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Shape Evolution of Metal Nanoparticles in Water Vapor Environment Beien Zhu, Zhen Xu, Chunlei Wang, and Yi Gao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00254 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 19, 2016
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Shape Evolution Environment
Nano Letters
of
Metal
Nanoparticles
in
Water
Vapor
Beien Zhu1, 2, Zhen Xu1, 2, Chunlei Wang1, 2 and Yi Gao1, 2* 1
Division of Interfacial Water and Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China. 2 Shanghai Science Research Center, Chinese Academy of Sciences, Shanghai 201204, China Supporting Information Placeholder metal nanoparticle? Actually, in early 2002 Hansen et al. had observed the water vapor induced dynamic shape changes of the supported copper nanocrystals.12 However, there is very limited work on this topic until now. Experimentally studying this kind of phenomenon requires advanced in-situ technique13, 14 which can hardly be performed in detail for all potentially interesting systems. Thus, to understand whether and how water vapor affects the shape of metal nanoparticles by a theoretical model is of primary importance and could provide a guiding tool for the tailored synthesis of desired nanocatalysts and their applications at given temperature and water vapor pressure. In this work, we develop a model based on the Wulff construction, density functional theory and Langmuir adsorption isotherm, which could explicitly predict the structure evolution of the metal nanoparticles in water vapor environment. Suggested by Georg Wulff in the beginning of the 20th century,15 Wulff construction has been successfully applied both in crystalline science16 and in characterizing the shapes of nanoparticles.17-22 The Wulff construction states that a crystal with the lowest surface energy can be constructed in this way: starting from a center point, O, a plane that is normal to the [hkl] vector is drawn at the distance of = , where C is a given constant and is the surface energy of unit area normal to the [hkl] vector. Once this process is repeated for all the Miller indexes, the space that lines inside all the planes gives the equilibrium shape for the crystal. When the nanoparticle is interacting with the gas-phase material, the surface tension should be corrected to be the interface
tension, :
= + ( ⁄ ) (1), where is the surface coverage, is the adsorption energy and is the area per surface atom. This equation was used to predict Au nanoparticles interacting with CO.23 However in a real case, is a temperature-, gas pressure- and -dependent value at equilibrium. Since is different for different [hkl] plane, should be facet anisotropic for given T and P. Apparently, to simulate the shape of nanoparticles under environment condition accurately, a proper description of (, , ) is necessary. For this purpose the Langmuir adsorption isotherm24 is applied, which describes the equilibrium coverage as a function of pressure: = (2), where K is the Langmuir isotherm constant, can be further described as:
ABSTRACT: The structures of the metal nanoparticles are crucial for their catalytic activities. How to understand and even control the shape evolution of nanoparticles under reaction condition is a big challenge in heterogeneous catalysis. It has been proved that many reactive gases hold the capability of changing the structures and properties of metal nanoparticles. One interesting question is whether water vapor such a ubiquitous environment could induce the shape evolution of metal nanoparticles. So far this question has not received enough attentions yet. In this work, we developed a model based on the density functional theory, the Wulff construction, and the Langmuir adsorption isotherm to explore the shape of metal nanoparticle at given temperature and water vapor pressure. By this model we show clearly that water vapor could notably increase the fraction of (110) facets and decrease that of (111) facets for 3-8 nm Cu nanoparticles, which is perfectly consistent with the experimental observations. Further investigations indicate the water vapor has different effects on the different metal species (Cu, Au, Pt, Pd) and size of the nanoparticles. This work not only helps to understand the water vapor effect on the structures of metal nanoparticles, but also proposes a simple but effective model to predict the shape of nanoparticles in certain environment. Keywords: Wulff Construction, water vapor, metal nanoparticle, density functional theory
It is well known that metal nanoparticles play an important role in heterogeneous catalysis. One of the main factors determining its functionality is the shape of the nanoparticle1 since the number of active sites is clearly shape-dependent.2 In recent years, many in-situ experiments have shown that the reaction conditions can change the morphologies of metal nanoparticles and consequently their catalytic properties dramatically.3-7 It has been widely accepted that to study the structure of nanoparticle under reaction condition is very necessary for understanding its catalytic property.8 So far, it has been found that reactive gases such like CO, O2 and NO can all induce the morphology changes of metal nanoparticles (Pt, Au, and etc.) and nanoalloys (PtCo, PtPd, AuNi, and etc.) as well (ref. 8-11 and references therein). One important question in this field is that can other environments besides the reactive gases do the same thing to nanoparticles. For example, can water vapor, one of the most ubiquitous environments on this planet, alter the shape of 1
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= !
∆#
$ %
& = exp (!
+,-. %/0,-. 01,. 2 $ %
) (3),
where 34 is the Boltzmann constant, 5 and 56 are the entropy of adsorbed molecule and of gas-phase molecule, respectively. The adsorption of water on the different facet is very complex since the adsorption energy of water is comparable to the hydrogen bond strength in water. Thus, the delicate balance between the water-water and the water-metal interaction strength determines the stability of water structures. We performed test examination which indicates the average adsorption energies per water of water trimer and hexamer are very close to that of water monomer on Cu surfaces (Table S2), which is consistent with the results shown in Ref. 36 table III. Thus, it is reasonable to use the product of water monomer adsorption energy ( ) and water surface coverage ( ) as the contribution of water-metal bonding to the metal surface tension. In the meanwhile, since the interfacial water monolayer on metal surface normally forms an ice-like structure,25, 26 so the entropy of ice is regarded as 5 in this work. We also use 78 instead of 1 in Eq. (2) and get: θ = 78 ;(1 + ) (4), for water adsorption. 78 is the average surface coverage of monolayer water on metal surface obtained from classical molecular dynamic simulations. The monolayer water structure on different metal surfaces are discussed in detail in our previous work,27 from which we obtain the value of 78 . It should be noticed that by employing these two treatments the lateral interactions between adsorbed H2O molecules have been considered implicitly although they are not involved in the Langmuir adsorption isotherm. Substituting Eq. (3) and Eq. (4) into Eq. (1), the interface tension of metal surface at given temperature and water vapor pressure can thus be determined if only and are calculated. In order to obtain and , the Vienna Ab initio Simulation Package (VASP)28 has been used to perform the spin-unrestricted density functional theory (DFT) calculations. The generalized gradient approximation (GGA)29 together with the projector augmented-wave method (PAW)30, 31 are employed. The cut-off of the plane-wave expansion is 400 eV. The convergence for the electronic self-consistent is set to 10-5 eV. Geometry optimizations are performed within a conjugate-gradient algorithm with a convergence criterion on forces (10-2 eV/Å). Three low-index ((100), (110), (111)) surfaces have been considered in this work. Notice that these low-index surfaces are enough for studying the shape change of metal nanoparticle in water vapor as observed in the experiment,12 while higher-index surfaces are needed for more detailed analysis such as the number of active sites. The Brillouin zone of the periodic 2-D (1x1) slab is modelled by a (16x16x1) Monkhorst-Pack grid of 3?@EG and >?@HG are used to approximatively characterize the relative area of (111) facets, (100) facets and (110) facets respectively. From Eq. (3) and Eq. (4) we can know that increases with the decreasing of T and the increasing of at fixed P ( , II and I as functions of T at 125 Pa are shown in Fig. S1). When the temperature is high (> 500 K), the water coverages on all the facets are close to 0, the structures of the Cu nanoparticles are composed mainly by the (111) and (100) facets which agree with the structure observed in ref. 12 under H2 condition. When the temperature starts to decrease, I increases firstly while II and remain I II small since J J (see Table S1). As a result, the >?@HG increases and >?@EF decreases in Fig. 2(a) when 450K < T < 520K. The shape of the Cu nanoparticle evolves from isomer J to isomer G in Fig. 1(c) which is exactly what Hansen et al. found in their experiment12. More interestingly, we find there is a region that >?@HG reaches almost 1 (380K < T < 420K). In this region the surface of the nanoparticle is dominant by (110) facets (isomer E, F in Fig. 1(c)). When the temperature continues to decrease, II starts to increase, >?@HG decreases and >?@EG
Figure 3, Structure isothermal diagrams of (a) Pd, (b) Pt and (c) Au nanoparticles with diameters of 8nm and corresponding structures. Besides the Cu nanoparticles, we also construct Au, Pt and Pd nanoparticles as mentioned above. These metal nanoparticles are commonly used catalysts in many chemical reactions, for example the CO oxidation. It has been recognized that not only H2O plays an important role in this reaction,38 but many reactions are performed in water or in a humid environment. Therefore, studying the shape change of Au, Pd and Pt in water vapor environment has a general importance in the research field of catalysis. Besides, since water vapor is one of the most normal environments on this planet, to understand the effect of water vapor on different metal nanoparticles has impacts in many other research and industrial fields, for example, the corrosion. The structural isothermal diagrams and typical structures of these nanoparticles with 8 nm diameters are shown in Fig. 3 (a), (b) and (c). The >?@EF , >?@EG and >?@HG of these metal nanoparticles are plotted as the functions of temperatures at P=125 Pa in Fig. 3
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2(b), (c) and (d). It can be seen the Pd nanoparticle shows a similar structure evolution pattern to the Cu nanoparticle from 500K to 380K that evolves from a cuboctahedral structure composed of (111) and (100) facets to a rhombic dodecahedral structure composed of (110) facets. However, at low temperature Pd nanoparticle reverts to its structure at high temperature, which is different to the case of Cu nanoparticle. The constructed Pt nanoparticle shows a cuboctahedral structure both at low and high temperature which evolves to a more spherical polyhedron with increasing fractions of (100) and (110) facets at 400K. From these results a considerable effect of water vapor on the shapes of Pt and Pd nanoparticles can be expected depending on the temperature and water vapor pressure. On the other hand, the effect of water vapor on Au nanoparticle is small due to the extremely small deference of the water adsorption energy on different facets, yet we still find a structure transformation between (111) facet and (100) facet. In this work we focus on the equilibrium shape of metal nanoparticles in water vapor environment. It should be noticed that the similar method can be used to study the morphology change induced by other gases as well. For example, in Ref. 12 the active shape changes of Cu in H2 and CO were reported. With calculations, we find the adsorption energies of CO on Cu facets (Cu(100): -0.90 eV; Cu(110): -0.96 eV; Cu(111): -0.72 eV) are much higher than those of H2O, while H2 molecule is found hardly adsorbed on Cu facets. We can roughly predict that H2 has less effect on the shape of Cu nanoparticle while CO may have a strong effect. Detailed structure constructions in other gases than H2O would be reported in our future work. In summary, we develop an environmental Wulff construction model by introducing the description of the adsorbate surface coverage as a function of temperature and pressure. Using this model we show water vapor is able to change the shape of Cu nanoparticle dramatically. The shape evolution process found in our study agrees very well with the experimental observations. Moreover, the shape change of the Cu nanoparticle caused by weak water-metal interaction is explained by the different degrees of water coverage on different facets depending on temperature and pressure. Beside the Cu nanoparticles, Au, Pt and Pd nanoparticles under water vapor conditions are constructed as well. We predict that Pd and Pt could reshape their structures dramatically as Cu under certain water vapor environment while Au may be different. By this work we hope to draw peoples’ attentions on the effect of water on the structure of metal nanoparticles which is crucial for the synthesis process and the catalytic activities. In the meanwhile we offer a simple but effective model to predict the shape of nanoparticle in certain environment. ASSOCIATED CONTENT Supporting Information All the data needed in our environmental Wulff Construction model: surface tension γhkl, adsorption energy Eads, atomic area Aat and maxim water coverage θML. The energy difference between dissociated adsorbed H2O and molecular adsorbed H2O are shown. Water coverage on the (111), (100) and (110) facets of a Cu nanoparticle with a diameter of 8 nm as a function of temperature at P=125 Pa. Surface site concentration of Cu nanoparticles with different sizes as function of temperature at P=125 Pa. Three surface sites are considered: (a) CN=9, (b) CN=8 and (c) CN
