PreparationandGasSensingPropertiesofNovelCdS-SupramolecularOrganogelHybridFilmsXiaHuiyunPengJunxiaLiuKaiqiangFangYu【精品論文大全】.doc

精品論文推薦preparation and gas sensing properties of novel cds-supramolecular organogel hybrid films xia huiyun，peng junxia，liu kaiqiang，fang yukey laboratory of applied surface and colloid chemistry of ministry of education，school ofchemistry and materials science，shaanxi normal university，xian （710062），prce-mail：abstracta novel cds-supramolecular organogel hybrid film with an unusual surface morphology has been fabricated by exposing a supramolecular organogel film containing cd(ac)2 in an h2s atmosphere at room temperature. the organogel film was prepared by spin-coating a lmog (low-molecular mass organic gelator) gel of dmethyl sulfoxide (dmso) onto a glass plate substrate. xrd, sem, eds,tg-dta, uv-vis, pl (photoluminescence) spectroscopy and pl lifetime measurements were employed to characterize the film. it was shown that the organogel film had functioned as a template to control the morphology of the final hybrid film. the quantities and sizes of the cds particles embedded in the organogel film can be easily altered by varying the initial concentration of cd(ac)2. importantly, the pl of the hybrid film is very sensitive to the presence of some organic monoamines and diamines. the selectivity and reversibility of the sensing process were investigated.keywords：cds，organic monoamines and diamine，lmog，hybrid films，sensing1introductionamines are a family of compounds, and have become intense pollutants due to their extensive uses in the preparation of fertilizers, pharmaceuticals, surfactants, biological buffers, and colorants, etc.1a. furthermore, volatile amines can be found in agricultural areas, and presence of them may be taken as an indicator of decayed food, as is the case of fish products1b, and thereby accurate and fast detection of amines is of great interests. it is well known that cds films and particles have been widely used in gas-sensing2. ellis and co-workers have embarked on the area for years, and found that n-cds or n-cdse in single crystal state can be used as a “l(fā)uminescent litmus test” to sense the presence of a variety of lewis acids and bases. the photoluminescence (pl) was found to increase when n-cds or n-cdse was exposed to lewis bases and decrease when exposed to lewis acids3. the direction of the pl response appears to reflect adsorbate-induced changes in the semiconductors depletion width and can be modeled by treating a near-surface region of this approximate thickness as non-emissive layer (dead-layer model). the photoconductivity and electrical conductivity of cds have been used for the detection of co and so2, respectively4, 5. in addition, nh3 can be also determined by monitoring thepl emission of a powered cds:te phosphor6.different from fluorescence sensors based upon organic fluorophores, the gas-sensing properties of cds are highly dependent upon its morphological and microstructural features, such as particle size, size distribution, shape and density, etc.7. recently, considerable efforts have been devoted to develop film sensors. this is because film sensors are, generally speaking, re-usable, and easy to be made into devices8. film sensors can be fabricated by employing vacuum evaporation, chemical vapor deposition (cvd), molecular beam epitaxy (mbe), sputtering, electrochemical co-deposition, and electrochemical atomic layer epitaxy (ecale), etc.9. however, many of these known methods suffer from one or more drawbacks such as prolonged post-process (e.g., annealing) time, expensive apparatus, and production of a large amount of wastewater, etc.9e.this work is supported by natural science foundation of china (nos. 20674048), doctoral program foundationof the ministry of education of china (nos. 20040718001, 306015).- 4 -chemical gels are interesting materials, in which gelator molecules are crosslinked into network structures and the solvent are fixed by the networks. it is these gels that have been used as templates for the “bottom-up” fabrication of various nano-structured inorganic and inorganic-organic composite materials10. it is believed that the gel networks can control and direct the crystallization and precipitation of the inorganic compounds formed within the gels, and result in composite materials with specific structures11. unlike chemical gels, physical gels formed by low-molecular-weight organic gelators (lmogs) are thermo-reversible, and of particular importance and interest because of the essential differences in properties and forming mechanisms12. very recently, these physical gels have been adopted as templates to produce various nano-structured materials, including rods, belts, tubes, balls etc.13. in addition, integration of inorganic nano-particles into the interior of a gel network have aroused great interest since this would result in inorganic/organic nano-composite materials, which would combine the advantages of inorganic, organic and nano-materials and offer opportunities to explore their novel collective mechanical, thermal, optical, magnetic, and electronic properties10,12.on the basis of above discussion and the work already conducted in our group14, an organogelfilm based upon a lmog was employed as a template for the preparation of organogel-cds hybrid film. it was found that the hybrid film occupies an unusual morphology, and the pl is selectively sensitive to the presence of some organic monoamines and diamines. the details are reported in this paper.2experimental details2.1 materialsallreagentsareanalyticallypure.cd(ac)2,thioacetamide,boc-l-alanine,cholesterol, dicyclohexylcarbodiimide (dcc), n,n-dimethylamino-pyridine (dmap), and sodium hydroxide were used as received. dichloromethane, tetrahydrofuran (thf), benzene, dimethyl sulfoxide (dmso), n-hexane and triethylamine (tea) were used after further purification by standard literature methods. other chemicals, such as methylamine aqueous solution, butylamine, octylamine, dodecylamine, aniline,n-methylaniline,n,n-dimethylaniline,acetylaniline,ethylenediamine(eda),1,3-propanediamine (pda), 1,4-butanediamine (bda), 1,5-pentanediamine, 1,6-hexanediamine (hda), pyridine, ammonia, water, hydrochloric acid, acetic acid, methanol, ethanol, formaldehyde, acetone and cyclohexane were used without further purification. water used throughout was deionized and then double distilled.2.2 preparation of a gel filmthe gelator, di-acid amides of di-cholesteryl l-alaninates, was prepared and characterized in the way as reported in a former report14c. in a typical preparation of the gel film, 0.0125 g of the gelator and 1 ml of dmso containing cd(ac)2 were placed into a sealed glass tube, and the system was heated in an oil bath until the gelator was dissolved. then, suck out 200 l of the hot solution, and drop it onto the surface of a pre-cleaned and heated glass plate ( 0.90 cm3.50 cm). finally, the glass plate wasgradually cooled to room temperature, and a stable organogel film was produced.2.3 preparation of organogel-cds hybrid filmafter 5 h aging, the film containing cd(ac)2 was transferred into a specially designed reactor. then, the reactor was connected to a clean h2s source via a drying tube. after two days reaction, a cds/organogel hybrid film was fabricated as evidenced by a change in the color of the film. the color of the hybrid film can be from light yellow to orange, depending upon the amount of cds produced, a property which can be employed to visually inspect the reaction degree. it is to be noted that the amounts of cds deposited in the hybrid films were controlled by varying the initial cd(ac)2concentrations, which are 5.010-3 m, 5.010-2 m, 5.010-1 m, 1.0 m, and saturated cd(ac)2 solutionin dmso, respectively. it is to be noted that there are no observable changes in the appearances and pl performances of the films after six months storage in a desiccator. the preparation of the hybrid films was schematically shown in scheme 1.3characterization methodsscanning electron microscopy (sem) and energy-dispersive x-ray spectroscopy (eds) measurements were performed with a philips, quanta-200 scanning electron microscopy spectrometer. the accelerating voltage was 15 kv, and the current was 10 ma. the film samples used for sem measurements were prepared by firstly drying in air at room temperature for 3 days, and then vacuum dried at 50 c for 24 h. the cracked cds/organogel film can be obtained by vacuum dried at 80 c for48 h. prior to examination, the film was attached to a copper holderbyusingconductive adhesive tape, and then it was coated with a thin layer of gold.x-ray powder diffraction (xrd) analysis was carried out byusingajapanrigakud/max- x-ray diffractometerwith cu k radiation at 35 kv and 40 ma. a scan rate of 0.02/s was applied to record the pattern in the 2 range of 10-80.thermo-gravimetricanalyses were performed using aperkin-elmer tga-7 instrument.the measurement was conducted in an atmosphere of air at a scan rate of 10 c/min. the filmswere dried at 50 c for two daysscheme 1. schematics of the preparation of a cds/organogel hybrid film.before measurement. then the powders were scraped carefully from the glass slides, and were used as samples for characterization.uv-vis absorption spectroscopy measurements were carried out on a perkin-elmer lambd 950uv-vis-nir spectrophotometer at room temperature.pl measurements were performed at room temperature on a time-correlated single photon counting fluorescence spectrometer (edinburgh instruments fls 920) with a front face method. the fabricated film was inserted into a quartz cell with the cds/organogel surface facing the interior of it. the position of the film was kept constant during each set of measurements.4pl quenchingto a cell with the film inside, 10 l of the solvent was carefully added, and the cell was sealed immediately. after that, the fluorescence spectrum of the film was recorded every 3 min. the quenching efficiency was calculated using the equation of (i0-i)/i0, where i0 and i stand for the plintensity of the film at the absence of quencher and that at the presence of the quencher, respectively.5results and discussion5.1 characterization of the filmsthe template film and the hybrid film have been characterized by measuring the morphologies, compositions, and pl behavior of them. figure 1 shows the sem images of the gel film, the cds/organogel film and the cracked cds/organogel film. it can be seen that the solvent-free gel filmconsists of relatively uniform cross-linked spherical microparticles, a commonly found morphology offigure 1. the sem images of an organogel film (a), a cds/organogel hybrid film (b), a higher magnification of the hybrid film (c), and the cracked hybrid film (d). (the initial concentration of cd(ac)2 is 0.50 m)aggregates of lmogs in organogels, of which the diameters are around 1.5 m (c.f. figure 1a)14. no doubt, the microparticles are the aggregates of the lmog molecules used to fabricate the gel film. as expected, the morphology of the solvent-free hybrid film is also characterized by microparticles, atypical result of template directed and controlled fabrication (c.f. figure 1b). however, it is to be noted that the diameters of the microparticles of this film are from 3 to 5 m, as shown in figure 1c, significantly larger than that of the template particles. the difference might be partially originated from the difference of the composition of the films because the hybrid particles should be harder than thetemplate particles, and result in less shrinkage during drying of the films. another reason might be精品論文推薦merger of the gel particles due to coagulation of the formed cds fine precipitates.it should be interesting to reveal whether the microparticles in the hybrid films are solid or hollow microspheres. figure 1d depicts the sem picture of the cracked hybrid film, which was prepared by long time vacuum drying it at a higher temperature. a typical hollow structure is obviously observed. it is worthwhile to mention that the sizes of the aggregates of the lmogs can be easily regulated by varying the conditions of the preparation, such as the concentration of the gelators, the temperature changing speed, the amount of the inorganic compound deposited etc.the hybrid films were further examined by eds and xrd analyses. eds analysis at different positions of the annealed films indicate that it is homogeneous in composition. relative atomic concentrations of cd and s are 49.36%and 50.64%, respectively, which is closeintensity (a.u.)to1:1stoichiometry(supporting information, figure s1). the annealed filmshad beenadoptedforthe compositionmeasurement toavoid contaminants from the solvent, dmso, which also contains sulfur. the xrd curves of the films are shown in figure 2. it is seen that there are four distinct diffraction peaks, which appear at 26.5,33.5, 44.1, and 51.5, respectively, inthe xrd curve of the hybrid film. the111200220311abfour diffractions can be indexed as (111), (200), (220), and (311) of a cubic cds phase by comparison with the data from jcpds no. 10-454. in contrast, there are no shark peaks appearing in the xrdcurve of the gel film. only a broad band10203040506070802 (degrees)figure 2. the xrd patterns of the cds/organogel hybrid film (a) and that of the organogel film (b) (the initial concentration of cd(ac)2 is 0.50 m) ).appeared at 2 values of 15 around, which is a clear evidence of presence of organics, corresponding to the organic nature of the gel films.the cds contents in the hybrid films have been determined by a thermo-analysis method. it wasfound, with no surprising, that the contents ofcds in the films are dependent upon the initial concentration of cd(ac)2 in the template gel provided sufficient h2s was supplied. for the films with 5.010-3 m, 5.010-2 m, 5.010-1 m,1.00 m, and saturated dmso solution of the salt as their initial cencentrations, 1.65, 3.23,pl (norm)6.42, 13.73, 21.38 wt% of cds were found in the final hybrid films, respectively.the steady-state pl spectra of five hybrid films with different contents of cds are depicted in figure 3. the emission spectrawere recorded with 400 nm as the excitation1.00.2a b c d e104counts (10 )86420400 450 500 550 600 650 700wavelength (nm)- 5 -wavelength. reference to the figure, reveals: (1) the emission is purely originated from cds because the background, organogel containing cd(ac)2, almost has no emission if compared450 500 550 600 650 700750wavelength (nm)figure 3. normalized photoluminescence spectra (ex=400 nm) of the organogel-cds hybrid films containing (a) 1.65 wt%, (b) 3.23 wt%, (c) 6.42 wt%, (d) 13.73 wt%, (e) 21.38 wt% of cds, respectively. the inset is the pl emission spectra of theorganogel containg cd(ac)2 (the red) and hybrid film (the black) (ex=400 nm).with those of the hybrid films, as shown in the inset; (2) the profiles of the emissions are broad and have no fine structures; (3) the maximum emission of the hybrid films shifts from 495 nm to 554 nm along with increasing the content of cds from 1.65 wt% to 21.38 wt%, an evidence of size effect. in literature, emissions in such a wavelength range are usually attributed to charge carrier recombination in trap states, which are related with low crystallinity, sulfur excess and other defects at the interface of the nano-crystals15. in fact, trap and band edge emissions are often observed together in cds nano-crystals15e,16, but the trap emission can be enhanced enormously by increasing the amount of defects16. in this way, the emission via the band-edge recombination mechanism can be completely suppressed. this might be the reason why the band-edge emission has not been observed in our samples. similar observations have been reported by luccio and co-workers in the studies of controlled nucleation and growth of cds nano-particles in a polymer matrix17. it is believed that the red-shift of the emission of the hybrid film should be a resultof increase of the sizes of the embedded cdsparticles. a similar explanation can be found in other literatures18.the red-shift of the pl emissions of thedf(r) (a.u)hybrid film is in parallel to the shift observed incthe diffuse reflectance (dr) uv-vis spectra (c.f.figure 4). reference to the figure, it is to beobserved that the band-edges of the spectrabaround 500 nm shift to longer wavelengths alongaewith increasing the initial concentration of the salt, cd(ac)2. this phenomenon has been- 11 -observed by others and was concluded that the edge of the spectrum shifts to shorter wavelength as the sizes of the cds micro-particles are getting smaller 19.400 500 600wavelength (nm)figure 4. diffuse reflectance uv-vis spectra of the cds/organogel films containing 1.65 wt% (a), 3.23 wt% (b), 6.42 wt% (c), 13.73 wt% (d), and 21.38 wt% (e) of cds, respectively.2.05counts (10 )1.51.0excitation wavelengths decreasing0.50.0300 400 500 600 700 800wavelength (nm)figure 5. the pl spectra of the cds/organogel (cds, 1.65 wt%)film excited at different wavele