Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Low Cost, Fast Solution Synthesis of 3D Framework ZnO Nanosponges Sarmad Naim Katea,† Špela Hajduk,‡ Zorica Crnjak Orel,*,‡ and Gunnar Westin*,† †
Department of Chemistry-Ångström, Ångström Laboratory, Uppsala University, 75121 Uppsala, Sweden National Institute of Chemistry, Hajdrihova 19, SI - 1001 Ljubljana, Slovenia
‡
ABSTRACT: An efficient, template-free solution-chemical route to nanostructured ZnO sponges is presented: A mixture of Zn(NO3)2·6H2O, Zn(OAc)2·2H2O, and triethanolamine in methanol was evaporated to a highly viscous liquid and rapidly heated to >200 °C for 1−3 min to achieve highly porous, nanocrystalline sponges of ZnO. The viscous precursor concentrate obtained on evaporation in air was characterized by TG, DSC, and IR spectroscopy, and the product ZnO sponges by XRD, SEM, TEM, and IR spectroscopy. The fast reaction forming ZnO started at 140 °C and finished within a few seconds. Scherrer analysis of the XRD peak broadening showed average crystallite sizes of 8 to 11 nm for ZnO prepared by annealing at 200−450 °C (3 min), while grain growth to 134 nm was observed from 500 to 900 °C (3 min). The ZnO powders obtained at 200−900 °C had cell dimensions of a = 3.25 Å and b = 5.21 Å, matching the ZnO literature data well. SEM and TEM analyses showed highly porous, bread-like 3D nanostructures built by ca. 30−70 nm thick walls of ZnO crystallites of the approximate average sizes given by the XRD Scherrer analysis. It seems that the crystal growth above 450 °C takes place within the ZnO 3D structure obtained at lower temperatures without much sintering of the larger porous structure.
1. INTRODUCTION Zinc oxide is a stable wide band gap semiconductor of low toxicity and cost with many attractive physical properties that also provides great possibilities to achieve novel or enhanced properties by doping.1−15 It has a wide range of applications in use and has the promise of many future applications including piezo-actuators, nanolasers, optoelectronic devices, UV blockers,1−15 solar cells,12−19 photoassisted cleaning of water and gases, water splitting for solar fuels,20−24 catalysts for fuel synthesis,25−28 batteries,14,29−32 and sensors.6,7,33−35 However, for the advancement of ZnO based devices reproducible and flexible processes to high quality ZnO in various desired architectures such as 1-, 2-, and 3-D structures with tailored sizes, exposed surfaces and porosity are necessary, since this allows for a detailed understanding of the various bulk and surface effects depending on the fine balance of crystal quality, defects and quantum size effects etc. For the ZnO devices to make any impact on society and environment through consumer products and energy conversion devices, the processes also have to use low cost precursors and be robust and scalable, which probably means solution based processing. Although there are an enormous amount of studies made on ZnO synthesis and virtually every possible type of processing has been studied, there is still a lack of processes that achieves the above-described goals for understanding and exploitation of ZnO based devices, even if there are some promising processes especially to thin film and nanoparticle and 1D wire arrays. © XXXX American Chemical Society
Therefore, there is still enormous interest paid to synthesis of well-controlled nanostructures of ZnO optimized for different applications. Here efficient processes to 3D structures with small, high quality ZnO crystallites and high porosity are of great value for solar energy applications, sensors, and batteries, but processes to such structures often depend on several steps and templates increasing complexity and cost, as well as make them tedious and hard to control. Although a large number of solution based synthesis routes have been reported, there remains to find efficient, direct processes that controllably achieve high quality ZnO nanocrystallites in open well-connected 3D networks, especially when not counting structures based on 50−500 nm wide wires in arrays of sea-urchin-like structures.36−39 Further, only very few routes yield important sizes below ca. 10−20 nm regardless of structure, and we fail to find routes that achieve crystallite sizes tunable over a wide range of sizes from very small to large ZnO grain sizes, while keeping a 3D overall structure. Various precipitation routes with or without surfactants or capping agents have been employed for ZnO nanoparticle synthesis40−44 which might be used to prepare at least dense 3D structures with a low degree of interparticle connection. Such structures, although 3D in nature, are not optimal for electron transport and access for ions or molecules to various Received: September 29, 2017
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DOI: 10.1021/acs.inorgchem.7b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
directing agent to achieve interesting narrow size distributed 670−1150 nm sized porous balls consisting of ZnO with primary average crystallite size of 16−22 nm.56 The structures can be tuned through the ratio of TEA to Zn. Similar porous, single-crystalline ZnO nanodisks with sponge-like structure, through the attachment of thin 100 nm crystalline nanodisks, have also been reported,63 as well as hollow spheres.64 Most of the sponge-like 3D ZnO structures have been formed with the addition of surfactants or templates, such as polyurethane, yielding open large pored structures with, e.g. Zn-nitrate, HMTA, and polyethylenimine (PEI) precursors.65−67 Porous spheres of 80−130 nm diameter and pore sizes of 2−35 nm were obtained by mixing zinc-hydroxide and concentrated ammonia, followed by addition of ethanol and polyacrylamide, and heating to 125 °C to cause precipitation and formation of ZnO, and a final annealing at 600 °C for 2 h.68 An emulsion process using aqueous ZnCl2 and NaOH together with triton X-100, PVP, or SDS has been adopted to prepare ZnO disks of 72 nm diameter and 15−30 nm thickness that were connected into 3D-like structures.53 Sponge-like ZnO nanostructures have also been prepared by first making Zn metal nanostructures by sputtering69,70 or deposition of a Zn powder emulsion29 and then convert them to a sponge-like ZnO nanostructure by oxidation with water vapor. These structures were shown to have promising properties for dye-sensitized solar-cells. However, the smallest ZnO crystallites were 20 nm.19,71 In most of these synthesis routes the 3D structure is only loosely connected through particles and the crystallite sizes are not easily controlled, and typically around 30 nm and up, in spite of the often elaborate multistep syntheses taking a long time. Herein we describe a novel ultrafast solution-chemical route to nanostructured continuous nanocrystalline 3D sponge-like structures with tunable ZnO crystallite sizes from 10 to 130 nm, which was first developed for preparation of nickel and cobalt metal sponges.72−74 It uses simple inorganic salts and only requires a temperature of 200 °C in air and an annealing time of 3 min. The microstructural and compositional evolution on heating to form the ZnO and annealing at temperatures up to 900 °C was studied by means of TGA, DSC, IR spectroscopy, powder X-ray diffraction, TEM-EDS, and SEM-EDS.
parts of the structure acting in solar-cells, sensors, and batteries, which makes them less effective and responsive and may also increase the structural changes on aging through sintering when in use. In the past few years different sizes and shapes of ZnO particles were achieved for the first time by Orel et al., combining the polyol and homogeneous precipitation methods at low temperature, and the influence of different polyols on the morphology of ZnO particles was presented.44−46 Techniques such as microwave synthesis have been seen as a promising tool for small, high quality ZnO particles, where Niederberger et al. have shown and discussed the mechanisms of synthesis of ZnO crystallites down to 8−9 nm in sizes using a nonaqueous condensation technique with benzyl alcohol.47−50 However, a critical study by Orel et al, comparing conventional and microwave heating, showed no changes in crystal phase, primary crystallite size, shape, and size distribution.51 The influence of the surfactant was shown in a comprehensive study of the soft-chemistry preparation of 20−250 nm sized particles by addition of p-toluene sulfonic acid where its catalytic and morphology-directing effect during formation of ZnO from zinc-acetylacetonate in boiling 1butanol was discussed in detail.52 Microemulsion syntheses has yielded down to 15−20 nm sized particles,53,54 while 5 to 15 nm sized oleate capped ZnO particles could be achieved in triethylene glycol,53 but such particles need to be cleaned from the organic groups for most applications.55 Autoclaving yielded down to 7 nm sized particles, according to analysis of the XRD peak broadening.56,57 Highly reactive metal organic compounds such as diethyl zinc have been used for preparation of down to 11 nm58 and 16 nm59 sized ZnO crystallites, respectively, and the grain growth on heating was studied.58 Self-sustained high temperature synthesis (SHS) using Znnitrate and glycine as fuel produced 50−1000 nm sized crystallites in a tufa-like structure.60 The powder was post=treated at 300 °C to remove residues of zinc cyanide and nitrate. This reaction is expected to yield over 1200 °C during a very short time and typically yields irregularly shaped powders with a wide size distribution. A surfactant assisted amine combustion method using Zn-nitrate, HMTA, and triton-X 100 yielded 22 nm sized ZnO crystallites after heating to 400 °C.61 The 3D structures based on 1D wires arrayed in different ways may be prepared from water solutions by the popular aqueous precipitation (ACG) route devised by Vayssieres et al., using the decomposition of hexamethyltetraamine (HMTA) to homogeneously yield NH3 and H2CO at ca. 95 °C in the presence of Zn-salts such as Zn-nitrate.36 The ammonia provide a solution with a pH (ionic strength) that makes the a/b surfaces stable and noncharged, while the c surface is charged and grown by adding Zn-ions from solution and thereby forms hexagonal wires of high crystal quality.36 The wire dimensions can be controlled down to ca. 50 nm in width and many microns in length, and various types of arrays can be obtained by controlling the number of nucleation points, leading to grass-like or sea-urchin-like structures.36 Different variations of solvothermal reactions using organic ligands as growth directing agents have been used to achieve powders with around 30 nm sized crystallites.62 Jiang et al. used this kind of route with Zn-nitrate and triethanolamine (TEA) as
2. EXPERIMENTAL SECTION Chemicals and Equipment Used. Methanol (Fischer Chemicals Analytical grade 99.99%), Zn(OAc)2 2H2O (ICN Biochemicals ACS Regent grade), Zn(NO3)2·6H2O (Sigma-Aldrich p.a. 99%), and triethanolamine (TEA) (Sigma-Aldrich p.a. 99%) were all used as purchased. The contents of the Zn-salt precursor paste and purity of the ZnO products obtained were studied by infrared (IR) spectroscopy, using a PerkinElmer Spectrum One instrument equipped with a KBr beamsplitter, DTGS/KBr detector and a Pike GladiATR diamond ATR, using a resolution of 2 cm−1 within the range 400−4000 cm−1. ATR correction was applied using the PerkinElmer Spectrum version 10.03.09.0139 software of the Spectrum One equipment. This makes the spectra more comparable to those obtained in transmission mode of KBr tablets or nujol mulls between KBr plates. Simultaneous thermogravimetric/differential thermal analysis was made by heating the Zn-paste in alumina crucibles to 700 °C, at 50 °C min−1, using a NETZSCH STA 409 PC Luxx thermal measurement apparatus (TGA/DTA). It was necessary to use small amounts of paste (ca. 10 mg) to avoid the very fast, gas evolving reactions taking place with the larger (ca. 17 mg) samples. The latter samples should be more similar to the furnace made batches but resulted in loss of some B
DOI: 10.1021/acs.inorgchem.7b02459 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry of the voluminous light product flying out of the crucible. Loss of material prevents a back calculus of the starting weight and steps taking place during the formation of the product ZnO. The DT response obtained with this instrument is not very well resolved and has a bit of delay at these high heating rates, which is why the weightloss during heating of the Zn precursor paste was also studied using a PerkinElmer Pyris 1 apparatus with a Pt pane, and differential scanning calorimetry (DSC) using a NETZSCH DSC 204 F1 phoenix apparatus with a closed Al cup with the lid perforated, in the temperature range 50−550 °C at 50 °C min−1. Although better in accuracy at low temperature, unfortunately some loss of material during the fast reaction step was unavoidable in this TG apparatus, and therefore, data from the two TG instruments had to be combined for a good picture of the low temperature reaction details and proper weight-loss. The crystallinity of the ZnO obtained on heating at 200−900 °C was studied with powder X-ray diffraction (PXRD) in a θ-2θ mode, using Cu Kα radiation in a Bruker D8 Advance instrument, equipped with an Ni-filter. The cell-dimensions were determined with the HighScore Plus 3.0.2 software employing the standard for ZnO (1011258 from COD). All peaks in the range 25−85° 2θ were included in the analysis. No significant difference in the ZnO celldimensions was observed when using the six first and strongest peaks at 30−65° 2θ, compared to all peaks. The ZnO crystallite sizes were determined through analysis of the XRD peak broadening using the Scherrer equation in the HighScore Plus 3.0.2 software, using the Kα1 peaks and the settings; Kα1 = 1.540598 Å, Kα2 = 1.5444426 Å, Kα2/ Kα1 = 0.5, and a spherical shape factor, K = 0.9. The instrumental peak broadening over the θ−2θ range investigated was obtained experimentally from a high quality α-Al2O3 standard and subtracted from the ZnO peak widths in the Scherrer analysis. An average of the six ZnO peaks; (hkl) 100, 002, 101, 102, 110, and 103 was used, and no preferential growth direction was observed. The overall microstructures of the ZnO obtained at 200, 450, and 800 °C were studied with a Zeiss Merlin Schottky field emission gun (FEG) scanning electron microscope (SEM), equipped with an AZtec EDS/EBSD energy dispersive spectrometer (EDS) with a silicon drift detector. The detailed microstructure was studied on samples made at 350 and 500 °C for 3 min, using a Jeol 2100F 200 kV transmission electron microscope (TEM), equipped with an EDS unit. The samples were prepared by adding fine powdered sample to a holey carbon grid, wetted by a drop of ethanol from the back-side. Synthesis of ZnO Sponges. The precursor solution having a 9:1 Zn-nitrate to acetate ratio and 0.3 TEA per Zn was prepared as follows: 12.197 g (41.00 mmol), Zn(NO3)2·6H2O was dissolved in 10 mL of methanol and 1.834 g (12.23 mmol) of triethanolamine (TEA) in 10 mL of methanol was added. In parallel, 1.000 g (4.555 mmol) of Zn(OAc)2·2H2O was dissolved in 5 mL of methanol and 0.203 g (1.37 mmol TEA in 1 mL of methanol) was added. After 15 min, the two solutions were mixed together, yielding an opaque white liquid which after 15 min was poured out on a Petri-dish and evaporated in air, until a highly viscous white paste was obtained. By the end of the evaporation the paste was worked with a spatula to achieve faster evaporation. The temperature was kept below 40 °C. The heat treatment to yield ZnO sponges was achieved by smearing out the Zn precursor paste on an aluminum plate, or with temperatures exceeding 600 °C, a steel plate, which was subsequently put into a muffle furnace set at 200, 250, 300, 350, 375, 400, 450, 500, 600, 700, 800, and 900 °C, for 3 min, respectively, before taking it out to cool. An investigation on the effect of annealing time was made by heating of the Zn paste to 350 °C for 1 and 2 min, respectively. The entire precursor paste is converted to ZnO in this reaction, but the isolated yield of porous ZnO when using the above-mentioned procedure was measured to be 96 wt %. The remaining material was obtained as a ZnO film stuck on the plate the paste was put on. However, one can imagine processing techniques such as spraying where the material is not in contact with a wall while the reaction takes place, which might be able to give close to 100% isolated yield of porous ZnO.
It should be mentioned that although we have not encountered any undesired self-ignition during drying, any mixture of nitrates and acetates should be regarded as potentially explosive, especially in large batches. Practical advice: The reaction is fast and probably yields up to 400−450 °C, while gases are evolved and an extremely voluminous ZnO powder is formed. Thus, if the reaction is not contained, it will fill much of a muffle furnace, which is why some, for example, tentlike structure restricting the powder from flying around and contaminating the furnace is useful. Also, if there is too much sample in a confined space and the temperature is low, the evolved gases that cannot escape will react with the ZnO product, yielding some ZnCO3 which has been seen by IR spectroscopy.
3. RESULTS AND DISCUSSION The process implies making a viscous paste of partially TEA coordinated Zn-nitrate and Zn-acetate that on rapid heating remains as a highly viscous liquid until fast chemical red-ox reactions of the nitrate and organics take place, rapidly evolving gas while forming a nanocrystalline ZnO sponge. The exothermic reaction is expected to evolve organic molecules, H2O, CO, CO2, NO, and NO2, during moderate local heating to temperatures around 450 °C and short enough time not to induce sintering. This process is new for ZnO but was originally developed for synthesis of nanostructured metal sponges.72−74 Studies of the Zn Paste and Its Decomposition. After solvent evaporation of the 9:1 nitrate:acetate ratio precursor liquid, a viscous white semitransparent paste remained. The IR spectra of the concentrate and ZnO sponges obtained on heating, are shown in Figure 1 and Figure 2, respectively. The
Figure 1. IR spectra of (from below) ZnCO3, ZnO sponge heated to 200 °C, 3 min (with the intensity strongly enhanced to show minor residues), TEA, Zn-precursor paste, Zn(OAc)2·2H2O in MeOH, and Zn(NO3)2·6H2O in MeOH. The graph has been deleted where it is unreliable due to strong peaks from the atmosphere.
peaks of the concentrate can be assigned as mainly due to O−H stretching (3330 and 3228 cm−1), H−O−H bending (1640 cm−1), and wagging, rocking, and twisting (
