Abstract
The present invention discloses a method for improving solar energy conversion efficiency using metal oxide photocatalysts having an energy band of core-shell structure for ultraviolet (UV) ray and visible light absorption, comprising a first process of forming a nanoparticle thin film layer; a second process of preparing a core-shell metal oxide on metal oxide nanoparticles by a plasma reaction under a hydrogen and nitrogen gas atmosphere, and a third process of depositing a transition metal on surfaces of core-shell metal oxide nanoparticles to produce a photocatalyst for energy conversion. A great amount of oxygen vacancies is formed in a shell region by the core-shell metal oxide to achieve effects of improving transfer ability of electron-hole pairs excited by light, and extending a wavelength range of absorbable light to a visible light region by changing a band-gap structure.
Claims
1. A method for manufacturing a transition metal oxide photocatalyst having a core-shell energy band structure to improve solar energy conversion efficiency by utilizing a wide range of sunlight from ultraviolet ray to visible light comprising: a first process of performing heat treatment on a metal oxide semiconductor having a band-gap to form a nanoparticle thin film layer; a second process of contacting a plasma ball including mixed gas in a substitutional NH or NHx radical state by a plasma reaction under a hydrogen and nitrogen gas atmosphere with a surface of nanoparticles of the nanoparticle thin film layer to simultaneously generate a NH functional group and oxygen vacancies formed by hydrogenation, so as to prepare a core-shell metal oxide capable of absorbing UV ray and visible light; and a third process of further depositing a transition metal on surfaces of core-shell nanoparticles of the nanoparticle thin film layer to produce a photocatalyst of metal oxide-transition metal having a HN-core-shell structure for energy conversion.
2. The method according to claim 1, wherein the metal oxide and the transition metal include at least one element selected from Ti, V, Fe, Cu, Zn, Ta, W and Bi.
3. The method according to claim 1, wherein the substitutional NH or NHx radical is a plasma ball formed of NH or NHx mixed gas by plasma treatment.
4. The method according to claim 1, wherein the gas plasma treatment in the second process is executed in a hydrogen gas to nitrogen gas ratio of 1:1, 1:2 or 1:3.
5. The method according to claim 1, wherein the second process is executed by contacting the plasma ball of substitutional hydrogen nitride mixed gas, which is formed by a plasma reaction in a single process at room temperature, to the surface of the metal oxide thin film or the particles, so as to form a core-shell structure.
6. The method according to claim 1, wherein a structure of the band-gap is changed by reacting the metal oxide with the gas to make the substitutional nitrogen and oxygen vacancies to form the mid-gap, and the interstitial hydrogen nitride raises a valence band maximum (VBM) level to decrease a size of the band-gap and extend a wavelength range in which electron-hole pairs are generated by sensing light, so as to absorb UV ray and light in the visible light region.
7. A transition metal oxide photocatalyst having a core-shell energy band structure to improve solar energy conversion efficiency by utilizing a wide range of sunlight from ultraviolet ray to visible light manufactured by the method according to claim 1.
8. The transition metal oxide photocatalyst according to claim 7, wherein the transition metal oxide photocatalyst is used to generate a solar energy compound selected from the group consisting of carbon monoxide, methanol, formic acid and methane from water and carbon dioxide using a light.
9. The transition metal oxide photocatalyst according to claim 7, wherein the transition metal oxide photocatalyst is used to generate a C1 compound selected from the group consisting of methanol, methane, formic acid and carbon monoxide by a photochemical reaction.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
(2) FIG. 1 is a view illustrating nitrogenation and hydrogenation of a metal oxide thin film sample through a mixed gas plasma reaction using a MPE-CVD device in a manufacturing process according to the present invention, wherein different colors of a ball generated through a plasma reaction depending on types of gases injected inside a reactor (hydrogen: dark blue, nitrogen: pink, hydrogen+nitrogen: violet) are shown through the view port. The different colors of each plasma reaction indicated that the different types of radicals were created depending on the reaction gases.
(3) FIG. 2 is a view illustrating a process of changing a TiO.sub.2 nanoparticle to a core-shell structure (HNTiO.sub.2) by mixed gas plasma treatment according to the present invention;
(4) FIG. 3 is a view illustrating energy levels created at the surface of the metal oxide nanoparticles by the mixed gas plasma treatment, and energy levels of Cu and Pt in co-catalyst used for carbon dioxide conversion, as compared to an energy level of reversible hydrogen electrode (RHE) level, as well as improved electron-hole transfer and separation effects resulting from the same;
(5) FIG. 4 is views illustrating: a) measured results of light absorbance before (bare TiO.sub.2) and after (HNTiO.sub.2) H.sub.2/N.sub.2 mixed gas plasma treatment converted into Tauc relationship, as well as photographs of samples; b) lattice distances measured along different color arrows shown in FIGS. 4c and 4d; and c) and d) TEM measurement photographs of bare TiO.sub.2 nanoparticle and HNTiO.sub.2 nanoparticle, respectively;
(6) FIG. 5 is views illustrating: a) NEXAFS measurement results of oxygen K-edge of samples treated with hydrogen (HTiO.sub.2), nitrogen (NTiO.sub.2) and H.sub.2/N.sub.2 mixed gas (HNTiO.sub.2) plasmas, respectively; b) NEXAFS measurement results of nitrogen K-edge of HNTiO.sub.2 sample, and an inset showing XPS measurement of nitrogen is orbital; and c) valance band (VB) XPS measurement results of the sample before and after H.sub.2/N.sub.2 mixed gas plasma treatment;
(7) FIG. 6 is views illustrating: a) a crystal model of anatase TiO.sub.2 crystal including both of substitutional or interstitial nitrogen and hydrogen; b) a crystal model of anatase TiO.sub.2 crystal including an oxygen vacancy (V.sub.o); c) substitutional nitrogen (N.sub.s); d) interstitial nitrogen (N.sub.i); e) interstitial hydrogen-bonded nitrogen (N.sub.iH); and f) substitutional hydrogen-bonded nitrogen (N.sub.sH);
(8) FIG. 7 is views illustrating: a) an electron state density of anatase TiO.sub.2 crystal including N.sub.s and N.sub.iH; and b) a mid-gap and c) a distribution of electrons at a VBM level, respectively, in the electron state density shown in FIG. 7a;
(9) FIG. 8 is views illustrating changes in photoelectrochemical catalytic properties of the sample along with changes in process conditions of the H.sub.2/N.sub.2 mixed gas plasma treatment such as: a) a plasma output power; b) a processing time; and c) a mixing ratio of H.sub.2 and N.sub.2 gases, respectively, wherein the above changes are expressed by photocurrent-voltage curves with 0.1 M NaClO.sub.4 (pH 7) aqueous solution;
(10) FIG. 9 is views illustrating: a) photoelectrochemical catalytic properties of a sample (HNTiO.sub.2) on which the gas plasma treatment is executed using the mixed gas of hydrogen and nitrogen in a mixing ratio of 2:1 at 500 W output power for 3 minutes and another sample (bare TiO.sub.2) without any treatment, which are expressed by the photocurrent-voltage curves with 0.1 M NaClO.sub.4 (pH 7) aqueous solution; and b) measured results of photoreaction efficiency for each wavelength range for each sample, and the insets showing photo-reactivity for each sample by normalizing an photocurrent amount in photocurrent-time curves.
(11) FIG. 10 is views illustrating amounts of C1 fuel products, that is: a) carbon monoxide; and b) methanol, which are converted from carbon dioxide and water through a photochemical reaction for 10 hours without any oxidant and reductant;
(12) FIG. 11 is views illustrating: TEM photographs of ZnO nanoparticles a) before and b) after mixed gas plasma treatment; and photocurrent-time curves showing photocurrent amounts generated from ZnO nanoparticles: c) under incident solar simulator; or d) under incident visible light, before and after mixed gas plasma treatment; and
(13) FIG. 12 is views illustrating: TEM photographs of CuO nanoparticles a) before and b) after mixed gas plasma treatment; and photocurrent-time curves showing photocurrent amounts generated from CuO nanoparticles: c) at a hydrogen generation potential; or d) at an oxygen generation potential, under incident solar simulator before and after mixed gas plasma treatment.
DETAILED DESCRIPTION OF THE INVENTION
(14) The present invention proposes a photocatalyst manufacturing method in which mass-production is possible and high effects are achieved at low costs by a simple method through hydrogenation and nitrogen/hydrogen (NH) treatment in a single process at room temperature within a short time. In particular, the present invention provides a treatment method of increasing the number of electrons/holes capable of participating a desired catalytic reaction, thereby improving catalytic properties of a metal oxide.
(15) The metal oxide proposed in the present invention is not particularly limited to titanium dioxide (TiO.sub.2) but may include any metal element having n-type semiconductor properties, and may be broadly employed to improve catalytic properties of the metal oxide consisting of at least one selected from Ti, V, Fe, Ni, Cu, Zn, Sn, Ta, W and Bi.
Example 1
(16) A method of fabricating a photoelectrochemical electrode is as follows. 100 mg of anatase TiO.sub.2 nanoparticles having a particle diameter of about 25 nm was sufficiently dispersed in 50 ml of acetone solution containing 20 mg of iodine dissolved therein. Two nickel foils were immersed in the prepared solution so as to face each other, and then, 100V DC was applied to the same for 1 minute, to form a TiO.sub.2 nanoparticle thin film layer having a thickness of about 250 nm on the nickel foil side of a cathode. The TiO.sub.2 thin film sample was subjected to heat treatment at 500 C. in an air atmosphere for 40 minutes to improve adhesion between the thin film layer and the nickel foil substrate and, at the same time, to remove organic impurities formed in the thin film fabrication process. A variety of metal oxides such as V, Fe, Cu, Zn, Ta, W or Bi, etc. metal oxide nanoparticles (<50 nm) may also be fabricated into a photoelectrochemical electrode by the same process as described above.
(17) The as-prepared metal oxide thin film was placed in a reactor of a chemical vapor deposition device (e.g. a microwave plasma enhanced chemical vapor deposition, MPE-CVD) for possible gas plasma treatment, and prepared to be under a vacuum atmosphere of 310.sup.3 Torr or less. Thereafter, while flowing a mixed gas containing H.sub.2 and N.sub.2 gases in a mixing ratio of 1:2 into the reactor at an overall 100 sccm flow rate, an outer surface of a plasma ball containing H and NH.sub.x radicals formed thereon has contacted with the surface of the metal oxide thin film by causing a gas plasma reaction under a condition of 500 W output power and controlling the same, followed by maintaining this state for 3 minutes. Herein, a distance between the plasma ball and the surface of the metal oxide thin film is controlled through atmospheric control inside the reactor. This distance is substantially different according to a thickness of the overall thin film including the substrate and the metal oxide layer, type and flow rate of a reactive gas, output power for plasma generation, etc., and therefore, may be variably controlled in a range of 1 to 30 Torr. Furthermore, a contact time between the plasma ball and the surface of the metal oxide thin film may also be variably controlled according to a desired degree of treatment. In addition, H.sub.2 and N.sub.2 gases may be controlled in a mixing ratio of 1:1, 1:2 or 1:3.
Example 2Fabrication of HNTiO2Cu Catalyst
(18) A method for fabricating a transition metal thin film serving as a co-catalyst for the HNTiO.sub.2 photoelectrochemical thin film electrode prepared by the above-described method is as follows. Using a RF magnetron sputtering device, a transition metal target such as Cu, Pt, Co, etc. was deposited on the prepared HNTiO.sub.2 thin film sample. After injecting argon gas, a pressure was adjusted to 12 mTorr and the deposition was performed under a RF power (Radio Frequency) of 100 W for 0 to 120 seconds. A weight of the deposited co-catalyst was measured using an ultra-micro balance and was controlled to be 1% or less as compared to TiO.sub.2 thin film. The transition metal-deposited sample was subjected to heat treatment under a temperature condition of about 100 C. and in nitrogen and hydrogen gas atmosphere for 1 hour, thereby improving adhesiveness and crystallinity.
Example 3Production of C1 Compound Using HNTiO2Cu Catalyst
(19) A measurement method of photochemical conversion efficiency from photochemical carbon dioxide to another carbon compound is as follows. In order to maximize the catalysis efficiency, platinum or copper (serving for reduction of carbon dioxide) and a cobalt oxide thin film were deposited on the fabricated HNTiO.sub.2 thin film using a RF magnetron sputtering device. After placing the prepared sample in a sealed stainless steel container, air was discharged and an inside of the container was became a saturated condition using carbon dioxide gas (99.9%) at a temperature of about 70 C. Then, after pouring a small amount of deionized water therein, the temperature was maintained at 70 C. for about 1 hour in order to sufficiently evaporate the reactants in the reactor. For photochemical catalytic reaction, a quartz glass at the top of the container was irradiated with light using a light simulator (AM 1.5G filter, 200 mWcm.sup.2) to conduct a reaction. Amounts of the generated substances, that is, carbon monoxide and methanol, respectively, were measured in real time by using measurement devices of a flame ionization detector (FID) and a pulsed discharge helium ionization detector (PDHID) in gas chromatography equipment.
Experimental Example
(20) FIG. 4 illustrates changes in light absorbance and crystallinity between a TiO.sub.2 nanoparticle (HNTiO.sub.2) on which mixed gas plasma treatment was executed according to the above-described method and a TiO.sub.2 nanoparticle (bare TiO.sub.2) without any treatment. As shown in the inset of FIG. 4a, a conventional white TiO.sub.2 thin film changed into a dark yellow color by the plasma treatment and showed a great increase in light absorbance characteristics in a visible light region after plasma treatment, as observed in the light absorbance curve of each sample in FIG. 4a, which was prepared using the Tauc relationship. As a result of calculating a size of the band-gap for each sample using x intercepts of the graph shown in FIG. 4a, it was observed that the bare TiO.sub.2 had 3.27 eV while HNTiO.sub.2 showed a decrease to 2.71 eV. Further, existence of an additional band-gap with a size of 1.92 eV was observed.
(21) FIGS. 4c and 4d illustrate transmission electron microscopy (TEM) photographs of a single nanoparticle in each sample, respectively. The bare TiO.sub.2 nanoparticle in FIG. 4c showed the same anatase (101) plane in both of the inside and the outside of the nanoparticle, however, the HNTiO.sub.2 in FIG. 4d showed that a crystalline structure of the anatase (101) plane is maintained on the inside while the outside thereof was observed to have irregularly altered crystalline properties. Further, FIG. 4b also illustrates results of an interplanar distance measured along the arrows shown in FIGS. 4c and 4d. Red and blue curves in FIG. 4b show the interplanar distance measured along the arrows with these colors, respectively, present in TEM photographs of HNTiO.sub.2 nanoparticle in FIG. 4d, while a black curve shows the interplanar distance measured along the arrow in FIG. 4c. All of the three curves have the same interplanar distance inside the particle of 0.351 nm to indicate the anatase 101 face. However, for HNTiO.sub.2, it was observed that the interplanar distance changes irregularly from the minimum of 0.325 nm to the maximum of 0.416 nm toward the outside thereof. Due to this phenomenon, a Fourier transformed TEM photograph as the inset of FIG. 4d illustrated that a white elliptical trace indicating the anatase (101) plane is observed around diffraction points. Meanwhile, the bare TiO.sub.2 showed that the interplanar distance is 0.354 nm near the surface thereof, which slightly increases as compared to the existing interplanar distance. Based on the above-observed results of the changes in light absorption properties and crystallinity, effects of the mixed gas plasma treatment proposed in the present invention have been mostly concentrated on the surface of particles, as shown in FIG. 2. On the other hand, the inside of the particle maintains inherent characteristics of the anatase phase TiO.sub.2.
(22) FIG. 5 illustrates measured results of changes in chemical states of elements forming the surface of TiO.sub.2 nanoparticles by the gas plasma treatment. Anatase phase TiO.sub.2 has titanium 3d orbital and oxygen 2p orbital hybridized by a crystal field effect to form a hybrid orbital level having an energy level in T.sub.g and e.sub.2g states above the Fermi energy level. Accordingly, in a near edge X-ray absorption fine structure (NEXAFS) curve obtained by measuring the chemical state of an oxygen K-edge in FIG. 5a, the bare TiO.sub.2 showed peaks indicating T.sub.g and e.sub.2g levels at a photon energy in a range of 530 eV to 535 eV. Further, it was observed that a and c peaks are present in a hybrid orbital level formed by hybridization of an oxygen 2p orbital and titanium 4s or 4p orbital at a high photon energy. Such specific peaks observed in the anatase phase TiO.sub.2 did not show any significant differences when performing an H.sub.2 gas plasma treatment (HTiO.sub.2). However, when performing an N.sub.2 gas plasma treatment (NTiO.sub.2), the positions of t.sub.g and e.sub.2g level peaks moved toward the right side and the depth of a valley between these two peaks was decreased. Further, it was observed that the form of the a and c peaks is significantly changed. The above changes are further increased when performing the H.sub.2/N.sub.2 mixed gas plasma treatment (HNTiO.sub.2), and a new peak was observed at the b position between the a and c peaks. As a result, an overall outline of the curve was altered into the morphology similar to the oxygen K-edge spectrum of surface-oxidized titanium nitride (TiN) or titanium oxynitride (TiON).
(23) Referring to the NEXAFS curve of a nitrogen K-edge of the HNTiO.sub.2 sample shown in FIG. 5b, it was observed that a nitrogen introduced by the plasma treatment is combined with a titanium to form a new orbital level. As described for the case of oxygen, two peaks found near 400 eV among such orbital levels exhibit a t.sub.g and e.sub.2g hybrid orbital level generated by the hybridization of the nitrogen 2p orbital and titanium 3d orbital. Likewise, a peak near 410 eV is also a specific peak which is generated in the hybrid orbital level and formed by the hybridization of the nitrogen 2p orbital and titanium 4s or 4p orbital. The inset of FIG. 5b shows the chemical status of the nitrogen 1 s orbital measured by X-ray photoelectron spectroscopy (XPS). Similar to the NEXAFS results, the nitrogen-titanium (NTi) combined peak was the highest peak and other nitrogen-oxygen (NO) and nitrogen-hydrogen (NH) combined peaks were further confirmed. Such nitrogen XPS and NEXAFS measured results demonstrated that nitrogen is introduced in two different modes, that is, an oxygen atom is substituted with nitrogen in an anatase crystal or nitrogen invades between oxygen and titanium atoms, and different energy levels are formed in relation to the respective introduction conditions. Meanwhile, it was found that the intensity of peaks indicating oxygen vacancy in the HNTiO.sub.2 sample sharply increases, compared to the bare TiO.sub.2, by measuring the chemical states of the XPS oxygen Is orbital.
(24) In order to determine the position of the energy level generated by such a nitrogen introduction effect as described above, a valance band XPS was measured and the measured result is shown in FIG. 5c. It was found that HNTiO.sub.2 has a VBM level at a 0.6 eV lower position, as compared to the bare TiO.sub.2. Further, due to an energy level newly formed at a 0.6 eV lower position than the VBM, a sharp peak was observed. As such, new energy levels discovered in HNTiO.sub.2 was predicted as energy levels resulted from the binding of nitrogen and hydrogen atoms to titanium atoms, and it was demonstrated that such new energy levels are formed by TiN.sub.s (substitutional nitrogen) bond and TiN.sub.iH (hydrogen bonded interstitial nitrogen) bond, respectively, according to a calculation method based on density functional theory (DFT) (see next paragraph and FIGS. 6 and 7). With reference to the results in FIG. 4a, assuming that a band-gap of the anatase phase TiO.sub.2 is 3.27 eV, band-gaps due to the energy level formed by the combination of TiN.sub.s and TiN.sub.i in the HNTiO.sub.2 sample measured by VB XPS have the size of 2.67 eV and 1.97 eV, respectively. It was determined that these results are very similar to 2.71 eV and 1.92 eV, respectively, which are the band-gap levels of HNTiO.sub.2 calculated on the basis of light absorption properties in FIG. 4a.
(25) FIG. 6 illustrates a theoretical crystal structure formed through the density functional theory in order to deduce an energy band structure of the anatase TiO.sub.2 containing nitrogen and hydrogen inserted therein. As shown in FIGS. 6c and 6d, it was estimated that the nitrogen may be present in substitutional or interstitial form at the position of oxygen in the anatase TiO.sub.2 crystal. However, as shown in FIG. 6e or 6f, the hydrogen could not be present in the crystal unless it has been bound to the nitrogen. In other cases, the hydrogen was bonded together to form a hydrogen molecule or was bound to oxygen to form H.sub.2O, thus being discharged out of the crystal. Among different crystal models including N.sub.s, N.sub.i, N.sub.sH, N.sub.iH bonds introduced therein separately or in combination, a band structure only in the crystal model including both of N.sub.s and N.sub.iH together has exhibited a value very similar to the band structure calculated using experimental values within an error range of 0.2 eV. In the band structure shown in FIG. 7a, as a result of reflecting a local distribution of the electron states at a mid-gap level (FIG. 7b) and a VBM (FIG. 7c) level into the crystal structure, it was found that TiN.sub.s bond forms the mid-gap level and TiN.sub.iH bond forms the VBM level, respectively. Meanwhile, a flat band level difference between two samples calculated by the Mott-Schottky measurement method was measured as 0.03 eV, therefore, it was determined that the CBM level of HNTiO.sub.2 was changed by 0.03 eV, compared to the bare TiO.sub.2.
(26) In other words, a structure of the band-gap is changed by reacting the metal oxide with the gas to make the substitutional nitrogen and oxygen vacancies to form the mid-gap, and the interstitial hydrogen nitride raises a valence band maximum (VBM) level to decrease a size of the band-gap and extend a wavelength range in which electron-hole pairs are generated by sensing light, so as to absorb UV ray and light in the visible light region.
(27) Referring to the above data, FIG. 3 illustrates a band-gap structure including all energy levels of HNTiO.sub.2 sample, as compared to a reversible hydrogen electrode (RHE) level. For reference, V.sub.o represents a characteristic of metal oxides hydrogenated at an energy level created at 0.8 eV below the CBM level if a density of oxygen vacancies in an n-type semiconductor metal oxide is too high. Due to such V.sub.o energy level, a color of the plasma treated sample became black. Briefly, because of TiN.sub.s, V.sub.o energy level formed on the surface of the particle shown in FIG. 3, there have been achieved effects of actively transferring electron-hole pairs excited in the particle to the surface of particles, and raising the VBM level of TiO.sub.2 by TiN.sub.iH to decrease a band-gap size, therefore, a wavelength range in which electron-hole pairs are generated by sensing light was extended to a wavelength range of about 470 nm in the visible light region.
(28) FIG. 8 illustrates conditions for H.sub.2/N.sub.2 mixed gas plasma treatment optimized according to a photoelectrochemical catalysis evaluation using 0.1 M NaClO.sub.4 aqueous solution (pH 7) as an electrolyte, a platinum counter electrode and an Ag/AgCl reference electrode. As a result of measuring a water-oxidation photocurrent amount with variations in a plasma generation output (FIG. 6a), a plasma treatment time (FIG. 6b) and a mixing ratio of H.sub.2/N.sub.2 gas (FIG. 6c), it was observed that the highest photocatalytic performance is achieved when a plasma ball generated using the H.sub.2/N.sub.2 mixed gas in a mixing ratio of hydrogen:nitrogen=1:2 with 500 W output power at room temperature contacts with the surface of the metal oxide thin film for 3 minutes to treat the same.
(29) FIG. 9 also illustrates comparison and evaluation results of photoelectrochemical catalysis reactivity of the sample (HNTiO.sub.2) with the plasma treatment under the optimum condition induced in FIG. 8 and the untreated sample (bare TiO.sub.2). In the photocurrent-voltage curve in FIG. 9a, it was observed that the photocurrent amount of the HNTiO.sub.2 sample emitting light at 1.23 V.sub.RHE (potential for degrading water and generating hydrogen and oxygen) has increased by 9 times or more, compared to the bare TiO.sub.2. FIG. 9b illustrates photoreaction efficiency with respect to wavelengths of light. Herein, it was observed that the bare TiO.sub.2 exhibits photoreaction in only the UV light range (300 nm to 400 nm) whereas HNTiO.sub.2 has an extended range of reaction up to the visible light region. In particular, it was found that an integral value of the visible light region (400 nm to 600 nm) curve only of the HNTiO.sub.2 sample is larger than an integral value of a curve as the sum of overall wavelength ranges of the bare TiO.sub.2. In fact, as a result of separating light with UV wavelength and light with visible light wavelength from each other, then, measuring the respective photocurrent amounts, the HNTiO.sub.2 sample showed the photocurrent amount (0.094 mA cm.sup.2) generated from the light only at the visible light wavelength, which is higher than the photocurrent amount of the bare TiO.sub.2 (0.041 mA cm.sup.2) generated by the light with the sum of UV light and visible light wavelengths. Meanwhile, photoreactivity of the HNTiO.sub.2 sample is gradually decreased as the number of waves per wavelength increases in the visible light region, then, rapidly approaches 0% from a point at 470 nm, and this result demonstrated that the VBM is raised due to the above-described TiN.sub.s binding to thus attain effects of decreasing the band-gap to about 2.7 eV.
(30) The inset of FIG. 9b illustrates a graph of chopped photocurrent time curve with normalization of their photocurrent intensity. Herein, it was observed that the HNTiO.sub.2 sample exhibits instant excitation and relaxation of the generated photocurrent depending on light irradiation and blocking thereof. On the other hand, the photocurrent of the bare TiO.sub.2 was very slowly increased to the maximum photocurrent value when becoming light, while being decreased slowly to the minimum value even if the light emission is blocked. The reason of such a phenomenon is that the bare TiO.sub.2 has a very low transportation rate of electrons and holes generated by the light from the inside to the outside of a particle, as compared to that of HNTiO.sub.2, therefore, a time required to reach a steady-state in the amounts of electrons and holes moving toward the outside of the particle is also much longer than that of HNTiO.sub.2. The above-described slow transfer phenomenon of the electron-hole has caused such a phenomenon that the electrons and holes still accumulated inside the particle without moving to the outside thereof are transferred to the outside and slowly decreased as if the photoreaction still exists even after the light is blocked. Further, referring to an HNTiO.sub.2 photocurrent curve, since an overshooting phenomenon occurs during light irradiation while a lower value of current amount than the dark current value in the steady-state is observed during light blocking, it was determined that the HNTiO.sub.2 sample involves a Shockley-Read-Hall (SRH) recombination. The SRH recombination is a phenomenon occurring when an electron donor or electron acceptor level is present at a deep level inside the band-gap of a semiconductor material. In the present case, this was observed as an effect occurring due to TiN.sub.i and V.sub.o energy levels shown in the graph of FIG. 3. Consequently, an electron carrier density is raised as oxygen vacancies are increased, and the electron-hole pairs generated by sensitization of the light in the visible light region are also increased. Furthermore, due to the energy level generated at the surface of the particle, the electrons and holes generated inside the particle are rapidly moved toward the outside. Finally, the number of electrons and holes participating in the oxidation/reduction reaction are increased to thus improve photocatalytic efficiency.
(31) FIG. 10 illustrates results of testing a production amount of a C1 compound fuel through a photochemical reaction in a stainless steel rector, in which carbon dioxide is saturated and sealed, and by using the present inventive material having a high photocurrent amount. According to the experimental method described in the example of the present invention, it was confirmed that a great amount of carbon monoxide and methanol was generated from carbon dioxide through a photochemical conversion reaction. In particular, it was demonstrated that a sample using copper metal as a co-catalyst (HNTiO.sub.2:Cu) exhibited very higher conversion efficiency by 25 times or more and 8 times or more, respectively, than other samples using a platinum catalyst (bare-TiO.sub.2:Pt, HNTiO.sub.2:Pt). Based on these results, it could be understood that photocatalytic efficiency of TiO.sub.2 material may be maximized according to the present inventive process.
(32) As shown in FIG. 10, conversion and production amounts of carbon dioxide and water into a fuel, that is, C1 compound such as a) carbon monoxide, b) methanol, etc. through the photochemical reaction for 10 hours without any oxidant and reductant are shown in Table 1 below.
(33) TABLE-US-00001 TABLE 1 Production amount per time (mol/g) Section a (CO; carbon monoxide) b (CH.sub.3OH; methanol) Bare TiO.sub.2Pt 0.45 0.07 HNTiO.sub.2Pt 4.75 0.22 HNTiO.sub.2Cu 12.67 1.79
(34) From the above results, it was confirmed that a great amount of C1 compounds, that is, a) carbon monoxide and b) methanol were generated from carbon dioxide through the photochemical conversion reaction by the experimental procedures described in the examples of the present invention. In particular, it was demonstrated that the sample using copper metal as a co-catalyst (HNTiO.sub.2:Cu) exhibited very higher conversion efficiency by 25 times or more and 8 times or more, respectively, than other samples using a platinum catalyst (bare-TiO.sub.2:Pt, HNTiO.sub.2:Pt). Alternatively, Cu and Zn among other transition metals have also accomplished the results similar to the above description. Based on the above-demonstrated results, it could be confirmed that the photocatalytic efficiency of TiO.sub.2 material was maximized by the present inventive process. Although not listed in Table 1 above, the C1 compound may further include methane CH.sub.4 and formic acid HCOOH.
(35) In order to confirm effects obtained by the mixed gas plasma treatment of the present invention through a test for evaluating applicability of the mixed gas plasma treatment to various metal oxide semiconductor materials other than TiO.sub.2, the same conditions of the examples according to the present invention were applied to zinc oxide (ZnO) nanoparticles and copper oxide (CuO) nanoparticles, and results thereof are shown in FIGS. 11 and 12. Similar to the case of the above-described TiO.sub.2 nanoparticles, the surface of particle became amorphous through the mixed gas plasma treatment to thus change the core-shell structure, which was determined by TEM measurement. A band-gap of pure ZnO nanoparticle was about 3.2 eV and, as same to TiO.sub.2, therefore, the photocurrent is formed only in UV light at a wavelength band of 400 nm or more. However, according to the mixed gas plasma treatment of the present invention, it could be found that the photocurrent is formed even in the light at a wavelength range of 400 to 700 nm in the visible light region (FIG. 11d). Further, it was determined that the plasma treated ZnO could generate the photocurrent in an amount increased by 7 times even under incident solar simulator (AM 1.5G) including both of UV ray and visible light, as compared to pure ZnO nanoparticles (FIG. 11c). Since the pure CuO nanoparticle has a band-gap of 1.3 eV, the light sensitive wavelength range is not increased by the plasma treatment, unlike TiO.sub.2 or ZnO nanoparticle. However, it was found that the amount of photocurrent generation was increased under the same incident light condition (FIG. 12c). Furthermore, it was demonstrated that the CuO nanoparticle originally exhibiting only p-type characteristics has also showed n-type characteristics through the mixed gas plasma treatment. Accordingly, as shown in FIG. 12d, CuO nanoparticle originally having hydrogen reduction ability could oxidize oxygen even at the potential of oxygen oxidation (1.23 V.sub.RHE) and generate photocurrent amount. Through such a series of experiments as described above, it was demonstrated that the mixed gas plasma treatment of the present invention can be applied to a broad range of metal oxide semiconductor materials.
(36) The terms or words used in the specification and claims of the present invention should not be construed as limited to a lexical meaning, and should be understood as appropriate notions by the inventor based on that he/she is able to define terms to describe his/her invention in the best way to be seen by others. Therefore, embodiments and drawings described herein are simply exemplary and not exhaustive, and it will be understood that various modifications and equivalents may be made to take the place of the embodiments.
(37) By further depositing a metal such as Cu or Pt on the surface of NH, H plasma treated core-shell metal oxide using the single process of the present invention, the resulting material may be utilized not only in a direct conversion catalyst for converting solar energy into a compound such as CO.sub.2 conversion but also in a cathode material for electrochemical energy conversion and storage fields, as well as other applications directly associated with metal oxide semiconductor catalysts such as a gas sensing catalytic material. Further, non-harmful effects to the human body and environment and fluorescent light sensitive properties may be practically utilized in a variety of applications including, for example: building interior/exterior materials; semi-permanent anti-fouling agents for garments, masks, etc.; offensive odor removers; preservatives; or tooth whitening agents. Moreover, due to strong UV reactivity and high organic matter oxidation effects, the metal oxide material of the present invention may be used as an air or water purification catalyst utilizing UV LED light.
(38) In addition, due to convenience and low costs of the single process, easiness in mass production and efficient improvement of characteristics, the present invention may be highly utilized in a broad range of applications.
