Heterogeneous Wet Chemical Synthesis of Superlattice-Type

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Heterogeneous Wet Chemical Synthesis of Superlattice-Type Hierarchical ZnO Architectures for Concurrent H2 Production and N2 Reduction C. M. Janet, S. Navaladian, B. Viswanathan, T. K. Varadarajan, and R. P. Viswanath* National Centre for Catalysis Research, Department of Chemistry, Indian Institute of Technology Madras, Chennai-600 036, India ReceiVed: September 8, 2009; ReVised Manuscript ReceiVed: January 9, 2010

Two wet chemical methods, namely, wet etching (heterogeneous) and chemical precipitation (homogeneous), have been exploited for the formation of hierarchical ZnO architectures without any specific templates, catalysts, or capping agents. As-synthesized ZnO have been extensively characterized by X-ray diffraction, UV-visible absorbance studies, Fourier transform infrared analysis, scanning electron microscopy, high-resolution transmission electron microscopy, energy-dispersive analysis of X-rays, selected area electron diffraction, photoluminescence (PL), and textural analysis. Wet etching resulted in the formation of self-assembled hexagonal plates with a disordered superlattice-type texture and ultimately ended up in crystallites with highdefect concentration showing a red PL emission. Mechanistic aspects of the growth process in both the methods have been analyzed, and a rational explanation is presented for their observed morphologies. ZnO prepared by both methods have been tested for their photocatalytic water splitting abilities in producing H2. Crystalline, spectral, textural, and activity analyses have been carried out for commercial ZnO for a comparison. Pt loaded ZnO was used for the concurrent hydrogen generation and dinitrogen fixation yielding a maximum of 86 µmol of ammonia/(h/0.1 g of catalyst), thus achieving the activation of N2 at room temperature and atmospheric pressure. Introduction Zinc oxide is a wide band gap semiconductor which has applications ranging in versatile fields such as photonics, electronics, magnetic materials, optics, etc. Due to its wide range of utilities, the synthesis of ZnO in any size or shape is always in demand and is undergoing tremendous changes with respect to the methodologies being adopted.1–3 Among semiconductors, ZnO has been thoroughly investigated because of its inherent ability to crystallize in various geometries resulting in hierarchical architectures.4–6 Catalytic applications of ZnO especially in photocatalysis are now being exploited widely for pollutant degradation studies. Only very few reports are available for its utility in photocatalytic water splitting for the generation of H2 even though many reports are there on thermochemical splitting of water using pure ZnO as catalyst.7–10 This may be due to the low surface area of bulk ZnO. Thermochemical water splitting on ZnO takes place via a two-step cyclic mechanism involving ZnO/Zn redox reactions. ZnO dissociates to form Zn in the first step followed by its hydrolysis in the second step forming H2. The first step is an energy intensive process, and hence, a more energy efficient photocatalytic process is worth exploring. Photocatalytic water splitting on ZnO takes place at room temperature and atmospheric pressure and does not undergo any chemical changes unlike thermochemical water splitting. Very few reports are available in the literature where ZnO is used for the photocatalytic H2 production.11,12 This may be due to the vulnerability of ZnO for photocorossion. Under the conditions of irradiation, photogenerated holes can oxidize the photocatalyst itself, leading to the dissolution of zinc oxide. However, it can be minimized to a certain extent by the use of suitable sacrificial agents. Zhang et al. showed that Na-EDTA, * Corresponding author, [email protected].

as sacrificial agent, suppressed the pollutant degradation activity to even 50% on ZnO.13,14 This indicates that in photocatalytic water splitting, where photogenerated electrons are the required species, sacrificial agents can enhance the activity by using up the holes in minimizing the photocorossion of ZnO as well as electron hole pair recombination. Sacrificial agents are reported to enhance the photocatalytic activity by a factor of 100 or more.15 Nanomaterials of ZnO with different shapes and sizes were proved to be interesting catalysts for energy as well as environmental applications.16–19 Wet chemical routes for the synthesis of nanostructures (materials with at least one dimension in the nanometer scale) are a valuable alternative to conventional processing and gas-phase synthesis, with known commercial applications.20 Water-based chemical methods offer numerous advantages such as environmentally benign, cheap and easy to handle starting products, low cost, simple equipment, and require only a low energy input. Moreover they allow the easy tailoring of synthesis parameters throughout the whole process, which may be exploited to achieve a more precise control of composition, shape, and size of the resulting material. Since the synthesis route determines the later properties of the material, the preparation method chosen is a very important issue when designing ZnO for a specific application. Lee et al. reported that there are four different parameters, kinetic energy barrier, temperature, time, and capping molecules, that can influence the growth pattern of crystals under non-equilibrium kinetic growth conditions in the solution-based approach.21 The two synthetic routes, adopted in the present study, basically differ in growth time, nature of precipitation (homogeneous or heterogeneous), and the available concentration of reagent at a given time. On the basis of the rate of formation, either a network of particles (slow) or similarly textured grains (fast)

10.1021/jp908683x  2010 American Chemical Society Published on Web 01/25/2010

Synthesis of Superlattice-Type Hierarchical ZnO Architectures of ZnO precursors are formed initially which on pyrolysis led to the respective ZnO architectures. Ammonia synthesis through dinitrogen reduction under room temperature and atmospheric pressure is an unrealizable task for scientists and technologists even today using conventional catalytic processes. But if light energy can be trapped suitably by materials which will absorb light and simultaneously bind dinitrogen, this could possibly show an alternative pathway for the fixation of nitrogen at room temperature and atmospheric pressure just like natural enzyme nitrogenase. Photocatalysts that can effectively split water can be used for the generation of H2 from water, and its concurrent use in the reduction of N2 can be realized in situ. Few attempts that have been made 2 decades ago were based on bulk materials of either TiO2 or CdS semiconductors.22–24 Semiconductor nanomaterials with modified properties are quite interesting for exploring photofixation of nitrogen. In the present study, a simple wet chemical etching method was adopted to prepare ZnO nanomaterials used for the photocatalytic splitting of water. The H2 thus formed was used for in situ reduction of N2 at room temperature and atmospheric pressure. Using cocatalysts such as Pt has been proved to be providing suitably placed Fermi energy levels for trapping the photogenerated electrons and thus reduce the possibility of electron hole pair recombination.16,25,26 Welldispersed Pt metal particles act as mini-photocathodes trapping the electrons that eventually reduce water to H2.15 Activation of dinitrogen also requires the presence of a suitable metal dispersed on the semiconductor photocatalyst. For simultaneous generation of H2 and reduction of N2, presence of a suitable cocatalyst, preferably a noble metal, is mandatory. Zinc oxide as-synthesized is found to form a disordered superlattice-type texture. Generally a superlattice is the one where the basic nanoparticles of any shape being self-assembled in an ordered manner giving rise to a network which can be considered as a bigger lattice with similar particles as its repeating unit.27–30 A chemical precipitation method has also been used to prepare ZnO to compare the effect of synthetic procedure on the morphology and properties. The precursor of ZnO in the present study was zinc oxalate. Zinc oxalate as a precursor for ZnO-based materials are reported for various applications.31–34 The use of the wet etching method and the precipitation method implemented in the present study differs in the growth kinetics of this precursor and thus becomes a determining factor in the final texture and morphology. Commercial ZnO obtained from M&B Laboratories, U.K., was also characterized and used for activity studies. Experimental Section All reagents used were of analytical grade and used as such without further purification. Zn(NO3)2 · 6H2O, K2C2O4 · H2O, and H2C2O4 · 2H2O were purchased from E. Merck, Germany. Zn plate (6 cm × 1 cm) was purchased from Alloy Tech, India. The commercial ZnO sample used in the present study was obtained from M&B Laboratories, U.K. All the reactions were carried out in deionized water. Synthetic Strategies. (i) Wet Etching Method. Zinc metal plate was immersed in 75 mL of a 5% solution of oxalic acid and stirred continuously with a magnetic stirrer for 4 h to obtain the precursor ZnC2O4 · 2H2O. The solution slowly turned turbid, and with time, more and more precipitate was formed. After 4 h, the Zn plate was taken out, the white precipitate that formed was allowed to settle, and the excess oxalic acid solution was decanted off. The precipitate that formed was washed, filtered,

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2623 and dried in an air oven at 60 °C for 4 h. Further it was calcined at 400 °C in air so as to obtain the self-assembled hexagonal ZnO. (ii) Precipitation Method. To 50 mL of 1 M Zn(NO3)2 · 6H2O a solution of 50 mL of 1 M K2C2O4 · H2O was added, and the solution was stirred rapidly for 10 min. The white precipitate formed was washed with deionized water, filtered, and dried in an air oven at 60 °C for 4 h followed by calcination in air atmosphere at 400 °C for 4 h. The material obtained has been further characterized and used for the catalytic applications. ZnO samples prepared by the respective methods are designated with its synthesis method or origin in parentheses. (iii) Preparation of Pt Loaded ZnO. A specific amount of ZnO was impregnated with H2PtCl6 solution in water so as to obtain a 1% Pt loading. Impregnated material was reduced in a tubular furnace in H2 flow at 400 °C for 4 h to obtain 1% Pt/ ZnO. Photocatalytic Activity Studies. Photocatalytic activity studies for the generation of H2 were carried out on as-synthesized, commercial as well as Pt-loaded ZnO samples. About 0.1 g of the photocatalyst was weighed and transferred to a doublewalled cylindrical quartz reactor containing 25 mL of deionized water and 10 mL of 0.001 mol of Na-EDTA solution which is acting as a sacrificial agent. The reactor was equipped with a water circulation facility in the outer jacket in order to maintain a constant temperature as well as for filtering off infrared rays. Before illumination by a UV source, the solution was stirred for homogeneity for about 30 min in the dark. Then the mixture was irradiated with photons from a 450 W Hg lamp (ORIEL Corp., USA). The reaction mixture was stirred at a constant speed during illumination by a magnetic stirrer. The evolved gas was collected over brine water using an inverted gas buret, and the H2 gas was analyzed through gas chromatography. The Pt metal loaded catalysts were used for the dinitrogen reduction studies. Pt loaded as well as unmodified ZnO have been tested for their photostability after H2 production analysis. The reactor solution was centrifuged after 3 h of reaction, and the centrifugate was concentrated and digested with 0.01 M nitric acid. ICP-OES analysis was performed for the estimation of zinc by recording the emission at 213.8 nm and comparing it to identically prepared standard and blank solutions.35 About 0.1 g of Pt/ZnO along with 25 mL of deionized water and 10 mL of 500 ppm Na2SO3 as a sacrificial agent was taken in the reactor. Catalyst was stirred for homogenization in dark before illumination and ultrahigh pure N2 was purged continuously through the reactor during illumination. The outgoing gas was collected in the 0.01 N H2SO4 solution kept in the trap. A schematic of the reaction setup for the ammonia formation is given in Scheme 1. After irradiation for an hour, the solution was centrifuged to remove essentially all the catalyst and the centrifugate was analyzed for ammonia. Estimation of ammonia in the reactor solution was done spectrophotometrically by indophenol test,36 and the trapped ammonia was detected by the volumetric titration with the standardized 0.001 N NaOH solution. Characterization. Characterization has been carried out using X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), highresolution transmission electron microscopy (HR-TEM), energydispersive analysis of X-rays (EDAX), selected area electron diffraction (SAED), BET surface area, and photoluminescence (PL) studies. Powder XRD patterns of samples were recorded using a Shimadzu XD-D1 diffractometer using Ni-filtered Cu KR radiation (λ ) 1.5406 Å). The specific surface area, pore size, and pore volume of the samples were measured using a

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SCHEME 1: Schematic Reaction Setup for Photocatalytic Ammonia Synthesis

Micromeritics ASAP 2020 instrument by adsorbing N2 at 77 K. Prior to the sorptometric experiment, the samples were degassed at 150 °C for 12 h. UV-visible absorption spectra were recorded using a Varian Cary 5E UV-vis-NIR spectrophotometer in the spectral range of 200-800 nm. Absorption spectra for catalyst samples were recorded as Nujol paste. The surface morphology of ZnO samples was analyzed by a FEI Quanta 200 scanning electron microscope operating at 30 kV. High-resolution transmission electron microscopy (HR-TEM) analysis was performed using a JEOL-3010 transmission electron microscope operating at 300 kV. Samples for TEM were prepared by dispersing the powdered sample in ethanol by sonication and then drip drying on a copper grid (400 mesh) coated with carbon film. PL measurements were carried out on a HITACHI 850-type visible ultraviolet spectrophotometer with a Xe lamp as the excitation light source. The FT-IR spectra for the samples were recorded with a Perkin-Elmer instrument in the range 400-4000 cm-1 at room temperature. Spectrophotometric analysis was carried out on a JASCO V-530 UV-visible spectrophotometer. Gas chromatographic analysis was done on a Nucon model 5765 gas chromatograph. ICP-OES analysis was performed to estimate the zinc in the reactor solution for Pt loaded catalysts using a Perkin-Elmer OPTIMA 5300 DV ICPOES system. Results and Discussion X-ray diffraction patterns for the ZnO prepared by wet etching method as well as direct chemical precipitation are given in Figure 1. Figure S1 shows crystalline and sharp peaks corresponding to the commercial ZnO (see Supporting Information). The observed diffraction pattern is in good agreement with that of the JCPDS file no. 79-0205 corresponding to the hexagonal wurtzite structure of ZnO. The commercial ZnO also showed the hexagonal lattice structure.37 The average crystallite size calculated using the Scherer equation from XRD is found to be 18, 19, and 22 nm for ZnO (wet etching), ZnO (precipitation), and ZnO (commercial), respectively. The lattice parameters were calculated for the three systems using the eq 1

1 4 1 ) 2 (h2 + hk + k2) + 2 (l2) dhkl2 3a c

samples of ZnO. In all the cases c/a values were less than 1.633, which indicates that as-synthesized ZnO as well as commercial ZnO are forming a distorted hexagonal lattice. High-temperature treatments may result in a closely packed ideal hexagonal lattice of ZnO. In general, ZnO is a wide band gap semiconductor with a band gap of 3.37 eV corresponding to an absorption onset of around 368 nm. UV-visible absorption spectra for all ZnO samples are given in Figure 2. Optical activity of a material is related to the morphology and particle size of the material, presence of impurities, concentration, type of defects, etc. Morphology will affect the shape of the intermediate energy levels when a semiconductor is concerned. And the size of the particles determines the energy gap between the valence band and conduction band of the material. The smaller the size of particle, the bigger will be the band gap. UV-visible absorbance studies of as-synthesized and commercial ZnO samples showed almost a similar absorption onset value centered around 400 nm. Band gap values corresponding to the samples were calculated from the absorption onset using the relation E ) hc/λ where E is corresponding to the energy gap, h is Planck’s constant, and λ is the wavelength corresponding to the onset of absorption. Calculated values are given in the Table 1. An expected blue shift is observed for the ZnO prepared by the wet etching method due to its lower crystallite size. Commercial ZnO, with its bigger crystallite, has the absorption onset value around 411 nm in the visible region. BET surface area studies reveal that the ZnO prepared by the wet etching method has the highest surface area (20 m2/g)

(1)

For an ideal hexagonal system the c/a value should be 1.633. Table 1 gives the c and a values calculated for three different

Figure 1. XRD patterns for ZnO prepared by (A) wet etching and (B) precipitation methods.

Synthesis of Superlattice-Type Hierarchical ZnO Architectures

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TABLE 1: Crystalline, Spectral, and the Textural Properties of Various ZnO Samples

sample

crystallite size (nm)

c (Å)

a (Å)

ZnO (wet etching) ZnO (precipitation) ZnO (commercial)

18 19 22

5.189 5.178 5.135

3.234 3.234 3.193

c/a

onset of absorption (nm)

band gap (eV)

surface area (m2/g)

pore volume (cm3/g)

1.605 1.601 1.608

396 401 411

3.13 3.1 3.02

20 15 3

0.18 0.10 0.01

and the commercial ZnO has the lowest surface area (3 m2/g). Adsorption-desorption isotherms and pore size distribution for ZnO samples are given in Figure 3 and Figure S2 in Supporting Information, respectively. As-synthesized ZnO samples showed a mesoporous nature arising due to the interparticle pores as evident from the isotherms as well as from the pore size distribution analysis. Commercial sample did not show any pores in it. Surface area and pore volume values are tabulated in Table 1. FT-IR spectra corresponding to zinc oxalate and ZnO prepared by the wet etching method as well as the precipitation method are given in Figure 4. Irrespective of the preparation method, all the spectra showed bands corresponding to the OH stretching arising due to the adsorbed water. Even after calcination at 400 °C both ZnO retained the adsorbed water on the surface. The bands around 880 cm-1 were due to the surface ligation of water molecules. Zinc oxalate showed two characteristic peaks around 1637 and 1364 cm-1 corresponding to the asymmetric and symmetric stretching of the carboxyl group, respectively. A weak peak around 780 cm-1 corresponds to the

Figure 2. UV-visible absorbance spectra for ZnO samples (A) ZnO (wet etching), (B) ZnO (precipitation), and (C) ZnO (commercial) and the corresponding band gap values are indicated in the inset.

Figure 3. BET adsorption-desorption isotherm for ZnO samples.

carboxyl bending frequency.38 A characteristic strong Zn-O stretching band around 470 cm-1 is observed in both the ZnO samples.39 A broad shoulder observed around 1650 cm-1 corresponds to the O-H bending vibration. A peak around 1400 cm-1 in ZnO may be due to the carbonate stretching which may arise due to the reaction between the surface oxygen in ZnO and the adsorbed CO2 generated during the oxalate decomposition. Zinc oxalate prepared by both methods showed adsorbed gaseous CO2 as indicated by the peak around 2534 cm-1. A broad peak around 1960 cm-1 is characteristic of oxalic acid, and it is absent in ZnC2O4 prepared by precipitation by K2C2O4.40 Zinc oxalate formed by the wet etching method showed broad peaks compared to the intense and narrow peaks observed for the zinc oxalate prepared by the precipitation method. This gives an indication that a smaller particle size distribution may be existing in ZnO prepared by the wet etching method which is further substantiated by the fact that this method allowed slow kinetics for the growth of the precursor. SEM images of the zinc oxalate precursor obtained from the wet etching showed a network-type surface morphology. SEM images of the zinc oxalate precursor are given in Figure 5. After calcination at 400 °C for 4 h in air, a three-dimensional assembly of hexagonal plates forming hexagonal bipyramidal microstructures were observed (Figure 6a). This assembly may be formed due to the inherent binding nature of the oxalate dianion. Each hexagonal plate forming self-assembled hierarchical structures can be seen in Figure 6b. The EDAX spectrum corresponding to the ZnO (wet etching) is given in Figure S3 in Supporting Information. But the analysis of SEM images of the zinc oxalate which is obtained from the direct chemical precipitation showed entirely different types of architectures with a central hole in it (Figure 7a). Even after calcination, it can be seen that the similar surface morphology is retained (Figure 7b) though the effect of the decomposition of oxalate is evident from the texture showing cracks and layers. This may be due to the CO2 evolution during the decomposition of oxalate.33 But the reason for the formation of architectures with a central hole is not clear. This may be due to a similar mechanism which is explained by Liang et al. which utilizes citrate as a capping agent.30 They proposed that citrate may serve as a surface modifier, presumably binding to the Zn2+ on the polar (0001) planes and, hence, a ZnO doughnut like structure with a central hole will be formed along the c axis.41 But in the present study, the precursor zinc oxalate itself is forming architectures with a central hole in it. As explained by Xu et al. this may be due to the formation of a mesocrystal.42 Perfect alignment and self-assembly of nanoparticles resulting in a mesocrystal may be due to the dipole fields that are generated during nucleation and growth. In the case of copper oxalate, the crystals were composed of nanometer building units and strings of nanodomains oriented along the principal axis of the particle, revealing that the lateral and basal faces of the layers are composed of stacked nanoparticle arrays with a specific thickness. Doughnut-like structures were formed when citrate was used as a surface modifier where mesocrystal formation along the principal axis was making an angle to the immediate neighbor resulting in concave- or convex-shaped

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Figure 4. FT-IR spectra of zinc oxalate and ZnO prepared by (A) the wet etching method and (B) the precipitation method.

Figure 5. SEM images of zinc oxalate precursor obtained by the wet etching method at different magnifications.

Figure 6. (a) SEM images of the ZnO prepared from the wet etching method showing hexagonal bipyramidal-shaped microstructures. (b) SEM image of the hexagonal-shaped plates forming such microstructures.

surfaces. Oxalate being shorter in its molecular dimensions binding of the layers is almost perpendicular to the principal axis forming flat surfaces having a central hole. Apart from this, a fast homogeneous precipitation achieved in the wet chemical precipitation resulted in forming a crystal lattice induced morphology for the zinc oxalate precursor formed. Zinc oxalate crystallizes in a primitive monoclinic lattice.34 A schematic of crystal lattice induced morphologies achieved in the present

study is given in Scheme S2 in Supporting Information. The EDAX spectrum of ZnO (precipitation) is given in Figure S4 in Supporting Information. Figure S5 in Supporting Information shows the surface texture of commercial ZnO. Bunches of particles can be seen from the image. Zinc oxalate prepared by wet etching shows the formation of hollow nanostructures in TEM images (Figure S6 in Supporting Information). It is the inherent nature of the metal


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Figure 7. (a) SEM image of zinc oxalate prepared by the precipitation method and (b) SEM image of the ZnO obtained by the precipitation method. Magnified images of the architecture formed before and after calcination are shown in the insets of (a) and (b), respectively.

Figure 8. HR-TEM images of ZnO (wet etching): (a) lattice images showing the fusion of the nanoparticles along the grain boundaries; (b) fusion of three prominent lattice planes along the grain boundaries.

oxalate complexes to appear like a hollow structure in the field of high-energy electrons as it transmits the electrons completely. ZnO prepared by wet chemical etching showed an array of anisotropic nanoparticles forming a hexagonal disordered type superlattice. A majority of anisotropic nanoparticles were also in hexagonal shape with particle sizes ranging from 10 to 60 nm. X-ray diffraction analysis also revealed the hexagonal crystal lattice for both wet chemical and chemically precipitated ZnO. In the case of ZnO formed by chemical precipitation, a majority of the anisotropic particles were either rodlike or hexagonal platelike in their morphology. But a well networked array is not formed in this method. In the wet etching method, the slow rate and the gradual release of Zn2+ and oxalate ions allowed the formation of a continuous network of zinc oxalate. During calcination, the decomposition of oxalate allowed the well connected particle to bind together forming the hexagonal plates of ZnO (Figure S7 and Figure S8 in Supporting Information). The HR-TEM image in Figure S7b in Supporting Information shows the interparticle pores that are 20-40 nm in diameter. Pore size distribution analysis also supports this observation. Generally a superlattice results in an ordered mesoporous texture due to the perfect alignment of isotropic particles. The nature of alignment of the anisotropic particles does not show a regular arrangement in the present case even though disordered arrangement aligns to produce hexagonal-

shaped plates. Due to the specific synthetic methodology, it is possible to incorporate grain boundaries in the ZnO which can act as trapping centers in photocatalytic applications. Figure 8a shows the lattice image along the grain boundary where the red arrows indicate the defects such as a missing row or atoms. The grain boundary is indicated by a blue line. Figure 8b shows the fusion of three prominent lattice planes (101), (100), and (002) along the interfaces creating the grain boundaries. This observation can be explained by the thermal fusion of adjacent particles, probably caused initially by ions diffusing along the interfaces to the points of contact between particles.32,33,43,44 This induces bridging and connection of the grains and results in the fusion of contiguous particles generating grain boundaries. The calculated lattice parameters from HR-TEM are in good agreement with that of the corresponding JCPDS file no. 790205. Parts a and b of Figure 9 show HR-TEM images of ZnO prepared by the precipitation method. Hexagonal and rodlike morphologies were observed. A lattice image of one of the hexagonal plate edges is shown indicating the presence of (002) planes corresponding to the d-spacing 0.26 nm (Figure 9c). The electron diffraction pattern showing the single crystalline nature for the platelike particle is given in Figure 9d. This is expected since no array is formed in this method due to the fast kinetics of the precipitation of the precursor material.

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Figure 9. (a) and (b) HR-TEM images of hexagonal and rodlike single crystalline particles of ZnO (precipitation). (c) Lattice images of a hexagonal particle with inset showing the corresponding lattice planes. (d) Electron diffraction pattern corresponding to the hexagonal particle.

Figure 10. (a) HR-TEM image of Pt loaded ZnO (wet etching), lower magnification showing the dispersed Pt clusters on the surface. Inset shows the magnified image of the supported Pt clusters. (b) HR-TEM image of the Pt cluster at the grain boundary.

Figure 10 shows the representative HR-TEM images of the Pt-loaded ZnO (wet etching). Pt clusters are found to be dispersed on the ZnO surface. The inset shows a magnified lattice image of a ZnO surface where the corresponding ZnO

facet is found to be (002) with a d-spacing of 0.26 nm. The d-spacing of Pt is measured to be around 0.23 nm, which matches that of the (111) plane of Pt (JCPDS no. 04-0802). Figure 10b shows another Pt cluster which is found to be at


Figure 11. Room temperature PL spectra of ZnO prepared by (A) the wet etching method and (B) the precipitation method.

grain boundary, and the corresponding ZnO background is analyzed to be (101) facet. As the Pt loading is done by the impregnation method on ZnO as a support, maximum loading can occur on both the exposed surface and inside the pores. Specific to the present method the defect centers and grain boundaries are also the preferred sites for metal loading as can be seen from the image. If Pt is deposited by means of any chemical or physical deposition methods either on a support film or substrate, low index planes of ZnO, that is (0001) facet is the preferred one.45 In the present case, Pt is found to be dispersed over all exposed surfaces and also in defect centers as shown in the images as expected for the impregnation method. It is the acid-base interaction between the acidic loading solution of Pt and basic ZnO which is the driving force for the formation of such a Pt-supported ZnO system.46 Deposition methods could be beneficial over the impregnation method especially in photocatalytic water splitting as it allows a facetspecific deposition on a support film or substrate. If more Pt can be deposited by any specific method on zinc rich facets (0001), photocatalytic water splitting activity is expected to be 2+ higher. It is because Zn will form the conduction band, where the photoelectrons will be available during illumination. And the electron trap, which is Pt cluster, if present on such a surface minimizes the recombination rate of electron hole pair consider-

J. Phys. Chem. C, Vol. 114, No. 6, 2010 2629 ably, thus enhancing the overall activity. Impregnation methods are preferred in general for the preparation of catalysts as dispersion of the active metal content can be achieved more effectively. It is quite economical and simple especially while handling noble metal salts for catalytic applications. Room temperature photoluminescence (PL) spectra were recorded for both ZnO samples with an excitation wavelength around 395 nm. All three ZnO samples gave characteristic emissions due to the defects incorporated in it. ZnO (precipitation) gave a strong emission centered around 576 nm, whereas ZnO (wet etching) showed only a very weak emission at 576 nm. However, it showed a strong emission around 614 nm, which was not observed for pure ZnO in general. These emissions can be attributed to the oxygen vacancies or excess zinc in the lattice. A series of weak emissions observed in the range of 600-700 nm is attributed to the deep trap states or excess surface states. The peak around 576 nm corresponding to the green emission of ZnO is due to the doubly ionized oxygen vacancies.19 In the case of ZnO (wet etching) the selfassembly formed by the fusion of the particles created grain boundaries with many defects. This may be the reason for the strong red emission.47 Generally, doped ZnO only could give rise to red emission. Doping, presence of impurities, and defects will create extra bands in the energy gap which are responsible for emission or absorption in the visible region. The color of the ZnO (wet etching) was off-white rather than the intense pure white observed for the ZnO (precipitation). This in itself shows the extent of defects in the sample as there was no other impurities present which was confirmed from the EDAX analysis. Hence, ZnO (wet etching) can be used in optoelectronic applications as the pure ZnO itself is capable of emitting red light. Figure 11 shows the room temperature PL spectra corresponding to the ZnO samples. The inset shows the interband energy levels responsible for red and green emissions in ZnO (wet etching) and ZnO (precipitation), respectively. A plausible mechanism for the formation of three-dimensional hexagonal bipyramidal ZnO architectures is given in Scheme 2. Through an induced self-assembly by bidentate oxalate ligand,48 the wet etching method yielded a networked array of zinc oxalate initially. During calcination, zinc oxalate decomposes forming ZnO while retaining the similar but more compact

SCHEME 2: Plausible Mechanism of Formation of Three-Dimensional Hexagonal Bipyramidal ZnO Architectures

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SCHEME 3: Molecular Model of a Hexagonal Wurtzite Lattice Showing the Prominent Planes in the Unit Cell of ZnO

network array forming the stable crystalline lattice of hexagonal ZnO. Each hexagonal plate thus formed can be considered as a disordered-type superlattice as in the present case; polydispersed particles are forming a lattice without a definite pattern. A schematic showing the hexagonal lattice of ZnO is given in Scheme 3. Three prominent planes which are scattered more along the given orientation during the measurement of X-ray diffractogram are indicated in the scheme. It can be seen from the unit cell that the (101) plane, which is more intense in XRD pattern, is atomically more dense. But the self-assembly caused in the present investigation is due to the surface modifier oxalate and it is a dianion. From the scheme it can be seen that the surface planes which are having more Zn2+ ions are {100} planes. Hence, it is more likely that oxalate binding or fusion of the particles will take place along the {100} planes and hence growth of particles can be along (002) through the c axis because these O2- rich (002) top or basal planes will be less hindered by oxalate anions. When such a crystallite forms hexagonal plates, it will have polar planes with a positive basal plane rich in Zn and negative basal plane rich in O2. Thus, the Zn-rich positive (0001) surface, being the more reactive surface, can attract new ZnO species or the opposite ionic species to its surface.41 The six side facets of a hexagonal plate are generally bound by the {1010} planes. In the ZnO crystal growth process, different families of planes follow the growth sequence (0001) > (1011) > (1010). Thus, normally, a one-dimensional growth of ZnO along the (0001) direction bound by six facets is expected. The energy of an individual plate is quite high, with two exposed flat planes, and they tend to align with each other to decrease the surface energy by reducing exposed areas. Hexagonal plates with well-matched crystal lattice and active surface are prone to fuse to each other, driven by the gaining of free energy and lattice-free energy.49 Directed by thermodynamic stability, each plate will bind a hexagonal plate of lower dimension as shown in Scheme 2. As the whole process is taking place in solid state, each hexagonal plate formed during the oxalate decomposition will induce the formation of another hexagonal plate of the oxalate network which subsequently

Janet et al. decomposes and forms a more compact plate of lower dimension. And the self-assembly will continue until a thermodynamically stable crystallite is formed. But in the case of zinc oxalate formed by the precipitation method, a monoclinic primitive lattice of zinc oxalate directs the formation of zinc oxalate plates of specific shape during the fast precipitation and will align one by one on top through oxalate bridging parallel to the adjacent layers along the c axis. Due to the fast kinetics, each particle formed will not have enough time to form a network. In the case of wet etching, the generation of Zn2+ ions from the Zn plate will be followed by its reaction with oxalate ions from the oxalic acid with the release of H2. This gradual and stepwise formation of ions in equimolar quantities will result in the formation of a network and can be regarded as a heterogeneous precipitation. But in the case of the precipitation method, homogeneous precipitation occurs due to the ready availability of Zn2+ and oxalate in the reacting medium forming similar particles simultaneously. During decomposition, this oxalate causes the formation of analogous anisotropic particles. Scheme S1 in Supporting Information shows the crystal lattice induced morphologies of ZnC2O4 and ZnO. Chemical reactions occurring in the two methods, namely, precipitation and wet etching are given in eqs 2 and 3, respectively. Equation 4 represents the decomposition reaction of zinc oxalate occurring at around 400 °C.50

Zn(NO3)2 + K2C2O4 f ZnC2O4 + 2KNO3

(2)

Zn + H2C2O4 f ZnC2O4 + H2

(3)

ZnC2O4 f ZnO + CO + CO2

(4)

Photocatalytic activity studies for the generation of H2 were carried out on as-synthesized and commercial ZnO samples. Generally, bulk ZnO will have poor surface area and this may be one of the reasons why ZnO is not sought for photocatalytic water splitting even though it has been used in pollutant degradation studies. The nanostructures of as-synthesized ZnO possess better surface area compared to the commercial ZnO which is used for comparison in the present study and bulk ZnO in general. ZnO prepared by wet etching showed more defects incorporated in it, which will be beneficiary in trapping the electrons or holes besides providing the active catalytic sites. Commercial ZnO (1% Pt) also was utilized for H2 production along with as-synthesized ZnO samples. Figure 12 gives the H2 production activities for various ZnO samples. ZnO prepared by wet etching showed maximum activity producing about 35 µmol of H2. Despite its lower surface area, ZnO (commercial) showed activity of the order of 30 µmol of H2. This observation may be due to the faster photocorrosion associated with the smaller particles of ZnO (wet etching) which overweighs the surface area effect.51 Commercial ZnO is more crystalline as observed from the sharp XRD signals (Figure S1 in Supporting Information) and it has exposed surface area compared to the surface area contributed from the pores unlike the as-synthesized samples which may also be the reason for the observed activity (Figure S2 in Supporting Information). H2 production activities measured for Pt-loaded ZnO (as-synthesiszed) showed a decrease in the amount of H2 produced than the unloaded catalyst, whereas Pt-loaded ZnO (commercial) showed a considerable decline in activity compared to the unloaded catalyst. This may be due to the lesser exposed surface area of photocatalyst due to the loading of the metal which will be more pronounced on


J. Phys. Chem. C, Vol. 114, No. 6, 2010 2631 of around 14 µmol/(h/0.1 g of catalyst) whereas Pt-loaded precipitated ZnO produced a total of 48 µmol of ammonia (Figure 12, Inset). The lesser activity observed for the commercial catalyst may be due to the unavailability of the exposed surface area due to the Pt loading. The activity for the N2 fixation can be improved by making use of Ru-loaded catalyst since, rather than H2 evolution, it needs a comparatively strong M-H bond and affinity for N2 which is more pronounced in the case of Ru. Conclusion

Figure 12. Photocatalytic H2 production activities for various ZnO and Pt-loaded ZnO samples. Inset shows the ammonia formed on Ptloaded ZnO samples.

SCHEME 4: Schematic Representation of Concurrent Hydrogen Production and Nitrogen Reduction

ZnO (commercial), which was originally having the lowest surface area. After the photocatalytic reaction was allowed to proceed for 3 h, zinc (Zn2+) in the reactor solution was estimated by ICP-OES analysis and is given in Table S1 in Supporting Information. In the case of unmodified ZnO, the estimated zinc leaching was more than that of Pt loaded ZnO, but in either case the leaching was observed to be negligible with the highest being 0.32% (w/w). In order to activate dinitrogen, it is necessary to have a suitable metal loaded on the ZnO which can bind N2 and thus weaken the strong N-N triple bond. Unlike conventional homogeneous and heterogeneous catalyzed activation of N2 producing ammonia by thermal energy where it needs a continuous supply of H2, the present strategy adopted has the advantage that H2 is generated in situ by photocatalytic water splitting and the same is used for the reduction of N2. In general Pt, Pd, Rh, and Ru are used for enhancing the H2 evolution by water splitting where the role of metal is to minimize the electron-hole recombination by acting as electron sinks. As far as H2 evolution is concerned, Pt has been proved to be the better option. Hence, Pt-loaded ZnO has been utilized in the present study. A schematic showing the whole process of water splitting at the semiconductor metal interface along with the N2 activation through the insertion of N2 to Pt-H bond is depicted in Scheme 4. Once dinitrogen is coordinated to metal and the bond is weakened, subsequent reduction by hydrogen will be facile and eventually ammonia will be formed during the irradiation.52 The maximum amount of ammonia formed was around 86 µmol/(h/0.1 g of 1% Pt ZnO (wet etching)) which is estimated by volumetric titration using the trap solution including the 1 µmol of ammonia trapped in the reactor solution which is detected by indophenol blue test (see Supporting Information). Pt-loaded commercial ZnO showed lesser activity of the order

Heterogeneous and homogeneous wet chemical syntheses of hierarchical ZnO architectures have been achieved without using any specific templates, catalysts, or capping agents with comparatively better surface area than the commercial ZnO. Among the two wet chemical methods, heterogeneous wet etching resulted in the formation of ZnO with a self-assembly of hexagonal plates which has a disordered superlattice-type arrangement of nanoparticles. UV absorption studies and textural analysis revealed that the ZnO which is obtained by wet etching showed a smaller crystallite size but incorporated many grain boundaries and in turn defects during the course of formation. Photoluminescence studies support the claim of high defect concentration in ZnO (wet etching) by showing a red emission which is absent in the ZnO (precipitation). Mechanistic aspects pointed out that a self-assembly driven by a mesocrystal formation is responsible for the hexagonal bipyramidal architectures of ZnO. Photocatalytic water splitting studies showed a maximum H2 production around 35 µmol/(h/0.1 g for ZnO (wet etching)) whereas the N2 photofixation on Pt-loaded ZnO (wet etching) has yielded around 86 µmol/(h/0.1 g of ammonia). Hence, it is worthwhile to explore the nanostructured semiconductors especially those which have suitable band positions and band gap for the fixation of N2 as ammonia with the H2 generated in situ by water splitting, as it provides the unique chance of dinitrogen activation at room temperature and atmospheric pressure. Acknowledgment. The authors thank CSIR for a research fellowship to C.M.J. The funding from the Department of Science and Technology (Government of India) for NCCR is gratefully acknowledged. We also acknowledge DST nanotechnology centre, IIT Madras for HR-TEM analysis. Supporting Information Available: ICP-OES analysis results, XRD pattern and SEM image of ZnO (Commercial), pore size distribuition of all ZnO samples, TEM images of zinc oxalate precursor and HR-TEM images of ZnO obtained by wet etching and precipitation methods and EDAX spectrum of the corresponding samples, schematic illustrations of the methodologies adopted in the present study, the crystal lattice induced morphology, and procedures adopted in the detection of ammonia. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744–16746. (2) Zhang, T.; Dong, W.; Brewer, M. K.; Konar, S.; Njabon, R. N.; Tian, Z. R. J. Am. Chem. Soc. 2006, 128, 10960–10968. (3) Kokotov, M.; Biller, A.; Hodes, G. Chem. Mater. 2008, 20, 4542– 4544. (4) Zhao, F.; Li, X.; Zheng, J. G.; Yang, X.; Zhao, F.; Wong, K. S.; Wang, J.; Lin, W.; Wu, M.; Su, Q. Chem. Mater. 2008, 20, 1197–1199. (5) Jung, S. H.; Oh, E.; Lee, K. H.; Yang, Y.; Park, C. G.; Park, W.; Jeong, S. H. Cryst. Growth Des. 2008, 8, 265–269.

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