Proof‐of‐Concept Studies Directed toward the Formation of Metallic Ag Nanostructures from Ag3PO4 Induced by Electron Beam and Femtosecond Laser

In this work, for the first time, the instantaneous nucleation and growth processes of Ag nanoparticles on Ag3PO4 mediated by femtosecond laser pulses are reported and analyzed. The investigated samples are pure Ag3PO4 sample, electron‐irradiated Ag3PO4 sample, and laser‐irradiated sample. Complete characterization of the samples is performed using X‐ray diffraction (XRD), Rietveld refinements, field emission scanning electron microscopy, and energy dispersive spectroscopy (EDS). XRD confirms that the irradiated surface layer remains crystalline, and according to EDS analysis, the surface particles are composed primarily of Ag nanoparticles. This method not only offers a one‐step route to synthesize Ag nanoparticles using laser‐assisted irradiation with particle size control, but also reports a complex process involving the formation and subsequent growth of Ag nanoparticles through an unexpected additive‐free in situ fabrication process.


Introduction
The properties derived from the interaction of electrons/waves with matter have a key role in modern science and engineering. Energetic particle and/or electromagnetic interactions with solid materials have been studied for several decades, and the www.advancedsciencenews.com www.particle-journal.com formation of Ag NPs on αAg 2 WO 4 with bactericidal proper ties; [17] ii) the synthesis of metallic Bi NPs with coexisting crys tallographic structures (rhombohedral, monoclinic, and cubic) on NaBiO 3 ; [33] iii) the formation of In NPs on InP; [34] and iv) the synthesis of Ag-Bi nanoalloys from inorganic oxide Ag 2 WO 4 and NaBiO 3 targets. [35] Both fs and electron beam irradiation strategies allow us to obtain Ag NPs with interesting techno logical applications such as photoluminescent and bactericide materials. A hybrid heterostructure formed by Ag NPs and Ag 3 PO 4 , Ag/ Ag 3 PO 4 , was, previously, successfully synthesized, and its appli cation as a highly efficient and stable plasmonic photocatalyst is validated. [36][37][38][39][40][41][42][43] In this context, recently we reported and ana lyzed the formation of Ag nanostructures on Ag 3 PO 4 induced by electron beam irradiation. [32] Herein, inspired by and as a continuation of these previous works, we report a systemic investigation of this phenomena and demonstrate a growth of Ag NPs on a surface of Ag 3 PO 4 induced by fs laser irradiation. The focus of this paper is threefold: i) to report, for the first time, the instantaneous nucleation and growth processes of Ag NPs on Ag 3 PO 4 mediated by fs laser pulses; ii) to investi gate samples that were pure Ag 3 PO 4 (pAP), electronirradiated Ag 3 PO 4 (eAP), and laserirradiated (lAP); iii) to compare this phenomenon with previous observations where these processes that occur on Ag 3 PO 4 are driven by an accelerated electron beam from an electronic microscope under high vacuum; and iv) to gain an improved understanding of this phenomenon and allow a finer control to future technological applications. Experimental techniques such as energydispersive Xray spec troscopy (EDS) and transmission electron microscopy (TEM) with a highangle annular dark field (HAADF) provide a valu able probe into the relationship between atomicscale structural and electronic perturbations produced by fs laser pulses and electron beam material modification. Figure 1 displays the Xray diffraction (XRD) patterns of the prepared samples. The diffraction peaks are all in agreement with the results reported in Inorganic Crystal Structure Data (ICSD) No. 1530, which are related to Ag 3 PO 4 phase with a bodycentered cubic structure (space group P-43n). [32,44] No secondary phases were observed, even after the electron or laser irradiation. The narrow profiles of the diffraction peaks are related to a longrange structural ordering in these sam ples. The pAP and eAP XRD patterns are similar; the principal differences are observed in the lAP sample. In this specific case, the XRD pattern is less resolved and certain diffraction peaks no longer appear, such as the ones referred to as the (220), (411), (332), (422), and (510) peaks. Thus, in the lAP sample, laser irradiation induces a higher structural disorder at the long range of the Ag 3 PO 4 crystalline structure. An analysis of the full width at half maximum (FWHM) of the most intense peak of the XRD patterns related to plane (210) was performed to understand the degree of order/disorder among the samples at long range. The pAP sample had an FWHM of ≈0.07°, causing this value to increase with different types of irradiation; the value was 0.016° for eAP and 0.025° for lAP. It was determined that when Ag 3 PO 4 is subjected to fs laser/electron beam irradi ation, a higher degree of disorder is added to the new material, caused by distortions in the crystal lattice of the Ag 3 PO 4 .

Results and Discussion
Rietveld refinements [45] were employed to understand the differences in the structural arrangements of the prepared sam ples. In this work, the refinements were performed through the general structure analysis program (GSAS), [46] assuming the spatial groups P-43n for the cubic structure of the cen tered body of Ag 3 PO 4 and adjusted to ICSD No. 1530. [32] The plots of the refinements in Figure S1 (Supporting Information) are in perfect agreement with the XRD patterns presented in Figure 1. Table 1 lists the fitting parameters (R wp , R p , R Bragg , and χ 2 ), which revealed an acceptable adjust between the theo retical and observed XRD patterns. The results obtained from the refinement revealed similar lattice parameters for all sam ples as indicated in Table 1. However, the volume of the unit cell linearly decreases from the microcrystalline pAP and eAP, and thereafter from the lAP, where the results indicate a small  tively. This could be because the structure underwent a process of cell contraction, possibly forming a high density of Ag vacan cies (V Ag ). The schematic representation of the unit cell of cubic lAp is illustrated in Figure 2.
Raman spectroscopy was performed as a complementary technique to XRD to evaluate the order/disorder of the short range. Figure 3 displays the Raman spectra obtained at room temperature for the pAP, eAP, and lAP samples. According to the analysis derived from the group theory, Ag 3 PO 4 exhibits 18 active modes in the Raman spectrum, corresponding to Γ = 2A1 + 4E + 12T2. [32,47] The bands at 77 and 223 cm −1 are associated with external translational and rotation modes of the [PO 4 ] clusters. The bending vibration modes related to the [PO 4 ] group were found at 406 and 551 cm −1 . The band at 709 cm −1 corresponds to symmetrical stretching vibrations of POP linkages in the [PO 4 ] clusters. The band located at 908 cm −1 is related to the symmetrical stretching vibrations of [PO 4 ], and asymmetrical stretching was verified at 951 and 1001 cm −1 . [32,[48][49][50][51] In the Raman spectra displayed in Figure 3, changes in the intensity of the vibrational modes related to the [PO 4 ] clus ters such as those at 223, 551, 908, 951, and 1001 cm −1 can be observed. This behavior is more pronounced for lAP, indicating an important shortrange disorder in this sample. Hence, a higher concentration of structural defects than pAP and eAP samples is provoked by the interaction of the fs laser. Xray photoelectron spectroscopy (XPS) measurements were performed to compare the pAP, eAP, and lAP samples. The Figure 4a indicates the presence of Ag, P, and O peaks, confirming the high degree of purity of the samples. Peaks related to C were also observed, which are related to the carbon pollution from the XPS instrument itself. [52,53] The highresolution XPS spectra in Figure 4b-d indicate two peaks with binding energies of ≈367 and 373 eV, attributed to Ag 3d5/2 and Ag 3d3/2 orbitals, respectively. [53,54] Moreover, each of these peaks could be fitted in two separate components, indicating the presence of Ag in varied valences. These asymmetric peaks were fitted as described in Figure 4b-d at 367.13/367.97 eV for Ag 3d5/2 and at 373.14/373.98 eV for Ag 3d3/2. The highintensity peaks at 367.13 and 373.14 eV correspond to Ag + ions, whereas the lowintensity peaks at 367.97 and 373.98 eV are associated with metallic Ag. [55][56][57][58][59][60] In the formed Ag/Ag 3 PO 4 heterostructure, the amount of metal Ag in the microcrystals samples is considerably variable, i.e., the metallic Ag content for the pAP, eAP, and lAP samples was calculated, considering a mean area of metallic silver on the surface of the samples, to be 18.86%, 23.12%, and 19.85%, respectively. A higher metal Ag 0 content is observed on the surface of the eAP sample, which is possibly associated with the larger amount fixed to the surface of the microcrystal; this is confirmed by the measurements of the scanning microscopy.

XPS spectrum displayed in
To examine the chemical characteristics of the P 2p bond, we verified the highresolution P 2p level spectra for all the samples analyzed. Figure S2 (Supporting Information) clearly indicates the spin-orbital division between the P2p1/2 and P2p3/2 peaks in the P 2p corelevel spectra, which corresponds to the characteristic binding energy of the P 5+ oxidation state in Ag 3 PO 4 ; the samples do not indicate significant differences.
Field emission scanning electron microscopy (FESEM) images of pAP, eAP, and lAP exposure are displayed in Figure 5. In all the samples, irregular spherical microparti cles with large size dispersion and aggregates can be observed. The particle dimensions for pAP, eAP, and lAP are 712, 468, and 432 nm, respectively. Further, it was possible to observe the nucleation and growth processes of metallic Ag NPs on the Ag 3    www.advancedsciencenews.com www.particle-journal.com reduction processes of the Ag + species are provoked by the electron beam and laser irradiations. The metallic Ag nano particles on the eAP sample are predominantly rod shaped, whereas the majority of the Ag nanoparticles on the lAP sample are composed of irregular spheres. The samples, pAP, eAP, and lAP, were also characterized by TEM. Figure 6a-c displays the obtained TEM images at low magnification. As in the case of Figure 5, the nucleation of Ag nanoparticles on the surface of the samples was also observed for eAP (nanorods) (Figure 6b) and lAP (irregular nanospheres) (Figure 6c). Moreover, owing to the high energy of the TEM analysis, the pAP also presented the initial stage of growth of Ag nanoparticles, which was included for comparison. To con firm this behavior of the nucleation of the Ag nanostructures, EDS analysis was also performed in two distinct regions of each image (Ag 3 PO 4 and Ag nanoparticles). Region 1 was selected in the center of the Ag 3 PO 4 microparticle; Region 2 was selected in the Ag NPs. The results confirmed the presence of Ag, P, and O in Region 1 and predominantly Ag in Region 2, con firming the presence of metallic Ag in all samples. Cu and C refer to the sample port grid.
Different morphologies of Ag NPs were observed for both samples, as indicated in Figure 7a,b. The eAP sample presents a 1D nanostructure in the form of nanorods, Figure 7a, whereas the lAP sample presents a 0D nanostructure in the irregular spherical shape, Figure 7b. This difference in morphology of Ag nanoparticles is caused by the differences in the formation process, leading to different time, temperature, and pressure  conditions. For fs laser irradiation, the sample can reach a pres sure of around 1010 Pa and a temperature of 1000 K, [17] thus causing the formation of a large number of confined photons in a small area. Moreover, when irradiated with fs pulses, the sample is in the steady state, that is, it does not present vibrational and rotational movements, since these occur in the second peak scale, which makes it difficult to follow the dynamic process of particle nucleation. [15,35] These Ag nanoparticles were analyzed using highreso lution TEM (HRTEM) in order to find the crystalline planes. The surface of these nanoparticles with light edges indicates a monocrystalline nature. This is confirmed by the reciprocal distances, with the strongest frame cubic metallic Ag nanoparti cles characterized by a (111) plane with an interplanar distance of 0.23 nm (Figure 7), corresponding to PDF 89-3722 in the JCPDS (Joint Committee on Powder Diffraction Standards). [32] In this part of the study, the initially formed Ag/Ag 3 PO 4 composite of eAP and lAP samples were irradiated by a gradual converging electron beam. For this experiment, the electron beam of the TEM was condensed over these particles to increase the current density and to promote structural transformations. Figure 8a,b displays the selected regions. An analysis of the results verifies that small and welldispersed nanoparticles were formed near the initial Ag nanorod (eAP) or Ag irregular spheres (lAP) a few nanometers away from the initial Ag NPs. The HRTEM images in the insets of both figures demonstrate an interplanar distance of 0.23 nm for these particles, which can be associated with the (111) plane of cubic metallic Ag. [61] Thus, the exposure of a secondary electron beam from TEM leads to the growth of metallic Ag from the initial particles obtained in the eAP and lAP samples. Figure 8a,b indicates that the dimensions of these particles are in the range of 2.26 and 3.36 nm for samples eAP and lAP, respectively. The Ag cations migrate from the matrix to the surface, resulting in structural and morphological modifications with the appearance of Ag vacancies. [32] The above results can be viewed as an example of the elec tron beam-induced fragmentation process of the initial Ag particles by exposure to a condensed electron beam. This is a wellknown process that results from the transfer of thermal energy and electric charge. [62,63] This phenomenon occurs in metallic samples that can melt and collapse into spherical units as the irradiating current density increases. The newly formed Ag NPs are expelled and "fly" in the carbon grid for several nanometers, as observed in the eAP and lAP samples. Figure 9 presents the proposed growth mechanisms of the Ag NPs. The structure was irradiated with different energy levels, originating two morphologies associated with different metal nanoparticles, a nanobastones morphology, related to electron irradiation, and irregular spheres for fs laser irradiation.
The universality of Ag NPs generation through electron beam and/or fs laser pulses naturally indicates the existence of a common mechanism, and in both cases, the interaction of the laser/electron beam on the Ag 3 PO 4 provokes the reduction of Ag + ion to form metal Ag 0 . When the surface of the Ag 3 PO 4 semiconductor is irradiated with an fs laser, the electrons of the valence band are excited to the conduction band. Because the electrons provide the bonding forces that hold atoms together in the lattice, if their distribution within the material changes significantly, the bonding forces can also change.  www.advancedsciencenews.com www.particle-journal.com presented in Figure 9. When the crystal of pAP is electronirra diated, there is a reduction of Ag cations along an ordered move ment from the bulk to the surface of the crystal. This promotes the formation of metallic Ag on its surface, which depends on the irradiation time and density of the injected electrons.
Conversely, laser irradiation promotes an instantaneous Ag growth. Owing to the ultrashort pulse duration, fs scale, the pAP crystal loses symmetry and the metallic Ag NPs grow rapidly in a disorderly manner. These two experiments prove that elec tron and photons produce the same effect over the irradiated material. In this manner, the photoreduction and electron reduction effects, followed by an atomic displacement and Ag crystallization, in the two experiments on the surface of a semiconductor crystal can be observed; see Figure 9b. The valence electrons responsible for the chemical bonds that hold the solids together are thus easily removed during the laser ablation process. This causes a mutually repulsive state between the atoms whose chemical bonds are broken, and the agglom erate explodes owing to mutual electrostatic repulsion of the ions.

Experimental Section
Synthesis-Synthesis of Ag 3 PO 4 Microcrystals: The methodology employed for the synthesis of the Ag 3 PO 4 microcrystals was a simple chemical precipitation at room temperature, as described in detail previously. [32]    www.advancedsciencenews.com www.particle-journal.com formation of yellow Ag 3 PO 4 precipitate. The mixture was then stirred for 10 min. The precipitates obtained were washed several times with deionized water and centrifuged to remove the by-products formed during the reaction. The resulting powder was dried at 60 °C for several hours under air atmosphere. The sample obtained in this procedure was referred to as the pAP. Synthesis-Electron Beam Irradiation of Ag 3 PO 4 : A Carl Zeiss DSM940A scanning electron microscope (Germany) with an accelerating voltage of 30 kV was used to irradiate the sample with electrons. In this procedure, the pAP powder was placed in a cylindrical sample holder. The electron beam exposure time was fixed at 5 min. The sample obtained here was referred to as the eAP.
Synthesis-Femtosecond Laser Irradiation of Ag 3 PO 4 : The fs laser irradiation scheme is displayed in Figure 10. The pure Ag 3 PO 4 (pAP) sample was irradiated with a Ti:sapphire laser (Femtopower Compact Pro, Femto Lasers) emitting 30 fs pulses, FWHM, with a central wavelength of 800 nm and a repetition rate of 1 kHz. An iris was used to obtain a 6 mm laser beam that was focused on the surface of a powder target of Ag 3 PO 4 using a 75 mm lens which allowed this study to obtain a focal spot in the processing plane with a diameter of about 21 µm FWHM. The pAP sample was placed at the bottom of a quartz cuvette attached to a 2D motion-controlled stage moving at a constant speed of 0.5 mm s −1 . To ensure that the laser interacted with the entire sample, the irradiation process was repeated four times after stirring the sample each time. A mean power of 10, 80, and 200 Mw, was used to irradiate the sample. These provided a fluence over the sample of about 3, 24, and 60 J cm −2 , respectively. Experimentally, it was observed that the results of the synthesized nanoparticles with these different laser parameters were similar. Previously, it was shown that the optimal conditions for the irradiation of semiconductors by femtosecond lasers result when the sample was irradiated with a mean power of 200 mW. [17,33] Finally, to ensure that the pulse duration of the laser at the focal plane was 30 fs, a user-adjustable postcompression stage, based on a pair of fused silica Brewster prisms,   www.advancedsciencenews.com www.particle-journal.com was employed to compensate for the dispersion in the beam delivery path. The sample obtained in this procedure was referred to as the fs lAP. Although for comparison with the previous papers, a femtosecond laser was employed, [17,[33][34][35] further studies could be conducted in order to elucidate if it is possible the formation of metallic Ag by irradiation of pAP with other kind of lasers, as it happens in other Ag-containing compounds. [64,65] Characterization: The pAP, eAP, and lAP samples were structurally characterized by powder XRD using a Rigaku D/Max-2500 PC diffractometer (Japan) with Cu Kα radiation (λ = 0.15406 nm). Data were collected in a 2θ range of 10-110° using a step scan rate and step size of 1° min −1 and 0.02°. Raman spectroscopy was performed using a Horiba Jobin-Yvon IHR550 (Japan) spectrometer coupled to a CCD detector and a Melles Griot, United States laser (USA), operated at 633 nm. XPS was performed using a Scienta Omicron ESCA+ spectrometer with a highperformance hemispheric analyzer (EA 125) with monochromatic Al Kα (hν = 1486.6 eV) radiation as the excitation source. The operating pressure in the ultrahigh vacuum chamber (UHV) during analysis was 2 × 10 −9 mbar. Energy steps of 50 and 20 eV were used for the survey and high-resolution spectra, respectively. The binding energies of all elements were calibrated by referencing to the C 1s peak at 284.8 eV.
The morphological features of Ag 3 PO 4 microparticles and the effect of electron beam and fs laser exposure on the growth mechanisms of metallic Ag NPs were first examined by FE-SEM with a Carl Zeiss Supra 35VP (Germany) microscope operating at 5 kV. In this procedure, the microscopic images were recorded as soon as possible to avoid influence of the electron beam during the FE-SEM characterization of the prepared samples. TEM and HRTEM microscopic images, as well as EDS analysis, were performed with an FEI TECNAI F20 (Netherlands) microscope operating at 200 kV. HAADF image and EDS mapping were recorded in scanning transmission mode. The samples were prepared by depositing small amounts of the powders directly onto holed carboncoated Cu grids to capture information regarding the nucleation and growth mechanisms of the metallic Ag.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.