Effect of Cation Intercalation on the Growth of Hexagonal WO3

May 3, 2012 - School of Computing, Engineering and Mathematics, University of Western Sydney, NSW 2751, Australia. J. Phys. Chem. C , 2012, 116 (21), ...
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Effect of Cation Intercalation on the Growth of Hexagonal WO3 Nanorods Li Chen,† Saiwei Lam,‡ Qinghua Zeng,§ Rose Amal,‡ and Aibing Yu*,† †

Laboratory for Simulation and Modelling of Particulate Systems, School of Materials Science and Engineering, The University of New South Wales, NSW 2052, Australia ‡ ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Sciences and Engineering, The University of New South Wales, NSW 2052, Australia § School of Computing, Engineering and Mathematics, University of Western Sydney, NSW 2751, Australia ABSTRACT: The growth mechanism of hexagonal tungsten oxide (h-WO3) nanorods is investigated using molecular dynamics simulation. The results show that cation intercalation has a great impact on the formation of h-WO3 nanorods, reflected from the attractive interaction between the growth species (polytungstate anion, W10O324−) and crystal faces of (001) and (100). In particular, an appropriate amount of intercalated cations not only accelerate the crystal growth but also induce the formation of onedimensional nanostructure of h-WO3 nanorods along the direction of [001]. An excess of intercalated cations would be unfavorable to the evolution of rod shape. Ammonium ion (NH4+) is found to be the most stable in a hexagonal tunnel, hence being effective in inducing the 1D morphology of h-WO3. The main findings from the simulations are also verified by experiments.

1. INTRODUCTION Hexagonal tungsten oxide (h-WO3) is of great interest because of its unique structure, that is, a hexagonal symmetry with empty hexagonal and trigonal tunnels in a framework of a vertex-shared WO6 octahedral.1 It has been widely used as an intercalation host to obtain hexagonal tungsten bronzes MxWO3 (M = Li+, Na+, K+, etc.) with a blue color. As an electrochromic material, h-WO3 can easily change from colorless to dark blue by applying electrical voltages. Recently, it has become the most promising candidate for large-scale energy saving devices (i.e., smart window).2−4 These devices allow the transmittance of visible light and solar energy to be modulated in architectural windows to adapt to external conditions. In addition, the performances of h-WO3 in gas sensors,5−7 catalysts,8−10 and batteries11 are also remarkable. As compared with bulk WO3, one-dimensional (1D) h-WO3 nanostructures have the advantages of fast electrochromic speed, high-contrast coloration, better sensitivity, and highcapacity retention. Yet, such advantages are dependent on the shape, size, and crystallinity of 1D h-WO3 nanostructures.11−15 Therefore, many efforts have been devoted to develop a wellcontrolled synthesis of 1D h-WO3. A proper understanding of the growth mechanism of h-WO3 nanorods is necessary and important to design a controllable synthetic strategy. In fact, many attempts have already been made in this area. Upon the success in hydrothermal synthesis of 1D h-WO3, a prevailing opinion holds that the inorganic salts (such as sulfate salts13,16or NaCl12) act as capping agents and can selectively adsorb onto particular crystal faces and inhibit the growth on these faces. Such selective adsorption accelerates the preferential growth, resulting in the formation of a 1D © 2012 American Chemical Society

structure. Another opinion suggests that the h-WO3 crystal can be regarded as a polar crystal that preferentially grows along its polar directions (c-axes) and finally grows into a rod morphology.12 Besides, it was reported that additive-free routes17,18 (e.g., using ion-exchange method to remove sodium ions) can also produce the h-WO3 nanorods under hydrothermal conditions. Being absent of additives, the growth is ascribed to the inherent anisotropic growth of the h-WO3 crystal. It is worthy to mention that the above growth mechanisms about the 1D h-WO3 nanostructure are based on limited experimental observation and findings. Such conclusions are arguable and even controversial. For example, Gu et al.13 reported that sulfate plays a very important role in inducing the formation of 1D h-WO3 structures. On the other hand, tungstic acid alone was observed to generate h-WO3 nanorods.17 However, using tungstic acid and sulfate acid in a hydrothermal synthesis, Wicaksana18 observed the formation of nanocube structures rather than nanorods. Therefore, the function of sulfate in h-WO3 rod shape evolution is doubtful. Likewise, the role of capping agent NaCl12 in the formation of h-WO3 nanorods is also in doubt because the h-WO3 nanorods were fabricated successfully in the absence of Na+ when using ionexchange method.18 The complexity of experiments makes it difficult to explore the growth mechanism of h-WO3 nanorods. In this regard, molecular dynamics (MD) simulation exhibits great advantages Received: February 7, 2012 Revised: May 1, 2012 Published: May 3, 2012 11722

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toward the fundamental understanding of such crystal growth. Some important mechanisms at an atomic level can be probed through the interactions between different species. For example, Prathab et al.19 examined the compatibility behavior of PMMA and its adhesion characteristics with other polymers and metal oxides by the polymer−polymer and polymer−metal oxide interactions. Pan and co-workers20 studied the adsorption behavior of Gly and Glu amino acids on hydroxyapatite (HAP) crystal faces to explore the formation of rod- and platelike morphologies of HAP crystal. Zeng et al.21 examined the growth mechanism of silver nanoparticles through comparing the interaction strength of surfactants with different crystal faces. By calculating the potential energies of the reduced matrix structures with a combination of different cation intercalation, Hao et al.22 identified a stable film composition of nickel hexacyanoferrate matrix. In this study, MD simulations will be used to investigate the effect of cation intercalation on the growth of h-WO3 nanorods. In particular, the interaction energies between growth species and main faces of h-WO3 crystal will be calculated with/without intercalated cations. An analysis of such interaction energies will offer some insight into the role of intercalated cation in promoting the formation of 1D h-WO3 nanostructures.

Figure 2. h-WO3 crystal structure: (A) pure crystal and (B) crystal with Na+ intercalation.

(111) are investigated. The cleaved surfaces are then expanded to around 30 × 30 Å2 after performing geometry optimization. Second, a 3D periodic cell containing a certain amount of adsorbates (i.e., growth species of W10O324−) is built with a size of 30 × 30 × 30 Å3. Finally, a 3D periodic system (Figure 3) is

2. NUMERICAL SIMULATION 2.1. Growth Species Model. It is well-known that various tungstate species can be formed in aqueous solution depending on the pH value.23−25 The Raman spectroscopy26 and polycondensation process24 indicate that the most stable species in acidic solution is H4W10O32, which has a O/W ratio mostly close to that of WO3. Thus, the structure of isopolyanions27 W10O324− is selected as the growth species (Figure 1) in this study.

Figure 3. Representative model of adsorbates interacting with h-WO3 (001) face.

built up, which consists of a surface layer obtained from the first step at the bottom, an adsorbate layer constructed from the second step in the middle, and a vacuum layer with the height of 100 Å on the top. 2.4. Simulation Conditions. All simulations are conducted using the Forcite Module of Materials Studio V4.3, from Accelrys Inc. The charge of atoms in various systems is determined using the charge equilibration (QEq) method. The bond stretching and bending as well as nonbond interactions are described by universal force field (UFF).28 Each system is first subjected to geometry optimization and then equilibrated by 100 ps MD simulation with a time step of 1 fs. The MD simulations are all performed in NVT ensemble at 453 K. 2.5. Analysis Method. In an attempt to explore the growth behavior of h-WO3 crystal, the interaction energies of growth species on different crystal faces are calculated. Such interaction energy is considered to be proportional to the growth rate of crystal surface. The greater the attractive interaction energy, the stronger the cohesion and the faster the face grows.29 Therefore, the morphology of a crystal can be predicted by comparing the interaction energies between growth species and various crystal faces.

Figure 1. Structure of W10O324− isopolyanions.

2.2. h-WO3 Crystal Model. The h-WO3 crystal model in Figure 2A is built with the data of h-WO3 from JCPDS card (no. 01-075-2187), which has a space group of P6/mmm and cell parameters of a = b = 7.298 Å and c = 3.899 Å. To investigate the effect of cation intercalation on the growth, another model of h-WO3 crystal with intercalated sodium ions (Figure 2B) is also created. The position of sodium ions is determined on the basis of minimized system potential energy achieved by geometry optimization. 2.3. MD Cell Construction. The simulation system is constructed in three steps. First, a 2D periodic crystal surface is cleaved from a h-WO3 crystal unit. Because h-WO3 crystal has hexagonal symmetry, the structures of some primary faces are exactly the same. Excluding those symmetrical faces, the primary h-WO3 crystal faces of (001), (100), (011), (110), and 11723

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The interaction energy Einteraction (kcal mol−1 Å−3) is calculated according to eq 1: E interaction = [Etotal − (Esurface + Eadsorbate)] × N /V

Table 1. Calculated Interaction Energies (kcal mol−1 Å−3) between One W10O324− and Pure h-WO3 Faces

(1)

h-WO3 faces interaction energy

where Etotal is the total potential energy of the system containing both adsorbate layer and surface layer, Esurface and Eadsorbates are the potential energies of the surface layer and adsorbate layer, respectively, N is the number of atoms contained in the surface layer, and V denotes the volume of the surface layer.

001 −77.86

100 −78.99

011 −58.60

110 −1.03

111 −16.13

the morphology importance verified by the BFDH and experiment,30 it will not be taken into account in the subsequent study. 3.3. Interaction between Growth Species and Crystal Faces of h-WO3 Intercalated by Cation (Na+). Because the interaction of growth species and the pure h-WO3 crystal has not demonstrated the possibility of the formation of h-WO3 rods, the other possible experimental factors should be considered. It is well-known that h-WO3 crystal structure can be easily intercalated by various cations (e.g., H+, alkali ions, or NH4+) due to its open tunnel structure.13,31 Dinatale32 has compared two tungsten bronzes [(NH4)xWO3] obtained from oxlic and citric systems, respectively. It reveals that the tungsten bronze obtained from the oxalic system has a higher ammonium content (x = 0.29) than that from citric system (x = 0.27). Moreover, the reduced W accounts for 2.0 and 3.4% of the total tungsten content in the products from oxalic and citric systems, respectively. This indicates that a higher ammonium content does not necessarily generate a greater proportion of substoichiometric phases within WO3 bulk. In other words, some NH4+ cations can enter into the h-WO3 tunnel and stay there with no W(VI) being reduced. There is no doubt over the fact that the aforementioned cations are present more or less in reaction solution during the hydrothermal synthesis of h-WO3. Hence, it can be speculated that the growth process of the h-WO3 crystal always accompanies the cation intercalation in a hydrothermal reaction. Moreover, the presence of cations in tunnel is found to be essential in forming h-WO3 structure. Szilagyi et al.31 demonstrated that the hexagonal framework of h-WO3 crystal cannot be maintained without some stabilizing ions or molecules in the hexagonal channels. Other residual ions (like Li+, Na+, H+, and K+) were also reported to assist in stabilizing the h-WO3 structure.13,33,34 It is therefore suspected that the intercalated cations were somehow involved in the growth of the h-WO3 crystal. To verify this, a series of MD simulations have been performed to probe the interactions between the growth species and the faces of the h-WO3 crystal with intercalated Na+. The calculated interaction energies are listed in Table 2. During MD simulation, the cations are observed to enter the tunnel only from (001) face to which the tunnel is opening. This suggests that the growth species on (001) face can always approach the inserting cations closely and are subjected to strong attractive interaction, while the interaction on the (100) face varies with the position from which the cations enter. As shown in Figure 5, a cation entering from the middle tunnel site on the (001) surface will be the farthest away from the (100) surface, leading to a weaker interaction between growth species and inserting cations. Provided that the cations enter into the different tunnels from the (001) face with equal opportunity, then the average interaction energy (E100) between growth species and (100) face can be approximated as:

3. RESULTS AND DISCUSSION 3.1. Growth Faces of h-WO3 Crystal. The Bravais− Friedel Donnay−Harker (BFDH) method is a geometrical calculation to deduce the crystal morphology by using the crystal lattice and symmetry. It generates a list of possible growth faces and their relative growth rates. In some cases, this method is not accurate enough due to the neglected energetics of the system. Yet, it is always useful for identifying important faces in the growth process. Interestingly, the morphology (Figure 4) and important faces [(001), (100), (010), (11̅0)] of

Figure 4. h-WO3 morphology calculated by the BFDH method: (A) top view of crystal morphology based on the crystal cell and (B) lateral view of crystal morphology.

h-WO3 crystal generated by the BFDH method exhibit good agreement with the experimental observations.30 It needs to be noted that the lateral faces (Figure 4B) of (010), (100), and (11̅0) are theoretically identical because of the high hexagonal symmetry of h-WO3 crystal structure. The two faces of (001) and (100) are therefore to be the focus in the following study. 3.2. Interactions between Growth Species and Pure hWO3 Crystal Faces. As mentioned before, polytungstate anion (W10O324−) is regarded as the growth species of h-WO3 crystal in this simulation. By investigating the interaction between polytungstate anion and h-WO3 crystal faces, the growth mechanism of h-WO3 crystal can be explored. A positive value of the interaction energy represents a repulsive interaction, while a negative value implies an attractive interaction, and the absolute value indicates the strength of an interaction. It is noticed that the calculated attractive interaction energies (Table 1) of growth species on (001) and (100) faces are very comparable, indicating that the 1D morphology may not be achieved because there is no face with significant growth over the others. In addition, the face (011) also appears to be active in the crystal growth. It is actually a pyramidal face between (001) and (010) faces. Because the (011) face does not possess

E100 = (E100 max + E100 min)/2 11724

(2)

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Table 2. Calculated Interaction Energies (kcal mol−1 Å−3) between One W10O324− and h-WO3 Faces of Na+ Intercalated Systems

a

system

no. of intercalated Na+

E001

E100_max

E100_min

E100

g (%)a

1 2 3 4 5

1 2 4 8 64

−91.36 −103.27 −126.88 −168.42 −430.32

−87.34 −97.18 −116.00 −133.88 −407.69

−78.99 −78.99 −78.99 −78.99 −407.69

−83.17 −88.09 −97.50 −106.44 −407.69

9.85 17.23 30.13 58.23 5.55

Note: g = (E001 − E100) × 100%/E100.

inserting cations is able to induce the formation of 1D h-WO3 nanorods, while an excess of cations is unfavorable to the formation of such nanorods. On the basis of the above simulation results, the overall growth mechanism of h-WO 3 nanorods upon cation intercalation can be explained as follows. Initially, a solution contains a large number of small particles or crystals of h-WO3. The average particle size will increase via Ostwald ripening process.35 On the other hand, with the increase of particle size, more tunnels are formed in particles. The cations contained in solution continuously enter into or extract from the tunnel of hWO3 crystal, altering the strength of attractive interaction of growth species on different surfaces. When the number of intercalated cations reaches an appropriate level, the interaction of growth species on (001) face is much stronger than that on other faces. Thus, anisotropic growth along [001] becomes dominant and finally leads to the formation of h-WO3 nanorods. That explains why using Li2SO4,13 K2SO4,16 NaCl,15 (NH4)2SO4,32 and even additives free17,18 (containing concentrated H+ and small amount of Na+) can obtain the 1D h-WO3 nanostructure without exception. In addition, the adverse effects of excess cations on the formation of h-WO3 nanorods are also reported in experiments. Excess NaCl15 was observed to generate a mixture of irregular nanorods and nanoparticles instead of uniform nanorods. As to the effect of H+ concentration, Wicaksana18 found that at pH 1−2, h-WO3 nanobundle structures emerged, while under very acidic conditions (pH < 0.3), nanocube structures were formed. 3.4. Stability of Cations Intercalated in Hexagonal Tunnel of h-WO3 Crystal. The stability of various cations in the hollow hexagonal tunnel of h-WO3 crystal is different. Our simulations show that only NH4+ is found to stay in tunnel and rotate in situ; all of the other cations are observed to move along the tunnel and pass through it. By comparing the mean square displacement (MSD) of various cations in tunnel of hWO3 crystal (Figure 6), it is found that the MSD curve of NH4+ is flat, indicating that NH4+ does not move much along the tunnel. In contrast, the cation that exhibits the highest diffusion

Figure 5. Various intercalating sites corresponding to different distances to the (100) surface.

where E100_max and E100_min are the interaction energies when the cations are located at the closest and farthest tunnel sites to (100) surface, respectively. The diameter of most synthesized nanorods/nanowires is in a range of 20−60 nm.13,17 The distance between the furthest tunnel site, located at the center of the nanorod from the cross-section view, and the (100) face is around 10−30 nm. The generated interaction by the cations at such a distance is very weak and can be ignored. Thus, at a low cation intercalation level, E100_min ≈ E100_p. Here, E100_p is the interaction energy of growth species on the (100) face of the pure h-WO3 crystal system (Table 1). On the other hand, at a very high cation intercalation level, the tunnel is almost or fully occupied as system 5 in Table 2. The difference between E100_max and E100_min is very small and can be neglected, that is, E100_min ≈ E100_max. It needs to be noted that the term g (in Table 2) was given by g = (E001 − E100) × 100%/E100, in which E001 and E100 are the interaction energies of the growth species on (001) surface and (100) faces, respectively. The term g represents the degree of such energy difference relative to the energies on the (100) face. The larger the g value, the greater the energy difference between (001) and (100) faces, the faster the growth of (001) over (100) face and the stronger the tendency of shape evolution toward 1D morphology. By comparing the systems with different numbers of Na+ intercalated, it is clear that the interaction energies of growth species on these two surfaces increase steadily with the number of Na+ inserted. This indicates the intercalated cations are able to accelerate the growth of the h-WO3 crystal. More importantly, when increasing the number of intercalated Na+ from 1 to 8, the increasing value of g suggests that the growth of the (001) face is much faster than other faces, such as (100), (010), and (110̅ ), and the 1D morphology of h-WO3 crystal with a length direction of [001] can eventually be formed. When the tunnel is completely (or nearly completely) occupied as system 5 in Table 2, the value of g drops dramatically, indicating that the interaction difference (g = 5.55%) of growth species on crystal surfaces is very small, and 1D growth may not be caused due to the absence of one leading growth direction. Therefore, an appropriate amount of

Figure 6. Mean square displacement of various cations in a hexagonal tunnel of h-WO3 crystal. 11725

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Table 3. Calculated Interaction Energies (kcal mol−1) between One Intercalating Cation and h-WO3 Crystal Lattice cations 37

ionic radius (Å) interaction energy

Li+

NH4+

H+

Na+

K+

Rb+

Cs+

0.60 −30.56

1.44 −30.40

0.28 −30.15

0.95 −29.91

1.33 −28.15

1.48 −27.87

1.69 −26.65

coefficient is H+, followed by Li+. The diffusion of other cations (Na+, K+, and Rb+) in the tunnel is comparable, slightly lower than that of Cs+. In an attempt to elucidate whether such instability is due to the strength of their interaction with the crystal lattice, a series of 3D periodic h-WO3 crystal cells of the size of 14.596 × 14.596 × 7.798 Å3 with different cation intercalation are built up. The calculated interaction energies between the inserted ion (in hexagonal tunnel) and the crystal lattice are listed in Table 3. Because the narrowest part of the hexagonal tunnel still has the large dimensions of diameter of 5.36 Å, the ions Li+ and H+, though possessing relative strong interactions with crystal lattice, become the fastest moving ions in the tunnel due to their small ionic radius (Table 3). In contrast, ions such as Na+, K+, Rb+, and Cs+, having larger ionic size but weaker interactions with the tunnel, are not stable in the tunnel. Only NH4+, with strong interaction and large ionic size, is found to be the most stable in the tunnel. Therefore, NH4+ can be a good stabilizer in maintaining the hexagonal tunnel structure, which is consistent with the experimental observations of Szilagyi et al.31 It should be noted that the above simulations regarding the stability of various cations in a hexagonal tunnel are conducted in vacuum. However, in actual hydrothermal conditions, the small cations such as H+, Li+, and Na+ are often in the form of hydrated ions.36 The diffusivity of these ions in a tunnel will be decreased largely due to the increased hydrated ionic size. Thus, they could be stable in a WO3 tunnel and be effective in inducing the 1D h-WO3 nanostructure as observed in experiments.13,15−18 Because the NH4+ cation is found to be the most stable in hexagonal tunnel, it is supposed to be effective in inducing the formation of 1D h-WO3. To verify that the cations, other than the anions, make the major contributions to the inducement of h-WO3 nanorods, a series of experiments with different ammonium salts have been carried out to synthesize h-WO3 nanostructures. The experimental details can be found in the Dinatale's method.32 Briefly, the reaction solutions of system A are prepared by first dissolving Na2WO4 into 20 mL of H2O, then adding HCl to adjust the solution pH to 1, and finally diluting the solution to 50 mL to give a final Na2WO4 concentration of 0.1 M. Systems B, D, and E are made by simply adding into the solution of system A with one type of ammonium salts [(NH4)2SO4, NH4Cl, or NH4NO3], respectively. The SEM images of the final products for the above systems are shown in Figure 7. Without ammonium salts, system A only gave the products of slablike morphology, consisting of 55% hexagonal and 45% orthorhombic WO3. Upon the addition of ammonium salts, the other three systems B, D, and E all produced the rod shape hexagonal WO3 crystals. Moreover, in system B with (NH4)2SO4, it was found that the h-WO3 nanorod structure started to collapse and form short fiberlike structures when increasing the (NH4)2SO4 concentration by five times (Figure 7C), which suggests an excess of cations is a disadvantage for the growth of h-WO3 nanorods. As shown in the aforementioned simulation results, at a high cation intercalation level, the tunnel could be fully occupied by

Figure 7. SEM images of hydrothermal products of h-WO3: (A) a nanoslab, without ammonium salts; (B) nanorods, 0.76 M (NH4)2SO4; (C) fiberlike structure, 3.8 M (NH4)2SO4; (D) nanorods, 0.76 M NH4Cl; and (E) nanorods, 0.76 M NH4NO3.

the inserted cations. The resulting interaction energies of growth species on (100) and (001) faces would be comparable, narrowing the energy gap between (001) and (100) faces, which is indicated by intensively droped g value of system 5 in Table 3. Thus, in the absence of single dominant growth direction, the 1D morphology could not be formed. Such experimental results not only verify our simulation prediction but also further demonstrate that only an appropriate amount of cations (such as NH4+) can promote the formation of hWO3 nanorods.

4. CONCLUSIONS MD simulation has been used to investigate the growth mechanisms of hexagonal tungsten oxide nanorods. In particular, the effects of cation intercalation in the hexagonal tunnel on the growth of nanorods are examined. The following conclusions can be drawn from this study. For a pure h-WO3 crystal system, the interaction energies between growth species and surfaces (001) and (100) are found to be comparable, which indicates that growth species do not adsorb preferentially on a specific surface, and thus, the 1D morphology may not be formed inherently. For the cation-intercalated h-WO3 crystal systems, the calculated interaction energies indicate that increasing the number of intercalated cations can cause the h-WO3 crystal to grow faster. Particularly, an appropriate amount of cations can promote a pronounced advantage in growth of (001) face over other main faces and shape the h-WO3 crystal into 1D morphology. However, an excess of cations is found to be unfavorable in 1D shape evolution. A comparison of mean square displacement of various cations in a hexagonal tunnel of h-WO3 crystal indicates that NH4+ is the most stable in the tunnel due to its strong interaction with crystal lattice and large ionic radius. Experiments with various ammonium salts confirm the effective role of NH4+ in inducing the 1D morphology of h-WO3 crystal. 11726

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AUTHOR INFORMATION

Corresponding Author

*Tel: +61 2 93854429. Fax: +61 2 93855956. E-mail: a.yu@ unsw.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Australia Research Council for the financial support of this work.

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dx.doi.org/10.1021/jp301210q | J. Phys. Chem. C 2012, 116, 11722−11727