BACKGROUND

Better carbon capture technologies are needed including ones that convert the carbon dioxide into useful products. While it is hard to assess the challenges of implementing carbon capture technologies, Van Noorden estimates that it could possibly double the cost of energy production [Van Noorden, R., Europe's untamed carbon. Nature, 2013. 493(7431 ): p. 141 -2; see, also, Rogelj, J., et al., Probabilistic cost estimates for climate change mitigation. Nature, 2013. 493(7430): p. 79-83]. Because worldwide demand for energy is projected to grow by 33% in the next two decades, the environmental imperative of reducing atmospheric carbon levels is seemingly totally at odds with the demands of economic growth [IEA, World Energy Outlook 2012. International Energy Agency, 2012, www.worldenergyoutlook.org.].

One way to bridge the conflicting realities of the increasing demands for inexpensive energy with the need to reduce CO2 emissions is to convert the gas into commercially valuable products that could offset some of the sequestration costs. Costentin and co-workers recently took a first step towards addressing this great dual environmental-energy challenge by demonstrating that the electrochemical conversion of stable inert CO2 into the relatively more reactive CO can be enhanced by the introduction of a local proton source [Costentin, C, et al., A local proton source enhances C0 ₂ electroreduction to CO by a molecular Fe catalyst. Science, 2012. 338(6103): p. 90-4.]. This is an important transformation, because it bypasses the high energy intermediate step of converting CO ₂ into a radical anion, which these investigators describe as, "quite unreasonable in terms of energy and activation." Their characterization is based on the fact that creating the radical anion requires about -2V of potential versus a standard electrode. Hence, a prejudice in the art exists against using anionic free radical CO ₂ .

Converting CO ₂ into milligram quantities of carbon requires the formation of nearly an Avagadro's number of carbon-carbon bonds and the elimination of a similar amount of oxygen atoms. Since CO ₂ will be the only source of carbon, this operation requires exposing the gas to an enormous amount of reducing potential, which will be in the form of larger and larger amounts of electric power. Because the aim is to lower C0 ₂ emissions in an economically desirable manner, it makes little sense to use C0 ₂ emitting fossil fuels to generate this electricity.

Terrestrial ecosystems apparently lack the ability to adapt to rapidly rising amounts of C0 ₂ [van Groenigen, K.J., C.W. Osenberg, and B.A. Hungate, Increased soil emissions of potent greenhouse gases under increased atmospheric C02.

Nature, 201 1 . 475(7355): p. 214-6; Caldeira, K. and S.J. Davis, Accounting for carbon dioxide emissions: a matter of time. Proc Natl Acad Sci U S A, 201 1 . 108(21 ): p. 8533-4; and Peters, G.P., et al., Growth in emission transfers via international trade from 1990 to 2008. Proc Natl Acad Sci U S A, 201 1 . 108(21 ): p. 8903-8].

Creative solutions to this problem require designing systems that sequester C0 ₂ in a way that potentially produces a desirable product, while not using energy sources that actually emit the gas as this would be self-defeating.

US Patent Publication No. 201 1 /0104029 discloses a photocatalytic

composite material for splitting carbon dioxide and forming small amounts of material which is said to be carbon black but without experimental evidence or controls for the material said to be produced.

A strong need exists for better processes for converting carbon dioxide into useful products.

SUMMARY

One approach to address these and other problems is by constraining the solution to a process that is solely driven by light that could potentially come from the sun and harnessing the electricity needed to drive the process from the light itself. Herein, it is described how elemental carbon in high purity can be created in different allotropic forms using CO ₂ as the only source of carbon and light (e.g., UV) as the only source of energy.

Embodiments described herein include methods of using systems and apparatuses, as well as the systems and apparatuses.

A first aspect provides for a method comprising: irradiating ultraviolet light on at least one solid composite electrode comprising at least one semiconductor material which is in contact with at least one liquid solution comprising carbon dioxide; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide; switching the state of the solid composite electrode between an open circuit mode and a closed circuit mode during the irradiating step; wherein the carbon dioxide is reduced to form an elemental carbon-containing reaction product .

A second aspect is a preferred embodiment which is a method comprising: in a photoelectrochemical apparatus, irradiating with UV radiation at least one semiconductor working electrode in contact with an aqueous solution comprising carbon dioxide; wherein the semiconductor working electrode comprises at least one semiconductor, at least one electronic conductor, and at least one binder; wherein at least one proton source is also present in the electrode and/or in the liquid solution comprising carbon dioxide; switching the state of the semiconductor working electrode between an open circuit mode and a closed circuit mode during the irradiating step to form an elemental carbon-containing reaction product which is at least 90 wt.% carbon.

A third aspect is a method for reducing carbon dioxide with UV light comprising fabricating a material into a solid conductive state that will transfer an electron to the carbon dioxide upon illumination with this radiation resulting in a buildup of electric potential that may be converted to electric current if this fabricated material is put into electrical contact with a counter electrode.

In one embodiment, the switching step comprises alternating steps of open circuit and closed circuit characterized by a time period of 1 minute to 480 minutes in open circuit mode. In one embodiment, the alternating step comprises alternating steps of open circuit and closed circuit characterized by a time period of 5 minutes to 240 minutes in open circuit mode.

In one embodiment, the switching step comprises alternating steps of open circuit and closed circuit characterized by a time period of 1 minute to 480 minutes in closed circuit mode. In one embodiment, the alternating step comprises alternating steps of open circuit and closed circuit characterized by a time period of 5 minutes to 240 minutes in closed circuit mode.

In one embodiment, power built up in the open circuit mode is at least 5 mW when switched to close circuit mode. In one embodiment, power built up in the open circuit mode is at least 15 mW when switched to close circuit mode.

In one embodiment, the method is carried out for at least 72 hours.

In one embodiment, the solid composite electrode is a composite comprising at least one semiconductor, at least electronic conductor, and at least one binder. In one embodiment, the solid composite electrode is a composite comprising at least one colloidal ZnS or CdS semiconductor, at least electronic conductor, and at least one binder.

In one embodiment, the solid composite electrode is a composite comprising at least one colloidal ZnS semiconductor, at least electronic metallic conductor, and at least one fluoropolymer binder.

In one embodiment, the UV light is part of electromagnetic radiation which is primarily UV radiation.

In one embodiment, the solution is an aqueous solution.

In one embodiment, the elemental carbon-containing reaction product is a carbon material which is at least 90 wt.% carbon. In one embodiment, the elemental carbon-containing reaction product is diamond-like carbon, graphene, amorphous carbon, or a mixture thereof. In one embodiment, the elemental carbon-containing reaction product is primarily diamond-like carbon. In one embodiment, the elemental carbon-containing reaction product is primarily graphene. In one embodiment, the elemental carbon-containing reaction product is primarily amorphous carbon.

In one embodiment, the proton source is a reducing agent.

In one embodiment, the proton source is present in the electrode. In one embodiment, the proton source is present in the electrode and is an organic compound. In one embodiment, the proton source is present in the electrode and is an aromatic organic compound having hydroxyl groups. In one embodiment, the proton source is present in the electrode and is hydroquinone or Raney nickel. In one embodiment, the proton source is present in the electrode and is hydroquinone. In one embodiment, the proton source is present in the liquid solution comprising carbon dioxide. In one embodiment, the proton source is present in the liquid solution comprising carbon dioxide and is replenished by electrolysis of water. In one embodiment, the proton source is present in the liquid solution comprising carbon dioxide and the proton source also is present in the electrode.

In one embodiment, the switching step is carried out with a time period adapted to produce carbon as the primary carbon-containing reaction product and produce one form of carbon as the primary type of carbon over another form of carbon. The reaction time can be also adapted to facilitate one form of carbon over another. In another embodiment, the fabricated material contains, but is not limited to, a semiconductor. In another embodiment, the semiconductor material contains, but is not limited to, ZnS, CdS or mixtures of ZnS and CdS.

In another embodiment, the fabricated material may contain, but is not limited to, Teflon (polytetrafluoroethylene) and/or other materials used provide structure.

In another embodiment, the fabricated material contains a conducting material including electronically conducting material. In another embodiment, the conducting material contains, but is not limited to, silver, carbon or some form of additive that makes the fabricated material conductive. In another embodiment, the fabricated material is fused to a conducting surface by methods that include but are not limited to, mechanical pressing, heat fusion, or other means.

In another embodiment, the counter electrode is but is not limited to a metallic substance such as titanium, platinum or other conductor, or a nonmetallic substance that can serve as a conductor, which may be but is not limited to a carbon-based material.

In another embodiment, the fabricated material and the counter electrode are put into electrical contact in a manner that enables irradiating the system while the electrical contact is intact (closed circuit mode) or not intact (open circuit mode).

In another embodiment, switching is carried out between open and closed circuit mode, which is done by computer, manual, or other control mechanisms.

In another embodiment, the method is carried out in alternating open and closed circuit mode for different periods of time. In another embodiment, the method is carried out for different periods of time between open and closed circuit mode which enables creating different reduction products of carbon dioxide.

In another embodiment, an electrochemical system is provided which is adapted to carry out the methods described and/or claimed herein.

Representative and preferred embodiments described and exemplified herein show that, for example, (1 ) a ZnS-Tf-Ag solid composite irradiated with UV photons can transfer electrons to C0 ₂ possibly from forming high energy free radical anions, (2) increasing the time of UV exposure can increase the number of free radicals and increase the number of holes in the composite, (3) this hole build up can be converted to electric power by conductively connecting the ZnS-Tf-Ag cathode to a platinum anode that can result in electrolysis of water and a flow of electrons back to the cathode refilling the holes caused by the UV light, (4) running the system for longer times in open circuit mode can result in increasing amounts of power when it is subsequently switched to closed circuit mode, (5) the generated power can be sufficient to reduce C0 ₂ to molecular carbon, and (6) different levels of power can result in the creation of different carbon allotropes.

At least one advantage for at least one embodiment is production of useful carbon products such as elemental carbon in different allotropes from carbon dioxide in a flexible, tunable process. Additional advantages for at least some embodiments include, for example, that the only input energy is UV light, which could potentially be harvested from the sun - no input electricity is used and thus there is no C0 ₂ producing energy inputs, which is often the case but contrary to the stated goals of reducing C0 ₂ .

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 . One embodiment showing a schematic for a photoelectrohemical apparatus using a Ag-filled ZnS cathode as working electrode and a Hg-arc lamp.

Figure 2A. An exemplary embodiment showing potential, current, and power when the system is run in a five minute cycle.

Figure 2B. An exemplary embodiment showing potential, current, and power when the system is run in a thirty minute cycle.

Figure 2C. An exemplary embodiment showing potential, current, and power when the system is run in a four hour (240 minute) cycle.

Figure 2D. In closed circuit mode, the current arises from the splitting of water and a flow of electrons back to the cathode, evolution of molecular oxygen, and a drop in pH from the release of protons.

Figure 3. After running the system in oscillating open-closed circuit mode for 3 days for either 5, 30, or 240 minutes periods, carbon was found to be deposited in milligram quantities on the cathode. Figures 4A and 4B. Two representative electron microscopy images and diffraction patterns for the system in high power mode are shown at 50,000x and 490,000x magnification. Particular regions are focused on for the 490,000x images (five regions for Figure 4A and three regions for Figure 4B).

Figures 5A and 5B. Representative electron microscopy images and one diffraction pattern (5A) for the carbon sample generated with the system run with a 30 minute period between open and closed circuit modes.

Figure 6. Representative electron microscopy images and diffraction pattern for the system run in low power mode - the 5 minute oscillation period.

Figure 7. Graph which shows pH change which increases when cell is open circuited and decreases when cell is short circuited (see also Figure 2D).

DETAILED DESCRIPTION

INTRODUCTION

References cited herein are incorporated by reference in their entirety.

The term "comprising" or "comprises" as used herein can be replaced in other embodiments with the terms "consisting essentially of" and "consisting of" as known in the art. Basic and novel characteristics of the inventions described herein are described and support use of these phrases.

The various elements as summarized above are described in further detail herein.

PHOTOELECTROCHEMICAL CELLS AND DEVICES

Photoelectrochemical cells or devices can be used to carry out the methods. These are known in the art and include three electrode devices including working electrode, counter electrode, and reference electrode, as well as the light source and medium for holding the solids, liquids, and gases used in the cell. Observation windows can be built into the cell. Small, medium, and large scale devices and cells can be used. An example is shown in Figure 1 and comprises, for example, a lamp, a reference electrode, a container for the working electrode, a container for the counter electrode, a connecting path between the two containers, a proton exchange membrane in the connecting path, and a potentiostat. The cells and devices can be adapted for batch reaction, continuous reaction, or semi-continuous reaction.

IRRADIATING STEP

Methods and equipment for irradiation and providing electromagnetic radiation, covering the electromagnetic spectrum, are known in the art. In a preferred embodiment, irradiating primarily with UV light is carried out, using a UV lamp. The UV or UV-Vis portion of the electromagnetic spectrum is preferred. For example, wavelengths of 200 nm to 1 ,200 nm can be used, extending below and above the wavelength of the visible range (which is approximately 400-800 nm). Factors such as the power, geometry, and wavelength of the light source such as the lamp can be adapted for a particular application in view of the larger system. For purposes herein, the term "light" is used broadly to cover visible light, UV light, and the like. The selection of the wavelength of light can be controlled by the selected semiconductor working electrode. Also, methods known in the art such as filters or monochromators can be used to control the wavelength which impacts the electrode.

In a preferred embodiment, the electromagnetic radiation is primarily UV radiation which includes UV-Vis radiation.

SOLID COMPOSITE OR SEMICONDUCTING WORKING ELECTRODE

Semiconducting working electrodes are known in the art including those used for photoelectrochemistry. For example, the semiconductor working electrode can create holes when it is irradiated as known in the art. The semiconductor can be selected to have a band gap which allows for the creation of holes and reaction with carbon dioxide.

Semiconductors include group IV semiconductors, group lll-V

semiconductors, group ll-VI semiconductors, and the like, as known in the art.

Examples of ll-VI semiconductors include sulfide, selenide, and telluride materials including, for example, CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, MoS ₂ , MoSe ₂ , MoTe ₂ , WS ₂ , WSe ₂ . Metal dichalcogenides can be used. The semiconductor can be in particle form and compounded with other components to form a solid, integral electrode structure. Average particle size can be, for example, less than one micron, or from about 5 nm to about 500 nm, or about 10 nm to about 250 nm, or about 25 nm to about 100 nm.

Composite electrodes can be used. Multi-layer electrodes can be used.

Various geometries can be used.

The semiconductor working electrode can be, for example, a composite comprising at least one semiconductor, at least electronic conductor, and at least one binder. In one embodiment, for example, the semiconductor working electrode is a composite comprising at least one colloidal ZnS or CdS semiconductor, at least electronic conductor, and at least one binder. In another embodiment, the

semiconductor working electrode is a composite comprising at least one colloidal ZnS semiconductor, at least electronic metallic conductor, and at least one

fluoropolymer binder.

The ZnS semiconductor can be used in various forms including zinc blende (band gap is 3.58 eV at 300K), wurtzite (band gap is 3.70 eV at 300K), and combinations thereof.

Electronic conductor for use in electrode formation are well-known in the art including conductive carbon (e.g., graphite) and metal materials including, for example, silver, gold, and copper. Additional examples of electronic conductors include conjugated polymers, whether doped or not, which can also function as a binder.

Binders including insulating or conductive binders for use in electrode formation are well-known in the art. Polymeric materials can be used including polyolefins, carbon backbone polymers, fluorinated polymers, and perfluorinated polymers and copolymers. Poly(tetrafluoroethylene) ("Teflon") can be used.

Additional examples of the binder include conjugated polymers, whether doped or not, which can function also as a conductor.

Additives can be used in forming the working electrode. Solvents can be used in forming the electrode to help with dispersion. Compounded materials can be shaped and dried to form solid electrodes with proper shape. LIQUID SOLUTION COMPRISING CARBON DIOXIDE

A liquid solution can be prepared which includes one or more solvents such as water and establishes a semiconductor-liquid interface. The water can be subjected to carbon dioxide mixing so that the carbon dioxide can diffuse in the liquid and participate in the electrochemical reactions at the electrode. As known in the art, carbon dioxide can dissolve in water providing mild acidic pH, forming carbonic acid. Bubbling of carbon dioxide throughout the liquid can be carried out.

Additives can be used in the liquid solution such as, for example, electrolytes or buffers, to control conductivity or pH.

In one embodiment, the solution is an aqueous solution.

PROTON SOURCE

The proton source can be used in a variety of embodiments. As used herein, the term "proton source" means that the proton source provides hydrogen atoms in the conversion of carbon dioxide to products, including ultimately elemental carbon products, in a liquid media via one or more intermediates.

In one embodiment, the proton source is present in the solid compound electrode. When the solid composite electrode is made, the proton source can be incorporated into the ingredients used to make the composite electrode. One skilled in the art can adapt the amount of the proton source to be included in the solid composite electrode. Hence, the electrode can be a source of both electrons and protons.

In another embodiment, the proton source is present in the liquid solution comprising carbon dioxide.

In another embodiment, the proton source can be present both in the solid composite electrode and in the liquid solution comprising carbon dioxide.

In one embodiment, the proton source is or comprises a reducing agent.

In one embodiment, the proton source can be an organic compound. The proton source can be a weakly acidic compound. The organic compound can be, for example, an organic aromatic compound, having one or more hydroxyl moieties such as hydroquinone. The organic compound, such as hydroquinone, optionally can be substituted. As known in the art, the hydroxyl moiety can be converted to a ketone moiety releasing protons. A material such as Raney nickel also can be a proton source. In one embodiment, the proton source is hydroquinone.

SWITCHING BETWEEN OPEN AND CLOSED CIRCUIT MODES

An electrochemical system is provided which allows for switching of the semiconducting working electrodes between open and closed circuit modes.

Potentiostats are known in the art and can be used. In open circuit mode, the working electrode is not able to have current flow due to the open circuit. Hence, for example, holes can build up in the working electrode when it is irradiated with light. In the closed circuit mode, the working electrode can participate in current flow creating power.

One can control the potentiostat to alternate the working electrode between open and closed circuit modes.

Time periods can be established for both the open circuit mode and the closed circuit mode. For example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of 1 minute to 480 minutes, or 5 minutes to 240 minutes, in open circuit mode. In another example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of 1 minute to 480 minutes, or 4 minutes to 240 minutes, in closed circuit mode. In another example, the switching step can comprise alternating steps of open circuit mode and closed circuit mode characterized by a time period of 1 minute to 480 minutes, or 4 minutes to 240 minutes, in both open and closed circuit modes. In general, longer cycle times can be used to produce sp ³ carbon and shorter cycle times can be used to produce sp ² carbon.

One can vary the process to control the amount of power developed. The power built up in the open circuit mode can be, for example, at least 5 mW, or at least 15 mW when switched to close circuit mode.

One can vary the time the process is carried whether continuously or intermittently. For example, the method can be carried out for at least 24 hours, 48 hours, or 72 hours.

One can adapt these and other parameters of the methods described herein to ensure that elemental carbon is the lead product formed. In one embodiment, oxygen gas is generated at the counter electrode in the closed circuit mode.

ELEMENTAL CARBON-CONTAINING REACTION PRODUCT

The process can produce an elemental carbon-containing reaction product at the working electrode. In a preferred embodiment, the reaction product is a carbon material. The reaction product can form on the working electrode and separated from the electrode and recovered for further use. The content of carbon in the reaction product can be measured to determine the purity of the carbon. Mixtures of different kinds of carbon materials can be formed.

In one embodiment, the carbon material is at least 90 wt.% carbon, or at least 95 wt.% carbon, or at least 99 wt.% carbon.

The carbon material can be crystalline or amorphous.

In some embodiments, the carbon is diamond-like carbon, graphene, amorphous carbon, or a mixture thereof.

In one embodiment, the carbon is primarily diamond-like carbon. In another embodiment, the carbon is primarily graphene. In another embodiment, the carbon is primarily amorphous carbon. The carbon product can be in sp ² form, or in sp ³ form, or a combination thereof.

Importantly, the switching step can be carried out with a time period adapted to form one form of carbon as the primary type of carbon over another form of carbon.

The following table provides some information about the formation of carbon and other reactants form carbon dioxide, electrons, and protons:

13 2C0 ₂ +8e +8H ⁺ C ₂ +4H ₂ 0 +599 -0.78 4

14 60CO ₂ +240e +240H ⁺ C ₆₀ +120H ₂ O (fullerene) -2978 0.129 4

15 nC0 ₂ +4ne +4nH ⁺ nCarbon +2nH ₂ 0 -n88 4

16 2C0 ₂ +8e +8H ⁺ C ₂ +4H ₂ 0 -176 0.228 4

17 4C0 ₂ +16e +16H ⁺ C ₄ +8H ₂ 0 -352 0.228 4

18 8C0 ₂ +32e +32H ⁺ C ₈ +16H ₂ 0 -704 0.228 4

19 16C0 ₂ +64e +64H ⁺ C ₁₆ +32H ₂ 0 -1408 0.228 4

20 32C0 ₂ +128e +128H ⁺ C ₃₂ +64H ₂ 0 -2816 0.228 4

21 60CO ₂ +240e +240H ⁺ C ₆₀ +120H ₂ O -5280 0.228 4

WORKING EXAMPLES

Additional embodiments are provided in the following non-limiting working examples, including Figures 1 -6.

MATERIALS AND METHODS

Experimental Setup:

Silver nanoparticles were produced by the citrate reduction method described in Hiriuki et al. [Naiki, H., et al., Single-Photon Emission Behavior of Isolated

CdSe/ZnS Quantum Dots Interacting with the Localized Surface Plasmon

Resonance of Silver Nanoparticles. J. Phys. Chem. C, 201 1 . 115: p. 23299-23304.] Here, 0.4 M of AgNO ₃ (Fischer) was dissolved in 2 liters of >18 MQcm-1 water. The solution was refluxed while 40 ml of 1 .1 M sodium citrate was added slowly. The reflux continued for one hour before another 40 ml of citrate solution was added. The reaction mixture was allowed to cool to room temperature overnight. The precipitated Ag nanoparticles were separated from the supernatant by centrifugation. They were washed and centrifuged several additional times to remove impurities. The Ag nanoparticles were dried at room temperature in vacuum.

The photoactive material was colloidal ZnS. It was prepared by drop wise addition of 0.05 M anhydrous sodium sulfide (Alfa Aesar) to a stirred solution of 0.05 M zinc perchlorate (Alfa Aesar) stabilized with 0.02 M SiO2 nanopowder [Inoue, H., et al., Effects of Size Quantization of Zinc Sulfide Microcrystallites on Photocatalytic Reduction of Carbon Dioxide. Chemistry Letters, 1990. 19: p. 1483-1486; Inoue, H., et al., Photoreduction of carbon dioxide using chalcogenide semiconductor microcrystals. Journal of Photochemistry and Photobiology A: Chemistry, 1995. 86: p. 191 -196.] (10-20nm Aldrich). The resulting suspension was centrifuged and resuspended twice. Several aliquots of the purified material were prepared simultaneously. Combined batches were dried at 50 ^Q C overnight in vacuo. The photocathodes used in this study were compounded from the photocatalyst (ZnS) a conductive substrate - either Ag nanoparticles or high purity graphite powder and a polymer binder - uncured PTFE obtained by drying 60% PTFE dispersion (DuPont) in a vacuum oven at 250 ^Q for 24 hours. The three dry ingredients were suspended in mineral spirits that was sonicated for 10 minutes.

The slurry was then poured onto a glass sheet. The wet solid mixture was kneaded with a spatula to knit the PTFE particles into a support matrix for the photocatalyst and conductive substrate. A film was formed by rolling out the mass to a thickness of about 0.5 mm. The film was then dried at room temperature. Final assembly was obtained by pressing a sheet of the active material onto either side of a perforated, roughened titanium plate that served as a current collector and to which the electrical connections were made. The photocathode was heated to 250 ^Q in vacuo for 24hr prior to use.

Transmission Electron Microscopy:

The material was mechanically crushed, and the powder that resulted from this action was placed in direct contact with a TEM grid with a carbon support with regularly spaced holes of 1 .2-micron diameter (quantifoilTM 1 .2/1 .3). TEM was performed on a Tecnai F20 G2 microscope equipped with an eucentric goniometer stage operated at 200 kV and a nominal magnification range of 290-700,000 X. Images were recorded on a 4k x 4k Gatan CCD camera. Magnification was calibrated [Houska, C.R. and B.E. Warren, X-ray study of the graphitization of carbon black. J.. Appl. Phys. , 1954. 25: p. 1503-1509.] using the 3.35 A spacing of commercial graphitized carbon (from Electron Microscopy Sciences, Hatfield, PA) as a calibration specimen, which indicated a post-magnification factor of 1 .43.

Eucentricity was monitored and refined if necessary when analyzing different regions of the grid under observation to avoid variations in magnification. Images were analyzed with Digital Micrograph software (Gatan, Inc).

SUMMARY OF ELECTROCHEMICAL SYSTEM

In preferred and exemplified embodiments, an electrochemical system was provided that can convert CO2 into various carbon forms including crystalline diamond-like carbon, graphene, or amorphous carbon using UV photons as the only source of energy. A ZnS-Teflon-Ag composite was fabricated as a cathode and connected through a computer controlled potentiostat to a platinum counter ion anode and run in an oscillating open-closed circuit manner while being illuminated with UV light as C02 was bubbled into the system. The UV photons caused electrons to be transferred from the ZnS to C02, converting some of the gas to what is hypothesized to be free radical anions and a buildup of holes in the ZnS when running the system in open circuit mode. Switching to closed circuit mode created a current, resulting in a flow of electrons to the cathode that refilled the holes in the ZnS.

In preferred embodiments, increasing the time of UV exposure in open circuit mode increases the population of holes on the ZnS. For example, irradiation for 5, 30, or 240 minutes in open circuit mode resulted in instantaneous powers of 6.55 mW, 7.85 mW, and 16.1 mW, respectively, when the system was switched to closed circuit mode.

The claimed subject matter is not limited by theory or mechanism, but it was hypothesized that this electric energy would provide enough reducing equivalents to convert C02 into carbon, or molecular carbon, via a radical anion and that the increasing power would result in the creation of different carbon allotropes. After three days of running the system in alternating open-closed circuit mode for the three different times, milligram quantities of amorphous carbon, graphene, or crystalline diamond-like carbon were deposited on the cathode at low, intermediate, or high power modes, respectively. These results demonstrated using sunlight to create valuable carbon products from C0 ₂ .

Additional description is provided with reference to the Figures 1 -6.

A schematic of an exemplary photo-electrochemical system is shown in Figure 1 . A UV photon will induce colloidal ZnS to transfer an electron to C02 creating a radical anion [Eggins, B.R., et al., Formation of two-carbon acids from carbon dioxide by photoreduction on cadmium sulphide. J. Chem. Soc, Chem.

Commun., 1988(16): p. 1 123-24; Eggins, B.R., et al., Photoreduction of carbon dioxide on zinc sulfide to give four-carbon and two-carbon acids. J. Chem. Soc, Chem. Commun., 1993(4): p. 349-50]. It was reasoned that if the oily ZnS could be incorporated into a conductive solid structure by combining it with Teflon and silver, UV radiation would cause a buildup of a population of holes resulting in a buildup of positive potential on this composite as carbon dioxide is converted into anionic free radicals. The ZnS-Tf-Ag sheet is the cathode of the system and can be labeled "W" for working electrode and connected through a computer controlled potentiostat to the platinum anode which can be labeled "C" for counter electrode. The potentiostat can be programmed to oscillate between connecting (closed) or breaking (open) this circuit. A hydrogen reference electrode which can be labeled "R" is used to monitor the changes in voltage between open and closed circuit mode while the cathode is continuously irradiated with UV light. Carbon dioxide was flowed into the system, and the lamp was on continuously irradiating the cathode with UV photons as C0 ₂ was bubbled into the solution.

Figures 2A, 2B, and 2C show the voltage, current and power at the instant the circuit is closed when the system is run in open circuit mode for 5, 30, and 240 minutes. Increasing the time of UV irradiation in open circuit mode does in fact increase the instantaneous power when the system is switched into closed circuit mode. In closed circuit mode electrolysis of water was observed as seen by the evolution of oxygen at the anode and a decrease in pH of the system (Figures 2D and 7).

When the system was run in open circuit mode, the population of holes in the ZnS increased as the time of UV exposure was increased. When the system was run in open circuit mode for 5 minutes (2A) about 0.47 Volts of potential were built up on the anode. This voltage was converted into current the instant that the circuit was closed. The instantaneous power the moment the circuit was closed was 6.55 milliwatts. When the system was run in open circuit mode for 30 minutes (2B), the instantaneous power at the moment the circuit was switched to closed mode was 7.85 mW. Running the system in open circuit mode for 4 hours resulted in 16.1 mW of power the moment the system was switched into closed circuit mode. In closed circuit mode, the current arises from the splitting of water and a flow of electrons back to the cathode, evolution of molecular oxygen (D), and a drop in pH from the release of protons. See Figures 2D and 7.

Creating too many holes in the ZnS could cause the material to become unstable. The oscillation, or switching, process prevented this, because the holes being created in the ZnS while the system is being run in open circuit mode are refilled with electrons when the system is run in closed circuit mode. Thus, the system can be run for extended periods of time, while continuously exposing the cathode to UV radiation. We ran the circuit continuously for three days oscillating between open and closed circuit mode.

For the 5, 30, and 240 minute oscillation times several milligrams of molecular (or elemental) carbon was found to be deposited on the cathode, which was confirmed by elemental analysis (Figure 3). Elemental analysis on the three samples indicates 95.6%, 93.7%, and 92.4% carbon, respectively.

The three samples were studied with electron microscopy. Two representative electron microscopy images and diffraction patterns for the system in high power mode are shown. The regular ordering seen within the images characteristic of crystalline carbon is confirmed by the clarity, spacing and symmetry of the diffraction patterns. Figures 4A and 4B are images of the carbon deposited when the system was run in 240 minute oscillation mode. The diffraction patterns show the dot separation, clarity, and symmetry characteristic of crystalline diamond-like carbon.

Figures 5A and 5B are the representative images of the carbon deposited when the system was run in 30 minute oscillation mode. Parts of the images, especially in Figure 5B are an almost transparent sheet of carbon that seems to be one or only a few layers thick and thus appears to be graphene. Parts of Figure 5A show how a few sheets may fold over on each other to create a braided structure. The diffraction pattern shown in Figure 5A looks like a four-leaf clover indicating some degree of molecular pattern that produces only blobs due to insufficient periodicity.

Figure 6 are the images of the carbon deposited when the system was run in 5 minute oscillation mode. There is no pattern whatsoever in either the images or the diffraction analysis indicating that this is amorphous carbon.