【精品论文】Potassium Tungsten Bronze Nanosheets-- Synthesis, Structure and Field Emission.doc

精品论文potassium tungsten bronze nanosheets: synthesis, structure and field emissionqin jingfang, zhang gengmin, xing yingjie5(department of electronics, peking university, beijing 100871)abstract: single crystalline potassium tungsten brone nanosheets were prepared by reducing potassium tungsten oxide nansheets grown on a w foil. the nanosheets have the thickness less than100 nm and micron-sized smooth surface. a coherent phase transformation from orthorhombic potassium tungsten oxide to hexagonal potassium tungsten bronze is demonstrated in this reaction.10field emission was measured for potassium tungsten oxide and potassium tungsten bronze nanosheet film before and after reduction. work function decrease and slight morphology change were observedfrom nanosheet film after reduction.key words: physical electronics; phase transformation; reduction; work function0introduction15two-dimensional nanomaterials have been a research focus for the opportunity of new scientific and technological breakthroughs recently. other than well studied graphene,1 metal oxides nanosheets also present novel ferromagnetic, magneto-optical, electrochemical, catalytic, and photoresponsive properties.2-4 with unique morphology of both ultrathin thickness and micron-sized smooth surface, metal oxide nanosheets attract much attention in applications of gas20sensor, photocatalysis and battery materials.5-7 in addition to the limited size, the dimensionalityof metal oxide nanosheets provides new realm in future electronic devices.2 metal oxide nanosheets exhibit ideal performance because of both ultrathin thickness and sufficient surface area for thorough and controllable chemical or physical interactions in these cases. in this communication, we show the phase transformation in potassium tungsten oxide by reducing the25thin nanosheets while maintaining their structure stability. after reduction treatment, metallic hexagonal tungsten bronze nanosheets are synthesized.ternary alkali metal-w-o compounds have been studied for a long time because of their chromic behavior, drude-type optical performance and superconducting properties.8-10 among these compounds, hexagonal alkali tungsten bronzes (mxwo3, 0x1/3) and metal tungsten30oxides (mxwo3+x/2) hold similar structure but quite different properties.11 for example, metaltungsten oxides with the general formula of mxwo3+x/2, are fully oxidized compound and lack of free electron. electrical measurement reveals insulator performance form metal tungsten oxide film. on the other hand, hexagonal alkali tungsten bronzes are reduced compounds which show metallic conductivity because of regularly located potassium ions inside the hexagonal channel.1235the structure complexity in alkali metal-w-o compounds is well-known for several decades.there are a large amount of structure choices by variation of location of potassium ions and edge/corner sharing style of wo6 octahedrons.13 such structure similarities likely instigate the assumption that by uptake or evolution of a small amount of oxygen, phase transformation will happen by oxidation-reduction method, i.e., synthesis of metal tungsten oxide from tungsten40bronzes, or vice versa. however, in these experiments, the reaction is believed by the nucleationfoundations: most of china (grant nos 2012cb932701, 2011cb933001), national natural science foundation of china (grant nos 61076057, 61171023), the specialized research fund for the doctoral program of high education (no. 20090001120024)brief author introduction:qin jingfang(1985-),female,doctor student,physical electrnics correspondance author: xing yingjie(1970-),male, associated professor,physical electronics. e-mail: - 12 -and growth of new phase, but not by a simple continuous and coherent phase transformation.14,15 it still needs to clarify whether continuous phase transformation can be realized in metal tungsten oxides.recently, nanostructured potassium-tungsten-oxygen materials attracted attention because of45their promising applications in gas sensor and heat ray coating.5,11 tetragonal potassium tungsten bronze nanosheets were reported by zhang et al.5 oriented potassium tungsten oxide nanowires were grown on w substrate by a physical heating method.16 potassium tungsten bronzenanowires can be synthesized by reaction of citric acid and k2so4 directly,17 or by annealing potassium tungsten oxide nanowires prepared by hydrothermal technique under h2/n250atmosphere.11 size- and shape-dependent properties are well-known in nanomaterials. it seemsthat simple phase transformation can occur in these nanomaterials by reduction. however, obvious diameter and length change was observed in nanowires after annealing.11 this fact means that continuous and coherent phase transformation does not happen in nanowires either. here, we demonstrate the synthesis of k0.3wo3 nanosheets reduced from k2w6o19 nanosheets on centimeter55sized substrate. to the best of our knowledge, this is the first evidence for phase transformationfrom potassium tungsten oxide to potassium tungsten bronze while keeping their morphology. we believe that an intermediate surface-area-to-volume ratio between micro-scale powders and nanoparticles/nanowires causes such a particular behavior. we also present the results of field emission from potassium tungsten bronze nanosheet film and potassium tungsten oxide nanosheet60film for the first time. it is observed that the turn-on voltage of field emission from tungsten bronze nanosheet film is lower than that of potassium tungsten oxide nanosheet film, which reflects the decrease of work function after reduction treatment. our results reveal that novel device based on one piece of ternary k-w-o nanosheet containing both metallic and semiconducting parts may be fabricated by controlled heating treatment.651experimental methodsa tube furnace with a horizontal quartz tube was used to prepare potassium tungsten oxide nanosheets. a piece of cleaned w foil (1.5cm1cm) was placed in the quartz tube as both substrate and w source. a silicon plate loaded kbr powder was put in the tube next to the w foil.the tube was then heated at 650 for 2 hours in the air and then cooled down to room temperature70naturally. then, this w plate was mounted between two graphite rods for resistance heating in a home-made vacuum chamber to reduce the potassium tungsten oxide nanosheets. after the chamber was evacuated by a rotation pump, hydrogen with a constant flow rate of 15 standard cubic centimeters per minute (sccm) was introduced to keep a pressure of about 26 pa. the w plate was heated by current joule heat under h2 atmosphere. the voltage and current supplied75outside the chamber are 62.5 v and 1.85 a, respectively. the temperature of w plate wasmeasured by an optical pyrometer. field emission was measured in a home-made system in which the base pressure is about 10-6 pa. a stainless steel plate was used as the anode to measure the i-v curve of the field emission. the distance between anode and cathode is about 200 m.the product morphologies were observed by scanning electron microscope (sem, fei80xl30sfeg). the crystal structure of products was analyzed by x-ray powder diffraction (xrd, dmax 2400). high resolution transmission electron microscope (hrtem, fei tecnai f20) equipped edx was used to investigate the structure and composition of the sample. x-ray photoelectron spectroscopy (xps) measurements were carried out with an imaging photoelectron spectrometer (kratos axis ultra). work function was measured in air by photoelectron yield85spectroscopy (riken keiki ac-2).90951002results and discussionpotassium tungsten oxide nanosheets were grown on a w foil in large scale. the surface of as-prepared w foil shows a white color. low magnification sem image (figure 1(a) reveals that products are nanosheets which were uniformly grown on the w substrate. more details of the nanosheets are shown in figure 1(b). it can be seen that most nanosheets have smooth surface and sharp or rectangle corner. the inset of figure 1(b) demonstrates a highly magnified image of the side wall of a curved nanosheet. the thickness of this nanosheet is measured as 37 nm. as shown in figure 1, all nanosheets have the length and width of about several tens micron and a thickness less than 100 nm. the crystal structure of the nanosheets was characterized by xrd analysis. figure 2(a) (top panel) shows the xrd spectra of nanosheet film. all strong peaks match the standard data of orthorhombic k2w6o19 phase (jcpds no. 31-1115), which confirms that the nanosheets are single crystalline potassium tungsten oxide.figure 1 sem images of as-prepared nanosheets. (a) low magnification and (b) high magnification sem image of k2w6o19 nanosheets. the inset in (b) indicates the thickness of a nanosheet.105110figure 2 characterizations of nanosheets before and after reduction. (a) xrd spectra indicating (top panel) k2w6o19 phase and (bottom panel) k0.3wo3 phase of nanosheets before and after reduction, the peak of wo2.83 in top panel comes from oxided w substrate; 18 sem image of thinnanosheets (b) before and (c) after reduction treatment.potassium tungsten oxide nanosheets were reduced under hydrogen atmosphere in a home-made stainless steel vacuum chamber. the color of sample surface became blue after treatment. the nanosheet film on the w plate was examined by xrd and sem again. xrd115120125130135patterns for treated nanosheets are demonstrated in figure 2(a) (bottom panel). all xrd peaks can be indexed to single crystalline hexagonal potassium tungsten bronze (jcpds no. 49-0541,k0.3wo3). the morphologies of nanosheets before and after reduction were observed by sem for comparison and those thinnest nanosheets were particularly examined. figure 2(b) (before reduction) and figure 2(c) (after reduction) shows that only slight contour change occurs on thinnest nanosheets. no obvious morphology change was found in thicker nanosheets. these facts mean that potassium tungsten bronze nanosheets can be prepared by reduction of potassium tungsten oxide nanosheets.it is well-known that unambiguous identification of crystal structure of hexagonal tungsten bronze (htb) and htb-like tungstate by xrd analysis is difficult.14,15 a general hexagonal structure for htb compounds is depicted as a tungsten-oxygen framework built up of layers containing corner-sharing wo6 octahedron which are located in six-side channels, in which potassium atoms occupied in the hexagonal channel. for htb-like tungstate, it is proposed that both potassium and oxygen atoms are located within the hexagonal channel, while the wo6octahedrons keeps constructing the tungsten-oxygen framework.14,15 some superstructures areformed in single crystalline htb-like tungstate comparing to the hexagonal tungsten bronze. for example, superstructure formed by repetition in b crystallographic direction by 2 times issupposed for explanation of the crystal structure of k2w6o19.14,15 such a slight structure variationmakes the similarity of xrd patterns between htb and htb-like tungstate. therefore, more detailed structure analyses were conducted by high resolution transmission electron microscopy to help the structure identification of k2w6o19 and k0.3wo3 nanosheets.140figure 3 characterizations of nanosheets before and after reduction by hrtem. (a) low magnification tem image (b) hrtem image (c) saed pattern (d) edx of a k2w6o19 nanosheet, parallel white lines indicate superstructure in a direction and the black square indicatesuperstructure in b direction in figure 3(b); (e) low magnification tem image (f) hrtem image(g) saed pattern (h) edx of a k0.3wo3 nanosheet.145150155160165170175180185figure 3(a) shows the low-magnification tem image of a k2w6o19 nanosheet. somestreak-type or ripple-type contrast observed in the image is induced by bending or deformation of the thin nanosheet. the hrtem characterization was demonstrated in figure 3(b). it can be clearly seen that the single crystalline nature of the nanosheet without amorphous sheath layer. the crystal fringes show two d-spacings of 0.371 nm and 0.356 nm corresponding to (002) and(200) planes, respectively. the selected area electron diffraction (saed) pattern of this nanosheet is shown in figure 3(c), in which two points corresponding to (100) and (002) planes are marked. these growth directions are also coincident with the strongest peaks in xrd spectrum. it is a little surprised that the point corresponding to (1/2 00) planes appears in figure 3(c) too. this factmeans that our nanosheets exhibit some long-range periods. with these hrtem image and saed pattern, we give direct evidences for some superstructures in potassium tungsten oxide now. evident regular four-fold superstructure in a crystallographic direction is observed throughout in nanosheet (marked by parallel white lines in figure 3 (b), resulting in the (1/2 00) point in saed image. the superstructure in b direction cannot be identified by crystal fringes directly because itis perpendicular to the nanosheet plane. however, by observing the thin edge region of the nanosheet (marked and magnified in figure 3(b), the contrast at the edge of the nanosheet suggests the existence of superstructure.19 it should be noted that such superstructures andelectron diffraction patterns were not observed from potassium tungsten oxide nanowires,11which means that 2d nanosheet is more ideal for structure investigation. the low-magnificationtem image, hrtem image and saed patterns of k0.3wo3 nanosheets were shown in figure3(e)(f)(g) for comparison. the streak-type or ripple-type contrast can be found in figure 3(e) too. figure 3(f) shows two sets of crystal fringes with d-spacings of 0.378 nm and 0.369 nm, which correspond to (002) and (110) planes, respectively. no large area containing superstructure can be found in figure 3(f). the saed pattern of a k0.3wo3 nanosheet is demonstrated in figure 3(g). it is very clear that most points reflecting superstructures disappear, which is in contrast with figure3(c). such a change in both hrtem image and saed pattern means that the structure transformation from k2w6o19 to k0.3wo3 takes place in nanosheet. the chemical composition analyses for nanosheets before and after reduction were conducted by edx and the results confirm the k-w-o composition. figure 3(d) (h) show the edx results of k2w6o19 and k0.3wo3nanosheets, respectively. w, k and o elements were detected from both samples, whereas cu andc peaks were from the copper grid.not only crystalline structure but also the oxidation state of w has been changed by the reduction treatment. k2w6o19 nanosheet has only w6+ state, and the formula can be written as k2o6wo3. on the other hand, k0.3wo3 has the formula of k0.3w0.76+w0.35+o3, which means theexistence of both w5+ and w6+ in the reduced nanosheet. xps was used to examine the oxidationstate in nanosheets before and after reduction. peaks at binding energy corresponding to potassium, tungsten and oxygen are detected. detailed information of the chemical state of the core level tungsten (w4f) is shown in figure 4.the curves can be fitted to two spin-orbit doublets (w4f5/2 and w4f7/2) with the interval of 2.1 ev. the sample of k2w6o19 nanosheets exhibits two peaks at37.7 and 35.6 ev figure 4(a), which can be assigned to w6+. for k0.3wo3 nanosheets, four peaksat 34.3, 36.4 and 35.6, 37.7 ev were attributed to w5+ and w6+, respectively (shown in figure4(b). all these peaks agree with reported values.11,17synthesis of potassium tungsten oxide nanosheets is demonstrated in this paper for the first time. many kinds of metal oxide nanosheets are prepared from layered materials with weak coupling between layers and strong in-plane bonds.2 because potassium tungsten oxide do not have such a layered structure, we do not think similar growth mechanism in our experiments. by190195200adjusting the location of kbr powders, the morphology of products can be changed. we observed many small nanosheets grew from the corner of v-shaped nanowire. we suggest that both ofv-shaped nanowire growth by vapor-liquid-solid (vls) mechanism and nanosheet deposition from the corner of v-shaped nanowire by vapor-solid (vs) mechanism occurred during the heating period in our experiments. detailed analyses of the growth model will be published elsewhere.figure 4 w4f core-level xps spectra: (a) k2w6o19 nanosheets and (b) k0.3wo3 nanosheets.continuous and coherent phase transformation from potassium tungsten oxide to potassiumtungsten bronze is proved from above results. this conversion is described by the followingequation:k x wo3+ x 2 + x2 h2 = k x wo3 + x2 h2 o205figure 5 sem image of nanosheets and nanowires: (a) before and (b) after reduction. white arrows indicate shape change of some nanowires.210215220225phase transformation from htb-like k2w6o19 to hexagonal k0.3wo3 needs that the framework built by wo6 octahedrons keeps stable, whereas