Patent Application: US-201013502956-A

Abstract:
a method for generating hydrogen is disclosed . the method includes substantially submersing nanowires having metallic nanoparticles into water , exposing the water submerged nanowires to light , and collecting hydrogen gas produced by photolysis from the exposure to light .

Description:
it takes 1 . 23 ev to cleave a water molecule . however , water is transparent to near infrared as well as visible radiation , and can only absorb photons in excess of 6 . 47 ev ( i . e ., in ultraviolet ). however , photolysis of water can be driven by visible light photons , if they are first absorbed by a molecule or semiconductor and the energy is transferred to electron - hole ( e − and p + , respectively ) pairs . this reaction is governed by the equation : where hω is the energy of a single photon , h and ω being the angular frequency of the photon and planck &# 39 ; s constant , respectively . subsequently , the holes generated in the valence band of the semiconductor oxidize water , following the equation : while the photogenerated electrons are channeled to the cathode to reduce h + as hence , the overall reaction is the splitting of water to oxygen and hydrogen according to both of the redox reactions above involve charge transfer across a solid - electrolyte interface . the mechanism of the charge transfer is tunneling or phonon assisted tunneling . the former has an appreciable probability only between states of the same energy . whereas , the latter is probable from higher to lower energy , creating phonons , through an intermediate localized ( e . g ., defect ) state . hence , a major challenge in an efficient photolysis cell , is the wise engineering of charge transfer pathways , which are favorable for electron / hole pairs . otherwise , electron / hole pairs can choose the competing path of recombination , dumping their energy into heat or luminescence . another undesirable competing path is the oxidation of the semiconductor . fig1 illustrates electron energy band diagram of what is considered by the photolysis literature as the “ ideal photolysis cell ”. the arrows indicate the direction of electron flow . the energy bands in the semiconductor are bent due to lining up of the fermi levels , ef , in the semiconductor and solution ( h + / h 2 ). hence , the semiconductor - electrolyte interface is equivalent to a schottky barrier . here , the bending is in the favorable direction , where the built - in field pushes a photogenerated hole towards the electrolyte for the oxidation reaction , while the electron is swept away in the opposite direction to be channeled to the metal . this way , the recombination of the electron / hole pair is also impeded . finally , the electrons leave the semiconductor through an ohmic contact , which was indium in the original demonstration by fujishima and honda . subsequently , the electrons are conducted to the counter electrode through a metal wire . the desired band bending in fig1 is attained if the work function of the n - type semiconductor is less than that of h + / h 2 , which is 4 . 5 ev below the vacuum level for ph = 0 . for a band bending of 0 . 5 ev , the semiconductor work function therefore should be 4 . 0 ev . shown in the same energy diagram , is h 2 o / o 2 located 1 . 23 ev below h + / h 2 . for the oxidation reaction to be favorable , the valence band edge , e v , should align with h 2 o / o 2 or lie lower . in the latter case , surface states located in the gap ( i . e ., defects ) mediate the phonon assisted tunneling as also depicted in the figure . although these states facilitate photolysis in terms of charge transfer , they may also be harmful if they also assist in electron - hole recombination . with a nanowire radius of less than 10 nm , in the embodiments described in the present disclosure , there will not be considerable band bending . further , the electron and hole energies can be in excess of band edges if the ballistic carrier transport dominates and thermalization is absent . noble metal nanoparticles allow for a fascinating phenomenon ; the metal electrons can collectively couple with an incident electromagnetic field , once the wavelength is ˜ 10 or more times larger than the nanoparticle size . these modes , known as localized surface plasmons ( lsp ), indeed strongly couple with incident photons , and concentrate them in the close vicinity of metal nanoparticles . as an example , these enhanced near fields lead to dramatically enhanced optical signals from molecules adsorbed on these particles . in the specific case of surface - enhanced raman scattering ( sers ) for example , the signal gains can be as gigantic as 10 12 enabling detection and imaging of single molecules . in 1996 , yet another attribute of lsp &# 39 ; s was discovered . stuart and hall demonstrated that metal nanoparticle layers can couple incident light into the waveguide modes of a thin si film ( i . e ., 160 nm thick ) on - insulator detector , leading to increased optical absorption . interestingly , the observed enhancement peaked at a certain wavelength , which was different than the lsp resonance wavelength . hence , the enhancement was not due to lsp modes directly . indeed , the enhancement peak showed negligible variation from au to ag to cu nanoparticles . in contrast , the lsp wavelengths for these three metals differ significantly . stuart and hall , on the other hand , observed a trend between the enhancement and lsp wavelength . they found that the enhancement factor increased as the lsp wavelength and the wavelength of peak enhancement came closer . these observations strongly suggested that the enhancement in absorption was due to coupling of light into a waveguide mode mediated by lsp modes . the present disclosure provides physical mechanisms , structural architecture , and fabrication technique for the realization of a novel fuel - generating ( e . g ., hydrogen ) photolytic device . in one embodiment , a device consists of a low band gap oxide semiconductor nanowire decorated with metal nanoparticles . the technology offers low - cost , high photolytic conversion energy and stability by making use of multifunctional nanostructures with unique electronic , photonic , and plasmonic attributes at the nanoscale . referring now to fig2 , a schematic diagram illustrating the operation of one embodiment of a semiconductor nanowire — metal nanoparticle conjugate system according to the present disclosure is shown . the present embodiment of the disclosed nanowire - nanoparticle conjugate device exploits a fortunate combination of effects or mechanisms unique to its structure . fig2 illustrates the essential electron excitation and transfer steps ( steps 1 to 6 ) and electron states in geometrical representation . fig3 is the complementary description in an energy diagram representation . the energy levels are drawn to scale and an energy scale bar is provided . in both representations , the steps are numbered the same . below is the description of the steps . ( 2 ) capture of electron by high work function metal nanoparticle ( e . g ., au ). this mechanism , being absent in bulk photolytic cells , facilitates efficient separation of electron - hole pairs . nanoparticles collect the photogenerated electrons efficiently by virtue of their being a high work function metal before channeling them to the reduction reaction . this aspect differs from having a counter metal electrode ( cathode ) in a bulk photolytic cell , where a semiconductor / cathode interface is absent . the energy offset at the semiconductor / metal interface serves as a check - valve and permits the passage of the electrons from semiconductor to metal only . ( 3 ) the hole is conducted to the nanowire / water interface , where it oxidizes water . equivalently , the hole steals an electron from the water . this reaction replenishes the h + and the electron needed for the reduction reaction , which produces h 2 . here the h 2 o / o 2 is located 5 . 73 ev below the vacuum level . assuming the conduction band edge of vanadia is ˜ 4 ev below the vacuum level as in other transition metal oxides and having measured the band gap as ˜ 2 . 2 ev , the valence band edge is estimated to be at ˜ 6 . 2 ev below the vacuum level . although h 2 o / o 2 and e v are offset by 0 . 2 - 0 . 3 ev , the electron transfer can occur through the surface states by dissipation of the excess energy to a phonon as argued in the photolysis literature . by virtue of the wire diameter being at the nanoscale , there is efficient transport of the photogenerated careers to nanowire / metal and nanowire / electrolyte interfaces for the reduction and oxidation reactions , respectively . because redox reactions occur at the nanowire and nanoparticle surfaces , high surface to volume nature of these structures at the nanoscale increases the fuel production rate . ( 4 ) metal nanoparticles also serve as the cathodes for channeling the electrons to h + . from fig3 , this step is inferred to be not efficient for au due to the energy offset . namely , h + / h 2 and au fermi level are 4 . 5 and 5 . 3 ev below the vacuum level . here , the electron transfer requires energy intake , which is difficult to happen with phonon absorption ( i . e ., boltzmann factor & lt ;& lt ; 1 ). however , the proof of concept of the present disclosure clearly shows photolysis and hydrogen production . the inventor explains this unexpected success in terms of a novel property of the metal nanoparticles . in case electrons are not efficiently channeled to h + , they accumulate in the au nanoparticle and charge it negatively . subsequently , the increasing negative electric potential will align the fermi level in au with h + / h 2 resulting in efficient channeling of electrons . this self - alignment of energy levels is the attribute of ultralow nanoparticle capacitance , where a charge of countable electrons can induce a potential up to 1 v or more . apart from the novel electronic steps described above , fig2 also illustrates two other novel mechanisms regarding efficient absorption of electromagnetic radiation in the semiconductor nanowires . these mechanisms are illustrated as ( 5 ) and ( 6 ) in fig2 . ( 5 ) in addition to functioning as cathodes , plasmonic metal nanoparticles decorating the nanowires serve as near - field concentrators enhancing absorption of light in the semiconductor nanowire . ( 6 ) the plasmonic nanoparticles also couple the incident radiation into the waveguide modes of the nanowires in the direction of the wire axis , maximizing optical absorption . this mechanism of light - trapping has great potential in benefiting nanowire based photolytic devices . once the incident radiation is coupled into waveguide modes propagating in the direction of nanowire axis , the dramatic gain in optical path length inside the semiconductor ( i . e ., along the nanowire length rather than width ) ensures efficient light absorption . in one embodiment , the decoration of nanowires with plasmonic nanoparticles may be accomplished by an exposure of the nanowires to a metal salt solution . in other words , the semiconductor nanowire may be multifunctional , and they may as well serve as a reducing agent for reduction synthesis of the metallic nanoparticles . as recently demonstrated by the inventor , v 3 o 7 . h 2 ) nanowires can reduce ag + and au + ions leading to the formation of metal nanoparticles attached to them . this simplicity translates to low cost in fabrication . because redox reactions occur at the nanowire and nanoparticle surfaces , high surface to volume nature of these structural components at the nanoscale increases the fuel production rate . further , photogenerated electrons and holes have to be transported from where they are generated in the nanowire to the nanowire / metal and nanowire / electrolyte interfaces for the reduction and oxidation reactions , respectively . therefore , length scale of the wire diameter being at the nanoscale , renders the charge transfer efficient . in particular , a new mode of transport , namely ballistic transport , takes place for transportation distances less than the electron mean free path . in this new regime , no thermalization of the photogenerated electrons to the conduction band edge of the semiconductor occurs , before they reach the nanowire - metal interface . similar argument can be iterated for the photogenerated holes . thereby , more efficient channeling of the photogenerated electrons and holes to the corresponding redox reactions is enabled . the present disclosure enables an efficient photolytic system based on nanoelectronics and nanophotonics . when the nanowire diameter approaches to electron mean free path , a new regime of charge transport , namely ballistic transport , takes place that increases the conversion energy . nanoparticle - mediated coupling of incident radiation to nanowire waveguide modes is also contemplated . in one embodiment , the present disclosure teaches a materials architecture consisting of nanowires and nanoparticles only . both material components are multifunctional as well as optically and electronically coupled . the technology to be developed is low cost , because all fabrication steps are based on solution synthesis that can be carried out to vast quantities . in one working example given below , the system employs a low bandgap oxide semiconductor , v 3 o 7 . h 2 o . the low band gap of v 3 o 7 . h 2 o ensures efficient absorption of visible light with no sacrifice in stability ( i . e ., no photocorrosion ). although the nanowires of the present disclosure are not limited to a particular semiconductor , the current example is accomplished by utilization of v 3 o 7 . h 2 o nanowires . additionally , in some embodiments , other structures such as carbon nanotubes , or nanofibers , may be utilized . in the present example , v 3 o 7 . h 2 o nanowires were prepared by supercritical drying of wet vanadia gels obtained via methods known in the art . in one embodiment , as shown in fig4 a mixture of 5 . 58 ml of deionized water and 11 . 34 ml of acetone is added to 2 . 4 ml of vanadium ( v ) tripropoxide , vo ( och 2 ch 2 ch 3 ) 3 . in order to slow down the gelation process , solutions are cooled in an ice bath until ice appeared in water / acetone solution and vanadium ( v ) tripropoxide became more viscous . prior to the mixing , water / acetone solution is shaken vigorously until ice chunks disappear . the solution is then added into vanadium ( v ) tripropoxide at once in order to initiate the gelation . in contrast to methods known in the art , the obtained mixture was not shaken . the mixture was then transferred to the sealed molds . the wet gels were aged for five days . after aging , gels are washed with anhydrous acetone by changing acetone once every 24 hours for four times . finally , the v 3 o 7 . h 2 o gel was supecritically dried with co 2 at 40 ° c . and 1200 psi . the v 3 o 7 . h 2 o gel is dissolved in de - ionized ( di ) water to a suspension of nanowires prior to nanoparticle synthesis . although supercritical drying may be replaced by ordinary drying of acetone in ambient conditions , the former provides the most efficient extraction of residual byproducts from the gelation reaction . as stated earlier , v 3 o 7 . h 2 o nanowires have a multifunctional role in the present disclosure . in addition to being the light absorber and carrier transporter , v 3 o 7 also serves as the reducer for the synthesis of plasmonic nanoparticles . an au - decorated v 3 o 7 . h 2 o nanowire suspension was synthesized by mixing of 1 ml of v 3 o 7 . h 2 o nanowire suspension in water ( 3 . 4 g / l ) with 1 ml of haucl 4 ( 0 . 002 m ). this step is also shown in the process flow diagram of fig4 . fig5 shows a micrograph of the v 3 o 7 . h 2 o nanowires decorated with au nanoparticles . the au nanoparticles are typically seen as darker “ dots ”. for the synthesis of ag nanoparticles , haucl 4 was replaced by agno 3 . in addition , it may be necessary to expose the nanowire and metal salt solution to electromagnetic radiation to enable reduction of the nanoparticles on the nanowires by photochemical reduction . referring now to fig6 , optical absorbance of the v 3 o 7 . h 2 o nanowires prior to metal nanoparticle synthesis is shown . a nanowire concentration of 1 . 7 g / l in water , from which band gap was deduced to be 2 . 18 ev is shown . referring now to fig7 , the additional ( enhanced ) absorbance in v 3 o 7 . h 2 o nanowires ( 0 . 34 g / l ), when they are decorated with au nanoparticles , is shown . interestingly , multiple resonances are found with no coincidence with the localized surface plasmon resonance for au nanoparticles ( i . e . ˜ 520 nm ). this observation indicates that the peaks correspond to plasmonic nanoparticle — mediated coupling of light into waveguide modes in the v 3 o 7 . h 2 o nanowires . photolysis was performed in septum - sealed 4 ml uv - vis optical cells enclosing the nanowire - nanoparticle conjugate device suspension . photolysis was conducted under led radiation of 405 , 425 , 470 , 525 , and 605 nm . in these demonstrations , the suspension was heated to 40 ° c . rapid gas bubbling , as depicted in fig8 , was immediately observed once the led radiation was turned on . prior to gc , the vial was filled with the nanowire - nanoparticle suspension leaving 1 ml air volume . subsequently , the air was purged by argon for 10 min through a pair of needles piercing the septum making sure no air is left inside the vial . next , the photolysis was started and continued for 30 min . for gas chromatography ( gc ) analysis , the photolysis was conducted with 470 nm led irradiation at an incident power of 11 mw . then , the vial was taken to the gc analysis , and then the collected gas mixture in the vial was injected to the gc system . gc was performed using an agilent technologies model 6890n gas chromatographer ( gc ). samples are injected via a hamilton 100 μl syringe model 1710sl , coupled to a gastight 22s / 2 ″/ 2 attachment , and 26s / 2 ″/ 5 needle . the system utilizes a supelco 1010 capillary column with dimensions 30 m × 320 μm × 15 μm ( length × inside diameter × nominal film thickness ) and uses argon as the carrier gas . a splitless inlet is used , which purges the flow at 12 . 1 ml / min at 1 minute into the testing . the gc has two separate systems for gas detection : the front detector using a flame ionization detector ( fid ) and the back detector using a thermal conductivity detector ( tcd ). for the detection of hydrogen , a tcd is required , due to the widespread use of hydrogen gas as the fuel in most fid units . the crucial parameters for the testing were determined through multiple measurements , and gas volumes introduced into the system . the initial temperature ( t i ) of the system was set at 32 ° c . and held for 12 minutes . due to the interaction of the gas with the system , a ramp is used at a rate of 30 ° c ./ minute and final temperature ( t f ) of 236 ° c . the flow rate of the carrier gas is initially 0 . 4 ml / min ramped to 0 . 8 ml / min at a rate of 0 . 1 ml / min 2 . the total time required for one measurement is 20 minutes . fig9 a and 9b show the gc spectrum after injection of 250 μl gas from the photolysis vial . the spectrum was found to be highly reproducible after several photolysis batches with the same suspension or different suspensions . the assignments for the peaks are shown by peak labels in the spectra . for comparison , spectra of air and 30 % h 2 ( 40 % co + 30 % co 2 ) are also provided . as different from the air , the photolysis sample shows the h 2 peak and a more intense moisture peak . the water peak is attributed to the evaporating water in the vial during photolysis . the n 2 in the photolysis sample is anticipated to be associated with the air , which occupies the dead volume of the gas syringe . same is observed for the 30 % h 2 reference sample , which intentionally contains no air . in the photolysis sample , the ratio of o 2 to n 2 is slightly larger compare to that in air . higher ratio of o 2 in the photolysis sample is credited to splitting of water to both h 2 and o 2 during photolysis . thus , the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein . while presently preferred embodiments have been described for purposes of this disclosure , numerous changes and modifications will be apparent to those of ordinary skill in the art . such changes and modifications are encompassed within the spirit of this invention as defined by the claims . 3 . c . a . grimes , s . ranjan , and o . k . varghese , “ light , water , hydrogen ,” springer science + business media , llc , new york , n . y . 10013 , usa ( 2008 ). 4 . k . rajeshwar , r . mcconnell , and s . licht , “ solar hydrogen generation : toward a renewable energy future ,” springer science + business media , llc , new york , n . y . 10013 , usa ( 2008 ) 5 . a . fujishima and k . honda , “ electrochemical photolysis of water at a semiconductor electrode ,” nature 238 , 37 ( 1972 ). 6 . l . i . halaoui , n . m . abrams , and t . e . mallouk , “ increasing the conversion efficiency of dye - 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