Simple two-step synthesis of Ag/ZnO nanoparticles with enhanced photocatalytic response

This study presents a simple two-step synthesis method for the fabrication of Ag/ZnO nanocomposites to improve the photocatalytic response of ZnO. The synthesis involves ZnO nanoparticles that were fabricated from the thermal decomposition of commercial zinc acetate. In order to produce Ag/ZnO nanoparticles in a simple two-step process, ZnO nanoparticles were mixed with Ag nanoparticle suspensions previously obtained by the laser ablation of solids in liquids technique at three different fluences. Structural characterization of ZnO powders revealed the presence of single phase wurtzite ZnO nanoparticles with crystal sizes of 20 nm. On the other hand, XRD patterns for a composite sample revealed the presence of signals associated to both ZnO and Ag suggesting that silver nanoparticles were attached to the ZnO particles surface. Optical characterization of the ZnO powders, carried out by UV–vis spectroscopy, showed a strong absorption band centered at 380 nm, which is associated to excitonic transitions in ZnO nanoparticles, whilst absorption measurements of silver nanoparticles colloids revealed the presence of a strong band centered near 412 nm. This band shifts to shorter wavelengths with increasing fluence from 2.6 to 6.2 J cm−2, indicating changes in nanoparticles size. Photocatalytic degradation tests of methylene blue under UV irradiation were carried out using pure ZnO, Ag colloids and Ag/ZnO nanoparticles. After the first 30 min of irradiation, it was observed that the silver nanoparticles reached degradation percentages of 16, 22 and 29% for samples synthesized at 2.6, 4.2 and 6.2 J cm−2, respectively. Meanwhile the ZnO sample reached a value of 13% after 30 min. Regarding the Ag/ZnO composite sample, the percentage of degradation after 30 min was 36%, demonstrating a considerable enhanced photocatalytic activity as compared to ZnO. After 24 h irradiation, Ag/ZnO degraded 95% of the methylene blue solution. It was observed that decorating ZnO with laser produced silver nanoparticles accelerates the photocatalytic response of ZnO by enhancing the activity at short times.


Introduction
Zinc oxide (ZnO) has been extensively studied in the last decades mainly for its excellent optoelectronic properties such as high exciton binding energy [1,2], high electrical conduction [3], ultraviolet light emissions [4,5], among others.Furthermore, as it happens with different materials, reducing size to produce ZnO nanoparticles (NPs), has shown to induce enhancement of the material properties [4] or the appearance of new interesting properties caused by quantum effects induced by size reduction [6].
Among all the possible applications of ZnO NPs, the photocatalytic degradation of organic dyes has become one of the most important due to the water pollution problems generated mainly by textile industries [7].
Moreover, it has been found that organic dyes can produce several toxic effects on living beings [8], making their removal from water an urgent need.One of the disadvantages of ZnO is that it has electron-hole recombination rates that are fast enough to prevent the oxidation-reduction processes used for degradation of dyes and, additionally, its wide band gap (3.3 eV) makes that activation occurs using UV radiation, limiting its application using sunlight.
In order to overcome the disadvantages of ZnO as a photocatalytic material, different approaches have been used, such as morphology modifications, size reduction, doping, decorating with metal NPs, among others [9][10][11][12].Taking these in consideration, decorating with NPs has become an excellent alternative since the interactions at the ZnO-metal interface can help to increase electron-hole recombination time, together with charge injection of photogenerated electrons from absorption processes in the semiconductor.In particular, using plasmonic nanostructures (Ag or Au NPs) has shown to be promising for the activation of the photocatalytic response in the visible range of the electromagnetic spectrum [12,13] and for charge transfer.On the other hand, reports using Zn-Mn bimetallic nanostructures under solar irradiation, have shown efficiencies higher than 65% for different dyes degradation [14].Based on this, the photocatalytic response enhancement on these systems is attributable to the joint properties of the semiconducting NPs and the plasmonic material.
Several techniques have been reported for the fabrication of nanocomposites.However, some processes may take too long and require several steps to achieve the final nanocomposite structures [15], very high voltages [1], high temperatures [15,16], the use of expensive precursors or several steps [17], sophisticated and exclusive synthesis equipment [18], etc.Another problem is the generation of hazardous waste from some chemical techniques that need several washes to clean the nanocomposite [19].
In this work, Ag/ZnO nanocomposites were synthesized by a simple and novel two-step process combining laser ablation of solids in liquids for silver NPs fabrication, and thermal decomposition of zinc acetate for ZnO NPs synthesis.The effect as photocatalytic agents for degradation of methylene blue under UV irradiation was evaluated, resulting in an increase of the activity for the Ag/ZnO sample as compared to the pure Ag and ZnO NPs.Although both the synthesis of ZnO by thermal decomposition of zinc acetate and Ag NPs by laser ablation of solids in liquids are well known techniques, the combination of the processes to fabricate in a simple two-step way Ag/ZnO nanocomposites with enhanced photocatalytic activity has not been explored.It is important to note that it is not necessary to perform additional treatments such as annealing, functionalizing, photoreduction, etc., nor the use of additional precursors such as silver nitrate to fabricate the composites.Furthermore, the use of sodium citrate on Ag NPs synthesis allows the successful decorating of ZnO showing synergistic effects, which is a known characteristic of composite materials.Sodium citrate has been widely used mainly in the food, personal care, and medicine industries, as a chelating agent, preservative, antioxidant, complexing agent, emulsifier, stabilizer, antimicrobial agent, pH regulator, etc.
With the use of a sodium citrate solution in the synthesis of silver NPs, colloidal stability is favored, the free electrons of the carbonyl group generate electrostatic stabilization in the Ag NPs and it also acts as a coordination agent in compounds with metal atoms that have free orbitals [20].Therefore, this salt improves the adhesion of the Ag NPs on the surface of ZnO NPs, allowing to form a stable nanocomposite.On the other hand, sodium citrate prevents silver from ionizing by keeping the suspension chemistry stable and it also prevents its oxidation.

Experimental details
ZnO powders were obtained by thermal decomposition of zinc acetate (ZnAc) dihydrate (Zn(CH 3 COO) 2 -2H 2 O) from Sigma-Adrich.ZnAc powders were annealed at 500 °C in a conventional furnace during 3 h in air conditions.The resulting powders were structurally characterized by x-ray diffraction using a Panalytical Empyrean diffractometer system.Samples were measured using the Bragg Brentano configuration with a step size of 0.02°and 30 s integration time.
For the synthesis of Ag NPs by laser ablation of solids in liquids, a 99.99% silver target was immersed in a 5 μM aqueous solution of sodium citrate and then ablated with the 1064 nm wavelength of a Quantel brilliant B, Nd:YAG laser.Three different fluence values were used for the ablation process: 2.6, 4.2 and 6.2 J cm −2 .A ZnO colloid, obtained by dispersing 0.01 g of ZnO powder in 30 ml of distilled water was prepared, afterwards 0.5 ml of the ZnO suspension was mixed with 0.5 ml of Ag NPs colloid to obtain Ag/ZnO NPs.The resulting colloidal suspensions along with Ag colloids were optically characterized by means of UV-vis spectroscopy using a Thermo Genesys spectrophotometer in the 300-800 nm range with a step size of 1 nm and using glass cuvettes with 1 cm optical length and 3.5 ml volume.The morphology and size of the Ag and Ag/ZnO NPs were studied by SEM using a Tescan Mira 3 scanning electron microscope at a 20 kV acceleration voltage.
Photocatalytic tests were performed by the degradation of a 5.78 × 10 −5 M methylene blue (MB) solution.For the experiments, 1 ml of the colloids was mixed with 1 ml of MB solution.Resulting solutions were let to rest before irradiation to reach adsorption-desorption equilibrium.
For the irradiation process, the UV source was a lights of America brand lamp model F17T8/BLB with 17 W of power and a color temperature of 30000 K, its maximum emission is centered at 360 nm.Samples were irradiated during 24 h, one aliquot of each sample was taken and measured by UV-vis spectroscopy 30 min and 24 h after irradiation began.

Results and discussion
The crystalline structure of the ZnO powder was evaluated by x-ray diffraction (XRD) analysis.The XRD pattern was obtained in a 2θ range between 20°and 80°(figure 1(a)).The pattern shows that NPs crystallized in the wurtzite phase of ZnO according to the JCPDS (36-1451).The crystallite sizes (d, in nm) were calculated from Scherrer's formula:

cos
Where, d is the average grain size, λ is the x-ray wavelength (CuKα), β is the full width at half maximum of the (101) peak and θ is the maximum Bragg diffraction peak (in radians).Data were taken using the diffraction planes (100), (002), (101), (102), (110), ( 103), ( 200), ( 112) and (201).The obtained average grain size was 20.4 ± 3 nm.It is worth noting that no secondary phases nor ZnAc residues were detected in the powders, which means that thermal annealing conditions are good for zinc acetate molecule decomposition, generating the breaking of organic bonds to produce ZnO.The colloidal suspensions of ZnO and Ag NPs were studied by UV-vis spectroscopy within wavelengths ranges of 300-800 nm and 300-600 nm, respectively.Figure 2(a)) shows that the absorption spectrum of ZnO powder suspended in distilled water has a peak at 378 nm, which arises from the presence of pure ZnO NPs [21].The band gap energy of ZnO was determined using the Tauc method and results are shown in figure 2(b)).The band gap value is close to 3.34 eV, which agrees well with the reported value for ZnO nanostructures [21,22].On the other hand, figure 2(c)) shows the UV-vis absorption spectra of Ag NPs synthesized at three different fluences.
The maximum absorption peak appears at wavelengths values between 406 and 412 nm, depending on the fluence.This difference in peak position is related to size changes and it will be discussed later.Furthermore, the absorption bands have different intensities; this behavior is attributed to the NPs concentration in the colloids, the higher the intensity, the higher the NPs concentration.In this sense, the most concentrated sample obtained in our experiments is the one synthesized at 4.2 J cm −2 [23].The absorption bands presented in figure 2(c)) are associated to the surface plasmon resonance absorption (SPR) of silver arising from the presence of silver NPs [24].
Figure 2(d)) shows the dependence of SPR peak position and FWHM on the fluence.Notice that the peak position shifts from 412 to 406 nm as fluence increases, whilst FWHM increases from 57 to 78 nm with fluence.SPR peak position shifts to shorter wavelengths are associated to particle size reduction [25], which means that NPs synthesized with a fluence of 2.6 J cm −2 , are expected to be bigger than those synthesized with 6.2 J cm −2 .On the other hand, an increase in FWHM relates to a widening of size distribution, thus, the effect of increasing fluence for silver NPs fabrication is to decrease particle size, broadening the size distribution.Table 1 shows the peak position and FWHM values for the different fluences.
Note that for the highest fluence value, the position at 406 nm indicates smaller NPs; however, the FWHM is the highest, indicating a broader size dispersion, which means that even though there are more smaller NPs, they are mixed with particles with different sizes in a higher proportion than NPs synthesized with lower fluences.
Considering UV-vis results, the Ag NPs chosen for the fabrication of Ag/ZnO nanocomposites was the colloidal sample synthesized with 4.2 J/cm 2 fluence, as it was the sample with higher concentration with a relative low size dispersion and NPs size.Figure 3 shows scanning electron micrographs of representative Ag and Ag/ZnO NPs. Figure 3(a)) shows that Ag NPs grew in spherical shapes with a wide range of sizes, which is consistent with the existence of size distributions as revealed by UV-vis measurements.The inset of figure 3(a)) shows a histogram in which it can be seen that the main size is 28 nm. Figure 3(b)) corresponds to the Ag/ZnO NPs, it can be seen that the NPs of Ag completely cover the surface of the ZnO particles.This surface coverage of ZnO with Ag NPs observed in figure 3(b)) should have an influence on the composite photocatalytic activity as compared with ZnO.
It is well established that changes in morphology lead to changes in surface area, which is a determinant factor for photocatalysis, however, in the present work, the morphology of the ZnO NPs remain unchanged in ZnO and Ag/ZnO samples.Instead, the surface area of ZnO was modified by adding Ag NPs to ZnO surfaces, which should induce ZnO surface area reduction.Despite this, the combination at the ZnO-Ag interface of different physical processes such as surface plasmon resonance and charge injection could result in the enhancement of the photocatalytic activity of individual ZnO and Ag NPs.
The Ag NPs samples synthesized at different fluences, the ZnO powder and the Ag/ZnO NPs were used in photocatalytic tests by degradation of methylene blue with 5.78 × 10 −5 M concentration.Figure 4 shows the UV-vis spectra of MB measured at 0.5 and 24 h of irradiation with ultraviolet light (maximum at 360 nm).
The intensity of the absorption band is related to the degradation of MB over time.The MB sample after 24 h does not show considerable degradation; whilst the samples of Ag NPs synthesized at different fluences show that, the increase in degradation is related to the increase in fluence.As it was discussed in figure 2 and table 1, three main findings can be mentioned: NPs size decreases with increasing fluence (SPR peak position shifts to shorter wavelengths); FWHM and thus, size dispersion increases with fluence and, there is an important increase of the absorption band maximum when changing from 2.6 to 4.2 J cm −2 followed by a decrease for 6.2 J cm −2 , to approximately the same observed value for the 2.6 J cm −2 sample.In view of the latter, particles size and size dispersion have the highest influence on MB degradation with silver NPs.The degradation mechanism will be discussed below.
The ZnO NPs and Ag decorated ZnO had a better degradation response than silver NPs alone, which was expected since it is known that silver NPs can act as better catalysts when they have a surrounding medium that  allows effective charge transfer to induce degradation of dyes [26].In the present work, ZnO decorated with Ag NPs was the sample that presented the highest MB degradation.
Figure 5 shows the percentage of degradation for each one of the samples in relation to the time that each exposure to UV radiation lasted.In the percentage of degradation of MB it is possible to see that, by itself, it presents negligible degradation compared to the samples.
Regarding degradation with silver NPs, it can be seen that for the first 30 min, degradation percentages increase from 15 to 26% with increasing fluence.As it was mentioned before, increasing fluence causes particles size decrease as revealed by the blueshift of the SPR absorption signals.Proposed mechanisms for MB degradation in literature [27,28] state that absorption of photons by silver NPs induces the production of a free electron plus a silver ion.This charge separation at the NPs surface induces reduction and oxidation processes by producing hydroxyl groups (OH-) bounded to the surface that subsequently generate the hydroxyl radicals needed for the dye molecules oxidation.In view of the latter, in our samples, a blueshift of the SPR absorption band, consequence of size reduction, would increase the charge separation due to the broad nature of the UV spectrum used for degradation, which has its maximum at 360 nm but it has important intensity near 400 nm.As irradiation time increased to 24 h, the increase in degradation percentage for Ag NPs trend was preserved, reaching values of 32, 45 and 60% for fluences of 2.6, 4.2 and 6.2 J cm −2, respectively.
Regarding pure ZnO NPs, note that initially the degradation percentage was 13%, which was lower than for silver NPs.Nevertheless, after 24 h, the percentage reached 90%.It is well established that the degradation mechanism in ZnO consists in the generation of electron-hole pairs at the surface by the absorption of photons with energies near the band gap of ZnO and a subsequent interaction between these electro-hole pairs with the MB molecules.This interaction includes the reduction and oxidation of the dye by electrons promoted to the conduction band and holes generated in the valence band, respectively [29].This need of electron hole pairs, requires recombination times sufficiently long for allowing the interaction with the dye molecules, which can be improved in ZnO by the addition of Ag NPs at the surface generating Ag/ZnO nanocomposites.The composites degradation mechanism will essentially be the same as the one for ZnO, however, there will exist charge (electrons) transfer from the ZnO conduction band to the Fermi energy level of silver, suppressing electron-hole recombination and increasing the available electrons for hydroxyl radicals generation at the composite/dye solution interface and thus, improving the photocatalytic activity [28,29].Furthermore, the SPR absorption from Ag NPs, increases the number of electrons available for reduction at the NPs surface, which further enhances the composites photocatalytic response.For the present work, it was observed that the composite sample reached degradation percentages higher than those presented for ZnO and Ag NPs.Notice that initial degradation of pure ZnO NPs is slower than the degradation induced by all the other samples, however, after 24 h ZnO degrades more efficiently than silver NPs.Meanwhile, the Ag/ZnO sample presents the fastest degradation rate for the first 30 min, reaching a value of near 40%, which can be useful for rapid degradation applications.

Conclusions
The present work shows the successful synthesis of ZnO powders decorated with Ag NPs using a simple two-step method.The photocatalysis tests show that the degradation effect of the ZnO powder is improved for short irradiation times when it is decorated with Ag NPs, and that the fluence used in the synthesis of Ag NPs is closely related to the concentration, particle size and size dispersion of the NPs obtained and therefore with the degradation of MB.It is important to mention that the degradation efficiency depends on the specific type of contaminant and the properties of the materials used.In addition, under certain experimental conditions, the combination of ZnO with Ag NPs can have benefits in terms of synergy and efficiency improvements.

Figure 1 (
b)) shows the XRD pattern of a representative Ag/ZnO composite sample obtained in the 2θ range between 30°and 60°.It can be noticed that together with the signals corresponding to ZnO, there are two small peaks centered at 38 and 44°corresponding to the (111) and (200) planes of cubic silver (JCPDS 87-0597) respectively.The presence of these signals indicates the successful combination of Ag and ZnO NPs.

Figure 1 .
Figure 1.X-Ray diffraction patterns of a) ZnO and b) Ag/ZnO powders.

Figure 2 .
Figure 2. (a) UV-vis absorption spectrum of ZnO, (b) Tauc plot for ZnO, (c) UV-vis absorption spectra for Ag NPs at different fluences and (d) SPR peak position and FWHM as a function of fluence (dashed lines are just a guide to the eye).

Figure 3 .
Figure 3. (a) Ag NPs and its size distribution, (b) ZnO decorated by Ag NPs.

Table 1 .
SPR peak position and FWHM for different fluence.