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Sensors and Actuators B 161 (2012) 209– 215

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

acile synthesis and ultrahigh ethanol response of hierarchically porous ZnOanosheets

exi Zhanga,b, Jianghong Zhaoa,∗, Haiqiang Lua,b, Li Lia, Jianfeng Zhenga, Hui Lia,b, Zhenping Zhua,∗

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road 27, Taiyuan 030001, PR ChinaGraduate University of Chinese Academy of Sciences, Beijing 100039, PR China

r t i c l e i n f o

rticle history:eceived 1 August 2011eceived in revised form 5 October 2011ccepted 10 October 2011vailable online 17 October 2011

a b s t r a c t

Zinc oxide (ZnO) nanosheets were successfully synthesized through a facile, economic, and low-temperature hydrothermal process, followed by annealing of the zinc carbonate hydroxide hydrateprecursors. The nanosheets are single crystals with hexagonal wurtzite and mesoporous structures. Gassensors based on these ZnO nanosheets exhibited ultrahigh response, fast response–recovery, and goodselectivity and stability to 0.01–1000 ppm (parts per million) ethanol at 400 ◦C. Extremely low concen-

eywords:nO nanosheetierarchical structurethanolpb-level response

tration ethanol (down to 10 ppb (parts per billion)) can be readily detected (S = 3.05 ± 0.21), which is thelowest detection limit to ethanol utilizing pure ZnO as sensing materials in a one-side heated gas sensorhitherto. The excellent ethanol-sensing performance of ZnO, particularly the ppb-level response, is mainlyattributed to its novel hierarchical structure, which has a large specific surface area, abundant meso-pores, single-crystal structure, plane-contact between sheets, three-dimensional network architecture,and characteristically small thickness.

. Introduction

Metal oxide semiconductors (MOS), such as SnO2, ZnO, TiO2,e2O3, In2O3, WO3, and so on, have great potential as gas-sensingaterials [1], owing to the fact that the chemical interaction of

as molecules with the semiconductor surface leads to remarkablehanges in the electrical conductivity. As one of the key wide-band-ap II–VI compound semiconductors, ZnO is a primary material toevelop metal oxide gas sensors [2] based on its capability of detect-

ng trace concentration gases [3]. Taking advantages of the smallize, high-density surface active sites, and large surface-to-volumeatios compared with their bulk counterparts, considerable atten-ion has been given to gas sensors utilizing nanoparticles [4] andne-dimensional nanostructures (1D) [5,6], including nanorods,anowires, nanobelts, nanotubes, and so on. MOS gas sensors basedn quasi-two-dimensional (quasi-2D) nanostructures have begunttracting the interest of researchers due to their comparable orven better gas sensing properties compared with nanoparticlesnd 1D nanostructures [7–18]. Up to now, some successes onas sensors fabricated from quasi-2D MOS nanostructures have

een achieved, such as ZnO [7–12], SnO2 [13–15], WO3 [16,17],nd Co3O4 [18]. However, the controlled synthesis of quasi-2DOS nanostructures as gas sensing materials with relatively large

∗ Corresponding authors. Tel.: +86 351 4048433; fax: +86 351 4041153.E-mail addresses: zjh sx@sxicc.ac.cn (J. Zhao), zpzhu@sxicc.ac.cn (Z. Zhu).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.10.021

© 2011 Elsevier B.V. All rights reserved.

surface area, single-crystal structure, 3D network architecture,characteristically small thickness, and so on, which can remarkablyenhance the gas sensing performance, is still a challenge. Comparedto nanoparticles and 1D nanostructures, the synthesis of quasi-2D nanostructures and the investigation of their shape-dependentphysical/chemical properties (e.g., gas sensing application) are stillat an early stage. Thus, a controlled synthesis of quasi-2D MOSnanostructures and an in-depth research on their gas-sensing prop-erties should be conducted. In addition, high-response gas sensors,especially at ppb levels, are increasingly gaining interest in thefields of environmental protection, exhaust monitoring, industrialcontrol, and so on. It is considered that quasi-2D MOS nanostruc-tures can provide an alternative solution to achieving ultrahighgas detection due to their high response shown at ppm levels andabove.

In the current work, hierarchically porous single-crystal ZnOnanosheets were successfully synthesized through a facile, low-cost, and low-temperature hydrothermal process, followed byannealing of the zinc carbonate hydroxide hydrate precursors at300 ◦C for 0.5 h in air. The precursors were initially formed asassemblies on glass slides during the hydrothermal process inaqueous solutions of urea and zinc chloride at 80 ◦C for 24 h.The sensor fabricated from the ZnO nanosheets showed excel-

lent ethanol-sensing characteristics, such as ultrahigh response,rapid response–recovery, and good selectivity and long-term sta-bility. Especially, the as-prepared sensor presented ppb-level gassensing performance, with a detection limit (S ≥ 3) of as low as

2 Actuators B 161 (2012) 209– 215

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0 ppb ethanol. Furthermore, the reasons for the ultrahigh ethanolesponse were also discussed.

. Experimental

.1. Preparation and characterization of materials

Zinc carbonate hydroxide hydrate precursors were synthe-ized through a facile hydrothermal process in aqueous solutionnder mild conditions, similar to a previously reported method19]. All chemicals were analytical grade reagents and useds received without further purification. In a typical synthesis,.2726 g ZnCl2 (0.002 mol) and 2.4024 g urea (0.04 mol) were addedo 40 mL deionized water under magnetic stirring for 10 min. Then,ilute HCl (2 wt.%) was added to the above solution until theH value was adjusted to 5. A piece of commercial glass slide45 mm × 25 mm × 1 mm) was then used as a substrate for theeposition of the precursors and placed in a 50 mL Teflon-linedtainless steel autoclave with the above solution. The autoclaveas sealed and maintained at 80 ◦C for 24 h. The white products on

oth sides of the glass slide were repeatedly washed with deionizedater, and then dried at 80 ◦C for 12 h. Finally, the white productsere scraped and heated at 300 ◦C for 0.5 h in air to obtain ZnO.

X-ray diffraction (XRD) analysis was conducted on a D8dvance Bruker X-ray diffractometer with a CuK� radiation

� = 0.15406 nm) operating at 40 kV. Field emission scanning elec-ron microscope (FE-SEM) images were performed on a JEOLSM-6700F microscope operating at 5 kV. Transmission electron

icroscope (TEM) images and selected area electron diffractionSAED) patterns were obtained on a JEOL JEM-2010 microscopeith an accelerating voltage of 200 kV. The specific surface areasere measured via the Brunauer–Emmett–Teller (BET) methodsing a N2 adsorption at 77 K after treating the samples at 100 ◦Cnd 10−4 Pa for 2 h using a Tristar-3000 apparatus. X-ray photoelec-ron spectroscopy (XPS) was performed by a Thermo ESCALAB 250PS spectrometer, using a monochromated aluminium (K� radi-tion: h� = 1486.6 eV) anode operated at 20 kV and 150 W with aesolution of 0.6 eV (beam spot size: 500 �m2). In the XPS mea-urements, the as-prepared ZnO powder was ground and mountedn the sample holder with double-sided adhesive tape, then thepectra were taken under ultrahigh vacuum conditions (workingressure in the analysis chamber was maintained below 10−6 Pa)t room temperature. To eliminate the effect of surface contamina-ion, the binding energy scale was calibrated by a XPS peak of C1sith a value of 284.6 eV.

.2. Sensor fabrication and measurement

The fabrication procedure for the one-side heated gas sensorss similar to the method used in previous reports [20–22]. The gas-ensing properties of the ZnO sample were determined using anW-30A gas sensitivity instrument (Hanwei Electronics Co. Ltd.,R China). The measurement followed a stationary-state gas dis-ribution process, wherein a given amount of test gas was injectednto a glass chamber and fully mixed with air. The response (S) foreducing gases is defined by the equation, S = Ra/Rg, where Ra andg are the resistances of the gas sensor in air and in the test gases,espectively. In the current study, the response or recovery timeas defined as the time taken for the sensor to achieve 90% of itsaximum response or to decrease to 10% of its maximum response,

espectively. The long-term stability of the sensor was determined

y repeating the test seven times at 400 ◦C in 60 days. In this work,our ZnO nanosheet sensors were prepared under the same prepa-ation procedure, and a response curve taken on average value ofhe four sensors was shown in the paper.

Fig. 1. XRD patterns of as-synthesized precursors (1) and final ZnO products (2).

3. Results and discussion

3.1. Characterization of the precursors and the final ZnO product

The crystal structures of the precursors (1) and the final ZnO(2) were characterized via XRD (Fig. 1). All peaks of the pre-cursors can be indexed to zinc carbonate hydroxide hydrate[Zn4CO3(OH)6·H2O, JCPDS card no. 11-0287] [19]. After heatingat 300 ◦C for 0.5 h in air, the precursors were fully transformedinto the hexagonal wurtzite ZnO (JCPDS card no. 36-1451),as shown in Fig. 1(2). No characteristic peaks for any otherimpurities were observed, indicating that all precursors were com-pletely transformed into ZnO. The crystallite size was calculatedfrom X-ray line broadening analysis using the Scherrer equation,D = 0.89�/( ̌ cos �), where D is the average crystal size in nm, �is the X-ray wave length (CuK�: 0.15406 nm), ̌ is the full widthat half-maximum (fwhm) of the peak, and � is the correspondingBragg diffraction angle. The mean crystallite size of the ZnO sample,which was calculated from the full width at half-maximum of thepeak (0 0 2) at 2� = 34.48◦, was approximately 14.91 nm.

The morphology of the as-formed ZnO was characterized viaFE-SEM (Fig. 2). Fig. 2(a) exhibits a typical image of the ZnO prod-ucts composed of random sheet-like nanostructures with 3–6 �mlengths and 10–40 nm edge thicknesses. These values are in goodagreement with the calculated crystallite size of 14.91 nm. Themagnified image (Fig. 2(b)) shows that the ZnO nanosheets withvery rough surfaces assembled into 3D network architectures,with lots of interspaces simultaneously forming between adja-cent sheets. A detailed structural analysis of individual nanosheetswas conducted using TEM and SAED. Fig. 2(c) shows a typicalTEM image of the random porous structure distributed in thenanosheets. Abundant pores 8–40 nm in diameter are present inthe ZnO nanosheets. The high-resolution TEM (HRTEM) image ofa single nanosheet displays resolved fringes with a separation of0.260 nm, which corresponds to the (0 0 2) lattice spacing of ahexagonal ZnO crystal (Fig. 2(d)). A representative SAED patternof the ZnO nanosheets (top right corner in Fig. 2(d)) recorded todetermine their exact structure further reveals their single-crystalnature.

The porous feature of the ZnO nanosheets was investigatedusing N2 adsorption–desorption measurements. The representa-tive isotherm and the corresponding Barrett–Joyner–Halenda (BJH)pore-size distribution plot (inset) of ZnO nanosheets are shown

L. Zhang et al. / Sensors and Actuators B 161 (2012) 209– 215 211

of the

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Fig. 2. FE-SEM (a), magnified FE-SEM (b), TEM (c) and HRTEM (d) images

n Fig. 3. A hysteresis loop observed at higher relative pressuresP/P0 = 0.7–1.0) is associated with the filling and emptying of meso-ores (2–50 nm diameter) via capillary condensation. According tohe IUPAC classifications, the loop indicates a hybrid of the type H1nd H3 profiles [19], which is ascribed to the presence of meso-

ores and slit-shaped pores (or plate-like particles), respectively.

udging from the FE-SEM and TEM observations, the mesopores inhe ZnO nanosheets contribute to the type H1 profile, whereas the

ig. 3. Typical N2 adsorption–desorption isotherm and Barrett–Joyner–HalendaBJH) pore size distribution plot (inset) of the ZnO nanosheets.

ZnO nanosheets. The SAED pattern is inserted in top right corner in (d).

interspaces between the nanosheets are responsible for the type H3profile. The BET surface area of the ZnO nanosheets was calculatedas 52.02 m2/g. The calculated pore size distribution (insert in Fig. 3)for the desorption branch of the N2 isotherm using the BJH methodindicates that the sample contains mesopores with diameters rang-ing from 5 to 50 nm, with two peaks at 9.0 and 45 nm. The porousnature of the hierarchical ZnO architecture makes it a promisingcandidate for gas-sensing applications because of its contributionto the increase in the specific surface area and gas diffusion andtransportation.

In principle, gas sensing is based on the oxidation–reductionreaction of test gases on the surface of MOS, which leads to anabrupt conductance change in the sensing materials. Hence, theefficiency of gas sensing is essentially influenced by surface struc-tures and the composition of the sensing materials. XPS analysiswas used to obtain the surface structural and compositional infor-mation of the as-prepared ZnO sample. The symmetrical peak ofZn2p3/2 centered at 1020.37 eV in Fig. 4(a) indicates that only onevalence state of Zn is present. However, the O1s photoelectron peakis asymmetrical and shows a visible shoulder (Fig. 4(b)). The O1s XPSpeak can be deconvoluted into three Gaussian component peakscentered at 529.97 eV (OL), 531.06 eV (OV), and 531.94 eV (OC) byfitting with the Gaussian function (insert in Fig. 4(b)). It is reportedthat the OL component of the O1s spectrum is attributed to the O2−

ions in the ZnO lattice, the OV component is associated with theO2− ions in oxygen-deficient regions within the ZnO matrix, andthe OC component is usually attributed to the chemisorbed anddissociated oxygen species or OH [23]. Given that the gas sensing

performance is closely related to the chemisorbed oxygen species,the high percentage of the OC component (15.53%) might suggestthe excellent sensitivity of the as-synthesized porous single-crystal ZnO nanosheets. The XPS spectrum reveals that the atomic

212 L. Zhang et al. / Sensors and Actuators B 161 (2012) 209– 215

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ig. 4. XPS spectra of the ZnO nanosheets: Zn2p3/2 peak (a) and O1s peak (b). Inseeparated from O1s.

omposition ratios of Zn:O on the surfaces of the ZnO nanosheetss 1.16:1, demonstrating that oxygen vacancies [24] exist in thenal ZnO products. Introducing more electron donor defects (VOnd Zni) might be beneficial to enhance the gas response, sinceuch more adsorbed oxygen would be ionized on the surface of

he semiconductor sensing materials [25].

.2. Ethanol-sensing properties of the ZnO nanosheets

The gas sensing properties of the ZnO nanosheets were eval-ated by first determining the optimum operating temperature.he average response of the ZnO nanosheet sensor to 200 ppmthanol was tested as a function of the operating temperatureFig. 5). The response continuously increases and reaches its max-mum at 400 ◦C, and then rapidly decreases as the temperature isurther increased. The maximum response to 200 ppm ethanol is98.43 ± 23.06, which is much higher than some reported resultsor ethanol sensors based on ZnO nanoparticles [4], and 1D [26–28]

nd quasi-2D [7–10] ZnO nanostructures. This phenomenon indi-ates that the ZnO nanosheets have the capacity to detect very lowven ppb level ethanol concentrations.

ig. 5. Average response of the sensor to 200 ppm ethanol versus operating tem-erature.

b): binding energy positions and percentages of three sub-peaks (OL, OV and OC)

Fig. 6(a) displays the correlation between the average responseof the ZnO nanosheet sensor and the ethanol concentration, whichranges from 0.01 to 1000 ppm at 400 ◦C. The response rapidlyincreases with increasing ethanol concentration, indicating that thesensor has not reached saturation at 1000 ppm. It is worth not-ing that the sensor displays high response to ethanol even at theppb level. The responses are 3.05 ± 0.21, 3.98 ± 0.34, 7.93 ± 0.64,and 16.55 ± 1.41 to 0.01, 0.1, 0.5, and 1 ppm ethanol, respectively,revealing that the detection limit (S ≥ 3) [29] of the as-preparedsensor is as low as 10 ppb. According to the gas sensing mech-anism of MOS sensors, the response is related to not only thestate of oxygen on the material surface but also the reductiveor oxidative ability of the test gases [30]. ZnO generally exhibitshigh response and ppb-level detection capability to strong oxidiz-ing gases, including NO2 [31–33] and O3 [34], or strong reducinggases such as H2S [35], trimethylamine [36], triethylamine [37],and so on. However, the ppb-level detection of ethanol by pureZnO sensors is rarely reported [38]. To the best of our knowl-edge, the 10 ppb detection limit for ethanol using only ZnO assensing materials in a one-side heated gas sensor is the lowestreported. The response (S) of MOS gas sensors is usually empir-ically represented as S = a[C]b + 1 [39,40], where a and b are theconstants and C is the concentration of the test gas. At a cer-tain operating temperature, the above equation can be rewrittenas log(S − 1) = b log(C) + log a. It can be seen that log(S − 1) has alinear relationship with log(C): Y = 0.5813 × X + 1.2191 (Fig. 6(b)).The response has a good linear relationship (R = 0.9935) withthe ethanol concentration (0.01–1000 ppm range) in logarithmicforms, suggesting that the porous single-crystal ZnO nanosheets areexcellent sensing materials for the fabrication of practical ethanolsensors.

The response–recovery behavior is an important characteris-tic that determines the performance of gas sensors. Fig. 7 showsa typical response-and-recovery curve of the sensor to ethanolconcentrations ranging from 0.01 to 1000 ppm at 400 ◦C. At a cer-tain ethanol concentration, the corresponding response increasesrapidly and reaches its equilibrium. Once the ethanol source isremoved, the response decreases quickly to the baseline. Thisphenomenon shows the high reproducibility of the sensor. Theresponse and recovery time for 200 ppm ethanol are 7 and 19 s,respectively, which indicates a fast response–recovery character-

istic.

The cross response was also tested by exposing the sensor to200 ppm potential interfering gases, including CH4, C5H12, C6H14,C6H12, C7H16, CH2Cl2, CHCl3, CCl4, C6H6, H2, and CO, at 400 ◦C

L. Zhang et al. / Sensors and Actuators B 161 (2012) 209– 215 213

Fig. 6. The average response versus ethanol concentration in the range of 0.01–1000 ppmview of the concentration below 5 ppm.

Fig. 7. Typical response and recovery characteristic curve of the sensor to ethanol intw

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he range of 0.01–1000 ppm at 400 ◦C. The low concentration region (0.01–1 ppm)as magnified as inset.

Fig. 8). The sensor exhibits considerably lower response values tohese gases than that to ethanol, indicating the excellent selectivityf the ZnO nanosheets.

The long-term stability of the sensors was also determined

Fig. 9). Clearly, the sensor shows a nearly constant response to00 ppm ethanol in 60 days, which confirms the high stability ofhe sensor.

ig. 8. The cross-response of the sensor to ethanol and other 11 interfering gases at00 ◦C.

at 400 ◦C (a) and the corresponding log(S − 1) − log(C) plot (b). Inset in (a): enlarged

It is considered that the excellent ethanol-sensing performance,especially the ppb-level response, results from the following fac-tors: (1) the adsorption capacity for ethanol molecules and oxygenspecies is extremely enhanced on the surface of the ZnO nanosheetsdue to their large specific surface area (52.02 m2/g); (2) gas dif-fusion and transportation are significantly improved due to the3D network architecture [41] and the porous nature [9–12] ofthe ZnO nanosheets; (3) the charge carrier (electron) transportproperties are considerably improved in both the grain and grainboundary regions because of the high single-crystallinity [11,12]and plane-contact characteristic between the nanosheets [10].Since the resistance of the sensing film is mainly controlled bythe inter-nanocrystal barrier at the contacts, the characteristicsof the contacts between the nanostructures play a crucial role inthe gas-sensing process. For nanorods or nanowires, the contactsbetween them are point-to-point with small touch areas. Even so,Kolmakov and co-workers have proved that the response valuerises monotonously with the increase of the contact points betweenSnO2 nanowires [42]. In the case of the ZnO nanosheet sensors, adistinct characteristic of the sensing film is that most of the con-

tacts between the sheets are plane-to-plane contacts (called “planecontact”). The touch area of plane-contact is larger, contribut-ing to improving the sensing properties [43]. (4) The “switchingeffect” is notably identified as a result of the characteristically small

Fig. 9. Long-term stability of the sensor to 200 ppm ethanol at 400 ◦C.

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imension (10–40 nm thickness), which might be comparable tohe Debye length (LD, which is about 7.5 nm at 325 ◦C for ZnO44]) based on the grain-control model [45]. This means that thentire nanosheet body is electron depleted in air, resulting in theargest resistance. Once exposed to ethanol, the previously trappedlectrons by oxygen are sent bank to ZnO due to the redox reac-ion between ethanol and the oxygen species, leading to a sharpecrease of the resistance and a great resistance variation i.e., gasensing signal. This obvious size-dependent (comparable to LD) gasesponse phenomenon is vividly called as “switching effect”. Actu-lly, the ultrahigh ethanol response of the ZnO nanosheets shoulde attributed to the combined actions of the aforementioned items.

. Conclusion

In summary, based on a facile, low-temperature (80 ◦C),nvironment-friendly (without any surfactant or organic solvent)ydrothermal method, porous single-crystal ZnO nanosheets wereuccessfully synthesized followed by annealing of the zinc carbon-te hydroxide hydrate precursors at 300 ◦C for 0.5 h in air. The ZnOanosheets exhibited ultrahigh response to ethanol, with a detec-ion limit of as low as 10 ppb at the optimal operating temperaturef 400 ◦C. The ultrahigh ethanol response of the as-synthesized ZnOanosheets is attributed to its large specific surface area, single-rystal structure, plane-contact between nanosheets, 3D networkrchitecture, and characteristically small thickness. Furthermore,he wide detection range, fast response–recovery characteristic,nd good selectivity and stability confirm that the porous single-rystal ZnO nanosheets are excellent sensing materials for theabrication of practical ethanol sensors. In addition, special atten-ion are expected to be paid to the controlled synthesis of quasi-2D

OS nanostructures and comprehensive study of their shape-elated physical/chemical properties, such as adsorption, catalysis,as-sensing, and photodegradation.

cknowledgements

This research was financially supported by the “BaiRen” pro-ram of Chinese Academy of Sciences and Autonomic Researchrogram (2008BWZ009) of Ministry of Science and Technology, PRhina.

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iographies

exi Zhang is currently a candidate for PhD degree in materialogy in the Stateey Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academyf Sciences (SKLCC, ICCCAS). His research is focused on synthesis and gas sensingroperties of low dimensional metal oxide semiconductors.

ianghong Zhao received her PhD degree in catalysis in 2005 from SKLCC, ICCCASnd then was appointed as an associate research fellow in 2007 there. Now, she isnterested in synthesis of carbon and semiconductor nano- and micro-materials andheir applications in the field of energy and environment, especially in photocatal-sis and electrocatalysis.

tors B 161 (2012) 209– 215 215

Haiqiang Lu received his MS degree in chemical engineering in 2005 from NanjingUniversity of Technology. He is currently a candidate for PhD degree in material-ogy in SKLCC, ICCCAS. His research is focused on synthesis and photocatalysis ofsemiconductor nanostructures.

Li Li received her MS degree in 2004 from Tianjin University, majored in synthesisof organic macromolecule polymers.

Jianfeng Zheng received his PhD degree in materialogy in 2009 from SKLCC, ICCCASin major of controllable synthesis and mechanism of one dimensional conductingpolymers. Recently, his research is focused on synthesis and design of nanostruc-tures, and their TEM characterizations.

Hui Li received her PhD degree in materialogy in 2009 from Jilin University. Sheis now a postdoctoral researcher in SKLCC, ICCCAS. Her research is focused on themechanism of hydrogen evolution driven by visible light through computer simu-lations.

Zhenping Zhu received his MS degree in 1992 from Wuhan University and PhDdegree in 2000 from SKLCC, ICCCAS. He jointed the ICCCAS in 1993 and wasappointed as an associated professor in 1997, followed by a full professor in 2000.

In 2002, he was admitted a research fellow of the Alexander von Humboldt Foun-dation with Prof. Dr. Robert Schlögl at Fritz-Haber-Institute of MPG, Germany. In2004, he re-joined ICCCAS and obtained the “Bairen” program support. His recentinterests are assembly of nanostructures, their related mechanism and applicationsin the realm of energy and environment.