Patent Application
20070122564
The present invention generally relates to a method and an apparatus for growing thin film, and more particularly for growing a composite metal sulphide photocatalyst thin film.
With development of highly efficient photocatalyst materials as reported in scientific researches, their uses and applications have been widely extended to different fields. For example, in a hydolysis reaction catalyzed by a photocatalyst, a water molecule is broken down to produce hydrogen, water and carbon dioxide for generating fuels such as methanol, methane and so on. The photocatalyst has also been used conventionally for creating a better amenity environment. For example, transparent titanium oxide (TiO2) film photocatalyst has been utilized under visible light or ultraviolet (UV) irradiation to decompose odors, bacteria and stains during the process of sterilization, oxidative decomposition, and deodorization.
Typically, most of the photocatalysts have been manufactured with material in the form of powders. Yet, it is more favorable, in terms of industrial applicability to manufacture a photocatalyst thin film which facilitates the design of industrial photoreactors. Japanese Patent JP2002-20108 has disclosed a method and apparatus for forming semiconductor thin film in aqueous solution. The photocatalyst thin film was formed by photochemical deposition (PCD) on a substrate that was irradiated with a light source. Japanese Patent JP2003-181297 has disclosed formation of a thin film-like photocatalyst by dipping a base material into the solution containing Zn and depositing ZnS, ZnO or the like on the surface of the base material by a chemical bath deposition (CBD) method.
However, none of the studies has been directed to manufacturing a composite metal sulphides photocatalyst thin film which is grown on a large scale basis.
In one aspect the invention provides a method for growing a composite metal sulphide thin film, comprising steps of: immersing a first carrier for photochemical deposition and a second carrier for chemical bath deposition in a reaction tub filled with an alkaline solution comprising at least a metal ion and a sulphur-based compound, wherein the second carrier is arranged vertical to a bottom surface of the reaction tub; and irradiating the first carrier with a light source producing light, such that the metal sulphide thin film is grown by photochemical deposition and chemical bath deposition on the first and second carriers, respectively, wherein the sulphur-based compound comprises thiosulfate (S2O32−) and thiourea (CSN2H4).
In another aspect the invention provides an apparatus for growing a composite metal sulphide thin film, comprising: a reaction tub having a first carrier and a second carrier held within the reaction tub, wherein the second carrier is held vertical to a bottom surface of the reaction tub; and a light exposure assembly comprising a frame holding a light source in such a way that the light source is over the reaction tub for irradiating the first carrier.
The foregoing summary, as well as the following detailed description of the invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings embodiments which are presently preferred. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
In the drawings:
The present invention proposes a method and apparatus for growing a composite metal sulphide photcatalyst thin film. Two deposition methods, including photochemical deposition and chemical bath deposition, are used in the invention for growing the photocatalyst thin film, such as (AgInS2)x/(ZnS)2(1-x), wherein x is 0-1. However, the invention should not be limited to growing AgInS2, ZnS or AgInS2/(ZnS)2 material. Other types of metal sulphides or composite metal sulphides grown by the method and apparatus described in details below are also within the scope of the present invention.
According to the invention, a method is provided for growing a composite metal sulphide thin film, comprising steps of: immersing a first carrier for photochemical deposition and a second carrier for chemical bath deposition in a reaction tub filled with an alkaline solution comprising at least a metal ion and a sulphur-based compound, wherein the second carrier is arranged vertical to a bottom surface of the reaction tub; and irradiating the first carrier with a light source producing light, such that the metal sulphide thin film is grown by photochemical deposition and chemical bath deposition on the first and second carriers; wherein the sulphur-based compound comprises thiosulfate (S2O32−) and thiourea (CSN2H4). In the photochemical deposition, a light ray or light beam from the light source irradiates the first carrier immersed in the alkaline solution comprising S2O32−, so that the thiosulfate may be excited by electron transition to generate electrons e− according to equations (1)-(3). In the method, the light source producing light preferably has a wavelength of less than 300 nm. Preferentially, S2O32− may be potassium thiosulfate or sodium thiosulfate having a concentration of about 0.05-0.1M in the solution. The reactions also generate chemical species such as SO32−, S3O62−, S4O62−, and sulfur atoms (S) in the alkaline solution based on the following equations.
2S2O32−+hv→S4O62−+2e− (1)
2S2O32−+hv→S+SO32− (2)
S2O32−+SO32−+hv→S3O62−+2e− (3)
Then, the sulphur atoms and the electrons in the alkaline solution react with the metal ions to form metal sulphides. For example, the sulphur atoms and the electrons may react with Zn2+, Ag+ and In3+ according to equations (4)-(7) to form zinc sulphide (ZnS), silver indium sulphide (AgInS2), silver sulphide (Ag2S) and indium sulphide (In2S3).
Zn2++S+2e−→ZnS (4)
Ag++In3++2S+4e−→AgInS2 (5)
2Ag++S+2e−→Ag2S (6)
2In3++3S+6e−→In2S3 (7)
In the chemical bath deposition, thiourea (CSN2H4) in the alkaline solution is used to produce sulphide ion (S2−) based on equations (8) and (9). Preferably, thiourea is added at a concentration of about 0.05-1M in the solution. The solution comprises ammonium nitrate (NH4NO3) and ammonium hydroxide (NH4OH) for adjusting pH of the alkaline solution. The alkaline solution is adjusted to a pH range, preferably 8-11 with NH4OH. The solution pH is further stabilized by adding a buffer solution containing NH4NO3 preferably at a dose of 0.01-0.5M.
CS(NH2)2+OH31 →SH−+CH2N2+H2O (8)
SH−+OH−→S2−+H2O (9)
Then, the sulphide ions react with the metal ions to form metal sulphides. For example, the sulphide ions may react with Zn2+, Ag+ and In3+ according to equations (10)-(13) to form zinc sulphide (ZnS), silver sulphide (Ag2S), indium sulphide (In2S3) and silver indium sulphide (AgInS2).
Zn2++S2−→ZnS (10)
2Ag++S2−→Ag2S (11)
2In3++3S2−→In2S3 (12)
Ag++In3++2S2−→AgInS2 (13)
Due to low solubility of the sulphide compounds, the chemical reactions of equations (10)-(13) occur simultaneously once the sulphide ions are generated. Instead of forming on the second carrier, most of the metal suphide formation would take place in the solution. Therefore, NH4OH is further added to metal ions, forming metal complexes M(NH3)yn+ according to equations (14)-(15). The metal complexes then react with the sulphide ions to form metal sulphides MSn/2 according to equation (16).
wherein M is a metal ion comprising at least one of Ag+, Zn2+, In3+ and (AgIn)4+, n is about 1-3 and y is about 2-6.
Since the chemical reaction of equation (16) takes place slowly at room temperature, the solution may be added with hydrazine (N2H4) and heated with a heating element to a temperature of about 30-90° C. for facilitating or speeding up the chemical reaction of equation (16). Preferably, the hydrazine may be added at a concentration of about 0.01-1M in the solution. In addition, S2O32− from the photochemical reaction may also react with the metal ions in the solution to form metal complexes according to equation (17).
wherein n is about 1-3, y is about 2-6 and z is about 2-6.
The metal complexes as formed in equation (17) then react with sulphide ions in the solution to form metal sulphides on the second carrier in accordance with equation (18). Thus, the chemical bath deposition should not be limited to the chemical reactions as defined by the equations (8)-(16), other chemical reactions of equations (17) and (18) are also applicable to the chemical bath deposition for forming the metal sulphide photocatalyst thin film on the second carrier.
wherein n is about 1-3, y is about 2-6 and z is about 2-6.
In accordance with one preferred embodiment, the solution comprises silver nitrate (AgNO3), indium nitrate (In(NO3)3), zinc nitrate (Zn(NO3)2), ammonium nitrate (NH4NO3), sodium thiosulfate (Na2S2O3) and thiourea (CSN2H4) in a mole ratio of m:m:2(1−m):(1 to 20)m:(100 to 2000)m:(9 to 100)m, wherein m is greater than zero up to about 1.
In accordance with another embodiment, the solution comprises silver nitrate (AgNO3), indium nitrate (In(NO3)3), zinc nitrate (Zn(NO3)2), NH4NO3, potassium thiosulfate (K2S2O3) and thiourea (CSN2H4) in a mole ratio of m:m:2(1−m):(1 to 20)m:(100 to 2000)m:(9 to 100)m, wherein m is greater than zero up to about 1.
To ensure the crystallinity of the thin film grown, a thermal process is further performed for curing the metal sulphide thin film. Preferably, the metal sulphide thin film is cured at a temperature of about 130° C. for two hours to remove water content within the thin film. Next, a thermal process is further performed in a high temperature furnace flushed with nitrogen gas. Preferably, a sintering process is performed on the metal sulphide thin film at a temperature of about 200-1000° C. for about 6-12 hours before cooling to a room temperature to yield metal sulphide thin film.
The invention also provides an apparatus for growing a composite metal sulphide photocatalyst thin film. Referring to
In accordance with one embodiment, the first carrier 11 and the second carrier 12 are held in the reaction tub 10 by a first carrier holder 13 and a second carrier holder 14, respectively, and the second carrier 12 is held vertical to a bottom surface 10a of the reaction tub 10. As shown in
Referring to
To ensure that a solution concentration is not changed by evaporation or contamination by pollutants, the reaction tub 10 may include a lid 18 to keep the solution closed in the reaction tub. The lid 18 may be made of transparent material to allow light having the wavelength of less than 300 nm to pass through. For example, the lid 18 may be made of a quartz glass lid or a glass lid.
The reaction tub 10 may further include a stirring component 19 adjacent the bottom surface 10a of the reaction tub 10 for stabilizing the solution concentration. The stirring component 19 may be a stirring member or a stirring device provided in the reaction tub 10 to stabilize the concentration of the solution.
The apparatus 1 may also include a temperature regulating assembly 30 for maintaining the reaction tub 10 at a temperature optimal for performing chemical bath deposition. For example, the temperature is kept at about 30-90° C. via the temperature control assembly 30 which comprises a thermostatic assembly 31 for keeping the temperature of the solution constant, a heating element 32 for heating up the solution, a temperature detector 33, and a temperature controller 34 for monitoring the temperature change of the solution. The temperature detector 33 is coupled to the thermostatic assembly 31 for controlling the temperature in the reaction tub, and may be a thermometer, a k-type thermocouple, J-type thermocouple or other devices for measuring the temperature of the thermostatic assembly 31. The reaction tub 10 is bathed in the thermostatic assembly 31, such as a steam bath containing water vapor, an oil bath containing silicon oil or a water bath containing water to keep the temperature of the solution constant.
The heating element 32 may be a heating plate, a heat rod, a heating filament, a heating belt, or other similar heating structure, and the heating element 32 may be switched on/off by the temperature controller 34 based on the temperature detected by the temperature detector 33. For example, when the temperature of the solution drops below the range of 30-90° C., the heating element 32 is turned on by the temperature controller 34 to heat up the solution via the thermostatic assembly 31. On the other hand, as the temperature exceeds the range, the heating element 32 is turned off by the temperature controller 34.
With respect to the apparatus 1 described above, a method for growing the composite metal sulphide photocatalyst thin film is also provided. The method for growing metal sulphide photocatalyst thin film comprises immersing first and second carriers 11 and 12 in a reaction tub 10 filled with an alkaline solution comprising at least a metal ion and a sulphur-based compound, wherein the second carrier 12 is arranged vertical to a bottom surface of the reaction tub 10. The sulphur-based compound is defined as a compound containing sulphur and comprises thiosulfate (S2O32−) and thiourea (CSN2H4).
In the alkaline solution, the metal ion comprises at least one of silver ions (Ag+), copper ions (Cu+), zinc ions (Zn2+), cadmium ions (Cd2+), indium ions (In3+), tantalum ions (Ta3+), titanium ions (Ti4+), CuIn4+, AgIn4+, and sulfate, nitrate and carbonate salt compounds thereof to provide cations in the subsequent photochemical deposition process and chemical bath deposition process. Preferably, the metal ion has a concentration of about 1×10−4M-0.5M in the alkaline solution to form metal complexes with a corresponding complexing agent. The alkaline solution may be prepared by adding the metal ion in a sulfur-based compound solution or adding the sulfur-based compound solution in an aqueous solution containing the metal ion.
As the first carrier 11 is irradiated with a light source 22 having a wavelength of less than 300 nm, the metal sulphide thin film is grown by photochemical deposition and chemical bath deposition on the first and second carriers 11 and 12.
In the photochemical deposition, light ray or light beam from the light source 22 passes through the lens assembly 24 to irradiate the first carrier 11 immersed in the alkaline solution comprising S2O32−, so that the thiosulfate may be excited by electron transition to generate electrons e−.
The invention will now be described in further detail with reference to the following specific, non-limiting examples.
Referring to
The first carrier 11 is immersed about 5 mm below the solution surface. The second carrier 12 having a plurality of substrates 12a is arranged vertical to a bottom surface of the reaction tub 10. As shown in
The reaction tub 10 is also provided with the stirring component 19, such as magnetite spinning at a rate of about 300 revolutions per minute (rpm) for maintaining a constant concentration of the solution. Similarly, as the solution is kept at a temperature of about 30-70° C. and a pH of about 8-11, thiourea in the solution is hydrolyzed to hydroxysulphide (SH−) and diazomethane (CH2N2) to generate sulphide ions which react with silver, indium and zinc to grow (AgInS2)x/(ZnS)2(1-x) composite thin film on the second carrier 12. Also, thiosulfate in the solution can react with cations or metal ions to generate metal complexes. Then, the metal complexes would react with sulphide ions to form (AgInS2)x/(ZnS)2(1-x) composite thin film on the second carrier 12. Thus, formation of metal complexes in the solution reduces the chance of metal sulphide formation in the solution. The efficiency for metal sulphide thin film formation is improved.
To ensure the crystalline structure of the thin film grown, a thermal process is further performed for curing the metal sulphide thin film. Preferably, the metal sulphide thin film is cured at a temperature of about 130° C. for two hours to remove water from the thin film. Next, a thermal process is further performed in a high temperature furnace flushed with nitrogen gas. Preferably, a annealing process is performed on the metal sulphide thin film at a temperature of about 600° C. for about 6 hours before cooling to room temperature to yield metal sulphide crystals.
After a AgInZn5S7 composite thin film grown by photochemical deposition is annealed at a temperature of about 600° C. for about 6 hours, the composite thin film is tested for light absorbance in terms of transmission percentage. The transmission percentage is a measure of light transmission for the composite thin film against light transmission for the substrate, such as glass.
Referring to
Similarly, a AgInZn7S9 composite thin film grown by chemical bath deposition is annealed at a temperature of about 600° C. for about 6 hours, and the composite thin film is tested for light absorbance in terms of transmission percentage.
Referring to
Referring to
Referring to
According to the present invention, the method and apparatus for growing metal sulphide thin film are provided. Both photochemical deposition and chemical bath deposition can occur simultaneously in the reaction tub according to the method and apparatus of the invention. Therefore, the metal sulphide thin film or composite metal sulphide thin film is grown on the first and second carriers with a shorter deposition time. Since thiosulfate in the solution is used for generating electrons and sulfur atoms in the photochemical deposition process and forming metal complexes in the chemical bath deposition process, a smaller amount of electroplating solution is used on both deposition processes and the efficiency for forming the metal sulphide photocatalyst is significantly improved. Accordingly, the method and apparatus are provided for forming a large-area metal sulphide thin film applicable to forming solar cell panels, photocatalyst thin films, photoreactors and optoelectronic substrate.
It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims.
