DNA-Mediated Fast Synthesis of Shape-Selective ZnO Nanostructures

Article pubs.acs.org/IECR

DNA-Mediated Fast Synthesis of Shape-Selective ZnO Nanostructures and Their Potential Applications in Catalysis and Dye-Sensitized Solar Cells Subrata Kundu* and U. Nithiyanantham Electrochemical Materials Science (ECMS) Division, CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi - 630006, Tamil Nadu, India S Supporting Information *

ABSTRACT: Shape-selective ZnO nanoparticles (NPs) with various morphologies have been synthesized within 2 min of microwave heating by the reaction of Zn(NO3)2·2H2O with NaOH in the presence of DNA. The size and shape of the materials can be tuned by controlling the molar ratio of Zn(II) salt to DNA and by altering the other reaction parameters. The role of DNA and other reaction parameters for the formation and growth mechanisms of different morphologies has been elaborated. The potentiality of the DNA−ZnO NPs has been tested in the catalysis reaction for the decomposition of toxic KMnO4, and the effect of different morphologies on the catalysis reaction has been examined. Moreover, the suitability of the materials is also tested for dye-sensitized solar cell (DSSC) applications, and it was observed that all the morphologies of ZnO NPs can be used as a potential anode material in DSSC applications.

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INTRODUCTION Research on nanocrystalline materials has received much attention by worldwide materials scientists due to their uncommon property compared to their bulk phase in the last several years.1 The intense investigations are stimulated by several envisaged application areas for this new class of materials. For example, metal, semiconductor, and oxide materials at the nanoscale dimension exhibit novel optical, electrical, and mechanical properties which are useful for different types of applications such as photovoltaic solar cells,2 light-emitting diodes,3 varistors,4 ceramics,5 devices,6 catalysis, and sensors.7 Thus, it is important to develop a facile approach for the synthesis of size- and shape-selective nanostructures in shorter time scale and in a controlled fashion. Zinc oxide (ZnO), a group II−VI semiconductor, has a direct wide band gap (Eg = 3.37 eV) and a notable piezoelectric behavior. ZnO nanomaterials have been extensively studied due to their unique properties and potential application in various fields such as photonic catalysis,8 nanogenerators,9 photocatalysts, and in solar cells.2,10,11 All these applications are strongly dependent on the size, shape, composition, and morphologies of the micro/nanostructures.12−14 Thus, the design of ZnO nanostructures a with variety of shapes plays a critical role in the basic fundamental research as well as the development of novel devices. Nature provides a wide range of biomacromolecules (e.g., proteins, peptides, amino acids and nucleic acids) which can act as scaffolds, templates, and stabilizers for the formation of new hybrid organic−inorganic nanostructures with a specific shape and unique properties.15−17 Among the different biomacromolecules, deoxyribonucleic acid (DNA) has been investigated widely as biological template to construct nanostructures with a specific shape and unique properties. DNA has a linear polynucleotide chain having a width of ∼2 nm and length of ∼0.34 nm per nucleoside subunit. The DNA molecule has two © 2014 American Chemical Society

binding sites; one is a negatively charged phosphate group, and other is the aromatic base molecule. Other important properties of DNA include (i) double helix rigid chain structure gives DNA higher mechanical strength,18 (ii) intermolecular interactions in DNA can be readily programmed and reliably predicted,19 (iii) the versatile chemical structure containing polymeric sequences allows DNA to self-assemble into complex structures such as cube, square, T-junctions, etc.20 Moreover, the backbone of DNA contains phosphate group and sugar molecule which binds with various metal cations or nanoparticles (NPs) by electrostatic interaction.21−23 There are four types of base molecules present in DNA, adenine (A), thymine (T), guanine (G) and cytosine (C) where A binds with T and G binds with C. This versatile chemical nature makes DNA an effective genetic material for programmed self-assembly. Different successful strategies have been employed for the synthesis of ZnO NPs including high-temperature methods, hydrothermal routes, template-directed synthesis, self-assembly of nanocrystals, chemical and physical vapor deposition (CVD and PVD), etc. Different shaped ZnO nanostructures such as nanowire,24 nanorod,25 nanoribbon,26 nanoplate,27 nanotube,28 and multipod29 have been reported. Xu et al. synthesized hierarchically assembled ZnO NPs in P-123 copolymer.30 Meulenkamp synthesized 2−7 nm ZnO NPs using LiOH and Zn-acetate as precursor material.31 Ahmed et al. synthesized uniform ZnO NPs by the thermal decomposition of Zn-oxalate nano rods.32 Several other research groups synthesized ZnO nanostructures using templates or electrochemical process or using other routes.33−37 Recently, Du et al. demonstrated a one-pot hydrothermal approach for selective synthesis of Received: Revised: Accepted: Published: 13667

January 28, 2014 August 1, 2014 August 13, 2014 August 13, 2014 dx.doi.org/10.1021/ie500398q | Ind. Eng. Chem. Res. 2014, 53, 13667−13679

Industrial & Engineering Chemistry Research

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Table 1. Detailed Final Concentrations of All the Reaction Parameters, Time of MW Irradiation, Solution pH, Particle Sizes, Shapes, etc. set no

stock conc. of DNA solution (M)

final conc. of DNA solution (M)

final conc. of Zn (II) ions (M)

final conc. of NaOH (M)

pH of the solution mixture

time of MW irradiation (min)

shape of the ZnO NPs

diameter of the ZnO NPs (nm)

shape distribution

1

∼2.5 × 10−3

4.9 × 10−5

8.0 × 10−2

2.0 × 10−1

6.84

2

wire -like

∼150 ± 15

2

∼2.5 × 10−3

2.1 × 10−5

8.6 × 10−2

1.3 × 10−1

6.54

2

flake-like

∼80 ± 10

3

∼2.5 × 10−3

1.0 × 10−5

8.6 × 10−2

1.3 × 10−1

6.32

2

flower-like

100% wirelike 90% flakelike 100% flowerlike

complex ZnO architectures.38 Tong et al. synthesized complex ZnO architectures with tunable morphologies and structures by modulating the base type and molar ratio of base to Zn2+ ions using a hydrothermal approach.39 Hu et al. describe a facile one-step ethanediamine (en)-assisted hydrothermal approach for the selective synthesis of ZnO architectures with different morphologies.40 They examined the role of different kinetic factors for the generation of various morphologies although it still remains a challenge to develop a facile, fast, and lowtemperature method without using any toxic chemicals and organic additives to design size and shape-selective ZnO nanostructures. To overcome the issues related to long reaction time, high temperature, or multiple-step reactions in conventional hydrothermal processes, we used a microwave solvothermal heating route which is quite faster, simpler, and energy efficient. Microwave heating has several unique advantages such as rapid and homogeneous heating, high reaction rate, enhanced reaction selectivity, and ability to generate uniform nucleation sites. Since the first discovery of microwave heating-assisted synthesis in 1986,41 the application of microwave heating in the synthesis of inorganic materials has been rapidly growing.42,43 The exact nature of microwave interaction with reactants during the synthesis of materials is still unclear and speculative. Energy transfer from the microwave to the materials is believed to occur through resonance or relaxation which results in rapid and concentrated heating. Overall, microwave heating can shorten the reaction time by a few orders of magnitude compared to conventional heating. Kundu et al. synthesized earlier uniform Au nanoprisms,44 Ag nanocubes,45 and CdS nanowires46 by microwave heating. Ma et al. synthesized ZnO nanostructures using microwave heating for 10 min by the reaction of Zn(NO3)· 6H2O with pyridine.47 To the best of our knowledge, there is little information available in the literature for the formation of shape-selective ZnO nanostructures using DNA as scaffold within 2 min of reaction time. Moreover, there are no reports for the potential application of these DNA−ZnO shapeselective nanostructures in catalysis study nor in dye-sensitized solar cells (DSSC) applications. In the present study, we report for the first time, the synthesis of shape-selective ZnO nanostructures within 2 min of microwave heating using DNA as a scaffold. The synthesis was done by the reaction of Zn(NO3)2·6H2O with NaOH in the presence of DNA under continuous stirring. The process exclusively generates uniform ZnO NPs with wire-like, flakelike, and flower-like morphologies. The size and shape of the synthesized NPs can be easily tuned by changing the concentrations of the reagents and controlling the other reaction parameters. The shape-selective DNA−ZnO NPs exhibit excellent catalytic properties and can be used as a potential anode materials for DSSC applications. For the

∼350 ± 50

catalysis study, we have tested the decomposition of toxic KMnO4 using DNA−ZnO NPs as catalyst, and the reaction generates the formation of nontoxic MnO2 NPs. The shapeeffect of the ZnO NPs for the catalysis study has also been investigated, and the order of catalytic activity follows as wirelike > flower-like > flake-like. Apart from the catalysis, a preliminary study has been conducted for DSSC applications, and the observed efficiency was found highest in the case of flower-like morphology compared to others. The proposed synthesis method is simple, reproducible, less time-consuming, and environmentally friendly.

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EXPERIMENTAL SECTION Reagents and Instruments. Double-stranded herring testes DNA with an average molecular weight ∼50 K bp (base pair), zinc(II) nitrate, hexahydrate [(Zn (NO3)2·6H2O] and sodium hydroxide (NaOH) were obtained from SigmaAldrich and used as received. Potassium permanganate (KMnO4), lithium iodide (LiI), iodine(I), hexachloroplatinic acid (H2PtCl6), ditetrabutylammonium cis-bis-(isothiocyanato)-bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II) (also called N719 dye) and fluorine-doped tin oxide (resistivity 7Ω/square) were purchased from Sigma-Aldrich. Acetonitrile was purchased from Fisher Scientific and used as received. Polyethylene glycol (PEG 400 MW) was purchased from S.D. Fine Chemical Limited. Absolute ethanol was purchased from the local Balaji Scientific Company, Karaikudi, Tamilnadu. Deionized (DI) water was used for the entire synthesis and catalysis work. The synthesized shape-selective DNA−ZnO nanomaterials were characterized using several spectroscopic techniques such as UV−visible (UV−vis) absorption spectra, transmission electron microscopy (TEM) analysis, field emission scanning electron microscopy (FE-SEM) analysis, energy dispersive X-ray spectroscopy (EDS) analysis, LASER Raman measurements, X-ray diffraction (XRD) analysis, Fourier transform infrared (FT-IR) spectroscopy analysis, photoluminescence (PL) study, and thermal analysis study. The details of all these instruments specifications are given in the Supporting Information (SI). Synthesis of DNA−ZnO Nanostructures Using Microwave Heating. ZnO nanostructures having different morphologies have been synthesized by the reaction of Zn(NO3)2· 6H2O and NaOH in the presence of aqueous DNA solution under microwave heating for 2 min. For a typical synthesis, 100 mL of DNA solution (conc. ≈ 3 mg/50 mL) was mixed with 50 mL of Zn (NO3)2·6H2O solution (conc. ≈ 0.4 M) and stirred well. Then the reaction mixture was placed in a microwave for 20−30 s and after that taken out from the microwave and stirred continuously. At that hot condition, we mixed 30 mL of 1 (M) NaOH solution gradually. After adding NaOH, a white colored Zn(OH)2 solution was formed. The solution mixture 13668

dx.doi.org/10.1021/ie500398q | Ind. Eng. Chem. Res. 2014, 53, 13667−13679


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Scheme 1. Overall Preparation Process for the Synthesis of Shape-Selective DNA−ZnO Nanostructures Using 2 Min of MW Heating

using 0.5 mL of polyethylene glycol and 9.5 mL of DI water using a mortar and pestle and mixed well. Now, the ZnO paste was spread over the conducting side of the FTO substrate covering an area of 0.6 × 0.6 cm2 and thickness of ∼12 μm using the doctorblade method. The thickness and area were controlled by using cellophane tape. Initially, the glass slide was dried at 80 °C in hot air oven for 30 min followed by sintering at 400 °C for 20 min. After the annealing process, the DNAbased ZnO thin film was immersed in the dye solution and left for 4 h. The dye-adsorbed ZnO thin film looks nearly maroon in color. Excess nonadsorbed dye was removed with ethanol. The counter electrode was fabricated by spin coating using hexachloroplatinic acid solution. Before the coating process, it is necessary to make two small holes (size of 1 mm diameter) over the FTO glass substrate. The Pt-coated thin film was annealed at 400 °C for 30 min. The electrolyte solution was prepared with 0.3 M LiI and 0.03 M of iodine solution in acetonitrile. The prepared electrolyte was injected into the space between the photoelectrode and the counter electrode through the predrilled hole, and the hole was closed by doublesided tape. The efficiency of the fabricated materials was measured using I−V characterization.

was placed in the microwave again and heated for 2 min with an intermittent pause for 2 min after every 30 s. After completion of the reaction, the solution mixture was taken out of the oven and stirred for another 20 min. Then the reaction mixture was cooled at room temperature (RT)and centrifuged 3−4 times at 8000 rpm, and the solid precipitate was dried at RT. Finally, the dried white mass was transferred to hot air oven and annealed for 1 h at 250 °C. The white-colored dried mass was collected and used for various characterization and application purposes. The solution exclusively contains ZnO NPs having wire-like shapes (set 1). We prepared other sets by changing the concentration of DNA to Zn(II) salt, and the final concentration of all the reagents, solution pH, particles sizes, shape are summarized in Table 1. Scheme 1 shows the stepwise formation process of three different sets of DNA−ZnO nanostructures. The DNA−ZnO nanostructures were characterized using UV−vis, TEM, FE-SEM, photoluminescence (PL), EDS, XRD, Raman, FT-IR, and thermal analysis studies, and the details for sample preparation techniques for various characterizations are given in the SI. Catalytic Study for the Decomposition of KMnO4 Using DNA−ZnO NPs As Catalyst. The catalytic activities of DNA−ZnO thin film have been tested for the decomposition of aqueous KMnO4 solution. A glass slide coated with ZnO thin film (dimension ∼2 cm2) was placed in an aqueous solution of KMnO4 (10 mL, 10−4 M) inside the quartz cuvette. The pH of the stock KMnO4 solution was 6.51, and the pH of the mixture of samples containing KMnO4 and DNA−ZnO thin films was maintained near to 7 by adding NaOH solution. The catalysis reaction was done at room temperature (RT) ∼30 °C. We did a preliminary study with as-synthesized DNA−ZnO NPs (not annealed) keeping similar reaction conditions. Moreover, a comparison study with three differently shaped DNA−ZnO NPs were also examined. Fabrication of Electrodes for DSSC Applications. For DSSC sample preparation, the thickness of the working electrode was measured by SJ-301 surface roughness tester. The conductivity (current−voltage, I−V) measurement was performed by using a solar simulator having model number SS80AAA under the light illumination of 1000 W/m2. The synthesized DNA−ZnO nanomaterial was used to make a paste

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RESULTS AND DISCUSSION UV−Vis Spectroscopic Analysis. Figure 1 shows the UV− vis absorption spectra of the different solution mixtures for the synthesis of ZnO NPs in DNA scaffold at RT. The UV−vis absorption spectrum provides a convenient way to investigate particle growth. Figure 1, curve A shows the absorption band of only aqueous DNA solution which has a band maximum at 260 nm due to absorption of aromatic base molecules on DNA. Aqueous Zn (NO3)2·6H2O salt solution has a λmax at 301 nm (curve B, Figure 1) due to ligand to metal charge transfer (LMCT) spectra.48 Mixing DNA with Zn(II) salt solution shows an absorption peak in the range 260−315 nm (curve C, Figure 1) due to interaction of positively charged Zn2+ ions with negatively charged DNA molecule. Curves D, E, and F in Figure 1 show the UV−vis spectra for shape-selective ZnO NPs and all the absorbance bands show similar types of spectral feature. Curve D, Figure 1 shows the excitonic absorption band for ZnO NPs on DNA having wire-like shapes. A sharp band 13669



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high magnified FE-SEM images of flower-like ZnO nanostructures. From the images we can clearly see that the nanoflowers are composed of small ZnO particles having an average diameter ∼20 ± 5 nm, whereas the average diameter of the nanoflowers is ∼350 ± 50 nm. Figure S-1 (SI) shows the transmission electron microscopy (TEM) images of three differently shaped ZnO NPs synthesized using DNA as scaffold. A and B of Figure S-1 in the SI show the low magnified TEM images of ZnO nanowires from the different parts of the sample where the insets of each image show the corresponding, more highly magnified image. The inset of Figure S-1B in the SI also shows the selected area electron diffraction (SAED) pattern which confirms the particles are crystalline in nature. The wires are not fully uniform, and the average diameters of the wires are ∼ 150 ± 15 nm. Figure S-1C and S-1D in the SI show the low and high magnified ZnO NPs having flake-like shapes. The average diameters of the flakes are ∼80 ± 10 nm. The inset of Figure S-1D in the SI shows the corresponding SAED pattern which confirms the crystalline nature of the particles. Figure S1E in the SI shows the low magnified TEM image of ZnO NPs having a flower-like structure where we can see a few nanoflowers assembled together, whereas Figure S-1F in the SI shows the image of a single nanoflower. Careful observation shows that the nanoflowers are composed of smaller sized spherical ZnO particles which are better viewed from the FESEM image in Figure 2. The inset of Figure S-1F in the SI shows the corresponding SAED pattern which reveals the particles are crystalline in nature. The average diameter of the ZnO nanoflowers are ∼350 ± 50 nm which consist of smaller ZnO particles having an average diameter ∼20−25 nm. From analyzing Figure 2 and Figure S-1 in the SI, we can confirm that our proposed reaction generates differently shaped ZnO particles by varying the reaction conditions and both the FESEM and TEM analysis match well with each other. Energy Dispersive X-ray Spectroscopy (EDS) Analysis. The energy dispersive X-ray (EDS) analysis was used to detect the elements present in the synthesized nanomaterials solution. Figure S-2 (in the SI) shows the EDS analysis of ZnO NPs on DNA. The spectrum consists of the peaks for Zn, O, Si, and P. The Zn and O peak came from the ZnO NPs samples, whereas the Si peak came from the glass substrate used to deposit ZnO NPs for EDS analysis during FE-SEM. The P peak came from the DNA which is used during the synthesis and acts as a stabilizing agent for the stabilization of ZnO NPs. X-ray Diffraction (XRD) and Thermal (TGA-DTA) Analysis. X-ray diffraction patterns of three differently shaped ZnO NPs on DNA scaffold are shown in Figure 3. In all the cases the nanomaterials are synthesized by microwave heating for 2 min. Curves A, B, and C of Figure 3 are the diffraction patterns for wire-like, flake-like, and flower-like ZnO NPs, respectively. In all the cases, the diffraction pattern consists of the peaks originating from the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202) planes of hexagonal wurtzite structure of ZnO and in good agreement with the JCPDS file number of ZnO (JCPDS 36-1451).31,49,50 At the lower 2θ region below 30° we have not observed any peak, which indicates the absence of any zinc hydroxide impurity in the sample. No peaks for any other phase of ZnO or impurity were observed in the experimental scale range. In all three cases, the diffraction pattern shows similar types of morphology although the intensity differs. Thus, the pure wurtzite structure of ZnO NPs on DNA scaffold has been successfully synthesized in our present preparation route. In

Figure 1. UV−vis spectrum of the different solution mixtures for the preparation of DNA−ZnO nanostructures under 2 min of microwave heating. (A) Absorption band of DNA only aqueous solution; (B) absorption band of aqueous of Zn (NO3)2·6H2O salt solution; (C) absorption band of the mixture of DNA with Zn(II) salt solution. (D), (E), and (F) excitonic absorption bands for ZnO NPs on DNA having wire-like, flake-like, and flower-like shapes, respectively.

appears at 371 nm due to the excitonic feature of ZnO NPs with a small peak at 260 nm for DNA. Curves E and F in Figure 1 show a red shifting of the excitonic band (compared to curve D), appearing at 384 and 303 nm and due to formation of differently shaped ZnO NPs. Flake-like ZnO NPs on DNA show the excitonic band at 384 nm (curve E), and the flowerlike ZnO shows the same band at 393 nm (curve F). The shifting of the absorption band clearly features the formation of differently sized and shaped NPs. All the absorption peaks of DNA−ZnO NPs are matches with the absorption band of bulk ZnO (at 373 nm, having large excitonic binding energy ∼60 meV). In all the samples, there exists a small peak at around 260 nm due to the presence of DNA. All other differently shaped ZnO NPs show the excitonic feature in the range 370− 395 which matches nicely with earlier literature.31,49,50 We calculated the band gap of our synthesized DNA−ZnO NPs which matches well with bulk ZnO (3.37 eV). There are various ways reported in literature for the measurement of a band gap where an easy and practical method is to equate Eg with the wavelength at which the absorbance is 50% of that at the excitonic peak (or shoulder), called λ1/2 as seen in Figure 1. This graphical procedure allowed calculations of Eg from the UV−vis spectra as reported by others.51,52 Field-Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) analysis. Figure 2 shows the field emission scanning electron microscopy (FE-SEM) images of ZnO NPs having different shapes. Parts A and B of Figure 2 show the low and high magnified FE-SEM images of wire-like ZnO NPs on DNA scaffold. From Figure 2A and 2B we can see that spherical ZnO NPs are grown over the DNA and generate the wire-like morphology. The nominal lengths of the wires are ∼1−2 μm, and the average diameter of the individual ZnO particles on the wires is ∼20−30 nm. The smaller size ZnO particles are assembled together and form the wire-like morphology. Figure 2C and 2D show the FE-SEM image of flake-like ZnO NPs at low and high magnification. The average size length of the flakes is ∼100 ± 20 nm. Figure 2E and 2F show the low and 13670



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Figure 2. Field emission scanning electron microscopy (FE-SEM) images of the three differently shaped DNA−ZnO nanostructures. (A) and (B) show the low and high magnified images of wire-like ZnO nanostructures; (C) and (D) show the low and high magnified image of flake-like ZnO nanostructures; (E) and (F) show the low and high magnified image of flower-like ZnO nanostructures.

spectra at Figure 4A, the emission bands are located at 423, 486 and 530 nm. In Figure 4A, curves a, b and c denote the emission band for wire-like, flake-like and flower-like ZnO NPs, respectively. The UV-emission band with an asymmetrical line shape is attributed to the near-band edge exciton and bound exciton emission. The broad visible emission band located at 529 nm is generally related to defects related to intrinsic defects such as oxygen vacancies, zinc vacancies, oxygen interstitials, and zinc interstitials.53 In the literature there are mainly two mechanisms proposed for visible emission.54 One is for

order to characterize the thermal stability and crystalline condition of ZnO NPs on DNA, a thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were examined with the as-synthesized ZnO samples and a detailed discussion with related Figure is given in the SI as Figure S-3. Photoluminescence (PL) study. Figure 4 shows the photoluminescence (PL) spectra of the ZnO NPs on DNA scaffold. Figure 4A shows the PL emission spectra at an excitation wavelength of 388 nm. Figure 4B shows the excitation spectra which peaks at 388 nm. In the emission 13671



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recombination of a shallowly trapped electron with a deeply trapped hole, and the other is due to recombination of a shallowly trapped hole with a deeply trapped electron. DNA− ZnO NPs with different shapes are highly crystalline in nature. Our PL results were compared with some other reports and those match nicely.53−55 LASER Raman Study. The LASER Raman spectra of the shape-selective ZnO NPs on DNA are shown in Figure S-4 (in the SI). For this study we have used 632.8 nm He−Ne LASER as an excitation source. Curves a, b and c denote the Raman spectra for wire-like, flake-like, and flower-like ZnO NPs, respectively. The spectral features are almost similar for all the cases. The main dominant sharp peak (for all the samples) appeared at 438 cm−1, and another less intense broad peak appeared at 331 cm−1 for all the samples. There is a small intense peak also appearing at 381 cm−1 for the flower-like shape. The Raman signal is normally very sensitive to the structure of crystal as well as to the defect in the crystal structure. The crystal structure of ZnO is wurtzite (hexagonal) which belongs to the C46ν space group having two formula units per primitive cell with all the atoms occupying the C3ν sites. The main dominant sharp peak with high intensity is located at 438 cm−1 and corresponds to the ZnO nonpolar optical phonon E2 mode, which is characteristic of the wurtzite hexagonal phase of ZnO.56,57 Another small intense peak at 331 cm−1 is the second-order Raman spectrum originating from the zone boundary phonons 3E2H-E2L, and another peak at 381 cm−1 (only observed for nanoflower-like structure) is leveled as A1T. Fourier Transforms Infrared (FT-IR) Spectroscopic Analysis. Parts A and B of Figure 5 show the Fourier transform infrared (FT-IR) spectra of bare DNA and DNAbound ZnO NPs at different scales. Figure 5A shows the wavenumber scale from 700 to 4000 cm−1, and Figure 5B shows the wavenumber scale from 400 to 4000 cm−1. In Figure 5A, ‘a’ denotes the FT-IR spectra for bare DNA, and ‘b’ denotes the FT-IR spectra for DNA-bound ZnO NPs having a wire-like shape. As an example we highlight the FT-IR spectra of the wire-like shape although other shaped particles were also tested and the results show similar types of spectra. A detailed comparison between both the FT-IR spectra not only supports the presence of DNA on the ZnO NPs surface but also indicates the nature of the probable interaction taking place among them. A number of specific parts in DNA such as the phosphate backbone and the organic base functionalities are the main candidates for their specific growth and stabilization action. From Figure 5A, in the range 900−1310 cm−1, three sharp peaks are observed (at 912, 1119, and 1310 cm−1) for DNA (curve a) which is attributed to the stretching vibration of the P = O group in DNA molecule. Those specific peaks either shifted or are much weaker for DNA-based ZnO (curve b). The peaks above 1310 cm−1 to 1820 cm−1 are assigned to the ν (C− O-X) mode. In that region we can observe a sharp peak at 1724 cm−1 for bare DNA sample due to carbon stretching which is shifted to 1713 cm−1 in DNA−ZnO samples; in addition there is new peak appearing at 1767 cm−1 for DNA−ZnO NPs. The stretching vibration appeareing in the range 2800−3650 cm−1 is due to the OH group. The OH group stretching appears for DNA at 3600 cm−1, whereas it shifted to 3442 cm−1 for DNA− ZnO NPs. Similarly, the C−H stretching for DNA only appeared at 2816 and 2888 cm−1, whereas for DNA−ZnO samples it appears at 2823 and 2886 cm−1 in addition to a new sharp band appeareing at 2985 cm−1. An extremely high intense

Figure 3. X-ray diffraction pattern of three different shapes of ZnO NPs on DNA scaffold. CurvesA, B, and C are the diffraction patterns for wire-like, flake-like and flower-like ZnO NPs, respectively.

Figure 4. Room-temperature photoluminescence (PL) spectra of shape-selective DNA−ZnO nanostructures. (A) shows the PL emission spectra at an excitation wavelength of 388 nm where a, b, and c denote the emission band for wire-like, flake-like, and flower-like DNA−ZnO NPs, respectively. (B) shows the PL excitation spectra peaking at 388 nm.

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Figure 5. Fourier transform infrared (FT-IR) spectra of bare DNA (curve a) and DNA−ZnO NPs (curve b) having wire-like shapes at different scales. (A) shows the wavenumber scale from 700 to 4000 cm−1, where (B) shows the wavenumber scale from 400 to 4000 cm−1.

peak at 450 cm−1 appearing for DNA−ZnO NPs (see Figure 5B, curve b) sample which is purely absent for bare DNA is attributed to the characteristic absorption of Zn−O bonds that matches nicely with the literature.58,59 From the above analysis we observed that the phosphate group in DNA mainly guides the interaction of DNA with Zn(II) salt by electrostatic interaction and helps the control of growth of ZnO NPs on DNA. As we discussed earlier, the backbone of DNA strand is made by attractive phosphate and sugar moieties, and they join together to form a phosphodiester bond which helps for the binding of ZnO with DNA. Moreover, the binding of ZnO with phosphate increases the polarity of the nearby C−O−X group and generates a change in absorption intensity of the C−O−X mode. So the above FT-IR analysis clearly states that ZnO NPs binds with the DNA molecule via the phosphate and aromatic base molecules and aids in their specific growth with different morphologyies. Mechanisms for the Formation of ZnO Nanostructures in DNA. On the DNA scaffold the shape-selective ZnO NPs are formed by the reaction of Zn(II) salt with NaOH under two min of microwave heating. To get the particles with uniform and definite shape we have conducted few control experiments for the formation of shape-controlled ZnO NPs on DNA by varying the concentration of Zn(II) salt, DNA, NaOH, and microwave heating time. The detailed discussion and related TEM figures (as Figure S-5) are given in the SI. The presence of DNA is extremely important for the shape-

controlled formation of ZnO NPs. In the absence of DNA, only spherical large-size ZnO particles are formed, and they aggregate due to absence of any stabilizer as observed from TEM analysis (not shown here). Moreover, in the absence of NaOH, no ZnO particles are formed in our experimental time scale. From the FT-IR analysis as discussed earlier we have observed that the phosphate group on DNA mostly binds with the Zn(II) ions and control the growth in different shapes. Initially, after addition of Zn(II) salts to aqueous DNA solution, Zn2+ ions are absorbed on DNA due to electrostatic interaction of oppositely charged species. Now, after addition of NaOH, Zn2+ salt forms either Zn(OH)2 or [Zn(OH)4]2−. Upon microwave heating, the above Zn hydroxide complexes break down to generate ZnO nuclei and with time, the small ZnO nuclei grow along the backbone of DNA chains and form the ZnO NPs. Scheme S-1 (in the SI) shows the schematic formation of shape-selective ZnO NPs on DNA. From Scheme S-1 in the SI, we can see initially that Zn(II) ions attach to DNA, then after NaOH is added, they form Zn(OH)2, and then both Zn(OH)2 and excess Zn(II) ions attach onto DNA. Now after heating, the Zn(OH)2 is dehydrated and forms ZnO and stabilizes on the DNA. At high DNA concentration, the ZnO nuclei homogeneously grow on the DNA chain and produce wire-like morphology. Once the DNA concentration is low and Zn(II) concentration is high, the ZnO particles assembled in flake-like structures. At comparatively less DNA and NaOH concentration but high Zn(II) salt concentration, 13673



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Figure 6. (A) shows the UV−vis spectra evolutions for the catalytic decomposition of KMnO4 using DNA−ZnO thin films; (B) the left side image indicates the pure KMnO4 solution, the middle image shows the faded color of KMnO4 solution at the middle of the reaction, and the right side image shows the colorless solution after completion of the reaction; (C) shows the ln Abs (at 546 nm) vs time (hours) plot for the decomposition of KMnO4 with DNA−ZnO thin films; (D) shows the XRD pattern of MnO2 NPs evolved by decomposition of KMnO4 solution; (E) shows the TEM image of the evolved MnO2 NPs via decomposition of KMnO4 using DNA−ZnO NPs.

the OH− ions and Zn(OH)2 produced the [Zn(OH)4]2+ growth units. When the OH− ions in the solution reached a certain concentration, the growth units dehydrated into quasispherical ZnO nanocrystals, which ultimately united into spherical aggregates to reduce the surface free energy. These quasi-spherical ZnO nanocrystals initially formed particles and can rapidly grow preferentially along the 001 direction and generate the anisotropic shapes. Moreover, the specific formation of flower-like architectures is generally associated with a synergistically driven assembly mechanism driven by polar molecular interactions and remnant magnetic moment interactions,62 the minimum magnetic anisotropic energy or surface free energy,63 site-specific anisotropic growth,64 selfassembly, and the Ostwald ripening process.65 In our present study we also believe that either site-specific anisotropic growth or the self-assembly process predominate to generate the structurally oriented different morphology such as flake-like or flower-like structures. The wire-like morphology probably generates due to the oriented growth along the DNA chain at higher DNA concentrations, while at other DNA concentrations the process generates the flake-like or flowerlike morphology as already described above. The growth of ZnO nuclei on DNA for shape-selective generation of ZnO NPs are dependent upon the concentration of DNA present in the solution. It is reported earlier that the shape of any nanocrystal depends mainly upon two parameters,66 one is the

the ZnO particles aggregate and generate flower-like structures which is composed of smaller sized spherical ZnO NPs. Hu et al. studied the formation of ZnO architecture by ethanediamine (en)-assisted hydrothermal approach and observed different shapes like nanocones, twinned nanoroses, dispersed microneedles, and flower-shaped architectures.40 The synthesis was done in a Teflon cup in a stainless steel-lined autoclave at 120 °C. Tong et al. described the morphological modulation of the ZnO flower-like architectures simply by regulating the base type and base/Zn2+ molar ratio within ∼2 h reaction time.39 Li et al. described the formation of ZnO nanorods and radial nanoneedles on a large scale using 1-D and 2-D coordination polymer as reactants using a Teflon-lined stainless autoclave at 140 °C for 24 h.60 Mclaren et al. reported a solution-based method for tailoring the degree of extended growth of ZnO nanoparticles (NPs) along the ⟨0001⟩ axis, giving regular hexagonal plate-like and hexagonal rod-like nanocrystals.61 The hexagonal plate-like particles were found to display >5 times higher activity in the photocatalytic decomposition compared to that by rod-shaped particles. This clearly suggests that the terminal polar (001) faces are more active surfaces for photocatalysis than the nonpolar surfaces (i.e., 100, 101). In the mechanistic point of view, as examples by Hu et al.40 initially, Zn2+ ions coordinated with en (ethylene diamine) molecules to form [Zn(en)2]2+. Afterward, the OH− ions reacted with Zn2+ to generate Zn(OH)2. The reaction between 13674



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Table 2. Details about the Catalysis Reaction and the Rate Constant Values name of the molecules potassium permanganate (KMnO4)

final conc. KMnO4 (M) 10−4 10−4 10−4

name and shape of the ZnO NPs used DNA−ZnO NPswire-like DNA−ZnO NPsflake-like DNA−ZnO NPsflower-like

amount of ZnO NPs used in the preparation of thin film (g)

time for complete decomposition (h)

first-order rate constant (k) (h−1)

correlation coefficient (R)

standard deviation (SD)

0.014

3.16

0.26

0.98

0.061

0.014

7

0.12

0.98

0.55

0.014

5

0.17

0.98

0.057

absorption maxima. Figure 6A, shows the UV−vis spectra evolution for the catalytic decomposition of KMnO4. The characteristic absorption bands of KMnO4 appeared at 310 and 350 nm (shoulder) and a set of other peaks at 505, 525, 546, and 566 nm, respectively, due to metal to ligand charge transfer (MLCT) transition (here oxygen to Mn VII transition).67 After immersing the ZnO-coated glass slide into the KMnO4 solution, the intensity of all the absorption peaks started to decrease. The progress of the reaction was monitored by UV− vis spectra where a steady decrease of all the specified bands maxima at 505, 525, 546, and 566 nm was observed, whereas an increase of absorption maxima with shifting of absorbance value was observed at the lower wavelength region. The overall reaction was studied at room temperature (∼28−30 °C), and the pink colors of KMnO4 changed to red, to reddish brown, to a deep-brown color, and finally precipitated as a brownish yellow color at the bottom of the container. From Figure 6A, it is seen that the decomposition reaction is completed in 190 min where the MLCT bands of KMnO4 totally disappeared and new peaks at a wavelength near 365−380 nm appeared due to the formation of MnO2 particles. There is a significant blue shifting of the absorption maxima as the obtained MnO2 was observed up to 150 min of the reaction in comparison to bulk MnO2 (λgap ≈ 380 nm) which is probably due to the presence of small-sized MnO2 particles.68 The overall reaction is completed in 3 h 10 min (∼190 min), and the pink color KMnO4 solution totally faded as seen in Figure 6B. In Figure 6B, the left side image indicates the pure KMnO4 solution, the middle image shows the faded color of KMnO4 solution at the middle of the reaction, and the right side image shows colorless solution after completion of the reaction. From the right side image we can see that the pink color KMnO4 totally degrades and at the bottom we can see yellowish colored MnO2 particles. Figure 6C shows the ln Abs (at 546 nm) vs time (hours) plot for the decomposition of KMnO4 using DNA−ZnO thin films as catalyst. From the linear plot we calculate the rate constant value which is 0.26 h−1. The correlation coefficient (R) value and relative standard of deviation for the measurement was 0.98 and 0.061 respectively. After completion of the reaction, the glass slides were taken out from the solution, and the yellowish precipitate at the bottom was separated out, washed several times in water and dried. The dried mass was analyzed by XRD which confirmed the formation of MnO2 particles. Figure 6D shows the XRD pattern of MnO2 having the JCPDS no. 18-802 and matches with earlier reports.69 Apart from XRD, we also examined the morphology of the MnO2 particles by TEM analysis and observed that the particles are spherical in shape having an average diameter ∼40 ± 10 nm (Figure 6E). The spherical particles are not fully dispersed and they formed an agglomeration structure. The shape-effect of the DNA−ZnO NPs with other shapes was also tested as preliminary experiment, keeping the reaction conditions same. We have

faceting tendency of the stabilizing agent (here DNA), and the other is the rate of supply of metal ions to the different crystallographic planes of ZnO. So we believe that Zn(OH)2 is dehydrated to generate the ZnO nuclei which grow along the DNA chain and results in the different morphology that in turn depends upon the specific concentrations of the DNA and the metal salt in the reaction mixture. Careful observation says that the wires and flowers are composed of small sized spherical ZnO particles and they assembled together in a specific fashion to generate their corresponding morphology. It is important to mention at this point we are not fully clear about the exact reaction mechanism for the shape-selective formation of ZnO on DNA, and further study in near future will help us to reach clear understanding about the growth mechanisms. Taking these three differently shaped ZnO NPs on DNA, we conducted two potential applications. First, we checked the catalytic decomposition reaction of toxic KMnO4 using ZnO thin films as catalyst. Second, we checked the suitability of the synthesized ZnO NPs as anode materials for DSSC applications. Catalytic Study for the Decomposition of KMnO4 Using Shape-Selective ZNO Nanostructures. The potentiality of the DNA−ZnO NPs as a catalyst has been studied for the catalytic decomposition of aqueous KMnO4 solution as an example. For the catalysis study we used both DNA−ZnO nanopowders and DNA−ZnO thin films. For a typical catalysis study, 10 mL of 10−4 (M) KMnO4 solution was taken, and 0.014 g of ZnO nanopowder was added and shaken by hand, and the decomposition reaction was monitored. It was found that the pink color of KMnO4 solution becomes faded with time and after 6−8 h, ∼90−92% of the KMnO4 color was degraded. After keeping the solution for a few more hours, a light yellowish precipitate was formed in the bottom of the container which is due to the formation of MnO2 particles generated via the decomposition of KMnO4. Now, when we used ZnO as powder so the catalyst particles also mixed with the product (here MnO2), separation of the catalyst from the product was very difficult. To overcome this problem, we prepared thin films of DNA−ZnO on glass substrate by spin coating, and details of the thin film preparation is given in the Experimental Section. It is important to be note that the overall reaction was studied taking DNA−ZnO NPs having wire-like shapes although the other two shapes of NPs also were tested as a preliminary study for comparison purposes. The approximate size of the glass substrate was 2 cm2. The whole substrate was coated by the DNA−ZnO NPs and used for the catalysis study. We measured the pH of the different solutions and solution mixtures for the catalysis study. The pH of KMnO4 solution was 6.51, and after mixing with ZnO NPs it was 6.40. Now, the progress of the reaction or the decomposition of KMnO4 was monitored using a UV−vis spectrophotometer by measuring the changes in the specified 13675



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seen that in case of the flower-like shape, the reaction completed in ∼5 h time whereas for the flake-like shape it completed in ∼7 h time. Thus, the order of catalytic decomposition of KMnO4 follows as wire-like > flower-like > flake-like, and the details of the concentration of reagents and the corresponding catalytic rate constants are given in Table 2. In homogeneous catalysis the catalytic rate mainly depends upon the rate diffusion of reactant molecule into the catalyst. Moreover, a few other parameters such as the size and shape of the catalyst, orientation of the catalyst particles, and surface area of the catalyst particles also contribute. So, it is very difficult to claim for which reason the actual catalysis rate is different. A detailed controlled experiment is necessary, which will be focused on in the near future. Furthermore, we also checked the reusability of the catalyst after washing and drying after the first run, and we have seen that the same catalyst can be reused for 4−6 cycles, although the catalytic efficiency is somewhat reduced. Application of the DNA−ZnO Nanostructures in DyeSensitized Solar Cells (DSSCs). Over the last few years a great deal of interest has been focused on the utilization of nanomaterials for making highly efficient dye-sensitized solar cells (DSSC). DSSC belong to a relatively new class of low-cost solar cells, which belong to the group of thin film solar cells. The idea was first discovered in the late 1970s; however, since a Swiss photochemist, Michael Grätzel, published a paper in Nature in 1991, reporting 7% efficiency,70 the interest in the system has grown enormously. DSSC are based on a semiconductor formed between a photosensitized anode and an electrolyte. This cell is composed of a porous layer of metal oxide NPs, covered with a molecular dye that absorbs sunlight. Sunlight passes through the cathode and the conductor, and then withdraws the electrons from the anode, at the bottom of the cell. These electrons travel through a circuit from the anode to the cathode, creating an electrical current. Charge separation occurs at the surfaces between the dye, semiconductor, and electrolyte. The dye molecules are quite small (nanometer sized), so in order to capture a reasonable amount of the incoming light the layer of dye molecules needs to be made fairly thicker than the molecules themselves. For this reason a nanomaterial is used as a scaffold to hold large numbers of the dye molecules in a 3-D matrix by increasing the number of molecules for any given surface area of a cell. Among the different oxide materials, ZnO and TiO2 have been used mostly in DSSC applications.71−76 In this study, we have used the different morphologies of DNA−ZnO particles to examine the performance of the photovoltaic conversion efficiency. Figure 7 shows the I−V characteristics curve for the different morphologies of DNA-based ZnO material. From the curve, the fill factor (FF) and the power conversion efficiency (η) can be calculated by using the following equations: FF =

Vmax × Imax Jsc × Voc

and

η=

Figure 7. I−V characteristics for the DSSC using DNA−ZnO having different morphologies of wire-like, flake-like, and flower-like structures.

Table 3. Details about the Different Parameters Used to Calculate the Efficiency of DSSC shapes of the DNA−ZnO nanostructures

Jsc (mA/cm2)

Voc (V)

FF

η (%)

wire-like flower-like flake-like

8.19 8.97 7.16

0.491 0.513 0.539

0.48 0.46 0.54

1.93 2.11 2.08

like morphology shows the highest efficiency compared to the other morphologies. The flower-like DNA−ZnO structure shows highest efficiency probably due to its high absorption of dye molecules due to its porous structure and high lightharvesting efficiency. Compared to wire-like, the flake-like structure has higher surface area and consequently more dye adsorption capacity, and the corresponding η is higher. The order of DSSC efficiency was found as flower-like > flake-like > wire-like. The results indicate that the dye having a larger absorption coefficient in the long wavelength region would be more beneficial in obtaining higher photocurrents for cells with higher surface area nanostructure. The higher Jsc value for the flower-like structure is due to the high surface area which increases in the injection current from the excited dyes to the conduction band of ZnO. On the other hand, the lower Voc value for the wire-like structure signifies the increase of electron back transfer between I3− ions and conduction band electrons in the ZnO electrode. From Figure 7 and Table 3 we did not observe much difference among the different morphologies of DNA−ZnO nanostructures. Generally, in the fabrication of DSSC, the dye sensitization plays a crucial role in order to obtain good solar conversion efficiency. Moreover, the sensitization process for ZnO is more complex due to the dissolution of the ZnO surface caused by the proton from the acidic carboxyl groups, which subsequently leads to the formation of Zn2+/dye complex in the nanostructured thin film. The dissolution of ZnO and Zn2+/ dye complex formation could cause the filter effect that reduces the conversion efficiency of the DSSC.77 Although these are the preliminary results, we are going to design more electrodes by controlling various parameters by making multilayered structures to get better efficiency which will be discussed in near future.

Jsc × Voc × FF Pin

where, Jsc is the short-circuit photocurrent density; Voc is the open-circuit voltage; FF is the fill factor; Pin is the input power density; Vmax and Imax are the maximum cell voltage and current, respectively, at the maximum power point. For three different thicknesses of the ZnO electrodes the results in terms of the above parameters are presented in Table 3. From Figure 7 and Table 3, it is observed that among the three different ZnO morphologies, the efficiency value is different and the flower13676



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(3) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Light-emitting diodes made from cadmium selenide nanocrystals and a semiconducting polymer. Nature 1994, 370, 354. (4) Lee, J.; Hwang, J.-H.; Mashek, J. J.; Mason, T. O.; Miller, A. E.; Siegel, R. W. Impedance spectroscopy of grain boundaries in nanophase ZnO. J. Mater. Res. 1995, 10, 2295. (5) Mayo, M.-J.; Chen, D.-J.; Hague, D. C. Nanomaterials: Synthesis, Properties and Applications; Edelstein, A. S., Cammarata, R. C., Eds.; Institute of Physics Publishing: Bristol, 1996; Chapter 8. (6) Kundu, S.; Liang, H. Photochemical synthesis of electrically conductive CdS nanowires on DNA scaffolds. Adv. Mater. 2008, 20, 826. (7) Kundu, S.; Wang, K.; Liang, H. Shape-controlled catalysis by cetyltrimethylammonium bromide terminated gold nanospheres, nanorods, and nanoprisms. J. Phys. Chem. C 2009, 113, 5150. (8) Chen, Y.; Bagnall, D.; Yao, T. ZnO as a novel photonic material for the UV region. Mater. Sci. Eng., B 2000, 75, 190. (9) Xu, F.; Sun, L. T. Solution-derived ZnO nanostructures for photoanodes of dye-sensitized solar cells. Energy Environ. Sci. 2011, 4, 818. (10) Zhang, Q. F.; Dandeneau, C. S.; Zhou, X. Y.; Cao, G. Z. ZnO nanostructures for dye-sensitized solar cells. Adv. Mater. 2009, 21, 4087. (11) Jang, E. S.; Won, J. H.; Hwang, S. J.; Choy, J. H. Fine tuning of the face orientation of ZnO crystals to optimize their photocatalytic activity. Adv. Mater. 2006, 18, 3309. (12) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-Dimensional nanostructures: Synthesis, characterization, and applications. Adv. Mater. 2003, 15, 353. (13) Lu, F.; Cai, W.; Zhang, Y. ZnO hierarchical micro/nanoarchitectures: Solvothermal synthesis and structurally enhanced photocatalytic performance. Adv. Funct. Mater. 2008, 18, 1047. (14) Comini, E.; Faglia, G.; Sberveglieri, G.; Pan, Z. W.; Wang, Z. L. Stable and highly sensitive gas sensors based on semiconducting oxide nanobelts. Appl. Phys. Lett. 2002, 81, 1869. (15) Scheibel, T.; Parthasarathy, R.; Sawicki, G.; Lin, X. M.; Jaeger, H.; Lindquist, S. L. Conducting nanowires built by controlled selfassembly of amyloid fibers and selective metal deposition. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4527. (16) Gerstel, P.; Hoffmann, R. C.; Lipowsky, P.; Jeurgens, L. P. H.; Bill, J.; Aldinger, F. Mineralization from aqueous solutions of zinc salts directed by amino acids and peptides. Chem. Mater. 2006, 18, 179. (17) Hinds, S.; Taft, B. J.; Levina, L.; Sukkhovatkin, V.; Dooley, C. J.; Roy, M. D.; Macneil, D. D.; Sargent, E. H.; Kelley, S. O. Nucleotidedirected growth of semiconductor nanocrystals. J. Am. Chem. Soc. 2006, 128, 64. (18) Wirtz, D. Direct measurement of the transport-properties of a single DNA molecule. Phys. Rev. Lett. 1995, 75, 2436. (19) Seeman, N. C. DNA nanotechnology: Novel DNA constructions. Annu. Rev. Biophys. Biomol. Struct. 1998, 27, 225. (20) SanMartin, M. C.; Gruss, C.; Carazo, J. M. Six molecules of SV40 large T antigen assemble in a propeller-shaped particle around a channel. J. Mol. Biol. 1997, 268, 15. (21) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. DNAtemplated assembly and electrode attachment of a conducting silver wire. Nature 1998, 391, 775. (22) Alivisatos, A. P.; Johnsson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Organization of ’nanocrystal molecules’ using DNA. Nature 1996, 382, 609. (23) Mirkin, C. A. Programming the assembly of two- and threedimensional architectures with DNA and nanoscale inorganic building blocks. Inorg. Chem. 2000, 39, 2258. (24) Pan, Z. W.; Dai, S.; Rouleau, C. M.; Lowndes, D. H. Germanium-catalyzed growth of zinc oxide nanowires: a semiconductor catalyst for nanowire synthesis. Angew. Chem., Int. Ed. 2005, 44, 274. (25) Greene, L. E.; Law, M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Low-Temperature wafer-scale

CONCLUSION In summary, shape-selective ZnO NPs with uniform morphology have been synthesized by 2 min of microwave heating. The ZnO NPs with wire-like, flake-like, and flower-like shapes have been prepared by the reaction of Zn(NO3)2·2H2O with NaOH in the presence of DNA under microwave heating. The size and shape of the particles can be tuned by controlling the molar ratio of Zn(II) salt to DNA and by changing the other reaction parameters. The specific role of DNA and other reaction parameters for the growth of ZnO particles with different morphologies has been examined in detail. The potentiality of the material has been tested by two different applications. First, in the catalytic application of the decomposition of toxic KMnO4 and the catalytic reaction results in the evolution of nontoxic MnO2 NPs in the medium. The DNA−ZnO NPs having a wire-like shape have been found to be the best catalyst among those of other morphologies. Second, a preliminary study has been conducted to examine the potentiality of the DNA−ZnO NPs for DSSC applications. It was found that all the differently shaped ZnO NPs can be used as a suitable anode material in DSSC and the flower-like morphology shows better efficiency compared to the others. The present synthesis process is very simple, fast, cost-effective, and environmentally friendly. Other than catalysis and DSSC, the synthesized DNA−ZnO NPs can be employed in other applications such as wastewater treatment, gas sensors, and templates to synthesize other materials with novel morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

The details about instruments used, preparation of samples for various characterizations, study with other reaction parameters, and figures related to TEM analysis, EDS analysis, TGA-DTA analysis, and LASER Raman study are provided. Scheme related to formation mechanism. 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]; [email protected]. Fax: +91-4565-227651. Tel: +91-4565-241487. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS U.N. wishes to thank CSIR-CECRI for research-internship fellowship. S.K. wishes to acknowledge Dr. Vijayamohanan K. Pillai, Director, and Dr. M. Jayachandran, HOD, ECMS Division, CSIR-CECRI for their continuous support and encouragement. The research funding from DST, SERB, New Delhi (DST Fast Track Project number SR/FT/CS-98/2011, GAP 7/12), support from the Central Instrumental Facility (CIF) and help from Mr. A. Rathishkumar (TEM in-charge, CIF) and Mr. J. Kenndy and Mr. R. Ravishankar (SEM incharge, CIF), CSIR-CECRI, Karaikudi are greatly appreciated.

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REFERENCES

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