AgCl Heterogeneous

Visualizing Redox Dynamics of a Single Ag/ AgCl Heterogeneous Nanocatalyst at Atomic Resolution Yimin A. Wu, Liang Li, Zheng Li, Alper Kinaci, Maria K. Y. Chan, Yugang Sun,† Jeffrey R. Guest, Ian McNulty, Tijana Rajh, and Yuzi Liu* Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Operando characterization of gas−solid reactions at the atomic scale is of great importance for determining the mechanism of catalysis. This is especially true in the study of heterostructures because of structural correlation between the different parts. However, such experiments are challenging and have rarely been accomplished. In this work, atomic scale redox dynamics of Ag/ AgCl heterostructures have been studied using in situ environmental transmission electron microscopy (ETEM) in combination with density function theory (DFT) calculations. The reduction of Ag/AgCl to Ag is likely a result of the formation of Cl vacancies while Ag+ ions accept electrons. The oxidation process of Ag/AgCl has been observed: rather than direct replacement of Cl by O, the Ag/AgCl nanocatalyst was first reduced to Ag, and then Ag was oxidized to different phases of silver oxide under different O2 partial pressures. Ag2O formed at low O2 partial pressure, whereas AgO formed at atmospheric pressure. By combining in situ ETEM observation and DFT calculations, this structural evolution is characterized in a distinct nanoscale environment. KEYWORDS: environmental TEM, in situ TEM, Ag/AgCl heterogeneous nanocatalyst, density functional theory

A

realization of in situ ETEM operating at atmospheric pressure.9,19,20 Hence, in situ visualization of gas−solid reactions on a single nanocatalyst at atmospheric pressure is achievable. Here, we report a direct visualization of redox process of a single Ag/AgCl heterostructure at the nanoscale using an in situ TEM equipped with a nanoreactor under atmospheric pressure. By combining in situ ETEM and DFT calculations, we were able to capture the local phase transition. This work focused on Ag/AgCl heterogeneous plasmonic photocatalysts, which has attracted substantial interest owing to two distinct characteristics. One is the Schottky junction between the metallic Ag nanoparticle and semiconductor AgCl interface.21 This Schottky junction leads to an intensive electric field inside the space charge region, which can enhance the separation of the photogenerated charge carriers within the photocatalyst.21 The other one is the localized surface plasmon resonance (LSPR) on the Ag nanoparticle, which enhances visible light harvesting due to the dipole formation and collective oscillation of surface plasmons under illumina-

tomic scale details in heterogeneous gas−solid catalyst are important for understanding the reactions and activity.1,2 In situ monitoring and controlling the gas− solid catalyst reactions at the nanoscale adds indispensable insight for the development of catalysts for a wide range of energy and environmental technologies.3 Although many in situ and operando X-ray spectroscopy techniques are available,4−8 it remains a great challenge to obtain atomic scale information on structural transformations and reactivity of nanocatalysts under actual reaction conditions. Measurements using X-ray spectroscopy cannot provide direct information on a single nanocatalyst due to the limited spatial resolution of X-ray techniques.9 In situ environmental transmission electron microscopy (ETEM) has the capability to monitor the gas− solid interaction on a single nanoparticle due to its high spatial and temporal resolution.10−12 Extensive efforts have been devoted to developing in situ ETEM techniques by modifying the transmission electron microscope (TEM) to accommodate an environmental cell using the differential pumping strategy;13−17 however, the column pressure was limited to millibars,13−18 which is well below the atmospheric pressure often used in general catalysis. The recent development of nanoreactors using nanofabrication techniques allows for the © XXXX American Chemical Society

Received: January 15, 2016 Accepted: March 2, 2016

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Figure 1. (a) Schematic of a nanoreactor for ETEM. The nanocatalysts are confined between two silicon nitride (Si3N4) membranes with the reacting gas flowing. The MEMS heating chip is built on Si3N4 to control the temperature. (b) Atomic model of Ag/AgCl heterostructure with an interface between Ag (111) and AgCl (100).

tion.22−25 This strong optical absorption in the visible light region can enhance the solar energy conversion efficiency.26 These nanoparticles also show great potential as photocatalysts for processes aimed at environmental protection, such as the degradation of organic dyes, reduction of hexavalent chromium, and oxidation of water pollutant phenol to CO2 and H2O.27 Recent studies have been focused on the mechanism of electron transfer between Ag and AgCl domains during the oxidation process. Two different electron transfer mechanisms have been proposed for the oxidation reaction with O2. Tang et al. showed an ultrafast surface plasmon resonance electron transfer process from Ag to the conduction band of AgCl. Subsequently, the electrons are trapped by O2 to form reactive oxygen radicals.28 On the other hand, Wang et al. showed that the surface plasmon resonance electrons remain in the Ag nanoparticle and then these electrons are trapped by O2 to form the reactive oxygen radicals.22 However, in particular, no direct observation has been made at the nanoscale to understand the redox reaction of Ag/AgCl nanocatalyst from the structural dynamics point of view. Here, we report on the visualization of redox dynamics of a single Ag/AgCl nanocatalyst at the nanoscale by using ETEM. The observations are consistent with a reduction of Ag/AgCl due to the formation of Cl vacancies and Ag+ ions accepting electrons. In the oxidation of Ag/AgCl, we found indications of a two-step process rather than the direct replacement of Cl by O.

with interface between Ag (111) and AgCl (100). This model was built based on the experimental observation for illustration. Ex Situ Characterization of Ag/AgCl Heterostructure. The sample synthesis method was reported elsewhere29 and briefly provided in the Materials and Methods section. Figure 2a and b show a low magnification overview TEM image and a

Figure 2. Ex situ characterization: (a) low magnification TEM image of Ag/AgCl nanoparticles. (b) Single Ag/AgCl nanoparticle. (c) High resolution transmission electron microscopy (HRTEM) image of AgCl lattice on the Ag/AgCl nanoparticle from the red region indicated in (b). (d) FFT image of (c). (e) HRTEM image of the Ag lattice on the Ag/AgCl heterostructure with an inset image showing Ag nanoparticle; scale bar of inset image is 5 nm. (f) FFT image of the blue region indicated in (e).

RESULTS AND DISCUSSION Experimental Setup and Atomic Model of Ag/AgCl Heterostructure. The ETEM experiment was realized by using a gas flow holder with nanoreactor on a JEOL 2100F operated with accelerating voltage of 200 kV. Figure 1a shows a schematic of in situ gas ETEM using a nanoreactor constructed through silicon nanofabrication.19,20 Two silicon chips were coated with Si3N4 membrane and separated by a spacer with thickness of 100 nm.20 A MEMS heating chip was employed to control the temperature. The Ag/AgCl nanoparticles were sandwiched in the nanoreactor. The gas flow rate controller and heating controller were connected to the nanoreactor via a Hummingbird holder (Hummingbird Scientific Inc.) to control the gas flow and the temperature. During the reaction, the structural evolution of the samples was monitored with TEM and the images were recorded by a CCD detector. Figure 1b shows an atomic model of Ag/AgCl heterogeneous structure

single Ag/AgCl heterostructure, respectively. Clearly, each heterostructure has two components with bright (AgCl) and dark (Ag) regions, identified as such due to the mass contrast. Figure 2c shows the lattice fringes from the brighter region, indicated with a red box in Figure 2b. The fast Fourier transform (FFT) of Figure 2c is shown in Figure 2d. Peaks separated by 45° with d spacing values of 0.20 and 0.28 nm were resolved; these correspond to AgCl (220) and (200) planes, respectively. It is consistent with the expectation that the bright regions is AgCl component. The dark counterpart in the heterostructure is shown in a HRTEM image in Figure 2e with an inset showing its morphology. The FFT of the blue region in Figure 2e, shown in Figure 2f, indicates polycrystallinity with resolved d spacings of 0.24 and 0.21 nm, corresponding to the Ag (111) and (200) planes, respectively. B

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ACS Nano Reduction of Ag/AgCl Heterostructure. In order to understand the reduction dynamics of Ag/AgCl heterostructure, in situ ETEM was performed at room temperature in conventional TEM column (not using the nanoreactor). The electron beam has two major functions in the experiment, acting both as the reducing agent and the imaging probe. The high energy electron beam acts as a reducing agent because it can donate electrons to the Ag/AgCl nanocatalyst. Movie S1 in the Supporting Information (SI) shows the AgCl reduction process. Figure 3 shows the sequential images extracted from

Figure 4. (a)−(g) show the pictures taken at room temperature to illustrate the Ag/AgCl lattice dynamics during the reduction: (a) the initial state of the Ag/AgCl heterostructure. (b) FFT image of the blue region in (a). (c) FFT image of the red region in (a). (d) Catalytic front propagation during the reduction. (e) FFT of the purple region in (d), indicating the propagation direction of the catalytic activity. (f) Final state of Ag/AgCl heterostructure after the reduction. (g) FFT image of (f), indicating that AgCl is completely reduced to Ag with the formation of Ag2O at room temperature. (h) SAED pattern at room temperature. (i) SAED pattern after heating in TEM column at 200 °C for 1 min. (j) EELS on the nanoparticle after heating in TEM column at 200 °C for 1 min to show the small amount of oxygen (red curve is smoothed).

Figure 3. Progress of Ag/AgCl reduction by the electron beam at a pressure of 10−5 Pa. (a)−(i) Time sequential images of the reduction dynamics. The brown line in (f) indicates the (200) AgCl crystal plane. The purple arrow along [100] indicates the propagation direction of the structural transition during the AgCl reduction.

AgCl nanocrystal, it appears to be on the order 10−20 nm on each side in the projected image in Figure 3. Therefore, the proposed mechanism is consistent with the observed increase in Ag nanoparticle size. A more detailed analysis was performed to understand the reduction dynamics. Figure 4a shows the initial state of the Ag/ AgCl heterostructure with lattice resolved. The FFT image is shown in Figure 4b corresponding to the blue region on Figure 4a. One can clearly resolve the AgCl (200) planes with d spacing of 0.28 nm. Figure 4c shows the FFT of the area indicated by red square in Figure 4a. It indicates that the Ag nanoparticle composed of multiple (111) twins.30 These results are consistent with the analysis of another Ag/AgCl particle shown in Figure 2. At 300 s, the dramatic crystal plane propagation began. In order to analyze the movement of structural transition front and its propagation direction, FFT imaging (shown in Figure 4e) was performed at the purple region in Figure 4d. The plane perpendicular to the propagation direction has a d spacing of 0.28 nm, which corresponds to the AgCl (200) plane; therefore, the propagation direction is identified as [100]. The final state (at 361 s) of Ag/AgCl heterostructure is shown in Figure 4f, in which all the AgCl lattice has been depleted. Also, from electron energy loss spectroscopy (EELS), we did not find any Cl signal from this nanoparticle. But there is very weak oxygen K edge (similar like Figure 4j). Because the scattered electrons

the Movie S1. From the 0 to 300 s, the nanoparticle did not show any dramatic structural change. Movie S2 was calculated from movie S1 with sequential FFT images (Figure S1) in the SI; this shows the lattice dynamics during the reduction process in reciprocal space. A dramatic change in the structure was observed after 300s. The AgCl facet was found to recede, whereas the Ag nanoparticle did not change significantly. The brown line in Figure 3f indicates the AgCl (200) lattice plane (obtained by analysis of FFT in Figure 4), which was the moving front of the structural transformation. The purple line indicates the propagation direction of the reaction. The speed of the propagation was found to be approximately 0.34 nm/s in this experiment. Eventually, at 355 s, the AgCl lattice disappeared completely. It is worth noting that the projected area of the Ag nanoparticle increased about 9% through comparing with the Figure 3a and i. This is likely due to extra Ag atoms from AgCl reduction diffusing and coating onto the surface of the existing Ag nanoparticle. On the basis of the calculations shown in the SI, if all the Ag atoms from a cubic AgCl nanocrystal measuring 14 nm on each side are deposited onto the surface of a 25 nm diameter Ag nanoparticle, the resultant increase in projected area is expected to be 9%. Although we do not have the exact initial 3D dimensions of the C

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corresponds to the generation of a backscatter electron (BSE). Emax is quantified as35

can interact with the particles in other areas to generate X-ray signals, which can be collected by energy-dispersive X-ray spectroscopy (EDS) detector, EELS is the better approach to characterize the composition of a single nanoparticle. The FFT image of Figure 4f is shown in Figure 4g. In addition to the dominant diffraction spots from Ag (111) planes, there are a couple pairs of spots corresponding to planes with lattice spacing ratio of 1.24 (the large lattice spacing is ∼0.33 nm). As expected, these can be assigned to the Ag2O (110) and Ag2O (111) planes. There are no such lattice planes in Figure S1, which shows the time series FFT during the reduction process. This suggests that a small amount of surface oxide only formed after 355 s, when the reduction process had concluded. The formation of surface oxides may be due to the inevitable existence of O2 molecules around Ag/AgCl nanoparticles even at low pressure in the TEM column. These O2 molecules can adsorb on Ag facets at very low pressure and lead to the formation of surface oxides with different stoichiometry that depends on the termination of the surface.31 If O2 is adsorbed on a close-packed crystal plane, it tends to form Ag2O, whereas the AgO could form if O occupies octahedral sites.31 With the dominant close-packed Ag (111) planes in our experiment, the O2 molecules tended to adsorb onto the Ag (111) surface and form Ag2O. Note that the formation of Ag2O in Figure 4g is at room temperature and total pressure of 10−5 Pa. The partial pressure of oxygen is not known, but previous DFT calculations have found thermodynamic favorability for forming surface Ag2‑xO on Ag(111) under an oxygen partial pressure as low as 10−12 Pa,32 so the formation of Ag2O under such conditions is plausible. AgO is not expected to form under this experimental condition, taking into account the limited concentration of oxygen and the kinetic hindrance for atomic rearrangement during oxidation.33,34 In order to evaluate the effect of temperature on the reduction process, the Ag/AgCl heterostructures were heated to 200 °C by using the conventional furnace-based heating holder in the TEM column for 1 min. Also, the electron beam was open for approximately 30 s in total to take the selected area electron diffraction (SAED) patterns before and after the heating process. Figure 4h shows the SAED pattern at room temperature, with Ag (111) (d spacing of 0.24 nm), Ag (200) (d spacing of 0.21 nm), AgCl (111), and AgCl (200) (measured AgCl (111)/AgCl (200) = 1.14) resolved. After heating in the TEM column at 200 °C for 1 min, Ag/AgCl heterostructure was reduced to Ag while Ag2O (110) and Ag2O (111) (measured Ag2O (110)/Ag2O (111) = 1.23) were also resolved in Figure 4i. Indeed the EELS in Figure 4j was acquired on the nanoparticle after the heating shows the presence of the oxygen K edge around 535 eV. The signal is weak, indicating that only a small amount of oxygen is present in the system. This confirms that Ag/AgCl heterostructure was reduced to Ag under the electron beam both at room temperature and 200 °C. However, the formation of Ag2O is much quicker at the elevated temperature. It took about 6 min to see the formation of Ag2O in Figure 4g at room temperature, whereas it only took about 1 min to observe the formation of Ag2O at 200 °C in Figure 4i. This confirms that Ag2O forms at low oxygen partial pressure after the reduction process. DFT calculations have been performed to shed light on this reduction process of Ag/AgCl heterostructures. During the reduction process, the amount of energy transferred from an incident high-energy electron to the atomic nucleus reaches its maximum Emax when the scattering angle θ = 180°, which

Emax

⎡ ⎤ m 2 1 + M0 Mc 2 ⎥ ⎢ = E0(E0 + 2m0c )/⎢E0 + ⎥ 2 ⎢⎣ ⎥⎦

(

2

)

≈ 2E0(E0 + 2m0c 2)/(Mc 2)

(1)

where E0 is the kinetic energy of the incident electrons, c is the speed of light, and m0 and M are the rest mass of electron and atomic nucleus being bombarded by incident electrons. At normal TEM operating voltages, Emax generally falls into the range of a few electron volts, which is comparable to the surface binding energies of metals, oxides, and halides. Therefore, electron-induced sputtering is highly likely to cause the instability of AgCl. To remove a surface atom through the sputtering process, the maximum energy transferred from the incident electron to the nucleus (Emax) needs at least to surpass the surface binding energy, that is, Emax ≥ Ebind, where Ebind represents the surface binding energy of the atom being removed. In the extreme case that Emax = Ebind, the minimum incident electron energy that is required to remove a surface atom is approximated by solving eq 135 E0min = (511 keV){[1 + (AE bind) /(561 eV)]1/2 − 1} (2)

where A is the atomic weight of the surface atom. Ebind is usually taken as the sublimation energy Esub, which is defined as the energy difference between the atom or molecule in gas and bulk phases.36,37 In the compound AgCl system, the surface binding energy is defined as vac E bind = (Esurf + Eatom) − Esurf

(3)

Evac surf

where is the energy of the surface with a Ag or Cl vacancy, Eatom is the energy of an isolated Ag or Cl atom, and Esurf is the energy of the clean surface. Using a 2 × 2 AgCl (100) surface cell, the surface binding energies of Ag and Cl on the AgCl (100) surface are calculated by DFT and tabulated in Table 1 together with the calculated Emin 0 . Table 1. Surface Binding Energy Ebind of Ag and Cl on AgCl Surface and the Minimum Incident Electron Energy Required for Surface Sputtering Emin 0 species

Ebind (eV)

Emin (keV) 0

Ag Cl

2.53 4.07

111.6 61.7

Clearly, in principle, both Ag and Cl atoms can be removed from the AgCl surface under the experimental conditions, which leads to the lattice decomposition and reduction of AgCl. It should be noted that Emax defined in eq 1 corresponds to the rare scenario that electrons are scattered backward after hitting the sample surface. For more general cases, the energy transferred from incident electrons to the surface atom can be written as35

E = Emax sin 2(θ /2)

(4)

For the experimental condition that E0 = 200 keV, by solving eq 1, the maximum energy that Ag and Cl atoms can receive from Cl incident electrons is EAg max = 4.86 eV and Emax = 14.80 eV, D

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ACS Nano Cl respectively. By substituting EAg max and Emax into eq 4 and letting E = Ebind, it is found that Cl atoms can possibly be removed from the surface for electron scattering angles ranging from 63.3° to 180°, whereas the removal of Ag atoms occurs in a smaller range of 92.4° to 180°. The angular distribution of atom removal decreases quickly as θ increases,38 as shown in SI Figure S4. To study the displacements of Ag and Cl atoms under the impact of incident electrons, we perform a series of DFT calculations in which surface Ag and Cl atoms are displaced vertically from the surface. Various displacements are tested and the energy change of the whole system due to the displacements is calculated, as shown in Figure 5. It can be

electrons. Egerton et al. studied the beam-induced damage on thin Ag sample using a 200 kV accelerating voltage and found that Ag atoms were removed on the beam-exit surface at a low rate, and Ag accumulation occurs within the incident beam on the entrance surface.41 In AgCl, Ag+ diffusion is facile and primarily attributed to interstitial diffusion.42 The reason for such an atom accumulation is attributed to the low surface diffusion barrier of Ag and the local surface curvature induced by sputtering.41 A previous study has also shown the Ag diffusion barrier on Ag surface is as low as 0.3 eV.43 Therefore, it is reasonable to expect the remaining Ag atoms in the original AgCl lattice to diffuse through the nanocrystal and onto the surface of the Ag particle to minimize the overall surface energy, as evidenced by the volumetric expansion of the Ag nanoparticle between Figure 3a and i. Thus, the reduction of Ag/AgCl nanocatalyst is most likely due to the formation of Cl vacancies while the Ag+ ions accept electrons. In our case, the migration and ejection of Cl atoms play an important role in the reduction process, which is due to momentum transfer from electron beam to the atoms. In the photocatalytic reduction, Cl migration and ejection also play an important role. It was reported that for Ag/AgCl, the calculated surface content of chlorine was 41.77 atom %, which was slightly higher than the total silver content at 40.48 atom %. Cl− will trap the holes to form radicals Cl0.44 It is expected that the Ag+ ions will trap the electrons to form Ag atoms. Under high energy electron beam illumination, surface plasmons of Ag nanoparticles will assist electron transfer in the catalytic reaction. In the current experiments, plasmon generation on the Ag nanoparticle is possible,45,46 and electron transfer can be assisted by electron beam. The phenomena we observed under the electron beam are analogous, though not identical, to photocatalytic reduction. Oxidation of Ag/AgCl Heterostructure. In Figure 4, we found the formation of Ag2O after the reduction process at a pressure of 10−5 Pa in the TEM column, which suggests a possible oxidation process at low pressure. In order to understand the oxidation dynamics of Ag/AgCl heterostructure at atmospheric pressure, in situ ETEM was performed in the nanoreactor at atmospheric pressure and under constant air flow rate. We heated the system to 200 °C to accelerate the oxidation reaction and to record the structural dynamics within a short period of time. The temperature of the reaction zone was controlled by a MEMS heating chip with a heating profile shown in Figure S2. The in situ oxidation Movie S3 can be found in the SI. Sequential images of the oxidation process in real space are shown in Figure S3, which were extracted from the in situ oxidation Movie S3. The Ag/AgCl heterostructure underwent dramatic structural change as seen in Figure S3. However, in the constant gas flow environment, the electron scattering is more significant, which makes HRTEM a great challenge. In order to capture the structure evolution, we performed SAED during the oxidation reaction. Figure 6a−c show the sequential images of SAED patterns that were extracted from the Movie S3. The signal of SAED is weak due to the electron scattering in the nanoreactor. At the initial state (0 s), we observed Ag (111) (d spacing of 0.24 nm), Ag (200) (d spacing of 0.21 nm), and AgCl (200) (d spacing of 0.28 nm). After 66 s, SAED was performed on the same particle and only Ag (111) (200) planes were observed. This indicates the Ag/AgCl heterostructure was reduced to Ag during the reaction by the electron beam. At 187s, based on SAED in Figure 6c, we believe that AgO had formed by indexing the diffraction spots as AgO (121) and (114) (AgO (121)/AgO

Figure 5. Energy change of AgCl surface as a function of the vertical displacement of Ag and Cl atoms. Vertical displacement of 0 Å corresponds to the clean surface.

seen that the total energies of the Ag-removed and Cl-removed systems begin to saturate when the atomic displacements reach ∼3.0 Å and ∼4.0 Å, respectively, indicating that the interaction of the surface and the detached atom becomes negligible at this distance. Before saturation, even if the same amount of energy is received from the incident electron beam by Ag and Cl atoms, Cl atoms always undergo larger vertical displacement from the surface. In fact, according to eq 4, Cl atoms receive more energy from the incident electrons compared with Ag atoms when other conditions like the scattering angles are the same, so it is clear that Cl atoms are more likely to escape from the AgCl surface. The effect of incident electron beam on the AgCl is now clear: from an energetic point of view, Cl atoms are more likely to receive enough energy to break surface bonds because the energy transferred from the electron beam surpasses Cl surface binding energy over a larger range of electron scattering angle when compared with Ag atoms. Moreover, Cl atoms have larger displacement from the surface due to the impact of incident electrons. Also, previous calculation shows the higher formation energy of Ag+ vacancies than Cl− vacancies.39 It therefore is reasonable to expect that the removal of Cl atoms is much faster than that of Ag atoms, which is consistent with our experimental finding. Thus, once Ag+ is formed with Cl vacancies, it will immediately accept electrons to form Ag atoms to minimize the energy. Previous DFT study also finds the silver atom adsorbed at a Ag site is more stable than that at a Cl site.40 Hence, we attribute the propagation of the reaction surface during the reduction process to the formation of Cl vacancies reaching a threshold concentration and diffusion of Ag atoms to the surface of existing Ag nanoparticle. As Cl atoms are removed at a higher rate, the remnant is mostly Ag+ ions that are then reduced to Ag atoms by accepting E

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surface vacancies, in order to maintain the charge balance and simulate the process subsequent to Cl removal by the electron beam. Because all the subsurface octahedral sites are occupied, the only possible sites for oxygen adatoms are the tetrahedral ones. However, after structural relaxation, the oxygen atom which is initially placed at the subsurface tetrahedral site stabilizes at the on-surface hollow site, which is surrounded by four Ag atoms. Such a result shows that it is not favorable for oxygen atoms to penetrate AgCl layer and form bulk Ag2O, even if the surface AgCl layer is defected. On the other hand, for Ag, it is reported that for low oxygen surface coverage, the energy barrier for an oxygen atom to enter the subsurface region is 0.86 eV,49 and for high oxygen surface coverage, the subsurface oxygen adsorption becomes more energetically favorable than on-surface adsorption. Therefore, based on the above analysis, silver oxides are easily formed in the Ag lattice, whereas oxygen subsurface adsorption is not favorable on AgCl surface due to its more close-packed nature. This provides an explanation as to why the Ag/AgCl nanocatalyst oxidation process is not a one-step process by a direct replacement of a Cl atom by an O atom but instead is a two-step process from Ag/ AgCl, to Ag, then to silver oxides.

Figure 6. In situ oxidation of Ag/AgCl heterostructure with O2 at atmospheric pressure. (a) SAED on a single particle at initial state. (b) SAED on the same particle after the 66 s reaction time with constant air flow. (c) SAED on the same particle after the 187 s of reaction time with constant air flow. (d) Image of the same particle after 293 s of reaction time with constant air flow. (e) FFT of the red region in (d). (f) EELS spectrum during the reaction. The red line was taken at 200 °C after 200 s of reaction time with constant air flow. The inset EELS spectrum shows the existence of oxygen in the nanoparticle as compared with the background signal from the constant air flow.

DISCUSSIONS Electron beams are known to cause atomic displacements, ionization, electron donation, and heating.50 The Ag/AgCl heterostructure has ionic bonding on the AgCl side and metallic bonding on the Ag side. Because Ag is the heavy atom, it is more difficult to cause the atomic displacement by momentum transfer from electron beam. In contrast, Cl is a light atom and point defects can be created by the electron beam via atom displacement, which is frequently observed in graphene, similarly composed of light atoms.51,52 The ease of Cl removal is also confirmed by our DFT calculations. However, Cl vacancies are difficult to distinguish due to being invisible to analytical techniques.53 Once enough Cl vacancies are created, major structural transformation can begin, that is, AgCl was reduced to Ag. Similar observations have previously been reported, such as the growth of Ag nanowires by reducing AgI in carbon nanotubes54 and the growth of Ag nanoparticle from a liquid AgNO3 source by an electron beam.55 The electron dose in our reduction experiment is above the dose for nucleation of Ag nanoparticles (0.5 electrons/s·Å2).55 This reduction threshold dose for nucleation only depends on the material itself but is independent of the beam current, pixel dwell time and magnification in microscope.56 We observed that the projected area of Ag nanoparticle increases after AgCl reduction as illustrated in Figure 3a and i. Radiation heating is possible in Ag metal nanoparticles but the temperature rise is usually insignificant.50 Indeed, the electron beam acts as a reducing agent in the reaction.56 To some extent, chemical reactions are accelerated in the electron beam environment as compared with a realistic reduction gas environment. This enables the observation of dynamics in a relatively short period of time. The reduction of metal ions happens preferably at defect sites. H. Chen’s group57 reported that the reduction of Au ions was likely at the ligand deficient sites, which was associated with crystal coalescence. In our experiments, the formation of Cl vacancies promotes the reduction of Ag ions accompanied by diffusion. During the oxidation process, as the DFT calculation suggests, it is not favorable for oxygen atoms to penetrate AgCl surface layer and form bulk silver oxides due to packed

(114) = 1.4, large d spacing is 0.29 nm) with angle of 47° in respect to each other. Besides, the Ag (111) (d spacing 0.24 nm) was also seen in Figure 6c, which indicates partial oxidation at 187 s. EDS is technically challenging for elemental analysis in concert with the nanoreactor due to the confined geometry between nanoreactor and detector. Also, as mentioned above that EELS can detect the elements on the localized nanoparticle. We measured the EELS on the nanoparticle at 200 °C at 200 s, shown as the red line in Figure 6f. A clear oxygen K edge is visible, indicating oxide formation on the nanoparticle before 200 s. Without the nanoparticle, the oxygen K edge can still be seen but is very weak from the background air gas at the same temperature, as shown in the inset of Figure 6f. The EELS spectra are normalized with the same background signal in Figure 6f. Figure 6d shows a sequential image after 293 s of reaction time. Figure 6e is the FFT of Figure 6d, showing the expected AgO lattice plane (02̅2) and (2̅20) with single crystallinity and 52° angle with respect to one another. Thus, the oxidation of Ag/ AgCl heterostructure is still a two-step process. Ag/AgCl was first reduced to Ag. Then the Ag nanoparticle reacted with O2 molecules to form AgO at atmospheric pressure. For the oxidation process, one essential prerequisite for the formation of transition metal oxide is that oxygen atoms should be able to penetrate the surface in order to form threedimensional oxides.47,48 On the basis of previous DFT studies,39 it is known that subsurface oxygen atoms prefers to reside in the octahedral sites when the subsurface adsorption initiates. AgCl has a rocksalt structure, Fm3̅m, with all the octahedral sites being occupied by either Ag or Cl atoms. As a result, pristine AgCl is immune from oxygen subsurface adsorption because it is difficult for oxygen atom to enter through the top AgCl layer, which would be accompanied by significant lattice distortion. To study the oxygen subsurface adsorption on a defected AgCl surface, we modeled the oxygen subsurface incorporation on an AgCl (100) slab with two Cl F

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DFT Calculations. DFT calculations are performed using the generalized gradient approximation (GGA) with the parametrization of Perdew−Burke−Ernzerhof (PBE)58 for the exchange-correlation functional, as implemented in the Vienna Ab-initio Simulation Package (VASP).59,60 The energetics associated with Ag and Cl removal on AgCl (100) surface are evaluated using projector augmented wave (PAW)61,62 potentials in conjunction with a plane-wave cutoff energy of 328 eV. The AgCl surfaces are modeled using (2 × 2) surface slabs with four atomic layers in which the bottom layer is kept fixed during the relaxation. The vacancy formation energy of Cl atom on the surface are tested using both four- and five-atomiclayer slabs and both yield essentially the same result. The slabs along the direction perpendicular to (100) surface are separated by a vacuum region of approximately 40 Å. The positions of all atoms, except the ones being fixed, are allowed to relax in all three directions until the force components acting on acting on each atom are less than 0.01 eV/ Å. All calculations are spinpolarized. The Brillouin-zone integration is performed using a (4 × 4 × 1) Monkhorst−Pack grid63 with broadening of the Fermi surface using Gaussian smearing technique with a smearing width of 0.05 eV. Our calculated lattice constant for AgCl is 5.59 Å, which is in good agreement with the experimental value of 5.55 Å.22 The surface binding energy of Ag or Cl atom on AgCl(100) is defined as

nature of the rocksalt structure. Thus, the formation energy of AgO by direct replacement of Cl by O is much higher than the two step oxidation process, that is, reduce Ag/AgCl to Ag first and then react with O2. On the basis of the experimental observation, the possible reaction formula can be summarized as follows. e−

Ag np/AgCl → Ag np/Ag + Cl0 (reduction) e−

Ag np/AgCl + O2 → Ag np/Ag + O2 + Cl0 e−

→ Ag np/Ag 2O + Cl0 (low oxygen partial pressure) e−

Ag np/AgCl + O2 → Ag np/Ag + O2 + Cl0 e−

→ AgO + Cl0 (atmospheric pressure)

where e− means electron beam accelerated by 200 kV.

CONCLUSIONS In summary, we studied the redox process of Ag/AgCl heterostructure using in situ ETEM with a nanoreactor. The reduction of Ag/AgCl is due to the formation of Cl vacancies while Ag+ ions accept electrons. Also, DFT calculations show that the removal of Cl atoms is much more likely than that of Ag atoms. This is the reason that AgCl can be reduced to Ag by the electron beam. A two-step oxidation process of Ag/AgCl has been observed. Rather than direct replacement of Cl by O, Ag/AgCl heterostructure is first reduced to Ag nanoparticle. The O2 molecules then react with Ag nanoparticle. Ag2O forms at low O2 partial pressure, whereas AgO primarily forms at atmospheric pressure.

vac E bind = (Esurf + Eatom) − Esurf

where Evac surf is the energy of the surface with a Ag or Cl vacancy, Eatom is the energy of an isolated Ag or Cl atom, and Esurf is the energy of the clean surface. The subsurface oxygen adsorption is also studied on a defected AgCl (100) surface with two Cl vacancies. The surface is modeled using a (3 × 3) five-layer slab containing 90 Ag atoms and 88 Cl atoms. The bottom layer is fixed throughout the relaxation. A cutoff energy of 400 eV is used and adjacent slabs are separated using a vacuum region of 12 Å. The calculations are performed without spin polarization. Brillouinzone integration is performed only at the Γ point considering the large size of simulation cell. To evaluate the size effect on the DFT results, the binding energies of Ag and Cl atoms on AgCl (100) surface are also calculated using 3 × 3 surface cell, which is shown in the SI Figure S5.

MATERIALS AND METHODS Synthesis of Ag/AgCl Heterostructure. The Ag/AgCl heterostructure was synthesized followed by a previous work.29 In a typical reaction, 0.35 mmol of NaCl (Fisher Chemicals) was mixed with 2.5 g of PVP (MW = 55 000, Sigma-Aldrich) in a three neck flask. Then, 12 mL of ethylene glycol was added into the flask with N2 gas flow to dissolve the powders assisted by heating the solution to 60 °C for 1 h under vigorous magnetic stirring. Then, 1 mL of freshly prepared ethylene glycol solution of AgNO3 (0.34 mmol) was dropwise added into the flask over 20 s. The reaction lasted another 2 h, and the reaction temperature was maintained at 60 °C. After the synthesis, the precipitate was purified by centrifuge using ethanol and later stored without light. TEM Experiments. For in situ ETEM, the as prepared Ag/ AgCl heterostructure was dispersed in ethanol and drop coated onto the Si chip with Si3N4 membrane by micropipette. To get a nanoreactor, the chip was assembled together with another one. The nanoreactor was pocketed in a Hummingbird gas flow holder. In situ reduction, oxidation, imaging, SAED, and EELS were performed on a JEOL 2100F operated at accelerating voltage of 200 kV and equipped with a Gatan imaging filter (GIF) system at the Center for Nanoscale Materials, Argonne National Lab. The air was used for the oxidation. Same microscope was used for ex situ TEM characterization by drop casting purified nanocatalyst solution onto a Formvar Au TEM grid.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b00355. Time series FFT of reduction process (Figure S1), in situ heating profile of nanoreactor during oxidation process (Figure S2), sequential images of Ag/AgCl nanocatalyst during oxidation process (Figure S3), simulated differential cross section (dσ/dΩ) as a function of scattering angle θ (Figure S4), DFT calculation using 3 × 3 surface cell (Figure S5). Calculation of changes in cross sectional area of Ag nanoparticle. (PDF) In situ reduction of Ag/AgCl nanoparticle, sped up 20 times (movie S1). (AVI) Time series FFT of Ag/AgCl heterogeneous nanocatalyst reduction, sped up 20 times (movie S2). (AVI) G

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In situ oxidation of Ag/AgCl nanocatalyst, sped up 20 times (movie S3). (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Present Address †

Department of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, PA 19122, United States. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS Use of the Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575.64 REFERENCES (1) Ertl, G. Elementary Steps in Heterogeneous Catalysis. Angew. Chem., Int. Ed. Engl. 1990, 29, 1219−1227. (2) Somorjai, G. A.; Marsh, A. L. Active Sites and States in the Heterogeneous Catalysis of Carbon−hydrogen Bonds. Philos. Trans. R. Soc., A 2005, 363, 879−900. (3) Yoshida, H.; Kuwauchi, Y.; Jinschek, J. R.; Sun, K.; Tanaka, S.; Kohyama, M.; Shimada, S.; Haruta, M.; Takeda, S. Visualizing Gas Molecules Interacting with Supported Nanoparticulate Catalysts at Reaction Conditions. Science 2012, 335, 317−319. (4) de Smit, E.; Swart, I.; Creemer, J. F.; Hoveling, G. H.; Gilles, M. K.; Tyliszczak, T.; Kooyman, P. J.; Zandbergen, H. W.; Morin, C.; Weckhuysen, B. M.; de Groot, F. M F. Nanoscale Chemical Imaging of a Working Catalyst by Scanning Transmission X-Ray Microscopy. Nature 2008, 456, 222−225. (5) Bañares, M. A. Operando Spectroscopy: The Knowledge Bridge to Assessing Structure-Performance Relationships in Catalyst Nanoparticles. Adv. Mater. 2011, 23, 5293−5301. (6) Buurmans, I. L. C.; Weckhuysen, B. M. Heterogeneities of Individual Catalyst Particles in Space and Time as Monitored by Spectroscopy. Nat. Chem. 2012, 4, 873−886. (7) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A. ReactionDriven Restructuring of Rh-Pd and Pt-Pd Core-Shell Nanoparticles. Science 2008, 322, 932−934. (8) Divins, N. J.; Angurell, I.; Escudero, C.; Perez-Dieste, V.; Llorca, J. Influence of the Support on Surface Rearrangements of Bimetallic Nanoparticles in Real Catalysts. Science 2014, 346, 620−623. (9) Li, Y.; Zakharov, D.; Zhao, S.; Tappero, R.; Jung, U.; Elsen, A.; Baumann, P.; Nuzzo, R. G.; Stach, E. A.; Frenkel, A. I. Complex Structural Dynamics of Nanocatalysts Revealed in Operando Conditions by Correlated Imaging and Spectroscopy Probes. Nat. Commun. 2015, 6, 7583. (10) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 1688−1691. (11) van der Veen, R. M.; Kwon, O.-H.; Tissot, A.; Hauser, A.; Zewail, A. H. Single-Nanoparticle Phase Transitions Visualized by Four-Dimensional Electron Microscopy. Nat. Chem. 2013, 5, 395− 402. (12) Browning, N. D. Electron Microscopy: Phase Transition Singled Out. Nat. Chem. 2013, 5, 363−364. (13) Gai, P. L.; Kourtakis, K. Solid-State Defect Mechanism in Vanadyl Pyrophosphate Catalysts: Implications for Selective Oxidation. Science 1995, 267, 661−663. H

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