US3655383A - Method for reproducing images of a solid photocatalyst with an oxidizing agent - Google Patents

Abstract

The present invention relates to an embodiment of the invention disclosed in U.S. Pat. No. 3,152,903 in which a radiation image is reproduced by exposing a carrier bearing a photocatalyst and an oxidizing agent to radiation to reduce a portion of the oxidizing agent present to a free metal defining a latent image which serves as a catalyst site for the subsequent development reaction of a reducing agent and the non-reduced oxidizing agent.

Description

ilnited States Patent Shepard et al.

[541 METHOD FOR REPRODUCING IMAGES OF A SOLID PHOTOCATALYST WITH AN OXIDIZING AGENT [72] Inventors: Joseph W. Shepard, St. Paul; Benjamin L.

Shely, White Bear Lake, both of Minn.

[73] Assignee: Minnesota Mining and Manufacturing Company, St. Paul, Minn.

[ Notice: The portion of the term of this patent subsequent to Feb. 25, 1986, has been disclaimed.

[22] Filed: Nov. 29, 1968 211 Appl. No.: 780,283

Related US. Application Data [63] Continuation of Ser. No. 517,469, Dec. 29, 1965, Pat. No. 3.429.706. which is a continuation of Ser. No. 221,329, Sept. 4, 1962, abandoned, which is a continuation-in-part of Ser. No. 809,927, Apr. 30, 1959, Pat. No. 3,152,903.

[151 3,655,383 [451 *Apr. 11, 1972 Primary Examiner-Norman G. Torchin Assistant Examiner-John L. Goodrow Att0mey-Kinney, Alexander, Sell, Steldt & Delahunt [57] ABSTRACT The present invention relates to an embodiment of the invention disclosed in US. Pat. No. 3,152,903 in which a radiation image is reproduced by exposing a carrier bearing a photocatalyst and an oxidizing agent to radiation to reduce a portion of the oxidizing agent present to a free metal defining a latent image which serves as a catalyst site for the subsequent development reaction of a reducing agent and the non-reduced oxidizing agent.

3 Claims, No Drawings METHOD FOR REPRODUCING IMAGES OF A SOLID PHOTOCATALYST WITH AN OXIDIZING AGENT This application is a continuation of our copending application Ser. No. 517,469, filed Dec. 29, 1965, now US. Pat. No. 3,429,706 which is a continuation of our copending application Ser. No. 221,329, filed Sept. 4, 1962, now abaondoned, which is a continuation-in-part of our prior application Ser. No. 809,927, filed Apr. 30, 1959, now US. Pat. No. 3,152,903.

The present invention relates to a novel and useful radiation-sensitive system. In one aspect the invention relates to the permanent reproduction of images or patterns on a surface by irradiation. in another aspect the invention relates to a new light-sensitive composition and to a reproduction surface or sheet made from such composition. In still another aspect the invention relates to a new and novel photographic process in which an image is reproduced directly without the conventional developing step.

Numerous processes are known for the light reproduction of images and for copying. One of the more common and typical of such processes is that known as the silver halide process. This process requires exposure of a sensitive film or paper to the light or image source followed by a separate step of wet developing of the image on the film or paper.

Another typical process is known as electrophotography, and this process depends upon the presence of a photoconductive material in the film or printing paper. As in the silver halide process, this process requires a separate step of developing of the image. If fixing is necessary or desirable, fixing may be done by a dry process, such as by heat.

The silver halide process and similar processes are considered more sensitive than the electrophotographic process. The disadvantage of the silver halide process, however, is the rather involved developing procedure. On the other hand, the disadvantage of the electrophotographic process is the low sensitivity thereof to the reproduction of images. Most of the processes require a separate step in addition to the developing step for the fixing of the image so that upon exposure to normal light conditions the image will not fade or the background will not darken. It is much to be desired, therefore, to provide a simpler process than the above with elimination of their disadvantages. It has been discovered that certain materials have a catalytic effect upon reactions when activated by irradiation. This photocatalytic effect is taken advantage of in accordance with the present invention.

An object of this invention is to provide a novel light-sensitive composition or combination of components.

Another object of this invention is to provide a novel process for graphic reproduction or copying of printed matter and the like.

Another object of this invention is to provide a process which directly reproduces the image or directly copies upon exposure to the object to be reproduced and does not require a separate step of developing.

Still another object of this invention is to provide a lightsensitive combination of components of increased sensitivity for the graphic reproduction of images and the like.

Still another object of this invention is to provide a dry process for graphic reproduction.

Still another object of this invention is to provide a process for directly reproducing transparencies.

Yet another object of this invention is to provide a novel copy-paper or film.

Another object is to provide a new photographic transparency film.

Still another object of this invention is to provide a dry graphic reproduction process which is receptive to a braoder light spectrum than heretofore possible.

Another object of this invention is to provide a new imagereproducing composition which can be developed and fixed in a single operation.

Still another object of this invention is to provide a new technique for permanently fixing or stabilizing a reproduced image.

Various other objects and advantages will become apparent to those skilled in the art from the accompanying description and disclosure.

According to this invention, the radiation-sensitive system comprises an image-forming combination of components or composition and a separate radiation-sensistive catalyst or photocatalyst chemically different from the image-forming composition. The radiation-sensitive system is usually in a dry or even anhydrous form and is supported by or is a part of a suitable inert carrier or sheet. The carrier sheet containing the radiation-sensitive system is then exposed to an image source or light source, and the image or pattern to be copied is reproduced either immediately upon exposure or upon subsequent development. In some instances, fixing or inactivation of the radiation-sensitive system is required so that upon viewing the reproduction, the image or reproduced matter will not fade or the background will not darken.

The image-forming or reactive components of the system are in the form of an irreversible oxidation-reduction reaction combination which is capable of initiation into reaction or of catalyzation by electron transfer thereto from the photocatalyst. The oxidation-reduction reaction combination or composition comprises two solid phases; a separate solid phase comprising an oxidizing agent and another separate solid phase comprising a reducing agent. During exposure to radiation, the oxidizing agent reacts with the reducing agent whereby a reproduction of the image or pattern results, which reproduction may be latent (invisible) or visible. Any redox combination of an oxidizing agent and a reducing agent having a negative-free energy under the exposure or development conditions suffices as the image-forming or reactive composition.

In one embodiment of the invention, the reaction of the oxidation-reduction combination upon exposure produces a product or products having a change in color or reflectance resulting in avisible reproduction. In another embodiment, the reaction product or products of the reaction of the oxidation-reduction combination upon exposure are not visible (latent image) but are made to react further with each other or with other materials in a subsequent development step to produce a visible reproduction by a change in color or reflectance. In both embodiments, the image-forming composition is substantially dry when exposed, and reaction is effected at least partially at the time of exposure under exposure conditions which are usually ambient conditions. Exposure may be effected at somewhat elevated temperatures, such as 50 C. Subsequent reactions, if necessary, are effected by heating above the exposure temperature or by wetting the exposed redox combination with water.

In addition to the image-forming combination or composition above-described, the radiation-sensitive system requires a third solid phase comprising, as a radiation-sensitive catalyst, a relatively nonreactive material which is not the same as the oxidizing agent or the reducing agent and can be activated into the transfer or release of electrons upon exposure to irradiation having a wave length below 5 microns, preferably below 1 micron, such as actinic light, X-rays or gamma rays. Electrons are transferred from the catalyst to the oxidationreduction combination, usually to the oxidizing agent as the electron acceptor, which electrons initiate or catalyze the reaction.

The radiation-sensitive system may comprise an admixture of the above three separate solid phases or may comprise separate layers of each, or layers of a combination of any solid phases, in any order. The radiation-sensitive catalyst phase may be applied as a layer bonded to a suitable carrier or inert substrate. Upon the catalyst layer is affixed the image-forming combination in the form of a single layer comprising an admixture of components or in the form of two separate adjacent layers of each of the components. Any one or all of the components of the radiation-sensitive system may also comprise, or be impregnated in, the carrier. Any one of the reactive components may be included with the catalyst layer and the other reactive component included in a separate layer adjacent and in contact with the catalyst layer.

Although inactivation of the image reproduction system is not required in all instances, depending upon the components of the system or type of irradiation source used, in many instances an inactivation or fixing operation is desirable or necessary Where the irradiation source is x-rays or gamma rays, visual observation of the image will not be carried out in the presence of such rays, and, therefore, inactivation may not be necessary. On the other hand, where the type of irradiation source is actinic light and the visual image reproduction is to be observed in the presence of such light, inactivation is required in most instances. Inactivation is carried out by inactivating the image-forming composition such as by removing or complexing at least one of the unreacted components, by inactivating the photocatalyst, or by the use of a combination of these methods.

In order to maintain the oxidizing agent and the reducing agent as separate solid phases, at least one of these reactive components is admixed with a material or binder which is incompatible or immiscible with that component or with the other component or with a second material or binder used with the other component. ln applying the present combination to an inert substrate, such as paper, the metal containing catalyst is applied or affixed to the surface, such as with an organic resin or binder. Then one of the components of the oxidation-reduction combination is admixed with an organic solvent or diluent containing dissolved therein an organic resin as a binder, which admixture is applied as a continuous layer over the catalyst layer and dried to form a solid layer. Upon this layer is applied the other reactive component from a solution thereof, the solvent of which solution is substantially immiscible with, but bondable to, the organic layer containing the other reactive component. Since most of the oxidizing agents and reducing agents are water-soluble, this latter layer may be applied from an aqueous solution containing dissolved therein a water-soluble binder to form a continuous layer when dried. There are various techniques which will become obvious to those skilled in the art for maintaining the reactive components and catalyst in separate phases and in a nonreacted condition while in admixture or in contact with each other while departing from the scope of this invention.

The reactive components of the radiation-sensitive composition are in actual contact or in mutually interreactive relationship with each other, but are physically distinct from each other. The catalyst is similarly in such close relationship with the reactive components as to be capable of transferring electrons to at least one of the components when the catalyst is activated by radiation.

As previously stated, the image-forming combination includes both an oxidizing agent and a reducing agent. The oxidizing agent in this composition is usually the image former, but not necessarily. Either organic or inorganic oxidizing agents may be employed as the oxidizing component of the image-forming combination. The preferred oxidizing agents comprise the metal salts, either inorganic or organic. Suitable metal salts include the salts of silver, mercury, lead, gold manganese (in the form of the permanganate), nickel, tin, chromium, platinum and copper. Examples of typical nonmetal salt oxidizing agents include the nonmetal organic salts and dyes comprising the tetrazolium salts, such as tetrazolium blue (33 dianisole-bislA (3,4 diphenyl) tetrazolium chloride) and red (triphenyl-tetrazolium chloride) and diphenyl carbazone, and Genacryl Red 68, a methine dye (CI 48020).

When zinc oxide is used as a photocatalyst, as will hereinafter be discussed, molecules or ions with reduction potential below oxygen in the electromotive series are useful as the oxidizing agent in either neutral or acid media. Thus, the salts of the reducible metal ions, Ag, Hg, Pb, Au, Pt and MnO can be used as the oxidizing agent with zinc oxide as the photocatalyst upon irradiation. in basic media, molecules or ions below zinc in the electromotive series can be used as the oxidizing agent when zinc oxide is used as the photocatalyst. Thus, the reducible metal ions, Ni Sn, Pb" and Cu, are suitable in salt form as the oxidizing agent with zinc oxide as a photocatalyst on exposure to irradiation.

When the above metal salts are used as the oxidizing agent, an organic nonmetal complexing agent may be used which will complex with the metal ion of the above metal salts. Thus, carbazone can be reduced to the carbazide and an image formed by complexing a metal ion with the carbazide. Also, additives may be used in combination with the oxidizing agent to change the character and tonal value of the image. Images often assume a darker and more dense tone when the metal ion of the oxidizing agent is complexed with another material. For example, the density of a silver image is increased by the use of a small amount of an organic complexing additive, such as an acid amide as, for example, formamide or acetamide, and phytic acid. Similarly, the density of a gold image is increased by use of acetamide.

The reducing agents of the image-forming composition which are chemically different from and physically separate from the oxidizing agents are organic reducing compounds, such as the oxalates, formates, substituted and nonsubstituted hydroxylamine and substituted and nonsubstituted hydrazine, ascorbic acid, aminophenols and the mono and dihydric phenols. The oxalates and formates are usually in the form of salts of the alkali earths and alkali metals, such as sodium, lithium and potassium. A preferred oxalate salt is sodium oxalate. A preferred formate is sodium formate. Examples of substituted hydroxylamines include phenyl hydroxylamine and benzyl hydroxylamine. An example of an aminophenol is Metol (l,4-rnethyl-p-aminophenol) and Elon (N-methyl-paminophenol sulfate); an example of a substituted hydrazine is phenyl hydrazine. Suitable mono and dihydric phenols include lonol (2,6-ditertiary-butyl-4-methyl-phenol), hydroquinone and catechol.

As previously stated, some of the oxidizing agents work best in acidic or basic media. Suitable acids which can be employed in admixture with the oxidizing agent and reducing agent as part of the image-forming composition include the carboxylic acids, such as oxalic acid and stearic acid. The basic media may be provided in the image-forming composition by the inclusion therein of an organic or inorganic base, such as ammonium hydroxide or sodium acetate, or any salt of a strong base and weak acid.

The selection of the particular oxidizing agent to be used with a particular reducing agent is, of course, determined in one aspect by the ability of either one or both of the compounds in their reacted form to show a change in light value (a change in color or reflectance), or to react with another compound resulting in a change in light value. The oxidationreduction potential (E.) for the reaction between the oxidizing agent (electron acceptor) and the reducing agent (electron donor) must be positive under the conditions of reaction. This can be calculated from the standard electrode potentials (E) for the half cells. Preferably, the oxidation-reduction potential (E.) for the reaction is at least +0.1 volt.

As previously stated, the light-sensitive catalyst or photocatalyst is a separate solid phase which may be combined with the ingredients in the image-forming composition or may form a separate layer or impregnated in the carrier. The photocatalyst is a material which will transfer electrons when activated by a radiation wave length below 5 microns. Such photocatalysts comprise both inorganic photoconductors and nonphotoconductors. Among the inorganic photoconductors which may beused are zinc oxide, indium oxide, zinc sulfide, cadmium sulfide and selenium. Of the photoconductors, the N-type is preferred, such as zinc oxide. Inorganic nonphotoconductors which act as photocatalysts are the polyvalent metal oxides including titanium dioxide and antimony trioxide. Of both the photoconductors and nonphotoconductors above described, the white materials are preferred to assure a white background.

It has also been found that certain fluorescent or phosphorescent metal compounds are also useful as In place of the ethylenediamine (C H N H,) and ammonia of the above compounds, such coordinating groups as guanidine, azido and nitrite may be used. Other readily reducible anions which may be used in place of those of the peroxydisulfate of the above complexes include tetrathionate, selenate and perchlorate.

A simple test may be used to determine whether or not materials have a photocatalytic effect. The material in question is mixed with an aqueous solution of silver nitrate and no reaction should take place in the absence of light. The mixture is then exposed to light at the same time that a control sample of an aqueous solution of silver nitrate alone is exposed to light, such as ultraviolet light. If the mixture darkens faster than the silver nitrate alone, the material is a photocatalyst. If, however, a reaction does take place in the absence of light, the materials should be coated out separately on a flat surface using a suitable binder in accordance with the procedure described herein. A control sheet is also made for comparison. The test is carried out on the sheets as described above.

The irradiation source is an important feature of the present invention. Ultraviolet light is one of the best radiation sources, and all of the photocatalytic materials are sensitive thereto. Incandescent light is a fair source of ultraviolet light. Fluorescent light is a better source of ultraviolet light. The photocatalysts are not usually sensitive to the entire actinic light range but may be made so by the use of a dye sensitizer, such as eosin, Seto FlavinT, Thioflavin, uranine, erythrosin, phosphine R, orthochrome P, Vasoflavin and dicyanine A. Radiation by x-rays or gamma rays is also effective in exciting the photocatalyst to transfer electrons.

The binding agent used to bind the image-forming composition and the photocatalyst to the carrier medium is also an important feature of the present invention. In general, these binders should be translucent or transparent so as not to interfere with the transmission of light therethrough. The preferred binders for the reactive components and catalyst are organic materials, such as solid polymers and resins. Suitable organic resins and copolymers include a copolymer of butadiene and styrene sold on the open market as Pliolite, polystyrene, chlorinated rubber, polyvinylchloride, nitrocellulose, rubber hydrochloride, polyvinylbutyral, polyethyleneglycol, carbowax, polyamide resin sold as Zytel-6l, hydroxyethyl cellulose, methyl cellulose and polyvinylpyrrolidone. The polartype binders which are water or alcohol-soluble are useful with one of the reactive components, such as binders including polyethyleneglycol, polyvinylbutyral, polyamide resin and carbowax and polyvinylpyrrolidone. These binders may be removed by dissolving the binder with water or alcohol and thus removing the oxidizing and/or reducing agent. This inactivates the carrier to further exposure to light as will be hereinafter discussed. The nonpolar binders or water or alcohol insoluble organic binders are useful with the other reactive component or both reactive components. These binders include polystyrene, chlorinated rubber, rubber hydrochloride, polyvinylchloride, nitrocellulose and Pliolite.

Any of these binders may be used for the catalyst phase, but the nonpolar binders are preferred. The binders may also be admixed and used in admixture for layer formation.

The carrier material or support upon which the photocatalyst and image-forming composition are deposited may be any suitable inert backing of sufficient strength and durability to satisfactorily serve as a reproduction. The carrier or support may be in the form of sheets, ribbon, roll or other suitable form for supporting the reproduction of the image. The carrier may comprise wood pulp paper, rag content paper, various synthetic plastics, such as cellulose acetate and polyethylene terephthalate (Mylar) in the form of film, cotton or wool cloth, metal foil and glass plate. The preferred form of the backing or carrier material is a thin sheet which is flexible and durable.

An example of a suitable white paper containing the irradiation-sensitive system of this invention comprises zinc oxide as a photocatalyst, silver nitrate as the oxidizing agent and image-forming material, and sodium formate or sodium oxalate as the reducing agent, all of which are bonded to the paper as a continuous uniform layer. A slurry is formed of these materials with an immiscible organic binder and diluent and coated on a wood pulp-type paper in a thickness of about 4 mils. Approximately equal proportions of the components of the system are used. A resin such as Pliolite (copolymer of butadiene and styrene) is useful as the binder since it securely adheres the components to the paper and to each other. Another method of construction is to apply an admixture of a zinc oxide and silver nitrate to the paper first in about a 2-mil thickness with a suitable binder, such as Pliolite, followed by a separate layer of the reducing agent sodium formate in an aqueous solution of a suitable water-soluble binder such as carbowax.

A negative film or transparency (developed) as a master is applied to the surface of the above paper containing the image-forming composition and catalyst. The film is then exposed to actinic light (500 lumens) for about 1 to 15 seconds. The paper and film are then removed from the presence of the actinic light, and film removed from the paper. The paper contains a reproduced black image (Ag) on the film, even though the paper has been dry throughout the procedure. When zinc oxide is omitted from the above process, no visible image is formed in 30 seconds of exposure. If in the above system the reducing agent is omitted, the rate of image formation is considerably slower unless the backing or the binder itself contains a reducing agent or is in itself a reducing agent.

In the above typical system using zinc oxide, ultraviolet light has been theorized to raise the electrons of the zinc oxide into its conduction band in accordance with the following equation:

ZnflO, light ZnQO' (8) Zn O is zinc oxide crystal; Zn is the excited electron; O is the hole (absence of electron). The silver ion of the oxidizing agent then apparently removes the electron from the zinc oxide conduction band in accordance with the following equation:

Zn Ag -Zn Ag (9) The hole created by the removal of the electron from the zinc oxide conduction band migrates to the surface and recombines with an electron from the organic reducing agent (electron donor) in accordance with the following equation:

01 HCOO O" CO (g) H (10) It may not be necessary that the electron actually be raised to the conduction band by the light, such as in zinc oxide or other photoconductive materials. Irradiation may sufficiently activate the electron of the photocatalyst such that it is in an excited state and loosely held to the photocatalyst. In such a condition, the electron is easily transferred to the oxidizing agent (electron acceptor) of the oxidation-reduction system to initiate an irreversible reaction by electron transfer. This is the case with nonphotoconductors, such as the fluorescent materials, the metal complexes and the nonphotoconductive metal oxides.

The probable theory for the action of the metal complexes as photocatalysts is that the metal ion can exist in more than one oxidation state, a nonionic ligand and an oxidizable anion. The irradiation of the complexes involves excitation of electrons in the anions to higher energy levels by the absorption of radiation wave lengths. The electrons thus excited become trapped in association with metal ions. The electrons, however, tend to return to their original state when irradiation ceases. If an oxidizing agent, an easily reducible compound, is present, the electrons are available by transfer to the oxidizing agent and initiation of the irreversible oxidation-reduction reaction occurs.

Mixtures of the various components of the system may be used as well as the single components. Thus, mixtures of two or more photocatalysts may be used. Also, mixtures of two or more oxidizing agents or two or more reducing agents may be used without departing from the scope of this invention. Even mixtures of binders may be used.

The oxidizing agent and reducing agent are usually used in substantially stoichiometric proportions. If desired, an excess of the oxidizing agent may be used without departing from the scope of this invention. The weight ratio of the image-forming composition, i.e. the combination of oxidizing agent and reducing agent, to photocatalyst is between about 10:1 and about 1:10; preferably 2:1 to 1:2. The binder is used in asufficient amount to effectively bind the various ingredients to the carrier surface. Generally, the weight ratio of binder to the material to be bound is between about 2:1 to about 1:5.

The thickness of the image-reproducing system on the carrier will vary between about 0.5 and about 8 mils. In case separate layers for the reactive components and the photocatalyst are used on the carrier base, the total thickness will be within the above range and the thickness of each layer will be about 0.5 mil to about 4 mils. The thickness of the inert carrier base or support, when in the form of a flexible sheet, is usually between about 5 and about 30 mils.

The exposure time will vary to a considerable extent and will depend primarily upon the type and intensity of light or irradiation source, the sensitivity of the oxidation-reduction reaction, and upon the sensitivity of the photocatalyst. In general, the time of exposure will vary between about 0.001 of a second and about minutes. Generally, the reproduction requires not more than about seconds exposure.

Normally, it is the oxidizing agent that reproduces the image. For example, a dark material may turn light upon reaction or a light material may turn dark. Also, a white material or colorless material may turn a color upon reaction or vice versa. Any change in the reflection of light from the surface as a result of the reaction between the reducing agent and the oxidizing agent constitutes a change in light value which causes a visual reproduction of the image, which reaction may be effected simultaneously with exposure or in a subsequent developing step.

As previously stated, in one embodiment of this invention the image after exposure is latent under ambient and normal conditions. In other words, there is no visible image formed upon exposure which is effected at a temperature not higher than about 50 C., usually at ambient conditions, even in the presence of the photocatalyst, although a reaction has taken place. In this embodiment, the exposed system is subsequently treated by heating above 50 C, or by wetting with water to cause further reaction. Upon further reaction, either the reducing agent or the oxidizing agent or some other reactive compound changes in light value so as to reproduce the image in those areas where the initial reaction has taken place; for example, where the initial reaction produces free metal in a small and invisible quantity which catalyzes the subsequent reaction to form the image.

The source of this free metal is the oxidizing agent as disclosed above. The oxidizing agent accepts an excited electron from the exposed photocatalyst according to reactions (8) and (9) above. Again the rate of image formation, here a latent image, is enhanced by the presence of an electron donor (reducing agent) at the time of exposure. As shown by reactions (8) and (9) reduction of a portion of the oxidizing agent to the metal proceeds by donation of electrons from the photocatalyst to the oxidizing agent at the exposure time. The function of the reducing agent if present at the time of exposure is explained by reaction (10) above, i.e., the reducing agent serves to provide electrons for recombining with the hole thus created. Thus, the reducing agent present at the time of light exposure serves as an electron source for the photocatalyst. The metal which is produced upon exposure of the photocatalyst forms the latent image, the metal then serving to catalyze the subsequent development reaction between the oxidizing agent and the reducing agent at the side of such metal to provide the developed, visible image. Thus, a carrier sheet bearing a photocatalyst and an oxidizing agent may be light exposed to reduce a portion of the oxidizing agent to a free metal defining a latent image, and this latent image may be developed by treating the exposed sheet with a reducing agent, e. g., hydroquinone. The oxidizing agent may be applied to the carrier sheet at any time prior to exposure. The oxidizing agent which was not reduced at the time of exposure reacts with the reducing agent added after exposure in the presence of the free metal to provide the visible image corresponding to the latent image. Preferably, prior to light exposure there is also present on the carrier sheet a reducing agent, e.g., an oxalate, in a separate solid phase which in its role as an electron source for the photocatalyst aids in formation of the latent image. A second reducing agent is then employed in the postexposure, development step which reacts with oxidizing agent to form additional free metal in the presence of latent image free metal catalyst.

The photocatalyst should be conditioned in the dark before exposure when the catalyst is sensitive to actinic light. Usually dark conditioning of the photocatalyst of l to 24 hours is desirable in such instances. After conditioning, the catalyst is not exposed to light prior to its exposure for reproducing the image.

Preferred image-forming compositions comprise (oxidizing agent and reducing agent): silver nitrate and sodium formate or oxalate, copper sulfate and sodium formate, or sodium oxalate, silver saccharin and hydroquinone, silver saccharin and Metol or Elon, tetrazolium blue and sodium formate or sodium oxalate, silver behenate and lonol, diphenyl carbazone and sodium oxalate or sodium formate, silver nitrate or copper sulfate and sodium formate and benzene diazonium fluoroborate as a stabilizer, gold chloride and sodium oxalate or sodium formate, and gold chloride and hydroquinone.

The image-forming combinations of this invention in dry condition are not nonnally considered light-sensitive in the absence of the photocatalyst. These reactive combinations, upon exposure to an ultraviolet light source such as a mercury arc lamp (2,0004,000 lumens) for a period of time from 5 to 10 minutes, do not show any sensitivity to the light and are therefore considered normally latent under ambient conditions. Some sensitivity may be observed, darkening of the composition, upon prolonged exposure of several hours to an intense light source with certain combinations, such as with silver nitrate and a reducing agent. In other combinations, such as with silver saccharin and a reducing agent, no sensitivity is observed even upon prolonged exposure to light when no photocatalyst is present. These redox combinations are in no way similar in sensitivity to the silver halide emulsion type of light-sensitive compositions or those which are aqueous or moist (in solution).

In addition to the above, the redox combinations may be divided into two classes, one class in which the redox combination results in a reproduction of the image simultaneously with exposure (print out system), and the other class in which the visible reproduction is made after exposure by a separate developing step, such as by heating.

As previously stated, inactivation of the radiation-sensitive system is required where the reproduced image will be observed under the same or similar light conditions used during exposure or when the system does not require a subsequent development step, such as heating or wetting. However, where the light conditions of observation are not the same as under exposure, such as exposure to X-rays or gamma rays or where special development is required, no stabilization or inactivation of the image-reproducing system may be necessary.

One method of inactivation is washing off one of the components of the image-reproducing system after exposure. Washing may be effected with water or any suitable solvent, such as an alcohol or a ketone. For example, a permanent copy of a photographic negative may be obtained by coating out the photocatalyst, such as zinc oxide, in a water-insoluble binder, such as Pliolite, and coating either the oxidizing agent or reducing agent on this surface with a water-soluble binder, such as polyethyleneglycol. After development of the image either at the time of exposure or in a subsequent step, the water-soluble film containing one of the reactive components can be washed off by holding under running tap water for several seconds. The reproduced image remains on the zinc oxide-Pliolite surface and is a permanent copy. By this method a permanent photographic print can be obtained in approximately 20 seconds, including all of the operations for making the print.

Another method for inactivation of the image reproduction system is by the use of heat in combination with the material capable of releasing an acid, i.e. either a Bronsted or Lewis classified acid, such as HCl, BF HF, PC1 and p-toluene sulfonic acid. In accordance with this procedure, metal ions that are above oxygen in the electromotive series, such as copper, are used to deposit an image from a basic media directly upon exposure to the image source. The metal salt oxidizing agent, the reducing agent and a basic additive are coated with suitable binders on top of a zinc oxide coated carrier. If separate layers are used for the reactive components, the layer containing the metal salt oxidizing agent will also preferably contain the basic additive. Since metals above oxygen in the electromotive series do not deposit in neutral media, the layer or layers forming the image-forming combination are neutralized after light development of the image therein which stabilizes the metal ion of the metal salt oxidizing agent. This is accomplished by releasing an acid by a heat-sensitive reaction after exposure.

For example, the radiation-sensitive system is heated to a temperature of about 100 to 250 C. A suitable composition that will release hydrogen chloride and thus neutralize the basic material upon heating is an admixture of mnitrobenzenesulfonyl chloride (acid-releasing compound) and phloroglucinol. This admixture is added to the layer containing the metal salt oxidizing agent of the photosensitive sheet. This method gives a dry reproduction, light-sensitive image system that does not require a development step and is heatinactivated to give a permanent stable copy. Other acidreleasing compounds include p-toluene sulfonic acid urea addition complex, p-acetamidobenzene diazonium fluoroborate, and m-chlorobenzene diazonium fluorophosphate.

Another method for releasing acids as a means of inactivation includes moistening of the system with water which results in the release of an acid in the system as above. This type of operation does not require heating. In this method of inactivation or fixing, diazonium fiuoroborate is used alone and is combined on the top layer with a methanol or watersoluble polyamide binder. Included in this top layer, of course, is the metal salt oxidizing agent composition. The lower layer of this system is a photocatalyst dispersed in a nonwater-soluble binder, such as Pliolite. The reducing agent may be included in the lower layer or be in another separate intermediate layer using a nonwater-soluble binder, but cannot be included in the top layer if the binder is water-soluble. Upon wetting the top layer containing the water-soluble binder with water, the fluoroborate decomposes, releasing BF of HF, thus neutralizing the basic media used in the image-forming composition and inactivating the composition to further sensitivity to light. No heating is required. A variation of the above two types of operations is the inclusion in the layer containing the metal salt oxidizing agent and the acid former a compound that liberates water at low temperatures, which water will react with the acid former to liberate the acid. A diazonium fluoroborate as the acid former will release BF upon heating to about C. in the presence of a hydrate, resulting in an inactivation system stable to further light sensitivity.

Another method of inactivation of the image reproduction system constitutes the chelation of the oxidizing agent or reducible metal ion by forming a very stable metal chelate with any of the unreacted metal ions of the oxidizing agent. The chelating compound is combined in the binder or layer containing the oxidizing agent. The chelating compound may also be used as a separate adjacent layer either above or below the layer containing the oxidizing agent. The chelating compound may also be admixed in a system where all of the components are mixed together with a single layer formation. In this method, the image reproduction system is exposed to develop the image and then heated at a temperature of about 120 to 250 C. to form the metal chelate with the unreacted metal ion of the oxidizing agent. The metal chelate formed must be nonlight-sensitive. A suitable chelating agent which may be used when copper is the metal ion of the oxidizing agent is salicylaldoxine. The copper-salicylaldoxine chelate formed upon stabilization of the system is light colored and very stable. Another chelating agent is benztriazole which may be used when silver is the metal ion of the oxidizing agent. Heating such a system to a temperature of about to 200 C. results in a black image on a stable white background which is no longer sensitive to light.

A simple test for determining whether the metal chelate is nonlight-sensitive is to expose the metal chelate to ultraviolet light. If the material does not darken after 5 minutes exposure, the chelating agent is suitable as a means for inactivation of the system.

Inactivation of the image reproduction system may also be accomplished by the application of pressure to the surface of the carrier. It has been found that pressure will desensitize the photocatalyst, such as zinc oxide, as a result of which it is no longer light-sensitized. Thus, the sheet containing the image as a result of exposure may be passed through rolls which exert pressure upon the sheet. Another method is to pass a bar under pressure across the surface of the sheet containing the image. Generally, at least 500 pounds per square inch pressure must be applied to the surface to deactivate the photocatalyst. It has been found that with zinc oxide, for example, passing a pencil or rod across the image surface with exertion of heavy hand pressure will deactivate the zinc oxide to further sensitization by actinic light. This type of inactivation is that which inactivates the photocatalyst rather than the imageforming composition.

Still another method of inactivation is to separately bind the photocatalyst and at least one of the reactive components on separate independent sheets. The sheets are then firmly pressed together and exposed to light. Thereafter the sheets are separated and the image is formed on either the photocatalytic carrier or the image-forming carrier, depending upon the type of image-forming composition used. One method is to coat one sheet with a pressure-sensitive adhesive containing the image-forming combination, and the other sheet is coated with the catalyst and a conventional binder. The sheets are pressed together and form a sufficient bond such that electrons may transfer from one sheet to the other. After exposure, the sheets are separated by pulling them apart. The two-sheet method has been found to be quite distinctive in that a transparency or negative can be formed immediately upon exposure. For example, the image-forming combination which is usually transparent may be coated upon a transparent backing or carrier, such as Mylar film. The second sheet is coated with a pressure-sensitive adhesive which contains a photosensitive catalyst in admixture therewith or which contains the photocatalyst dusted on the surface. The sheets are pressed together and the combined sheets are then exposed to an image source, such as through a negative. After exposure, the sheets are separated and a transparency is produced upon the Mylar film containing the image-forming composition. As a modification of the above, the image-forming compound, such as the oxidizing agent, is coated on the first sheet with a transparent binder. The second sheet is coated with an adhesive containing both the photocatalyst and the other component of the image-forming composition, such as the reducing agent. Various other combinations as will become apparent from the above are within the scope of this invention. The above methods of forming transparencies are simple and inexpensive and are particularly adapted to use by the amateur photographer.

The system of the present invention may be particularly adaptable to amateur photography. In accordance with the present invention, a composition of this invention is placed upon a paper backing in roll form and directly placed in the camera. The image is formed immediately upon exposure and the only remaining step in order to obtain a print is the inactivation of the composition. This may be done by the amateur photographer by removing the exposed print in the dark and washing with water as above-described. When using zinc oxide as the photocatalyst, inactivation may also be achieved by using hand pressure with a pencil over the surface of the print. The camera itself can be constructed to have the film pass through small pressure rollers to desensitize the print. Other modifications or alterations are obvious for adaptation to conventional cameras.

The following Examples are offered as a better understanding of the present invention and are not to be construed as unnecessarily limiting thereto. In the Examples, the zinc oxide used was New Jersey Zinc Companys zinc oxide of the U.S.P.- 12 or Red Seal No. 9 type and was prepared by the French process of burning zinc metal in air, and the titanium dioxide was Mercks analytical reagent grade.

EXAMPLE I A dispersion of (42 parts by weight) photoconductive zinc oxide and (three parts by weight) sodium oxalate (reducing agent) in a solution of 11 parts by weight) Pliolite, (23 parts by weight) acetone, (24 parts by weight) toluene was ballmilled for 12 hours. This dispersion was coated 4 mils thick on a transparent Mylar film as a flexible support and dried at room temperature with a subsequent dark-adapting period of 12 hours. A top layer, containing five parts by weight silver nitrate (oxidizing agent), 20 parts by weight of water-soluble binder material carbowax (20-M), and 75 parts water was coated in the dark on the white zinc oxide layer in a thickness of about 3 mils and allowed to air-dry in the dark. The dried sheet gave an image in to seconds exposure to a mercury arc lamp (3,000 lumens). This sheet was fixed by washing away'the unreacted silver nitrate along with the water-soluble binder. The image of reduced silver clings to the surface of zinc oxide in Pliolite and remains intact with a white background on the nonimage areas.

Substitution of a wood pulp paper as a support for the Mylar plastic film gave similar results to the above. Also, any commercial sun lamp will give a' satisfactory source of ultraviolet light for exposure.

Other water-soluble binders, such as polyvinylpyrrolidone, hydroxyethyl cellulose and methyl cellulose, can be used in place of carbowax for the top layer containing the oxidizing agent. Other normally water insoluble or nonwater-soluble organic binders, such as polystyrene and polyvinylchloride, work equally well as binders for the photocatalytic layer containing the reducing agent in place of Pliolite.

EXAMPLE [I A dispersion of (42 parts by weight) zinc oxide in (l l parts by weight) Pliolite, (three parts by weight sodium oxalate, (23 parts by weight) acetone and (24 parts by weight) toluene was ball-milled for 12 hours. The zinc oxide and the sodium oxalate are immiscible with the Pliolite and the solvent. This dispersion was coated 4 mils thick on Mylar film as a flexible support and dried at room temperature with a subsequent dark-adapting period of 12 hours. An aqueous solution containing dissolved therein 4 parts by weight copper sulfate, 20 parts by weight of water-soluble binder carbowax (20-M), one part by weight of hexamethylenetetramine (basic media), and 75 parts water, was coated in the dark on the zinc oxide layer to a thickness of about 2 mils and allowed to air-dry in the dark. The dried sheet gave a permanent image in 30 seconds exposure to a mercury arc lamp (3,000 lumens). This sheet was fixed by washing away the top layer along with water-soluble binder and basic media. The image sheet was stabilized by washing away the basic media which was necessary for the oxidation-reduction reaction between the sodium oxalate and the copper sulfate.

EXAMPLE III A dispersion of 42 parts by weight of zinc oxide in l 1 parts by weight of Pliolite, 23 parts by weight of acetone, 24 parts by weight of toluene and three parts by weight of sodium oxalate is ball-milled for 12 hours. This dispersion is coated 4 mils thick on a Mylar support and dried at room temperature with a subsequent dark adapting period of 12 hours. An imageforming layer containing 1 part by weight gold chloride, 24 parts by weight polyvinylpyrrolidone, and 75 parts by weight methanol is coated to 3 mils and allowed to air dry in the dark. This sheet is exposed to a mercury arc lamp for 20 seconds with the formation of a latent image. The exposed sheet is dipped in a solution of hydroquinone with an immediate visual development of the image in the light-struck areas. This is an example of latent image formation by light with subsequent visual development of the latent image by an external reducing agent. The sheet is then washed to remove the unreacted image-forming material and water-soluble binder to give a stable sheet.

EXAMPLE IV A dispersion of 42 parts by weight of zinc oxide in l 1 parts by weight of Pliolite, 23 parts by weight of acetone, and 24 parts by weight of toluene is ballmilled for 12 hours. This dispersion is coated 4 mils thick on a Mylar support and dried at room temperature with a subsequent dark adapting period of 12 hours. An image-forming layer containing 1 part by weight gold chloride, 24 parts by weight polyvinylpyrrolidone, and 75 parts by weight methanol is coated to 3 mils and allowed to air dry in the dark. This sheet is exposed to a mercury arc lamp for 20 seconds with the formation of a latent image. The exposed sheet is dipped in a solution of hydroquinone with an immediate visual development of the image in the light-struck areas. This is an example of latent image formation by light with subsequent visual development of the latent image by an external reducing agent. The sheet is then washed to remove the unreacted image-forming material and watersoluble binder to give a stable sheet.

EXAMPLE V A dispersion of 42 parts by weight zinc oxide in l 1 pans by weight Pliolite, 23 parts by weight acetone, 24 parts by weight toluene and three parts by weight sodium oxalate is ball-milled for 12 hours. This dispersion is coated 4 mils thick on a Mylar support and dried at room temperature with a subsequent dark adapting period of 12 hours. The coated sheet is dipped in an alcoholic 3 percent solution of silver nitrate. The excess is wiped off and the sheet dried. The sheet is exposed to light through a transparency in a projection device. No visible image is discernible. Subsequent to exposure, the sheet is dipped in an aqueous 1,4-methyl paramino-phenol sulfate (Elon) bath and the latent image is developed in a second to a reflected optical density of greater than 1.0.

sheet is exposed to light through a transparency in a projection device. No visible image is discernible. Subsequent to exposure, the sheet is dipped in an aqueous 1,4-methyl paraminophenol sulfate (Elon) bath and the latent image is developed in a second to a reflected optical density of greater than 1.0.

EXAMPLE VII A dispersion of 44 parts by weight of photoconductive French process zinc oxide powder, 36 parts by weight of 30 percent by weight of Pliolite in toluene, three parts by weight of sodium formate, 30 parts by weight of acetone, and 4 X 10 grams of Phosphine R (C.I.46055) per gram of zinc oxide as a 2 percent by weight alcoholic solution is ball-milled for 12 hours. The dispersion is coated 4 mils thick (wet) on a paper base and dried at room temperature with a subsequent dark adapting period of 12 hours. The exposed sheet is swabbed with a 2 percent alcoholic solution of silver nitrate and allowed to dry. The sheet is exposed via a projection transparency to a tungsten source. The light striking the surface is 100 foot candle seconds. Subsequent to exposure, a 1 percent solution of 2,2'-dinapthyl-para-phenylene-diamine is applied to the exposed sheet. The sheet is washed with a thiourea stabilizer after a dense image is formed.

EXAMPLE VIII A dispersion of 44 parts by weight of photoconductive French process zinc oxide powder, 36 parts by weight of 30 percent by weight of Pliolite in toluene, 30 parts by weight of acetone, and 4 X 10 grams of Phosphine R (C.I.46055) per gram of zinc oxide as a 2 percent by weight alcoholic solution is ball-milled for 12 hours. The dispersion is coated 4 mils thick (wet) on a paper base and dried at room temperature with a subsequent dark adapting period of 12 hours. The exposed sheet is swabbed with a 2 percent alcoholic solution of silver nitrate and allowed to dry. The sheet is exposed via a projection transparency to a tungsten source. The light striking the surface is I foot candle seconds. Subsequent to exposure, a .1 percent solution of 2,2-dinapthyl-para-phenylenediamine is applied to the exposed sheet. The sheet is washed with a thiourea stabilizer after a dense image is formed.

EXAMPLE IX A dispersion of 38 parts by weight of titanium dioxide powder, (anatase form), 16 parts by weight of 30 percent by weight Pliolite in toluene, three parts by weight sodium oxalate, three parts by weight of polystyrene, three parts by weight silver nitrate, 40 parts by weight toluene and 4 X grams of Phosphine R (C.I.46055) per gram of oxide as a 2 percent weight alcoholic solution is ball-milled for 12 hours. The dispersion is coated 3.0 grams/ft. (dry) on a 45 pound lowed to dark adapt for a period of 12 hours. The sheet is exposed to a tungsten source with 100 foot candle seconds balling on the surface. The latent image is developed with an aqueous solution of an aminophenol reducing agent. The developed sheet is further washed to stabilize.

EXAMPLE X A dispersion of 38 parts by weight of titanium dioxide powder, (anatase form), 16 parts by weight of 30 percent by weight Pliolite in toluene, three parts by weight of polystyrene, 3 parts by weight silver nitrate, 40 parts by weight toluene and 4 X 10" grams of Phosphine R (C.I.46055) per gram of oxide as a 2 percent weight alcoholic solution is ball-milled for 12 hours. The dispersion is coated 3.0 grams/ft. (dry) on a pound Crocker-Hamilton paper which had been subbed with a 0.2 gram/ft. layer of cellulose acetate. The layer is dried and allowed to dark adapt for a period of 12 hours. The sheet is exposed to a tungsten source with 100 foot candle seconds balling on the surface. The latent image is developed with an aqueous solution of an aminophenol reducing agent. The developed sheet is further washed to stabilize.

EXAMPLE XI A dispersion of (42 parts by weight) zinc oxide and (three parts by weight) sodium oxalate in (11 parts by weight) Pliolite, (23 parts by weight) acetone and (24 parts by weight) toluene was ball-milled for 12 hours. This dispersion was coated 4 mils thick on a Mylar support and dried at room temperature with a subsequent dark-adapting period of 12 hours. This dispersion was coated 4 mils thick on a Mylar support and dried at room temperature with a subsequent dark-adapting period of 12 hours. An image-forming layer containing I part by weight gold chloride, 24 parts by weight polyvinylpyrrolidone and 75 parts by weight methanol was coated in the dark to 3 mils thick and allowed to air-dry in the dark. This sheet was exposed to a mercury arc lamp (3,000 lumens) for 20 seconds with the formation of a latent image (reduction of small amount of gold chloride to gold). The exposed sheet was dipped in a solution of hydroquinone with an immediate visual development of the image in the light-struck areas. This is an example of latent image formation by light with subsequent visual development of the latent image by an external reducing agent. The sheet was then washed to remove the unreacted image-forming material and water-soluble binder to give a stable sheet. A Mylar film support with the above system gave similar results.

EXAMPLE XII The effect of the reducing agent in causing an increase in image density for a given exposure is exemplified by the following Table I. In the runs of the Table, a l-watt projection lamp (500 lumens) was used as the light source. In Runs 1 through 5 of Table I, the paper compositions were prepared in accordance with the procedure of Example I with modifications as to the photocatalyst and omission of reducing agent as indicated. The tabulation of values in the columns below the time of exposure in seconds of Table I is the change in optical density from unexposed to exposed, and the higher values are the most desirable.

TABLE I 1 Change in Optical Density=(O.D.Ex weO.D.

Exposure seconds Run Compositions 2.5 5 10 15 20 30 l Zinc oxide, silver nitrate .01 .02 .035 .055 .08 .13 .23 275 2 Zinc oxide, silver nitrate, sodium oxalate 0.55 .08 115 .24 355 50 3 Titanium dioxide, silver nitrate 025 .04 05 065 .08 11 18 .23 Titanium dioxide, silver nitrate, sodium oxalate .035 .05 .07 095 12 15 25 .37 5 (No photocatalyst), silver nitrate. sodium oxalate 0.00

1 Change in optical density for indicated exposures to tungsten lamp.

Crocker-Hamilton paper which had been subbed with a 0.2 gram/ft. layer of cellulose acetate. The layer is dried and al- I-Iaving described our invention, we claim: 1. A method for making a reproduction of a radiation image which comprises (1) providing a sheet comprising an inert carrier bearing (a) a solid photocatalyst phase comprising a metal containing substance activatable into transfer of electrons to an electron acceptor by a wave length below 1 micron, and (b) a reducible phase comprising an oxidizing agent which accepts electrons from said photocatalyst to form a free metal; (2) exposing said sheet to a radiation image to cause said photocatalyst to donate electrons to said oxidizing agent to reduce a portion thereof to a free metal defining a latent image; and (3) post-exposure treating said sheet with a reducing agent reactable with the unreacted portion of said oxidizing agent in the presence of said free metal whereby said latent image is developed to a visible image.

2. The method of claim 1 wherein said photocatalyst is at least one member selected from the class consisting of zinc oxide and titanium dioxide.

3. The method of claim 1 wherein said oxidizing agent is at least one member selected from the class consisting of a gold salt and a silver salt.

4. The method of claim 1 wherein said reducing agent is at least one member selected from the class consisting of a dihydric phenol and an aminophenol.

5. A method for making reproduction of a radiation image which comprises (1) providing a sheet comprising an inert carrier bearing (a) a solid photocatalyst phase comprising a metal containing substance activatable into transfer of elec- *trons to an electron acceptor by a wave length below 1 micron, (b) a reducible phase comprising oxidizing agent which accepts electrons from said photocatalyst to form a free metal, and (c) a solid oxidizable phase in non-reactive relationship with said reducible phase, said oxidizable phase comprising a reducing agent capable of donating electrons to said photocatalyst; (2) exposing said sheet to a radiation image to cause said photocatalyst to donate electrons to said oxidizing agent to reduce a portion thereof to a free metal defining a latent image; and (3) post-exposure treating said sheet with a second reducing agent reactable with the unreacted portion of said oxidizing agent in the presence of said free metal whereby said latent image is developed to a visible image.

6. The method of claim 5 wherein said photocatalyst is at least one member selected from the class consisting of zinc oxide and titanium dioxide.

7. The method of claim 5 wherein said oxidizing agent is at least one member selected from the class consisting of a gold salt and a silver salt.

8. The method of claim 5 wherein said reducing agent is at least one member selected from the class consisting of a dihydric phenol and an aminophenol.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N a 6 5 383 I Dated April ll. 1972 Inventofls) Joseph W. Shepard and Benjamin L. Shely It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as show-n below:

On the title page, under the right hand column, the phrase "3 Claims, No Drawings" should read 8 Claims,

No Drawings I Col. 7, line 5, "absorption" should read adsorption Col. 8, line 13, "side" should read site Col. 14, in Table I under heading 2.5, Run No. 2 "0.55

should read .055

Signed and sealed this 5th day of September 1972.

(SEAL) Attest LDUARD M .FLETGHl-JH, JR

ROBERT GOTTSCHALK At 50 s ting; Offi ce r Commissioner of Patents 'ORM PO-1050 (10-69) USCOMM'DC 6O376-P69 U.Si GOVERNMENT PRINTING OFFICE: 1959 0-366-31

Claims (7)

1. 2. The method of claim 1 wherein said photocatalyst is at least one member selected from the class consisting of zinc oxide and titanium dioxide.
2. 3. The method of claim 1 wherein said oxidizing agent is at least one member selected from the class consisting of a gold salt and a silver salt.
3. 4. The method of claim 1 wherein said reducing agent is at least one member selected from the class consisting of a dihydric phenol and an aminophenol.
4. 5. A method for making reproduction of a radiation image which comprises (1) providing a sheet comprising an inert carrier bearing (a) a solid photocatalyst phase comprising a metal containing substance activatable into transfer of electrons to an electron acceptor by a wave length below 1 micron, (b) a reducible phase comprising oxidizing agent which accepts electrons from said photocatalyst to form a free metal, and (c) a solid oxidizable phase in non-reactive relationship with said reducible phase, said oxidizable phase comprising a reducing agent capable of donating electrons to said photocatalyst; (2) exposing said sheet to a radiation image to cause said photocatalyst to donate electrons to said oxidizing agent to reduce a portion thereof to a free metal defining a latent image; and (3) post-exposure treating said sheet with a second reducing agent reactable with the unreacted portion of said oxidizing agent in the presence of said free metal whereby said latent image is developed to a visible image.
5. 6. The method of claim 5 wherein said photocatalyst is at least one member selected from the class consisting of zinc oxide and titanium dioxide.
6. 7. The method of claim 5 wherein said oxidizing agent is at least one member selected from the class consisting of a gold salt and a silver salt.
7. 8. The method of claim 5 wherein said reducing agent is at least one member selected from the class consisting of a dihydric phenol and an aminophenol.