Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets

Article pubs.acs.org/JPCC

Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets and Shape Induced Enhanced Photocatalytic Performance Ramireddy Boppella, K. Anjaneyulu, Pratyay Basak, and Sunkara V. Manorama* Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Council of Scientific and Industrial Research, Indian Institute of Chemical Technology (CSIR-IICT) and CSIR-Network Institutes for Solar Energy (CSIR-NISE), Hyderabad, Andhra Pradesh, India S Supporting Information *

ABSTRACT: This study reports on the formation of unique oriented ZnO structures (face oriented hexagonal discs, 3D-trapezoids, rings, doughnuts, and hemispheres) with tunable percentage of exposed polar facets synthesized via a simple hydrothermal route in aqueous base environment. The significance of the synthetic strategy is the generation of exotic structures without using any templates/structure directing agents and successful realization of morphologies with increased polar to nonpolar facet ratio. Detailed investigation reveals that the size and shape of ZnO microstructures can be conveniently tailored by systematically exercising control on the choice of precursor (zinc source), concentration of reactants, use of Trizma as a base (pH control) which is also seen as structure directing agent. The Trizma base with an appreciably short pH range effectively controls the rate of hydrolysis, regulates nucleation, and restrains rapid growth leading to these well-defined ZnO structures. Photocatalytic degradation of methylene blue as a model system was used to showcase the morphology-dependent enhanced photoactivity under UV-light. The enhanced photocatalytic performance could be attributed to the higher fraction of exposed (0001) and (0001̅) ZnO polar facets present. by the surface atomic rearrangements.1,10,11 Surface atomic arrangements and coordination modulate the crystal facets in different orientations. Consequently, the reactivity of the photocatalyst significantly varies on the exposed crystal facet.12,13 Recently, Jang et al. have reported that terminal polar facets are much more reactive than thermodynamically stable nonpolar facets due to the higher surface energy of the former.1,11 Owing to the intrinsic anisotropy in the growth rate of ZnO, one-dimensional hexagonal rods have been predominantly synthesized that offer fewer exposed polar (0001) and (0001̅) facets.14,15 In such cases, however, it is difficult to assess an explicit relationship between the face orientation and the photocatalytic activity. Assuming that a ZnO rod could be sliced into many shorter discs, the relative number of polar facets (0001) and (0001̅) can be increased significantly.16 Thus, synthesizing ZnO with preferential growth patterns to obtain higher percentage of exposed polar facets is the key to enhance its photocatalytic activity. Successful tuning of morphology to obtain higher polar to nonpolar facets ratio is desirable to increase the percentage of reactive sites, and this remains a synthesis challenge. Control of the size, shape, and preferred orientation of ZnO nanostructures to tailor its chemical and physical properties for optimum reactivity and selectivity is also very important.17−20 Although considerable efforts are being

1. INTRODUCTION Semiconductor photocatalysts have attracted attention worldwide owing to their envisaged potentials in mitigating environmental issues in addition to their prospects in nextgeneration energy conversion/storage devices.1−3 Photocatalysis offers a great advantage for environmental remediation with possibilities to detoxify noxious organic pollutants. Primarily, the elimination of toxic and hazardous substances effectively for the reuse of wastewater has remained a major challenge. In this perspective, semiconductor based photocatalysis is being projected as a viable cost-efficient method for environmental abatement.3 Over the years, several types of micro- or nanostructured semiconductor metal oxide photocatalysts such as SnO2,4 Fe2O3,5 ZnO,1 and TiO23 have been developed and studied for their remediation efficiencies using model systems. Of these, TiO2 and ZnO are the two most extensively investigated materials because of their lower cost, nontoxicity, biocompatibility, and high thermal and chemical stability.6,7 ZnO presents impressive photocatalytic activity and is recognized as a suitable alternative to TiO2.8,9 Though ZnO is a wide-band-gap material (Eg = 3.37 eV), nevertheless it has attracted attention for its ability to generate hydrogen peroxide photocatalytically which effectively degrades several pollutants and viruses. It is now well understood that the photocatalytic reaction occurs primarily at the interface where the pollutants come in contact with the active surface of the photocatalyst. Hence, the photocatalytic properties of ZnO largely depend on the shape and surface morphologies affected © 2013 American Chemical Society

Received: November 20, 2012 Revised: January 30, 2013 Published: February 11, 2013 4597

dx.doi.org/10.1021/jp311443s | J. Phys. Chem. C 2013, 117, 4597−4605

The Journal of Physical Chemistry C

Article

Figure 1. Low-resolution SEM images of as-synthesized ZnO hexagonal microstructures (a) ZTR-1, (b) ZTR-2, (c) ZTR-3, (d) ZTR-4, ringlike structures (e) ZNTR-1, doughnuts (f) ZNTR-2, (g) ZNTR-3, and hemispheres (h) ZNTR-4. The scale bar is 4 μm in all panels.

which was then added to the above zinc precursor solution. A white precipitate formed instantly, and the solution was stirred for 30 min to obtain a homogeneous mixture. The resulting mix was transferred in a 100 mL Teflon autoclave and heated to 120 °C for 12 h. Subsequent to the completion of the reaction time, the autoclave was allowed to oven cool to room temperature; the precipitate was collected, washed repeatedly with water to get rid of organic components, and finally dried at 60 °C in an oven. In an alternate method of ZnO synthesis, zinc nitrate hexahydrate was used as the zinc precursor while the rest of synthetic protocol described above was followed as such. Samples synthesized using Zn(Ac)2 and Zn(NO3)2 are coded as ZTR and ZNTR, respectively. 2.2. Evaluation of Photocatalytic Activity. The photocatalytic activity of the as-prepared ZnO was evaluated by the photoassisted degradation of methylene blue (MB) aqueous solution at room temperature under UV-light (400 W, 360 nm, high pressure Hg vapor lamp, SAIC, India). In a typical reaction, 0.03 g of catalyst was dispersed in 30 mL of aqueous MB (1 × 10−5 M) in a glass reactor vessel equipped with a water jacket for effective heat dissipation. Prior to irradiation, the suspension was magnetically stirred for 30 min in the dark to stabilize and equilibrate the adsorption of MB on the surface of ZnO. The stable aqueous dye−ZnO suspension was then exposed to UV-light irradiation under continual stirring. Aliquots of 5 mL were withdrawn at regular time intervals to carry out the constituent analysis on a Varian Cary 5000 UV− vis−NIR spectrophotometer. The concentration of MB (C/C0) was estimated from the absorbance obtained. A blank run without ZnO reaction carried out under similar conditions was used for comparative evaluation.

devoted to synthesize ZnO crystals with controlled morphologies that can possibly provide highly reactive facets, relatively few attempts to correlate the effect of oriented ZnO morphology on the photocatalytic activity have been reported.13,21 In the present article, we describe in detail our successful attempts in achieving appreciable control on the fraction of exposed polar facets during the synthesis of a series of hexagonal prismatic ZnO structures with unique morphologies. Hierarchical structures including twin discs, 3D-trapezoids, selfassembled rings, doughnuts, and hemisphere morphologies could be achieved by simple hydrothermal routes depending on the choice of precursor, reactant concentration, pH, time, temperature, and pressure without the usage of any additives/ templates. Of particular interest was our successful realization of morphologies with tunable polar to nonpolar facet ratio; the twin discs and 3D-trapezoids that provide predominantly exposed (0001) and (0001̅) planes. The precursor and the pH of reaction mixtures are found to play critical roles in the formation of self-stacked hexagonal ZnO, twin discs, 3Dtrapezoids, and hemispheres. The degradation of methylene blue under the UV-illumination is used as a model system to showcase the photocatalytic activity, and results are compared with the conventional ZnO microrods. The twin discs, 3Dtrapezoids, and hemispheres exhibit significant enhancement in the photocatalytic activities. A detailed study on the optical properties and reactivity reveals a strong dependence on the morphology induced photoactivity.

2. EXPERIMENTAL DETAILS 2.1. Materials and Methods. All chemicalszinc acetate dihydrate (Zn(Ac)2, Zn(CO2CH3)2·2H2O), zinc nitrate hexahydrate (Zn(NO3)2·6H2O), and Trizma base ((HOCH2)3C− NH2), hereafter referred to as TB)were purchased from Sigma-Aldrich and used as received. In a typical preparation of zinc oxide, the required amount of zinc acetate dihydrate was dissolved in 22 mL of water under vigorous magnetic stirring to obtain a precursor solution. A requisite amount of TB was dissolved separately in another beaker with 20 mL of water,

3. RESULTS AND DISCUSSION Controlled synthesis of a series of hexagonal prismatic ZnO structures with unique morphologies is successfully achieved under hydrothermal condition. Detailed analyses on the synthesized structures are carried out using electron microscopy in the scanning and transmission mode along with selected area electron diffraction patterns. Figure 1a−h details a 4598



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Overall, it could be inferred from the SEM studies that Trizma base (TB) concentration does play a very crucial role in the growth of ZnO crystals and mediates the orientation of the planes. The morphology and size of the synthesized ZnO crystals were further investigated using transmission electron microscopy and selected area diffraction pattern to confirm the crystal structure. Figure 2a,b shows the TEM images of ZTR-2 and

series of low-resolution scanning electron microscopy (SEM) images obtained for different precursors (Zn(Ac)2, Zn(NO3)2) and concentrations of Trizma base (TB) in the reaction mix. As can be observed, the fine-tuning of the preferred polar face orientation for the ZnO crystals in aqueous solution of Zn(Ac)2 and Zn(NO3)2 can be achieved by varying concentration of TB at 120 °C. Different morphologies can be selectively obtained within a certain pH range, where TB exercises control on both the pH and as a shape-directing agent (see Table 1). Table 1. Samples Synthesized at Different Concentrations of Trizma Base, Corresponding pH Change Observed as a Function of Trizma Base Concentration, and Their Surface Area pH of the reaction sample

Zn/TB mole ratio

initial

final

BET surf. area (m2 g−1)

ZTR-1 ZTR-2 ZTR-3 ZTR-4 ZNTR-1 ZNTR-2 ZNTR-3 ZNTR-4 ZNR

1:1.5 1:2 1:2.5 1:3 1:1.5 1:2 1:2.5 1:3

7.28 7.96 8.23 8.65 6.11 6.62 6.96 7.23

6.5 6.7 7.1 7.2 6.1 6.2 6.2 6.4

4.3 8.4 27.55 32.02 32.46 94.7 15.82 14.87 16.55

Figure 2. TEM and their corresponding SAED pattern of assynthesized ZnO structure (a) ZTR-2, (b) ZTR-4, (c) ZNTR-1, and (d) ZNTR-3. Inset shows the SAED of corresponding samples.

ZTR-4 samples while Figure 2c,d corresponds to the ZNTR-1 and ZNTR-3 ZnO samples. The inset SAED patterns provided in each micrograph confirms the wurtzite ZnO structures with hexagonal lattices. The diffraction pattern of samples ZTR-2 and ZTR-4 exhibit hexagonal shapes having plane (1̅010) at the edge and plane (0001) as the top and bottom facet. Close inspection of Figure 2c,d in the transmission mode reveals further details on the microstructure which was not apparent in the SEM studies. Hierarchical self-assemblies of smaller ZnO particles in micrometer-sized structures (ring and doughnut shapes) are very evident from these images. Figure 3a represents typical X-ray diffraction patterns for the hexagonal faceted twin discs (ZTR) and rings (ZNTR) of the synthesized materials. All the diffraction peaks of synthesized samples can be ascribed to (100), (002), (101), (102), (110), and (103) planes corresponding to the hexagonal wurtzite ZnO crystals and are in good agreement with the standard JCPDS No. 36-1451. Raman studies also confirm the phase purity and degree of crystallization for the as-synthesized samples. Figure 3b shows the representative Raman spectra collected at room temperature for the synthesized samples (ZTR, ZNTR) in the range of 50−800 cm−1. All samples exhibit similar scattering, which corresponds to the characteristic bands of the hexagonal wurtzite phase of ZnO. The two high-intensity peaks observed at ∼99.7 and ∼441.02 cm−1 are attributed to the low and high E2 modes of nonpolar optical phonons, respectively, typical of wurtzite phase.22,23 Weaker peaks observed at ∼332.7 and ∼383.11 cm−1 correspond to the E2H−E2L multiphonons and A1T modes, respectively. The broad and suppressed peak at ∼578.4 cm−1 is assigned to the E1L mode, which possibly relates to the structural defects owing to the interstitial oxygen vacancies in the ZnO samples.24,25 The composition and oxidation states of the synthesized materials were further confirmed by XPS. As-synthesized ZnO exhibits binding energies at 1021.8 and 1044.8 eV, which

The use of Zn(Ac)2 precursor as a function of increasing TB concentration is sequenced in Figure 1a−d. Micrograph Figure 1a shows formation of hexagonally prismatic structures when the zinc acetate to TB mole ratio was 1:1.5. The diameters of hexagonal discs are ∼5−6 μm, and these discs are observed to be stacked symmetrically in pairs along with some irregular shapes interspersed. Interestingly, coverslips like overgrowth are seen a-top the exposed hexagonal sides almost uniformly throughout the samples. The pairs together apparently resemble layered hexagonal rods with an average length of ∼7−9 μm. When the TB concentration was increased to 1:2 mol, high aspect ratio well-formed hexagonal discs were obtained with defined edges and are self-stacked symmetrically to form conjoined twin discs (Figure 1b). The diameters of these hexagonal discs are ∼7 μm with edge length of ∼3−3.5 μm. Upon further increase of TB to 1:2.5 (Figure 1c) and 1:3 (Figure 1d) mole ratios, a noticeable change in the morphology was observed with formation of truncated hexagonal conelike shapes approaching 3D-trapezoids. There is also a significant reduction of the axial growth along with the tendency to pair up. On the other hand, contrasting morphologies in the shape of ring, doughnuts and hemispheres of ZnO crystals were obtained from zinc nitrate precursor. The effect of TB concentration in the reaction medium can be observed in Figure 1e−h. When the Zn(NO3)2 to TB ratio was 1:1.5, ringshaped ZnO was observed with outer diameter of ∼640 ± 50 nm and inner diameter of 100 ± 50 nm. For the ratios 1:2 and 1:2.5, the ZnO particles still maintained their ring-like morphology, but the inner diameter of the ring decreased, giving a doughnut-like appearance, while at higher Zn(NO3)2/ TB ratio (1:3) resulted in the formation of uniformed hemispheres. Upon further increase in the TB concentration, the size of the hemispheres appeared to increase considerably with significant agglomeration and irregular morphology. 4599



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Figure 3. (a) X-ray crystallographic studies of the ZnO particles obtained for different Zn precursor with 1:2 ratio of TB base. (b) Corresponding Raman spectral analysis.

corresponds to Zn 2p3/2 and Zn 2p1/2 electrons in the Zn2+ oxidation state, respectively (see Supporting Information Figure 4). The O 1s spectrum shows the existence of oxygen in two different states, with binding energies at 530.7 and 532.8 eV that can be assigned to the lattice-oxygen and the oxygendeficient regions within the ZnO matrix, respectively.26,27

hexagonal disks and rings can be proposed. The amino group of TB is basic in nature, which can be effectively used to control the desired pH of the reaction media. Thus, increasing TB concentration leads to an increase in pH of the reaction system. Additionally, TB in aqueous medium provides the necessary hydroxide ions for the formation of ZnO along with the presence of a cationic counterpart (ammonium cation).

4. GROWTH PROCESS AND STRUCTURAL EVALUATION To evaluate the growth mechanism and effect of TB concentration on ZnO formation, we carefully carried out reaction/process optimization by monitoring the parameters involved following multiple experiments and using a wide range of techniques. The key reaction parameters that are necessary to control and the underlying mechanism to obtain the desired morphology of ZnO (hexagonal disks, hemispheres, ring shapes) can be rationalized as follows. The immediate formation of a white turbid complex formed initially at room temperature when TB was added to the zinc precursor was analyzed by XRD and FT-IR (see Supporting Information Figures 1 and 2, respectively). Our deduction shows the formation of Zn(OH)2 particles unlike an earlier report on the formation of zinc amine complex with polyamines.27 As discussed in the preceding section, the concentration of TB was found to play a significant role in determining the morphology of ZnO formation (Figures 1 and 2). Our investigations indicate the choice of zinc precursor also influence the formation of the ZnO structure. Zinc salts such as Zn(Ac)2 used in this study yields hexagonal structures while ring-shaped ZnO was obtained when Zn(NO3)2 is used as the precursor. This observation strongly indicates the structure directing effect of Trizma base during the nucleation and growth processes. To demonstrate that the reaction process in presence of TB is exclusive, we have carried out multiple experiments in different sets of conditions, reactants, and structure-directing agents. In the thermal treatment of aqueous zinc precursors (Zn(Ac)2, Zn(NO3)2) under identical experimental conditions without the presence of TB, no ZnO could be obtained. The addition of NaOH to aqueous zinc acetate (pH ∼11.2) and zinc nitrate (pH ∼10.3) in Zn2+ to NaOH mole ratio of 1:2 resulted in dumbbell shapes and microrods of ZnO, respectively (see Supporting Information Figure 3). Further increase in NaOH concentration leads to the formation of irregular shaped ZnO microstructures. This drastic change in the morphology could be ascribed to a large difference in the initial and final pH of the reaction system.28 On the grounds of the above observation, a plausible mechanism on the formation and growth process of the

(HOCH 2)3 C−NH 2 + H 2O → (HOCH 2)3 C−NH3+ + OH− Zn 2 + + 4OH− → Zn(OH)4 2 −

Zn(OH)4 2 − → ZnO + H 2O + 2OH−

Zinc cations are known to readily react with hydroxide anions to form stable tetrahedral Zn(OH)42− complexes, which act as the growth units for the ZnO structures.29,30 The solution pH controls the rate of hydrolysis of the zinc precursor. Shinde et al. suggested that, in the intermediate pH range between 7 and 10, insoluble Zn(OH)2 is formed which redissolves at higher pH (>10).29 In our protocol for synthesis, the pH of reaction system was maintained between ∼7 and 9. Hence, we do observe the initial formation of Zn(OH) 2 , which subsequently condenses and transforms into ZnO at higher temperatures. In addition, pH of the reaction mixture shows an obvious influence on the final shape of the particles. Large change in the pH values not only alters the nucleation rates but also accelerates the crystal growth. Any difference in the growth rates of different crystallographic planes would be reflected in the final geometry of crystals. When the pH variation in initial and final stages of the reaction is small, the nucleation rate is appreciably slower, leading to well-defined particle shapes and sizes.28,31 Subsequent to our significant observations made from the electron microscopy studies, the realization of diverse morphology can be comprehended based on the controlled nucleation and directed growth under the specific reaction conditions (choice of precursors, reactant concentrations, pH, counterions, etc.). With increasing concentration of the TB and its corresponding pH, the morphology of the synthesized ZnO changed from hexagonal stacked discs with interspersed irregular shapes-to-uniform hexagonal twin discs and finally into truncated hexagonal cones (3D trapezoids). This shape transformation can be understood in terms of the differences in the growth rates of various crystal faces at different pH values. In general, depending on the structural anisotropy and surface electric polarity of ZnO, the growth rate is higher for [0001], 4600



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Figure 4. High-magnification TEM images of the as-synthesized samples using zinc nitrate precursor as a function of Trizma base concentration in mole ratios: (a) 1.0:1.5, rings; (b) 1.0:2.0, (c) 1.0:2.5; doughnuts (d) 1.0:3.0, hemispheres. The images of synthesized samples indicate that smaller crystallites (primary particles) adjoins into larger shapes through oriented growth and Ostwald ripening process. Scale bar provided in all micrographs is 100 nm.

decreases for (1̅010), (101̅0), (11̅00), and (1̅100), and is lowest for (0001̅) under normal conditions.32,33 In the pH range 6−9, ZnO nuclei are hexagonally prismatic, and the crystals grow in both axial and equatorial direction simultaneously.19,34 Under this pH condition, the ZnO crystals grow preferentially along the (101̅0) or (011̅0) equatorial crystal planes that forms the side edges of the hexagonal discs (prismatic faces).35 Although with increase in the base concentration, the particles hold the hexagonal prismatic structure, they show significant changes radially. This is probably due to the small difference between the initial and final pH value of the reaction mixture that alters the nucleation rate and growth kinetics leading to well-defined but different morphologies. Moreover, in our experiments, the Trizma base possibly also serves as an effective capping agent for the ZnO nuclei and plays an important and parallel role in the growth orientation. The −OH groups of TB molecule can preferentially get adsorbed on the positive surface of (0001) Zn2+ plane and prevents the rapid growth along the ⟨0001⟩ direction.30 This condition promotes ZnO crystal growth along the six equatorial directions symmetrically, which leads to the formation of hexagonal discs. Further, the choices of precursor do show the obvious influence as observed in our studies and also discussed in earlier reports.36 Liang et al. reported the inhibition effect of adsorbed acetate ions on the surface of Zn2+ substituting hydroxyl anions along the ⟨0001⟩ direction. It is now wellunderstood that acetate ions in the solution have high affinity of adsorption on the ZnO surface compared to other anions.37−39 The high proportion of polar plane can thus be related to the strong suppression of crystal growth along ⟨0001⟩ and relative enhancement of crystal growth along the ⟨011̅0⟩ direction.11 Although the exact mechanism of stacking or pairing of discs is debatable, it is proposed that the surface charge of these polar faces with opposite polar direction interact and assemble together during initial growth and forms stacked discs, resulting in neutralization of local polar charges.11,40 However, the positively charged Zn2+-terminated (0001) and negatively charged O2−-terminated polar (0001̅) surfaces have high surface energies.41 Hence, we believe that coming together of these polar surfaces would be energetically unfavorable unless the surface charge is compensated by a passivation agent, in our case, possibly the combination of both Trizma base and acetate anions.40,42 That the crystal growth strongly depends on the structure of the material, the surface chemistry of the particles resulting from ions in the solution, the interface between the crystals, and surrounding solution28,43 is also reflected in the formation of ZnO rings and hemispheres. Zeshan et al. stated that the tendency of anion adsorption on surface of ZnO affect the growth kinetics and resulting particle morphologies.43 These

adsorbed anions can dictate the material structure, shape, and size by altering the nucleation rate and growth kinetics. Hence, the formation of rings and hemisphere-shaped ZnO crystals comprised of smaller primary particles as observed in TEM images, although a new observation was not surprising with the choice of zinc nitrate as an alternate precursor. The growth mechanism and resulting morphology of these ZnO rings and hemispheres can be interpreted in the context of nucleation, Ostwald ripening, and oriented attachment. Oriented attachment crystal growth phenomena have been often observed for nanoparticles, where primary crystallites can be joined together into larger ones.44−46 Unfortunately, it was noticed that during this process Ostwald ripening also usually occurred simultaneously.47,48 The Ostwald ripening mechanism driven by minimization of the overall surface energy47 results in the rings, doughnuts, and hemispherical shapes consisting of larger primary particles at the expense of smaller particles. The higher magnification TEM images (Figure 4) of rings, doughnuts, and hemispherical shaped ZnO reveal that the structures are composed of several aggregated smaller crystallite domains (primary particles) yielding larger morphologies. Initially, ZnO nuclei are understood to grow via the diffusive mechanism, resulting in smaller particles, and the growth process starts with self-assembly of these ZnO nanocrystallites through an oriented attachment mechanism.28 The nanocrystallites possess higher surface energies and easily aggregate at an early stage of the growth process under hydrothermal conditions. Analysis of the SAED patterns provided as the inset in Figure 2c,d reveals that the polycrystalline domains are oriented differently for the ring-like and hemispherical ZnO assemblies. The formation of these new structures as rings and hemispheres can be attributed to an imperfect oriented attachment mechanism.49 Different but adjacent crystallographic planes oriented randomly self-assemble and adjoin to yield spherical structures via an Ostwald ripening process form the basis of this mechanism.50 It is reported that the inner region possessing significantly higher surface energies have a stronger tendency to partially redissolve, reorient, redeposit, and adjoin via Ostwald ripening process to help formation of ring-like structures. With the increase in concentration of TB we observe that the dissolution effect of inner region reduces reflecting the final morphology (formation of doughnuts/ hemispheres) and hints of considerable surface roughness.

5. OPTICAL PROPERTIES AND PHOTOCATALYTIC ACTIVITY The effect of morphology on the optical properties of synthesized ZnO samples was evaluated from the diffusereflectance (UV-DRS) studies. As can be observed from Figure 4601



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Figure 5. (a) Diffused reflectance UV spectra of the as-synthesized samples collected in the absorbance mode. The band edge provides an estimate of the optical band gap. (b) Photoluminescence spectra of as-synthesized samples recorded at an excitation wavelength of 360 nm and room temperature.

Figure 6. (a) UV/vis spectra of methylene blue over ZTR-4 sample at different irradiation times under UV light. (b) Change in methylene blue concentration (photocatalytic degradation) as a function of UV-light irradiation time over different ZnO samples as catalysts. The catalyst-free condition is denoted by MB in the figure.

5a, the absorption edge of the ZTR-1 sample is ∼414 nm, which corresponds to a band gap of 2.99 eV. For all other samples, the observed absorption band edge is found to be ∼398 nm, corresponding to a band gap of ∼3.11 eV. Overall, no noteworthy change was inferred in the optical assessment that relates to the change in morphology of the synthesized samples. Interestingly, even though the crystals synthesized are notably larger in size (micrometers), all the samples show a considerable red shift of the optical band gap when compared to that of bulk ZnO (∼3.34 eV). This observation can be rationalized considering the increased percentage of polar facets present in the synthesized samples, consequently with more oxygen defects. Earlier reports have suggested that large fraction of polar planes can generate significant number of oxygen defects which can appreciably influence the optical properties.21,51 It has been reasoned that these oxygen defects can form shallow levels within the band gap, leading to the redshift.51 Photoluminescence (PL) spectra of hexagonal discs were measured using a He−Cd laser as an excitation source presented in Figure 5b. The room temperature PL spectra of the ZnO samples excited at 360 nm showed two intense emission bands centered at ∼398 and 409 nm along with a less intense band centered at ∼468 nm. While the UV emission corresponds to the near-band-edge emission resulting from the exciton recombination, the blue emission peak is usually attributed to the zinc vacancy, with instances of being referred to oxygen vacancy as well.26,52,53 However, the origin of defect

emissions in ZnO is still an unresolved question, in spite of a large number of reports. The polar planes easily generate oxygen vacancies and are expected to prevent electron−hole recombination, thereby resulting in an increased photoinduced activity.54 That in our synthesized samples oxygen vacancies exists was evident and confirmed by Raman and XPS analysis as discussed in the preceding section. Morphology induced enhanced photocatalytic performance of the hexagonal prismatic ZnO and Zinc oxide rings were studied following the degradation kinetics of methylene blue (MB) under the UV light as a model system. The results of the MB degradation in a series of experimental conditions are summarized in Figure 6. In absence of catalyst, only a minimal decrease in the concentration of MB was detected. The characteristic absorption of the methylene blue seen at 663 nm was selected for monitoring the adsorption and photocatalytic degradation process. A rapid decrease in the initial absorbance and finally disappearance of this peak in the presence of synthesized ZnO samples could be observed (Figure 6a). The photocatalytic effect and degradation of MB were obvious under UV irradiation in the presence of synthesized ZnO, and the samples do exhibit different photoactivities depending on the morphology and surface area. Comparative analysis shows, for the ZTR-3 sample, the dye degradation was 97% within 40 min while for the ZTR-1 and ZTR-2, the degradation was 93% and 88%, respectively. ZNTR sample, on other hand, degrades 80% of dye within 40 min. Among all the synthesized samples, 4602



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surface area. In addition, the ZnO particles were found to be structurally very robust and stable even when treated with 0.1 M HCl solution at 90 °C for 12 h. No apparent change was observed (see Supporting Information Figure 5) and the findings indicate that the synthesized samples are appreciably resistant to acid corrosion. It has been suggested that such chemical stabilities of the ZnO samples can also be partially held responsible for the higher photocatalytic activity of the ZnO observed.8

ZTR-4 shows the best photoctalytic activity and completely degrades the MB solution within 40 min under the UV illumination. To demonstrate the morphology induced superior photocatalytic performances of hexagonal ZnO discs and ring-like microstructures over the other morphologies synthesized, photocatalytic activity of ZnO rods (ZNR) was also studied. It is evident from our findings that these ZnO rods show relatively poor photocatalytic activity when compared to hexagonal discs and rings. The schematic illustration (Figure 7) represents the trend of increasing photocatalytic activity as observed for the synthesized samples studied.

6. CONCLUSIONS In summary, we demonstrate a successful strategy for synthesis of ZnO with preferentially exposed polar facets in unique morphologies such as self-stacked hexagonal discs, rings, and hemispheres. An appreciable control on the fraction of polar facets is achieved by a simple hydrothermal method in aqueous base environment. Choice of precursor, concentration of reactants, use of Trizma as a base and structure-directing agent, mild basic conditions, slower rate of hydrolysis, nucleation and growth all play equally important roles in determining the final morphology and fraction of the exposed crystal facets of hexagonal ZnO. The photocatalytic activities of these controlled ZnO superstructures were evaluated albeit with a model system, methylene blue. The photocatalytic studies reveal not only enhanced performance but also a significant dependence on the shape and fraction of polar faces present in the samples. We believe that this approach along with our findings on shape and directionality control can possibly be adopted and extended to other materials as well to create unique morphologies yielding excotic physicochemical properties.

Figure 7. A schematic illustration representing the correlation between polar facets and photocatalytic activity trends observed experimentally for the synthesized ZnO structures.

The superior photoactivity of hexagonal discs can be attributed to their exotic structural features with predominantly exposed active polar facets. It has been earlier demonstrated that highly exposed terminal polar surfaces of ZnO crystals could induce high photocatalytic activity. Jang et al. and Mclarn clearly suggested that compared to other planes the exposed (0001) and (0001̅) polar faces are more active surfaces for photocatalysis.8,17 As demonstrated in the present study, all the synthesized ZTR samples do show the presence of highly ordered and cleanly exposed polar faces (see Figures 1 and 2). Thus, our observation, ZTR-1, ZTR-2, and ZTR-3 all showing better photocatalytic efficiency with ZTR-4 showing the best activity (ZTR-4 ≥ ZTR-3 > ZTR-2 > ZTR-1), is consistent with their surface area and percentage of exposed polar facets. The comparison of BET surface area of the ZNR (16.55 m2/g), ZNTR-2 (94.7 m2/g), and ZTR-1 (4.5 m2/g) reveals the importance of polar planes of ZnO. The BET surface area of ZNTR-2 is ∼20 times higher than the ZTR-1 and ∼4 times higher than ZNR. Conventionally, the higher surface area is beneficial for the higher catalytic activity. Yet the photoactivity of ZTR-1 was found to be better than that of ZNTR-2. Though the ZNR sample possesses higher surface area compared to that of ZTR-1, the photocatalytic activity was significantly less than the ZTR-1 sample. This too can be understood on the basis of percentage of polar facets present, which definitely decreases for ZNR (ZnO microrods) due to there larger particle size to that of ZTR-1.8 Analysis of photoluminescence spectra along with Raman and XPS also confirms the presence of more oxygen vacancies in samples that have higher ratio of polar planes which supports higher photocatalytic efficiencies. Exposed surfaces of polar facets induce rapid generation of H2O2 in situ under light irradiation that accounts for the high photoactivity of ZnO. These results clearly demonstrate that ZTR samples with its higher percentage of exposed polar facets possess superior photocatalytic activity than the ZnO rods with fewer exposed polar facets (shown in Figure 7). These observations together provide conclusive evidence that the surface area is not the only parameter that determines the photoactivity of ZnO. The percentage of exposed polar faces bears more influence on the photoactivity of ZnO than its

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ASSOCIATED CONTENT

S Supporting Information *

Details of characterization techniques and evaluation parameters, XRD and FT-IR spectra of Zn(OH)2, XPS spectra, SEM images of ZnO samples treated with hydrochloric acid, and photocatalytic activity of ZNTR series samples. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +91-40-27193225; Fax +9140-27160921. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.R.R. and K.A. sincerely acknowledge UGC and CSIR for the senior research fellowship. The authors deeply appreciate the help extended by Dr. Rohit Kumar Rana, Dr. B Sreedhar, and Dr. V Jayathirtha Rao, Scientist, IICT, for their support in the evaluation of the materials. P.B. and S.V.M. appreciate the strong support of CSIR-IICT, Hyderabad, CSIR-NISE, and acknowledge CSIR, New Delhi, under the TAPSUN programme for the research grant.

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REFERENCES

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Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets

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