Microwave-assisted synthesis of layered basic zinc acetate nanosheets and their thermal decomposition into nanocrystalline ZnO
© Tarat et al.; licensee Springer. 2014
Received: 14 August 2013
Accepted: 22 December 2013
Published: 8 January 2014
We have developed a low-cost technique using a conventional microwave oven to grow layered basic zinc acetate (LBZA) nanosheets (NSs) from a zinc acetate, zinc nitrate and HMTA solution in only 2 min. The as-grown crystals and their pyrolytic decomposition into ZnO nanocrystalline NSs are characterized by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL). SEM and AFM measurements show that the LBZA NSs have typical lateral dimensions of 1 to 5 μm and thickness of 20 to 100 nm. Annealing in air from 200°C to 1,000°C results in the formation of ZnO nanocrystalline NSs, with a nanocrystallite size ranging from 16 nm at 200°C to 104 nm at 1,000°C, as determined by SEM. SEM shows evidence of sintering at 600°C. PL shows that the shape of the visible band is greatly affected by the annealing temperature and that the exciton band to defect band intensity ratio is maximum at 400°C and decreases by a factor of 15 after annealing at 600°C. The shape and thickness of the ZnO nanocrystalline NSs are the same as LBZA NSs. This structure provides a high surface-to-volume ratio of interconnected nanoparticles that is favorable for applications requiring high specific area and low resistivity such as gas sensing and dye-sensitized solar cells (DSCs). We show that resistive gas sensors fabricated with the ZnO NSs showed a response of 1.12 and 1.65 to 12.5 ppm and 200 ppm of CO at 350°C in dry air, respectively, and that DSCs also fabricated from the material had an overall efficiency of 1.3%.
81.07.-b; 62.23.Kn; 61.82.Fk
KeywordsZnO Nanocrystalline LBZA Gas sensor Solar cell
ZnO nanomaterials have attracted significant attention over the past 12 years due to a wide direct band gap (3.37 eV), a large exciton binding energy, a large piezoelectric constant and the availability of a vast range of nanostructure shapes . In the last decade, a variety of different techniques have been used to produce ZnO nanoparticles (NPs). Chemical bath synthesis  is a widespread method due its simplicity and low temperature. However, it is a lengthy process, requiring hours or even days. Microwave-assisted solution phase growth, with the microwave energy delivered to the chemical precursors through molecular interactions with the electromagnetic field, leads to rapid reactions. ZnO nanostructures have been produced through microwave-assisted growth in minutes, including nanowires and nanosheets (NSs) [3–5], but the microwave-assisted fabrication of layered basic zinc acetate (LBZA) crystals has not been reported. The thermal decomposition of LBZA into ZnO is an efficient route for low-cost mass production of ZnO nanomaterial, especially for applications requiring a high surface-to-volume ratio [6, 7]. In a previous publication, we described the growth of LBZA nanobelts and their subsequent decomposition into interconnected ZnO NPs and demonstrated their potential for gas sensing . However, the growth of the LBZA NBs took 20 h, similar to previously reported LBZA growth studies [9, 10]. Here, we report on the fabrication of LBZA NSs using a conventional microwave, with the process taking only 2 min. The physical, chemical and optical properties of the LBZA NSs and the ZnO NSs obtained by subsequent air annealing are investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction (XRD) and photoluminescence (PL). We also demonstrate the promising potential of this novel growth process for practical applications by fabricating and testing gas sensing devices and dye sensitized solar cells (DSCs) using ZnO NPs evolved from the NSs.
Without any further purification (purity ≥ 99.0%), 0.1 M Zinc acetate dihydrate (Zn(CH3COO)2.2H2O), 0.02 M zinc nitrate hexahydrate (Zn (NO3)2.6H2O) and 0.02 M Hexamethylenetetramine (HMTA, (CH2)6 N4) from Sigma Aldrich Co. Ltd. (St. Louis, MO, USA) were dissolved in 60 ml deionized water. The resulting solution had a pH of 6.8. It was then placed in a commercial microwave oven at maximum power (800 W, 2,450 MHz) for 2 min. The oven capacity was 25 l and the dimensions of the cavity were 281 × 483 × 390 mm3. This resulted in the formation of a white suspension. The structure and morphology of the products were characterized using AFM (NanoWizard® II NanoScience, JPK Instruments, Berlin, Germany), field emission SEM (Hitachi S4800, Hitachi High Technologies, Minato-ku, Tokyo, Japan), XRD (Bruker D8 diffractometer, Billerica, MA, USA) using CuKα radiation and fitted with a LynxEYE detector and photoluminescence (PL) using a He-Cd laser with a wavelength of 325 nm and a Ocean Optics USB2000+ spectrometer (Dunedin, FL, USA), blazed at 500 nm and calibrated using a standard 3,100 K lamp. The excitation power density was approximately 3 mW/mm2 for all samples, and the PL spectra were corrected for the detection response of the spectrometer. The PL was performed at room temperature and in air and the XRD diffractogram acquired in θ-2θ mode.
Sample preparation for AFM and SEM consisted of dropcasting a 10-μl droplet of the diluted LBZA NSs suspension on clean silicon wafers followed by drying at 60°C. For the PL characterization, the as-grown product was filtered using a vacuum filtration system. A white thin membrane subsequently formed on the filter paper after drying the product at 60°C for 1 h. The LBZA NSs (either in filtered membrane form or deposited on silicon) were then air annealed in a tube furnace at temperatures from 200°C to 1,000°C for 10 min.
Samples for the resistive gas sensing tests were fabricated by dropcasting 10 μl of the as-grown LBZA suspension onto alumina substrates comprised of a Pt-interdigitated electrode and a Pt track heater at the back. The NSs were left to sediment on to the substrate and form a film for 1 min after which the drop of suspension was removed and the sensor was annealed at 400°C in air for 30 min. The response of the ZnO NSs to CO was measured in dry air using a custom built gas flow apparatus (details are published elsewhere ) under a 400-sccm total flow and at 350°C.
To make DSCs, vacuum filtration was used to separate the grown product from the growth solution, adding a 1:1 volume mix of ethanol to deionised water when almost dry. The resulting LBZA NS paste was then spread onto FTO glass using a spatula, following by annealing at 400°C. The DSCs were then fabricated by a method reported elsewhere  using a dye solution made up of cis-bis(isothiocyanato)bis(2,2-bipyridyl-4,4-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium2 in a 1:1 volume mix of ethanol to deionised water. The electrolyte solution was 0.1 M LiI, 0.6 M tetrabutyl ammonium iodine (TBAI), 0.5 M 4-tert butylpyridine (4-TBP) and 0.1 M I2 In 3-methoxypropionitrile (MPN). The DSCs were characterized using a PV Measurements QEX10 quantum efficiency measurement system (Boulder, CO, USA) and a Newport Oriel AAA solar simulator (Stratford, CT, USA).
Results and discussion
SEM size measurement of the crystallite size for ZnO NSs evolved from LBZA NSs annealed at different temperatures and their standard deviation
Average size (nm)
Standard deviation (nm)
We report a novel technique for the production of ZnO nanocrystalline NSs through thermal decomposition of LBZA NSs. The LBZA NSs were produced by a low-cost, high-yield and low-temperature microwave-assisted aqueous technique in only 2 min. The NSs are mostly rectangular in shape with sides of 1 to 5 μm and a minimum thickness of 20 nm, with a structure typical of lamellar growth. Partial thermal decomposition into ZnO occurs after annealing in air at 200°C and is complete after 400°C, producing ZnO nanocrystalline NSs. Annealing at higher temperatures results in an increase of the nanoparticle size within the NSs and sintering was observed after 600°C. The NSs keep their shape even after annealing at 1,000°C. PL data show a significant deep level emission comprising several distinct transitions. The exciton to deep level intensity ratio was highest at 400°C and decreased at higher temperatures and with longer annealing times at 400°C. The shape of the deep level band was also altered by the annealing temperature. ZnO NSs produced by annealing at 400°C were used to fabricate DSCs and resistive gas sensors. The DSCs showed an overall efficiency of 1.3% whilst the response of the sensors at 350°C was 1.65 and 1.13 at 200 and 12.5 ppm, respectively. These results highlight the potential of the material for device applications.
Deep level emission
Dye-sensitized solar cells
Energy-dispersive X-ray spectroscopy
Incident photon to charge carrier efficiency
Layered basic zinc acetate
Near band edge
Scanning electron microscopy
This work was supported by the Royal Society (TGGM), the Welsh European Funding Office (RAB, MWP, DRJ, CJN), the Engineering and Physical Science Research Council (DTJB, AT). KEM and RM gratefully acknowledge support from the National Science Foundation CBET-0933719.
- Wang ZL: Zinc oxide nanostructures: growth, properties and applications. J Phys Condens Matter 2004, 16: R829-R858. 10.1088/0953-8984/16/25/R01View Article
- Baruah S, Dutta J: Hydrothermal growth of ZnO nanostructures. Sci Technol Adv Mater 2009, 10: 013001. 10.1088/1468-6996/10/1/013001View Article
- Unalan HE, Hiralal P, Rupesinghe N, Dalal S, Milne WI, Amaratunga GAJ: Rapid synthesis of aligned zinc oxide nanowires. Nanotechnology 2008, 19: 255608. 10.1088/0957-4484/19/25/255608View Article
- Chen Y-C, Lo S-L: Effects of operational conditions of microwave-assisted synthesis on morphology and photocatalytic capability of zinc oxide. Chem Eng J 2011, 170: 411–418. 10.1016/j.cej.2010.11.057View Article
- Peiró AM, Domingo C, Peral J, Domènech X, Vigil E, Hernández-Fenollosa MA, Mollar M, Marí B, Ayllón JA: Nanostructured zinc oxide films grown from microwave activated aqueous solutions. Thin Solid Films 2005, 483: 79–83. 10.1016/j.tsf.2004.12.030View Article
- Hosono E, Fujihara S, Kimura T, Imai H: Growth of layered basic zinc acetate in methanolic solutions and its pyrolytic transformation into porous zinc oxide films. J Colloid Interface Sci 2004, 272: 391–398. 10.1016/j.jcis.2003.10.005View Article
- Cui QY, Yu K, Zhang N, Zhu ZQ: Porous ZnO nanobelts evolved from layered basic zinc acetate nanobelts. Appl Surf Sci 2008, 254: 3517–3521. 10.1016/j.apsusc.2007.11.044View Article
- Tarat A, Majithia R, Brown RA, Penny MW, Meissner KE: Synthesis of nanocrystalline ZnO nanobelts via pyrolytic decomposition of zinc acetate nanobelts and their gas sensing behavior. Surf Sci 2012, 606: 715–721. 10.1016/j.susc.2011.12.010View Article
- Zhang Y, Zhu F, Zhang J, Xia L: Converting layered zinc acetate nanobelts to one-dimensional structured ZnO nanoparticle aggregates and their photocatalytic activity. Nanoscale Res Lett 2008, 3: 201–204. 10.1007/s11671-008-9136-2View Article
- Song R-Q, Xu A-W, Deng B, Li Q, Chen G-Y: From layered basic zinc acetate nanobelts to hierarchical zinc oxide nanostructures and porous zinc oxide nanobelts. Adv Funct Mater 2007, 17: 296–306. 10.1002/adfm.200600024View Article
- Sch R, Quintana M, Johansson EMJ, Hahlin M, Marinado T, Hagfeldt A: Preventing dye aggregation on ZnO by adding water in the dye-sensitization process. J Phys Chem C 2011, 115: 19274–19279. 10.1021/jp206052tView Article
- Tang L, Ding X, Zhao X, Wang Z, Zhou B: Preparation of zinc oxide particles by using layered basic zinc acetate as a precursor. J Alloys Compd 2012, 544: 67–72.View Article
- Morioka H, Tagaya H, Kadokawa J, Chiba K: Studies on layered basic zinc acetate. Mater Sci 1999, 8: 995–998.
- Poul L, Jouini N, Fiévet F: Layered hydroxide metal acetates (metal = zinc, cobalt, and nickel): elaboration via hydrolysis in polyol medium and comparative study. Chem Mater 2000, 12: 3123–3132. 10.1021/cm991179jView Article
- Lin S, Hu H, Zheng W, Qu Y, Lai F: Growth and optical properties of ZnO nanorod arrays on Al-doped ZnO transparent conductive film. Nanoscale Res Lett 2013, 8: 158. 10.1186/1556-276X-8-158View Article
- Zhang Z, Yuan H, Gao Y, Wang J, Liu D, Shen J, Liu L, Zhou W, Xie S, Wang X, Zhu X, Zhao Y, Sun L: Large-scale synthesis and optical behaviors of ZnO tetrapods. Appl Phys Lett 2007, 90: 153116. 10.1063/1.2712512View Article
- Djurišić AB, Choy WCH, Roy VAL, Leung YH, Kwong CY, Cheah KW, Gundu Rao TK, Chan WK, Fei Lui H, Surya C: Photoluminescence and electron paramagnetic resonance of ZnO tetrapod structures. Adv Funct Mater 2004, 14: 856–864. 10.1002/adfm.200305082View Article
- Djurišić AB, Leung YH, Tam KH, Hsu YF, Ding L, Ge WK, Zhong YC, Wong KS, Chan WK, Tam HL, Cheah KW, Kwok WM, Phillips DL: Defect emissions in ZnO nanostructures. Nanotechnology 2007, 18: 095702. 10.1088/0957-4484/18/9/095702View Article
- Hsieh P-T, Chen Y-C, Kao K-S, Wang C-M: Luminescence mechanism of ZnO thin film investigated by XPS measurement. Appl Phys A 2007, 90: 317–321. 10.1007/s00339-007-4275-3View Article
- Djurisić AB, Leung YH: Optical properties of ZnO nanostructures. Small 2006, 2: 944–961. 10.1002/smll.200600134View Article
- Sheng YJ, Lin YZ, Jiao HS, Zhu M: Size-selected growth of transparent well-aligned ZnO nanowire arrays. Nanoscale Res Lett 2012, 7: 517. 10.1186/1556-276X-7-517View Article
- Law M, Greene LE, Johnson JC, Saykally R, Yang P: Nanowire dye-sensitized solar cells. Nat Mater 2005, 4: 455–459. 10.1038/nmat1387View Article
- Seung HK, Daeho L, Hyun Wook K, Koo Hyun N, Joon Yeob Y, Suk Joon H, Grigoropoulos CP, Sung HJ: Nanoforest of hydrothermally grown hierarchical ZnO nanowires for a high efficiency dye-sensitised solar cell. Nano Lett 2011, 11: 666–671. 10.1021/nl1037962View Article
This article is published under license to BioMed Central Ltd. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.