Mesoporous Ag@WO3 core–shell, an investigation at different concentrated environment employing laser ablation in liquid

In this study, silver-tungsten oxide core–shell nanoparticles (Ag–WO3 NPs) were synthesized by pulsed laser ablation in liquid employing a (1.06 µm) Q-switched Nd:YAG laser, at different Ag colloidal concentration environment (different core concentration). The produced Ag–WO3 core–shell NPs were subjected to characterization using UV–visible spectrophotometry, X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive spectroscopy, electrical analysis, and photoluminescence PL. The UV–visible spectra exhibited distinct absorption peaks at around 200 and 405 nm, which attributed to the occurrence of surface Plasmon resonance of Ag NPs and WO3 NPs, respectively. The absorbance values of the Ag–WO3 core–shell NPs increased as the core concentrations rose, while the band gap decreased by 2.73–2.5 eV, The (PL) results exhibited prominent peaks with a central wavelength of 456, 458, 458, 464, and 466 nm. Additionally, the PL intensity of the Ag–WO3-NP samples increased proportionally with the concentration of the core. Furthermore, the redshift seen at the peak of the PL emission band may be attributed to the quantum confinement effect. EDX analysis can verify the creation process of the Ag–WO3 core–shell nanostructure. XRD analysis confirms the presence of Ag and WO3 (NPs). The TEM images provided a good visualization of the core-spherical shell structure of the Ag–WO3 core–shell NPs. The average size of the particles ranged from 30.5 to 89 (nm). The electrical characteristics showed an increase in electrical conductivity from (5.89 × 10−4) (Ω cm)−1 to (9.91 × 10−4) (Ω cm)−1, with a drop in average activation energy values of (0.155 eV) and (0.084 eV) at a concentration of 1.6 μg/mL of silver.


Experimental work
We have synthesized Ag-WO 3 core-shell NPs by a two-step process of laser ablation in water.Initially, a 1064 nm Nd:YAG pulsed laser beam was used to ablate a square silver target plate with dimensions of (0.8*0.8 mm) and high purity of (99.9%) that was submerged in 3 ml of distilled water inside a glass container without the use of any chemical additives.A repetition rate of 1 Hz, 10 focal length of the lens and a pulse width of 15 ns were used.The silver target was cleaned before immersion and irradiation by dipping it in acetone and washing it in pure water.The laser ablation procedure was conducted on the Ag target using a laser fluence of 6.12 J/cm 2 and number of laser pulses (200, 250, 300, 350, and 400).The experiment was conducted at different concentrations of silver.Resulting in the production of nanoparticles in the form of a suspension (0.36, 0.76, 1.2, 1.6, and 1.96 μg/ mL).Step two, the silver target was subsequently substituted for a tungsten target in the solution, to produce the Ag-WO 3 colloidal core-shell.Using 1064 nm, 1 Hz, 10 focal length lens with a constant laser fluence of 76.34 J/ cm 2 laser pulses (300), respectively.Figure 1 illustrates the schematic diagram depicting the process of forming Ag-WO 3 core-Porous shell NPs by laser ablation in water (Table 1).Figure 2 depicts an image of newly formed colloidal Ag-WO 3 core-shell NPs.It is evident that increasing the number of laser pulses caused an increase in core concentrations and change in the color of the solution, transitioning from light yellow to deep yellow.This change in color indicates a variation in particle size according to the number of laser pulses.The concentrations of the Ag colloidal nanoparticles were determined by estimating the weight of the Ag target before and after ablation by laser.The concentration was estimated as a function of the laser pulses number using a five digit (digital scale) precision weighing instrument that can measure weight to an accuracy of 0.00001 g.The following formulae were used.Five distinct concentrations of (Ag) nanoparticles where used [95][96][97] .
(1) �M = (m 1 − m 2 ) µg  where: m 1 and m 2 denote the target's mass prior to and subsequent to ablation, respectively.The formula for calculating the concentration is as follows [98][99][100] : where: V stands for the liquid's amount.
In order to study the structural characteristics of Ag-WO 3 core-shell NPs deposited on silicon substrate, XRD measurement was used (XRD-6000, Shimadzu, X-ray diffractometer).In this work, all samples were processed using the FE-SEM (ARYA Electron Optic) equipment, which featured an energy dispersive X-ray (EDX), to accomplish the advantages indicated before.The TEM (type CM10 pw6020, Philips-Germany) was used to analyze the size and form of Ag-WO 3 core-shell NPs.Using a UV-Vis double beam spectrophotometer (Shimadzu UV-1800), the optical absorbance of the colloidal nanoparticles solution was documented.We measured the resistance (R) values of the Ag-WO 3 core-shell samples using a Kiethly electrometer and an excitation wavelength of 325 nm, as part of the Pl analysis.

Results and discussion
The optical properties and the energy gap of the Ag-WO 3 core-shell NPs were determined by means of ultraviolet spectroscopy.The absorption spectrum of Ag-WO 3 core-shell NPs samples was obtained in Fig. 3 using a fluence of 76.43 J/cm 2 , 400 laser pulses, and a wavelength of 1064 nm.The samples were prepared with varying concentrations of silver (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL) and a constant fluence of 6.12 J/cm 2 .Figure 3 shows the optical absorption spectra of the Ag-WO 3 core-shell nanoparticles that were produced by employing pulse    101,102 .The intensities of all distinctive peaks, which were seen in the Ag-WO 3 NPs samples, exhibit an increase when the amounts of silver particles are increased.The observed peaks ascribed to the presence of Ag-NPs.Furthermore, it was observed that there were minor absorption peaks at around 311, 312, 313, 314, and 315 nm, indicating the presence of WO 3 nanoparticles for the samples with concentrations of 0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL, respectively.After the incorporation of Ag NPs into WO 3 NPs, the absorption band edge of bare WO 3 is redshifted (toward a longer wavelength).The presence of both peaks provided evidence for the creation of the Ag-WO 3 core shell NPs [103][104][105] .This result gives a clear indication of the effect of Ag nanoparticles due to the effect of SPR coming from Ag (core) nanoparticles.Consequently, the bandgap energy of the material decreased as a result of the redshifted of absorption band edges produced by the incorporations of Ag NPs.The band gaps of the samples at different concentrations of silver (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL) are presented in Fig. www.nature.com/scientificreports/absorption band gap follows a power law when incident photon energy is greater than the band gap and above the exponential [106][107][108][109] : where E g is the optical bandgap, α is the absorption coefficient, n is an exponent, β is the edge with parameter and hʋ is the incident photon energy.It can be observed from Fig. 4 that the optical band gap energy decreases to 2.75, 2.73, 2.62, 2.55, and 2.5 eV as the concentration of Ag increases.These values are very close to those stated in a previous study 110 , the band gap tailoring in core-shell NPs was attributed to its shape and the quantum confinement effect 111,112 .The interfaces of Ag-WO 3 NPs have a significant impact on the processes of charge transfer and separation 113,114 .This is attributed to the presence of Ag NPs, which function as localized surface plasmons (LSP).This process credits the combination of electromagnetic waves to the oscillations of electrons 115 .This procedure enables the reduction of the band gap in the core cell structure.As a result, the adjusted band gap allows for increased interaction between visible light and the core-shell compared to the regulated WO 3 .The core-shell has promising optical and electrical properties because to its ability to fine-tune the band gap features 116,117 .Consequently, the bandgap energy of the material decreased as a result of the redshifted of absorption band edges produced by the incorporations of Ag NPs.
The flat peak in surface plasmon resonance typically refers to the collective oscillation of electrons at the interface between a metal and a dielectric material when excited by incident light.The resonance condition occurs when the momentum of incident photons matches the momentum of the surface plasmons.
The SPR response may not exhibit a sharp peak, making it challenging to identify the resonance position.This could be due to various factors, such as broadening of the SPR peak due to particle size distribution, polydispersity, or other experimental conditions.
Figure 5 displays the photoluminescence (PL) spectrum of core-shell NPs consisting of silver (Ag) and tungsten oxide (WO 3 ).The photoluminescence (PL) studies revealed increased intensities in all the Ag-WO 3 samples created using varying amounts of silver (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL).The spectrum shows excitation bands located at 456, 458, 458, 464, and 466 nm, which correspond to energy gaps of about 2.71, 2.7, 2.7, 2.67, and 2.66 eV, respectively.The energy gap value approximated using PL data is marginally greater in magnitude compared to what was ascertained using UV-vis data 118 .Further, it differs from the energy gap of pure WO 3 .The PL intensities of the Ag-WO 3 NPs peaks exhibited a significant increase compared to the PL peak of WO3.Additionally, these peaks show a minor shift, which may be ascribed to the enhanced photoluminescence seen in the NCs structure.This improvement is due to the incorporation of Ag-NPs.These results are consistent with UV-visible results.Ag-WO 3 CS-NPs' emission energy and PL emission wavelength are displayed in Table 2 as a function of Ag concentrations.
Figure 6 displays the diffraction peaks corresponding to the core-shell samples.These peaks were generated using a fluence of 76.34 J/cm 2 and 400 laser pulses.The figure also displays a wavelength of 1064 nm with different concentrations of silver (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL).This figure further displays the prominent peaks that can be ascribed to hexagonal WO www.nature.com/scientificreports/nanoparticles, according to pdf number 870720.Due to the relatively high diameter of the core, significant diffraction peaks are generated.XRD identification of the core-shell configuration [119][120][121] showed that these results are in line with the study 122 .When exposed to X-ray radiation, the greater concentrations of aggregated silver nanoparticles resulted in a greater degree of reflection 123,124 .For the WO 3 sample, two peaks appeared at 2θ: 28.92° and 58.84°.While 28.92° corresponds to the (122) plane according to (JCPDS # 201323) triclinic WO 3 phase structure, 58.84° corresponds to the (220) plane hexagonal WO3 phase structure.In addition, the XRD patterns of Ag-WO 3 core/shell NPs exhibit a marginally greater intensity than those of shell NPs (Fig. 6).This may be the result of incorporation Ag core NPs causing an increase in particle size and crystallinity.Table 3 illustrates   where λ is the X-ray wavelength, β is the full width at the half maximum, k is the constant 0.89 < k < 1 change with Miller indices and crystallite shape, but is frequently close to 0.94, and θ is the diffraction angle [128][129][130] .The dislocation density was calculated using the formula (3) and the Microstrains were determined using Eq. ( 5).The table demonstrates a small augmentation in crystalline size when the concentration of silver particles is increased.Conversely, the dislocation density and Microstrains exhibited a reduction due to the aforementioned factor.The dislocation density in lines/m2 can be determined by employing the equation [131][132][133] : (d) Lattice strain or Microstrains (η).
The lattice strain is caused by lattice imperfections such dislocations, vacancies, interstitials, and substitutional.These defects cause the atoms to be displaced from their original places in the crystal structure, as a result, the lattice plane d-spacing may be varied.Microstrains will occur during the production of the thin film.This strain may be estimated using the equation below [134][135][136][137] .
The film deposition circumstances will have an impact on structural factors such grain size, crystallinity, and crystal structure.
Figure 7 shows the FESEM images of Ag-WO 3 core-shell NPs samples as function of Ag-NPs concentrations.The Ag-NPs concentrations in the samples varied from 0.36 to 1.96 μg/mL.As seen in Fig. 7.The Ag-WO 3 core-shell structure displays a noticeable augmentation in particle size as the concentration of silver increases.Furthermore, the particles have a well-defined spherical morphology that aligns precisely with the findings of the study [138][139][140] .The shape, size and agglomerated particles of nanoparticles are determined by the concentrations of Ag-NPs, as seen in Fig. 7. Specifically, the size of WO 3 outer shell nanoparticles rises proportionally with higher concentrations of Ag-NPs.Additionally, the size of the Ag core also increases in tandem with the Ag-NPs concentrations.This aligns with the findings reported in a prior investigation 141 .The (SEM) images reveal the presence of core/porous-shell structures in the microspheres.The pores inside the shell are densely and uniformly distributed, as seen from the distinct color difference.The shell exhibited a high degree of porosity and was characterized by its minimal thickness.The pictures given below depict various morphological characteristics of both the core and the shell layer.The outer layer, WO 3 , has a higher degree of surface roughness in comparison to the inner layer, Ag.  www.nature.com/scientificreports/Chopra 142,143 .The presence of tungsten (W), oxygen (O), and silver (Ag) in the core shell system was by this figure.Supporting the EDX results of Ag-WO 3 core-shell NPs, Fig. 8a,e illustrate the mapping outcomes of Ag, WO 3 , and O, respectively.Ag NPs showed high intensity in EDX due to the increase laser pulses and increasing Ag concentration.Table 4 illustrates the weight percentages of the elements contained in the samples, as well as the stoichiometries of WO 3 .The stoichiometric ratio and weight percentage appear to exhibit an upward trend when the concentration of silver nanoparticles increases.Figure 9a-e depicts transmission electron microscopy (TEM) pictures of Ag-WO 3 core-shell NPs samples, illustrating the relationship between the concentration of silver and the observed characteristics.The form and size of NPs are influenced by the concentration of silver, as depicted in the figure.The size of the silver core exhibits a positive correlation with the concentration of silver.In this study, the transmission electron microscopy (TEM) images demonstrate the production of Ag nanoparticles of varying sizes, as well as the presence of some  www.nature.com/scientificreports/monodisperse Ag particles that are enveloped by a WO3 shell.Additionally, the presence of both aggregated and agglomerated silver particles was noted as increasing the concentration of silver.This matches what was stated in the results of the FE-SEM.Besides, pictures provide confirmation that the central Ag particles possess spherical morphologies.Table 5 presents data on the average particle size of Ag-WO 3 core-shell and Ag core.
The results indicate that when the concentration of silver nanoparticles was raised from 0.36 to 1.96 μg/mL, the particle size of the Ag-WO 3 core-shell climbed from 30.5 to 89 nm, while the size of the Ag core increased from 28 to 81 nm.Thus, the manipulation of core size is a viable approach for modulating the overall dimensions of metal-oxide nanoparticles, hence exerting a significant influence on the characteristics of the resulting nanocomposites.This is consistent with what was reported in these studies [144][145][146] .Electrical measurements of WO 3 NPs and Ag-WO 3 core-shell samples at various Ag concentrations (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL) were studied in order to achieve the optimum sample conductivity (lowest resistance).The relation of resistance (R) as a function of temperature (T) was determined, as shown in Fig. 10.The reported resistance values of the Ag-WO 3 core-shell were high at low temperatures and gradually decreased as the temperature of each sample increased.This is the typical situation for semiconducting materials, as they possess a temperature coefficient that is negative (wherein the resistance decreases as the temperature increases).At thermal equilibrium conditions (room temperature), the initial reading was obtained for each sample to evaluate the resistivity of Ag-WO 3 manufactured at different Ag concentrations.As shown in this figure, the sample prepared with a concentration of 1.6 g/mL of silver yielded the lowest resistance of approximately 47.1 MΩ at 150 °C.The electrical resistance of the created samples was reduced on a regular basis as compared to the WO3 sample.Because (Ag) nanostructures are metals characterized by elevated levels of free carriers, the conductivity (σ) was raised.
Figure 11 shows the logarithmic conductivities (lnσ) of the carrier's curve as a function of reciprocal temperature (1000/T) for WO 3 and Ag-WO 3 -produced samples with various Ag concentrations (0.36, 0.76, 1.2, 1.6, and 1.96 μg/mL).The corresponding electrical conductivities (σ) of the samples were determined using the equation [147][148][149] : The equation shows the resistivity (ρ) in (Ω cm).Based on the figure, the electrical conductivity of the Ag-WO 3 core-shell increases as the concentration of silver increases.This rise in conductivity was due to an  www.nature.com/scientificreports/increase in carrier concentration and mobility 150 , whereas the electrical conductivity (σ) values for the sample prepared with 1.96 μg/mL of silver were decreased.This result is due to the aggregation of (Ag) nanostructures in extremely concentrations, which reduced the carrier mobility.The drop in mobility was directly proportional to the decrease in electrical conductivities [151][152][153] .This is consistent with what was stated in the previously work [154][155][156] .Table 6 show the activation energy of Ag-WO 3 with different Ag concentrations for current work and previous work 156,157 .
The Arrhenius relation was used to calculate the activation energy [158][159][160] : where the conductivities of carriers are denoted by the symbol σ, σ° represents the temperature of the independent portion conductivity, Ea denotes the necessary amount of energy for activation, then tn, constant of Boltzman (K), and the temperature, denoted by the letter T, is expressed as a value in Kelvin (k) unit.The activation energy values as a function of Ag concentration in the Ag-WO 3 samples are listed in Table 6.
The figure of merit (F.O.M.) of Ag-WO 3 core-shell samples created with various silver concentrations is displayed in Fig. 12.They were computed using Eq. ( 9) which was provided by Iles and Soclof [161][162][163] : The absorption coefficient is represented by the symbol (α) and is the conductivity of electricity.The requirement for calculating the F.O.M. was to realize the optimal electrical conductivity (σ) as a function of the absorption coefficient (α) of Ag-WO 3 that was made by varying the amount of Ag (0.36, 0.76, 1.2, 1.6, and 1.96 μg/ mL).The concentration of 1.6 μg/mL of silver used in the preparation of the Ag-WO 3 core shell is considered a merit figure, as the incorporation of silver (Ag) was intended to improve the semiconductor material's properties.

Conclusion
We are successively synthesizing a Novel Ag-WO 3 core-shell nanostructure using a two-step laser ablation process in deionized water.The impact of varying core concentrations (Ag-NPs) on the structural, electrical, morphological, and optical characteristics of Ag-WO 3 core-shell NPs have been investigated.XRD analysis has

Figure 1 .
Figure 1.Schematic diagram illustrating the process of forming Ag-WO 3 core-Porous shell NPs using pulsed laser ablation in water.

Figure 3 .
Figure 3. Absorption spectrum of Ag-WO 3 nanoparticles with various Ag nanoparticle concentrations.

Figure 5 .
Figure 5. PL spectrum of Ag-WO 3 nanoparticles with various Ag nanoparticle concentrations.

Figure 10 .
Figure 10.Temperature-dependent resistance of WO 3 NPs and Ag-WO 3 core-shell synthesized with various Ag concentrations.

Table 1 .
Explained different concentrations of colloidal silver versus number of laser pulses.

Table 2 .
The Ag-WO 3 core-shell NPs' emission energy and PL emission wavelength as a function of Ag concentrations.

Table 4 .
Ag-WO 3 -NPs stoichiometry and weight percent of elements.

Table 5 .
Presents the average particle size of Ag-WO 3 -NPs.

Table 6 .
Arrhenius plot curve of ln (σ) versus (1000/T) of WO 3 NPs and Ag-WO 3 core-shell synthesized with various Ag concentrations.The activation energy of Ag-WO 3 with different Ag concentrations for current work and previous work.