Preparation of NickelâAluminum-Containing Layered Double

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Preparation of Layered Double Hydroxide Films by Secondary (Seeded) Growth Method and their Electrochemical Properties Fazhi Zhang, Li Guo, Sailong Xu, and Rong Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b00619 • Publication Date (Web): 02 Jun 2015 Downloaded from http://pubs.acs.org on June 8, 2015

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Preparation of Nickel, Aluminum-containing Layered Double Hydroxide Films by Secondary (Seeded) Growth Method and their Electrochemical Properties. Fazhi Zhang*, Li Guo, Sailong Xu, and Rong Zhang State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Keywords: layered double hydroxide, thin film, secondary growth, seeded growth, evolution process, electrode material.

ABSTRACT

Thin Films of nickel, aluminum-containing layered double hydroxide (NiAl-LDH) had been prepared on nickel foil and nickel foam substrates by secondary (seeded) growth of NiAl-LDH seed layer. The preparation procedure consists of deposition of LDH seeds from a colloidal suspension on the substrate by dip coating, followed by hydrothermal treatment of the nanocrystals to form LDH film. The secondary-grown film is found to provide a higher crystallinity and more uniform composition of metal cations in film layer than the in situ-grown one on seed-free substrate under the identical hydrothermal conditions. A systematic

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investigation of film evolution process reveals that crystallite growth rate is relatively fast for the secondary-grown film, due to the presence of LDH nanocrystal seeds. Electrochemical performance of the resulting NiAl-LDH films as positive electrode material was further assessed as an example of their practical applications. The secondary-grown film electrode delivers improved rechargeable, discharge-capacity and cycling stability compared with the in situ-grown one, which can be explained by existence of unique microstructure for the former. Our findings show an example for the effective fabrication of LDH film with controllable microstructure and enhanced application performance through a secondary (seeded) growth procedure.

INTRODUCTION Layered double hydroxides (LDHs, a family of anionic clays) have recently attracted increasing attention in view of their potential applications in the fields of catalysis, adsorption, additives in polymers, drug delivery, environmental remediation, and energy storage. 1-3 LDHs have the structural units which are made from stacks of brucite-like positively charged octahedral sheets [M2+1-xM3+x(OH)2] (where M2+ and M3+ are divalent and trivalent cations, respectively) and the corresponding exchangeable anions An- as well as water molecules in the interlayer to balance the positive charges.4,5 For the purpose of broadening new applications in practical devices, LDH crystallites should be organized into aligned two-dimensional arrays or films on various substrates.6,7 Over the last decade, LDH films have gained much attention in several areas including superhydrophobic films,8 anti-corrosion coatings for metals and alloys,911

heterogeneous catalysts,12 functional films for use in optical, electrical and magnetic devices.13-

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Up to now, there are two different ways to prepare LDH films: physical deposition and in situ

growth on substrates such as metals, inorganic materials, and polymers.6,7 The former technique, mainly consisting of layer-by-layer assembly, sol-gel spin coating, and solvent evaporation, uses


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colloidal nanocrystals or delaminated nanosheets of LDH as building blocks which are immobilized on a substrate via physical interactions. Meanwhile, the in situ growth technique has been adopted for fabricating LDH films by means of direct one-step crystallization of LDH crystallites on a substrate. Recently, the electrochemical deposition method has been proved to be a very attractive procedure to prepare LDH films on metallic substrates.20-22 Because of the presence of chemical bonding between LDH crystallites and substrate, the in situ-grown LDH films have a relatively strong adhesion between the two phases. Numerous studies have shown that the nucleation and growth processes involved in forming an inorganic film have a significant effect on the film microstructures, such as crystal composition, grain sizes, crystalline orientation, etc.23-25 Recently, based on the experimental results of formation process evolution, a rough twostage formation process was commonly proposed for the synthesis of LDH by coprecipitation: 2628

colloidal amorphous aluminum hydroxide (AAH) is formed from the aluminum precursor salt

solution at the early stage; subsequently, the amorphous hydroxides are transformed into the crystallites of oxide-hydroxide aluminum, accompanying the continuous incorporation of surrounding Mg2+ into the sheet for forming LDH crystallites. From the above studies we can reasonably infer that LDH film upon in situ growth under the hydrothermal crystallization condition may suffer from problems including the localized segregation of components and possible loss of stoichiometry, which could result in a significant decrease in application properties. Thus, fabrication of LDH film with improved performance for specific applications by developing novel strategy still remains a priority and a challenge. In this study, we report the synthesis of NiAl-LDH films on nickel foil and nickel foam substrates by means of a secondary (seeded) growth procedure. To this day, several classes of film materials have been fabricated by this technique, such as films of zeolites, metal organic


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frameworks, metal oxides, metal sulfides, and polymers.23-25 The secondary growth method for film fabrication relies on the deposition of preformed micro- or nanoparticles to form closely packed layers, followed by secondary treatment of the seed particles to yield continuous films. Herein the nanosized NiAl-LDH seed particles were prepared using a modified two-step crystallization procedure29 which involving a fast coprecipitation in a colloidal mill with vigorous stirring30 followed by controlled hydrothermal treatment. Compared with the conventional coprecipitation, the advantage of this method is that it gives stable and homogeneous LDH suspensions with controllable particle sizes, which should facilitate the deposition of LDH nanocrystallites into uniform layer on substrate. For comparison, NiAl-LDH films were prepared by in situ growth technique on substrate without seed layer. The formation process of both LDH films upon secondary (seeded) growth and in situ growth, such as the evolution of phase composition, structure, and particle morphology during the whole process was verified and the formation mechanism was postulated based on the characterization results. To explore prospective applications, we further evaluated the electrochemical performance of the secondary-grown NiAl-LDH film as positive electrode material and compared with that of the in situ-grown specimen. Recently, developing high-performance nickel hydroxide-positive electrodes for application in Ni-Cd and Ni-H2 secondary batteries has become an exciting direction in the field of electrical energy storage. Numerous studies have demonstrated that formation of Ni-containing LDH crystallites by introducing bi- or trivalent cations, such as Co2+, Zn2+, Al3+, Mn3+ and Fe3+, into the nickel hydroxide lattice can result in an enhanced stability of the α-Ni(OH)2 phase.3,31-36 Herein, the electrochemical performance of the secondary-grown NiAl-LDH film was assessed, which exhibiting an improved rechargeable, discharge-capacity and cycling stability, compared with the in situ-grown specimen. To the best of our knowledge,


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this is the first report dealing with the fabrication of LDH film by secondary (seeded) growth technique and provides valuable insight on the microstructure-performance relationship of this type film as electrode material.

EXPERIMENTAL SECTION Materials. Nickel foil and nickel foam were purchased from Beijing General Research Institute for Nonferrous Metals. The chemicals including ethanol, acetone, NaOH, Na2CO3, (NH2)2CO, NH4F, Al(NO3)3•9H2O, and Ni(NO3)2•6H2O are of analytical grade and were used without further purification. Deionized water was used in all the experimental processes. Preparation of colloidal LDH suspension. For the synthesis of colloidal NiAl-LDH precursor with Ni/Al molar ratio 2.0, 20 ml of mixed salt solution containing Ni(NO3)2•6H2O (2.0 mmol) and Al(NO3)3•9H2O (1.0 mmol) and 80 mL basic solution of NaOH (12.0 mmol) and NaCO3 (1.04 mmol) were firstly poured out into a colloidal mill with vigorous stirring for 10 min. The resulting slurry was centrifuged and washed twice with deionized water to remove the excess free metal salts and alkali, and manually dispersed in 20 ml of deionized water. Subsequently, the aqueous suspension was transferred into a 100 ml stainless steel autoclave with a Teflon lining. The autoclave was then placed in a preheated oven, followed by hydrothermal treatment at 100 ºC for 16 h. Nanosized LDH crystallites formed were collected and washed by centrifugation and suspended in deionized water. The concentration of LDH slurry was about 2.0% and it can remain as a stable suspension for several months. Deposition of seed layer by dip coating. Ni foil (5 cm × 2 cm × 0.1 mm) or Ni foam (5 cm × 2 cm × 1.5 mm), cleaned with acetone, ethanol, and deionized water in sequence before being used, were immersed in and slowly withdrawn from the above obtained LDH suspension in order to form a seed layer on the substrate surface. The coated substrate is then dried at 60 ºC. The


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withdrawal rate was 200 μm/s. Two successive dip coatings were completed in order to obtain a seed layer with high surface coverage. LDH films fabricated by secondary (seeded) growth and in situ growth. In a typical crystallization procedure for secondary growth, 12 mmol Ni(NO3)2•6H2O, 4 mmol Al(NO3)3•9H2O, 16 mmol NH4F, and 40 mmol CO(NH2)2 were firstly dissolved in 80 ml of deionized water. The mixed solution was transferred into a 100 ml Teflon-lined stainless-steel autoclave. Then, the seeded Ni substrate was immersed in solution and leaned against the container, which were sealed and maintained at 120 ºC for different reaction times from 0.5 to 16 h. The substrate was then withdrawn, rinsed with deionized water and dried at 60 ºC. For comparison, in situ growth technique was employed on Ni substrate without seed layer under the identical hydrothermal conditions as described above. Characterization. X-ray diffraction (XRD) patterns were recorded on a UItima III diffractometer, using Cu Kα radiation (λ = 0.15418 nm), with a scan step of 0.02° and a scan range between 3°and 70°. The morphologies of the samples were investigated using a scanning electron microscopy (SEM) instrument (Zeiss SUPRA 55), equipped with an energy dispersive X-ray spectrometer (EDX), with an accelerating voltage of 20 kV. The composition data for the samples of LDH seed particles and LDH films were obtained by EDX analyses. Photon correlation spectroscopy (PCS, Nanosizer Nano ZS, Malvern Instruments) was used to analyze the particle size distribution of LDH suspensions from 0.6 to 6000 nm, in which the peak position and the width at the half-maximum were automatically calculated. The transmission electron microscopic (TEM) images were obtained on JEOL JEM-2010 transmission electron microscope at an acceleration voltage of 200 kV. Fourier transform infrared spectra (FTIR) were collected on Bruker Vector 22 spectrometer within 4000-400 cm-1 at a resolution of 4 cm-1 by


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KBr pellet technique. X-ray photoelectron spectrometer (XPS) measurements were recorded on an ESCALAB 250 X-ray photoelectron spectrometer, operating at a typical pressure of about 2 × 10−9 Pa using Al Kα X-rays as the excitation source (1486.6 kV) in 0.05 eV energy step size. Electrochemical testing. The electrochemical measurements were carried out at room temperature in a three-electrode glass cell containing a 6 mol L-1 KOH aqueous solution as the electrolyte. The galvanostatic charge/discharge tests were conducted on a LAND CT2001A cell performance test system. Fresh NiAl-LDH film fabricated by secondary (seeded) growth or in situ growth on the Ni foam substrate was used as the working electrodes, together with a nickel mesh counter electrode and an Hg/HgO reference electrode. The electrode was charged at a current density of 30 mA g-1 and discharged at the same density until the potential was -1 V vs Hg/HgO. Cyclic voltammetry (CV) measurements were performed between -0.1 V and 0.6 V at a scan rate of 0.5 and 5 mV s-1 vs. Hg/HgO on an electrochemical workstation (Corrtest CS350, China) with Hg/HgO as the reference electrode and Pt foil as the counter-electrode. Herein, the current densities and discharge capacities per gram were all calculated according to the weight of LDH accurately measured by weighing the Ni foam before and after hydrothermal treatment, a loading mass of about 10 mg cm-2 is achieved.

RESULTS AND DISCUSSION Preparation of LDH films by secondary (seeded) growth. Firstly, the colloidal NiAl-LDH suspension was prepared using a modified protocol with a two-step process described in the literature.29 XRD pattern of the resulting LDH crystallites exhibits a series of (00l) reflections at low 2θ values, characteristic of a layered structure (Figure S1, see Supporting Information). The intercalation of CO32- anions into LDH gallery is confirmed by the (003) reflection located at 11.7º, together with the appearance of the characteristic υ band of CO32- at 1360 cm−1 in FT-IR


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spectrum (Figure S2). The lateral dimension of LDH seed crystallites from TEM image is about 15-30 nm, which is very close to the hydrodynamic diameter determined by the laser particle size analyzer (Figure S3). It is difficult to observe the thickness (the stacking direction, perpendicular to the layers) of LDH seed platelets by TEM. We estimated the thickness value from the values of the full-width at half-maximum (fwhm) of the (003) and (006) diffraction peaks by means of the Scherrer equation [L = 0.89λ/(θ) cos θ, where L is the crystallite size, λ is the wavelength of the radiation used, θ is the Bragg diffraction angle, and �(θ) is the fwhm].37 The value calculated from the Scherrer equation is about 4 nm. Subsequently, the NiAl-LDH suspension was adopted for deposition of LDH seed layer by dip coating. XRD pattern (Figure S1c) and SEM image (Figure S3d) of the LDH seed layer reveals the well (00l)-oriented assemblies of LDH nanoplatelets parallel to the substrate surface.

Figure 1. XRD patterns of NiAl-LDH films on Ni foil substrate prepared by secondary growth (left) and in situ growth (right) method hydrothermally treated for different reaction times, with ‘*’ indicating the peaks from the Ni substrate.


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The precursor seed layer on Ni foil was subjected to secondary growth in order to prepare intergrown NiAl-LDH film. XRD patterns of the film specimens after hydrothermal treatment at 120 ºC for different reaction times are shown in Figure 1. It can be seen the weak peaks of LDH phase for specimen after 3 h (Figure 1, left). The characteristic reflections gradually sharpen with the prolonging reaction time, indicating the increase of LDH crystallinity. Meanwhile, XRD pattern of 3 h-prepared LDH film upon in situ growth on Ni foil substrate without seed layer displays broad peaks centered at 2θ positions characteristic of pseudoboehmite aluminum hydroxide gel (Figure 1, right).38 The characteristic reflections of LDH phase, such as (003), (012) and (110)/(113) peaks, appear after the reaction time was extended beyond 5 h. We can also observe the reinforcement of the reflections with increasing reaction times. Obviously, the film specimens prepared by secondary growth have a higher crystallinity than those prepared by in situ growth with the identical crystallization time. SEM top-view images of LDH films prepared by both techniques with different crystallization times are shown in Figure 2. All film specimens are predominantly perpendicular to the substrate. In the case of LDH films upon secondary growth (Figure 2, left), we can see a nest-like microstructure film composed of interlaced plates after crystallization time 3 h. Sharp blade-like crystal sheets are formed and the size of the microcrystals gradually increases to about 500 nm after 5 h. The resulting film has a compact and continuous morphology. We can clearly observe the edges of sheet-like microcrystals forming the interlaced assemblies on the substrate. A closer arrangement of LDH crystallites can be obtained for the film with particle size about 700 nm after 7 h. The evolution of the surface morphology for the secondary-grown LDH films with increasing crystallization time from 10 to 16 h was found to be insignificant by the SEM observation. Meanwhile, we can observe an alveolate-like layer with irregularly shaped flakelets


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on the substrate for the in situ-grown LDH film with crystallization time 3 h (Figure 2, right). The LDH microcrystals with characteristic hexagonal plate-like morphology appear after 5 h. It is obvious that crystallite growth occurred much more slowly than the secondary-grown film.

Figure 2. SEM top-view images of NiAl-LDH films on the Ni foil substrate by secondary growth (left) and in situ growth (right) method hydrothermally treated for different reaction times.


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Figure 3. SEM cross-sectional images and the corresponding energy dispersive X-ray line scan of NiAl-LDH films on the Ni foil substrate by secondary growth (left) and in situ growth (right) method hydrothermally treated for different reaction times.


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SEM cross-sectional images of LDH films prepared by secondary growth with different crystallization times are shown in Figure 3, left. We can identify the different domains corresponding to the LDH microcrystal layer and the substrate surface. LDH crystallites are accumulated with a multilayer structure from the support to the outer surface, exhibiting an orientation in which the ab-faces of the platelets are perpendicular to (i.e., c-axis parallel to) the substrate. The thickness of the LDH layer is about 6.5 μm with crystallization time 3 h and increases to about 7.6 μm after 16 h. In a similar manner to the secondary-grown films, the in situ-grown ones were assembled with the crystallites oriented with their ab-faces perpendicular to the substrate. Moreover, both kinds of LDH films obtained with the identical crystallization time have the comparable layer thickness measured by SEM. The component distribution of metal cations in the film layer along the cross-sectional direction of the specimens was determined by SEM-EDX spectroscopy (Figure 3). The calculated Ni/Al molar ratio are listed in Table 1. For the 3 h-prepared LDH film upon secondary growth, the Ni/Al molar ratio ranges from 1.5 to 2.3 along EDX depth profile over the LDH layer, and the mean Ni/Al ratio is about 2.0. Meanwhile, the in situ-grown film exhibits a lower Ni/Al ratio 1.5, indicating the decreased Ni species in the LDH sheets. Ni/Al ratio for the in situgrown LDH film ascends larger with a longer reaction time. Both LDH films with crystallization time 16 h have the same Ni/Al ratio 2.2. In addition, we can observe that the Ni/Al ratio for the in situ-grown LDH film declines along EDX depth profile from the exterior to the interior of the specimen, demonstrating the uneven distribution of metal cations along the film layer. However, the similar Ni/Al ratio can be found for the secondary-grown LDH films over film layer along EDX depth profile, especially for the film specimens prepared after a longer crystallization time.


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Table 1. Ni/Al molar ratios of LDH films on the Ni foil substrate by secondary growth and in situ growth method hydrothermally treated for different reaction times.

Ni/Al ratio over LDH layer along EDX depth profile mean Ni/Al

from the bottom

sample*

ratio# 1 μm

2 μm

3 μm

4 μm

5 μm

6 μm

LDH film (sg3)

1.9

2.0

2.3

2.0

2.3

/

2.0

LDH film (is3)

1.6

1.6

1.4

1.4

1.4

/

1.5

LDH film (sg7)

1.8

1.9

2.1

1.9

1.9

1.9

2.2

LDH film (is7)

1.4

1.6

1.5

1.8

1.7

1.8

1.6

LDH film (sg10)

2.1

2.1

2.2

2.2

2.2

2.1

2.2

LDH film (is10)

1.4

1.7

1.8

2.6

2.3

2.5

1.8

LDH film (sg16)

2.0

2.0

2.1

1.9

2.1

2.0

2.2

LDH film (is16)

1.4

1.4

1.9

2.2

2.2

2.3

2.2

* sg: prepared by secondary growth method; is: prepared by in situ growth method; The number in parenthesis shows the reaction time (hour) for hydrothermal treatment. # determined by the total Ni and Al contents along EDX depth profile. In order to obtain more information on the chemical composition of the superficial layer and further insight into the evolution process of NiAl-LDH films prepared by both methods, XPS spectra of Ni 2p and Al 2p for the film specimens with different hydrothermal treatment times


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are given in Fig. 4 and 5, respectively. The binding energy (BE) of main Ni 2p 3/2 peak for the LDH films observed at about 856.5 eV is assigned to the Ni2+ species in NiAl-LDH (Figure 4, upper).39 It can be seen that the Ni 2p3/2 BE values for three specimens upon secondary growth are approximately in the same region, suggesting the electronic state of Ni2+ in LDH film crystals are similar. However, the intensities of the main Ni 2p3/2 peak obviously increase after prolonging the reaction time, due to the change of the film microstructure resulting from a relatively closer arrangement of crystallites with a compact morphology (Figure 2, left). Meanwhile, for the LDH films by in situ growth, the BE shifts obviously to a lower value by 0.7 eV (from 857.2 to 856.5 eV) with prolonging the reaction time from 3 h to 16 h, which is accompanied by the increase of the Ni 2p3/2 peak intensity (Figure 4, lower). On the other hand, the broad Al 2p signals in Figure 5 can be fitted to two peaks. For the film specimen prepared by seeded growth after 3 h, a main peak at 74.0 eV should be the evidence of the appearance of aluminum oxide layers 26,40 with a small shoulder located at about 76.4 eV which is related to the Al3+ species in the form of Al-OH of the amorphous phase

41

(denoted as Al3+low and Al3+high,

respectively). We can recognize a slight shift of Al3+low BE value from 74.0 to 73.7 eV, with an increase of peak intensity after prolonging the reaction time from 3 h to 16 h. At the same time, the peak intensity of Al3+high declines rapidly at the later crystallization stage, accompanied by the shift to a higher position. The XPS spectra of Al 2p for the film specimens by in situ growth show the same trend. It is noted that a weak Al3+high peak at 78.8 eV can be seen for the film specimen by in situ growth after 16 h, whilst this peak almost vanishes for the secondary-grown film after the same reaction time.


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Figure 4. XPS spectra of Ni 2p for NiAl-LDH films on the Ni foil substrate by secondary growth (upper) and in situ growth (lower) method hydrothermally treated for different reaction times.

Figure 5. XPS spectra of Al 2p for NiAl-LDH films on the Ni foil substrate by secondary growth (upper) and in situ growth (lower) method hydrothermally treated for different reaction times.


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Based on the above XPS results, it is concluded that the electronic atmosphere of Ni and Al atoms in the LDH films may be changed with prolonging the reaction time, which is caused by the formation of LDH phase by incorporation of Ni2+ with aluminum hydroxide in the layer. In the case of in situ-grown LDH film, after the Ni2+ ions are incorporated into the sheets forming NiO6 octahedral, the binding energy can be shifted to lower region as the crystallinity and electron cloud density surrounding the nickel nuclei increase (Figure 4, lower). Meanwhile, the Al3+low BE is also shifted to lower values for the Ni2+ ions that were introduced to the aluminum hydroxide during this period. Simultaneously, the Al3+high BE is also shifted to higher values with a rapid decrease of peak intensity, indicating a continuous dissociation of AAH gel supplying the necessary nutrients for the crystallization and growth of LDH crystallites (see next subsection). The larger decrease of the Ni 2p and Al 2p BEs for the in situ-grown LDH film than those for the secondary-grown one upon increasing the reaction time demonstrates the crystallite growth rate is relatively slow for the former, which is coincident with the above results of XRD and SEM. Formation mechanism of LDH films. Nucleation and growth behavior in colloidal crystallization can significantly affect the assembly of crystal layers and synthesis of ordered films. Further observation of the evolution of structure and crystallite morphology of LDH film at very beginning stage of the crystallization (with a reaction time shorter than 3 h) was carried out for obtaining a good deal of insight into the fundamental processes for formation of LDH film. No reflection peaks can be found for the specimens by in situ growth after 0.5, 1 and 2 h except for those from the Ni substrate (Figure S4, right). Only a layer of AAH gel can be found in SEM image at early stage of crystallization after 0.5 h (Figure S5, right). The urea decomposition method has been adopted for preparation of LDHs by our group and others. 26,42,43 Urea is a very weak Brønsted base (pKb ＝ 13.8) which is highly soluble in water. The


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temperature-controlled hydrolysis of urea is suggested to give the required gradual increases in both pH and concentration of carbonate ions. According to Adachi-Pagano et al., two steps of pH variation are evidenced during precipitation of MgAl-LDH by this method: at the earlier stage the pH undergoes a steep increase, then it reaches a plateau but it continues to increase slowly. 42 During the first step at low pH conditions, hydrolysis of urea produces OH- anions which react immediately with Al(H2O)63+ to induce the formation of aluminium aquohydroxo complexes. Because the precipitation pH of Al(H2O)63+ is lower than that of Ni2+,44 sequential hydrolysis and polycondensation of Al(H2O)63+ and urea kinetically favor the precipitation of AAH gel. In the second step, the rate of OH- consumption is faster than its production by urea hydrolysis, leadings to a low degree of super saturation during precipitation. During this period, Ni2+ is incorporated in the solid hydroxide under a dissolution/precipitation mechanism. Very recently, it was revealed that such colloidal AAH with nanosphere configuration preorganized from basic aluminum acetate compound in ammonia aqueous solution is an essential prerequisite for inducing the construction of NiAl-LDH spherical architecture.45 With prolonging reaction time to 2 h, a large number of particles with curved plate-like morphology appear, indicating the gradual transformation into the aggregates of oxide-hydroxide aluminum. The formation of numerous nanoflakes on the substrate surface results in an alveolate-like structure. Subsequently, LDH nucleation may occur at the gel-solution interface where Ni2+ and Al3+ species are present in abundance. In addition, it is important to recognize that LDH nuclei are formed in bulk solution and the nuclei then grow into small crystals. In fact, the light green solution with formation of a green precipitate on the bottom of stainless steel autoclave was observed after crystallization. Accordingly, these adhered LDH nuclei, formed heterogeneously at the gel-solution interface and homogeneously in bulk solution, are crystallized in the basic


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solution. The continuous dissociation of AAH gel can supply the necessary nutrients for the crystallization and growth of LDH crystallites. We note that the mass relocation starting from the AAH surface region. LDH phase crystallization consequently proceeds toward the AAH phase interior until the crystals eventually bond with the Ni substrate. Therefore, the interlaced accumulation of LDH platelets restricts the growth by each other, forming a final nest-like film configuration.

Figure 6. Schematic illustration of proposed evolution process of NiAl-LDH films via in situ growth (upper) and secondary (seeded) growth (lower). Based on the above experimental results, the formation mechanism of NiAl-LDH film via in situ growth is proposed and represented schematically in Figure 6, upper. It is noted that the LDH crystallites at the outer surface of the film can further grow and enhance their crystallinity with longer reaction time, as illuminated by the gradual increase in the intensity of basal XRD reflections (Figure 1, right). However, the enwrapped crystallites in the interior region of the film and near the substrate by the restricting growth of the interlaced accumulation of LDH platelets may be prevented from further growth without supplying of essential Ni species. As shown in Table 1, the distribution of Ni metal cations in the film layer for the in situ-grown specimens is declined along EDX depth profile from the exterior to the interior of the film, which may have a negative effects on the application properties resulting from the localized segregation of components and loss of stoichiometry.


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This crucial problem can be migrated by using secondary growth technique consisting of deposition of LDH seed layer followed by hydrothermal treatment of the microcrystals to form the LDH film. As indicated in Table 1, similar Ni/Al ratio can be found for the secondary-grown LDH films over film layer along EDX depth profile. Extensive research has been carried out so far on the use of the secondary growth technique for fabrication MFI zeolite membrane with controllable microstructure characteristics, whereby nucleation can be effectively decoupled from film growth.24,46,47 It is worth noting that control growth process by the secondary crystallization is exclusively suitable for the MFI film obtained from clear sols. However, in the case of NiAl-LDH film, the AAH gels appear at early stage of the crystallization for 0.5 h, partly overlapping the existing LDH seeds (Figure S5, left). It is found that there are a large number of voids at some regions of the LDH layer. Meanwhile, the LDH crystallites become irregularly shaped with smaller particle size (Figure S5, inside). At the same time, we can see the decrease of the characteristic (003) XRD reflection of LDH for the 0.5 h-prepared film specimen, compared with that of the LDH seed lay (Figure S4, left). These results indicate that LDH seed crystallites can be dissolved upon hydrothermal treatment. We propose that the released Ni2+ and Al3+ ions may re-nucleate on the substrate where a high concentration of Al3+ ions exists. The new crystallites grow larger with increasing the hydrothermal treatment time, and the dissolving of the existing seeds appears to allow for faster formation of the LDH film because of the supplying of the Ni2+ ions as nutrients. It had been revealed that release of Al3+ cations from alumina is promoted by adsorption of cations such as Ni2+ and Zn2+.48 Accordingly, in the case of NiAl-LDH film upon secondary growth, the Al3+ dissolution from AAH gels would be promoted by the release of Ni2+ cations from LDH seeds with the driving force of the coprecipitation of Al3+ with the Ni2+ as an LDH phase. Thus, the crystallization proceeds fast by the growth of the


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re-nucleated LDH nanocrystallites, along with the adhered ones which were initially nucleated heterogeneously at the AAH gel-solution interface and homogeneously in the bulk solution (Figure 6, lower). More importantly, the re-nucleation and growth of LDH crystallites in the interior region of the film and near the substrate can result in the homogeneous composition of the LDH layer determined by EDX spectroscopy. Effect of substrate on growth. The effect of substrate on growth of NiAl-LDH film was investigated by performing secondary growth experiments on Ni foam under the identical reaction conditions. Indeed, nickel foams containing appropriate microstructure have been adopted wildly as substrate materials for porous electrodes and catalysts. SEM images confirm that both NiAl-LDH films composed of interlaced plates are formed on Ni foam substrate by secondary growth and in situ growth (Figure S6). On closer inspection, however, the secondarygrown film has larger crystal sheets with a more compact morphology, by comparison with the in situ-grown one. Moreover, the intensities of characteristic XRD peaks of LDH phase are stronger for the former (Figure S7). Electrochemical testing of LDH films. Figure 7 shows the cyclic voltammograms (CVs) of both electrodes with NiAl-LDH film/Ni foam prepared by secondary growth and in situ growth method at a scan rate of 0.5 mV s-1. Similarly shaped CVs were obtained for the two electrode potential cycling: a unique couple of anodic peaks (the oxidation potential Epa and the oxygenevolution potential EOE) and cathodic peak (the reduction potential Epc) are observed for each electrode. The potential difference between the anodic and cathodic peaks, Epa - Epc, is 124 and 156 mV for the secondary-grown and in situ-grown film electrode, respectively, demonstrating the more reversible electrode reaction and better proton diffusion efficiency for the former (Table S1). In addition, the peak intensities of Epa and Epc for the secondary-grown film electrode


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are much stronger than those of the in situ-grown film electrode, that is, the energy density in the former is higher. In addition, we measured the CV curves of both LDH films at scan rate of 5 mV s-1 (Figure S8). At such a relatively higher scan rate, the potential difference Epa - Epc is 206 and 226 mV for the secondary-grown and in situ-grown film electrode, respectively, illustrating the film obtained by secondary growth possesses a relatively higher reversibility. Moreover, the difference between the oxidation peak potential and the oxygen evolution potential, EOE - Epa, can be used to assess the charging efficiency of electrodes. Table S1 shows the value of EOE - Epa increases from 158 mV for the in situ-grown film electrode to 189 mV for the secondary-grown one, allowing the later to be charged fully (i.e., complete oxidation of Ni2+ to Ni3+) before oxygen evolution.

Figure 7. Cyclic voltammograms (CVs) of electrodes with NiAl-LDH films on Ni foam, prepared by secondary growth and in situ growth method after hydrothermal times 16 h, at a scan rate of 0.5 mV s-1.


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Figure 8 shows the charge-discharge curves of both NiAl-LDH film electrodes in the third cycle. The charge curve of the secondary-grown film electrode displays a lower and longer voltage plateau than that of the in situ-grown film electrode, which is well separated from the oxygen evolution reaction at the end of the charging. Simultaneously, the discharge curve of the former exhibits a higher and longer discharge voltage plateau. The highest discharge capacity of 216 and 173 mAh g-1 is obtained for the secondary-grown and in situ-grown film electrode, respectively. The conjunction of a low charge plateau on charging and a high discharge plateau on discharging comes as an indication of a smaller polarization during charge and discharge for the secondary-grown film electrode.49 According to the literature, both the utilization of active material and the discharge capacity of NiAl-LDH electrode material increased as the crystallinity improved, which might be explained by the reduction of the polarization magnitude of the material due to a more ordered structure which led to a rapid solid-state reaction rate. 3,35,36 As demonstrated by the above experimental results, the secondary-grown film has a relatively higher crystallinity and more uniform composition of metal cations in film layer than the in situgrown one. Such unique microstructure with an ordered but less turbostratic space distribution between the layers can give a relatively well ordered water molecules and anions in the interlayer spacing and thus provides a reduced resistance against proton diffusion leading to a higher discharge capacity.50,51


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Figure 8. The typical charge-discharge curves for electrodes with NiAl-LDH films on Ni foam, prepared by secondary growth and in situ growth method after hydrothermal times 16 h, at a current density of 30 mA g-1.

Figure 9. Gravimetric discharge capacity as a function of cycle number for electrode with NiAlLDH films on Ni foam, prepared by secondary growth and in situ growth method after hydrothermal times 16 h. The electrode was charged and discharged at a current density of 30 mA g-1.


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The evolution of the gravimetric discharge capacity as a function of the cycle number for both film electrodes is shown in Figure 9, which comprises two steps: the first step consists of a rapid increase of the discharge capacity reaching the maximum value at the third cycle, which corresponds to the activation time; the second step starts fading from the maximum, which determines the deterioration rate of the electrode. For the in situ-grown film electrode, the discharge capacity declines straightly without any constant step until the end of 50 chargingdischarging cycles. Instead, the discharge capacity of the secondary-grown film electrode reaches its minimum at the 10th cycle and ends up with a constant value of 160 mAh g-1, holding a certain cycling stability. The above electrochemical measurements show that the secondarygrown film delivers improved rechargeable, discharge-capacity and cycling stability and thus provides an encouraging overall performance as electrode material which is superior to the in situ-grown one. Further improvement of the secondary growth technique for preparation of NiAl-LDH film, such as optimizing the crystal orientation and handling the pore structure, is required in order to further enhance the electrochemical properties.

CONCLUSIONS In this study, a secondary (seeded) growth technique was developed for preparation of NiAlLDH films on nickel foil and nickel foam substrates, which consisting of the deposition of preformed LDH seed nanocrystals from a colloidal suspension to form closely packed layers, followed by hydrothermal treatment of the seed layers to yield LDH films. For comparison, NiAl-LDH films were also prepared by in situ growth technique using seed-free substrates. It is revealed that the crystallite growth for the LDH films prepared by secondary growth occurred much fast than that by in situ growth. Moreover, the secondary-grown film has a relatively higher crystallinity and more uniform composition of metal cations in film layer. Furthermore,


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we propose an explanation for the role of the seed microcrystals in LDH film growth. For the preparation of LDH films by secondary growth, LDH seed crystallites were dissolved upon hydrothermal treatment at the very beginning stage of the crystallization, and the released Ni2+ and Al3+ ions may re-nucleate on the substrate where a high concentration of Al3+ ions exists. The crystallization proceeds fast by the growth of the re-nucleated NiAl-LDH microcrystallites, along with the adhered ones which were initially nucleated heterogeneously at the AAH gelsolution interface and homogeneously in the bulk solution. Furthermore, the secondary-grown film delivers improved rechargeable, discharge-capacity, and cycling stability and thus provides an encouraging overall performance as electrode which is superior to the in situ-grown one. These results can be explained by the existence of unique microstructure for the former which may provide a reduced resistance against proton diffusion. Supporting Information. XRD pattern, FT-IR spectrum, TEM image, and profile of particle diameter distribution by PCS of NiAl-LDH seed crystals; XRD pattern and SEM image of NiAlLDH seed layer on Ni foil; XRD patterns and SEM top-view images of NiAl-LDH films hydrothermally treated for 0.5 h, 1 h and 2 h; XRD patterns and SEM top-view images of NiAlLDH films on Ni foam substrate; CVs of electrodes with NiAl-LDH films on Ni foam, prepared by secondary growth and in situ growth method after hydrothermal times 16 h, at a scan rate of 5 mV s-1. Comparison of parameters of CVs for NiAl-LDH films on Ni foam by secondary growth and in situ growth method at a scan rate of 0.5 mV s-1. This material is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author *Fazhi Zhang. Fax: (+86) 10-6442-5385. E-mail: [email protected]


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21376019), the National Basic Research Program of China (the 973 Program; No. 2011CBA00506), and the Fundamental Research Funds for the Central Universities (No. YS1406).


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