Patent Application: US-201414252248-A

Abstract:
a method of fabricating an inorganic fibrous membrane , the method comprising the steps of : grafting sulfonated graphene oxide onto a scaffold of inorganic nanofibers to form a suspension of heterojunctions of the sulfonated graphene oxide and the scaffold ; filtering the suspension through a support to obtain heterojunctions on the support ; drying the heterojunctions on the support ; and removing the support to obtain the inorganic fibrous membrane .

Description:
exemplary embodiments of the invention will be described with reference to fig1 to 13 below . in a first exemplary embodiment of the method ( 100 ) of fabricating the inorganic nanofibrous membrane , cryptomelane - type manganese oxide ( k - oms - 2 ) nanowires 10 were used as a scaffold for synthesis of the inorganic nanofibrous membrane . the k - oms - 2 nanowires 10 were synthesized via a hydrothermal method . in a typical procedure , 19 . 1 mmol of k 2 so 4 , 38 . 2 mmol of k 252 o 8 , and 19 . 1 mmol of mnso 4 . h 2 o were dissolved in 80 ml of deionized water . the solution was then transferred to a 125 ml teflon - lined stainless - steel autoclave . the autoclave was sealed and heated in an oven at 250 ° c . for 4 days . the resulting black precipitate was suspended in 1000 ml of deionized water , and stirred vigorously for 12 h . after thorough washing with deionized water to remove remaining ions present in the product , the sample was dried at 105 ° c . for 24 h . graphene oxide ( go ) was synthesized according to the modification of hummer &# 39 ; s method 19 from natural graphite . subsequently , sulfonated go sheets go - so 3 h were prepared by sulfonating ( 102 ) the go using sodium 2 - chloroethanesulfonate hydrate under ultrasonic conditions , and so 3 h groups were formed on the go sheet in this process . to synthesize the crosslinking agent of go - so 3 h , 100 mg of go sheets , 0 - 3 g of sodium 2 - chloroethanesulfonate hydrate and 0 - 1500 mg of naoh were added into 500 ml deionized water , and the suspension was subjected to ultrasonication for 3 h for reaction to take place . subsequently , 2 ml of concentrated hno 3 was injected into the suspension . after stirring the mixture or suspension for 30 min , the resultant product was centrifuged and washed with ethanol for three times . finally , the product was dispersed into 200 ml deionized water . the sulfonated go sheets were grafted ( 104 ) onto the k - oms - 2 nanofibers to form a hierarchical structure of heterojunctions of the go - so 3 h and the scaffold , as shown in fig1 . the go - so 3 h sheets have a strong affinity for the k - oms - 2 nanowires due to the coordination action between the sulfonic acid group and carbonic acid groups of go - so 3 h and the mn center of the k - oms - 2 nanowire . finally , the go grafted k - oms - 2 nanowires were fabricated into a nanofibrous membrane through flow - directed assembly ( 106 ) by filtration . to do so , a suspension of the synthesized hierarchical k - oms - 2 / go - so 3 h heterojunctions was first well dispersed under ultrasonic agitation . then , the suspension was filtered on a vacuum - filtration setup with a glass filter ( advantec , gc - 50 , 0 . 45 μm ), and the hierarchical k - oms - 2 / go - so 3 h heterojunctions will form a compact cake layer on the glass filter which acts as a support for the heterojunctions . thickness of the membrane can be controlled by the volume of the pulp suspension used / filtered . after drying ( 108 ) the supported heterojunctions at 105 ° c . for 1 day , a free - standing membrane was formed after removal of the glass filter support ( 110 ). the fabricated k - oms - 2 / go - so 3 h nanofibrous membrane exhibits high permeability and selectivity in water filtration process . a typical procedure to fabricate the graphene based nanocomposites membrane is illustrated in fig2 . importantly , the presence of functionalized go can act as a cross linker to assist the interweaving of inorganic nanofibers of the scaffold , resulting in the reduction of membrane pore sizes and the enhancement of the membrane rejection rate . it is also worth noting that the superhydrophilic properties of the k - oms - 2 nanowires and go - so 3 h would enhance the permeability of the synthesized nanofibrous membrane . 6 the morphology of the synthesized materials was characterized with tem , and some representative images are summarized in fig3 a to 3d . as shown in fig3 a , the synthesized go sheet is a transparent thin film with diameters of a few micrometers . the wrinkles and folds can be clearly observed , showing the two dimensional structure of the go sheet . the sulfonated go sheet or go - so 3 h is shown in fig2 b , which reveals that the go sheet kept its original morphology after the sulfonation reaction . the go sheets and go - so 3 h sheets are confirmed as single layer sheets by afm patterns ( fig4 and 5 ). in addition , the go - so 3 h sheet is smaller than the go sheet due to the physical effect of ultrasound , indicating that go - so 3 h sheets have a relatively large contact area with k - oms - 2 nanowires as compared to go sheets . thus , the sulfonic acid group has more chance of forming coordinate bonds with the mn centre of k - oms - 2 nanowires , which would facilitate the grafting of go - so 3 h onto k - oms - 2 nanowires . k - oms - 2 nanowires were prepared via a hydrothermal method . 6 in a typical process , mn 2 + was oxidized by s 2 o 8 22 under constant pressure and temperature for 4 days . as shown in fig3 c , the k - oms - 2 nanowire is about 100 nm in diameter . hrtem ( inset of fig2 c ) reveals that the d - spacing of 0 . 48 nm corresponds to { 002 } planes of monoclinic k 2 - x mn 8 o 16 , which further confirms that the prepared nanowire is k - oms - 2 . fig3 d shows that the go - so 3 h sheets were successfully grafted onto the k - oms - 2 nanowire , constructing a hierarchical heterojunction . owing to the affinity of the sulfonate acid group toward the k - oms - 2 nanowire , go - so 3 h can be used as a cross linker to combine the k - oms - 2 nanowires more tightly , which facilitates the fabrication of a nanofibrous membrane with good selectivity . in previous work , 18 it was found that inorganic nanomaterials could be detached from go sheets by the addition of naoh solution . as shown in fig6 a , the go sheets cannot be grafted onto k - oms - 2 nanowires at a ph of less than 11 . however , the strong nucleation capability of the sulfonic acid group can ensure the stability of the k - oms - 2 / go - so 3 h composite in basic conditions ( fig6 b ), which can extend its applications in water purification . xrd analysis of the synthesized materials is shown in fig7 a . diffraction patterns of go and go - so 3 h show the { 001 } peak of graphite oxide centered at 2θ = 11 . 9 ° and 10 . 5 °, corresponding to interlayer spacings of 7 . 43 å and 7 . 58 å respectively . 20 it can be seen that the interlayer spacing of go - so 3 h is slightly larger than that of the original go , due to the introduction of the ethane sulfonic acid group . moreover , the xrd pattern of go - so 3 h shows a weak broader peak from the graphitic { 002 } diffraction plane centered at 28 = 21 . 95 °, which results from the disordered stacking of functionalized graphene sheets . 2 ° this is caused by the decrease of oxygen containing groups under basic conditions and the transformation from epoxy group to ether group during the functionalization process . 21 , 22 the xrd pattern of k - oms - 2 / go - so 3 h shows clear diffraction peaks from the k - oms - 2 crystalline phase ( jcpds 44 - 1386 ), which are very similar to the k - oms - 2 nanowires . no marked reflections from the { 001 } diffraction plane of go - so 3 h are observed because the regular stacking of go - so 3 h is destroyed by the intercalation of k - oms - 2 nanowires . 18 , 20 fig7 b shows the ftir spectra of go , go - so 3 h , k - oms - 2 and k - oms - 2 / go - so 3 h . the ftir spectrum of go indicates that the broad band ranging from 3600 cm − 1 to 3000 cm − 1 and the band near 1631 cm − 1 can be assigned to the h — o — h stretching vibrations of adsorbed water molecules . 23 the bands at 1730 cm − 1 and 1039 cm − 1 are related to the c ═ o and c — o stretching vibrations of cooh groups , 24 indicating the graphite was oxidized into hydrophilic go with hydroxyl and carboxyl groups . for the spectrum of go - so 3 h , the weak band centered at 1265 cm − 1 and a weak shoulder band centered at 1162 cm − 1 ( inset of fig7 b ) are respectively attributed to the c — o — c bond stretching vibration and the s ═ o stretching vibrations of sulfonic acid , 25 , 26 which reveals that the so 3 h group was successfully grafted onto the go sheet by etherification . for the spectrum of k - oms - 2 , the bands centered at 716 cm − 1 and 531 cm − 1 result from the characteristic vibrations of the o — mn — o bonding . 27 after combining with go - so 3 h , the ftir spectrum of k - oms - 2 / go - so 3 h shows a clear band centered at 1390 cm − 1 , which can be attributed to the coordination between mn and the carboxylic group from go - so 3 h . 28 however , no clear band connected to the coordination between the so 3 h group of go - so 3 h and the mn center can be distinguished , due to the small number of sulfonic groups in the sample . to further identify the so 3 h group in the synthesized k - oms - 2 / go - so 3 h sample , an xps measurement was conducted . the survey spectrum of the k - oms - 2 / go - so 3 h indicates that the sample contains mn , c , o , k , and s , as shown in fig8 . fig9 a to 9c show the high resolution xps spectra of c 1s taken for the different synthesized samples , and the gaussian curve fittings of c 1s were performed to describe the different carbon bonds in the synthesized materials . although both the c — oh groups and the ho — c ═ o groups appeared in all three samples , the c — oh groups are considerably decreased for go - so 3 h and k - oms - 2 / go - so 3 h , indicating that the c — oh groups were partly replaced by c — o — so 3 h groups during the sulfonation reaction . furthermore , a single s 2p peak at 168 ev in the high resolution xps spectrum in fig9 d confirms the existence of a so 3 h group in the k - oms - 2 / go - so 3 h composite . 29 , 30 fig9 e shows the high resolution of mn 2p , and a gaussian curve fitting of mn 2p 3 / 2 displays that the mn 2p 3 / 2 binding energies were centered at 641 . 7 ev and 643 . 0 ev , representing mn 3 + and mn 4 + respectively . 31 the strong nucleophilic effect of the so 3 h group in go - so 3 h toward mn atoms can effectively contribute to the combination of k - oms - 2 and go - so 3 h together with coordination between the carboxylic acid groups and mn atoms . an inorganic nanofibrous membrane was fabricated using the synthesized k - oms - 2 / go - so 3 h heterojunctions via a filtration process . a digital photograph of the synthesized free - standing nanofibrous membrane is shown in fig1 a . the 35 mm - diameter membrane was fabricated via a filtration process , and larger diameter membranes could also be produced . the inset of fig1 a indicates that the synthesized membrane possesses excellent flexibility , which can be freely shaped by tweezers . fig1 b shows a top view fesem image of the membrane , which reveals a relatively flat topology with no observed cracks . from the high - resolution fesem image ( fig1 d ), it can be observed that the open porous network was formed by overlapping and interweaving of the ultra long hierarchical k - oms - 2 / go - so 3 h heterojunctions , and the membrane pore sizes range from 0 . 05 μm to 0 . 2 μm . compared to the k - oms - 2 nanowire membrane without calcination ( fig1 ), the membrane in fig1 d shows much more compact nanowire bundles , due to the presence of go - so 3 h as a cross linker , which downsizes the membrane pore and thus increases the rejection capacity of the membrane in the water purification process . go - so 3 h sheets can enable the k - oms - 2 nanowires to bind together and thus form a free - standing and flexible membrane , which was further confirmed by the afm image in fig1 . in addition , a cross - sectional sem image ( fig1 c ) of the synthesized membrane reveals that the membrane is composed of many layers , and each layer is assembled by the bundles of hierarchical k - oms - 2 / go - so 3 h heterojunctions . the tightly interwoven k - oms - 2 / go - so 3 h heterojunctions can endow the membrane with a compact functional layer . to investigate the permeability of the k - oms - 2 / go - so 3 h membrane , membrane fluxes of deionized water under different transmembrane pressures ( tmp ) were studied in a lab - scale dead - end filtration apparatus . as shown in fig1 a , the permeate flux of the k - oms - 2 / go - so 3 h membrane is highly correlated with tmp ( r 2 = 0 . 991 ) since the only resistance present in the experiments is the intrinsic membrane resistance ( r m ). the k - oms - 2 nanowires were tightly combined because of the crosslinking effect of go - so 3 h , and the k - oms - 2 / go - so 3 h membrane flux increased proportionately with increasing tmp . however , in the absence of go - so 3 h , the degree of binding of the nanowires varied with increasing tmp , resulting in the poorly fitted regression line ( r 2 = 0 . 937 ) in fig1 a . although the k - oms - 2 / go - so 3 h membrane has a lower permeability than the k - oms - 2 membrane due to the presence of the go - so 3 h sheets , the go - so 3 h sheets can enhance the separation efficiency of the nanofibrous membrane . standard polystyrene ( ps ) microsphere solutions with different particle sizes were filtered by the synthesized membranes . as shown in fig1 b , the synthesized k - oms - 2 / go - so 3 h membrane displays a much higher separation efficiency than the k - oms - 2 nanowire membrane , and the retention rates of ps microspheres increase with increasing particle size . since the pore size of a membrane can be defined as the diameter of latex microspheres which are 90 % retained by the membrane , 32 the pore size of the k - oms - 2 / go - so 3 h membrane can be characterized as being around 0 . 2 μm , classifying it under the microfiltration membrane category . fig1 c shows that the 0 . 2 μm ps microspheres can be retained and subsequently accumulated to form a cake layer on the surface of the membrane , and it is predictable that pollutants with larger particle sizes ( larger than 0 . 2 mm ) would be efficiently removed by the membrane . the synthesized k - oms - 2 / go - so 3 h membrane is milking the profits from both the k - oms - 2 nanowires and the go - so 3 h sheets , and consequently possesses excellent permeability and selectivity . in conclusion , there has been successfully synthesized a hierarchical k - oms - 2 / go - so 3 h heterojunction , and a free - standing , flexible nanofibrous membrane was further fabricated using a filtration method . it is important to note that go - so 3 h can act as a cross linker to combine the k - oms - 2 nanowires , which enhances the rejection capacity of the membrane in the filtration process . although the effect of go - so 3 h sheets on the membrane performance can be further investigated , there is no doubt that the synthesized membrane possesses good permeability and selectivity in the water purification process . as a microfiltration membrane , it exhibited excellent rejection capacity on pollutants with particle sizes larger than 0 . 2 μm . it is thus believed that the synthesized membrane can have great potential in membrane applications . moreover , this work provides a novel methodology towards the fabrication of nanofibrous membranes using other inorganic nanomaterials with one dimensional structures . the advantages of using the k - oms - 2 / go - so 3 h nanofibrous membrane are : ( 1 ) the membrane can be fabricated without a calcination process which can reduce the formation of pinholes and cracks within the membrane as well as energy cost , ( 2 ) sulfonated go sheets act as a crosslinking agent which downsize the membrane pores and thus increase the rejection rate of the membrane in water purification processes , ( 3 ) the membrane exhibits excellent flexibility which can be freely shaped , and ( 4 ) due to the super - hydrophilic property of the k - oms - 2 nanowires and go - so 3 h , the membrane possesses great permeability . whilst there has been described in the foregoing description exemplary embodiments of the present invention , it will be understood by those skilled in the technology concerned that many variations in details of design , construction and / or operation may be made without departing from the present invention . for example , other variants of nanofibrous membranes may be fabricated using go - so 3 h as the crosslinking agent and another material as the inorganic nanofibrous scaffold . accordingly , other inorganic nanofibrous materials such as tio 2 nanofiber , mno 2 nanofiber , sno 2 nanowire , srtio 3 nanofiber , and fe 3 o 4 nanofiber can be chosen as the scaffold of the membrane , which would endow the synthesized membranes with differential properties for other multifunctional applications such as catalysis , adsorption , oxidation and disinfection . exemplary variants of the synthesized membranes are listed below : mno 2 / go - 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