Received 27th March 2016 Olexiy A. Balitskii,* Dariusz Moszynski´ b and Zareen Abbasc
Accepted 10th May 2016
DOI: 10.1039/c6ra07938e www.rsc.org/advances
Heavily doped tungsten oxide nanoparticles with localized surface plasmon resonances (LSPRs) were recently highlighted as potential substitutes for noble metals in the field of plasmonic applications. Herein, oxygen deficient, spherically shaped WO3x nanocrystals (NCs) were synthesized with a pronounced visible LSPR absorbance peak instead of a broadband tail usually observed for WO3x nanowires. Although the tuning of the plasmon resonances was achieved mainly by changing the nanocrystals composition or solvent refraction index, we demonstrate this via the interfacial charge donation/ extraction. In an aqueous NCs dispersion, the LSPR peak was either blue shifted in an acidic solution up to 80 nm or bleached by a basic solution making the NCs appropriate for sensing applications.
1. Introduction
Colloidal nanocrystals (NCs) exhibiting localized surfaces plasmon resonances (LSPRs) generally represented by various noble-metals1,2 have recently been complemented by heavily doped semiconductors.3–5 LSPR semiconductor NCs are usually
Inself-activated either by cation vacancies (in CuyCh , ¼ 2xCh,6–8 Cu11,x129,
2 Ch S, Se, Te) or anion vacancies in metal oxides.
The defect concentration is engineered to be high enough for producing hole or electron degenerated materials. Owing to the tunable carrier concentration/light absorbance, semiconductor plasmonic NCs are highly desirable for many application ranging from photothermal therapy,13,14 engineering of conductive parts of microchips,15 sensing16 or catalyses17 to the realization of absolute blackbody conditions.18 Recent developments in solvothermal methods of particle synthesis for heavily doped metal-oxides NCs19,20 have made it possible to obtain aqueous dispersible LSPR nanoparticles. Moreover, these methods did not require the use of complex surfactant molecules, which usually affected plasmons in metal chalcogenides.21 Among the various plasmonic oxides, WO3x seems to be closest to metallic nanoparticles in terms of the LSPR properties such as spectral position of absorbance and carriers concentration.5 In particular, in W18O49 NCs, the surface plasmons can enhance Raman scattering to the same level as noble metals.22 Herein, we pursued the goal to tune the LSPRs in W18O49 NCs by interfacial charge transfer between the NCs and aqueous media of different pH, resulting in a scalable LSPR shi. A similar task for noble metal plasmonic NCs was realized by changing the NCs surrounding media.23 This can include several types of surfactant, sensitivity to each other as well to the pH, and shrinkage or elongation under specic chemical conditions. As mentioned above, a main intrinsic LSPR characteristic (carrier concentration) is locked on the synthesis stage for such NCs. In contrast to other semiconductor LSPR NCs, wherein the defect concentration can be varied continuously by the synthesis procedures, WO3x is characterized by several discrete stable defect stoichiometries only, namely, W18O49 has the highest possible oxygen deciency and only the tungsten suboxide is obtained in isolated specic monoclinic crystalline phase.24 The successful synthesis procedures implied the fabrication of nanocrystalline W18O49, including solution combustion,24 thermal evaporation,25 microwave-assisted26 and solvothermal procedures.19,20,22,27,28 The plasmonic absorbance in such particles is expected in the visible region and should not be tunable only by the refractive index of the surrounding medium, but also by interfacial charge transfer. This study demonstrates later for the W18O49 NCs, providing a tool for pH
sensing by tuning the LSPR band.
2. Materials and methods
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Tungsten hexacarbonyl (W(CO)6, 97%) was purchased from Alfa Aesar. Absolute ethanol, methanol, 2-propanol, sodium hydroxide, pH buffer solutions (all of the ACS grade) and perchloric acid (70%) were ordered from Merck and Fluka, respectively. The water used was of Milli-Q quality. All chemicals were used without purication.
For the synthesis of WO3x nanocrystals, we modied solvothermal procedure recently developed for nanowires by the Xue group.20 The purpose of such modication was to grow spherically shaped particles with eliminated longitudinal plasmon frequencies. The last ones in the case of nanowires, broadly distributed by aspect ratio, transform into broadband vis-NIR tail, which is unsuitable for LSPR tuning. An alternative way to observe the LSPR band is a chemical surface modication of WO2.72 nanorods being rather complicated and comprising surfactant amine functionalization followed by the stepwise oxidation of nanoparticles, as shown by Grauer and Alivisatos.29 The synthesis procedure used in this study was as follows: 50 mg of W(CO)6 (0.14 mmol) handled in a glovebox was added to absolute ethanol with an amount of 30 mL. Aer 30 min stirring at room temperature, the mixture yielded a slightly yellow but transparent solution. The solution was then transferred to a 40 mL polyetheretherketone lined stainless steel autoclave, heated up to 160 C and maintained at this temperature for 20 hours. The NCs were isolated from solution by centrifugation at 4.5 krpm for 7 min followed by redispersion in ethanol. The washing procedure was repeated at least three times and was the same for the transfer of NCs to other alcohols or aqueous solvents.
The UV-vis-NIR absorbance was measured in a 10 mm optical pathlength cuvette using a Thermo Scientic Evolution 60S Spectrophotometer. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS) were carried out on a ZEISS ULTRA 55 machine, operating at 5/15 kV. X-ray diffraction patterns and X-ray photoelectron spectra were recorded using Siemens D5000 diffractometer and SES 2002 spectrometer with Cu Ka and Al Ka X-rays sources, respectively. For SEM, XRD and XPS, the NCs solution was drop casted onto Si wafers and plastic slides. pH dispersions were prepared using perchloric acid, sodium hydroxide or standard ACS buffer solutions.
3. Results and discussions
The synthesized NCs were broadly size-distributed in the range of 100–1400 nm, with a mean size of 440 nm and were almost all of a regular spherical shape (Fig. 1a–c). Similar W18O49 nanospheres were obtained solvothermally19 from tungsten ethanolate, elucidating the precursor concentration to be a key factor in size/shape control. The NCs composition was determined by EDX (Fig. 1c and d). The W to O atomic ratio was close to 2.7, corresponding to an oxygen deciency in a sub-stoichiometric tungsten oxide WO3x with x ¼ 0.3. In addition, EDX also detected some carbon and silicon originating from substrate and residuals of the precursors or solvents. The elemental maps (Fig. 1c) show the homogeneity of NCs in composition. The XPS data were in rather good agreement with EDX. The spectrum of the core W 4f level (Fig. S1†) exhibited distinct peaks of tungsten ions, W4+ and W6+, which are typical for W18O49 stoichiometry.28 A ratio of W 4f peaks area, corresponding to different tungsten oxidation states determines the relative concentrations of W and O without the need for O 1s line and ASF data and conrms the oxygen deciency in WO3x: x ¼ 0.265 0.005. This value is in slight disagreement with the one obtained by EDX because XPS tests just a surface layer of about 1.5 nm in thickness coinciding with an average electron escape depth. In addition, O 1s line reveals hydroxyl signal originated either from residuals of the solvent or from the adsorbed species. As it will be shown later, hydroxyl groups can cure oxygen vacancies and affect the oxidation state of tungsten. W18O49 crystallinity was monitored by XRD. All the diffraction peaks (Fig. 1e) were perfectly matched with those of monoclinic W18O49 (P2/m space group, JCPDS no. 84-1516). In the case of anisotropic
Fig. 1 Representative (a) and detailed (b) SEM images, EDX elemental maps (c) and spectrum (d), XRD pattern (e) of WO3x NCs.
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nanowires,22 the diffraction peaks resulting from the planes of the preferable growth direction (i.e., (010) plane) were narrower. Herein, we observed an opposite trend; all the peaks were diffused showing almost isotropic growth of NCs.
One of the distinctive manifestations of semiconductor plasmons is their tunable optical absorbance (Fig. 2). The optical density minimum at about 350–360 nm followed towards the blue area by an absorption onset, which correlates with W18O49 nanowires data29 and originated by the band gap of W18O49. The slightly weaker absorbance (l > 500 nm) is due to oscillations of the free electrons, formed in the conduction band by anion vacancies. Thus, we tested the optical absorbance of the NCs dispersed in different polar protic solvents (Fig. 2). For the sample, dispersed in ethanol a well-resolved LSPR peak appears at a wavelength of 556 nm. The position of
the LSPR maximum in W18O49 NCs correlates almost linearly with the solvent refraction index (Fig. 3 and Table 1) compared to that of Cu2xS.6 That could be explained by the absence of surfactants, affecting the refraction on NC-solvent boundary. The Milli-Q water dispersion of NCs does not match the value of that tendency, showing up to 20% of free electrons losses (Table 1). The effect of the different pH on the LSPR properties of NCs is presented in Fig. 4. The addition of acids to a NCs solution gradually changes the LSPR peak, which was found to lose some part of the resonance quality and was blue-shied up to 80 nm. This shi was enhanced by decreasing the pH to 4. NCs
dispersed in acidic solutions substantially vary neither in size nor in shape elucidating the insignicance of the etching effects (Fig. S2†). In addition, the resonance wavelength is slightly tuned back to the red by the higher pH values, but we could not demonstrate the complete reversibility of the tuning. The blue
Fig. 2 UV-vis-NIR absorbance of WO3x nanocrystals – LSPRs in different protic polar solvents.
Fig. 3 UV-vis absorbance of WO3x nanocrystals – shifts of LSPR maximum with changing of solvent refractive index.
Table 1 Parameters of WO3x nanocrystals plasmon resonancesa
Sample lsp
(nm) usp (s1) up (s1) Nn (cm3)
WO3x–C3H7OH
WO3x–C2H5OH
WO3x–CH3OH
WO3x–HOH 542 3.5 1015 6.00 1015 1.76 1022
556 3.4 1015 5.98 1015 1.75 1022
575 3.3 1015 5.96 1015 1.71 1022
613 3.0 1015 5.42 1015 1.43 1022
a sffi up2 2 sNffi ee2
usp ¼ þ g -surface plasmon, up ¼ -bulk plasmon
1 23m 30me
frequencies; g-spectral line width, Ne and me – concentration and effective mass of electrons,30 respectively. The dielectric constants were taken as 3m ¼n2 for the corresponding wavelength.
Fig. 4 UV-vis-NIR absorbance of an aqueous solution of WO3x nanocrystals – LSPRs tuning versus pH of the solvent. The band in the NIR at about 975 nm was not connected with LSPR as being an intrinsic overtone/combination water absorbance.31
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shi in the extinction frequencies for the Au nanoparticles deposited on the electrode under a negative potential has recently been reported in ref. 32. The metal oxide surface acquires positive charge at pH values lower than the pH at the point of zero charge (PZC) and negative at pH values larger than the PZC. In the literature, different PZC values have been reported for WO3. In one study33 the PZC is given at pH value of 2, whereas in another study,34 it is given at pH ¼ 4–5. This means that at pH values >2 or >5 the surface of WO3x will be negatively charged. Therefore, the blue shi observed in the absorption frequencies in this study most probably originates from the negatively charged surface of tungsten oxide. However, we would like to mention that the data on the surface charging of WO3x is scarce in the literature and more study needs to be performed.
The addition of a base such as NaOH (Fig. 4) resulted in NCs solution bleaching and a red shi of the absorption band offset was observed simultaneously (l > 400 nm). As a result, the concentration of free electrons at the bottom of the conductivity band was lowered enough not to manifest Moss–Burstein effect for degenerated materials.6 As a consequence, it was assumed that the base solution either traps electrons from NCs conduction band or cures oxygen intrinsic defects in tungsten suboxide. In both cases, the electron density decreases to the level inappropriate for LSPR observance. By changing the pH to more acidic, the overall tuning of LSPR almost matches the absorbance of NCs dispersed in methyl alcohol characterized by the similar water refractive index (Fig. 3). We assume that the increase in free electrons concentration limited to that one of 1.7 1022 cm3 (Table 1). The addition of protons just cures the trap states appeared during the NCs surface–water interaction, altering the carrier concentration by 18% to the value of the assynthesized sample.
4. Conclusions
We demonstrate a simple and facile modication of hydrothermal method employed to share control of plasmonic WO3x nanospheres. In contrast to well-studied WO3x nanowires with a broad vis-NIR plasmonic absorbance tail, spherical NCs possess an easily tunable LSPR band. Considering that the LSPR absorbance is of reasonable optical density and its essential blue shi in acidic pH solutions, we suggest the possible applicability of such plasmonic particles for the quantitative sensing of acidic media and the qualitative elucidation of the presence of bases.
Acknowledgements
One of the authors (O.A.B.) acknowledges the support from the Swedish Institute in form of a Scholarship for Postdoctoral research.
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