WO2009065179A1 - Nanosheets with band gap modification agent and method of production thereof - Google Patents

Nanosheets with band gap modification agent and method of production thereof Download PDF

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WO2009065179A1
WO2009065179A1 PCT/AU2008/001727 AU2008001727W WO2009065179A1 WO 2009065179 A1 WO2009065179 A1 WO 2009065179A1 AU 2008001727 W AU2008001727 W AU 2008001727W WO 2009065179 A1 WO2009065179 A1 WO 2009065179A1
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metal oxide
band gap
modification agent
equal
group
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PCT/AU2008/001727
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French (fr)
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Gao Qing Lu
Lianzhou Wang
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The University Of Queensland
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Definitions

  • the present invention generally relates to nanosheets and a method of production thereof. More particularly, the invention relates to metal oxide nanosheets coupled with a band gap modification agent, their uses and methods of production thereof.
  • titania nanosheets Exfoliated titania (Ti 0 .9iO 2 ) nanosheets have been extensively investigated due to their unique physicochemical properties and potential functionalities (Sasaki, T., et al. J. Am. Chem. Soc. 1996, 118, 8329-8335; Sasaki, T., et al. Chem. Mater. 1997, 9, 602-608; Sasaki, T. Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682-4689; and Tanaka, T.; et al. Chem. Mater.
  • Exfoliated Ti 0 9 i O 2 nanosheets generally have a lateral length of about several hundred nanometres and thickness of about 0.75 nm. These nearly two-dimensional nanosheets can be thought of as paper-like building blocks for the fabrication of a variety of nanostructures.
  • T1 0.91 O 2 nanosheets have been studied for application in photocatalysis, photoelectrochemical water splitting, photodegradation and superhydrophilicity (Choy, J-H., et al. J. Mater. Chem. 2001 , 11 , 2232-2234; Choy, J-H., et al. Chem. Mater. 2002, 14, 2486-2491 ; Wang, L.Z., et al J.
  • T1O.91O2 nanosheets involve a photoexcitation process, which is highly dependent upon the electronic structure of the Ti 09I O 2 nanosheets. Exfoliated Tio .9 i0 2 nanosheets have a sharp optical absorption peak in the UV light range.
  • metal oxide nanosheets are ineffective as photocatalysts in the visible light range and/or are expensive to produce.
  • the invention provides a method of producing metal oxide nanosheets coupled with a band gap modification agent.
  • a method of producing metal oxide nanosheets coupled with a band gap modification agent including the steps of: a) protonating a layered metal oxide to form a protonated layered metal oxide; b) exfoliating the protonated layered metal oxide to form metal oxide nanosheets; and c) coupling the metal oxide nanosheets with a band gap modification agent to form metal oxide nanosheets coupled with the band gap modification agent.
  • the metal oxides may be any metal oxide having a layered structure which may or may not have intercalated cations within the layer space.
  • the metal oxides may also be doped with a non-metal dopant, such as, for example, nitrogen, fluorine, boron, carbon, sulphur, iodine and phosphorous.
  • the metal oxide may be selected from titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, tantalum and vanadium oxides and mixed oxides thereof.
  • the metal oxides may also be selected from the group having formula (I):
  • A is a cation selected from the group comprising lithium, sodium, potassium, calcium, magnesium, rubidium, caesium and francium
  • M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, 5 zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;
  • D is a dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is an value selected between equal to or greater than 0 and equal to or less than 8; O y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and less than 8; and z-x is always a value greater than 0. 5
  • the protonation step of the method may be carried out by adding the metal oxide to an acidic solution.
  • the acidic solution may be selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO 3 ) and periodic acid (HIO 4 ).
  • hydrochloric acid nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO 3 ) and periodic acid (HIO 4 ).
  • the acidic solution is selected from hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid.
  • the acidic solution is preferably 0.001 M to 15M hydrochloric acid.
  • the protonated layered metal oxides may be selected from the group having formula (II):
  • A is a cation selected from the group comprising lithium, sodium,0 potassium, calcium, magnesium, rubidium, caesium and francium;
  • M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;
  • D is a dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine;
  • n is an value either equal to or greater than m and is a value greater than
  • m is an value selected greater than 0 and equal to or less than 8; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and less than 8; and z-x is always a value greater than 0.
  • the exfoliating step of the method may be carried out by mixing or adding an exfoliating agent to the protonated metal oxide.
  • the exfoliating agent is preferably an organic compound selected from the group comprising tetraalkylammonium hydroxides, such as tetrabutylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetramethylammonium hydroxide.
  • the metal oxide is preferably in contact with the exfoliating agent for a period of between 1 hour and two weeks. More preferably for 1 to 10 days. Most preferably approximately 7 days.
  • the exfoliation step is preferably carried out at a temperature of between room temperature and 6O 0 C. Most preferably the exfoliation step is carried out at room temperature.
  • the band gap modification agents may be one or more agents which make or cause the metal oxide nanosheets to absorb visible light once coupled.
  • the band gap modification agent may be any inorganic or organic chemical or compound which when adsorbed onto the surface of the metal oxide nanosheets extends or increases the light absorption of the metal oxide nanosheets in the visible light range.
  • the band gap modification agent is preferably a semiconductor or a chromophore. If the band gap modification agent is a semiconductor with the band gap smaller than 3.5 eV, then the band gap modification agent may be selected from the group comprising quantum dots; conductive nanowires, nanotubes or nanorods; inorganic dopants, such as metal impurities; or the like.
  • the band gap modification agent may be any inorganic or organic compound which is capable of absorbing visible light.
  • the band gap modification agent may be an inorganic molecule, such as iodine, bromine, fluorine, nitrogen, phosphorus and the like; or an organic molecule such as porphyrin and its derivatives, ruthenium based dyes and the like.
  • the band gap modification agent is iodine, preferably in clusters.
  • the band gap modification agent is preferably dissolved or suspended in a suitable solvent such as ethanol, methanol, water or the like, prior to coupling with the metal oxide nanosheets.
  • the method may further comprise the step of separating the metal oxide nanosheets coupled with a band gap modification agent.
  • the separation step may require the addition of a flocculant.
  • the flocculant may be any known flocculant which is suitable for separating suspended inorganic particles from solution.
  • the flocculant may be a natural organic product such as starch, guar or the like, acidic solution, a solution comprising inorganic or organic salts which may provide cations such as Li + , Na + , K + , Mg 2+ , Ca 2+ , Al 3+ and the like; nanoclusters, such as Keggin type ions; inorganic nanoparticles; organic macromolecules such as dyes and the like, which assist with the separation of the metal oxide nanosheets coupled with a band gap modification agent.
  • the invention provides metal oxide nanosheets coupled with a band gap modification agent having a formula (III): M y O z-x D x [ G ] R (III) wherein:
  • M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;
  • D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine;
  • y is a value greater than 0 and equal to or less than 8;
  • z is a value greater than 0 and equal to or less than 8;
  • x is a value equal to or greater than 0 and less than 8;
  • G is a band gap modification agent
  • R is a value greater than 0 and equal to or less than 9.
  • the band gap modification agent is preferably selected from the group comprising: semiconductors, such as quantum dots, conductive nanowires, nanotubes or nanorods, inorganic impurities or dopants, such as metal impurities; and chromophores, such as iodine, bromine, fluorine, nitrogen, phosphorus and the like, or organic molecules such as porphyrin and its derivatives, ruthenium based dyes and the like.
  • the band gap modification agent is a chromophore selected from the group comprising iodine, bromine, fluorine, nitrogen and phosphorous.
  • the metal oxide nanosheets coupled with a band gap modification agent of formula (III) is preferably Ti 0.9 i ⁇ 2 [l 2 ] R wherein R is a value greater than 0 and equal to or less than 9.
  • the invention provides use of metal oxide nanosheets coupled with a band gap modification agent as a photocatalyst.
  • the invention provides a method of coating a substrate with metal oxide nanosheets coupled with a band gap modification agent, including the step of applying to a substrate at least one layer of metal oxide nanosheets coupled with a band gap modification agent of formula (III).
  • the substrate may be any substrate suitable for supporting a photocatalytic film.
  • the substrate is glass, quartz glass, silicon wafer or ITO glass.
  • the metal oxide nanosheets coupled with a band gap modification agent may be applied to the substrate using a dip coating method.
  • the substrate remains in a suspension of metal oxide nanosheets coupled with a band gap modification agent for a period of between 1 minute and 2 hours. More preferably the substrate is placed in the suspension for a period of between 15 to 60 minutes. Most preferably the substrate remains in the suspension for approximately 20 minutes before being rinsed and dried.
  • the substrate may also have one or more layers of binder applied thereto.
  • the binder may be an organic polyelectrolyte or charged inorganic nanocluster.
  • the polyelectrolyte may be selected from the group comprising polyethylenimine (PEI); poly(allylamine hydrochloride); and poly(diallyldimethylammonium) chloride.
  • the charged inorganic nanoclusters may be selected from the group comprising partially hydrolysed nanoclusters of metal hydroxides and oxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.
  • metal hydroxides and oxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.
  • the binder is preferably applied using a dip-coating method in which the substrate is placed in a solution of binder for a period of between 1 minute and 2 hours.
  • the substrate is placed in the solution of binder for a period of between 15 to 60 minutes. More preferably the substrate remains in the solution of binder for approximately 20 minutes before being rinsed and dried.
  • the method of coating a substrate may further include applying additional and/or alternating layers of binder, such as polyelectrolyte or inorganic nanocluster, and metal oxide nanosheets coupled with a band gap modification agent
  • a further aspect of the invention provides a substrate coated with metal oxide nanosheets coupled with a band gap modification agent of formula (III).
  • FIG. 1 is a schematic illustration of an embodiment of the invention
  • FIG. 2 shows UV-visible absorption spectra comparing Ti 0 . 9 iO 2 nanosheets coupled with different concentrations of iodine clusters: wherein the amount of iodine clusters coupled to the nanosheets is a) 0mol%; b) 2mol%; c) 4mol%; d) 6mol%;
  • FIG. 3 illustrates XRD patterns for the " I ⁇ 0.91 O2 nanosheets coupled with different amounts of iodine clusters, namely a) 0mol%; b) 2mol%; c) 4mol%; and d) 6mol%;
  • FIG. 4 is a schematic illustration of the structure of a Ti 0.9 i ⁇ 2 nanosheet coupled with iodine clusters and illustrates the interlayer space
  • FIG. 5 is a comparison of BET surface area analysis results for the
  • FIG. 6 is XPS spectra of the Tio .9 i ⁇ 2 nanosheets coupled with different amounts of iodine clusters, namely a) Omol%; b) 2mol%; c) 4mol%; and d) 6mol%;
  • FIG. 7 is a graph of binding energies calculated for the Tio. 9 i0 2 nanosheets coupled with different amounts of iodine clusters, namely a) Omol%; b) 2mol%; c) 4mol%; and d) 6mol%;
  • FIG. 8 is a schematic illustration of a structure of an embodiment of the invention (Ti 0 .9iO 2 [bin);
  • FIG. 9 is a graph comparing the catalytic decomposition of rhodamine using an intermediate, protonated nitrogen doped titanate, and Ti 0 . 9 i ⁇ 2 nanosheets coupled with iodine clusters.
  • the metal oxide nanosheets coupled with one or more band gap modification agents may be produced by: forming a protonated layered metal oxide by adding a layered metal oxide to an acidic solution; exfoliating the protonated layered metal oxide to form a suspension of metal oxide nanosheets; adding a solution of a band gap modification agent to the suspension; and allowing the band gap modification agent to be absorbed onto or coupled with the metal oxide nanosheets to form metal oxide nanosheets coupled with a band gap modification agent.
  • Step 2 Protonated layered metal oxide Exfoliating Exfoliated metal oxide nanosheets (H m A n-m M y O z-x D x ) * (M y O z . x D x )
  • step 1 leads to the complete or partial replacement of any intercalated cations (A) with protons in the layered metal oxide to form protonated metal oxide of formula (II).
  • the protonated metal oxide nanosheets may be separated and dried.
  • the exfoliation step illustrated in step 2, involving the addition of an exfoliating agent to the protonated metal oxide of formula (II), is preferably carried out in suspension. This step results in the exfoliation of the protonated metal oxide through the exchange of protons with cations from the exfoliating agent which alleviates or eliminates the attractive forces between the layers of the metal oxide to form metal oxide nanosheets.
  • the metal oxide nanosheets may be coupled with the band gap modification agent(s) by adding a solution or suspension comprising the appropriate band gap modification agent(s) to a suspension of metal oxide nanosheets.
  • the metal oxides of step 1 may be any metal oxide having a layered structure which may or may not have intercalated cations within the layer space.
  • the metal oxides may also be doped with a non-metal dopant, such as nitrogen, fluorine, boron, carbon, sulphur, iodine and phosphorous.
  • the metal oxide may be selected from titanium, lanthanum, niobium, tungsten, nickel, iron, cobalt, tantalum and vanadium oxides and mixed oxides thereof.
  • the metal oxide may be a compound of formula (I), described above.
  • the method may further include the step of reacting a metal oxide precursor with a dopant to form a doped metal oxide, which may be summarised as follows:
  • the metal oxide precursor is preferably a compound having the formula (IV):
  • A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium;
  • M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; n is a value greater than or equal to 0 and equal to or less than 8; and y and z are independently a value greater than 0 and equal to or less than 8.
  • the metal oxide precursor of formula (IV) is Cso .68 Ti 1. a 3 O4 .
  • the dopant may be any compound or composition which is capable of donating the appropriate dopant atoms to form a non-metal doped metal oxide.
  • the non- metal dopant may be an inorganic or organic compound, in solid, liquid or gas form.
  • the dopant is a gas which may be selected from one or more of the following: nitrogen, ammonia, methane, ethane, propane, butane, gas comprising B x H y , carbon monoxide, carbon dioxide, hydrogen sulphide or fluorine.
  • the dopant is a gas it is preferably supplied together with an inert or non- reactive gas, such as air, argon, helium, or hydrogen.
  • the dopant gas and the non-reactive gas are present in a 1 :1 volume ratio.
  • the dopant when it is an organic compound it may comprise one or more of the following C 6 Hi 2 N 4 , CO(NH 2 )2, CS(NH 2 ) 2 , triethylamine, (NH 4 ) 2 CO 3 ⁇ C 25 H 31 N 3 , Ci 2 H 22 On, C 25 H 3 oO 5 , C 6 Hi 2 , CeHi 2 O 2 , CeHi 2 BNOa, 0 7 HsBF 4 O 2 , C 7 H 7 BO 4 , H 3 N-BH 3 , C 6 H 5 N(C 2 Hs) 2 BH 3 , CS(NH 2 ) 2 , C 7 H 7 SO 2 , C 7 Hi 2 O 2 S, C 6 H 4 S, C 4 CI 2 F 6 , C 4 H 2 F 2 N 2 , C 4 H 8 BrF, C 4 H 9 I, C 5 H 3 IO 2 , C 5 H 3 FI, or C 6 H 13 I.
  • C 6 Hi 2 N 4 CO(NH 2 )2, CS(NH
  • the dopant may also be selected from one more inorganic compounds or solutions including carbon, boron, H 3 BO 3 , sulphur, (NH 4 ) 2 S, iodine, HIO 3 , HIO 4 , NH 4 I, or NH 4 IO 3 .
  • the metal oxide precursor when it is calcined with the dopant it is also in the presence of one or more non-reactive or inert gases, selected from the group comprising oxygen, hydrogen, argon, helium, and air.
  • one or more non-reactive or inert gases selected from the group comprising oxygen, hydrogen, argon, helium, and air.
  • the doping step is preferably carried out by calcining the metal oxide precursor in contact with the dopant.
  • the metal oxide precursor is calcined with the dopant at a temperature of between 200 0 C to 1800 0 C for a period of between 30 minutes and 5 days.
  • the metal oxide precursor is calcined in contact with the dopant at a temperature of between 600 0 C and 1000 0 C for a period of between 30 minutes and 3 days.
  • the metal oxide precursor may be calcined in contact with the dopant at a temperature of about 700 0 C for a period of about 60 minutes.
  • the doped metal oxide as described above, has a layered structure in which metal oxide layers may be intercalated with a cation (A). The dopant stoichiometrically replaces oxygen from the metal oxide layers to form the non- metal doped metal oxide.
  • the metal oxide precursor may be formed by heating a cation donor precursor with a metal oxide donor, which may be summarised as follows: calcination Cation donor precursor + Metal oxide donor ⁇ A n M y O z
  • the cation donor precursor is an alkali earth metal salt or an alkali metal salt selected from the group comprising: alkali metal halides; alkali earth metal halides; alkali metal sulphides; alkali earth metal sulphides; alkali metal sulphates; alkali earth metal sulphates; alkali metal carbonates; alkali earth metal carbonates; alkali metal nitrates; alkali earth metal nitrates; alkali metal hydroxides; alkali earth metal hydroxides; alkali metal acetates; alkali earth metal acetates; alkali metal dimethenylamine (AN(CH 2 ) 2 ); alkali earth metal dimethenylamine (AN(CH 2 ) 2 ); alkali metal oxide; alkali earth metal oxides; alkali metal chlorate; alkali earth metal chlorate; alkali metal phosphate and/or alkali earth metal phosphate.
  • the metal oxide donor is selected from the group comprising metal oxides or hydroxides, including TiO, Ti 2 O 3 , Ti 3 O 5 , TiO 2 , TiO x Ny, TiO x Cy, Ti(OH) 4 .xH 2 O; mixed oxides of titanium, such as lanthanum titanium oxide; niobium oxides and mixed oxides thereof, such as calcium niobium oxide; nickel oxides, cobalt oxides, ferric and ferrous oxides, tantalum oxides; vanadium oxides; and tungsten oxides; metal nitride compounds, such as titanium nitride (TiN), niobium nitride, tantalum nitride and vanadium nitride; metal carbide compounds, including titanium carbide (TiC); metal cyanamide compounds, including titanium cyanamide (TiC x Ny); metal boride compounds including titanium diboride (TiB 2 ); metal sulphide compounds, including titanium sulphide, including
  • the cation donor precursor and the metal oxide donor may be calcined at a temperature of between 500 0 C to 1200 0 C for a period of between 0.5 and 40 hours.
  • the cation donor precursor and the metal oxide donor are calcined at a temperature of between 600 0 C and 1000 0 C for a period of between 2 hours and 30 hours.
  • the cation donor precursor and the metal oxide donor are most preferably calcined at a temperature of about 75O 0 C for a period of about 20 hours.
  • the metal oxide nanosheets when coupled with the band modification agent may be used in suspension to form coatings and films. Alternatively they may be separated and dried.
  • Step 3 of the method outlined above and subsequent application of the metal oxide nanosheets coupled with a band gap modification agent is schematically represented in Figure 1.
  • the flocculant may include an acidic solution; a solution comprising inorganic or organic salts which may provide cations such as Li + , Na + , K + , Mg 2+ , Ca 2+ , Al 3+ and the like; nanoclusters, such as Keggin type ions; inorganic nanoparticles; and organic macromolecules such as dyes and the like, which are capable of providing cations to the non-metal doped metal oxide nanosheets or capable of encouraging the re-ordering of the nanosheets.
  • an acidic solution a solution comprising inorganic or organic salts which may provide cations such as Li + , Na + , K + , Mg 2+ , Ca 2+ , Al 3+ and the like
  • nanoclusters such as Keggin type ions
  • inorganic nanoparticles such as organic macromolecules such as dyes and the like, which are capable of providing cations to the non-metal doped metal oxide nanosheets or capable of encouraging the re
  • the metal oxide nanosheets coupled with band gap modification agents may be used in photocatalytic applications, such as self cleaning coatings, decomposition of organic compounds, and hydrogen production from the photocatalytic splitting of water.
  • a layered metal oxide precursor of Cso 6 ⁇ Tii 83 ⁇ 4 was prepared by mixing caesium carbonate (CS 2 CO 3 ), with titania (Ti ⁇ 2 ), and calcining at 75O 0 C in air for 20 hours. Approximately 60 - 70 grams of white crystalline caesium titanate (Cso 6 ⁇ Tii 83 O 4 ) was collected.
  • the protonated form of the nitrogen doped titanate (Ho ⁇ Tii 83 O 4 ) was prepared by the ion-exchange of the caesium in Cso ⁇ Th 83 O 4 with protons by placing the Cso esTii 83 O 4 in 1 M HCI solution for three days.
  • the Ho ⁇ sTh 83 ⁇ 4 was separated and dried.
  • the resultant yellow H 0.6 sTi 1.83 O 4 (1.2g) was dispersed in an exfoliating agent, tetrabutylammonium hydroxide (TBA + OH " ) solution (300 cm 3 , 0.2M) 1 and was shaken for more than 7 days at room temperature to exfoliate the protonated nitrogen doped titanate into a yellow colloidal suspension of Tio .9 iO 2 nanosheets.
  • TSA + OH " tetrabutylammonium hydroxide
  • iodine 100 mg was first dissolved in 300 ml_ of ethanol solution, and then different amounts of the iodine solution was added into the 100 mg Tio .9 iO 2 nanosheets suspension and stirred for approximately 2 hours to obtain the Ti 0.9 i ⁇ 2 nanosheets coupled with different concentrations or loadings of iodine clusters, namely, (a) 0mol%; (b) 2mol%; (c) 4mol%; and (d) 6mol% of iodine.
  • the iodine-loaded Ti 09 i ⁇ 2 nanosheets were then flocculated by adding acid to form restacked layered structure coupled with iodine. The resultant flocculent was recovered by filtration and dried at room temperature.
  • the UV-Visible absorption spectra were recorded for these coupled nanosheets (a) to (d), as above.
  • the comparative UV-Visible absorbance spectra are shown in Fig 2. It can be seen that as the amount of band gap modification agent, i.e. iodine clusters, increases so does the absorption in the visible range.
  • Fig 3 illustrates comparative XRD patterns for the compounds (a) to (d), described above.
  • the interlayer spacing was calculated from the XRD patterns and found to be between 1.22nm to 1.46nm.
  • band gap modification agents or iodine in this particular example, form clusters or group together. These clusters are believed to adhere or adsorb onto the surface of the metal oxide nanosheets. It is also believed that when the coupled nanosheets are separated from solution and dried, that the iodine clusters create a porous structure between the layers of nanosheets.
  • the interlayer structure is believed to be mesoporous in nature.
  • a hypothetical structure of Ti 0.9 iO 2 nanosheets coupled with iodine clusters is illustrated in Fig 4, which also illustrates the interlayer spacing "d" between nanosheets.
  • the BET surface area results show that samples of small amounts of iodine clusters coupled to the titania nanosheets have similar BET surface area to that of uncoupled titania nanosheets.
  • higher coupling with iodine clusters also leads to higher visible light absorption.
  • XPS spectra were recorded for (a) to (d) and are summarised in Fig 6.
  • the binding intensities were also measured; these are illustrated in Fig 7. Both of these figures appear to support the hypothesis that the iodine is coupled to the titania nanosheets in distinct clusters.
  • the coupled nanosheets of this example once separated from solution, form a bilayer structure of TiO ⁇ layers with iodine clusters between the layers.
  • the structure of these coupled nanosheets is illustrated schematically in Fig 8.
  • the photocatalytic ability of th.e Tio.9 1 O 2 nanosheets coupled with iodine clusters was compared to that of an intermediate, protonated nitrogen doped titanate (HTiON), by examining the activity of the respective photocatalysts in the decomposition of rhodamine.
  • HTiON protonated nitrogen doped titanate
  • a solution of rhodamine was formed and the respective photocatalysts added to the solution.
  • the 20 mg of iodine coupled titania nanosheets were added per 100 ml of rhodamine solution.
  • the protonated nitrogen doped titanate photocatalysts was added in a concentration of 100 mg per 100 ml of rhodamine solution.
  • the Tio . gi ⁇ 2 nanosheets coupled with iodine clusters was much more effective at the photodecomposition of the rhodomine dye, as illustrated in Fig 9.

Abstract

Metal oxide nanosheets coupled with a band gap modification agent having a formula (III): MyOz-xDx [ G ]R (III) wherein: M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and equal to or less than 8; G is a band gap modification agent; and R is a value greater than 0 and equal to or less than 9, and method of production thereof.

Description

NANOSHEETS WITH BAND GAP MODIFICATION AGENT AND METHOD OF PRODUCTION THEREOF
Field of the Invention
The present invention generally relates to nanosheets and a method of production thereof. More particularly, the invention relates to metal oxide nanosheets coupled with a band gap modification agent, their uses and methods of production thereof.
Background Art
Increasing attention is being directed to nanoparticles and nanosheets and their potential use in photocatalytic reactions. One notable example is titania nanosheets. Exfoliated titania (Ti0.9iO2) nanosheets have been extensively investigated due to their unique physicochemical properties and potential functionalities (Sasaki, T., et al. J. Am. Chem. Soc. 1996, 118, 8329-8335; Sasaki, T., et al. Chem. Mater. 1997, 9, 602-608; Sasaki, T. Watanabe, M. J. Am. Chem. Soc. 1998, 120, 4682-4689; and Tanaka, T.; et al. Chem. Mater. 2003, 15, 3564-3568). Exfoliated Ti0 9i O2 nanosheets generally have a lateral length of about several hundred nanometres and thickness of about 0.75 nm. These nearly two-dimensional nanosheets can be thought of as paper-like building blocks for the fabrication of a variety of nanostructures. T10.91O2 nanosheets have been studied for application in photocatalysis, photoelectrochemical water splitting, photodegradation and superhydrophilicity (Choy, J-H., et al. J. Mater. Chem. 2001 , 11 , 2232-2234; Choy, J-H., et al. Chem. Mater. 2002, 14, 2486-2491 ; Wang, L.Z., et al J. Phys. Chem. B 2004, 108, 4283-4288, Tanaka, T., et al Adv. Mater. 2004, 16, 872-875; Kim, T. W., et al. Adv. Fund. Mater. 2007, 17, 307-314). All of these potential applications of T1O.91O2 nanosheets involve a photoexcitation process, which is highly dependent upon the electronic structure of the Ti09IO2 nanosheets. Exfoliated Tio.9i02 nanosheets have a sharp optical absorption peak in the UV light range. As a result exfoliated Ti091O2 nanosheets require the use of external sources of UV light in order to irradiate the Ti0 9iO2 so that it may be effective as a photocatalyst. The need to use external irradiation sources adds to the expense of using these Ti0giθ2 nanosheets in photocata lytic applications.
Known metal oxide nanosheets are ineffective as photocatalysts in the visible light range and/or are expensive to produce.
Summary of the Invention
In one aspect the invention provides a method of producing metal oxide nanosheets coupled with a band gap modification agent.
In a non-limiting example, there is provided a method of producing metal oxide nanosheets coupled with a band gap modification agent, including the steps of: a) protonating a layered metal oxide to form a protonated layered metal oxide; b) exfoliating the protonated layered metal oxide to form metal oxide nanosheets; and c) coupling the metal oxide nanosheets with a band gap modification agent to form metal oxide nanosheets coupled with the band gap modification agent.
The metal oxides may be any metal oxide having a layered structure which may or may not have intercalated cations within the layer space. The metal oxides may also be doped with a non-metal dopant, such as, for example, nitrogen, fluorine, boron, carbon, sulphur, iodine and phosphorous. The metal oxide may be selected from titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, tantalum and vanadium oxides and mixed oxides thereof. The metal oxides may also be selected from the group having formula (I):
AnMy02-xDx (I) wherein: A is a cation selected from the group comprising lithium, sodium, potassium, calcium, magnesium, rubidium, caesium and francium; M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, 5 zirconium, hafnium, molybdenum, chromium, tantalum and vanadium;
D is a dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is an value selected between equal to or greater than 0 and equal to or less than 8; O y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and less than 8; and z-x is always a value greater than 0. 5 The protonation step of the method may be carried out by adding the metal oxide to an acidic solution. For example, the acidic solution may be selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO3) and periodic acid (HIO4). 0
Preferably the acidic solution is selected from hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid. In a particular example, the acidic solution is preferably 0.001 M to 15M hydrochloric acid. 5 According to a particular example, the protonated layered metal oxides may be selected from the group having formula (II):
HmAn-mMyOz-xDx (II) wherein:
A is a cation selected from the group comprising lithium, sodium,0 potassium, calcium, magnesium, rubidium, caesium and francium;
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is an value either equal to or greater than m and is a value greater than
0 and less than or equal to 8; m is an value selected greater than 0 and equal to or less than 8; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and less than 8; and z-x is always a value greater than 0.
The exfoliating step of the method may be carried out by mixing or adding an exfoliating agent to the protonated metal oxide. For example, the exfoliating agent is preferably an organic compound selected from the group comprising tetraalkylammonium hydroxides, such as tetrabutylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide, and tetramethylammonium hydroxide.
The metal oxide is preferably in contact with the exfoliating agent for a period of between 1 hour and two weeks. More preferably for 1 to 10 days. Most preferably approximately 7 days.
The exfoliation step is preferably carried out at a temperature of between room temperature and 6O0C. Most preferably the exfoliation step is carried out at room temperature.
The band gap modification agents may be one or more agents which make or cause the metal oxide nanosheets to absorb visible light once coupled. The band gap modification agent may be any inorganic or organic chemical or compound which when adsorbed onto the surface of the metal oxide nanosheets extends or increases the light absorption of the metal oxide nanosheets in the visible light range. The band gap modification agent is preferably a semiconductor or a chromophore. If the band gap modification agent is a semiconductor with the band gap smaller than 3.5 eV, then the band gap modification agent may be selected from the group comprising quantum dots; conductive nanowires, nanotubes or nanorods; inorganic dopants, such as metal impurities; or the like.
If the band gap modification agent is a chromophore, then the band gap modification agent may be any inorganic or organic compound which is capable of absorbing visible light. The band gap modification agent may be an inorganic molecule, such as iodine, bromine, fluorine, nitrogen, phosphorus and the like; or an organic molecule such as porphyrin and its derivatives, ruthenium based dyes and the like.
Most preferably the band gap modification agent is iodine, preferably in clusters.
The band gap modification agent is preferably dissolved or suspended in a suitable solvent such as ethanol, methanol, water or the like, prior to coupling with the metal oxide nanosheets.
The method may further comprise the step of separating the metal oxide nanosheets coupled with a band gap modification agent. The separation step may require the addition of a flocculant.
The flocculant may be any known flocculant which is suitable for separating suspended inorganic particles from solution. The flocculant may be a natural organic product such as starch, guar or the like, acidic solution, a solution comprising inorganic or organic salts which may provide cations such as Li+, Na+, K+, Mg2+, Ca2+, Al3+ and the like; nanoclusters, such as Keggin type ions; inorganic nanoparticles; organic macromolecules such as dyes and the like, which assist with the separation of the metal oxide nanosheets coupled with a band gap modification agent.
In another aspect the invention provides metal oxide nanosheets coupled with a band gap modification agent having a formula (III): MyOz-xDx [ G ]R (III) wherein:
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and less than 8;
G is a band gap modification agent; and
R is a value greater than 0 and equal to or less than 9.
The band gap modification agent is preferably selected from the group comprising: semiconductors, such as quantum dots, conductive nanowires, nanotubes or nanorods, inorganic impurities or dopants, such as metal impurities; and chromophores, such as iodine, bromine, fluorine, nitrogen, phosphorus and the like, or organic molecules such as porphyrin and its derivatives, ruthenium based dyes and the like.
Most preferably the band gap modification agent is a chromophore selected from the group comprising iodine, bromine, fluorine, nitrogen and phosphorous.
The metal oxide nanosheets coupled with a band gap modification agent of formula (III) is preferably Ti0.9iθ2[l2]R wherein R is a value greater than 0 and equal to or less than 9.
In yet another aspect the invention provides use of metal oxide nanosheets coupled with a band gap modification agent as a photocatalyst.
In a further aspect the invention provides a method of coating a substrate with metal oxide nanosheets coupled with a band gap modification agent, including the step of applying to a substrate at least one layer of metal oxide nanosheets coupled with a band gap modification agent of formula (III).
The substrate may be any substrate suitable for supporting a photocatalytic film. Preferably the substrate is glass, quartz glass, silicon wafer or ITO glass.
The metal oxide nanosheets coupled with a band gap modification agent may be applied to the substrate using a dip coating method. Preferably the substrate remains in a suspension of metal oxide nanosheets coupled with a band gap modification agent for a period of between 1 minute and 2 hours. More preferably the substrate is placed in the suspension for a period of between 15 to 60 minutes. Most preferably the substrate remains in the suspension for approximately 20 minutes before being rinsed and dried.
Preferably the substrate may also have one or more layers of binder applied thereto. The binder may be an organic polyelectrolyte or charged inorganic nanocluster.
The polyelectrolyte may be selected from the group comprising polyethylenimine (PEI); poly(allylamine hydrochloride); and poly(diallyldimethylammonium) chloride.
The charged inorganic nanoclusters may be selected from the group comprising partially hydrolysed nanoclusters of metal hydroxides and oxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.
The binder is preferably applied using a dip-coating method in which the substrate is placed in a solution of binder for a period of between 1 minute and 2 hours. Preferably the substrate is placed in the solution of binder for a period of between 15 to 60 minutes. More preferably the substrate remains in the solution of binder for approximately 20 minutes before being rinsed and dried.
The method of coating a substrate may further include applying additional and/or alternating layers of binder, such as polyelectrolyte or inorganic nanocluster, and metal oxide nanosheets coupled with a band gap modification agent
A further aspect of the invention provides a substrate coated with metal oxide nanosheets coupled with a band gap modification agent of formula (III).
Brief Details of the Drawings
In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings provided by way of example which illustrate preferred embodiments of the invention, and wherein:
FIG. 1 is a schematic illustration of an embodiment of the invention;
FIG. 2 shows UV-visible absorption spectra comparing Ti0.9iO2 nanosheets coupled with different concentrations of iodine clusters: wherein the amount of iodine clusters coupled to the nanosheets is a) 0mol%; b) 2mol%; c) 4mol%; d) 6mol%;
FIG. 3 illustrates XRD patterns for the "IΪ0.91O2 nanosheets coupled with different amounts of iodine clusters, namely a) 0mol%; b) 2mol%; c) 4mol%; and d) 6mol%;
FIG. 4 is a schematic illustration of the structure of a Ti0.9iθ2 nanosheet coupled with iodine clusters and illustrates the interlayer space
"d"; FIG. 5 is a comparison of BET surface area analysis results for the
TΪ0.91O2 nanosheets coupled with different amounts of iodine clusters, namely a) Omol%; b) 2mol%; c) 4mol%; and d) 6mol%;
FIG. 6 is XPS spectra of the Tio.9iθ2 nanosheets coupled with different amounts of iodine clusters, namely a) Omol%; b) 2mol%; c) 4mol%; and d) 6mol%;
FIG. 7 is a graph of binding energies calculated for the Tio.9i02 nanosheets coupled with different amounts of iodine clusters, namely a) Omol%; b) 2mol%; c) 4mol%; and d) 6mol%;
FIG. 8 is a schematic illustration of a structure of an embodiment of the invention (Ti0.9iO2 [bin); and
FIG. 9 is a graph comparing the catalytic decomposition of rhodamine using an intermediate, protonated nitrogen doped titanate, and Ti0.9iθ2 nanosheets coupled with iodine clusters.
Description of the Preferred Embodiments
The metal oxide nanosheets coupled with one or more band gap modification agents may be produced by: forming a protonated layered metal oxide by adding a layered metal oxide to an acidic solution; exfoliating the protonated layered metal oxide to form a suspension of metal oxide nanosheets; adding a solution of a band gap modification agent to the suspension; and allowing the band gap modification agent to be absorbed onto or coupled with the metal oxide nanosheets to form metal oxide nanosheets coupled with a band gap modification agent.
The method can be summarised as follows: Protonation Step 1 Layered metal oxide p- Protonated layered metal oxide
(AnMyOz-xDx) (HmAn-mMyOz-xDx)
Step 2 Protonated layered metal oxide Exfoliating Exfoliated metal oxide nanosheets (HmAn-mMyOz-xDx) * (MyOz.xDx)
Coupling Step 3 Exfoliated metal oxide nanosheets ^ Coupled metal oxide nanosheets
(MyOz-xDx) MyOz.xDx [G]R
It is believed that the protonation or ion exchange of step 1 leads to the complete or partial replacement of any intercalated cations (A) with protons in the layered metal oxide to form protonated metal oxide of formula (II).
If required, the protonated metal oxide nanosheets may be separated and dried.
The exfoliation step, illustrated in step 2, involving the addition of an exfoliating agent to the protonated metal oxide of formula (II), is preferably carried out in suspension. This step results in the exfoliation of the protonated metal oxide through the exchange of protons with cations from the exfoliating agent which alleviates or eliminates the attractive forces between the layers of the metal oxide to form metal oxide nanosheets.
The metal oxide nanosheets may be coupled with the band gap modification agent(s) by adding a solution or suspension comprising the appropriate band gap modification agent(s) to a suspension of metal oxide nanosheets.
The metal oxides of step 1 may be any metal oxide having a layered structure which may or may not have intercalated cations within the layer space. The metal oxides may also be doped with a non-metal dopant, such as nitrogen, fluorine, boron, carbon, sulphur, iodine and phosphorous. The metal oxide may be selected from titanium, lanthanum, niobium, tungsten, nickel, iron, cobalt, tantalum and vanadium oxides and mixed oxides thereof. Alternatively the metal oxide may be a compound of formula (I), described above. When the method of the invention utilises a metal oxide of formula (I), the method may further include the step of reacting a metal oxide precursor with a dopant to form a doped metal oxide, which may be summarised as follows:
doping
AnMyO2 ^ AnMyOz-xDx
The metal oxide precursor is preferably a compound having the formula (IV):
AnMyO2 (IV) wherein:
A is a cation selected from the group comprising lithium, sodium, potassium, rubidium, calcium, magnesium, caesium and francium;
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; n is a value greater than or equal to 0 and equal to or less than 8; and y and z are independently a value greater than 0 and equal to or less than 8.
Preferably the metal oxide precursor of formula (IV) is Cso.68Ti1.a3O4.
The dopant may be any compound or composition which is capable of donating the appropriate dopant atoms to form a non-metal doped metal oxide. The non- metal dopant may be an inorganic or organic compound, in solid, liquid or gas form.
Preferably the dopant is a gas which may be selected from one or more of the following: nitrogen, ammonia, methane, ethane, propane, butane, gas comprising BxHy, carbon monoxide, carbon dioxide, hydrogen sulphide or fluorine. If the dopant is a gas it is preferably supplied together with an inert or non- reactive gas, such as air, argon, helium, or hydrogen. Preferably, though not necessarily, the dopant gas and the non-reactive gas are present in a 1 :1 volume ratio.
Preferably, when the dopant is an organic compound it may comprise one or more of the following C6Hi2N4, CO(NH2)2, CS(NH2)2, triethylamine, (NH4)2CO3ι C25H31N3, Ci2H22On, C25H3oO5, C6Hi2, CeHi2O2, CeHi2BNOa, 07HsBF4O2, C7H7BO4, H3N-BH3, C6H5N(C2Hs)2 BH3, CS(NH2)2, C7H7SO2, C7Hi2O2S, C6H4S, C4CI2F6, C4H2F2N2, C4H8BrF, C4H9I, C5H3IO2, C5H3FI, or C6H13I.
The dopant may also be selected from one more inorganic compounds or solutions including carbon, boron, H3BO3, sulphur, (NH4)2S, iodine, HIO3, HIO4, NH4I, or NH4IO3.
Preferably, when the metal oxide precursor is calcined with the dopant it is also in the presence of one or more non-reactive or inert gases, selected from the group comprising oxygen, hydrogen, argon, helium, and air.
The doping step is preferably carried out by calcining the metal oxide precursor in contact with the dopant.
Preferably, the metal oxide precursor is calcined with the dopant at a temperature of between 2000C to 18000C for a period of between 30 minutes and 5 days.
More preferably, the metal oxide precursor is calcined in contact with the dopant at a temperature of between 6000C and 10000C for a period of between 30 minutes and 3 days.
The metal oxide precursor may be calcined in contact with the dopant at a temperature of about 7000C for a period of about 60 minutes. The doped metal oxide, as described above, has a layered structure in which metal oxide layers may be intercalated with a cation (A). The dopant stoichiometrically replaces oxygen from the metal oxide layers to form the non- metal doped metal oxide.
It will be appreciated that the conditions for the doping will vary depending on the type of dopant used to exchange with oxygen in the metal oxide precursor.
If it is desirable for the metal oxide precursor to contain intercalated cations within the layered structure of the metal oxide precursor, the metal oxide precursor may be formed by heating a cation donor precursor with a metal oxide donor, which may be summarised as follows: calcination Cation donor precursor + Metal oxide donor ► AnMyOz
Preferably the cation donor precursor is an alkali earth metal salt or an alkali metal salt selected from the group comprising: alkali metal halides; alkali earth metal halides; alkali metal sulphides; alkali earth metal sulphides; alkali metal sulphates; alkali earth metal sulphates; alkali metal carbonates; alkali earth metal carbonates; alkali metal nitrates; alkali earth metal nitrates; alkali metal hydroxides; alkali earth metal hydroxides; alkali metal acetates; alkali earth metal acetates; alkali metal dimethenylamine (AN(CH2)2); alkali earth metal dimethenylamine (AN(CH2)2); alkali metal oxide; alkali earth metal oxides; alkali metal chlorate; alkali earth metal chlorate; alkali metal phosphate and/or alkali earth metal phosphate.
Preferably the metal oxide donor is selected from the group comprising metal oxides or hydroxides, including TiO, Ti2O3, Ti3O5, TiO2, TiOxNy, TiOxCy, Ti(OH)4.xH2O; mixed oxides of titanium, such as lanthanum titanium oxide; niobium oxides and mixed oxides thereof, such as calcium niobium oxide; nickel oxides, cobalt oxides, ferric and ferrous oxides, tantalum oxides; vanadium oxides; and tungsten oxides; metal nitride compounds, such as titanium nitride (TiN), niobium nitride, tantalum nitride and vanadium nitride; metal carbide compounds, including titanium carbide (TiC); metal cyanamide compounds, including titanium cyanamide (TiCxNy); metal boride compounds including titanium diboride (TiB2); metal sulphide compounds, including titanium sulphide (TiS2); metal halide compounds such as titanium halide, including TiBr4 TiCI41TiCI3, TiF3, TiF4, TiI4; metal phosphide or phosphate compounds, including titanium phosphide (TiP); metal sulphate compounds, including titanium sulphates including Ti2SO4.xH2O, Ti2(SO4)3, and TiOSO4.xH2SO4; metal suicide compounds including titanium suicides (TiSi2); and/or organometalic compounds such as organic titanium compounds, including Ti(OCH(CH3)2)4, Ti[O(CH2)3CH3]4, Ti(OCH3)4.(CH3OH)x.
The cation donor precursor and the metal oxide donor may be calcined at a temperature of between 5000C to 12000C for a period of between 0.5 and 40 hours.
More preferably, the cation donor precursor and the metal oxide donor are calcined at a temperature of between 6000C and 10000C for a period of between 2 hours and 30 hours.
The cation donor precursor and the metal oxide donor are most preferably calcined at a temperature of about 75O0C for a period of about 20 hours.
The metal oxide nanosheets when coupled with the band modification agent may be used in suspension to form coatings and films. Alternatively they may be separated and dried.
Step 3 of the method outlined above and subsequent application of the metal oxide nanosheets coupled with a band gap modification agent is schematically represented in Figure 1.
It may be desirable to "reorder" the metal oxide nanosheets coupled with band modification agents to form layered structures. This reordering can be carried out by separating and drying the coupled nanosheets. The nanosheets may be separated by evaporating any solvent present or through the addition of a suitable flocculant.
The flocculant may include an acidic solution; a solution comprising inorganic or organic salts which may provide cations such as Li+, Na+, K+, Mg2+, Ca2+, Al3+ and the like; nanoclusters, such as Keggin type ions; inorganic nanoparticles; and organic macromolecules such as dyes and the like, which are capable of providing cations to the non-metal doped metal oxide nanosheets or capable of encouraging the re-ordering of the nanosheets.
The metal oxide nanosheets coupled with band gap modification agents may be used in photocatalytic applications, such as self cleaning coatings, decomposition of organic compounds, and hydrogen production from the photocatalytic splitting of water.
The below example describes a preferred embodiment with reference to the formation of nitrogen doped titania nanosheets coupled with clusters of iodine. It will be appreciated that other forms of the metal oxide nanosheets coupled with a band gap modification agent may be formed using similar methods and without departing from the scope of the invention.
Example 1
A layered metal oxide precursor of Cso6δTii 83θ4 was prepared by mixing caesium carbonate (CS2CO3), with titania (Tiθ2), and calcining at 75O0C in air for 20 hours. Approximately 60 - 70 grams of white crystalline caesium titanate (Cso6βTii 83O4) was collected.
The protonated form of the nitrogen doped titanate (HoββTii 83O4) was prepared by the ion-exchange of the caesium in CsoββTh 83O4 with protons by placing the Cso esTii 83O4 in 1 M HCI solution for three days. The HoβsTh 83θ4was separated and dried. The resultant yellow H0.6sTi1.83O4 (1.2g) was dispersed in an exfoliating agent, tetrabutylammonium hydroxide (TBA+OH") solution (300 cm3, 0.2M)1 and was shaken for more than 7 days at room temperature to exfoliate the protonated nitrogen doped titanate into a yellow colloidal suspension of Tio.9iO2 nanosheets.
100 mg of iodine was first dissolved in 300 ml_ of ethanol solution, and then different amounts of the iodine solution was added into the 100 mg Tio.9iO2 nanosheets suspension and stirred for approximately 2 hours to obtain the Ti0.9iθ2 nanosheets coupled with different concentrations or loadings of iodine clusters, namely, (a) 0mol%; (b) 2mol%; (c) 4mol%; and (d) 6mol% of iodine. The iodine-loaded Ti09iθ2 nanosheets were then flocculated by adding acid to form restacked layered structure coupled with iodine. The resultant flocculent was recovered by filtration and dried at room temperature.
The UV-Visible absorption spectra were recorded for these coupled nanosheets (a) to (d), as above. The comparative UV-Visible absorbance spectra are shown in Fig 2. It can be seen that as the amount of band gap modification agent, i.e. iodine clusters, increases so does the absorption in the visible range.
Fig 3 illustrates comparative XRD patterns for the compounds (a) to (d), described above. The interlayer spacing was calculated from the XRD patterns and found to be between 1.22nm to 1.46nm.
It is believed that the band gap modification agents, or iodine in this particular example, form clusters or group together. These clusters are believed to adhere or adsorb onto the surface of the metal oxide nanosheets. It is also believed that when the coupled nanosheets are separated from solution and dried, that the iodine clusters create a porous structure between the layers of nanosheets. The interlayer structure is believed to be mesoporous in nature. A hypothetical structure of Ti0.9iO2 nanosheets coupled with iodine clusters is illustrated in Fig 4, which also illustrates the interlayer spacing "d" between nanosheets. It is believed that as the nanosheets are dried they re-order or restack to form a layered metal oxide structure coupled with the iodine clusters. Indeed, restacking of a layered structure is achieved by the flocculation of iodine coupled nanosheets with protons, followed by filtration and drying.
The BET surface area for each of the compounds (a) to (d) was measured. The results are summarised in Fig 5. It is believed that these results support the hypothesis that the metal oxide nanosheets couple with iodine clusters when flocculated and form a pillared mesoporous type arrangement.
The BET surface area results show that samples of small amounts of iodine clusters coupled to the titania nanosheets have similar BET surface area to that of uncoupled titania nanosheets. The higher the amount of coupling of iodine clusters the more marked the reduction in the BET surface area. However, higher coupling with iodine clusters also leads to higher visible light absorption.
It would therefore appear possible to tailor the surface area and visible light absorption and properties of the metal oxide nanosheets coupled with a band gap modifying agent to suit particular applications.
The XPS spectra were recorded for (a) to (d) and are summarised in Fig 6. The binding intensities were also measured; these are illustrated in Fig 7. Both of these figures appear to support the hypothesis that the iodine is coupled to the titania nanosheets in distinct clusters.
As a result of the BET and XPS data it is believed that the coupled nanosheets of this example, once separated from solution, form a bilayer structure of TiOβ layers with iodine clusters between the layers. The structure of these coupled nanosheets is illustrated schematically in Fig 8.
The photocatalytic ability of th.e Tio.91O2 nanosheets coupled with iodine clusters was compared to that of an intermediate, protonated nitrogen doped titanate (HTiON), by examining the activity of the respective photocatalysts in the decomposition of rhodamine. A solution of rhodamine was formed and the respective photocatalysts added to the solution. The 20 mg of iodine coupled titania nanosheets were added per 100 ml of rhodamine solution. Whilst the protonated nitrogen doped titanate photocatalysts was added in a concentration of 100 mg per 100 ml of rhodamine solution. The Tio.giθ2 nanosheets coupled with iodine clusters was much more effective at the photodecomposition of the rhodomine dye, as illustrated in Fig 9.
It is to be understood that the above embodiments have been provided only by way of exemplification of this invention, and that further modifications and improvements thereto, as would be apparent to persons skilled in the relevant art, are deemed to fall within the broad scope and ambit of the present invention described herein.
In the specification and the claims the term "comprising" shall be understood to have a broad meaning similar to the term "including" and will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. This definition also applies to variations on the term "comprising" such as "comprise" and "comprises".

Claims

Claims:
1. A method of producing metal oxide nanosheets coupled with a band gap modification agent, including the steps of: protonating a layered metal oxide to form a protonated layered metal oxide; exfoliating the protonated layered metal oxide to form a suspension of metal oxide nanosheets; and coupling the metal oxide nanosheets with a band gap modification agent to form metal oxide nanosheets coupled with the band gap modification agent.
2. The method of claim 1 , wherein the protonation step is carried out by adding the layered metal oxide to an acidic solution.
3. The method of claim 2, wherein the acidic solution is selected from the group comprising hydrochloric acid, nitric acid, sulphuric acid, phosphoric acid, hydrofluoric acid, hydroiodic acid, hydrobromic acid, acetic acid (HAC), perchloric acid, iodic acid (HIO3) and periodic acid (HIO4).
4. The method of claim 1 , wherein the layered metal oxide is selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium oxides and mixed oxides thereof.
5. The method of claim 1 , wherein the layered metal oxide is selected from the group having formula (I):
AnMyOz-χDx (I) wherein:
A is a cation selected from the group comprising lithium, sodium, potassium, calcium, magnesium, rubidium, caesium and francium;
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group comprising boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; n is an value selected between equal to or greater than 0 and equal to or less than 8; y is a value greater than 0 and less than or equal to 8; z is a value greater than 0 and less than or equal to 8; x is a value greater than 0 and less than 8; and z-x is always a value greater than 0.
6. The method of claim 1 , wherein the exfoliation step of the protonated metal oxide is carried out by adding to the protonated metal oxide an exfoliating agent selected from the group comprising tetramethylammonium hydroxide, tetraethylammonium hydroxide, tetrapropylammonium hydroxide and tetrabutylammonium hydroxide.
7. The method of claim 6, wherein the metal oxide is in contact with the exfoliating agent for a period of between 1 hour and two weeks.
8. The method of claim 7, wherein the metal oxide is in contact with the exfoliating agent for a period of approximately 7 days at a temperature of between ambient and 6O0C.
9. The method of claim 1 , wherein the band gap modification agent is a semiconductor or a chromophore.
10. The method of claim 1 , wherein the band gap modification agent is a semiconductor selected from the group comprising quantum dots, conductive nanowires, nanotubes, nanorods and inorganic dopants.
11. The method of claim 1 , wherein the band gap modification agent is a chromophore selected from the group comprising inorganic molecules including iodine, bromine, fluorine, nitrogen, phosphorus; or organic molecules including porphyrin and its derivatives, and ruthenium based dyes.
12. The method of claim 1 , wherein the band gap modification agent is clusters of iodine.
13. Metal oxide nanosheets coupled with a band gap modification agent having a formula (III):
MyOz-xDx [ G ]R. (Ill) wherein:
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to or less than 8; z is a value greater than 0 and equal to or less than 8; x is a value equal to or greater than 0 and equal to or less than 8; G is a band gap modification agent; and
R is a value greater than 0 and equal to or less than 9.
14. The metal oxide nanosheets coupled with a band gap modification agent of claim 13, wherein the band gap modification agent [G] is a semiconductor selected from the group comprising quantum dots, conductive nanowires, nanotubes, nanorodes and inorganic dopants.
15. The metal oxide nanosheets coupled with a band gap modification agent of claim 13, wherein the band gap modification agent is a chromophore selected from the group comprising iodine, bromine, fluorine, nitrogen, phosphorus, porphyrin and its derivatives, and ruthenium based dyes.
16. The metal oxide nanosheets coupled with a band gap modification agent of claim 13, wherein the metal oxide nanosheets coupled with a band gap modification agent has a formula Ti09iθ2[l2]R wherein R is a value greater than 0 and equal to or less than 9.
17. Use of the metal oxide nanosheets coupled with a band gap modification agent of claim 13 as a photocatalyst.
18. A method of coating substrates with metal oxide nanosheets coupled with a band gap modification agent, including the step of coating a substrate with metal oxide nanosheets coupled with a band gap modification agent of formula (III):
MyOz-xDx [ G ]R (III) wherein:
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to less than 8; z is a value greater than 0 and equal to or less than 8; x is a valve equal to or greater than 0 and equal to or less than 8;
G is a band gap modification agent; and
R is a value greater than 0 and equal to or less than 9.
19. The method of claim 18, wherein the substrate is selected from the group comprising glass, quartz glass, silicon wafer and ITO glass.
20. The method of claim 18, wherein the method further includes the step of adding one or more layers of binder.
21. The method of claim 20, wherein the binder is selected from an organic polyelectrolyte or charged inorganic nanoclusters.
22. The method of claim 20, wherein the binder is an organic polyelectrolyte selected from polyethylenimine, poly(allylamine hydrochloride) or poly(diallyldimethylammonium) chloride.
23. The method of claim 20, wherein the binder is charged inorganic nanoclusters selected from the group comprising partially hydrolysed nanoclusters of metal hydroxides including aluminium oxides and hydroxides, chromium oxides and hydroxides, cobalt oxides and hydroxides, ferric oxides and hydroxides, ferrous oxides and hydroxides, nickel oxides and hydroxides, tungsten oxides and hydroxides manganese oxides and hydroxides, titanium oxides and hydroxides, zirconium oxides and hydroxides and vanadium oxides and hydroxides.
24. The method of claim 20, wherein the binder is applied to the substrate prior to coating with a layer of the non-metal doped metal oxide nanosheets of formula (III) and/or between layers of the non-metal doped metal oxide nanosheets of formula (III).
25. The method of claim 20, wherein the binder is applied to the substrate by placing the substrate in a solution of binder for a period of between 1 minute and 2 hours.
26. The method of claim 18, wherein the metal oxide nanosheets coupled with a band gap modification agent are applied to the substrate by placing the substrate in a suspension of the metal oxide nanosheets coupled with band gap modification agent for a period of between 10 minutes and 2 hours.
27. A substrate coated with metal oxide nanosheets coupled with a band gap modification agent of formula (III):
MyOz-xDx [ G ]R (III) wherein:
M is one or more metal ions selected from the group comprising titanium, lanthanium, niobium, tungsten, nickel, iron, cobalt, calcium, barium, zirconium, hafnium, molybdenum, chromium, tantalum and vanadium; D is a dopant selected from the group of boron, carbon, nitrogen, fluorine, sulphur, phosphorus and iodine; y is a value greater than 0 and equal to less than 8; z is a value greater than 0 and equal to or less than 8; x is a valve equal to or greater than 0 and equal to or less than 8; G is a band gap modification agent; and R is a value greater than 0 and equal to or less than 9.
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CN106000474A (en) * 2016-06-07 2016-10-12 河南大学 Preparation method and application of porphyrin/titanium dioxide uniformly co-assembled nanosphere
EP3406760A1 (en) * 2017-05-25 2018-11-28 United Technologies Corporation Aluminum-chromium oxide coating and method therefor
CN108807928A (en) * 2018-06-25 2018-11-13 宁德新能源科技有限公司 A kind of synthesis of metal oxide and lithium ion battery
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CN110116011A (en) * 2019-04-19 2019-08-13 广州城市职业学院 A kind of photochemical catalyst and preparation method thereof for organic matter in food industrial wastewater of degrading
CN111203255A (en) * 2020-01-16 2020-05-29 西安交通大学 N-doped CdPS3Preparation method of two-dimensional nanosheet photocatalyst
CN111203255B (en) * 2020-01-16 2021-05-28 西安交通大学 N-doped CdPS3Preparation method of two-dimensional nanosheet photocatalyst
CN112499684A (en) * 2020-12-04 2021-03-16 合肥工业大学 Multilayer WO based on ion repulsion action dispersion stripping3Method of nanosheet
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