Laser and electron beam-induced formation of Ag/Cr structures on Ag2CrO4

The interactions of silver chromate (Ag2CrO4) with a femtosecond (fs) laser and electron beam irradiations were investigated. For the first time, the growth and coalescence of metallic Ag nanoparticles (NPs) on an Ag2CrO4 surface via fs laser irradiation can be reported. Furthermore, electron beam irradiation causes a segregation process of Ag NPs in which Ag nanofilaments are obtained. The Ag2CrO4 particles were characterized using X-ray diffraction (XRD), micro-Raman spectroscopy (Raman), field-emission scanning electron microscopy (FE-SEM), high-resolution electron microscopy, and energy-dispersive X-ray spectroscopy (EDS). According to the results, the fs/electron beam irradiations of Ag2CrO4 are environmentally friendly, fast, and effective methods to produce metallic Ag NPs. Both strategies have enormous potential for the synthesis and morphological control of Ag NPs on templates or biotemplates, core–shells, decorations, and composites based on Ag2 xCrO4:Ag materials, which are difficult to synthesize using conventional chemical and physical methods.


Introduction
Nanoscale-manipulations of atoms are possible through the ever-increasing advances of modern technologies, such as electron microscopy 1,2 and ultra-short laser radiation, 3 which open up a number of exciting possibilities for the nanoscale-controlled in situ fabrication of nanoparticles (NPs).][6][7] These changes cause unexpected effects on their physical and chemical properties, e.g., photoluminescence emission, 8 photodegradation, 9 antimicrobial activity, 10 and the growth of nanostructures. 10,115][16][17][18][19] Both techniques have gained enormous attention owing to their ability to investigate the structureproperty relation of nanomaterials and their chemical interactions with solid surfaces.
Our research group investigates Ag-containing semiconductors that have attracted significant attention owing to their rich history in technical applications, e.g., in catalysis, 44 photocatalysis, 45 for gas sensors, 46 and antimicrobial agents. 45In this context, a variety of Ag-containing oxides such as Ag 2 WO 4 in their different polymorph forms a-Ag 2 WO 4 , 20 b-Ag 2 WO 4 , 21,47  and improve the separation rate of the photogenerated holes and electrons, thereby enhancing the photocatalytic activity. 54n this context, recently, we have presented the scale-up of the formation of Ag NPs on a-Ag 2 WO 4 with bactericidal properties via fs laser irradiation, 55 the laser-induced formation of Bi NPs with different crystallographic structures, 56 and the formation of In NPs provoked by laser/electron beam irradiation.57 In the current work, the semiconductor under study is Ag 2 CrO 4 , which is a promising inorganic material because of its unique electronic and crystal structure that enables it to be a good photocatalyst.[58][59][60][61][62][63][64][65] Our group performed the synthesis, characterization, and determination of the photoluminescence properties of Ag 2 CrO 4 microcrystals.66 In addition, the formation and growth of Ag NPs on Ag 2 CrO 4 induced by electron irradiation on an electron microscope was investigated.53 It is well known that the photo-and electrochemical properties of semiconductor films used in photovoltaics, photocatalysis, and photoelectrolysis are greatly enhanced by the deposition of Ag NPs onto their surfaces.The Ag NPs are employed as a high visible-light-driven photocatalyst and microbial agent 10,49,50,52,67 to enhance the performance.The improved properties appear owing to the surface plasmon resonance (SPR) effect and extended visible-light absorption that is present in metallic Ag NPs.69 This study is intended to complement our previous studies. The gl is two-fold: (i) to demonstrate, for the first time, that fs laser irradiation can be used for the formation of Ag NPs on Ag 2 CrO 4 surfaces, and (ii) to study the evolution behavior and growth mechanisms of Ag NPs induced by electron beam and fs laser irradiation.

Synthesis of Ag 2 CrO 4 microparticles
Two Ag 2 CrO 4 samples were synthesized using the precipitation method without any surfactants.The chromate precursor solution was prepared in 50 mL of distilled water, to which 1.0 Â 10 À3 mol of K 2 CrO 4 was added (99.5% purity; Quı ´mica Especializada Erich Ltda-QEEL).The solution was magnetically stirred until it turned homogeneous and yellow.The silver precursor solution was prepared with 2.0 Â 10 À3 mol of AgNO 3 (99.8%purity; Alfa Aesar) in 50 mL distilled water.The solution was magnetically stirred until it turned homogeneous and transparent.The addition of the silver precursor solution to the chromate precursor solution was carried out with dropwise additions with a rate of 10 mL min À1 .The resultant solution was magnetically stirred for 20 minutes.The precipitate was washed four times with water and once with acetone and afterward dried in a stove at 60 1C for 4 hours.

Structural characterization of Ag 2 CrO 4 microparticles
The Ag 2 CrO 4 was structurally characterized by X-ray diffraction (XRD) using a D/MAX-2500 PC diffractometer (Rigaku) with Cu Ka radiation (l = 1.5406Å).The data were recorded in the normal routine for 2y values ranging from 151 to 601 with a scanning velocity of 21 min À1 .Micro-Raman measurements were conducted using an iHR550 spectrometer (Horiba Jobin Yvon, Japan) coupled to a CCD detector and an Ag ion laser (MellesGriot, USA) operating at 514.5 nm with a maximum power of 200 mW and a fiber microscope.The measurements were performed in the range of 30-1000 cm À1 .

Fs laser irradiation
To perform the fs laser irradiation, the sample had to be compressed into a pellet form.The resulting Ag 2 CrO 4 pellets were irradiated with a Ti:sapphire laser (Femtopower Compact Pro, Femtolasers), which delivers 30 fs full-width at halfmaximum (FWHM) pulses at a central wavelength of 800 nm and a repetition rate of 1 kHz.In addition, a programmable acousto-optic filter (Dazzler, Fastlite) was used to ensure a precise pulse compression at the sample.A laser beam with a mean power of 200 mW was focused onto the surface of the pellet target with a 75 mm lens to obtain a focal spot with a diameter of 20.6 mm FWHM.The silver chromate sample was placed at the top of a quartz slide attached to a twodimensional motion-controlled stage moving at a constant speed of 0.45 mm s À1 in the focus plane perpendicular to the laser beam.Fig. 1a and b present a scheme of the experimental procedure and obtained results, where the interaction between the fs laser radiation and Ag 2 CrO 4 promotes the creation of a plasma plume that leads to the segregation of Ag nanoparticles.6][57] In addition, when we are dealing with pulsed laser sources that interact with a moving target, as the light is not delivered in a continuous way, it is more common to talk about the overlapping between the pulses on the sample.In this way, a large overlap means more investment of energy delivered to a single area in a long period of time, and a minimal overlap means a minor investment of energy delivered to a single area but in a short period of time.For the current experiment, the irradiation time is controlled by the size of the beam and the speed of the platform.The size of the focal spot on the sample surface was about 20.7 microns.The laser has a 1 KHz repetition rate and the motion velocity of the platform was 0.45 mm s À1 .These parameters indicate that there is an overlapping of the spot size of around 95% between consecutive pulses.Considering the large overlapping used in the current experiment, in comparison with electron beam excitation, while using fs laser irradiation there is still plenty of room towards crossing the line between basic research and the final industrialization of the process.

Morphological characterization of the samples
The morphology of fs laser-irradiated and non-irradiated samples was observed using a field-emission scanning electron microscope (FE-SEM) (Inspect F50 model; FEI Company, USA) working at 5 kV.Transmission electron microscopy (TEM) images are obtained using a high-resolution (HR) JEM-2100 LaB6 (Jeol) microscope with an acceleration voltage of 200 kV coupled with INCA Energy TEM 200 (Oxford Instruments) to perform energy-dispersive X-ray spectroscopy (EDS).For both the nonlaser irradiated powder and the laser irradiated powder, in the case of FE-SEM, the samples were prepared by depositing a small amount of powder directly onto the FE-SEM sample holder, and for the TEM the powder was suspended in water and a droplet of the mixture was deposited onto the carboncoated Cu grid (TEM grid).After the evaporation of water, it was submitted to high vacuum and analyzed via TEM.enhancing domains of different electron density distributions (low and high) in the crystalline network. 71

Raman spectra
Furthermore, a micro-Raman analysis was performed to investigate the degree of structural order-disorder in the short-range (Fig. 4).The micro-Raman spectrum presents three peaks in the high-frequency region at 781, 812, and 855 cm À1 .These peaks are related to the symmetric stretching vibrations of Cr-O bonds in the [CrO 4 ] clusters and their positions are in good agreement with the literature. 72The orthorhombic structure of Ag 2 CrO 4 has 36 Raman-active modes with mechanical representation G = 11A g + 7B1 g + 11B2 g + 7B3 g , of which 18 are internal modes [2n 1 , 4n 2 , 6n 3 , 6n 4 ] and 18 are external modes [6 rotational (R) and 12 translational (T) modes]. 53Not all Ramanactive modes were experimentally observed owing to weak intensity and/or merging of modes with close wavenumbers.

FE-SEM images
To investigate the morphological alterations resulting from the fs laser irradiation, FE-SEM micrographs were analyzed.The nonirradiated particles present a high agglomeration rate when observed under low magnification (Fig. 5a).However, when these particles are analyzed at medium and high magnifications (Fig. 5b  and c), they appear faceted and formed mainly by irregular polyhedrons.The particles of the fs laser-irradiated sample (Fig. 5d-f) are faceted and present irregular polyhedra (slightly different from those observed in the non-irradiated sample).
To analyze the effects of fs laser irradiation on the morphology and the particle size distribution more deeply, a count of 200 particles of each sample was taken, and the results are presented in Fig. 6.The distribution curve in each histogram, the mean height, and the width of the particles were calculated using a log-normal distribution curve.The non-irradiated sample (Fig. 6a and b) showed a very narrow particle size distribution.
The particles have a size distribution range between 0.10 mm and 0.60 mm.In addition, the non-irradiated particles exhibit a mean height and a width of (0.39 AE 0.09) mm and (0.30 AE 0.07) mm, respectively.On the other hand, the fs laser-irradiated sample (Fig. 6c and d) presented a particle size distribution ranging from 0.45 mm to more than 2.0 mm, and a mean height and a width of (0.92 AE 0.31) mm and (0.76 AE 0.24) mm, respectively.These values show that a significant increase in the particle size distribution of the fs laser-irradiated sample relative to the non-irradiated sample has occurred.The results reveal that the coalescence process has taken place and that the particles' growth has been stimulated by fs laser-irradiation.
By plotting the height of each particle with its own width (Fig. 7), a considerable increase in the dispersion of particles from the non-irradiated to fs laser-irradiated samples can be observed.In addition, the width/height ratios of the particles are similar (0.77 for the non-irradiated particles and 0.83 for the irradiated particles), which suggests that the coalescence process did not occur along a preferable unidirectional orientation or oriented attachment. 73 The coalescence process could be observed using microscopes.In Fig. 5f, the numbers 1, 2, and 3 present the neck growth between contacting particles, which indicate an early stage of the sintering process. 74As other groups have previously reported, the interaction between high intensity fs laser radiation and a semiconductor powder may promote a microsintering process. 75The whole interaction can be thought to occur in three stages: In the first stage, the laser irradiation promotes the detachment of electrons, ions and another bigger species out of the surface of the microparticles that interact with the laser radiation, leading to the creation of a plasma plume.In the second stage, there is no more interaction with the laser because the temporal width of the pulses used in the current article is 30 fs, but in the counterpart, the plasma plume still exists (lifetime in the region of ns), which interacts with the remaining powder bed.The plasma plume can reach extremely high temperature and pressure values, 1000 K and 10 10 Pa, respectively. 76As the melting temperature of Ag 2 CrO 4 is around 938 K, 77 such conditions can promote the melting of the microparticles in the powder bed that interact with the plasma.
In the third stage, the plasma plume gets extinguished and the species that formed the plasma plume cool down and form nanostructures, and the melted microparticles belonging to the powder bed also cool down and solidify forming bigger microparticles.A graphical representation of these stages can be found in the ESI.†

HR-TEM images
In order to investigate the effects of fs laser irradiation on the nanostructure growth and stoichiometric modifications of the original material, HR-TEM images were analyzed.Fig. 8    of Ag (100%).Region (e) shows the lowest concentration of Ag (63.1%) and the highest concentration of Cr (24.6%).After a conversion of the elementary percentages into molar fractions to estimate the stoichiometry of the material, we noticed that in all regions, the stoichiometric coefficients do not match the Ag 2 CrO 4 coefficients.These results suggest that in addition to metallic silver, fs laser-irradiation may form other products, such as simple metal oxides of silver (Ag w O x ) and chromium (Cr y O z ).
The morphology of the sample irradiated by the fs laser remained unchanged during the observation via TEM.This indicates that the stem growth is a consequence of the interactions between the sample and the fs laser irradiation.Since the fs laser-irradiated sample remained unchanged during the presence of the electron beam of the TEM, it can be assumed that the stems are thermodynamically stable because the sample received inside the TEM high-vacuum chamber energy doses of the order of 1.2 Â 10 8 kJ m À2 , which would be more than sufficient to alter the sample previously irradiated with the fs laser.
According to the EDS measurements (Fig. 8) the fs laser irradiation provokes the formation of Ag 2Àx Cr 1Ày O 4Àz and metallic Ag NPs.Therefore, aside from the sintering process the interplay between the species forming the above mentioned plasma plume might also lead to the formation of metallic Ag NPs and simpler metallic oxides. 55Moreover, the presence of different Ag nanostructure morphologies could be attributed to a welding phenomenon that is a consequence of the interaction between newly created Ag NPs with further incoming fs laser pulses.As it has been extensively reported, when the distance between two or more metal NPs is sufficiently short, hybridized plasmon modes appear producing an enhancement of the near field in the interparticle gaps, reaching enough energy to melt them, which leads to the union of the nanostructures. 78,79n addition, the non-irradiated sample was also exposed to the electron beam irradiation of the TEM and the corresponding results are presented in Fig. 9. Here, the interaction of the sample with the electron beam irradiation was completely different to that of the fs laser-irradiated sample.As soon as the electron beam interacted with Ag 2 CrO 4 (Fig. 9a), the sample decomposed rapidly, forming products that adhered to the Cu-C grid of the TEM, where the sample was placed onto (Fig. 9b).
In order to understand the influence of the electron beam irradiation, a quantitative analysis was performed, as shown in Fig. 10.Fig. 10a and b display the Ag 2 CrO 4 sample before and after electron beam irradiation, respectively.
The EDS characterization was performed for the two regions of Fig. 10b: region (1) already existed and region (2) formed as a consequence of the electron beam irradiation on the sample.
As soon as the electron beam irradiation reached the sample, the formed product spread instantly along the entire Cu-C grid of the TEM.The grid worked as a template and the formed product covered the grid over a few micrometers.The EDS characterization (Fig. 10c) shows different atomic proportions of Ag (66.1% and 39.1%), Cr (30.7% and 23.0%) and O (3.2% and 37.9%) in regions 1 and 2, respectively.When we converted the elementary percentages into molar fractions to determine the stoichiometry of the products (Fig. 10e), we noticed that in both regions, the stoichiometric coefficients do not match the Ag 2 CrO 4 coefficients.These results indicate that there may be other products formed by the electron beam, in addition to metallic silver, already observed.
The HR-TEM analysis displayed in Fig. 10d illustrates that the interplanar distances found in the formed product (2.58Å and 2.59 Å) are related to the (0 4 0) plane of the orthorhombic structure of Ag 2 O 3 , according to the JCPDS No. 40909.Furthermore, additional values of interplanar distances were found that could not be identified as metallic Ag, Ag 2 CrO 4 , or simple oxides such as Ag 2 O or CrO 3 .This confirms our hypothesis that there are other products of unknown composition being formed simultaneously to the metallic Ag.Hence, it was neither possible to detect a proper stoichiometry for Ag 2Àx Cr 1Ày O 4Àz nor an estimated ratio between formed oxides and metallic Ag.This is because the amounts of Ag, Cr, and O with respect to each formed compound cannot be determined and it is not possible to distinguish the amount of metallic Ag from the total Ag amount found in the sample by using only an EDS analysis.
The Ag NP growth process and spreading through the TEM grid can be understood as a mass transport phenomenon induced by the electron beam.The atom and particle migration along the grid occurs via diffusion and/or convection owing to the electrical potential generated by the electron beam.When the electron beam interacts with the sample, the electrons decelerate and part of the energy dissipates via repeated random scattering and absorption within an interaction volume of the specimen.This energy loss, due to inelastic collisions, is mostly used in the generation of electron-hole pairs.The generated free carriers move and redistribute around the electron-irradiated region, forming relatively stable defects, also known as selftrapped excitons (STEs).The STE formation is followed by strain and bond-breaking processes that accelerate and induce anatomic diffusion in comparison with the environmental thermal diffusion, without electron beam irradiation. 80The thermal energy inside the high-vacuum chamber is 7.73 million times higher than the environmental thermal energy.This condition enables a mass transport along the TEM grid.
As displayed in Fig. 3, Ag 2 CrO 4 is composed of three different clusters; two of them with a predominant ionic character that act as network modifiers: [AgO 4 ] and [AgO 6 ]; and one with a predominantly covalent character, which acts as a network former: [CrO 4 ].We believe that the interaction between Ag 2 CrO 4 and the different radiation forms, electronic or photonic, promotes the transitions of electrons from the occupied state (below the Fermi level) to the unoccupied state (usually above the Fermi level) in the oxygen atoms of the clusters.This causes an electron redistribution.Hence, stabilization of the crystal occurs, causing oxidation/reduction reactions.In this case, the clusters of Ag ([AgO x ], x = 4 and 6), as constituent building blocks of Ag 2 CrO 4 absorb this electronic excess, thereby reducing Ag + to Ag 0 and obtaining a non-stoichiometric oxide.As Ag 2 CrO 4 is an n-type semiconductor, it becomes an n/p-type semiconductor at the sites where the formation of Ag 0 occurs owing to the formation of internal defects which is due to the occurrence of Ag vacancies.
The control over the growth process during fs laser or electron beam irradiation is still limited.Nevertheless, an increment in the photon/electron current typically initiates or enhances the Ag growth process, which is still localized on particular nucleation sites, where Ag is drawn from distant regions of the particles.However, the fs laser and electron beam irradiation-driven processes induce the formation of non-stoichiometric compounds and/or compounds with an unknown stoichiometry.][83] Both products, Ag 2Àx Cr 1Ày O 4Àz and metallic Ag, induced by the fs laser and electron beam irradiation, are stable compounds.Their formation processes are extremely fast and deserve more profound investigations.In the future, we will investigate the effect of electron beam irradiation at cryogenic temperatures.Thus, it will be possible to determine if the mass transport occurs via convection or diffusion of atoms and to estimate the diffusion coefficients, which will help to understand the mass transport kinetics.Overall, these studies can provide a straightforward route for the design, iteration, and production of Ag and/or other metal NPs in semiconductor frameworks to develop promising applications.

Conclusions
The fs laser and electron beam irradiation of materials is a well-developed green process for the synthesis of metal NPs.In this study, we observed for the first time the growth of Ag NPs in Ag 2 CrO 4 induced by fs laser irradiation.In addition, and g-Ag 2 WO 4 48 have been studied.Various Ag-based compounds such as b-Ag 2 MoO 4 , 49 Ag 3 PO 4 , 50,51 b-AgVO 3 , 52 and Ag 2 CrO 4 53 have shown the ability to form Ag NPs under electron beam irradiation.The Ag NPs' growth on the semiconductor templates could enhance the surface plasmon resonance effect, which can dramatically amplify the absorption of visible light

Fig. 2
Fig. 2 presents the XRD patterns of the Ag 2 CrO 4 microparticles prepared using the aforementioned methodology.All diffraction peaks can be perfectly indexed to an orthorhombic structure with space group Pnma and without any deleterious phases within the detection limits of the diffractometer.The well-defined peaks indicate long-range structural order and crystallinity.The XRD patterns are in good agreement with the Inorganic Crystal Structure Database (ICSD) Card no.16298 and literature. 53,70Fig. 3 illustrates the orthorhombic structure of the Ag 2 CrO 4 microparticles.The figure is produced using the Visualization for Electronic and Structural Analysis (VESTA) program, version 3.3.2., using lattice parameters and atomic positions obtained from the crystallographic information file (CIF) no.16298.According to Silva et al., 66 the Ag 2 CrO 4 structure is composed of elongated octahedral [AgO 6 ] clusters and distorted off-centered [AgO 4 ] tetrahedra and [CrO 4 ] clusters.The tetrahedral clusters interact with the octahedral [AgO 6 ] ones, generating polarization at the short and medium distance, which is reflected throughout the crystalline lattice.These structural and electronic displacements promote the formation of induced and permanent dipoles between the different clusters,

Fig. 1
Fig. 1 Representation of the procedure employed to irradiate Ag 2 CrO 4 , where the generation of the plasma plume takes place (left), with formation of Ag 2Àx CrO 4 :Ag (right).(a) Experimental procedure and (b) schematic representation of the obtained results.

Fig. 6
Fig. 6 Average height and width distribution of Ag 2 CrO 4 microparticles non-irradiated (a and b) and fs laser-irradiated (c and d).
depicts a representative micrograph of the laser-treated samples.Four different regions are highlighted.Region (b) shows a zoomed-in particle near the stems.Through the zoom, it was possible to examine the lattice planes, which show that the reciprocal distances corresponding to the strongest reflections have the diffraction index (111).This matches with Ag according to the Joint Committee on Powder Diffraction Standards (JCPDS) No. 65-2871.Region (c) corresponds to the stems formed via fs laser irradiation.The EDS spectra indicate that this region exhibits 80.6% of Ag, 1.8% of Cr and 17.6% of O in its atomic composition.By contrast, region (d) exhibits only the presence

Fig. 8 (
Fig. 8 (Region a) TEM images of the Ag 2 CrO 4 microparticles irradiated with fs laser, (Region b) high resolution of the structure formed by fs laser radiation, (Region c) EDS characterization of the stems, (Region d) EDS characterization between the stems and the Ag 2 CrO 4 microparticle and (Region e) EDS characterization of the Ag 2 CrO 4 microparticle.

Fig. 9
Fig. 9 TEM images of the non-irradiated Ag 2 CrO 4 sample (a) before electron beam irradiation and (b) after electron beam irradiation.

Fig. 10
Fig. 10 TEM images (a) before electron beam irradiation and (b) after electron beam irradiation.(c) EDS characterization of the central (1) and peripheral (2) regions of the Ag 2 CrO 4 microparticle and (d) high resolution TEM of an Ag 2 CrO 4 microparticle.