Au纳米粒子提高到二氧化锡纳米带的CO氧化

Materials Chemistry and Physics 103 (2007) 132–136 Au nanoparticles enhance CO oxidatio b , Y a Sh , Chin f Tec uary 2 Abstract Au nanop al ox work, an ind elow novel Au/Sn CO o property me es on of CO oxida tectu the surface o d by Au/SnO2 int © 2007 Else Keywords: A 1. Introdu Catalyti scientific researches since Haruta et al. reported that gold becomes active once its particle size is reduced to a few nanome- ters (nm) [1–2]. Many previous investigations indicate that Au nanoparticles are active for several chemical reactions, such as propylen reaction; C H2 contain detected on spheric pre to improve persed onto or Fe2O3 [1 reaction en ticles, the support dep able electro between su enhanced c nanoparticl ∗ Correspon E-mail ad t the ts an e oxy main factors affecting reaction kinetics include the size of Au nanoparticles, the type of supported oxides, and contact angle between Au nanoparticles and oxides. The Au nanoparticles in the range of 2–4 nm in size are found to be most active for many 0254-0584/$ doi:10.1016/j e oxidation; NO reduction by CO; water gas shift O oxidation; and selective purification of CO from ing gases [3]. The active oxygen species are even gold nanoparticles under mild conditions-at atmo- ssure and temperature of 60–80 ◦C [4]. In most cases, catalytic abilities, Au nanoparticles are usually dis- some metal oxides, including reducible (TiO2, NiO ]) and irreducible (Al2O3) oxides [5]. As for catalytic hanced by metal oxides attached with Au nanopar- proposed mechanisms can be classified in terms of endence. The first mechanism is based on the vari- nic structure induced by strain and charge transfer pport materials and Au nanoparticles [6–8]. The atalytic abilities include the activations of both Au es and supported materials. The second one consid- ding author. Tel.: +86 2423971883; fax: +86 2423998660. dress: lh qian@hotmail.com (L.H. Qian). catalytic applications [12]. The active site of oxidation reaction has been suggested to be either on the metal or at specific ensem- ble sites around the interfaces between the metal and the support. Therefore, the most considerations are to control the microstruc- ture of Au nanoparticles and optimize interfacial character and fraction of Au/metal oxide. As for Au/metal oxide nanostructure, it is detrimental to reduce the size of Au nanoparticles and increase the fraction of Au/oxide interface in order to improve their catalytic abilities. As we all know, SnO2 nanobelt contains high surface-to-volume ratio, which is widely applied as CO sensor at low tempera- ture, even around room temperature [13–16]. So SnO2 nanobelt has a great potential to use as catalysis support and enhance CO oxidation. In this work, Au nanoparticles with a scale of several nm are deposited onto the surface of individual SnO2 nanobelt, and this composite nanostructure is applied to enhance CO oxidation. Electrical property measurement is used to inspect the occurrence of oxidation reaction on the surface of SnO2 nanobelt. – see front matter © 2007 Elsevier B.V. All rights reserved. .matchemphys.2007.02.001 L.H. Qian a,∗, K. Wang a, H.T. Fang enyang National Laboratory for Materials Science, Institute of Metal Research b Materials Science and Engineering School, Harbin Institute o Received 10 February 2006; received in revised form 6 Jan articles with a diameter of several nanometers (nm) attached onto met ividual SnO2 nanobelt is decorated by Au nanoparticles with a scale b O2 complex nanostructure is configured to detect the occurrences of asurement is used to detect the captures and releases of oxygen speci tions. As compared with the same SnO2 nanobelt without the archi f Au/SnO2 nanostructure are attributed to catalytic reactions improve erfaces. vier B.V. All rights reserved. u nanoparticles; SnO2 nanobelt; CO oxidation; Catalytic reactions ction c properties of Au nanoparticles have attracted many ers tha reactan provid n onto SnO2 nanobelt . Li a, X.L. Ma a ese Academy of Sciences, Shenyang 110016, PR China hnology, Harbin 150001, PR China 007; accepted 5 February 2007 ides are considered as excellent catalytic materials. In this 5 nm by means of conventional thermal evaporation. This xidations at low temperature region (<250 ◦C). Electrical the surface of SnO2 nanobelt, reflecting the occurrences res of Au nanoparticles, the enhanced CO oxidations on Au nanoparticles with small size and the introduction of support can provide active oxygen [9], or stabilize d intermediates [10–11]. The role of the support is to gen species, which is related to the temperature. The L.H. Qian et al. / Materials Chemistry and Physics 103 (2007) 132–136 133 2. Experimental procedures A mixture of elemental tin (6.5 g, 99%) and Fe(NO3)3 (3.9 g, 98.5%) pow- ders was arranged in the center of an alumina crucible. Five percent of H2 balanced by Ar flowed through the tube continuously with a constant rate of 50 sccm. The chamber pressure was kept at 4.7 × 104 Pa by fine adjustment of gas valve and continuous pumping. The temperature of furnace was kept at 850 ◦C for 45 min, and then further increased to 1200 ◦C for 60 min. After the fur- nace was cooled to room temperature, a white wool-like product was formed on the inner wall of the tube. The detailed microstructure characterizations revealed that the as-synthesized products consist of tetragonal rutile structure [17]. After removing some impurities (Fe2O3) introduced during chemical vapor deposi- tion, a few nanobelts were dispersed into ethanol by ultrasonic vibration, then a droplet was scattered on the surface of quartz sheet. A mask was used to cover the center segment of an individual nanobelt, and the nanobelt with relative large scale can be selected by Leica MPS 30 optical microscope because single crystal nanobelt was transparent and bright (the width was around hundreds of nm, and the length was tens of microns). A continuous Au film was deposited on both terminals as isolate electrodes for electrical conductance measurements. After the mask was removed, Au nanoparticles were coated onto the upper surface of nanobelt by thermal evaporation. In this step, the atomic flux during vapor deposition was finely controlled as around 2 × 1018 m−2 s−1 by adjusting the distance between sample and Au substrate, and the duration was 10 s. In order to determine the size of Au nanoparticles, SnO2 nanobelts dispersed into ethanol were dipped onto Cu grid coating amorphous C film. After Au nanoparticles were evaporated onto the surface of SnO2 nanobelt, transmission electron microscopy (TEM) observations were carried out in a JEOL 2010 microscope at an accel- erating voltage of 200 kV. Several different evaporation times (10, 30 and 50 s) were selected to check morphological change of Au nanoparticles during depo- sition. Electrical property measurements were carried out on the conventional pico-ampere source/measurement system, and a self-designed chamber and gas supply system were used to change the environments during electrical property measurements. Synthesized dry air contained the same N2 and O2 composition as the ambient air, and 1000 and 500 ppm CO were balanced by pure N2. 3. Results and discussions The size and morphology of Au nanoparticles attached onto SnO2 nanobelt are checked by TEM observations as shown in Fig. 1. For Au/SnO2 nanostructure after 10 s depositions illus- trated in Fig. 1(a), Au nanoparticles are uniformly dispersed on the surface of SnO2 nanobelt. Statistical distribution analysis indicates that the sizes of Au nanoparticles range from 2 to 5 nm, and the mean grain size is about 3 nm. Selected area electron diffraction (SAED) pattern in Fig. 1(b) shows that Au nanopar- ticles with random orientations decorate the surface of single crystal SnO2 . Energy dispersive X-ray spectroscopy (EDS) in Fig. 2 indicates that the content of Au element is about 3–5% (atomic ratio). Fig. 1(c) is a SnO2 nanobelt attached with Au nanoparticles after 30 s depositions. The scales of Au nanoparti- cles are not uniform in comparison with Au/SnO2 nanostructure for 10 s depositions. For Au nanoparticles marked by arrows lat- Fig. 1. TEM n SnO 50 s (d), respe nano SnO2 nanobe photographs of Au-decorated SnO2 nanobelt. Au nanoparticles are deposited o ctively. Selected area electron diffraction (SAED) pattern in (b) shows that Au lt. 2 nanobelt by means of thermal evaporation for 10 s (a), 30 s (c), particles with random orientations are dispersed on the surface of 134 L.H. Qian et al. / Materials Chemistry and Physics 103 (2007) 132–136 Fig. 2. The energy dispersive X-ray spectroscopy of Au/SnO2 nanostructures after 10 s evaporation of Au. tice fringes are obvious in some corners, while the fringes in the residual area are not appeared. These morphological traces imply that the aggregation of several Au nanoparticles occurs during the evaporation. Once increasing the evaporation time to 50 s, some nanoparticles exhibit a dog-bone and elliptical shapes as represented in Fig. 1(d). So it can be concluded that the coalescence of Au nanoparticles becomes significant. Fig. 3 sh edge of ind legible latt gle crystal. which can to the diffe the surface the sizes of tact angles Fig. 3. A TE SnO2 nanobe Fig. 4. An diverse obv smaller we Due to the ergy dist enc dete mbu rticl nmen SnO ion. of t of an des, nano . 5 represents electrical response of an individual SnO2 lt without Au decorations at different testing tem- res. A constant voltage of 1 V is supplied during the ows that three Au nanoparticles are adhered on the ividual nanobelt (evaporation time is 10 s), and the ice fringe implies that SnO2 nanobelt is perfect sin- Lattice fringes of Au nanoparticles are not attained, be attributed to unfavorable orientations, or related rent focus conditions between Au nanoparticles and of SnO2 nanobelt. Further observations indicate that Au nanoparticles are a little different, and the con- between Au nanoparticles and SnO2 nanobelt are face en So the depend To CO co nanopa enviro vidual oxidat minals image electro of this Fig nanobe peratu M image of Au nanoparticles attached on the edge of individual lt. The applied evaporation time is 10 s. experiment with the tim 200 ◦C, cu after switch temperatur rent with am changed cu into the cha are attribut nanobelt. A pick up ele O(ads)− or O occur: O2(gas) + 2 O2(gas) + e individual SnO2 nanobelt bridges two isolate Au electrodes. iously. Au nanoparticle with smaller size represents tting angle, and the angle ranges from 50◦ to 90◦. se particles are clung onto the same nanobelt, inter- between Au and SnO2 should be almost the same. inct wetting angle might be correlated to the size e of surface energy of Au nanoparticles. rmine catalytic abilities of Au nanoparticles to stion, the same nanobelt with and without Au es decorations is selected to detect the response to tal change. In this work, electrical response of indi- 2 nanobelt is applied to reflect the extent of CO A continuous Au film is applied to contact two ter- he nanobelt by vapor depositions. Fig. 4 is a digital individual SnO2 nanobelt bridging two separate Au and detailed observations determine that the width belt is about 400 nm. s, but the environment in the chamber is changed e. Clearly, when the temperature is stabilized around rrent flow through SnO2 nanobelt does not change ing the air to 1000 or 500 ppm CO. Once the testing e is increased up to 250 ◦C, a slight variation of cur- bient conditions can be observed. The magnitude of rrent is about 50% once 1000 ppm CO was conducted mber. The distinct currents in different atmospheres ed to the interaction between gas molecules and SnO2 t certain temperature, oxygen molecules in the air can ctrons from the conduction band of SnO2 and form 2(ads)− species [18], and the following reactions will e → 2O(ads)−, (1) → O2(ads)−. (2) L.H. Qian et al. / Materials Chemistry and Physics 103 (2007) 132–136 135 Fig. 5. The re pulses at 200 CO is about 0 According species are O(ads)− is f O2(ads)− be of oxygen s the electron layer is for reduction o of adsorbed field penet surface def When g as explaine of oxygen CO(gas) + O 2CO(gas) + The concen is reduced introductio between O current var tions occu bare SnO2 at 250 ◦C, SnO2 nano To explo bustion, the is employe spheres. B 5 nm exhib to the reac tion time is an individu tures. At 15 nanostructu The c ◦C by and a , and than f cur ICO nano ut 4 ectab when per can 000 than ratur eak articl rap to ions s. Es etec sponse of individual Au-free SnO2 nanobelt towards CO and air and 250 ◦C with a constant voltage of 1 V. The flux of dry air and .5 l/min. to the previous work, the types of the adsorbed mainly correlated to the temperature. In most cases, ormed when the temperature is above 200 ◦C, while comes dominant around 100 ◦C. The chemisorptions pecies at the surface defects of SnO2 nanobelt lead to exchange between surface and the bulk. A depletion med on the surface of SnO2 nanobelt, resulting in the f electrical conductance in the air. The concentrations oxygen species and Debye length (a measure of the ration into the bulk) depend on the temperature and ects [19]. as flow is switched from air to CO, oxidation reaction d in the following formulae will induce the remove species from the surface of SnO2 nanobelt: (ads)− → CO2 + e, (3) O2(ads)− → 2CO2 + e. (4) tration of oxygen species on the surface of nanobelt Fig. 6. and 250 1 V, CO to CO larger ratio o where SnO2 is abo no det found ing tem change R for 1 higher tempe very w nanop The related figurat system of the d and some electrons will return the bulk with the n of CO. It can be concluded that the interaction (ads)− and CO dominates current change. Obviously, iations are well related to the extent of CO oxida- rred on the surface of SnO2 nanobelt. As for the nanobelt, the current change with 50% is detected implying that the occurrence of CO oxidation onto belt. re the effect of Au/SnO2 nanostructure on CO com- same SnO2 nanobelt coated with Au nanoparticles d to detect electrical response to the diverse atmo- ecause Au nanoparticles with a scale smaller than it high catalytic ability for CO oxidation according tion mechanisms reported in Ref. [12], the evapora- selected as 10 s. Fig. 6 shows that current change of al Au-decorated SnO2 nanobelt at different tempera- 0 ◦C, the obvious current variation through Au/SnO2 re can be detected when the atmosphere is switched ture can be the gas swi Almost no as describe prompt res nanopowde many facto volume of work, CO o around ind mental con tracing the chamber ba limited vol container v In this st sity of carr of SnO2 na urrent flowing through Au-decorated SnO2 nanobelt at 150, 200 switching the atmosphere in the chamber. The applied voltage is ir pulses have the same flux as 0.5 l/min. the changed magnitude in 1000 ppm CO is a little that immersed into 500 ppm CO. One can define the rent between different environments as R = ICO/Iair, and Iair are defined as the currents flowing through belt in CO and in air, respectively. The value of R when 1000 ppm CO is used. It must be noted that le current change with gas switch within 1 h can be the applied temperature is 100 ◦C. Once the operat- ature is increased to 200 ◦C, the much larger current be measured in the same conditions. The value of ppm CO pulse can approach to 14, which is much the detected value at 150 ◦C. Once the operating e is elevated to 250 ◦C, the current change becomes , which might be related to the aggregation of Au es and their reduced catalytic abilities [20]. id current response with atmospheric switch is well the large gas flow rate with 0.5 l/min and the con- of gas pipe and SnO2 nanobelt in our measurement pecially, the input gas pipe is arranged in the vicinity ted sample, so that the complete Au/SnO2 nanostruc- exposed in the new environments immediately once tches from synthetic air to CO atmosphere or reverse. detectable delay of current change can be observed d in Figs. 5 and 6. It is worth mentioning that the ponse is different from CO oxidation rate studied in rs or bulk catalytic substances, which is controlled by rs, such as the penetration speed of gas into the total measured sample, actual reaction rate and etc. In this xidation should be localized within a small volume ividual Au/SnO2 nanostructure. Our present experi- figurations cannot detect CO oxidation correctly by evolution of output atmospheric species from the sed on conventional infrared gas analyzer due to the ume of an individual nanobelt and the relatively larger olume. udy, the measured current is mainly related to the den- iers in SnO2 nanobelt. Therefore, the current change nobelt scales with the discrepancies between the num- 136 L.H. Qian et al. / Materials Chemistry and Physics 103 (2007) 132–136 bers of captured and released electrons per unit surface area at different environments. Air pulse induces the adsorption of oxygen, while oxygen species are removed from the surface of SnO2 nanobelt in accompany with CO oxidation. So the mag- nitude of current change reflects the extent of CO oxidation on nanobelt surface. Certainly, operating temperature affects the adsorption of oxygen and CO oxidation, which is evidenced by the detectable current change of Au-free SnO2 nanobelt at 250 ◦C in c ◦ By cont rent chang after the in is attained magnitude nanobelt is their differe duction of distributed and the co evaporation ticles supp low temper the size of interface. I 5 nm, and t obtained du enhances o especially So we can decorated S can be attai kinds of ox diffusion o mer factor length), and induce the is impossib is proposed and SnO2 n onto SnO2 the detectio time. 4. Conclu Au nano decorate in This novel oxidation a erty measu oxygen spe occurrence of CO oxidation. In comparison with the same SnO2 nanobelt without the architectures of Au nanoparticles, larger variations of electrical properties with gas environments at lower temperature are observed, which is attributed to the catalytic reaction enhanced by Au nanoparticles and the introduction of Au/SnO2 interfaces. Acknowledgements ack e Fo 125 na ( ould unda ajor B61 nd D trica nce Haru n, J. C Valde Haru D. Hu . Car Griffi Okum ruta, C Mavr Giorg 01) 1 Sanch t, U. L Schu atal. . Mo 3. . Liu 03) 2 Haru Law, 02) 2 Kolm 03) 9 Kolm tt. 5 (2 . Qia ys. 10 . Ma Willia Barsa Wolf, . Ku omparison with no response at 200 C. rast, for the Au-decorated SnO2 nanobelt, the cur- e with a magnitude of four times can be obtained troduction of CO at 150 ◦C. Much larger change when the temperature rises up to 200 ◦C, and the can approach to 14 times. Because the same SnO2 employed to detect the occurrence of CO oxidation, nt properties at 200 ◦C can be attributed to the intro- Au/SnO2 interfaces. Au nanoparticles are uniformly on the surface of SnO2 nanobelt as shown in Fig. 1(a), ntinuous thin film is not formed yet during thermal . According to previous investigations, Au nanopar- orted by metal oxides can enhance CO oxidation at ature [21]. The catalytic reaction greatly depends on Au nanoparticles and the configuration of Au/SnO2 n this work, the size of Au nanoparticles is below he well-bonded interfaces between Au and SnO2 are ring thermal evaporation. This novel nanostructure xidation reaction occurred at Au/SnO2 interfaces, facilitates CO oxidation at low temperature region. find the large current change even at 150 ◦C for Au- nO2 nanobelt. Unfortunately, no obvious response ned at 100 ◦C, which might be related to the different ygen specimens absorbed on SnO2 surface or slow f oxygen species and CO at low temperature. The for- will reduce the thickness of depletion layer (Debye even the complete remove of oxygen species cannot obvious current change (in fact, the complete remove le). On the other hand, catalytic reaction oxidation to occ