ARTICLE pubs.acs.org/Langmuir
Optical and Photocatalytic Properties of Single Crystalline ZnO at the AirLiquid Interface by an Aminolytic Reaction Mukta V. Vaishampayan,†,‡ I. S. Mulla,†,§ and Satyawati S. Joshi*,‡ †
Physical and Materials Chemistry Division, National Chemical Laboratory, Homi Bhabha Road, Pune411008, India Department of Chemistry, University of Pune, Pune411007, India § Centre for Materials for Electronics and Technology (C-MET), Panchavati, Pune 411008, India ‡
bS Supporting Information ABSTRACT: Crystalline flowerlike ZnO was synthesized by an aminolytic reaction at the airliquid interface in an aqueous media at an alkaline pH. A thin visible film was formed at the airliquid interface by self-assembly of flowerlike ZnO. Diffraction studies show rearrangement of the single crystalline units at the airliquid interface leading to the formation of nanobelts. These nanobelts overlap systematically to form petals of the flowerlike structure; individual petals get curved with time. Each nanobelt is found to be single crystalline and can be indexed as the hexagonal ZnO phase. The organic product formed in the aminolytic reaction and dissolutionreprecipitation mechanism is the driving force for the formation of flowerlike ZnO at the airliquid interface. A clear relationship between the surface, photocatalytic, and photoluminescent properties of ZnO is observed. The flowerlike structure exhibits a blue shift (3.56 eV) in the band emission as compared to bulk ZnO (3.37 eV). The photodegradation of methylene blue over the flowerlike ZnO catalyst formed at the airliquid interface and in the sediments shows enhanced photocatalytic activity. The sub-bands formed due to surface defects facilitate separation of charge carriers increasing their lifetime, leading to enhanced photocatalytic activity of flowerlike ZnO.
’ INTRODUCTION Organization of nanocrystals into multidimensional superlattices can be achieved by self-assembly of molecules and particles into the ordered nanostructures using chemical forces. The properties of the self-assembled nanostructure vary significantly as compared to the nanoscale building blocks. At low temperature, this type of self-assembly is driven by the organic ligand and vectorially regulates the growth of nanostructures at the airliquid interface. Synthesis of ZnO nanostructures with controlled size and shape is very important in controlling their physical, chemical, and optical properties for a wide variety of technological applications. ZnO is one of the most extensively studied functional oxides with a direct wide band gap (3.37 eV) and a high exciton binding energy (60 meV), which makes it one of the most important functional materials. The chemical, electrical, and optical properties of ZnO depend on its dimension, morphology, and crystallinity. ZnO has been synthesized by different synthetic methods such as pulsed laser deposition (PLD), chemical vapor deposition (CVD), radio frequency (RF) magnetron sputtering, spray pyrolysis, hydrothermal, solgel processing, and other solution based methods.16 Low temperature solution based methods are cost-effective and used for the synthesis of ZnO with well-defined morphologies containing various crystalline nanoparticles with narrow size distribution.710 r 2011 American Chemical Society
There are few reports related to the synthesis of ZnO at an interface.1113 The airliquid interface provides a different environment for the growth of ZnO with variable morphology and properties. A number of methods can be applied to obtain an ordered functional material at the interfaces;1417 however, allowing the structure to self-assemble is a very attractive proposition. Tang et al.11 synthesized monodispersed ZnO troughs at the airwater interface using zinc acetate and hexamethylenetetramine in aqueous solution at 90 °C. The method is template free, additive free, and economic. The ZnO produced was with different morphologies. It was obtained by changing various preparative parameters such as initial reactants and its concentration, heating time, and stirring time. The nucleation and growth of troughlike ZnO nanomaterial was explored at the airwater interface. Troughlike ZnO shows a blue shift of the band emission from 3.37 eV in bulk ZnO to 3.48 eV. Masuda and Kato12 reported highly c-axis oriented standalone, self-assembled ZnO films at 60 °C using zinc nitrate hexahydrate and ethylenediamine. Shortell et al.13 synthesized one-dimensional capped ZnO nanoparticle assemblies at the airwater interface at low temperature. The synthesized rodlike dodecanethiol micelles contained clusters of zinc oxide Received: May 13, 2011 Published: September 02, 2011 12751
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Table 1. Sample Code with Different Period and Corresponding pH of the Solution Sr. no 1
sample code ZM4 (as-prepared flowerlike ZnO) ZnO film at airliquid interface
ZnO
liquid
in the
between
aging time
pH of the
(days)
solution
0
9.25
sediments the two layers
2
AL1
S1
F1
7
9.24
3
AL2
S2
F2
14
9.21
4
AL3
S3
F3
21
9.19
5
AL4
S4
F4
28
9.08
nanoparticles at the airwater interface. These nanoparticles with rodlike micelles were aligned into ordered arrangements of parallel rods forming and ordered monolayer film at the air water interface. This was obtained by maintaining a moderate surface pressure of 16 mN/m prior to and during deposition in the LangmuirBlodgett trough. Currently, research has been aimed at the self-assembly of nanoscale building blocks into three-dimensional hierarchical architectures. In continuation with our previously reported synthesis of flowerlike ZnO at low temperature (4 °C),18 we have studied the effect of aging time on structural, morphological, optical, and photocatalytic properties of flowerlike ZnO. In the present study, single crystalline nanobelts form flowerlike ZnO at the airliquid interface at room temperature with zinc acetate dihydrate (ZA) and ethanolamine (EA) as the precursor, wherein the amide obtained in the aminolytic reaction and dissolution reprecipitation approach are the driving forces for the formation of ZnO at the airliquid interface. Photoluminescence studies show blue shift in the band emission for all ZnO samples. Visible photoluminescence observed may be attributed to the defects in ZnO lattice. Degradation of methylene blue (MB) over the ZnO catalyst with UV irradiation is studied to evaluate the photocatalytic activity of flowerlike ZnO and to correlate the effect of presence of defect states on the photocatalytic activity.
’ EXPERIMENTAL DETAILS All the reagents used in the experiments were of analytical grade and utilized without further purification. Synthesis was carried out as reported earlier.18 In a typical procedure, 0.1 M ZA solution was prepared by dissolving 5 g of ZA in 200 mL of double distilled water. Then 5.56 mL of H2N(CH2)2OH EA was added dropwise to the above solution with constant magnetic stirring, maintaining the solution at 4 °C with a ZA/EA molar ratio of 1:4. The product precipitated out immediately with the addition of EA, turning the solution cloudy. Once the addition of EA was complete, the product obtained for each sample was constantly stirred for 10 min, maintaining the temperature at 4 °C. Four sets of such samples were synthesized under exactly identical conditions and aged for different time periods consistently at room temperature, and no air flow was exerted over the surface. A thin white layer appeared at the airliquid interface with time and also sediments at the bottom, thus turning the solution clear. This clear solution became intensely colored upon further aging. The product at the airliquid interface was collected on a glass slide at different time intervals from each set followed by drying at room temperature in vacuum, and used for further characterization. The sediments at the bottom were collected by
centrifugation, and the solution was separated (Table 1 shows the sample codes with different aging period). To study the photocatalytic property of flowerlike ZnO catalyst, 5 mg of catalyst was dispersed in 50 mL of 106 M aqueous MB solution in a quartz reactor having a water jacket for cooling. The dispersion was irradiated by UV light under constant magnetic stirring in a photochemical reactor (Srinivasan-Griffin Rayonet) type “PSAW” with 400 W power. This instrument comprised eight ultraviolet tubes of wavelength 253.7 Å fitted in a heavy metal enclosure with an inbuilt magnetic stirrer. Aliquots were collected at various time intervals to monitor degradation of MB. Finally, the aliquots were centrifuged and the UVvis absorption spectra were scanned. The chemical, structural, and morphological characterization of the samples was performed using various characterization techniques. X-ray diffraction (XRD) analysis of powder samples was done on an ‘X’ Pert Pro diffractometer, operating at a voltage of 20 kV (with Cu Kα radiation, λ = 1.5401 Å) in the range (2θ) from 5° to 90° with a scanning rate of 5°/min. Fourier transform infrared (FTIR) spectroscopy was used to study the chemical composition of the sample, in order to derive a plausible growth mechanism. Scanning electron microscopy (SEM) was used to study the morphology of the sample. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were obtained on a Tecnai F30 FEG machine operated at 300 kV. Samples were prepared by placing a drop of suspension of the sample in ethanol on the carbon coated copper grid followed by evaporation of the solvent. UVvis spectra and room temperaturephotoluminescence (RT-PL) spectra were scanned on Perkin-Elmer Lamda 650 UV/vis spectrophotometer and PerkinElmer-LS-55-photoluminescence spectrometer, respectively.
’ RESULTS AND DISCUSSION The aqueous solution of ZA became cloudy after addition of EA and flowerlike ZnO precipitate was formed. Once the EA was added to the aqueous solution of ZA, Zn-EA chelate was formed initially as given in eq 1. 2þ Znþ2 ðaqÞ þ 3fH2 NðCH2 Þ2 OHgðlÞ a ½ZnðH2 NðCH2 Þ2 OHÞ3 ðaqÞ
ð1Þ The equilibrium of eq 1 shifts to left when an excess of EA reacts with water to release OH(aq) ions and H3N+(CH2)2OH(aq) (EAH+(aq)) ions as given in eq 2 along with lattice energy from the hexagonal lattices of water formed at 4 °C. H2 NðCH2 Þ2 OHð1Þ þ H2 Oð1Þ f OH ðaqÞ þ H3 Hþ ðCH2 Þ2 OH ðaqÞ þ 2v
ð2Þ
Zn2+(aq), CH3COO(aq), and OH react to form zinc hydroxyacetate nanobelts {Zn(OH)x(CH3COO)y 3 zH2O} according to eq 3. Znþ2 ðaqÞ þ CH3 COOðaqÞ
þ OH ðaqÞ f ZnðOHÞx ðCH3 COOÞy 3 zH2 OðsÞ V
ð3Þ
At 4 °C, water possesses the highest density with tiny hexagonal lattices formed in liquid water. The reaction of amine with water is an exothermic reaction and releases OH ions and also the lattice energy from the hexagonal lattice of water. This released energy favors the conversion of zinc hydroxyl acetate (Zn(OH)x(CH3COO)y 3 zH2O) nanobelts to ZnO according to 12752
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Zn(OH)2 and Zn(OH)42 as given in the eq 5. The concentration of Zn(OH)2 in the sediments increases until the seventh day (S1) and later on again decreases as can be seen in Figure 1a. 2 ZnOðsÞ þ H2 OðlÞ þ OH ðaqÞ a ZnðOHÞ2ðsÞ V þ ½ZnðOHÞ4 ðaqÞ
ð5Þ
Figure 1. (a) XRD of flowerlike ZnO along with Zn(OH)2 and Zn(OH)x(CH3COO)y 3 zH2O in the sediment samples (// indicates zinc hydroxyl acetate, and / indicates zinc hydroxide peaks). (b) XRD of ZM4 and ZnO samples at airliquid interface (# indicates the direction of growth).
eq 4 as reported earlier.18 2v
ZnðOHÞx ðCH3 COOÞy 3 zH2 OðsÞ f ZnOðsÞ V þ CH3 COO ðaqÞ þ H2 OðlÞ
ð4Þ
The EAH+(aq) ions (eq 2) bind to the negative polar plane of ZnO formed to give a flowerlike structure. This is confirmed from the fact that when the sample is washed with an organic solvent, the flowerlike structure is not retained because of the simple reason that the ligand gets washed away in the organic solvent (see Figure 1 in the Supporting Information). The ligand not only provides an organized surface for structure formation but also induces a vector growth on the surface, the direction of which differs from the characteristically preferred direction of the unit cell. ZnO precipitate formed settles down with time, and a thin film appears at the airliquid interface with crystalline wurtzite ZnO structure. The XRD profile in Figure 1 clearly shows formation of ZnO at the airliquid interface and in the sediments. The sediment samples show formation of orthorhombic zinc hydroxide (Zn(OH)2 (JCPDS no. 76-1778), along with ZnO (JCPDS file no. 36-1451). Under alkaline conditions, ZnO solubilizes as
Also the solution becomes intensely colored with time as can be seen in the photograph in Figure 2. According to Goux et al.,19 at room temperature, the pKa values of ZnO and Zn(OH)2 are 16.8 and 16.5, respectively. Hence, it should be possible theoretically to prepare ZnO at room temperature, but experimentally due to kinetic aspects Zn(OH)2 is the dominant species formed at room temperature.20 Thus, in the present case, though we get hexagonal wurtzite at 4 °C; the Zn(OH)2 species dominates as the system attains room temperature. Figure 1a shows the formation of Zn(OH)2 as the dominant species, but later with aging its intensity decreases drastically due to the formation of Zn(OH)42-. Zn(OH)42 formed is soluble in water which in turn may interact with the free acetate and EAH+ ions in the solution to form soluble complex structures giving (colorless f yellow f orange; see Supporting Information, Figures 2 and 3) an intense orange color to the solution. The concentration of Zn(OH)42 increases with time as the color of the solution becomes more and more intense. The decrease in the pH of the solution as given in Table 1 supports the above statement. Due to close pKa values, there is competition between the formation of Zn(OH)2 and ZnO, and hence, dissolutionreprecipitation mechanism is favored. XRD study of ZnO at the airliquid interface suggests that the growth of ZnO is dominated in the (001) direction for the AL2 and AL3 samples (peak marked with # in Figure 1b). However, with time, growth along the (001) direction is suppressed in AL3 while the intensity of the (100) and (101) peaks increases as compared to AL2. As per eq 5, ZnO solubilizes as Zn(OH)2 and Zn(OH)42 may get adsorbed on the surface of ZnO, suppressing the growth in the (001) direction for the AL3 sample. The broad amorphous peak observed in Figure 1b between 20° and 30° (2θ) is due to the glass sample holder. A film of ZnO at the airliquid interface is formed by rearrangement of flowerlike ZnO by the amide formed in the aminolytic reaction (eq 6). As per our previous studies,18 the formation of ZnO goes through an aminolytic reaction as given in eq 6,
wherein the amide formed facilitates formation of a thin layer of ZnO at the airliquid interface. Initially, the amide formed brings along with it flowerlike ZnO to the surface of the liquid and forms a layer, and with time it grows toward the liquid side. The molecules are also held together by the intermolecular and intramolecular hydrogen bonding. This fact is confirmed from the FTIR spectra. Figure 2 shows comparative FTIR spectra of as-prepared ZnO (ZM4), ZnO at the airliquid interface and in the sediments. The broad band observed at 3370 cm1 is due to the hydrogen bonded OH and NH stretching frequencies. This broad band also includes the CH stretching frequencies. 12753
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Figure 2. FTIR spectra showing the functionality changes in the layer at airliquid interface and in the sediments as compared to as-prepared flowerlike ZnO.
This band is common in all the FTIR spectra. This broad spectrum is sharper for the layer at the airliquid interface, indicating a decrease in the hydrogen bonded OH functionality. The FTIR of the layer at the airliquid interface and in the sediment shows major differences. The band at 2935 cm1 corresponds to the methylene (>CH2) CH asymmetric/ symmetric stretch, and the band at 2850 cm1 is attributed to the methyl (CH3) CH asymmetric/symmetric stretch; the intensity of these two bands is significant in the layer at the airliquid interface and is insignificant in the sediments. The weak band at 1640 cm1 in the sediment and ZM4 may be assigned to the NH bending in the primary amine. This band disappears in the layer formed at the airliquid interface, and a distinct band appears at 1750 cm1. This frequency is attributed to the amide group (CONH, eq 6). The bands at about 1555 and 1400 cm1 are attributed to COO stretching modes in the acetate group, indicating the presence of acetate groups attached to ZnO.18 The band at 1388 cm1 is due to the H3N+ ion in H3N+CH2CH2OH (eq 2), and the 1340 cm1 frequency is due to CN stretching. The weak bands in the region 8001200 cm1 are attributed to CO, CC single bond, and NO aliphatic amine oxide stretching, and from NH bending in primary amine, indicating the role of amine in the formation of flowerlike structures. The broad band at 895 cm1 is clearly visible in the case of sediments, whereas for the layer at the airliquid interface a single peak disappears and a multiplet
appears. The formation of a broad band suggests intermolecular and intramolecular bonding due to the presence of amine and hydroxyl groups. The presence of the NH and OH stretching frequencies for the ZnO samples in the FTIR spectra prove the role of EA in the formation of flowerlike structures of zinc oxide and supports the formation of ZnO layers at the airliquid interface by rearrangement through amide (eq 6). The band at ∼480 cm1 is attributed to ZnO. Thus, we propose the formation of ZnO at the airliquid interface through two processes: (1) aggregation of flowerlike ZnO due to the presence of amide formed in the aminolytic reaction in aqueous medium directing ZnO formation at the air liquid interface and (2) further growth of flowerlike ZnO toward the liquid side by dissolutionreprecipitation mechanism. Figure 3 shows SEM images of ZnO at the airliquid interface and of the sediments. The SEM images of ZnO at the airliquid interface clearly show the aggregation of flowerlike structures. It can be seen that the flowerlike structures aggregate by stacking of one layer over the other. With time, these layers get attached to one another by the intermolecular and intramolecular hydrogen bonding. Each petal is of ∼1 μm in length with a tapering tip. The SEM images of ZnO in the sediments show the formation of sheetlike structures along with the flowerlike structures. This is probably due to dissolutionreprecipitation at alkaline pH. Figure 4 shows the TEM images of flowerlike ZnO formed at the airliquid interface. Each individual flowerlike structure is 12754
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Figure 3. SEM images of samples at the airliquid interface and in the sediments collected at different time intervals.
formed by overlapping of nanobelts. The SAED pattern for the nanobelt indicates that the nanobelt is single crystalline and can be indexed as the hexagonal ZnO phase. This is in accordance with the XRD results implying that the flowerlike ZnO film consists of nanobelts with a single crystal hexagonal phase. The SAED pattern also shows diffraction from zinc hydroxy acetate groups as well as its presence along with ZnO in the layer at the airliquid interface. This is also supported by the FTIR results. With time, the surfaces of the nanobelts get curved due to the fact that once the layer is formed at airliquid interface, further growth of ZnO layer takes place from the solution side and hence the surface of the nanobelt goes on getting a curvature. The TEM images for the S1 and S4 samples clearly indicate that the individual flowerlike structures merge to form sheetlike structure in an alkaline medium by dissolutionreprecipitation mechanism. Figure 5 shows the RT-PL spectra of ZM4, ZnO at the airliquid interface and in the sediments. Figure 5a,b shows the solid state RT-PL spectra of ZnO with an excitation wavelength of 325 nm. The sharp peak at 349 nm corresponds to the band emission. There is no shift in the emission peak position with time in the layer at the airliquid interface as well as in the sediments. The band gap of ZnO is calculated to be 3.56 eV. Thus, we assign a blue shift in the band emission from 3.37 eV in bulk ZnO to 3.56 eV in the flowerlike ZnO. The confinement effect observed may be attributed to the typical flowerlike
morphology, wherein each flowerlike structure is formed from crystalline nanobelts. Thus, it supports synthesis of flowerlike ZnO with modified electronic properties as compared to bulk ZnO. Figure 5c shows solid state RT-PL spectra of the flowerlike ZnO at the airliquid interface, and Figure 5d shows the RT-PL spectra of the flowerlike ZnO at airliquid interface dispersed in ethanol with an excitation wavelength of 370 nm. Figure 5c shows defect related peaks at 405 nm (3.07 eV) and 425 nm (2.92 eV). The intensity of the peak at 405 nm is seen to increase for the AL2 and AL3 samples. There is no significant change in the intensity of the peaks in the AL1 and AL4 samples. The defect related peaks are clear and intense as compared to ZM4. The other defect related peaks at 448 nm (2.77 eV), 473 nm (2.63 eV),21 and 522 nm (2.38 eV)22 disappear in the layer at the airliquid interface. Figure 5d shows a single broad peak at 420 nm (2.96 eV) for all the samples obtained from the air liquid interface (AL1-AL4), thus showing a blue shift in the emission peak from 440 nm (2.82 eV) for ZM4 to 420 nm in ZnO at the airliquid interface. Also other defect related peaks in the visible region disappear in the ZnO samples at the airliquid interface. Figure 5e shows solid state RT-PL spectra of the flowerlike ZnO in the sediments, and Figure 5f shows the RTPL spectra of the same dispersed in ethanol. Figure 5e shows the increase in the intensity of the defect related peaks in the sediment layer as compared to ZM4. S1 (Figure 5f) shows a 12755
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Figure 4. TEM images and SAED pattern of samples at the airliquid interface and in the sediments collected at different time intervals.
blue shift from 440 nm (2.82 eV) for as-prepared ZnO to 420 nm (2.96 eV) with complete disappearance of the other defect related peaks. Then onward with time (S2S4) a systematic red shift occurs along with the appearance of other defect related peaks. As we reported earlier,18 photocatalytic activity follows a pathway through the formation of hydroxyl radical as an oxidizing intermediate. UV irradiation of ZnO catalyst leads to the formation of electron (e(CB)) and hole (h+(VB)) pairs. Some of the electronhole pairs recombine, while others lead to the formation of hydroxyl radical which is a strong oxidizing agent for the dye. Figure 6 shows the degradation of MB over ZnO catalyst. The photocatalytic activity of the samples at the airliquid interface shows higher catalytic activity than that of the asprepared ZnO (ZM4) sample; this may be attributed to the defects corresponding to 3.07 and 2.92 eV energy. These high energy defects promote charge separation in the sample which increases the catalytic activity through hydroxyl radical formation. Also, significant interaction of adsorbates on the surface may probably lead to high photocatalytic activity. The general behavior of the photocatalytic activity of ZM4, AL1, and AL4 is the same during the initial time; more precisely there is a steady decrease followed by a plateau. However, AL4 catalyst shows the highest photocatalytic activity; more than 80% of the dye gets oxidized within 10 min, and after 10 min slow degradation occurs
and requires nearly 50 min to get nearly complete degradation. In comparison, the sedimented samples steadily degrade the dye and require nearly 50 min for complete degradation of the dye. Thus, the increase in the photocatalytic activity of the layer at the airliquid interface with respect to ZM4 can be attributed to the increased intensity and blue shift of the defect related peak in the visible region. S1 and S4 have comparable photocatalytic activity. If we consider the surface area of ZM4 (31 m2/g), S1 (14 m2/g), and S4 (12 m2/g), the increase in the catalytic activity is not dependent on the surface area in the present case. Thus, the increase in catalytic activity is attributed to the defects present on the surface of ZnO synthesized. The visible emission observed in the RT-PL spectra of ZnO (Figure 5) in addition to the UV excited emission peaks are due to the intrinsic or extrinsic defects. The emission is dependent on the type of defect rather than the morphology. According to the earlier reported work, the defects can greatly affect the optical, electronic, magnetic, and photocatalytic properties of ZnO.2326 Thus, we have correlated photoluminescence and photocatalytic activity. Understanding this relationship will provide insight into the origin of defect emission and chemistry behind photocatalysis. And hence this will advance our ability to utilize ZnO. Figure 5c shows the RT-PL spectra of solid state ZnO at the airliquid interface. The emissions corresponding to ∼3.07 eV 12756
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Figure 5. Solid state RT-PL spectra of ZM4 in comparison with (a) ZnO at the airliquid interface and (b) in sediments with λexc = 325 nm, (c,d) ZnO at the airliquid interface and in sediments with λexc = 370 nm. (e, f) RT-PL spectra of ZM4, ZnO at the airliquid interface, and in the sediments dispersed in ethanol and excited at a wavelength of 370 nm.
(405 nm) and 3.03 eV (410 nm) are mainly due to the zinc vacancy (VZn), and the emission at 2.92 eV (425 nm) is due to zinc interstitial.25,2729 Zinc vacancies create holes in the valence band (VZn+ hVB+) which facilitate the photocatalytic degradation according to the mechanism studied by us previously.18 If we compare the RT-PL spectra of AL1 and AL4, the intensity of the emission peak at ∼405 is higher for the AL4 sample. This contributes to the high catalytic activity of the ZnO samples at the airliquid interface. The as-prepared ZnO and the sediment samples show a very weak emission at ∼410 nm (∼3.03 eV)
which indicates a decrease in the emission related to the zinc vacancy. The low energy emissions are also attributed to the presence of Zn(OH)2 in ZnO. Figure 1a clearly indicates the presence of the Zn(OH)2 along with ZnO in the sediment samples. FTIR of the samples (Figure 2) indicates the presence of hydrogen bonded OH groups on the surface of the ZnO samples. The broad band due to hydrogen bonded OH groups for the as-prepared ZnO and the sediment samples becomes sharp for the ZnO layer at the airliquid interface. Due to the presence of Zn(OH)2 in the sediment samples and the surface 12757
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Figure 6. Comparative photocatalytic activity of the degradation of MB over as-prepared flowerlike ZnO, ZnO samples in the layer at the airliquid interface and in the sediments.
adsorbed OH on as-prepared ZnO, the luminescence at 410 nm is quenched30 compared to the samples at the airliquid interface. The other low energy defect related emission peaks (∼473 and ∼522 nm) present in the sediment layer are absent in the ZnO samples at the airliquid interface. Moreover, in nanocrystalline ZnO, the sub-bands formed due to surface defects facilitate separation of charge carriers increasing its lifetime, leading to enhanced photocatalytic activity.
’ GROWTH MECHANISM The growth rate of a crystal face is usually related to its surface energy, if the same growth mechanism acts on each face. The fast growing faces have high surface energies disappear in the final morphology, and vice versa. This treatment assumes that the equilibrium morphology of a crystal is defined by the minimum energy resulting from the sum of the products of the surface energy and the surface area of all exposed faces (Wulff rule).31 The driving force for this spontaneously oriented attachment is the elimination of the pairs with a high surface energy leading to the substantial reduction in the free energy of the surface from the thermodynamic viewpoint.3234 Wurtzite ZnO being a polar crystal, Zn forms a positive polar plane (0001) and O forms a negative polar plane (000i). Zn2+ and O2- ions are tetrahedrally coordinated and stack alternatively along the c-axis, and thus, ZnO grows along the c-axis. When EA is added in the aqueous solution, it gets hydrolyzed according to eq 2 and forms the EAH+ molecule. Thus, by coulomb interaction, the EAH+ molecule gets adsorbed on the negative polar plane, retarding the growth of ZnO along the negative polar plane. Therefore, an optimum amount of EA is used which covers the side surfaces of ZnO crystal, enhancing growth along the (001) direction. When EA concentration is low, that is, inadequate to cover the whole surface, Oswald ripening takes place and thereby the role of EAH+ in the growth of ZnO crystal results in the formation of flowerlike structures where individual petals are formed by the overlay of nanobelts as seen in Figure 4. Initially the ZnO thin layer is formed by the diffusion of ZnO particles at the airliquid interface via amide (eq 6). This is supported by the FTIR results. Further the growth units could diffuse only onto the ZnO contacted with water and grow freely. Growth does not occur on the ZnO crystal facing toward air.
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Due to the alkaline conditions, dissolutionreprecipitation occurs, favoring growth of ZnO on the surface in contact with the aqueous solution. Thus, the petals are curved laterally on the side in contact with the aqueous solution (Figure 4). Dissolution reprecipitation mechanism can be confirmed from the RT-PL spectra of the samples in the sediments and the layer at the airliquid interface. The enhanced color intensity of the aqueous solution with time seen in the photograph (Figure 2) supports the dissolutionreprecipitation mechanism. The growth of ZnO units is favored in the (001) direction as can be clearly observed from the XRD of AL2 and AL3 samples. The growth of ZnO at airsolution interface takes place in the (001) direction to minimize the total surface energy35 of the polar planes. After 21 days (AL3), though the growth in the (001) direction is favored, the (100) and (101) peak intensity is also seen to be increased as compared to the XRD after 14 days (AL2). After 28 days (AL4), the growth is seen to take place along all the crystallographic directions.
’ CONCLUSIONS The facile low temperature (4 °C) synthesis using ZA and EA produces flowerlike ZnO. Flowerlike ZnO synthesized in this study self-assembles at the airliquid interface via the amide formed in the aminolytic reaction and by dissolution reprecipitation mechanism. TEM characterization shows that each petal of the flowerlike structure is formed from single crystalline nanobelts with hexagonal ZnO and zinc hydroxyl acetate phase. Simultaneous evolution of Zn(OH)2 and ZnO in the sediments is evidenced from the XRD and FTIR studies. The RT-PL of as-prepared flowerlike ZnO, ZnO at airliquid the interface and in the sediments shows a blue shift in the band emission from 3.37 eV (bulk ZnO) to 3.56 eV. Flowerlike ZnO at the airliquid interface and in the sediments shows a blue shift from 440 to 420 nm with respect to as-prepared flowerlike ZnO in the defect related peaks in the visible region. The photodegradation of MB over the flowerlike ZnO catalyst formed at the airwater interface and in the sediments shows enhanced photocatalytic activity as compared with as-prepared flowerlike ZnO. Almost 80% of the dye is degraded by the AL4 sample in 10 min. The high catalytic activity of AL4 and other samples at the airliquid interface and in the sediments is attributed to the zinc vacancy (3.06 eV) and other low energy sub-bands (3.03, 2.92, 2.77, 2.63, and 2.38 eV) formed due to defects. ’ ASSOCIATED CONTENT
bS
Supporting Information. Figure 1 shows SEM images of as-prepared flowerlike ZnO washed with ethanol and with an organic solvent which proves the role of ethanolamine as structure directing agent. Figure 2 shows the photograph of the separation of layers at the airliquid interface and in the sediments from the as-prepared flowerlike ZnO with time; also the solution between the layers becomes intensely colored with time. Figure 3 gives the UVvis absorption spectra of the solution between the layers with time. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: +91-020-25691728. Telephone: +91-020-25601394-573, 569, 532. E-mail:
[email protected]. 12758
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’ ACKNOWLEDGMENT M.V.V. wishes to thank the Council of Scientific and Industrial Research (CSIR), Delhi, India, for the Senior Research Fellowship. I.S.M. is grateful to CSIR, Delhi for awarding Emeritus Scientist Scheme.
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dx.doi.org/10.1021/la203006n |Langmuir 2011, 27, 12751–12759
