Plasma-deposited reactive species assisted synthesis of colloidal zinc-oxide nanostructures

A surface-wave microwave discharge is applied to deposit reactive oxygen and nitrogen species (RONS) into the liquid subsequently used as a medium for laser ablation of a Zn metallic target. It is shown that during laser ablation in plasma-treated liquids the H2O2 concentration decreases, while in deionized water (DIW) significant H2O2 is produced. Meanwhile, the pH—initially adjusted by applying reductive metals—increases in the acidic liquids and decreases in the alkaline ones. During months of storage the pH of colloids stabilize around pH 6, which insures the long-term stability of RONS. It is demonstrated that in DIW metallic Zn NPs are created, which gradually oxidize during storage, while in the plasma-treated liquids ZnO NPs are produced with the mean size of 18 nm. In the alkaline plasma-treated liquid the NPs form large aggregates, which slows the dissolution of NPs. In the acidic and neutral solutions besides NPs nanosheets are also formed, which during storage evolve into nanosheet networks as a result of the dissolution of NPs. The band gap of the colloidal ZnO is found to decrease with the formation of aggregates and nanosheet networks. The ZnO NPs ablated in plasma-treated liquids exhibit a high-intensity visible emission covering the green-to-red spectral region. The photoluminescence spectra is dominated by the orange-red emission—previously not detected in the case of laser-ablated ZnO NPs and attributed to the interstitial Zn and oxygen sites—and the yellow emission, which can be attributed to the OH groups on the surface. It is shown that during months of storage, due to the dissolution of NPs and formation of nanosheets, the intensity of the visible emission decreases and shifts to the blue-green spectral region.


Introduction
The production of zinc oxide nanoparticles (NPs) has been extensively studied in the last decades by focusing on different aspects, physical and chemical characteristics, such as band gap, particle size and composition-related photoluminescence (PL).ZnO NPs are found in many application fields, such as opto-electronics [1], photonics [2] and biomedicine.Their main applications include photocatalysis [3,4], photovoltaics [5], transparent electrodes [6], piezoelectric transducers, gas sensors [7], bioimaging probes, drug delivery.Several applications are related to the optical properties of ZnO, which is primarily defined by the direct bandgap, that is ≈ 3.4 eV in the bulk form.The main features are the exciton-related emission in the UV region observable even at room temperature and the PL in the visible range, which can be attributed to the intersticial zinc and oxygen sites, as well as zinc and oxygen vacancies.
The synthesis of ZnO NPs by laser ablation in liquids (LAL) has the great advantage of providing residue-free colloid as compared to the chemical synthesis.An extensive review of the nanostructure's formation by laser ablation of different targets in liquids is given by Yan and Chrisey [8] and Kanitz et al [9].During ablation of a pure Zn target, the liquid environment not only confines the laser-produced species, but also provides reactivity to induce oxidation.Therefore, this process gives rise to three main questions: (i) the oxidation of formed NPs during and after the process, (ii) the growth of particles and (iii) the ablation yield.
In the last decade few studies have been dedicated to the monitoring and controlling the oxidation of Zn NPs produced by pulsed laser ablation of pure Zn metal plate in water [10,11] and ethanol solution [12].The in-situ monitorring of the colloid conducted by Camarda et al [10] through the absorption spectra has indicated that during ablation metallic NPs are created, which are gradually oxidize due to their reaction with the liquid to produce Zn/ZnO core-shell NPs .The complete oxidation of the zinc in the colloid solution prepared with 10 min ablation is observed at 10 min after the end of ablation.Reich et al [11] through the ex-situ analysing of the colloid prepared with 1 min ablation have reported the full oxidation of the NPs to take about 30 min.Through the in-situ monitoring of the PL emission of the colloids Camarda et al [10] have also shown that the earliest superficial oxidation of Zn produces a defect-free ZnO, and the subsequent slower oxidation process creates a defective ZnO material.In order to control the oxidation process Camarda et al [12] have proposed the use of ethanol and have shown, that with addition of ethanol to water the oxidation degree can be decreased, while in pure ethanol metallic Zn NPs are obtained and preserved after the end of ablation process.It has also been shown that ethanol/water ratio also controls the concentration of the oxygen vacancies inside the ZnO NPs.Similar results have been obtained by Zeng et al [13] by using sodium dodecyl sulfate (SDS) aqueous solutions, stating that ZnO-Zn composite NPs are prepared with the ratio of Zn/ZnO depending on the SDS concentration, namely high SDS concentration corresponds to high relative amount of Zn NPs existing as the core in the core/ shell nanostructures, whereas low SDS concentration leads to high ZnO ratio.Furthermore, the decrease of NP size with the increase of SDS has also been demonstrated and attributed to two different effects, as follows.The higher SDS concentration leads to the higher surface coverage of SDS molecules on the particle surface and hence, to their stronger spatial separation, while the increase of surface charges lead to enhancement of the electrostatic repulsive force among colloidal particles.
The NPs prepared by pulsed laser ablation in deionized water (DIW) have a rather large size distribution because of the post-ablation coalescence of nanoclusters.Along with the studies of Zeng et al [13], several ionic environments have been tested in order to control the growth and the stability of the ZnO NPs.Usui et al [14] have produced ZnO NPs by laser ablation in different surfactant solutions: cationic cetyltrimethylammonium bromide (CTAB), anionic SDS, amphoteric lauryl dimethylaminoacetic acid (lauryl betaine, LDA), and nonionic octaethylene glycol monododecyl ether (OGM).It has been found that the average particle size and the standard deviation of particle size decreased with increasing amphoteric and nonionic surfactant concentrations, in particular, the average size abruptly decreased in both cases when the surfactant concentrations exceeded the critical micelle concentration.It has been proposed that a sufficient number of LDA and OGM surfactant molecules suppress the growth and aggregation of ZnO NPs.Negatively charged molecules surround ZnO NPs due to electric attractive force between them and the formation of micelles is supposed to prevent aggregation and growth of ZnO NPs.He et al [15] have investigated the growth of NPs by controlling their charge through the pH of the solution, using HCl and NaOH, to inhibit the coalescence of particles during the ablation process.They have found that the NPs produced in surfactant-free HCl and NaOH solutions had comparable average size and standard deviation to those produced in LDA surfactant solution.These results indicate that the population of the large NPs present in DIW under identical ablation conditions was suppressed under the acidic and basic media.On the other hand, when using NaCl solution of pH 7.15, the average particle size and the standard deviation of particle size have been found larger than those prepared in water.
The ionic environment besides the growth control has also been found to contribute to the morphological changes of the ablated NPs.Ishikawa et al [16] have observed the formation of ZnO columnar single crystals by pulsed laser ablation in DIW and surfactant aqueous solutions of LDA and CTAB at 80 o C. It has been proposed that the ZnO particles produced by laser ablation were dissolved at a temperature higher than 60 o C followed by the crystalline growth to columnar structure .While large ZnO columnar crystals were obtained in DIW, the crystals prepared in surfactant solution were smaller than those in DIW due to inhibition of crystalline growth by surfactant adsorption on ZnO surfaces.Similar studies have also been conducted by Navas et al [17] by preparing ZnO nanostructures through laser ablation in ammonium nitrate and CTAB from metallic zinc and ZnO powder pellet, respectively, at different liquid temperatures.As a result, nanostructures of different morphology have been obtained, while there is no indication of formation of NPs.He et al [18] have shown that using CTAB solution can lead to the formation of spindle-like ZnO aggregates, which in fact are composed of many well-defined NPs.
The morphological changes of the ZnO nanostructures have also been detected during ageing of NP colloids.Zeng et al [19] have observed treelike structure formation through ageing, which has been also accompanied by a significant enhancement of the visible emission.The enhancement of the green emission has also been observed when acidic and alkaline solutions (HCl and NaOH), respectively, have been used as ablation environment [15].He et al [18] have found that the enhanced green emission obtained with the use of CTAB has been suppressed during few days of ageing.
Plasma-treated water (PTW), which recently has been receiving a lot of attention from the plasma medicine and plasma agriculture community, encompasses both ionic and oxidative species, while the pH can be varied from acidic to alkaline [20].The long-living reactive oxygen and nitrogen species (RONS) deposited through the plasma-liquid interaction are the nitrite (NO − 2 ), nitrate (NO − 3 ) and hydrogen peroxide (H 2 O 2 ), which define the characteristics of the PTW's, such as, antimicrobial and antibacterial [21][22][23][24], and anticancer [25][26][27][28][29] properties.In the field of agriculture the plasma-treated liquids have been shown to have potential in seed disinfection, and in improving seed germination, the growth and stress tolerance of plants [30][31][32][33].Furthermore, the RONS-enriched liquids have been suggested to be possibly used as green fertilizer by providing nitrogen nutrient for plants both in soil and soilless media [34][35][36][37][38], while having also the advantage of reducing the pathogenic bacteria and microflora [39] more specifically in soilless media.Besides the nitrate, several metal ions, such as Mg, Zn, Fe, Cu play beneficial roles in plant physiology and are also constituents of nutrition media [31].Thus enriching the plasma-treated liquids with metal ions, e.g.Mg 2+ and Zn 2+ , or with ion-releasing NPs may add positive effects to those of RONS.Furthermore, regarding the biomedical applications the Mg 2+ and Zn 2+ can contribute to the activation of enzymes playing role in the biochemical reactions.
The aim of the present work, which focuses on the synthesis of zinc NP in RONS-enriched plasma-treated liquids, is twofold: revealing (i) the effect of the RONS and of the pH, respectively, on the properties of the created NPs and (ii) the effect of the ablation process and of the zinc NPs, respectively, on the stability of the RONS.For this purpose, the NPs are synthesized by laser ablation in plasma-treated liquids of different RONS composition and pH.Additionally, a new ablation setup is tested, with the aim to enhance the ablation yield.

Deposition of RONS into the liquids
The RONS, specifically nitrate, nitrite and hydrogen peroxide, are deposited into the liquid by using a surface-wave microwave discharge ignited with the help of a surfatron launcher (Sairem, Surfatron 80) and operating in open air, as illustrated in figure 1.The surfatron is a wave launcher of a coaxial structure terminated by a short-circuit plunger that performs both field shaping and impedance matching.The microwave field is coupled into the surfatron with a capacitive coupler, while the field shaping is provided by the circular gap near the front plate of the surfatron.
The discharge is generated in a quartz tube of outer diameter (O.D.) 6 mm and inner diameter (I.D.) 4 mm, using as a main gas Ar at gas flow rate of 2 slm.The discharge is sustained by the surface-wave launched with the gas breakdown at the gap position and traveling on the dielectric quartz tube and plasma column boundary.The length of the quartz tube emerging from the surfatron is 15 mm and the input power is chosen 25 W (the reflected power is less than 1 W).The reflected power is minimized by impedance matching achieved with the setting of the plunger's and capacitive coupler's position, according to the theory described in [40].At these conditions the plasma column exceeds the quartz tube with a plasma plume outside the tube being long enough to allow different contact points with the liquid surface, and thus assuring the control of active species formation in the liquid phase.As previously shown in [20,41] with increasing the distance between the discharge tube and the liquid surface the deposition of H 2 O 2 in the liquid is decreased, while that of nitrite and nitrate are increased.
The RONS are deposited into 32 ml liquid filling a Berzelius beaker of D = 38 mm and H = 52 mm up to 2 mm from its edge.The beaker is placed on a magnetic stirrer (IKA lab disc) with the stirring speed set to 60-100 rpm.The treatment distance for the liquid positioned below the plasma plume is defined as the distance between the liquid surface and the edge of the quartz tube, as shown in figure 1. Different treatment distances are chosen between 5 mm and 15 mm, in order to obtain different deposited [H 2 O 2 ]/[NO − 3 ] relative concentrations [41].At the liquid contact the plasma streamers cover a 6 mm diameter circular area.
During RONS deposition the acidification of the liquid occurs 3 .In the case of the DIW treated with the surface-wave microwave discharge the pH can drop to about pH 3.5 [41], which insures a very efficient reaction between the H 2 O 2 and NO − 2 already during the deposition phase 4 .In order to avoid the acidification and obtain also alkaline solutions, we make use of the effect of the reductive metals, which can reduce the H + ions responsible for the decrease of pH.As previously shown in [20], by treating together the DIW with Zn or Mg powder, the pH of the treated water can be set to about 6 and 9, respectively.
In the present study RONS are deposited by treating three different systems: (i) DIW, (ii) DIW with Zn powder and (iii) DIW with Mg powder.The DIW is produced with the ELGA Purelab Option-R 7 purifier and is characterized by total organic carbon (TOC) <20 ppb, Bacteria <1 CFU/ml, Inorganic-Typical >15 MΩcm.The powder grains used are of irregular shape and their size vary for Zn from 10 to 200 µm and from 40 to 600 µm for Mg.Powder is added to the water with a spoon of 0.0125 ml volume, corresponding to 17 mg Zn and 6.6 mg Mg. 3 The nitrate and nitrite are created from the dissolved NO and NO 2 molecules through the following reactions: NO 2(aq) reaction peroxynitrous acid is formed, whose lifetime under acidic conditions is short and rapidly decays by homolysis ONOOH → NO 2 + OH, and by izomerization ONOOH → NO − 3 + H + to nitric acid.Peroxynitrous acid can be stabilized in basic medium ONOOH → ONOO − + H + , pH > 6.8 [42].
The concentration of the long-lived active species: NO − 2 , NO − 3 and H 2 O 2 , and the pH of samples are measured with QUANTOFIX ® test strips (Nitrate/nitrite 500, 10-500 mg l −1 NO − 3 , 1-80 mg l −1 NO − 2 ; Peroxide 25, 0.5-25 mg l −1 ; Peroxide 100, 0.5-25 mg l −1 ).The strips are evaluated with the QUANTOFIX ® Relax unit (by Macherey-Nagel, GmbH ), which allows quantitative analysis with high accuracy and reproducibility.The strips (each package) are calibrated with standard solutions of relevant molarity: 0.025-0.5 mM NaNO 2 , 0.1-0.7 mM NaNO 3 and 0.25-2 mM H 2 O 2 .The contribution of the nitrite to the nitrate test field's signal is also calibrated with the NaNO 2 solution and tested with NaNO 2 and NaNO 3 mixture solutions [43].Here we use the advantage of the strips of makeing possible to follow the ageing of samples with 1 min time resolution without any significant consumption of the sample, and allowing the characterization of large amount of samples within a short period of time.The liquids are analysed immediately before and after the laser ablation.The samples are also analysed by UV-VIS absorption spectroscopy, as described in the following section.

Synthesis and analyses of colloidal NPs
The colloidal NPs are synthesized by laser ablation of a metallic Zn target (purity: 99.9%) in DIW and different RONSenriched water solutions prepared as described in the pervious section.The RONS-enriched water solutions prepared by the plasma treatment of the DIW with metal powder are filtrated before laser ablation by using a syringe filter housing 0.2 µm pore membrane.
The target placed in a beaker of 55 mm height containing 20 ml liquid is covered with a 38 mm liquid column.The 1064 nm Nd:YAG laser (Quantel Q-smart 450 mJ) is focused with a 250 mm lens and the target is positioned from the lens at 125 mm distance, i.e. in the half focus as shown in figure 2. The energy delivered to the target surface is 320 mJ, the beam width on the target surface is 6 mm.The target is fixed tilted with the surface at a 60-degree angle with respect to the laser axis, which allows the ablated material to leave the surface in a direction different than the laser axis.The sequence of images taken during the ablation process presented in figure 3 show a circular convective motion of the ablated material in the liquid volume.This motion of the ablated material is found to decrease the shielding effect of the NPs, which in general causes the decrease of the NP production [45].The reduction of the shielding effect is further ensured by the chosen laser wavelength, which is very far away from the Surface Plasmon Resonance (SPR) of ZnO (that is close to the third harmonic of the 1064 nm laser).In this way, using 6 ns pulses at 20 Hz repetition rate during 60 s ablation a very dense colloid is achieved.The possible effect of the oblique laser beam incidence on the cavitation bubble has been also hypothesized by [11].A reduction of the bubble symmetry can lead to an asymmetry of bubble collapse, which can be beneficial for enhanced material emission.With the target being positioned in the half focus, the creation of deep holes on the target surface is avoided.The deep holes created as a result of focusing the laser on the target (can act as a rough surface) can trap the bubbles, which can delay the bubble collapse and make possible the laser beam to be reflected from the bubble surface.The synthesized colloidal solutions are characterized by optical absorbance measured with the UV-Vis spectrophotometer (Shimadzu) in a quartz cuvette with 10 mm path length and a volume of 3 ml covering the 190-800 nm spectral range.During measurements the dionized water is used as a reference.The UV-VIS absorption spectra can indicate the presence of both the RONS and the NPs in the colloidal solution.
The morphology of the NPs in the colloidal solution are studied by scanning electron microscopy (SEM) images.Small aliquots (some µL) of the NP-colloids are dispensed onto a Si substrate.After evaporation of the solvent, the electron micrographs of the dry residues of the samples are recorded with a TESCAN MIRA3 field-emisson scanning electron microscope by using the secondary electron detector with an acceleration voltage of 20 kV.
The PL of the colloidal NPs are measured using for the sample illumination a Laser-Driven Light Source (Energetiq) with different spectral bandpass filters in the UV region.The light is focused onto a 10 mm×10 mm quartz cuvette holding the colloid.The emitted light is collected perpendicularly to the excitation beam and focused into the optical fiber connected to an Avantes spectrometer (AvaSpec-ULS2048x64TEC-EVO) with a 200 µm entrance slit.

Effect of laser ablation on the concentration of RONS
In the case of RONS-enriched solutions a major question is, how the laser ablation process influences the species concentrations.During the laser ablation in the DIW the production of H 2 O 2 is observed, as follows: in the 20 ml DIW with an ablation time of 30 s concentrations of 3-4 mg l −1 are obtained, while in the case of a 60 s ablation the concentration ranges between 1-3 mg l −1 , and it further decreases with the ablation time.The presence of H 2 O 2 is also indicated by the absorption spectra (discussed in the next section) with the absorbance increasing at wavelengths below 220 nm.The dissociation of water during laser ablation of titanium has been observed by Kumar and Thareja [44] through the presence of OH bands in the emission spectra.The production of H 2 O 2 has been recently detected by Kalus et al [45] in the case of ablation of Au, Pt, Ag, Cu, Fe, Ti and Al targets, as well as the catalytic decomposition of H 2 O 2 by the different metals.the reducing ability of created ZnNPs, as described in the previous paragraph.In the case of the alkaline PT(W+Mg) the pH drops during the ablation process close to neutral due to the reduction of OH − ions concentration through their reaction with the dissolved Mg 2+ ions and the Zn ions escaped from the laser plasma, respectively.Overall, the pH of the freshly prepared colloids range between 6.5 and 7.5.
It is well know that the composition of plasma-treated liquids change during storage, more significantly under acidic conditions.It has previously been shown that the plasmatreated liquids can be stabilized at neutral pH by using reductive metals or NPs [20,43].Figures 6(a In the following sections the effect of RONS on the properties of the colloidal NPs is presented.

Optical characterization of the colloidal NPs
The colloidal NPs are first of all characterized by UV-VIS absorption spectroscopy.Figures 7(a) and (b) show the absorption spectra of the 30 s and 60 s, respectively, colloids prepared from DIW and plasma-treated liquids.The figures also show the absorption spectra of the parent plasma-treated liquids, which indicate the presence of nitrate, nitrite and H 2 O 2 [46,47] preserved also in the colloids, as discussed in the previous section 5 .Consequently, the absorption spectra of colloids are the convolution of the absorption of the RONS dominating the wavelength region of 190-250 nm (as indicated by the absorption of the parent plasma-treated liquids), and the absorption of the colloidal nanostructures exhibiting characteristic features at wavelength larger than 240 nm.The absorption spectra show a clear difference between the colloids prepared from DIW and plasma-treated liquids, respectively, and clearly indicate the increase of the NPs concentration with the ablation time.In the case of DIW the absorbance increases with decreasing the wavelength and features one peak at 242 nm, which corresponds to the SPR peak of metallic Zn NPs.This indicates, that with the ablation of a metallic Zn target in DIW for 60 s metallic Zn NPs are obtained.Additionally, the spectra of the DIWbased colloids also show the production of H 2 O 2 (as already discussed in the previous section) indicated by the increase of the absorption at wavelength below 220 nm.
In the case of the plasma-treated liquids the absorption spectra do not exhibit an SPR peak, instead, a broad exciton related peak appears at around 350 nm, followed by a sharp decrease.This absorption is characteristic of ZnO nano-sized particles.In the case of the PTW and PT(W+Zn)-based colloids, figure 7(b), the exciton peak is less pronounced and is shifted to lower wavelengths, while there is a sharp increase in the absorbance at wavelength below 340 nm.This type of absorbance has been found by Goh et al [48]-while studying the absorbance of monodisperse particles with sizes in the range of 15-40 nm-to be characteristic for particles of 15 nm size.Similar conclusions have also been drawn by Fazio et al [49] in the case of NPs produced by laser ablation in water, when comparing the calculated extinction cross section (based on the multipole expansion of the electromagnetic field in the framework of the transition matrix formalism) with the measured absorption spectra.They have also shown that the small particles influence the absorption in the 250-350 nm wavelength range, while above 400 nm the scattering due to the larger particles become significant.Furthermore, with increasing the size of the aggregates composed of lower dimension particles the exciton peak becomes more pronounced, while the absorbance at wavelengths shorter than the absorption edge is becoming constant.This type of behaviour is observed in the case of the PT(W+Mg)-based colloid, indicating the presence of larger aggregates.Overall, there is a significant difference between the colloidal NPs synthesized in PTW and PT(W+Zn), and PT(W+Mg), respectively, regarding the aggregation of particles.The differences can be the result of the ionic environment of the parent liquids, which influences the growth and stability of the colloidal NPs, as already mentioned in section 1.
Comparing the two plasma-treated liquids, PTW and PT(W+Mg), the total RONS (H 2 O 2 , NO − 3 and NO − 2 ) and the nitrate/nitrite concentrations, respectively, are very similar, while the pH of PTW is about 4.5 and that of PT(W+Mg) is 9.6.Accordingly, the difference between the colloidal NPs synthesized in PTW and PT(W+Mg) showed by the absorption spectra may be explained with the pH of the parent liquids and that of the colloids.During laser ablation the pH of the liquids change, and while the pH of the PTW-based colloid increases reaching values of about 6.7, that of the PT(W+Mg)based colloid decreases, however it stays above pH 7. The effect of the solution's pH on the ZnO production through laser ablation has been previously studied by He et al [15] using HCl (pH 5.36), NaOH (pH 11.98) and NaCl (pH 7.15) solutions, respectively.The absorbance of NPs prepared in HCl and NaOH solutions have indicated the production of smaller particles than in the DIW, and the measured absorption spectra exhibit same features as those of the PTW and PT(W+Zn)based colloids.On the other hand, the absorbance of NPs prepared in NaCl (pH 7.15) solution is similar to those prepared in PT(W+Mg), indicating the aggregation of NPs.In general, at low pH the surface of NPs can be charged positively by the adsorption of protons (MOH + 2 ), whereas it will be negatively charged at high pH owing to depletion of protons (M-O − ).These ions can electrostatically stabilize the NPs and also have size quenching effect [50].The surface charge and hence the electrostatic stability reaches a minimum close to the isoelectric point (pI) at pH = pI, which in the case of ZnO NPs is reported to be around 9.2 [51] and 9.5 [15].
During ageing the aggregation of NPs is expected, which can be confirmed by absorption measurements.Figure 8 shows the absorbance of the DIW and different plasma-treated liquid base colloids at 30 min after the end of ablation and at one week of storage at room temperature, respectively.In the case of the PT(W+Mg)-based colloid (figure 8(a)) no significant changes are found, except for the decrease of the total absorbance resulting from the sedimentation.In the case of PT(W+Zn) and PTW (figure 8(b)) the aggregation of NPs is manifested in the change of the absorption below the band edge, i.e. the sharply increasing absorption changes into a plateau, which also results in a more pronounced exciton peak.Nevertheless, the most significant changes are found in the case of DIW-based colloids (figure 8(b)), where initially Zn metallic NPs are prepared, which during storage, through their reaction with the liquid become Zn/ZnO core-shell NPs or fully oxidized ZnO NPs, as suggested also by Camarda et al [10] and Reich et al [11], respectively.The absorbance of DIW-based colloid clearly indicates the transition from the Drude-like behaviour of metallic Zn into a band gap behaviour of ZnO, namely, the increasing absorption changes into  a plateau below 350 nm and exhibits a sharp decrease above 350 nm.
The UV-VIS absorption spectra of the colloids provide also information about the band gap of the ZnO NPs.The band gap energy can be estimated from the absorption spectra with the method proposed for amorphous semiconductors by Tauc et al [52].The Tauc plot is based on the assumption that the energy-dependent absorption coefficient can be expressed as follows: (αhν) γ = A(hν-E g ), where h is the Planck constant, ν is the photon's frequency, E g is the band gap energy, and A is a constant.The γ factor for the direct allowed electron transition is 2. According to the equation, by transforming the absorption spectra into the energy-dependent absorption plotted against the photon energy, the x-axis intersection point of the linear fit of the Tauc plot gives an estimate of the band gap energy.Figure 9 shows the Tauc plot of different plasma-treated liquids based colloids.The linear fits indicate the band gap energies of NPs, which are also listed in table 1.Although in DIW metallic Zn NPs are created, as shown in figure 7, within two hours the oxidation of the NPs can already be detected with the appearance of the excitonic peak, which makes possible the estimation of the band gap.The band gap in the case of NPs created in DIW is calculated to be ≈ 3.36 eV, which agrees well with the results obtained by Kim et al [53] for NPs created by laser ablation in neat dionized water.Similar band gaps are also obtained in the case of the NPs created in PTW and PT(W+Zn), while in the case of PT(W+Mg)-based colloids a significantly lower 3.10 eV band gap is found.It is also found, that the ablation time does not influence significantly the band gap, in accordance with the results of Kim et al [53].Although the PT(W+Mg) solution also contains Mg 2+ ions, the band gap does not indicate a significant doping with Mg, since according to the studies of Wang et al [54] that would result in an increase of the band gap (2% of Mg doping increases the band gap from 3.3 eV to 3.35 eV).
According to the characteristics of absorbances discussed in the previous paragraph, the lower band gap obtained in the case of the NPs created in PT(W+Mg) may be attributed to the larger aggregates found in the PT(W+Mg)-based colloids.Fazio et al [55], by conducting a detailed investigation of the ZnO nanostructures prepared by ns-pulsed laser ablation in water, have found that the optical band gap increases with decreasing particle size, i.e. the band gap has changed from 3.34 eV to 3.16 eV with the particles mean size increasing from 15 nm to 50 nm.Very similar conclusions have also been drawn by Goh et al [48] by studying the absorbance of monodisperse particles with sizes in the range of 15-40 nm.
Regarding the ageing of the colloids, it is found that the band gap decreases in the case of all the colloids, as shown in table 1, which may indicate the aggregation of the particles.The further decrease of the band gap has been obtained with the heat treatment of the colloids.Since during the heat treatment the RONS concentrations have not changed significantly, the further aggregation of the particles is expected to occur.The morphological characterization of the colloidal NPs is given in the next section.

Morphological characterization of the colloidal NPs
The absorption spectra have indicated that the dominant size of particles in the PTW and PT(W+Zn)-based colloids is around 15 nm, while in the case of PT(W+Mg) either larger particles or larger aggregates are formed.Figure 10 shows the SEM images of the ablation product from the PT(W+Zn) and PT(W+Mg)-based colloids.The particle size distributions determined from the SEM images show that there is no significant difference between the two types of colloids, in what concerns the particle sizes.The particle size distribution of the PT(W+Mg)-based colloid, shown in the last panel of figure 10 and approximated with a log-normal distribution, indicates that the mean size of particles is 18 nm with geometric standard deviation (GSD) of 1.7, and there are very few particles larger than 40 nm.It can be concluded that in the case of laser ablation in PT(W+Mg) the aggregation of particles occurs already at a very early stage, which is reflected on the absorption spectra measured within minutes after the colloid preparation.The recorded SEM images are further used to identify the effect of ageing and that of heat treatment, respectively, on the morphology of the colloidal NPs.The SEM images of the aged PT(W+Mg) and PT(W+Zn)-based colloids, shown in figure 11, indicate that while in the case of PT(W+Mg)based colloid there are only particle aggregates, in the case of PT(W+Zn) besides particles nanosheets can also be identified.A more abundant formation of nanosheets is observed after a 50 min heat treatment of samples in the ultrasonic bath, where the samples are gradually heated and reach 65 o C within 30 min.Figure 12 shows the SEM images in the case of the PT(W+Zn)-based colloid prepared with a 30 s ablation time, where the self-assembly of nanosheets and the formation of flowerlike structures is captured.The thickness of the nanosheets are determined to be in the 5-20 nm range.In the case of colloids prepared with a 60 s ablation time, which are more dense than the 30 s colloids, the nanosheets evolve into a porous network structure-as shown in figure 13-indicating a stronger assembly.While in the case of the DIW-based colloid only dense nanosheet networks can be identified, in the case of the PT(W+Zn)-based colloid ZnO NPs can also be observed in the porous nanosheets network (see figure 13).The formation of nanosheets has previously been reported by Yan et al in the case of laser ablation of Zn in solutions [56] and by Pan et al [57] in solvothermal synthesis of ZnO.With the solvothermal method nanosheet-assembled nanoflowers and separated nanosheets with thickness of around 6 nm have been obtained by using ultrasonic pretreated zinc nitrate in ethanol solution [57].With a 20 min pulsed laser ablation of the zinc metal target self-assembled nanosheets have been produced in the aqueous solution of SDS with the addition of ethanol.
The formation of nanosheets during ageing is expected to occur due to the dissolution of the ZnO NPs.The dissolution of the smallest 4 nm NPs in different aqueous environments has been investigated by Bian et al [51].They have demonstrated that the smallest ZnO NPs show the greatest propensity for dissolution, and the dissolution decreases with increasing pH.Furthermore, it has been also shown that aggregation can decrease dissolution rates of NPs and in some cases can completely quench the process [58].This can explain the inhibition of the nanosheets formation in the case of PT(W+Mg)-based colloids, where large aggregates are formed during the ablation process, while the colloid is alkaline.

PL of colloidal NPs
In general, the liquid environment in which the nanostructures have been prepared, as well as the morphology and size of the structures strongly influence the PL of the colloidal nanostructures.The ZnO structures are characterized by a near-bandedge (excitonic) ultra-violet emission and at least one broad band visible emission due to the deep levels (called DLE) [59].The DLEs are related to zinc and oxygen defects, such as: the oxygen and zinc vacancies, as well as interstitial zinc and oxygen sites, as also listed in e.g.[59].
He et al [15] have shown that the green emission intensity is strong compared to the exciton emission intensity when ZnO have been produced in acidic or basic media, while the green emission intensity is somewhat weaker than the exciton emission intensity when ZnO is produced in salt solution (pH 7.15) or DIW.The lowest green emission has been found at pH 9.5 which is in accordance with the minimum of the electrostatic stability reached at the isoelectric point, as discussed in [50].He et al [15] have also demonstrate that the greento-exciton emission intensity ratio progressively increases as the particle size decreases.On the other hand, Zeng et al [19] have found the enhancement of the visible-in the blue and green region-emission due to the aggregation of the particles occurred during ageing.In the case of ZnO NPs prepared by laser ablation in neat water Kim et al [53] have obtained a yellow-green emission with comparable intensity to that of the exciton emission.
Comparing to the PLs previously reported [14,15,19,53], the PL of the ZnO nanostructures prepared by laser ablation in plasma-treated (RONS-enriched) liquids exhibit a significantly broader visible emission, as it is shown in figure 14.The visible emission covers the green, yellow and orange spectral region, comparing to which the violet-blue emission 6 previously observed in the case of ZnO-Zn composite NPs prepared by laser ablation of zinc in pure water and SDS aqueous solutions [13]-is very weak, as also shown in figure 14(b).The peak intensity of the exciton emission is about half of the peak intensity of the yellow emission both in the cases of the plasma-treated liquids and DIW-based colloids.
The emission spectra of the plasma-treated liquids based colloids are dominated by two main peaks at 564 nm and 602 nm.The orange-red emission peaking at 602 nm has not been previously reported in the case of NPs created by LAL.This emission has been identified to be related to the interstitial oxygen sites O i [62,63], and it is believed to be due to the band transition from zinc interstitial (Zn i ) to oxygen interstitial (O i ) defect levels in ZnO.Xu et al [64] has explained by the full potential linear muffin-tin orbital method, that the position of the O i level is located approximately at 2.28 eV below the conduction band, while the position of the Z i level is theoretically located at 0.22 eV below the conduction band [65].Therefore it is expected that the band transition from Zn i to O i level is approximately 2.06 eV [62] 7 .The Zn i and O i defect levels are also proposed to be related with the green emission [62].The electron paramagnetic-resonance analysis conducted by Vlasenko and Wathins [69] have demonstrated that the green emission consists of two transitions, with electron-hole recombinations from conduction band to V O level (544 nm) and from Zn i level to V O level (554 nm).In the case of plasmatreated liquids the green emission is less significant, which can be explained with the oxygen rich environment the nanostructures are prepared in [10].On the other hand, the colloids exhibit a strong yellow emission centering around 563 nm.This emission has previously been attributed to the transition from the conductive band to the single positively charged oxygen vacancy V + O [63,70].Furthermore, the yellow emission has also been suggested to occur due to the presence of OH groups on the surface by Djurišić et al [71] when investigating hydrothermally grown nanorods.The presence of surface hydroxides is obvious in case of NPs originating from LALs.
In the case of DIW-based colloid the emission intensity is significantly lower and the emission is shifted towards the green spectral region, with less significant emission in the orange-red.The DIW is less oxygen rich environment, which may explain the lower density of interstitial oxygen sites, which could contribute to the orange emission.The PL spectra exhibits similar features as the PL of the of nanowall arrays presented by Feng et al [72].
Although the different emissions can been attributed to the presence of defects, it has also been shown, that the PL of ZnO depends on the morphology of the nanostructures.Djurišić and Leung [73] have shown the PL being dominated by green for shells, by yellow for ribbons/combs and by orange for nanorods.The morphology dependence of the PL may also explain the change of the emission of the colloids after several months of storage.The PL spectra of six months old plasma-treated liquids based colloids presented in figure 15 exibit similar features as the PL of nanosheets presented by Vempati et al [59] .The SEM image also presented in figure 15 indicates the dominance of nanosheets also in the case of PT(W+Mg)-based colloids.Nevertheless, the visible emission, which in this case has similar intensity as the exciton emission, can also be attributed to the presence of defects levels.Namely, the blue emission with two identified peaks (around 404 nm and 423 nm) can be related to the transition from the conductive band to the V Zn and O i , respectively [61].The green emission centered at around 510 nm has originally been related to the oxygen vacancies [74], however investigations suggest the of Zn i -related complexes [75].

Conclusions
Colloidal zinc-oxide NPs have been prepared by ns-laser ablation of a Zn metallic target in RONS-enriched plasmatreated liquids.A new laser ablation set-up has been designed, where the target surface is positioned in the half-focus of the laser focusing lens and at 60-degree angle with respect to the laser axis.This results in oblique laser incidence with large beam width, which leads to asymmetric bubble collapse and thus favours enhanced material ablation.Furthermore, the ablated material leaves the target in a direction different than the laser axis, and a self-developed convective motion of the ablated material insures the decrease of the shielding effect.
The RONS have been deposited into DIW by using an argon surface-wave microwave discharge operating at 25 W input power.The composition of the plasma-treated liquids has been set by the treatment distance i.e. the discharge tubeliquid surface distance.The pH of the plasma-treated liquids has been adjusted by applying reductive, Zn and Mg, metals.The reductive metals are used in powder form and filtrated from the liquid before the laser ablation process.The different plasma-treated liquids are named as PTW for the plasmatreated DIW, PT(W+Zn) and PT(W+Mg) for the plasmatreated DIW with Zn and Mg powder, respectively.By using plasma-treated liquids with different H 2 O 2 to nitrate/nitrite concentration ratio, the work has focused on two main questions (i) the stability of the RONS affected by the ablation process and the created reductive zinc NPs, respectively, and (ii) the stability and properties of the created NPs affected by the RONS and the pH, respectively.
It has been found that the laser ablation process has a significant effect only on the concentration of H 2 O 2 , namely the concentration decreases with the ablation time.During laser ablation the H 2 O 2 can decompose through the oxidation of the ablated material, and through the reaction with the laser plasma electrons.It has been found that in the nitrate/nitrite dominated plasma-treated liquids the H 2 O 2 concentration can decrease with up to 50% during a 60 s ablation.A significant formation of the nitrite has only been detected in the case of the PTW, which originally does not contain any NO − 2 .Contrary to the plasma-treated liquids, in the case of RONS-free DIW the H 2 O 2 is produced with a concentration of up to 5 mg l −1 .With the laser ablation of the Zn target, the pH of the liquids also changes as follows, the pH of acidic liquids increases to about 6.5-7, while that of the alkaline PT(W+Mg) decreases to pH 7.5.And finally, during the long-term storage the RONS concentrations are found to be quasi-stable.
By analysing the UV-VIS absorption spectra of the colloids, it has been shown that in DIW metallic Zn NPs are created, which gradually oxidize during storage, while in the case of plasma-treated liquids ZnO NPs are produced.The produced particles are predominantly smaller than 40 nm with the mean size of 18 nm.The SEM images have indicated that while in the case of the alkaline PT(W+Mg) the particles form large aggregates, in the case of the acidic and neutral solutions besides NPs nanosheets are also formed.During storage the nanosheets evolve into nanosheet networks as a result of the dissolution of the NPs, which can be significantly enhanced by heat treatment.In the PT(W+Mg)-based colloids, due to the presence of the large aggregates, the dissolution of the NPs occurs at a lower rate.The band gap of the ZnO nanostructures calculated from the absorption spectra also reflects the formation of the particle aggregates and nanosheet networks, respectively.It has been shown that the band gap can decrease from 3.35 eV-characteristic for NPs with mean size of 15 nm-to 3.00 eV with the increase of aggregates and nanosheet networks, respectively.
Regarding the PL of the ZnO NPs, it has been found that the NPs ablated in plasma-treated liquids have a visible emission with significantly higher intensity than those produced in DIW.Furthermore, the emission covers a wider spectral region, namely from green to red.The orange-red emission, which is attributed to the transition from Zn i to O i level, has previously not been detected in the case of laser-ablated ZnO NPs.The plasma-treated liquids, that contain high concentration of RONS, favour the formation of interstitial oxygen sites.Due to the rich oxygen environment, the green emission related to the oxygen vacancies is less significant.On the other hand, the NPs exhibit a strong yellow emission, which can be attributed to the OH groups on the surface.It has been further found that during months of storage, due to the dissolution of NPs and formation of nanosheets, the intensity of the visible emission decreases and shifts to the blue-green spectral region.

Figure 1 .
Figure 1.Schematic of the liquid treatment with the surface-wave microwave discharge ignited with the surfatron wave launcher.The inset shows the operation of the argon discharge in contact with the liquid.

Figure 2 .
Figure 2. Schematic of the ablation set-up.

Figure 3 .
Figure 3. Sequence of images from a 60 s laser ablation covering the time interval of 2 to 7 s, illustrating the convective motion of the material ablated from a tilted target.

Figure 4
shows the evolution of H 2 O 2 , nitrate and nitrite concentrations in the PTW with Zn powder (PT(W+Zn)) during the laser ablation process in the case of two types of filtrated PT(W+Zn) regarding the ratio of H 2 O 2 to nitrate/nitrite.In both cases the decrease of the H 2 O 2 concentration occurs with the increase of the ablation time, and the absolute concentration drops with the same amount within a 90 s ablation process.During ablation the H 2 O 2 decomposition can occur both through the oxidation-H 2 O 2 + Zn → ZnO + H 2 O-of the Zn target and ablated material, respectively, and the dissociation by electron impact.Along the drop of the H 2 O 2 concentration, a slight decrease of the NO − 3 concentration and a less significant increase of the NO − 2 concentration can be observed.Since

Figure 4 .
Figure 4.The concentration of reactive oxygen and nitrogen species after different length of laser ablation in the PT(W+Zn).The RONS have been deposited into PT(W+Zn) using treatment distances of 6 mm (a) and 12 mm (b), respectively.

Figure 5 .
Figure 5.The RONS concentrations and the pH before and after the 60 s laser ablation in different plasma-treated liquids.The liquids have been treated at a 5.5 mm distance.
) and (b) present the change of the RONS concentrations during a 10 days storage at room temperature in the case of colloids prepared from different PT(W+Zn) and PT(W+Mg), respectively.The evolution of species concentrations is followed for three different plasma-treated liquids-regarding the [H 2 O 2 ]/[NO − 3 ] concentration ratio-prepared under the following conditions: (i) initial gas mixture 2000 sccm Ar, treatment distance 5.5 mm, (ii) 2000 sccm Ar, 12 mm and (iii) 2000 sccm Ar-25 sccmN 2 , 10 mm.In the cases of both the PT(W+Zn) and PT(W+Mg)based colloids similar trends can be observed.The pH of the colloids slightly decrease and reach values between 6.1 and 6.6 within 10 days.The RONS concentrations change significantly in the case of the nitrate/nitrite dominated colloids.When the nitrate concentration is quasi-equal to that of the H 2 O 2 , the nitrate concentration decreases , while the nitrite and H 2 O 2 concentrations increase.Similar trends are obtained also in the case of PTW-based colloids, data not presented here.In the case of DIW-based colloids, containing only H 2 O 2 , the concentration slightly increases, while the pH decreases to around 6.4.Analysing the colloids after 6 months of storage it is found that the pH of the PTW and PT(W+Zn)-based colloids stabilize at around pH 5.8-5.9, while that of the PT(W+Mg)-based colloids around 6-6.2.At the same time the reactive species concentrations do not change significantly.

Figure 6 .
Figure 6.The RONS concentrations after the laser ablation and after 10 days of storage of the colloids, respectively, in the case of PT(W+Zn) (a) and PT(W+Mg) (b) prepared under the following conditions: (i) initial gas mixture 2000 sccm Ar, treatment distance 5.5 mm, (ii) 2000 sccm Ar, 12 mm and (iii) 2000 sccm Ar-25 sccm N 2 , 10 mm.

Figure 7 .
Figure 7. UV-VIS absorption spectra of different plasma-treated liquids (thinner lines in the 200-250 nm spectral range) and the corresponding colloids (thicker lines) prepared with 30 s (a) and 60 s (b) laser ablation.The liquids have been treated with a 2 slm Ar discharge at a 5.5 mm distance.

Figure 8 .
Figure 8. UV-VIS absorption spectra of the DIW and plasma-treated liquids based colloids with 30 min after the ablation process (thicker lines) and after one week of storage (thinner lines).

Figure 9 .
Figure 9.The Tauc plot for the plasma-treated liquids based colloids.The linear fits indicate the band gap energies of the NPs.

Figure 10 .
Figure 10.The SEM images of the ablation product from the PT(W+Mg)-based and PT(W+Zn)-based colloids, respectively, and the particle size distribution determined from the SEM image in the case of the PT(W+Mg)-based colloid.

Figure 11 .
Figure 11.The SEM images of the ablation product from the 10 days old PT(W+Mg)-based and PT(W+Zn)-based colloids, respectively.

Figure 12 .
Figure 12.The SEM images illustrating the self-assembly of nanosheets in the case of the PT(W+Zn)-based colloid prepared with a 30 s ablation time.The colloid has been heat treated as described in the text.

Figure 13 .
Figure 13.The SEM images in the case of the heat treated PT(W+Zn)-based and DIW-based colloids, respectively.The colloids have been prepared with a 60 s ablation time.

Figure 14 .
Figure 14.Emission of the DIW-based and plasma-treated liquids based colloids using 351 nm excitation (a) and UV excitation in the 275-375 nm spectral range (b).The photoluminescence is measured after 4 weeks of storage of colloids.

Figure 15 .
Figure 15.Photoluminescence of the six months old plasma-treated liquids based colloids for UV excitation in the 275-375 nm spectral range, and the SEM image of the corresponding PT(W+Mg)-based colloid.

Table 1 .
Band gap of ZnO nanoparticles ablated in different type of liquids.