Modifying Surface Chemistry of Metal Oxides for Boosting Dissolution

Jul 24, 2017 - M.S. acknowledges support from the National Natural Science Foundation of China (Grants Nos. 11374028 and U1330112), the National ...
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Modifying Surface Chemistry of Metal Oxides for Boosting Dissolution Kinetics in Water by Liquid Cell Electron Microscopy Yue Lu,† Jiguo Geng,† Kuan Wang,† Wei Zhang,§ Wenqiang Ding,† Zhenhua Zhang,† Shaohua Xie,‡ Hongxing Dai,‡ Fu-Rong Chen,*,∥ and Manling Sui*,† †

Institute of Microstructure and Properties of Advanced Materials and ‡Department of Chemistry and Chemical Engineering, Beijing University of Technology, Beijing 100124, China § Department of Materials Science, Jilin University, Changchun 130012, China ∥ Department of Engineering and System Science, National Tsing Hua University, Kuang-Fu Road, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: Dissolution of metal oxides is fundamentally important for understanding mineral evolution and micromachining oxide functional materials. In general, dissolution of metal oxides is a slow and inefficient chemical reaction. Here, by introducing oxygen deficiencies to modify the surface chemistry of oxides, we can boost the dissolution kinetics of metal oxides in water, as in situ demonstrated in a liquid environmental transmission electron microscope (LETEM). The dissolution rate constant significantly increases by 16−19 orders of magnitude, equivalent to a reduction of 0.97−1.11 eV in activation energy, as compared with the normal dissolution in acid. It is evidenced from the high-resolution TEM imaging, electron energy loss spectra, and first-principle calculations where the dissolution route of metal oxides is dynamically changed by local interoperability between altered water chemistry and surface oxygen deficiencies via electron radiolysis. This discovery inspires the development of a highly efficient electron lithography method for metal oxide films in ecofriendly water, which offers an advanced technique for nanodevice fabrication. KEYWORDS: metal oxide, dissolution, liquid cell, transmission electron microscope, electron beam lithography the dissolution rate constants of CeO2,10 Fe2O3,11 and CuO12 are only in the range of 10−10−10−16 in solutions at a pH between −1 and +4. Therefore, reducing the dissolution activation energy Ea should be another effective pathway to trigger the fast dissolution of metal oxides, which mainly alters the surface structure of materials, such as the introduction of oxygen deficiencies.13,14 It is envisaged that the atomic structures at the oxide/water interface play a decisive role in influencing the dissolution ability of metal oxides.9,15 The dissolution process requires disruption of multiple metal oxide bonds to form the aquo complex with cations/anions surrounded by water molecules.13 Previous reports have shown that the low-coordinate oxygen or oxygen vacancies on metal oxide surfaces determine the dissolution thermodynamics and kinetics of metal oxides.8,13,16−19 Thus, how to manipulate the oxygen−metal

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ontrolling dissolution of metal oxides in aqueous solution requires an accurate understanding of physical/chemical scenarios at the oxide/water interfaces,1 which may undoubtedly offer the technological applications in range of fields, such as the lithography of oxide semiconductors,2 drug carriers,3 nuclear power protection,4 glass stability,5 and the creation of new materials.6 Theoretically, the dissolution rate (v) of metal oxides in an acidic solution can be expressed as v = k[H3O+]n,7 where n is the reaction order and [H3O+] represents the steady-state hydronium concentration. Here, the reaction rate constant (k) is sensitive to the dissolution activation energy (Ea) because of the exponential function correlation given by k = A exp(−Ea/ kBT), where kB is the Boltzmann constant and A is the frequency related pre-exponential factor which depends on the temperature.8 It has been reported that a fast dissolution of calcite in water was triggered at microscale by using the X-ray irradiation to locally increase the acidity in water, and the dissolution reaction fronts of calcite was successfully explored.9 However, in general, the rate constants for most dissolutions of metal oxides are very small even in strong acid. For example, © XXXX American Chemical Society

Received: April 17, 2017 Accepted: July 24, 2017 Published: July 24, 2017 A

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Figure 1. Electron beam effect for metal oxides in water. (a) Diagram of the enriched hydronium ions (H3O+) in radiolytic water induced by electron beam in the irradiated area. The electron beam also excites oxygen atoms to introduce vacancies (Ov) on the surface of a metal oxide, which is immersed in the water. The dissolved metal cations are surrounded by polar H2O molecules to form the metal aquo complex [M(H2O)6]+Z that diffuses into the water. (b) As the Ov-contained surface of metal oxides is exposed to higher [H3O+], the surface ions at oxide/H2O interface are easier to be pull out into the polar water, which alters the dissolution route and equivalently decreases the energy barrier. (c) The activation energy required for dissolution is expected to be reduced by ΔEa for the Ov-contained surface in radiolytic water, which is equivalent to an increase of the rate constant.

RESULTS AND DISCUSSION Synergistic Effect of Electron Beam on Water and Metal Oxides. The synergistic mechanism of an electron beam on water and metal oxides is shown in a schematic diagram (Figure 1a). First, the irradiation of electron beam can increase the H3O+ concentration as well as the other radical species (such as eaq−, H•, H2, H2O2, OH•, O2, etc.) in the radiolytic water.28 Simultaneously, the energetic electron beam interacts with the metal oxides and a desorption of the oxygen atoms takes place via the radiolysis of core-hole Auger decaying process.26,27 The desorbed positive charged oxygen ions or neutral oxygen atoms may react with the radical species of eaq−, H•, or H2 to form O2, OH•, or H2O in water. As a result, oxygen deficiencies are left on the metal oxide surface.26,27 As the oxygen-deficient surfaces (or the metal-rich surfaces) of metal oxides are exposed to the water with a higher H3O+ concentration (Figure 1b), the surface ions at the defective oxide/H2O interface are easier to pull out by the polar water molecules, which can alter the dissolution route and decrease the energy barrier (Figure 1c). The activation energy required for the dissolution of metal oxides with oxygen deficiencies is expected to be decreased, which is equivalent to an increase in the rate constant. The introduction of oxygen deficiencies by electron irradiation may not only enhance the surface-related dissolution of metal oxides but also lead to the reduced phase transformation of oxides, as is experimentally evidenced in Figure 2. In this experiment, insoluble cerium dioxide (CeO2) was put into water (pH = 7) and encapsulated inside a selfaligned wet (SAW) cell21 (Materials and Methods and Figure S1) for in situ LETEM observation. In Situ Observation for the Dissolution of CeO2 NPs in Radiolytic Water. As we all know, CeO2 is very sensitive to its oxygen deficiencies and easy to transform into different crystal structures of CeO2−x,29 which has been widely used in light filters and catalysis.30,31 Here, the CeO2 with a high diffusivity of oxygen32 serves as a representative metal oxide to show the phase transition evidence of introducing oxygen deficiencies

bonds or even to stimulate the generation of oxygen deficiencies on the surface of metal oxides is the immediate concern toward boosting the dissolution by altering the behavior of chemical reactions for many metal oxides. With the development of an advanced liquid environmental electron microscope, a great opportunity has been offered to in situ study the dissolution mechanism of nanoparticles in liquid. Recently, the growth and dissolution mechanisms of metal nanoparticles in water have been disclosed in real time.20−22 However, until now, it has been a great challenge to manipulate the dissolution process of metal oxides with nano or even angstrom resolution via lowering of the dissolution activation barrier. In this work, we employed, in particular, an electron beam as the irradiation source to stimulate the introduction of oxygen deficiencies on the metal oxide surface in a liquid environmental TEM (LETEM),23,24 to in situ study the accelerating dissolution process of metal oxides in pure water. By manipulating dose rate of electron beam, we can boost the dissolution of metal oxides with a significant increase in the dissolution rate constant to 16−19 orders of magnitude, equivalently, a decrease in activation energy of 0.97−1.11 eV as compared with the dissolutions in acid. As electron beam interacts with the metal oxides in water, a radiolysis process leads to desorption of oxygen atoms from the metal oxide surfaces and hence a corresponding structural change,25,26 such as via the Knotek−Feibelman (KF) process to create a metalrich surface.27 Such changes will cause a reduction in activation energy or an increase in rate constant. At the same time, high energy electron also can decompose the water molecules to generate locally higher H3O+ concentration for dissolving the metal oxides.28 Furthermore, this extraordinary control ability in dissolving the metal oxides inspires us to develop a direct, highly efficient electron beam nanolithography method for metal oxide films in pure water, which will offer an opportunity for future applications in the nanofabrication of metal oxide devices. B

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Figure 2. Dissolution characteristic of CeO2 NPs in radiolytic water showing a transformation to lower oxygen coordinated phase Ce2O3. (a− c) Dissolution of the CeO2 NPs as a function of irradiation time (see movie S1). The morphology of the red circled NP1 and NP2 changes from a cube to a polyhedron shape. (d) Projection area of NP1−NP4 changes as a function of the dissolution time. (e) EELS for CeO2 NPs in different dissolution time. Reduction for the ratio of M4/M5 intensities is observed during the dissolution process, which gives an indication of the presence of oxygen deficiencies. (f) HRTEM video frame for CeO2 NPs (see movie S2). (g−j) HRTEM images (upper) and the corresponding FFT patterns (lower) for the NP5 marked in (f), indicating a phase transition from cubic CeO2 to hexagonal Ce2O3. Note that the NP5 in water is rotated a little bit. (k) Projection area in (f−j) vs dissolution time. Different colors represent the different phases of CeO2 or Ce2O3; the crystal structure of the NP5 cannot be identified within ∼12−24 s because of the loss of two-dimensional lattice fringes by nanoparticle rotation in water. (l) Proposed phase transition mechanism for CeO2 (left, along [110]) to Ce2O3 (right, along [100]) based on their inheritance relationship from (g) to (j), in which the lattice planes of CeO2-(11̅1) and Ce2O3-(011) are indicated by red and black lines, respectively. In this mechanism the oxygen deficiencies in CeO2 necessarily participate in the intermediate state to achieve the inherited structure for Ce2O3. The blue ellipses represent the feasible oxygen deficiencies sites.

an expected evidence for the aggravation of oxygen deficiency during the dissolution of CeO2 in radiolytic water.31 High-resolution TEM (HRTEM) images and the corresponding fast Fourier transform (FFT) patterns (Figure 2f−j and movie S2) also reveal a sequential phase transition for CeO2 NPs from the cubic crystal structure CeO2 (Figure 2f−h) toward a low oxygen-coordinate phase (hexagonal Ce2O3, Figure 2i,j) during the dissolution process. The phase change of CeO2 to a hypoxia state confirms the local aggravation of oxygen deficiency during the dissolution of CeO2 in radiolytic water. Both the FFT analysis and the EELS result are consistent with giving support for the evidence of the phase transformation from CeO2 to Ce2O3 due to generation of oxygen deficiencies under electron beam irradiation. Figure 2k shows the corresponding dissolution kinetics of the NP5 (Figure 2f−j

under the electron beam irradiation in water as shown in Figure 2 and movie S1. First, the morphology of CeO2 nanoparticles (NPs) changes from a {100} faceted cube shape to a polyhedron shape during the dissolution process (red dash circled NP1 and NP2 in Figure 2a−c). The projection area of four NPs (NP1−NP4) vs dissolution time ( Figure 2d) shows that the dissolution rate (∼0.27 ± 0.03 nm2/s or 0.6 M/s, Supporting Information SI-1.1) is weakly dependent on the initial particle size. A detail analysis of the electron energy loss spectrum (EELS) shows a successive rising of Ce-M5 edge accompanying by the reduction of Ce−M4 during the dissolution of CeO2 NPs at 0, 280, and 470 s clearly (Figure 2e), which reveals a change of valence state for the cerium ion from tetravalent to trivalent state. The EELS result implies a possibility of the phase transition from CeO2 to Ce2O3. This is C

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Figure 3. Dissolution kinetics of metal oxides in radiolytic water. (a) Dissolution rate v (M/s) vs electron dose rate in the range of 2−5000 e−· Å−2·s−1 for CuO, VO2, Fe2O3, and CeO2 (lines with dots in different colored shape). (b) Logarithm of dissolution rate vs logarithm of radiolytic hydronium concentration [H3O+]. The values of kr‑water and n are extracted from the fitting of eq 2. The dash lines (in different colors) represent the dissolution kinetics of the corresponding metal oxides (CeO2, CuO and Fe2O3) in acids from previous reports.10−12

and movie S2). There is a turning point (starting at ∼28 s in Figure 2k) in the dissolution kinetics curve for the NP5, which is in fact a general case for most of the NPs when approaching to the end of the dissolution process (Figure S2). For the first 12 s, it confirms that the crystal structure of the NP5 is the cubic CeO2. After 24 s, the crystal structure of the NP5 is identified as the hexagonal Ce2O3. However, between 12 and 24 s, the structure of the NP5 cannot be identified by FFT analysis due to the loss of two-dimensional lattice fringes in the HRTEM video frames. It may be induced by a global rotation of the NPs in water before or during the phase transformation from CeO2 to Ce2O3. It is worthy to note that one-dimensional crystal fringes in the HRTEM video frame at 16 s (movie S2) are very similar to the ones at 29 s, which indicates that the phase transition of the NP5 may take place at some point before 16 s. In addition, by analyzing the dissolution processes of all NPs in movie S2, we noticed that the phase transition is unidirectional from CeO2 to Ce2O3, and there is not any obvious reversible transformation from trivalent to tetravalent cerium ions even though there are oxidizing agents in the radiolytic water. Thermodynamically, it is not favorable to oxidize Ce2O3 back to CeO2 at room temperature and normal atmospheric environment (for details, see the Supporting Information SI-1.2). Based on the FFT analysis of HRTEM images in Figure 2g− j, the structure transition from CeO2 to Ce2O3 obeys an inheritance of CeO2-(111̅ )/Ce2O3-(011). Evidently, the ball− stick model (Figure 2l) shows that a possible transformation from a higher oxygen coordinated cubic phase to a lower oxygen coordinated hexagonal phase while maintaining the plane registration as proposed in Figure 2l. In this mechanism, due to the high diffusivity of oxygen for CeO2 (∼1.1 × 10−14 cm2/s, at room temperature),32 the oxygen deficiencies at the surface can diffuse quickly into the inner lattice of nanoparticles to render the phase transition from CeO2 to Ce2O3. This means that the oxygen deficiencies necessarily participate in the intermediate state to maintain for the inheritance relationship from CeO2 to Ce2O3. Dissolution Kinetics of Metal Oxides in Radiolytic Water. Here we analyzed quantitatively for the increase of H3O+ concentration under electron beam irradiation.28 The steady-state H3O+ concentration, [H3O+], in the radiolytic water depends on the dose rate I28

[H3O+] = 5.11 × 10−6 × I 0.51

(1)

with an experimental dose rate of 5 × 103 e− Å−2 s−1, and the H3O+ concentration at steady-state is calculated to be ∼10−3.4 M (pH is ∼3.4). It implies that the H3O+ concentration is enriched by electron radiolysis from 10−7 to 10−3.4 M in the irradiated area. As previously reported, however, the dissolution rate of CeO2 is extraordinarily slow even in a strong acidic solution (∼1.39 × 10−8 M/s at a pH of −1.2, see Table S1),19,20 which is much smaller than the observed dissolution rate of 0.6 M/s from Figure 3a. On the other hand, the soluble criterion for metal oxides has been reported to be that the hydrolysis equilibrium constant pKa must be greater than the pH value of solution (Supporting Information SI-1.3).33,34 This implies that the hydronium concentration in radiolytic water at a pH of 3.4 in this case is not high enough to drive the dissolution of CeO2 NPs (pKa ≈ −1.1). Here, we also checked other two metal oxide materials with pKa < pH = 3.4, Fe2O3 (pKa ≈ 2.2) and VO2 (pKa ≈ -0.67), which both fail to satisfy the solubility criterion (Supporting Information SI-1.3), but in fact they are found to be soluble in the radiolytic water, even with an extraordinary enhancement for dissolution rate as compared with the referred data in acids (Table S1). All of these results give implicit support that the increase of acidity concentration [H3O+] due to electron irradiation is not enough for boosting the superdissolution of metal oxides. Furthermore, the superdissolution of the CeO2 NPs was even observed in a polar aprotic solvent of the N,N-Dimethylformamide (DMF) under the irradiation of electron beam (Figure S3), where there is no any H3O+ in the solvent. Therefore, the surface oxygen deficiencies, created by electron beam irradiation, play a more dominant role in the superdissolution of metal oxides, then the surface metal ions easily dissolve into the solution with the assistance of the polar solvent molecules (such as H2O and DMF). Alternatively, a change of the dissolution route, i.e., the reaction rate constant k, may have to take stronger responsibility for the superdissolution of metal oxides in our experiments. Figure 3a shows a plot of the experimental dissolution rate vs [H3O+] concentration in the radiolytic water for four metal oxides. The [H3O+] can be altered with different dose of the electron beam as shown in eq 1. Figure 3b shows the replotted logarithmic function of the dissolution rate vs [H3O+], based on eq 2. D

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Figure 4. Lateral and longitudinal etching of VO2 film in radiolytic water. (a) Lithographic pattern of the Chinese characters of “China” with an electron beam of 45 nm in diameter. The dose rate of these patterns is manipulated at ∼5000 e− Å−2 s−1. (b) The pseudocolor map for thickness variation of the patterned area in (a). The corresponding color bar for thickness is shown on the right-hand side. (c) Schematic diagram shows that H3O+ may laterally diffuse out of the e-beam irradiation area (red arrows) inducing broader etched patterns on VO2 film. (d) Image of the illuminated area of the electron nanobeam spot on the dry VO2 film showing the spot size of about 16 nm in diameter. (e−g) Bright filed images of the etched patterns on the VO2 film immersed in radiolytic water. The same spot size of 16 nm with a dose of 5000 e− Å−2 s−1 is used during etching process for 1, 20, and 60 s, respectively. (h) Intensity profiles of the beam spot image (green profile) and the etched patterns (blue, black and red profiles) in d−g, respectively. (i) Diameter of the etched patterns versus the irradiation time. The value of diameter was averaged from the fwhm’s of the intensity profiles along different radial directions for each etched pattern in e−g. The error bar for each diameter value was estimated from the standard deviation. The lateral etching rate for VO2 film is estimated at ∼1.8 nm/min based on the diameter broadening with the increase of irradiation time.

log(v) = log(k) + n log[H3O+]

regarded as unchanged in radiolytic water. The relative reduction in activation energy ΔEa for superdissolution with the aid of electron radiolysis as compared with the ones in acid is given in

(2)

Then, both the reaction rate constant k and the reaction order n for the dissolution of metal oxides with the aid of electron radiolysis can be extracted from the fitting in Figure 3b, as 2.04 × 105 and 1.27 for VO2, 3.39 × 107 and 1.86 for CuO, 2.51 × 108 and 2.27 for Fe2O3, and 2.00 × 103 and 1.05 for CeO2, respectively. As compared with the referred rate constants in acid (kacid), 6.98 × 10−10 for CuO (pH = 4), 5.08 × 10−10 for Fe2O3 (pH = 0), and 1.35 × 10−16 for CeO2 (pH = −1.2) (Table S1),10−12 the dissolution rate constant kr‑water in radiolytic water is ∼16−19 order of magnitude greater than the kacid accordingly for three metal oxides (CeO2, CuO and Fe2O3). Reduction for the Dissolution Activation Energy of Metal Oxides. For the exponential function correlation of k = A exp(−Ea/kBT), the pre-exponential factor A is a temperature dependent parameter.8 However, it is reasonably accepted that the electron radiolysis will not alter noticeably the temperature of water;7,9 therefore, the pre-exponential factor A can be

ln(k r‐water /kacid) = ΔEa /kBT

(3)

where kr‑water/kacid is ∼4.86 × 1016 for CuO, 4.94 × 1017 for Fe2O3, and 1.48 × 1019 for CeO2 (see Table S1). The ΔEa can be calculated to be ∼0.97 eV for CuO, 1.03 eV for Fe2O3 and 1.11 eV for CeO2 in this case. First-Principle Calculations. Above all, just the increase of [H3O+] in radiolytic water cannot lead to the superdissolution of metal oxides, while the introduction of oxygen deficiencies on the metal oxide surface may take great effort to achieve the superdissolution by modifying the surface chemistry and altering the dissolution route. Dissolution is a chemical process taking place at the oxide/water interface and can be boosted by the alteration of surface deficiency state.6,14 To further verify the contribution of surface oxygen deficiencies for the E

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ACS Nano superdissolution of metal oxides, we first quantified the crosssection of inelastic scattering of electron beam for dried metal oxides (Table S2). With the aid of electron radiolysis, oxygen atoms are quickly desorbed from the metal oxide surfaces (Supporting Information SI-1.6), which leads to the formation of oxygen deficiencies on the exposed atomic surface in every moment (i.e., the oxide/water interface) during the metal oxide dissolution process.25−27 Furthermore, we performed the first principle calculations (Materials and Methods) to confirm that the pre-existing oxygen vacancies on the surface of metal oxides can effectively lower the energy barrier for neighbor metal atom desorption; for example, the decreases are 0.84 eV for Ce on CeO2 and 1.08 eV for Cu on CuO, respectively. The calculated reduction in the energy barrier induced by the pre-existing oxygen vacancies is on the same order of magnitude with the measured reduction in activation energy. The atomic excitation and the introduction of oxygen deficiencies via electron radiolysis may create the active sites to boost the unusual superdissolution for metal oxides. Meanwhile, the high concentration of oxygen defects on the irradiated metal oxide surfaces would further change the surface energy and strain field, which can aggravate the dissolution far from equilibrium.9,14 Water−Base Electron Beam Lithography Method for Metal Oxide Film. The electron beam induced effect has attracted wide attention due to its promising applications in nanotechnology.10 Taking full advantage of electron beam enhanced dissolution, we here demonstrate a direct and highly efficient electron beam writing method for patterning metal oxide films in pure water environment (Figure 4a,b), which is a green and eco-friendly electron beam nanolithography method. In order to intuitively exhibit the accurate control ability for metal oxides dissolution, we have designed a time-dependent etching experiment on a VO2 film in radiolytic water (Figure 4c). The longitudinal etching rate along the direction of incident electron beam is mainly attributed to the beam effect, while the etched patterns may be degraded due to the laterally diffused hydroniums to attack the metal oxides (red arrows in Figure 4c). The spot size of incident beam is ∼16 nm in diameter as shown by the illuminated area on a dry VO2 film in Figure 4d. Parts e−g of Figure 4 show the etched patterns on the same VO2 film immersed in radiolytic water after illumination for 1, 20, and 60 s, respectively, with a dose rate of 5 × 103 e− Å−2 s−1. The VO2 film (thickness of ∼30 nm as shown in Figure S1d,e) almost completely dissolved in the depth direction after 1 s, since the longitudinal etching rate of VO2 is ∼28 nm/s (Figure 3). The profiles of the beam spot image and the etched patterns are shown in Figure 4h. The average full width at half maximums (fwhm’s) of the intensity profiles along different radial directions for each etched pattern are plotted against time in Figure 4i. The laterally broadening rate is estimated from the fitted slope of Figure 4i to be ∼1.8 ± 0.3 nm/min. It is evident that the lateral broadening rate is about a thousandth of the longitudinal etching rate to guarantee the spatial resolution as good as the size of electron beam within a few nanometers, which can be used for the water-base electron beam lithography method as shown in Figure S4.

modifies the surface chemistry and boosts the superdissolution of metal oxides in radiolytic water, which is associated with a dissolution rate constant of 16−19 orders of magnitude higher than that for the normal dissolution in acid. The increase in dissolution rate constant is equivalent to a decrease of activation energy by 0.97−1.11 eV, which is on the same order as the reduced energy barrier calculated from the first principle simulation. The in situ high-resolution structural identification and EELS analyses confirm the existence of oxygen deficiencies on metal oxides, which govern the thermodynamics and kinetics of the chemical reactions at oxide/water interface for the superdissolution. It inspires the development of a direct and highly efficient electron lithography method for nanodevice processing in water, which will be of great benefit to the ecofriendly applications for the precise control nanotechnology.

MATERIALS AND METHODS Fabrication of the Self-Aligned Wet (SAW) Cell and Liquid Loading. In situ observation of metal oxide dissolution was carried out in the liquid cell−SAW cell. The SAW cell is composed of two parts: out-frame and in-frame, both using 250 μm thick silicon wafers with holes (100 μm in diameter) as the braced frames, covered by low stress silicon nitride membranes (20 nm in thickness) as the observation windows, as shown in Figure S1a. During liquid loading, first a droplet of 20 μL of pure water or metal oxide NPs dispersed water solution was dropped into the reservoir of out-frame. The inframe was finally sealed hermetically on the out-frame with glue. A liquid layer with a thickness in the range of 10 nm to 1 μm can be controlled between these two silicon nitride membranes. In the electron beam lithography process, the metal oxide thin film was deposited onto the in-frame for etching. The SAW cell can be placed in a conventional TEM holder. Fabrication of the Vanadium Dioxide (VO2) Films. The VO2 films were deposited on the membranes of in-frames by radio frequency (RF) magnetron sputtering (Figure S1a,b). A vanadium disk (99.99 wt % in purity) with a diameter of 60 mm was used as the water-cooled target. High-purity Ar (99.5 wt %) and O2 (99.5 wt %) gases with a flow ratio of 3:2 were introduced into the sputtering chamber at a pressure of 0.5 Pa via two separate mass flow controllers. The in-frame chips were laid on the sample holder, which was about 50 mm distant from the target and rotated continuously during the sputtering process. The sputtering power was set to be 100 W, and the target voltage was monitored at a constant of 380 V. The deposition lasted for 50 s without bias and heating applied. Then the sputtered inframe chip was taken out and annealed at 300 °C for 1 h in a muffle furnace. The VO2 film was a polycrystalline structure with a space group of P42/ncm; see Figure S1c. The thickness of VO2 film was detected by using the electron energy loss spectrum as shown in Figure S1e,f. In Situ Observation of Metal Oxides Dissolution in the Liquid Environmental TEM. All of the experiments were carried out with 200 kV JEOL-2010 and JEOL-2010F microscopes at the room temperature. The SAW cells were loaded into the standard TEM holder. The electron beam can transmit through the silicon nitride membranes and the metal oxide dispersed water layer or the metal oxide film (as shown in Figure S1a). The dissolution process is simultaneously observed and recorded. To achieve the lithographic patterns as shown in Figure 4a,b, the electron beam was converged (use the spot size of 5 or the nanobeam model on JEOL 2010 and JEOL 2010F) and manually shifted by manipulating X and Y knobs in the TEM panel. First-Principle Calculation Method. All calculations are performed by using the linear combination of atomic orbital and spin-polarized density functional theory method, implemented in DMol3 package (Accelrys Software, Inc.).35,36 The generalized gradient approximation with Perdew−Burke−Ernzerhof (PBE) functional form with effective core potential and double-numerical basis set with

CONCLUSION In conclusion, by applying the state-of-art liquid environmental TEM technique, we successfully control the chemical reaction at oxide/water interface by manipulating electron beam to excite oxygen vacancies at the metal oxide surface. This scenario F

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ACS Nano polarization function are adopted in the calculations.36−39 The realspace global cutoff radius is set to be 4.5 Å. A supercell containing a 3 × 3 × 3 unit cells of oxides is used for the surface models, and the thickness of vacuum layer is chosen as 20 Å. Only a Γ-k point is used to describe the Brillouin zone for geometric optimization, and a 6 × 6 × 1 k point is used for electronic structure calculations.40 The smearing parameter is set to be 0.005 hartree. Test calculation indicates that the results do not change when the smearing parameter is set zero. Oxygen vacancies in the oxide surface were simulated by removing one O atom from top layer of a p(2 × 2) surface. The effects of the interactions between defects in neighboring supercells were mini-mized by performing additional calculations for the p(3 × 3) surface. The latter gives the same results as the p(2 × 2) surface. The results for different supercell sizes are used to extrapolate to the dilute limit, for which the defect−defect interactions are minimized. Calculations for odd charge states include the effects of spin polarization. The energy of oxygen vacancy formation ΔEO is calculated by using the equation ΔEO = Evac + EO − Est, where EO is the energy of one isolated O atom, Evac is the energy of a surface model with one oxygen vacancy per supercell, and Est is the energy of the same supercell without vacancy. The metal atom−substrate binding energy Ebm−s can be estimated by Ebm−s = Es + Em − E, where Em is the energy of one isolated metal atom and Es and E are the energy of surface model without and with the metal atom.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b02656. Materials and description of movies (PDF) TEM movie for the dissolution of CeO2 NPs (AVI) HRTEM movie for the dissolution of CeO2 NPs (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongxing Dai: 0000-0003-1738-0348 Manling Sui: 0000-0002-0415-5881 Author Contributions

Y.L., M.S., and F.-R.C. conceived and designed the experiments. Y.L. performed the experiments. Y.L., K.W., W.D., Z.Z., and S.X. contributed materials, F.-R.C. fabricated the SAW cell, J.G. and Y.L. performed the simulations, and M.S., F.-R.C., H.D., W.Z., and Y.L. cowrote the paper. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge useful discussions with F. Q. Huang and L. M. Liu. M.S. acknowledges support from the National Natural Science Foundation of China (Grants Nos. 11374028 and U1330112), the National Natural Science Fund for Innovative Research Groups (Grant No. 51621003), and the National Key Research and Development Program of China (Grant No. 2016YFB0700700). M.S. acknowledges the Cheung Kong Scholars Programme of China. F.-R.C. acknowledges support from NSC 96-2628-E-007-017-MY3 and NSC 101-2120-M007-012-CC1. G

DOI: 10.1021/acsnano.7b02656 ACS Nano XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsnano.7b02656 ACS Nano XXXX, XXX, XXX−XXX