇@PVP (Fig. 1d) was dispersed very well in water without obvious changes in shape and size. Moreover, X-ray diffraction (XRD) analysis was further employed to identify the phase composition and crystallographic structure of the samples. As shown in Fig. 2a, the XRD pattern of the as-obtained blue powder was well matched with the standard powder diffraction pattern of W₁₇O₄₇ as a reference (JCPDS card no. 79-0171). And the energy dispersive spectrum (EDS) results indicated that there was little sulfur on the surface of W₁₇O₄₇ (as Fig. S1†). The UV-vis-NIR absorption spectroscopy (Fig. 2b) indicated that the products have a broad absorption between 700 nm and 3300 nm, suggesting its potential applications for light absorption and photothermal conversion in a broad region, which is favorable for the convenient choice of various diode laser for photothermal tests.

Fig. 1 TEM and HRTEM images of W₁₇O₄₇ nanoneedles. (a) Low magnification; (b) high magnification; (c) HRTEM of hydrophobic W₁₇O₄₇ nanoneedles; (d) TEM image of W₁₇O₄₇@PVP dispersed in water.

Fig. 2 XRD pattern (a) and UV-vis-NIR absorption spectrum (b) of W₁₇O₄₇ nanoneedles.

To investigate the different factors for the growth of W₁₇O₄₇ nanoneedles, different experimental conditions such as solvent ratios, reaction time as well as material dosage were systematically studied. Initially, when fixed the total solvent volume (10 mL) while varying the volume ratios of oleic acid to 1-octadecene under identical conditions. It was found that the volume of oleic acid plays important roles on the morphology of nanomaterials. When the ratio was less than 2 : 8 (Fig. 3a and b), nanomaterials with various lengths were obtained (Table S1†). When 2 mL of oleic acid was injected into this system, nanomaterials with uniform size were acquired, as shown in Fig. 3c. However, keeping increasing the volume of oleic acid would lead to nanomaterial aggregation (Fig. 3d–g). If no 1-octadecene was used, entangled network of the nanomaterial could be obtained (Fig. 3h). To acquire the uniform size and morphology, we chose to use 2 mL of oleic acid in the following experiments.

Fig. 3 TEM images of W₁₇O₄₇ nanostructures prepared with different ratios of oleic acid to 1-octadecene. The ratios of oleic acid (mL) to 1-octadecene (mL) are (a) 0.5 : 9.5, (b) 1 : 9, (c) 2 : 8, (d) 3 : 7, (e) 5 : 5, (f) 7 : 3, (g) 8 : 2 and (h) 10 : 0.

Then the amount of sulfur powder was evaluated. In the absence of sulfur powder, nanowire was obtained (Fig. 4a). When 0.01 mmol sulfur powder was added into the reaction system, the sample was observed as fine nanoneedles (Fig. 4b). With the dosage of sulfur powder increasing, the morphologies of the samples stayed almost unchanged as seen in Fig. 4c–f, which showed the dosage of sulfur powder including 0.05 mmol, 0.10 mmol, 0.20 mmol and 0.30 mmol. The length, width and aspect ratio of W₁₇O₄₇ nanostructures with different dosage of sulfur power were shown in Table S2.† The aspect ratio was small with various length when sulfur powder under 0.20 mmol. It seems that the sulfur powder in this system has significant impact on the morphology of W₁₇O₄₇ by means of controlling the growth direction of W₁₇O₄₇ nanoneedles.

Fig. 4 TEM images of W₁₇O₄₇ nanostructures prepared with different quantity of sulfur powder. The dosages of sulfur powder are (a) 0, (b) 0.01, (c) 0.05, (d) 0.10, (e) 0.20 and (f) 0.30 mmol.

As reaction time is an important factor affecting the crystallinity of W₁₇O₄₇, we also investigated the influence of reaction time on the nanostructures. The results revealed that W₁₇O₄₇ nanoneedles showed different morphology and aspect ratios under different reaction time, as shown in Fig. 5 and Table S3.† It appears that the W₁₇O₄₇ was formed by gradually etching bulk materials. When the reaction time was too short, still large portion of unreacted materials were left. As the reaction time increased to 1 h, the needle-like W₁₇O₄₇ with uniform size was observed, however, the long needles gradually fell apart as the reaction time was prolonged to 2 h, indicating that the reaction time is of crucial importance for achieving uniform W₁₇O₄₇ nanoneedles. As a result, 1 h was chosen as the reaction time for the synthesis of W₁₇O₄₇ nanoneedles.

Fig. 5 TEM images of W₁₇O₄₇ nanoneedles with different reaction time intervals. The reaction time is (a) 10 min, (b) 30 min, (c) 1 h and (d) 2 h, respectively.

As mentioned before, the W₁₇O₄₇ nanoneedles had a broad UV-vis-NIR absorption from 700 nm to 3300 nm, which is favorable for the photothermal tests via different diode laser. Prior to further investigation, the photothermal performance of W₁₇O₄₇ nanoneedles was carefully performed. The 808 nm irradiation light was frequently adopted in photothermal imaging and photothermal therapy because of its good dodging of absorption by water, blood and tissues. As a result, an excellent penetration and good photothermal efficacy is highly desirable via 808 nm irradiation. Under continuous irradiation (808 nm) with a power density of 1.0 W cm⁻², the photothermal conversion profile shows the temperature changes of PVP coated W₁₇O₄₇ nanoneedles with different irradiation time (Fig. 6a). In addition, the temperature enhancement increased gradually along with the increment of W₁₇O₄₇ nanoneedle concentration and could reach 25 °C within 6 min when the concentration of W₁₇O₄₇ nanoneedles was 6.6 mg mL⁻¹. As a control, the temperature change of pure water was increased only 0.7 °C under 808 nm light irradiation (1.0 W cm⁻²) within 6 min. This result suggests that the 808 nm light can efficiently dodge the absorption of water, which is highly preferable for in vivo/in vitro photothermal imaging and photothermal therapy because this weak absorption will cause low side effects to normal biosamples. In another aspect, the increase of power densities would enhance the temperature increments when with fixed amount of W₁₇O₄₇ nanoneedles (6.6 mg mL⁻¹), as shown in Fig. 6b. We also tested the cytotoxicity of W₁₇O₄₇@PVP nanoneedles in Fig. S2,† indicating high biocompatability of the as-prepared W₁₇O₄₇@PVP nanoneedles and great potential to be used in biomedical applications such as photothermal imaging and photothermal therapy.

Fig. 6 Photothermal conversion profiles. (a) Temperature evolution of W₁₇O₄₇@PVP colloids with different concentration (0.066–6.6 mg mL⁻¹) versus irradiation time under irradiation of 808 nm NIR light (1.0 W cm⁻²). (b) Temperature evolution of W₁₇O₄₇ nanoneedle colloids (6.6 mg mL⁻¹) versus irradiation time with different irradiation power densities under 808 nm NIR light.

Conclusions

In summary, we developed a facile one-pot wet chemical strategy for the synthesis of monodispersed W₁₇O₄₇ nanoneedles with uniform shape and size. After systematically investigating different experimental factors such as solvent ratios, sulfur powder dosage and reaction time, we found that sulfur powder and oleic acid played an important role in the controlled synthesis of W₁₇O₄₇ nanostructures with nanoneedle morphology. After being transferred into aqueous solution by coating with PVP, the W₁₇O₄₇@PVP was explored for the photothermal imaging, indicating its potential applications for photothermal detection and photothermal therapy.

Acknowledgements

This research was supported in part by the National Natural Science Foundation of China (Grant no. 21475007, 21275015 and 21505003). We also thank the support from the “Innovation and Promotion Project of Beijing University of Chemical Technology”, the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology, and the High-Level Faculty Program of Beijing University of Chemical Technology (buctrc201507), and BUCT Fund for Disciplines Construction and Development (Project No. XK1526)”.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01567k

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