Understanding the formation and growth of Ag nanoparticles on silver chromate induced by electron irradiation in electron microscope: A combined experimental and theoretical study

synthesized


Introduction
Silver chromate, Ag 2 CrO 4 , exhibit an orthorhombic structure belonging to the space group Pnma at room temperature [1], the constituent clusters of this system are elongated (AgO 6 ) octahedral, distorted off-centered (AgO 4 ) tetrahedral, and (CrO 4 ) tetrahedral. The oxygen atoms are coordinated to the Ag atoms to generate a three-dimensional network [2]. In the electronic structure, the upper portion of the valence band (VB) is raised by the hybridization of Ag 4d and O 2p orbitals, while the position of the conduction band (CB) bottom controls the antibonding interaction between the Cr 3d and O 2p orbitals, resulting in a narrow band gap of ∼1.80 eV [3]. This low value can enable strong absorption in the visible spectrum [2], which might help in achieving excellent photocatalytic activity under visible light [2,[4][5][6].
Recent advances in in situ electron microscopy techniques have allowed direct observation of an interesting behavior of silver compounds, such as α-Ag 2 WO 4 , i.e. by exposure to electron beams of an electronic microscope and under high vacuum. These compounds undergo in situ nucleation of Ag nanofilaments on the α-Ag 2 WO 4 surface [7][8][9] , and in this study, we have elucidated nonclassical growth mechanisms of Ag. Nevertheless, similar phenomena take place in β-Ag 2 MoO 4 [10], and Ag 3 PO 4 crystals [11].
In this paper, we present a combination of experimental and theoretical studies devoted to rationalizing early events associated with the formation of Ag nanoparticles on Ag 2 CrO 4 . First, we synthesized this material using the co-precipitation method, and their crystal structure were characterized through experimental techniques, such as X-ray diffraction (XRD) with Rietveld analysis, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS), micro-Raman (MR) spectroscopy, and ultravioletvisible (UV-vis) diffuse reflectance spectroscopy. Subsequently, we observed the electron-irradiation-induced formation of Ag nanoparticles on the Ag 2 CrO 4 surface. To complement and rationalize experimental measurements, the quantum theory of atoms in molecules (QTAIM) developed by Bader and co-workers [12][13][14] was used in order to obtain a mechanism capable of explaining the electron redistribution induced by the excess electrons in both the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jssc bulk and surface of Ag 2 CrO 4 . In addition, this theory was used to determine the relationship between the structural and electronic changes in both Ag-O and Cr-O bonds of the constituent clusters: (AgO 6 ), (AgO 4 ), and (CrO 4 ).

Synthesis of Ag 2 CrO 4
Ag 2 CrO 4 microcrystals were synthesized, without the use of a surfactant, via the co-precipitation method. For this synthesis, 50 mL of distilled water were added to 1 Â 10 À 3 mol of Na 2 CrO 4 Á 2H 2 O (99.5% pure, Alfa Aesar) and stirred at room temperature until total solution was obtained. Parallel to this process, 2 Â 10 À 3 mol of AgNO 3 (99.8% pure, Alfa Aesar) were dissolved in 50 mL of distilled water. When both solutions turned homogeneous, both temperatures were increased to approximately 90°C. A silver salt solution was then added to the chromate salt solution by means of a constant drip process, thereby leading to the formation of precipitates. This suspension was stirred at $90°C for 30 min. The precipitates were collected by centrifugation, washed several times with water and ethanol, and dried at 60°C for 24 h.

Characterization
The Ag 2 CrO 4 sample was structurally determined by XRD (DMax/2500 PC diffractometer, Rigaku) using monochromatized Cu-Kα radiation (λ ¼1.5406 Å). The diffraction pattern was performed in the Rietveld routine over 2θ range from 10 to 110°with a scanning velocity of 1°min À 1 , and a scan step width of 0.02°. The phase analysis conducted via the Rietveld method [15] was performed by using the general structure analysis system (GSAS) software [16,17]. The shapes, sizes, visualization of the Ag nanoparticles and element distribution of the Ag 2 CrO 4 microcrystals were observed by FE-SEM (Inspect F50, FEI) operated at 5 kV, and TEM operating at 200 kV and energy-dispersive spectroscopy (EDS) (Tecnai G2TF20, FEI). Furthermore, TEM images were obtained from samples prepared by drying droplets of the as-prepared samples from an acetone dispersion sonicated for 10 min, and then deposited on the Cu grids. MR measurements were conducted by using a modular Raman spectrometer (model RMS-550, Horiba, Jobin Yvon), with Ar-laser excitation at 514 nm, and a fiber-microscope operating at 400-1100 cm À 1 .

Computational details
We have studied the geometric and electronic structure of Ag 2 CrO 4 , and have derived a mechanism sequence using electron irradiation as relevant to early events for the formation and growth of Ag filaments from AgO 6 and AgO 4 and CrO 4 clusters, as its constituent polyhedral. First-principles total-energy calculations were performed within the density-functional-theory (DFT) framework using the VASP program [18,19]. The Kohn-Sham equations were solved by means of the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional [20]. In addition, the electron-ion interaction was described by the projector-augmentedwave pseudopotentials [20,21]. The plane-wave expansion was truncated at a cut-off energy of 400 eV and the Brillouin zones were sampled through Monkhorst-Pack special k-point grids; these ensure the geometrical and energetic convergence for the Ag 2 CrO 4 structures considered in this work. Vibrational-frequencies were calculated at the Γ point within the harmonic approximation, and the dynamical matrix was computed via numerical evaluation of the first-derivative of the analytical atomic gradients. Furthermore, as increase the number of electrons in the bulk structure was increased, and the volume of the unit-cell and the atomic positions of all the crystal structures were optimized simultaneously. Bader [12,22] determined the relationship between the charge density topology and elements of molecular structure and bonding. This relationship, i.e., Bader's quantum theory of atoms in molecules (QTAIM) [12,14], is now a well-established tool used to analyze the electron density, describe interatomic interactions, and rationalize chemical bonding. Atomic charges were calculated using integrations of the charge density (n) within the atomic basins, Ω, and subtracting the nuclear charge, Z, of the corresponding atom.
The computations performed in this study can be very helpful achieving a thorough understanding of the distribution of excess electrons in this material; the relation between this distribution and the structural and electronic evolution is provided. Theoretical calculations are also used to determine the effect of electron irradiation on the strength of the Ag-O and Cr-O bonds. We will discuss the role of this analysis, performed on both experimental and theoretical results, in determining the structural and electronic structure of Ag 2 CrO 4 and hence in explaining the Ag nucleation process.
The theoretical (DFT calculations) estimated

Q4
values of the lattice parameters of the Ag 2 CrO 4 structure concur with those obtained via Rietveld refinement are summarized in Table 2. Others theoretical and experimental data have been collected in Table 2 for comparison purposes.

UV-vis absorption spectroscopy and DOS analysis
The optical band gap energy (E gap ) was determined by using the method proposed by Kubelka and Munk [23]. This methodology is based on the transformation of diffuse reflectance measurements to estimate E gap values with good accuracy within the limits of assumptions when modeled in three dimensions [24]. The Kubelka-Munk Eq. (2) for any wavelength is given as: where F(R 1 ) is the Kubelka-Munk function or absolute reflectance of the sample. In that case, magnesium oxide (MgO) was used as the standard sample in the reflectance measurements. In Eq. (2), is the reflectance of an infinitely thick sample), k is the molar absorption coefficient, and s is the scattering coefficient. The optical band gap and absorption coefficient of semiconductor oxides [2] that have a parabolic band structure, are calculated by using the following equation: where α is the linear absorption coefficient of the material, hν is the photon energy, C 1 is a proportionality constant, E gap is the optical gap, and n is a constant associated with the different types of electronic transitions, n ¼ 0.5 (direct allowed), n ¼1.5 (direct forbidden), n¼ 2 (indirect allowed), and n ¼3 (indirect forbidden), respectively. Xu et al. [3] reported that the silver chromate crystals exhibited an optical absorption spectrum governed by indirectly allowed electronic transitions between the valence and conduction bands. Therefore, in this work, a value of n¼ 2 was used in Eq.
(3). Finally, using the remission function described in Eq. (2) and with the term k ¼2α and C 2 is proportionality constant, we obtain the modified Kubelka-Munk equation that is given as follows: 1.70 eV was determined from the plot of [F(R 1 )hν] 2 against hν (Fig. 2a) of the Ag 2 CrO 4 crystals.
The computed band structure and total DOS projected onto the atoms and orbitals of the Ag 2 CrO 4 structure are shown in Fig. 2b, in order to facilitate the comparison between the theoretical and experimental results. Indirect and direct band gap values of 1.37 eV (from Γ to T) and 1.51 eV (from Γ to Γ), respectively, are obtained from the calculations. This difference is attributed primarily to the distortions on both the [AgO y ] (y¼6, or 4) and [CrO 4 ] clusters at short and medium-range. An analysis of the projected DOS reveals that the top of the VB is formed mainly by the hybridization between the Ag 4d and O 2p orbitals; however, the bottom of the CB is formed mainly by the hybridization between the Cr 3d and O 2p orbitals. The major contribution to the maximum of VB is produced by O 2p and Ag 4d z 2 , 4d xz and 4d xy states, showing an overlap between O 2p and Ag 4d orbitals stronger than that between O 2p and 3d Cr ones. Besides, the bottom of CB is derived mostly from Cr 3dxz states and Ag 5s states in minor extent, with its bottom located at T-point of the Brillouin path.

Raman scattering analyses
Raman spectroscopy is considered a powerful technique to estimate the local structural order or short-range order of solids. According to group-theory analysis, the allowed representation for each Wyckoff position of the Ag 2 CrO 4 structure, described by the Pnma space group, exhibits 36 Raman-active modes. These modes correspond to the decomposition at the Γ point (i.e., Γ¼11A g þ 7B 1g þ 7B 3g þ11B 2g ). The most intense peaks and considerably weaker ones occur at wavenumbers of 750-900 cm À 1 and 350-450 cm À 1 in the high-and medium-frequency regions, respectively. These peaks correspond to the respective stretching and bending modes of the CrO 4 group, as previously reported in the literature [25][26][27]. The spectrum collected from the Ag 2 CrO 4 ( Fig. 3) exhibits broad vibrational modes, which are indicative of short-range structural disorder. Furthermore, the most intense peaks occur in the high-frequency region, at 780-816 cm À 1 , and are attributed to the symmetric stretching vibrations of Cr-O bond in the [CrO 4 ] clusters (A g mode). Small shifts in the Raman mode positions arise from various factors such as the preparation method, average crystal size, interaction forces between the ions, or the degree of structural order in the lattice. The modes in the region between 350 and 450 cm À 1 that were not experimentally observed were predicted by the first-principles calculation, suggesting that their intensity may be low in the Raman spectra. Fig. 4(a.1-6 and b.1-6) shows a time-resolved series of FE-SEM images obtained under high vacuum (1 Â 10 À 5 Pa) during the growth of Ag nanoparticles after the absorption of energetic electrons on the Ag 2 CrO 4 surface at different times: 0-5 min. Fig. 4 (c.1-6) also shows the size distribution of Ag nanoparticles after 5 min of image acquisition. As observed in Fig. 4, when the absorption of energetic electrons by the Ag 2 CrO 4 surface occurs, the nucleation of Ag nanoparticles generating Ag vacancy defects occurs, which are related to the different average size distributions of particles. The formation of Ag nanoparticles continues as the exposure time increases. However, the growth process does not have a preferred region but rather occurs in all regions irradiated by the electron beam .   1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66   67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100  101  102  103  104  105  106  107  108  109  110  111  112  113  114  115  116  117  118  119  120  121  122  123  124  125  126  127  128  129  130  131 132

TEM image and EDS pattern analyses
Proving the Ag nanoparticles growth on the material surface, Fig. 5 shows the TEM images and EDS characterization of Ag 2 CrO 4 and the composition of Ag, Cr and O in the sample. Point 1, which is located in the Ag filament, has a Ag content of 100% Ag. Point 2, which is located in the vicinity of the interface, is composed of $ 83.92% Ag, 14.64% O, and 1.42% Cr. However, points 3 and 4, which are located in the internal region of the Ag 2 CrO 4 , are composed of 78.25% Ag, 6.76% O, 14.98% Cr, and 78.42%Ag, 6.01% O, and 15.57% Cr, respectively.
The electron beam irradiation leads to the formation of defects, such as Ag vacancies ( ′ ) V Ag or oxygen vacancies ( . . in the Ag 2 CrO 4 microcrystals. The electron absorption effect on the crystal surface generates an n-type semiconductor with Ag vacancies, described by equations as follows: The formation of Ag 0 promotes Ag-nanoparticle growth on the surface of the Ag 2 CrO 4 microcrystal. According to (Eqs. (5) and 6), the microcrystal is an n-type semiconductor characterized by the oxygen vacancies. Electron irradiation of the material induces the formation of Ag vacancies, and thereby transforms the specific region to a p-type semiconductor. In fact, the size of the Ag nanoparticles increases with increasing concentration of Ag vacancies. The material is therefore transformed from an n-type semiconductor to an n-type semiconductor with p-type semiconductor in specific regions, owing to the electron-induced formation of internal defects. This formation and the corresponding structural modifications of the [AgO 6 ] and [AgO 4 ] clusters are confirmed via the theoretical analysis.   67  68  69  70  71  72  73  74  75  76  77  78  79  80  81  82  83  84  85  86  87  88  89  90  91  92  93  94  95  96  97  98  99  100  101  102  103  104  105  106  107  108  109  110  111  112  113  114  115  116  117  118  119  120  121  122  123  124  125  126  127  128  129  130  131 132 Fig. 4. FE-SEM images of the Ag 2 CrO 4 microcrystals (left) and growth of the Ag nanoparticles on the Ag 2 CrO 4 surface at 0, 1, 2, 3, 4, and 5 min (right), respectively; histograms of Ag particle size distribution at different times.

Electronic structure analysis
In light of the experimental results shown in Figs. 4 and 5, theoretical calculations were performed in order to understand the phenomenon of electron absorption that leads to the growth of Ag nanostructures. A maximum of seven electrons were introduced into the octahedral [AgO 6 ] and tetrahedral [AgO 4 ] and [CrO 4 ] clusters of the orthorhombic Ag 2 CrO 4 structure (see Fig. 6).
The theoretically estimated values of the lattice parameters, and bond lengths as a function of the number of electrons (N) are shown in Table 3. As the table shows, the lattice parameters and the lengths of the Ag1-O and Ag2-O bonds increase significantly with increasing number of electrons; however, the opposite trend is observed in the case of the Ag1-Ag2 bond.
For  Fig. 7. Theoretical calculations indicate that Ag atoms of [AgO 6 ] and [AgO 4 ] clusters have a higher tendency to be reduced rather than the Cr atoms. Particularly at N ¼6, Ag1 and Ag2 atoms are practically reduced (from 0.81e to 0.01e) and almost reduced (from 0.74e to 0.14e), whereas for Cr atoms a constant value of the electron density is maintained (slight decrease of 0.25 units at N ¼7).
Consequently, the electron excess imposed in the material is transferred from one cluster to another through the Ag 2 CrO 4 lattice, so that the formation and growth processes of Ag involve adjacent [AgO 6 ] and [AgO 4 ] clusters.

Conclusion
In this work, we report the synthesis of Ag 2 CrO 4 via a simple precipitation method and, the first-ever in situ formation and growth of Ag nanoparticles on the surface of the Ag 2 CrO 4 . The XRD pattern present that the silver chromate microcrystals have orthorhombic structure with a space group Pnma and well-defined diffraction peaks, which indicate a structural order and crystallinity at long range, confirmed by the Rietveld refinement data. The material also exhibited short range structural disorder, as indicating by the Raman-active mode. Moreover, the E gap value, determined from UV-vis measurements, corresponded to the energy level between the VB and CB. The growth of the Ag-nanoparticles during electron beam irradiation, was monitored in real-1  2  3  4  5  6  7  8  9  10  11  12  13  14  15  16  17  18  19  20  21  22  23  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  41  42  43  44  45  46  47  48  49  50  51  52  53  54  55  56  57  58  59  60  61  62  63  64  65  66 time; the growth of the nanoparticles increased with increasing irradiation time. This growth was evaluated by using an FE-SEM and a TEM with EDS capabilities. In fact, these techniques were used to characterize the microcrystals and the Ag nanoparticles obtained.
Defects on the [AgO 6 ] and [AgO 4 ] clusters are generated by electron-induced modifications in the structure, and provide favorable conditions for the formation of Ag nanoparticles. In addition, these modifications in the interior material region under electron irradiation are capable of changing the n-type semiconductor to an n-type semiconductor with p-type semiconductor in specific regions. The results of theoretical calculations performed in this work, exhibit close correspondence to the Ag-nanoparticle growth observed experimentally.