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Abstract
We report a simple route to assembling gold (Au) nanoparticles (NPs) on the surface of one-dimensional tungsten oxide (WO3) nanorods (NRs) through electrostatic interactions. Transmission electron microscope (TEM) imaging, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, ultraviolet-visible (UV-Vis) and diffuse reflectance UV-vis absorption spectroscopy were used to investigate the morphology, structure, surface characteristics, and linear optical properties of the Au NP/WO3 NR heterostructures. TEM images, XRD, and UV/Vis spectroscopy results confirmed the successful decoration of Au NPs on the WO3 NRs. The structure and elemental chemical states of the WO3 NRs were retained during the self-assembly process. The bandgap of the WO3 NRs became wider after attachment of the Au NPs owing to the interaction of the dipole moments of the WO3 NRs and Au NPs under the induction of light. The optical limiting (OL) and nonlinear optical (NLO) properties of the resulting Au NP/WO3 NR heterostructures were studied using an open-aperture Z-scan technique in the nanosecond regime with a 532-nm laser. The introduction of Au NPs strongly influenced the competition of the saturable absorption and nonlinear scattering (NLS) in the WO3 NRs. The Au NP/WO3 NR heterostructure had superior NLO activity to that of un-decorated WO3 NRs. The main factor contributing to the enhanced NLO effect of the Au NP/WO3 NR heterostructures was a combination of free carrier absorption, NLS, and efficient charge/energy transfer at the Au NP/WO3 interface. Our findings show that Au NP/WO3 NR heterostructures are promising candidates for optical limiters to protect sensitive instruments and human eyes from damage caused by high power lasers.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nanostructured tungsten oxide (WO3), an archetypal n-type semiconductor, has drawn considerable attention owing to its relatively narrow bandgap (2.4–2.8 eV), low toxicity, stable physicochemical properties, resistance to photocorrosion, and the high oxidation power of its valence band (VB) holes. Notably, the chemical and physical properties of WO3 can be tailored by adjusting their nanostructures to satisfy the specific needs of new applications; hence, these structures are candidates for a wide range of photocatalytic and photoelectronic applications [1–3]. Unfortunately, there are still some key factors such as rapid charge recombination and low visible light absorption that limit the practical use of WO3 as a photocatalyst [4]. Thus, to address these limitations of WO3 as a photocatalyst, it is commonly doped or modified with metallic dopants, such as Pt, Au, Ag, and Pd to improve the photocatalytic process owing to the formation of rectifying Schottky junctions at the interfaces to alter electrical transport behavior of WO3 [5]. Furthermore, metal NPs have high reactivity and enhanced catalytic efficiency, together with surface plasmon absorption (SPR) and nonlinear optical (NLO) properties. These features suggest potential for optoelectronic, photocatalytic, and photovoltaic applications [6].
Recently, optical limiting (OL) materials have drawn interest for potential use as novel optical functional materials for protecting sensitive optical systems and human eyes from laser damage in both military and civilian applications. Ideal optical limiters should be fully transparent to light at low and moderate intensities but fully opaque to high intensity light. Several types of OL materials, such as organopolymers (including phthalocyanine [7,8], porphyrins and conjugated polymers [9,10]), carbon nanomaterials (including fullerenes, carbon black, and nanotubes, nano-onions, and nanodots) [11–14], and noble metal nanomaterials [15,16], have been widely investigated. Notably, passive OL mechanisms include nonlinear absorption (NLA), nonlinear scattering (NLS), and nonlinear refraction (NLR) have drawn attention [7–16]. For example, carbon nanotubes have unique broadband OL properties from the visible to the near infrared in the nanosecond regime, which arise from NLS because of the formation of scattering centers consisting of solvent bubbles and ionized carbon microplasma [13]. Phthalocyanine and porphyrins have a unique OL response to both nanosecond- and picosecond-laser pulses owing to extensive π-electron conjugation, which gives NLA properties [7,8]. Despite intense research, the capabilities of existing optical limiters have yet to meet the diverse needs of practical applications in harsh environments. Thus, various strategies have been developed, such as elemental doping, incorporation of materials in solid matrices, precise structural design, and hybridization of two OL components to construct composite heterostructures with improved OL performances. Among these approaches, construction of heterostructures has drawn considerable attention because the OL activities of materials can be efficiently improved through simple covalent or non-covalent combinations [17–21].
However, there have been few reports on OL effects of decorated WO3 with metal nanoparticles (NPs) to date. Metal NPs, such as Au and Ag, with strong SPR have shown great potential as NLO materials, especially for ultrashort pico-femtosecond laser pulses [22]. The generally accepted underlying OL mechanisms for metal NPs are NLA for pico-femtosecond laser pulses and/or NLS for nanosecond laser pulses. In our previous work, we revealed that energy/charge transfer from the excited Au NPs to the CNTs or graphene can enhance the OL effect [23]. Recently, we confirmed that one-dimensional WO3 nanorods (NRs) have good OL performance in response to at 532-nm nanosecond laser pulses [24,25], therefore, we expected that combining WO3 nanorods with Au NPs would lead to some interesting and notable consequences, including OL enhancement. On the basis of these promising studies and the need to explore new OL materials, here we describe a simple method for constructing Au NPs assembled on the surface of 1-dimensional (1D) WO3 NRs for OL applications. We predicted that the combination of 1D WO3 NRs with opto-electronically active metal NPs would lead to some interesting effects, including OL enhancement.
Recently, considerable attention has been devoted to fabricating Au NP/WO3 NR nanocomposites [26–30]; however, most of the synthetic strategies reported to date involve surface modifications that require relatively complex and/or delicate procedures. Under some conditions, achieving an intimate interfacial contact between the components at the nanoscale level poses a great challenge, which leads to unfavorable performance. Thus, it would be highly desirable to develop a simple and efficient strategy to construct well-defined Au NP/WO3 NR heterostructures without surface modification, and ensure the structural integrity and excellent properties of the individual components.
Herein, uniform decoration of Au NPs on WO3 NRs was achieved through electrostatic interactions, as illustrated in Fig. 1. Importantly, the assembly procedure did not involve surface modification, which ensured structural integrity and excellent properties of the individual components. This approach might be extended to fabricating a series of metal/semiconductor composite nanostructures with tailored electronic/optical properties. We used an open-aperture (OA) Z-scan technique to investigate the OL and NLO behaviors of the as-prepared Au NP/WO3 NR composites. The heterostructures had an OL effect that was superior to that of the WO3 NRs alone. The enhanced OL effect is attributed to the combination of NLS and enhanced NLA, which resulted from efficient charge/energy transfer at the Au NP/WO3 NR interface.
2. Experiment
2.1 Preparation of Au NP/WO3 NR heterostructures
2.1.1 Preparation of WO3 NRs
The WO3 NRs were synthesized by a mild hydrothermal method [31]. First, 3 mmol of Na2WO4·2H2O was dissolved in 15 mL of deionized water, followed by the addition of 3 g of Na2WO4, with vigorous stirring until a uniform transparent solution was obtained. The 3 M H2SO4 was added dropwise to the above solution to maintain the pH at approximately 1.8. The resulting mixture was transferred to a Teflon-lined autoclave for a hydrothermal reaction for 6 h at 160 °C, and then allowed to cool to room temperature. The obtained precipitate was centrifugated in deionized water and absolute ethanol several times to remove excess ions. The final product was attained by drying at 80 °C.
2.1.2 Preparation of Au NP/WO3 NR heterostructures
We developed a synthetic route to obtain heterogeneous nucleation of Au NPs on the unmodified WO3 NRs using the reported method to obtain Au NPs [32]. First, 0.015 g of WO3 was dissolved in 20 mL of deionized water with magnetic stirring at 95 °C to form a suspension. Then, 2 mL of 1 wt% HAuCl4·4H2O and 4 mL of 1 wt% sodium citrate were sequentially added dropwise into the mixture. The mixture was stirred at the reaction temperature for 25 min, and gradually changed from a white to red-black color. The reactant was centrifuged in deionized water and absolute ethanol repeatedly to free impurity ions. Finally, the obtained black product was dried in a vacuum oven at 50 °C.
2.2 Characterization
The morphologies of the samples were investigated with a transmission electron microscope (TEM, JEOL-2100F) operating at an accelerating voltage at 200 kV. The samples were homogeneously dispersed in alcohol by ultrasonication and dropped onto the Cu grid (consisting of C, Cu) for examination. Crystallographic information for each sample was acquired by X-ray diffraction (XRD, D8-Advance) using a Cu Kα radiation source (λ = 0.15418 nm) over a 2θ range of 10° to 80°. X-ray photoelectron spectroscopy (XPS, Escalab 250, ThermoFisher Scientific Co.) was based on Al Kα radiation as the excitation source. The binding energy of each element was corrected to the standard C 1s contamination peak (284.8 eV). Linear optical properties of the samples were analyzed with ultraviolet-visible (UV-vis) absorption spectroscopy (Shimadzu UV-2600 UV spectrophotometer) and diffuse reflectance ultraviolet-visible (DR UV-vis) absorption spectroscopy (Lambda 950 spectrophotometer). In the former text, the liquid samples were placed in a 10-mm quartz cuvette for direct measurements. In the latter case, a certain amount of barium sulfate (BaSO4) powder was placed on a substrate and flattened with a glass column before the experiment as a reflectance standard, and then a handful of the sample powder was placed onto the standard white plate followed by the press process. In this way its diffuse reflection absorption spectra was determined.
2.3 Z-scan measurements
The NLO and OL behaviors of the WO3 NRs and Au NP/WO3 NR heterostructures were evaluated by the OA Z-scan technique [33]. A nanosecond-light source irradiated by a Dawa-S laser (Beamtech) was a Q-switched Nd:YAG pulsed laser system having a pulse width of 7 ns, a repetition frequency of 10 Hz, a beam waist radius of ω0 of 22 μm, and a Rayleigh length of 2.86 mm. The energy of a single pulse was adjusted to be approximately 30, 50, 70, and 90 μJ, respectively. Each sample was uniformly dispersed in ethanol by ultrasonication and loaded into a 1 mm-thick cuvette. The sample-containing cuvette was mounted on a translation stage that allowed for movement along the z-axis. All measurements were performed at room temperature.
3. Results and discussion
3.1 Morphology
In present work, we demonstrated a simple route to assembling Au NPs on the surface of WO3 NRs through electrostatic interactions without chemically modifying the surface of individual Au NPs or WO3 NRs. Figure 2 shows representative TEM and high-resolution transmission electron microscope (HRTEM) images of the WO3 NRs and Au NP/WO3 NR heterostructures. The bare WO3 NRs had a smooth surface and clear interplanar spacing [Fig. 2(a)], suggesting good crystallinity of resulting WO3 NRs. After electrostatic self-assembly procedure, a substantial amount of Au NPs with an average size of approximately 10 nm were deposited on the surface of the WO3 NWs, as shown in [Figs. 2(b) and 2(c)], confirming the successful formation of the Au NP/WO3 NR assembly with Au NPs bound to the surface of the NRs. The HRTEM image shown in Fig. 2(d) provides more details of the heterostructure. Two distinct crystal lattice spacings of 2.1 and 2.5 Å, correspond to (200) and (111) planes of face-centered-cubic (fcc) Au. Furthermore, another lattice fringe with a spacing of approximately 3.7 Å, was indexed to the (220) plane of cubic crystals of WO3. Thus, the TEM and HRTEM images confirmed that the Au NP/WO3 NR nanostructures were synthesized through a simple solution phase method without chemical modification of the surface of individual Au NPs or WO3 NRs.
Fig. 2. (a) TEM image of WO3 NRs and (b), (c), (d)TEM and HRTEM images of Au NP/WO3 NRs heterostructures.
3.2 Structure
The crystal structure of the Au NP/WO3 NR heterostructures was characterized by XRD, as shown in Fig. 3. The XRD patterns of the Au NPs and WO3 NRs are also included for comparison. For Au NPs, four diffraction peaks at 38.184°, 44.392°, 64.576°, and 77.547° correspond to the (111), (200), (220), and (311) crystallographic planes of cubic Au, with lattice constants a = b = c = 4.0786 Å (JCPDS Card No.02-1095) and the space group of Fm-3 m(225). The reflection pattern of the pristine WO3 NRs was indexed to cubic WO3·0.5H2O, with lattice constants a = b = c = 10.27 Å (JCPDS Card No.44-0363) and the space group of Fd-3 m (227). Strong and sharp diffraction peaks suggested high crystallinity of the WO3 NRs. Four diffraction peaks from the Au NPs were also detected in the Au NP/WO3 NR heterostructures, which confirmed the successful self-assembly of Au NRs onto the WO3 NR framework. Notably, two weak diffraction peaks at 12.727° and 32.741° are attributed to WO2Cl2 present in the WO3 NRs, which disappeared in the Au NP/WO3 NR heterostructures. The residual WO2Cl2 was likely subject to the following reaction [34]:
We also used XPS to examine the surface element composition and elemental chemical valence states of the Au NP/WO3 NRs. A survey spectrum of the Au NP/WO3 NR heterostructure [ Fig. 4(a)] contained peaks from W, O, Au, and C, which confirmed the self-assembly of Au on the WO3 NRs. The C signal derives from carbon tape used in the XPS measurements. A high-resolution O 1s spectrum [Fig. 4(b)] of Au NP/WO3 NR had the same appearance as that of the unmodified WO3 NRs, with a peak at 530.09 eV associated with lattice oxygen in WO3. A high-resolution W 4f spectrum [Fig. 4(c)] of the Au NP/WO3 NR was also comparable to that of the WO3 NRs, with a spin-doublet of W 4f located at 37.9 and 35.7 eV, assigned to W 4f5/2 and W 4f7/2, respectively. These signals are consistent with the W6+ oxidation state [35]. Notably, there were no other peaks or humps between W 4f5/2 and W 4f7/2 that might indicate the presence of any WOx phase other than WO3. The analogous high-resolution O 1s and W 4f spectra of the WO3 and Au NPs/WO3 NRs suggested that the structure and elemental chemical states of WO3 NRs were retained during the self-assembly. The high-resolution Au 4f spectrum of the Au NP/WO3 NRs, shown in Fig. 4(d), featured two peaks at 84.2 and 87.87 eV, which correspond to the Au 4f7/2 and Au 4f5/2 signals of metallic Au. The XPS results confirmed that decoration of Au NP on the WO3 NRs occurred through only electrostatic interactions.
Fig. 4. (a) Survey XPS spectra of WO3 NRs and Au NP / WO3 NR heterostructures; (b), (c) W4f spectra of WO3 NRs and Au NP / WO3 NR heterostructures, respectively. (d) Au 4f spectra of Au NP / WO3 NR heterostructures.
3.3 Linear optical properties
The linear optical properties of the Au NP/WO3 NR heterostructures were examined by UV-vis and DR UV-vis absorption spectroscopy and the results are shown in Fig. 5. For the WO3 substrate and Au NP/WO3 NR heterostructure [Figs. 5(a) and 5(b)], a broad band from 200 to 450 nm is attributed to photoexcitation of the bandgap of WO3. Apparently, the Au NP/WO3 NR heterostructures had remarkably enhanced light absorption in the visible region (450–800 nm) compared with the WO3 NRs. Furthermore, a broad absorption band centered at approximate 535 nm corresponds to the SPR of the Au NPs [36–38], which derived from collective resonance of electrons caused by matching of their vibrational frequency with that of incident light. This result further verified the successfully deposition of Au NPs on the WO3 NRs. In addition, an apparent red-shift of the absorption band edge of the WO3 NRs (ca. 12 nm) and the peak maxima of the Au NPs (from 520 to 535 nm) were observed for Au NP/WO3 NR heterostructures and are similar to those features of the bare WO3 NRs and isolated Au NPs [Fig. 5(a)]. This result is likely attributed to charge transfer between the Au NPs and semiconductor [39] and the SPR-induced photosensitization effect of the Au NPs. Furthermore, the SPR of metallic NPs depends on their size, shape, and composition, and on the dielectric properties of the surrounding matrix [40]. Therefore, the change in the position of the absorption bands is likely because of the larger average size of the Au NP/WO3 NR heterostructures compared with those of the undecorated WO3 NRs and isolated Au NPs.
Fig. 5. (a) UV-vis absorption spectra of Au NPs, WO3 NRs and Au NP / WO3 NR heterostructures in the solution; (b) UV-vis diffuse reflectance spectra (DRS) of WO3 NRs and Au NP / WO3 NR heterostructures; (c),(d) plot of the transformed Kubelka–Munk function versus the energy of light of WO3 NRs and Au NP / WO3 NR heterostructures, respectively.
The band gap of the WO3 NRs and WO3 NR/Au NP hybrids can be determined from the following equation [41]:
where α, h, ν, and Eg represent the absorption coefficient, Planck’s constant, frequency of light, and the band gap, A and n are constants, respectively. We set n to be 2 for the indirect bandgap of WO3. The band gap energy Eg values for the WO3 NRs and WO3 NR/Au NP hybrids were determined from plots of (ahν)2 versus hν. Figures 5(c) and 5(d) show the diagram used to calculate the band gaps. From the extrapolation of the linear region of the Tauc plots in Figs. 5(c) and 5(d), the values of Eg were determined to be to be 2.48 and 2.73 eV for the WO3 NRs and WO3 NR/Au NP heterostructures, respectively. We attribute the widening of the WO3 NR bandgap to the interaction of the dipoles between WO3 and Au lying closer to WO3 (less than 5 nm) under the induction of light and the size of Au NPs (8–12 nm), which may induce quantum size effects.3.4 NLO behavior
The Z-scan technique is a reliable, rapid, and sensitive technique, which is widely used to characterize the OL and NLO properties of materials, such as NLA, nonlinear refraction, and NLS. To examine the OL and NLO performances of the Au NP/WO3 NR heterostructures, we applied the OA Z-scan technique with a ns laser as the excitation source (532 nm, 7 ns, 10 Hz), at an excitation energy of 30–90 µJ. Sample concentrations were adjusted to obtain a similar linear transmittance of 70% at 532 nm in 1 mm-thick cells based on a UV-vis spectrometer to ensure the equivalent absorbance. The OL and NLO activity of the original WO3 NRs was also measured for comparison purposes. The normalized transmission spectra of the WO3 NRs and Au NP/WO3 NRs in ethanol under different excitation pulse energies are shown in Fig. 6.
Fig. 6. OA Z-scan curves of WO3 NRs, Au NP / WO3 NR heterostructures at 30 μJ (a), 50 μJ (b), 70 μJ (c), and 90 μJ (d).
Apparently, the OA Z-scan curve of WO3 NRs has a symmetrical peak about the focus point at low laser input energy (30 µJ), which is attributed to saturable absorption (SA). As the energy increases (>50 µJ), a symmetrical valley induced by reverse saturable absorption (RSA) appears. Therefore, the WO3 NRs display energy dependent SA and RSA. Such phenomena have also been reported in black phosphorus [42,43], transition metal sulfides [44,45], and noble metal nanoparticles [46–48]. Previous reports have confirmed that the transfer is caused by competition between SA and NLS [49]. When WO3 NRs are excited by the weak 532-nm pulse laser (<30 µJ), most of the ground-state electrons are pumped into an excited state, and the valence band does not compensate enough electrons because of the weak interband transition. The combination of these two factors directly reduces the electronic population of the ground state. This condition leads to ground-state absorption bleaching, which ultimately reduces photon absorption and increases transmittance, hence SA occurs. When the pump is strong enough (>50 µJ), excited electrons are pumped to a higher excited state and a strong valley (RSA) results from free carrier absorption (FCA) together with thermally induced microbubbles scattering. The SA, FCA, and NLS become comparable at a suitable incident light energy and the transmission remains constant. As the pump becomes stronger, the FCA and NLS effects become more pronounced, resulting in only the RSA process. However, at the same laser energy (30 µJ), the Au NP/WO3 NR heterostructures show only RSA behavior [Fig. 6(a)], which implies that the encapsulated Au NPs strongly influence competition between the SA and FCA and NLS in the WO3 NRs. To confirm the contribution of NLS to the observed OL activities, we also performed NLS experiments and collected a fraction of the light scattered by the samples at a forward planar angle of 45° to the plane of the incident light. Figure 7 indicates that the scattered energy markedly increased with the input laser energy for both WO3 NRs and Au NP/WO3 NRs. As expected, the NLS of Au NP/WO3 NRs apparently outperforms that of the WO3 NRs, indicating the enhanced NLS induced by the Au NPs improves the NLS effect during the competition process and therefore only RSA activity appears in the Au NP/WO3 NR hybrid nanostructures.
Fig. 7. Nonlinear scattering of WO3 NRs and Au NP/WO3 NR samples in ethanol solution at 532 nm. The linear transmittance of each sample was individually adjusted to 70%.
At irradiation energies higher than 50 µJ, only RSA is observed in both bare WO3 NRs and the Au NP/WO3 NR heterostructures because the FCA and NLS effects become more pronounced and completely suppresses SA. In the OA Z-scan, the transmittance of the sample is determined during its translation through the focal plane of a tightly focused beam. As the sample moves closer to the focus, the beam intensity increases and nonlinear effects occur that decrease the transmittance because of NLA and NLS. The depth of the valley in the Z-scan curve directly determines the extent of the NLO performance. Hence, the valley became deeper and stronger as OL takes place at a higher excitation fluence (Fig. 6). Furthermore, the valley of the Au NP/WO3 NR heterostructures was deeper than that of the unmodified WO3 NRs owing to enhanced NLS and NLA, confirming the enhanced OL performance after modification by Au NPs. The measured OA Z-scan curves were well described as a third-order nonlinear process and the nonlinear extinction coefficient β was calculated from the propagation equation [33]:
Table 1. Nonlinear extinction coefficient β obtained by theoretical fitting of experimental data for WO3 NRs and Au NP / WO3 NR at different input intensities.
Two mechanisms are generally accepted to explain the OL effect of materials: NLA and NLS. For optical nonlinearities in semiconductors, NLA based on FCA, TPA, and multi-photo absorption (MPA), is the major mechanism. Generally, the NLO behavior of semiconductors is mainly attributed to TPA and FCA. TPA means that electrons usually have to absorb energy equivalent to the difference between two energy levels in order to transition from a low energy level to a high energy level. FCA originates from absorption of electrons in the conduction band or holes in the valence band that absorb photon energies in the same band. Semiconductors have FCA at wavelengths in the linear absorption range, whereas TPA is observed at wavelengths outside the linear absorption region [50]. FCA is attributed to ESA of electrons in the conduction band and holes in the valence band. These free carriers (electrons and holes) are generated by interband transitions caused by linear absorption [51]. Absorption by a sample may occur via TPA when the photon energy of the laser pulse is equal to 2hν. Linear absorption spectra of the samples show linear absorption peaks for the WO3 nanostructures located at 200–300 nm. However, the photon energy of the 532-nm laser was lower than the 2hν required for excitation. The NLA of the Au NP/WO3 NR heterostructure is therefore attributed to FCA.
Because the excited wavelength (532 nm) is close to the resonance of Au NP, we propose that the improvement of the OL performances in the Au NP/WO3 NRs is also related to SPR processes in the Au NPs at 532 nm resulting in an increase in the NLS properties of the WO3 NRs. Metal NPs, such as gold and silver, with surface plasmon properties produce SPR under light irradiation at a specific frequency. The light of the SPR is generally divided into two parts: surface plasmon absorption and resonance light scattering. The sum of these two parts is the extinction spectrum. The absorbed light is converted into heat and the scattered light becomes light radiation of the same frequency, which enhances the local electromagnetic field. In the present composites systems, the absorbed photon energies expanded the Au NPs into a micro-plasma state in the sub-nanosecond range and subsequently induced a scattering center. The absorbed heat was transferred to the solvent to form microbubbles near the boiling temperature. These microplasma and microbubble scattering centers around the metal particles accompanied with resonance light scattering of Au NPs resulted in a stronger NLS; hence, the NLO and OL effects in the Au NP/WO3 NR heterostructures are enhanced compared with those of the WO3 NRs.
Additionally, efficient charge/energy transfer at the Au NP and WO3 NR interface might also contribute to the observed enhancement in NLO and OL. The surface assembly of Au NPs enhanced light absorption intensity from the WO3 NRs in the range of 450–800 nm for the Au NP/WO3 NR heterostructure. We posit that efficient charge/energy transfer at the Au/WO3 interface also has a major role in the optical nonlinearity of Au NP /WO3 NRs. Because photo-induced electron transfer may produce a charge-separated excited state that enhances the OL effect. The work function of Au has been determined to be 5.1 V (vs. Vac) and the electron affinity (χ) of WO3 is ca. 3.50 V (vs. Vac). As a result, when Au is intimately integrated with WO3, electrons in WO3 transfer to the Au NPs, resulting in negatively charged Au NPs, and at the region surface of WO3, a positive charge space layer is formed, thereby generating a built-in electric field directed from inside to the interfacial of WO3. Therefore, the valence changes of the WO3 NRs induced by photo-induced excitation may contribute to the heterostructure electron system of Au NP/WO3 NRs and enhance the NLO effect.
4. Conclusions
We successfully deposited homogeneous Au NPs onto the surface of WO3 NRs and formed Au NP/WO3 NR heterostructures based on a simple solution method without modifying the surface of individual Au NPs or WO3 NRs. The TEM, XRD, and UV-vis spectroscopy results confirmed the formation of Au NP/WO3 NR heterostructures. XPS measurements indicated that the structure and elemental chemical states of WO3 NRs were retained during the self-assembly and the decoration of the Au NPs on the WO3 NRs via electrostatic interactions. However, the bandgap widened after assembly of the Au NPs owing to the interaction of the dipole between the WO3 NRs and Au NPs moving closer to WO3 under the induction of light. The OL and NLO properties of the resulting Au NP/WO3 NRs heterostructures were studied by the OA Z-scan technique. The introduction of Au NPs strongly influenced the competition of SA and NLS in the WO3 NRs and only RSA behavior was observed. Furthermore, the Au NP/WO3 NR heterostructure offered superior NLO activity compared with that of undecorated WO3 NRs. The main factors contributing to the enhanced NLO effects in the Au NP/WO3 NR heterostructure were the combination of the FCA and NLS of the WO3 NRs and enhanced NLS induced by Au NPs with efficient charge/energy transfer at the Au NP/WO3 interface. The unique structure and interesting electro-optical properties of the Au NP/WO3 NRs heterostructure, makes these conjugates interesting materials for applications in nonlinear optics.
Funding
Fujian University of Technology (GY-Z15002); Natural Science Foundation of Fujian Province (2016J01663); the Youth Natural Fund Key Project of Fujian Province (JZ160462); Central guidance for local science and technology development project (2018L3001).
Disclosures
The authors declare no conflicts of interest.
References
1. M. Aslam, I. M. I. Ismail, S. Chandrasekaran, and A. Hameed, “Morphology controlled bulk synthesis of disc-shaped WO3 powder and evaluation of its photocatalytic activity for the degradation of phenols,” J. Hazard. Mater. 276, 120–128 (2014). [CrossRef]
2. D. B. Hernandez-Uresti, D. Sánchez-Martínez, A. M. D. L. Cruz, A. Sepúlveda-Guzmán, S. Sepúlveda-Guzmán, and L. M. Torres-Martínez, “Characterization and photocatalytic properties of hexagonal and monoclinic WO3 prepared via microwave-assisted hydrothermal synthesis,” Ceram. Int. 40(3), 4767–4775 (2014). [CrossRef]
3. Y. Lu, J. Zhang, F. F. Wang, X. B. Chen, Z. C. Feng, and C. Li, “K2SO4-assisted hexagonal/monoclinic WO3 phase junction for efficient photocatalytic degradation of RhB,” ACS Appl. Energy Mater. 1(5), 2067–2077 (2018). [CrossRef]
4. J. Fu, Q. Xu, J. Low, C. Jiang, and J. Yu, “Ultrathin 2D/2D WO3/g-C3N4 step-scheme H2-production photocatalyst,” Appl. Catal., B 243, 556–565 (2019). [CrossRef]
5. L. Kang, X. Y. Liu, A. Wang, L. Li, Y. Ren, X. Li, X. Pan, Y. Li, X. Zong, H. Liu, A. I. Frenkel, and T. Zhang, “Photo-thermo Catalytic Oxidation over a TiO2-WO3-Supported Platinum Catalyst,” Angew. Chem. Int. Ed. (2020).
6. L. Liu, X. Zhang, L. Yang, L. Ren, D. Wang, and J. Ye, “Metal nanoparticles induced photocatalysis,” Natl. Sci. Rev. 4(5), 761–780 (2017). [CrossRef]
7. N. Nwaji and T. J. Lumin, “Nanosecond optical nonlinearities in low symmetry phthalocyanine nanoconjugates studied using the Z-scan technique,” J. Lumin. 192, 1167–1179 (2017). [CrossRef]
8. A. Tuhl, H. Manaa, S. Makhseed, N. Al-Awadi, J. Mathew, H. M. Ibrahim, T. Nyokong, and H. Behbehani, “Reverse saturation absorption spectra and optical limiting properties of chlorinated tetrasubstituted phthalocyanines containing different metals,” Opt. Mater. 34(11), 1869–1877 (2012). [CrossRef]
9. H. Rath, J. Sankar, V. Prabhuraja, C. Tavaekere, A. Nag, and D. Goswami, “Core-modified expanded porphyrins with large third-order nonlinear optical response,” J. Am. Chem. Soc. 127(33), 11608–11609 (2005). [CrossRef]
10. C. Sauteret, J. P. Hermann, R. Frey, J. Ducuing, R. H. Baughman, and R. R. Chance, “Optical nonlinearities in one-dimensional-conjugated polymer crystals,” Phys. Rev. Lett. 36(19), 1162 (1976). [CrossRef]
11. G. Brusatin and R. Signorini, “Linear and nonlinear optical properties of fullerenes in solid state materials,” J. Mater. Chem. 12(7), 1964–1977 (2002). [CrossRef]
12. I. M. Kislyakov and C. S. Yelleswarapu, “Nonlinear scattering studies of carbon black suspensions using photoacoustic Z-scan technique,” Appl. Phys. Lett. 103(15), 151104 (2013). [CrossRef]
13. S. Ghosh, S. M. Bachilo, and R. B. Weisman, “Advanced sorting of single-walled carbon nanotubes by nonlinear density-gradient ultracentrifugation,” Nat. Nanotechnol. 5(6), 443–450 (2010). [CrossRef]
14. D. Z. Tan, Y. Yamada, S. F. Zhou, Y. Shimotsuma, and K. Miura, “Carbon nanodots with strong nonlinear optical response,” Carbon 69, 638–640 (2014). [CrossRef]
15. A. I. Ryasnyanskiy, B. Palpant, S. Debrus, U. Pal, and A. L. Stepanov, “Optical nonlinearities of Au nanoparticles embedded in a zinc oxide matrix,” Opt. Commun. 273(2), 538–543 (2007). [CrossRef]
16. H. H. Mai, V. E. Kaydashev, V. K. Tikhomirov, E. Janssens, M. V. Shestakov, M. Meledina, S. Turner, G. V. Tendeloo, V. V. Moshchalkov, and P. J. Lievens, “Nonlinear optical properties of Ag nanoclusters and nanoparticles dispersed in a glass host,” J. Phys. Chem. C 118(29), 15995–16002 (2014). [CrossRef]
17. B. S. Zhao, B. B. Cao, W. L. Zhou, D. Li, and W. Zhao, “Nonlinear optical transmission of nanographene and its composites,” J. J. Phys. Chem. C 114(29), 12517–12523 (2010). [CrossRef]
18. T. Östreich, K. Schönhammer, and L. J. Sham, “Theory of exciton-exciton correlation in nonlinear optical response,” Phys. Rev. B 58(19), 12920–12936 (1998). [CrossRef]
19. F. Nan, S. Liang, X. L. Liu, X. N. Peng, M. Li, Z. J. Yang, L. Zhou, Z. H. Hao, and Q. Q. Wang, “Sign-reversed and magnitude-enhanced nonlinear absorption of Au-CdS core–shell hetero-nanorods,” Appl. Phys. Lett. 102(16), 163112 (2013). [CrossRef]
20. Q. Y. Ouyang, H. L. Yu, X. Zheng, Y. Zhang, C. Y. Li, L. H. Qi, and Y. J. Chen, “Synthesis and enhanced nonlinear optical properties of graphene/CdS organic glass,” Appl. Phys. Lett. 102(3), 031912 (2013). [CrossRef]
21. A. Wang, Y. Wang, Z. P. Huang, F. Zhou, J. B. Song, Y. L. Song, L. L. Long, M. P. Cifuentes, M. G. Humphrey, L. Shao, J. D. Zhang, and C. Zhang, “Covalent functionalization of reduced graphene oxide with porphyrin by means of diazonium chemistry for nonlinear optical performance,” Sci. Rep. 6(1), 23325–23334 (2016). [CrossRef]
22. M. H. Mezher, A. Nady, R. Penny, W. Y. Chong, and R. Zakaria, “Z-scan studies of the nonlinear optical properties of gold nanoparticles prepared by electron beam deposition,” Appl. Opt. 54(33), 9703 (2015). [CrossRef]
23. Q. Jing and Z. Chan, “Construction of graphene -Au nanocomposite system and its optical limiting effect,” Infrared Laser Eng. 44(9), 2757–2760 (2015).
24. D. D. Huang, C. Zheng, L. Huang, X. K. Wu, and L. Chen, “Linear and nonlinear optical proprties of ultrafine WO3 nanorods,” Optik 156, 994–998 (2018). [CrossRef]
25. D. D. Huang, C. Zheng, W. Li, Q. H. Guo, and W. Z. Chen, “Morphology related nonlinear optical response of tungsten trioxide nanostructures to nanosecond laser pulses,” Opt. Mater. 88, 451–457 (2019). [CrossRef]
26. M. Szabó, P. Pusztai, A.-Ri. Leino, K. Kordás, Z. Kónya, and Á. Kukovecz, “Synthesis and characterization of WO3 nanowires and metal nanoparticle-WO3 nanowire composites,” J. Mol. Struct. 1044, 99–103 (2013). [CrossRef]
27. F. Xu, Y. Yao, D. Bai, R. Xu, J. Mei, D. Wu, Z. Gao, and K. Jiang, “Au nanoparticle decorated WO3 photoelectrode for enhanced photoelectrochemical properties,” RSC Adv. 5(74), 60339–60344 (2015). [CrossRef]
28. J. R. G. Navarro, A. Mayence, J. Andrade, F. Lerouge, F. Chaput, P. Oleynikov, L. Bergström, S. Parola, and A. Pawlicka, “WO3 Nanorods Created by Self-Assembly of Highly Crystalline Nanowires under Hydrothermal Conditions,” Langmuir 30(34), 10487–10492 (2014). [CrossRef]
29. S. Vallejos, T. Stoycheva, P. Umek, C. Navio, R. Snyders, C. Bittencourt, E. Llobet, C. Blackman, S. Monizb, and X. Correig, “Au nanoparticle-functionalised WO3 nanoneedles and their application in high sensitivity gas sensor devices,” Chem. Commun. 47(1), 565–567 (2011). [CrossRef]
30. Q. Xiang, G. F. Meng, H. B. Zhao, Y. Zhang, H. Li, and W. J. Ma, “Au Nanoparticle Modifified WO3 Nanorods with Their Enhanced Properties for Photocatalysis and Gas Sensing,” J. J. Phys. Chem. C 114(5), 2049–2055 (2010). [CrossRef]
31. J. Wang, E. Khoo, P. S. Lee, and J. Ma, “Controlled Synthesis of WO3 Nanorods and Their Electrochromic Properties in H2SO4 Electrolyte,” J. J. Phys. Chem. C 113(22), 9655–9658 (2009). [CrossRef]
32. Y.-C. Yeh, B. Creran, and V. M. Rotello, “Gold nanoparticles: preparation, properties, and applications in bionanotechnology ,” Nanoscale 4(6), 1871–1880 (2012). [CrossRef]
33. M. Sheik-Bahae, A. A. Said, D. J. Hagan, M. J. Soileau, and E. V. Stryland, “Simple analysis and geometric optimization of a passive optical limiter based on internal self-action,” IEEE J. Quantum. Elect. 1105(12), 146–153 (1989).
34. M. Bortoluzzi, C. Evangelisti, F. Marchetti, G. Pampaloni, F. Piccinellid, and S. Zacchini, “Synthesis of a highly reactive form of WO2Cl2, its conversion into nanocrystalline mono-hydrated WO3 and coordination compounds with tetramethylurea,” Dalton Trans. 45(39), 15342–15349 (2016). [CrossRef]
35. M. Sun, N. Xu, Y. W. Cao, J. N. Yao, and E. G. Wang, “Nanocrystalline tungsten oxide thin film: preparation, microstructure, and photochromic behavior,” J. Mater. Res. 15(4), 927–933 (2000). [CrossRef]
36. S. Farsinezhad, S. P. Banerjee, B. B. Rajeeva, B. D. Wiltshire, H. Sharma, A. Sura, A. Mohammadpour, P. Kar, R. Fedosejevs, and K. Shankar, “Reduced ensemble plasmon line widths and enhanced two-photon luminescence in anodically formed high surface area Au-TiO2 3D nanocomposites,” ACS Appl. Mater. Interfaces 9(1), 740–749 (2017). [CrossRef]
37. Z. J. Lin, X. H. Wang, J. Liu, Z. Y. Tian, L. C. Dai, B. B. He, C. Han, Y. G. Wu, Z. G. Zeng, and Z. Y. Hu, “On the role of localized surface plasmon resonance in UV-Vis light irradiated Au/TiO2 photocatalysis systems: pros and cons,” Nanoscale 7(9), 4114–4123 (2015). [CrossRef]
38. J. Y. He, J. X. Lu, N. Dai, and D. M. Zhu, “Surface plasma resonance spectra of Au nanoparticles formed from dewetted thin films,” J. Mater. Sci. 47(2), 668–676 (2012). [CrossRef]
39. X. D. Lin, V. Uzayisenga, J. F. Li, P. P. Fang, D. Y. Wu, B. Ren, and Z. Q. Tian, “Synthesis of ultrathin and compact Au@MnO2 nanoparticles for shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS),” J. Raman Spectrosc. 43(1), 40–45 (2012). [CrossRef]
40. V. Amendola, R. Pilot, M. Frasconi, O. M. Maragò, and M. A. Iatì, “Surface plasmon resonance in gold nanoparticles: a review,” J. Phys.: Condens. Matter 29(20), 203002 (2017). [CrossRef]
41. J. Zhang, Z. Liu, and Z. Liu, “Novel WO3/Sb2S3 Heterojunction Photocatalyst Based on WO3 of Different Morphologies for Enhanced Efficiency in Photoelectrochemical Water Splitting,” ACS Appl. Mater. Interfaces 8(15), 9684–9691 (2016). [CrossRef]
42. J. Wang, Y. Hernandez, M. Lotya, J. Coleman, and W. J. Blau, “Broadband nonlinear optical response of graphene dispersions,” Adv. Mater. 21(24), 2430–2435 (2009). [CrossRef]
43. J. W. Huang, N. N. Dong, S. F. Zhang, Z. Y. Sun, W. H. Zhang, and J. Wang, “Nonlinear absorption induced transparency and optical limiting of black phosphorus nanosheets,” ACS Photonics 4(12), 3063–3070 (2017). [CrossRef]
44. M. S. Neo, N. Venkatram, G. S. Li, W. S. Chin, and W. Ji, “Synthesis of PbS/CdS core-shell QDs and their nonlinear optical properties,” J. Phys. Chem. C 114(42), 18037–18044 (2010). [CrossRef]
45. N. Dhasmana, D. Fadil, A. B. Kaul, and J. Thomas, “Investigation of nonlinear optical properties of exfoliated MoS2 using photoacoustic Zscan,” MRS Adv. 1(47), 3215–3221 (2016). [CrossRef]
46. V. Liberman, M. Sworin, R. P. Kingsborough, and M. Rothschild, “Nonlinear bleaching, absorption, and scattering of 532-nm-irradiated plasmonic nanoparticles,” J. Appl. Phys. 113(5), 053107 (2013). [CrossRef]
47. K. S. Krishna, C. S. S. Sandeep, R. Philip, and E. Muthusamy, “Mixing does the magic: a rapid synthesis of high surface area noble metal nanosponges showing broadband nonlinear optical response,” ACS Nano 4(5), 2681–2688 (2010). [CrossRef]
48. C. Zheng, W. Z. Chen, Y. Y. Huang, X. Q. Xiao, and X. Y. Ye, “Graphene oxide-noble metal (Au, Pt, and Pd) nanoparticle composites as optical limiters,” RSC Adv. 4(75), 39697–39703 (2014). [CrossRef]
49. V. Sreeramulu, K. K. Haldar, A. Patra, and D. N. Rao, “Nonlinear optical switching and enhanced nonlinear optical response of Au-CdSe heteronanostructures,” J. Phys. Chem. C 118(51), 30333–30341 (2014). [CrossRef]
50. Y. X. Li, N. N. Dong, S. F. Zhang, X. Y. Zhang, Y. Y. Feng, K. P. Wang, L. Zhang, and J. Wang, “Giant two-photon absorption in monolayer MoS2,” Laser Photonics Rev. 9(5), 427–434 (2015). [CrossRef]
51. S. B. Lu, L. L. Miao, Z. N. Guo, X. Qi, C. J. Zhao, H. Zhang, S. C. Wen, D. Y. Tang, and D. Y. Fan, “Broadband nonlinear optical response in multilayer black phosphorus: an emerging infrared and mid-infrared optical material,” Opt. Express 23(9), 11183–11194 (2015). [CrossRef]
