Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
  • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
  • G01N27/28—Electrolytic cell components
  • G01N27/30—Electrodes, e.g. test electrodes; Half-cells
  • G01N27/308—Electrodes, e.g. test electrodes; Half-cells at least partially made of carbon

Definitions

• the invention relates generally to a diamond microelectrode for electrochemical use, the use of a microelectrode and a method of making a microelectrode.
• Diamond microelectrodes for electrochemical use are used for, for example, the monitoring of electrochemical reactions in which oxidation or reduction reactions occur at an electrode.
• Diamond microelectrodes for electrochemical use are distinct from electrodes in electronic devices and thermionic or field emission devices.
• the microelectrode enables electrons to be transferred either from the microelectrode to an ion (or other species) in solution or from the ion (or other species) in solution to the microelectrode, thereby changing the charge state of the ion (or other species).
• Electrodes in electronic devices simply allow the passage of the electrons from one solid electrical conductor to another.
• Electrodes in thermionic devices (such as cathode ray tubes) and field emission devices such as cold cathodes) allow electrons to exit from a material surface into a vacuum.
• Boron-doped diamond electrodes are known for use in electrochemistry. Boron- doped diamond electrodes have been developed for the destruction of waste by electrochemical means (e.g. US Patent Number 5,399,247), for chemical synthesis (e.g. Panizza et al, 'Electrochemical oxidation of phenol at boron-doped diamond electrode. Application to electro-organic synthesis and wastewater treatment', Ann. Chim., 92 (2002), 995-1006), and for chemical analysis (e.g. EP1651951 ). D. J. Zhong et al, 'Fabrication and electrochemical characterization of boron-doped diamond interdigitated array disc electrode', Proc. IMechE. Part E: J.
• Process Mechanical Engineering 221 (2007), 201 -205, describes a device comprising an interdigitated array of boron-doped diamond electrodes that are deposited by chemical vapour deposition (CVD) on an undoped diamond layer, itself deposited by CVD on a silicon wafer. Both the undoped and doped diamond layers are rough and there is a height difference between the surfaces of the doped and undoped layers.
• CVD chemical vapour deposition
• an interdigitated electrode array comprising a diamond substrate with conductive electrodes that are optionally conductive diamond is proposed for use as a solid state radiation detector.
• a microelectrode for electrochemical use comprising:
• connection members that extend from the second surface of the electrically non-conductive diamond plate into the electrically non-conductive diamond plate, such that each of the connection members is electrically connected to respective ones of the working electrodes.
• the arrangement of the working electrodes and the electrically conductive connection members allows the working electrodes to be addressed from the opposed second surface of the microelectrode.
• the microelectrode may be used in an environment that is chemically aggressive without risk that the electrical connections to the microelectrode will be attacked by the environment.
• substantially flush it is meant that the root-mean-square roughness R q measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conducting diamond plate is 50 nm or less.
• R q The definition of R q is well known in the art and may be found in Tribology: Friction and Wear of Engineering Materials' by I. M. Hutchings (published by Edward Arnold (London), 1992, pages 8-9). For completeness the R q is defined as:
• y is the deviation of the surface height from the mean height at a distance x from the origin
• L is the overall length of the profile under examination.
• a method of using a microelectrode for electrochemical use for characterising or monitoring an electrochemical reaction or process comprising:
• electrically conductive diamond material contained within said recesses and being substantially flush with the first surface of said diamond plate, the electrically conductive diamond material in each of said at least two discrete recesses providing respective working electrodes in use;
• connection members that extend from the second surface of the electrically non-conductive diamond plate into the electrically non-conductive diamond plate, such that each of the connection members is electrically connected to respective ones of the working electrodes;
• a method of making a microelectrode for electrochemical use comprising the steps of:
• the electrically non-conductive diamond plate may be rectangular when viewed in plan. Other shapes viewed in plan are possible including square, circular, oval, parallelogram-shapes, rhombus-shaped, kite-shaped, trapezoidal or any other regular or irregular shape.
• the lateral dimensions electrically non-conductive diamond plate may be any suitable size. Typically lateral dimensions will range from of order 1 mm to several tens of mm depending upon the intended dimensions and separation of the working electrodes.
• the thickness of the electrically non-conductive diamond plate may be any suitable thickness, typically between 0.1 mm and 10 mm.
• the electrically non-conductive diamond plate may comprise single-crystal diamond or polycrystalline diamond.
• the electrically non-conductive diamond plate may comprise natural or synthetic diamond.
• the synthetic diamond may be produced by a chemical vapour deposition (CVD) process (often referred to as “CVD diamond” or “CVD diamond material") or a high-pressure/high- temperature (HPHT) process (often referred to as “HPHT diamond” or “HPHT diamond material”).
• the electrically non- conductive diamond plate may comprise polycrystalline synthetic diamond produced by a CVD process (referred to hereinafter as "polycrystalline CVD diamond” or “polycrystalline CVD diamond material”).
• the electrical resistivity of the electrically non- conductive diamond plate is 10 6 ohm. cm or greater.
• the electrical resistivity may be 10 8 ohm. cm or greater, alternatively 10 10 ohm. cm or greater, alternatively 10 11 ohm. cm or greater.
• the electrically conductive diamond material contained within the recesses may be electrically conductive CVD diamond material.
• the electrically conductive diamond material contained within the recesses may be electrically conductive polycrystalline CVD diamond material.
• the electrically conductive diamond material contained within the recesses may be made electrically conductive by any suitable means, for example by doping the diamond with, for example, boron, phosphorus, sulphur, lithium, or any other suitable element.
• the electrical conductivity of the electrically conductive diamond material is provided by doping with boron, the material referred to herein as "boron-doped diamond material".
• the preferred method of doping the diamond material is to add the dopant during the synthesis process.
• the electrically conductive diamond material contained in the recesses will also be predominantly single crystalline in nature.
• the electrically conductive diamond material contained in the recesses will also be polycrystalline in nature.
• the electrically non-conductive diamond plate contains additional recesses that contain electrically conductive diamond material to provide one or more counter electrode(s) and/or reference electrode(s) in addition to the two or more working electrodes already described.
• the electrically conductive diamond material making up any of the working electrodes, any counter electrodes, or any reference electrodes may be boron-doped CVD diamond.
• the concentration of boron in the electrically conductive diamond material may optionally be 10 18 atoms/cm 3 or greater, alternatively 10 19 atoms/cm 3 or greater, alternatively 10 20 atoms/cm 3 or greater, alternatively 10 21 atoms/cm 3 or greater.
• the concentration of boron atoms in the electrically conductive diamond material may be 10 22 atoms/cm 3 or lower, alternatively 10 21 atoms/cm 3 or lower, alternatively 10 20 atoms/cm 3 or lower.
• the concentration of boron in the electrically conductive diamond material may be between 10 18 atoms/cm 3 and 10 21 atoms/ cm 3 , alternatively may be between 10 19 atoms/cm 3 and 10 21 atoms/cm 3 .
• the electrical resistivity of the electrically conductive diamond material that provides any of the working electrodes, any counter electrodes, or any reference electrodes may be about 10 4 ohm. cm or lower, alternatively 10 ohm. cm or lower, alternatively 5 x 10 "1 ohm. cm or lower, alternatively 2 x 10 "1 ohm. cm or lower, alternatively 1 x 10 "1 ohm. cm or lower, alternatively 1 x 10 "2 ohm. cm or lower, alternatively 10 "3 ohm. cm or lower.
• the electrical resistivity of the electrically conductive diamond material that provides any of the working electrodes, any counter electrodes, or any reference electrodes may be in the range between 5 x 10 "1 ohm.
• An advantage of minimising the electrical resistivity of the material comprising the layer of electrically conductive diamond material is that as the electrical resistivity decreases the potential drop between any two points on a working electrode also decreases, so giving the microelectrode spatially more uniform performance. Whilst for some embodiments the electrical resistivities of the electrically conductive diamond material that provides the respective working electrodes are substantially the same, this need not necessarily be the case.
• the working electrodes are those electrodes between which, during use of the microelectrode, an electrochemical reaction or process is monitored, or an electrochemical property of an electrolyte is determined. At any time during use at least one working electrode is an anode and at least one working electrode is a cathode. "In use" in this context means when there is a non-zero potential difference between the working electrodes.
• microelectrodes are for performing cyclic voltammetry.
• cyclic voltammetry the voltage applied to one working electrode relative to the voltage applied to the other working electrode is swept from zero to a positive value, back through zero to a negative value before returning to zero. Consequently each working electrode is an anode for part of the cycle and a cathode for part of the cycle.
• first working electrode is biased relative to a second working electrode such that anions (negatively charged ions) are attracted to the first working electrode
• the first working electrode is an anode.
• a first working electrode is biased relative to a second working electrode such that cations (positively charged ions) are attracted to the first working electrode
• the first working electrode is a cathode.
• the recesses in the first surface of the electrically non-conductive diamond plate may be any suitable shape.
• any recess may be an elongate recess, for example a channel- shaped elongate recess.
• elongate it is meant that the length of the recess is typically 2 or more, alternatively 5 or more, alternatively 10 or more, alternatively 20 or more times the greater than the mean width of the recess.
• the length of the recess may typically be 0.5 mm or greater, alternatively 1 mm or greater, alternatively 3 mm or greater, alternatively 5 mm or greater, alternatively 8 mm or greater, alternatively 10 mm or greater.
• the layout of the recesses in the first surface typically corresponds to a preselected desired ultimate electrode pattern. Electrode patterns include branched electrodes, such as interdigitated electrodes and unbranched electrodes. Where the electrode pattern includes unbranched electrodes, the electrodes may be straight or curved or a combination of straight sections and curved sections.
• recesses are generally channel-shaped, such channel shaped recesses may for example be generally "U”-shaped or “V”-shaped or “V”-shaped with a flat bottom in cross-section (profile).
• the recesses when containing electrically conductive diamond material provide working electrodes. Recess configurations other than channel-shaped ones that could provide a desired pre-selected working electrode configuration could be deduced by the person skilled in the art.
• the depth of the recesses in the electrically non-conductive diamond plate may be 300 ⁇ or less, alternatively 100 ⁇ or less, alternatively 30 ⁇ or less, alternatively 10 m or less, alternatively 3 ⁇ or less, alternatively 1 ⁇ or less, alternatively 300 nm or less, alternatively 100 nm or less, alternatively 30 nm or less.
• the depth of the recesses in the electrically non-conductive diamond plate may be 10 nm or more, alternatively 30 nm or more, alternatively 100 nm or more, alternatively 300 nm or more, alternatively 1 ⁇ or more, alternatively 3 ⁇ or more, alternatively 10 ⁇ or more, alternatively 30 ⁇ or more, alternatively 100 ⁇ or more.
• the depth of the recesses in the electrically non-conductive diamond plate may be between 1 ⁇ and 300 ⁇ , alternatively between 1 ⁇ and 100 ⁇ , alternatively between 100 nm and 3 ⁇ , alternatively between 1 ⁇ and 100 ⁇ , alternatively between 1 ⁇ and 30 ⁇ , alternatively between 1 ⁇ and 10 ⁇ , alternatively between 3 ⁇ and 300 ⁇ , alternatively between 3 ⁇ and 100 ⁇ , alternatively between 3 ⁇ and 10 ⁇ , alternatively between 10 ⁇ and 300 ⁇ , alternatively between 10 ⁇ and 100 ⁇ , alternatively between 10 ⁇ and 30 ⁇ .
• One factor to consider in selection of the recess depth is the method involved in manufacturing the microelectrode, and in particular in making, and filling the recesses, and if necessary planarising any over-filled recesses so that the electrically conductive diamond material is substantially flush with the diamond plate surface.
• Deeper recesses disadvantageously take longer to make and fill than shallower recesses, but the margin for error in planarising any overfilled recesses is advantageously larger for deeper recesses than for shallower recesses.
• over-filled it is meant that the thickness of electrically conductive diamond material that is deposited in the recess is greater than the depth of the recess. A recess having such a thickness of material is said to be “over-filled”.
• the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate may be about 1 ⁇ or greater, alternatively about 3 ⁇ or greater, alternatively about 10 ⁇ or greater, alternatively about 30 ⁇ or greater, alternatively about 100 ⁇ or greater.
• the width of the recess where the side of the recess meets the surface of the electrically non-conductive diamond plate may be 2000 ⁇ or less, alternatively 1000 ⁇ or less, alternatively 300 ⁇ or less, alternatively 100 ⁇ or less, alternatively 30 ⁇ or less, alternatively 10 ⁇ or less, alternatively 3 ⁇ or less.
• the width of a recess may or may not be greater than its depth.
• the ratio of the width of the recess where the sides of the recess meet the surface of the diamond plate, w, to the depth of the recess, d, i.e. w/d is 1 or greater, alternatively 2 or greater, alternatively 5 or greater, alternatively 10 or greater.
• the recesses it is advantageous for the recesses to have a width that is substantially greater than the depth in embodiments in which the recesses are filled with electrically conductive diamond material using a CVD process and the width of the recesses is greater than about 100 ⁇ .
• the recesses have a width where the sides of the recess meet the surface of the diamond plate of 10-20 ⁇ and a depth of 10-20 ⁇ , such that the width is equal to or greater than the depth.
• the cross sectional profiles of the recesses in which the working electrodes are formed may be any shape and may vary with position and may be the same or different for different working electrodes.
• the cross-sectional profile of one or more of the recesses is of substantially constant depth and is the same for all working electrodes. This situation has an advantage that it is easier to fill the recess uniformly. Another advantage is that avoids the need to over-fill shallower parts of a recess to ensure that deeper parts of the recess are completely filled.
• the maximum width of a recess (at any point measured in its depth) is 150% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate.
• the maximum width of a recess is 120% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate.
• the maximum width of a recess is 1 10% or less of the width of the recess where it meets the surface of the electrically non-conductive diamond plate.
• the maximum width of a recess is the same as the width of the recess where it meets the surface of the electrically non-conductive diamond plate.
• the cross sectional profile of a recess may have substantially straight sides.
• the angle that the straight sides of the recess make with the first surface of the diamond plate may be in the range 90° ⁇ 10°, alternatively 90° ⁇ 5°, alternatively 90° ⁇ 3°, alternatively 90° ⁇ 2°, alternatively 90° ⁇ 1 °.
• the base of the recess may also be substantially straight and similarly at an angle of 90° ⁇ 10°, alternatively 90° ⁇ 5°, alternatively 90° ⁇ 3°, alternatively 90° ⁇ 2°, alternatively 90° ⁇ 1 °from the straight sides.
• An advantage of the angle that the sides the recess make with the surface of the diamond plate being close to 90° is that the width of the recesses, and hence separation between the working electrodes is constant with depth. This is advantageous in terms of the method of manufacture of the microelectrode, because it means that if the electrode is planarised after the conductive diamond material is deposited in the recesses then the variation in the width of the electrode at the surface after different extents of planarisation is reduced which provides tolerance in the planarisation process to achieve a particular electrode separation.
• the width of the recess is narrower at its base than where the sides of the recess meet the first surface of the electrically non-conductive diamond plate.
• the difference in the width may be in the range 0.1 % to 25%.
• the width of the recess at the base may be less than 100% and 99.9% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 99% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non-conductive diamond plate, alternatively less than 100% and 98% or more of the width of the recess where the sides of the recess meet the surface of the electrically non-conductive diamond plate, alternatively less than 100% and 97% or more of the width of the recess where the sides of the recess meet the first surface of the electrically non- conductive diamond plate, alternatively less than 100% and 95% or more of the width of the recess where
• the width of the recess is narrower at its base than where the side surfaces of the recess meet the first surface of the electrically non-conductive diamond plate. This is because the material adjacent to the edge of the recess at the first surface of the electrically non-conductive diamond plate is better supported by the sloping side surfaces compared to a recess with side surfaces that are perpendicular, or close to perpendicular, to the first surface of the electrically non-conductive diamond plate causing the structure to be less weak, thereby allowing a microelectrode having a small separation between a working electrode and an adjacent working electrode to be fabricated more easily.
• one side of the recess may be at an angle of approximately 90° to the first surface of the electrically non-conductive diamond plate and one side of the recess may be at an angle of less than 90° to the first surface (measured within the recess) such that the width of the base of the recess is less than the width of the recess where the sides of the recess meet the first surface.
• This embodiment represents a compromise between embodiments with two substantially vertical sides in which the material separating a working electrode from another working electrode may be structurally weak and embodiments with two sloping sides in which more precision is required in any planarisation process applied to remove any electrically conductive diamond material overfill, in order to achieve a particular separation between the working electrodes.
• This embodiment finds particular application for embodiments where the electrodes are closely spaced, for example where two working electrodes are parallel and unbranched.
• the recesses may be substantially uniform in depth or vary in depth over their length.
• substantially uniform the inventors intend that over 75% or more of the length of the recess, the depth of the recess deviates by 10% or less from the average depth of the recess.
• suitable means include contact and non-contact surface profilometers.
• the separation of the recesses will in general eventually constitute the separation between the working electrodes (the anode and the cathode).
• the desired separation of the working electrodes is in some applications dependent upon the reaction or property that the device is intended to monitor.
• the microelectrode of the invention may comprise multiple sets of working electrodes having different separations, for example a pH monitor may have one pair of closely spaced working electrodes for monitoring the pH of low conductivity electrolytes and a second pair of widely spaced working electrodes for monitoring the pH of high conductivity electrolytes. The presence of multiple pairs of working electrodes having different separations enables the microelectrode to have a wider sensing range.
• the separation of the working electrodes measured at the surface of the electrically non-conductive diamond plate may be 0.5 ⁇ or greater, alternatively 1 ⁇ or greater, alternatively 10 ⁇ or greater, alternatively 20 ⁇ or greater, alternatively 30 ⁇ or greater, alternatively 50 ⁇ or greater, alternatively 100 ⁇ or greater, alternatively 300 ⁇ or greater, alternatively 1 mm or greater, alternatively 3 mm or greater, alternatively 10 mm or greater, alternatively 20 mm or greater.
• the separation of the working electrodes measured at the surface of the electrically non-conductive diamond plate may be 50 mm or less, alternatively 30 mm or less, alternatively 20 mm or less, alternatively 10 mm or less, alternatively 3 mm or less, alternatively 1 mm or less, alternatively 300 ⁇ or less, alternatively 100 ⁇ or less, alternatively 30 ⁇ or less, alternatively 10 ⁇ or less, alternatively 3 ⁇ or less, alternatively 1 ⁇ or less.
• the separation between the working electrodes is between about 0.5 ⁇ and 300 ⁇ . In an embodiment of the invention the separation between the working electrodes is between 3 ⁇ and 300 ⁇ . Alternatively invention the separation between the working electrodes is between 10 ⁇ and 100 ⁇ .
• the problem of fragility may be exacerbated by the recess having side surfaces that are perpendicular to the first surface of the electrically non-conductive diamond plate and may be ameliorated by the recess having side surfaces that are not perpendicular to the first surface of the electrically non- conductive diamond plate as previously described.
• the separation at the surface of the diamond plate between the recesses forming the working electrodes is substantially constant.
• substantially constant the inventors mean that for 60% or more of the length of the electrode or the maximum deviation from the mean separation is 10% or less.
• the electrically conductive diamond material is substantially flush with the first surface of the electrically non-conductive diamond plate, by which we mean that the root-mean-square roughness, R q , of the surface measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate is 50 nm or less.
• the root-square-mean roughness, R q is even less; for example for some embodiments, the root-mean-square roughness, R q , of the surface measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate is 20 nm or less.
• the root-mean-square roughness, R q of the surface measured across any interface between a working electrode and the electrically non-conductive diamond plate is 10 nm or less, alternatively 5 nm or less, alternatively 2 nm or less. Low values of R q measured across the interface are advantageous as this minimises the disruption in flow of electrolyte across the surface between the working electrodes giving more accurate results.
• An interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate can also be characterised in terms of the height difference between the surface of the electrically non-conductive diamond plate and the surface of the working electrode.
• the height difference measured across any interface between a working electrode in the first surface and the first surface of the electrically non-conductive diamond plate is 10 ⁇ or less.
• the height difference is 1 ⁇ or less, alternatively 0.1 ⁇ or less, 10 nm or less, alternatively 5 nm or less, alternatively 3 nm or less, alternatively 2 nm or less, alternatively 1 nm or less, alternatively 0.5 nm or less.
• the height difference may be measured using an instrument of the type used to measure other surface characteristics (e.g. R q ), such as a contact or non-contact profilometer.
• each electrically conductive connection member is provided that connects the working electrode to the second surface of the electrically non-conductive diamond plate.
• each electrically conductive connection member comprises a cavity in the electrically non-conductive diamond plate that extends between the second surface of the electrically non-conductive diamond plate and the working electrode.
• each electrically conductive connection member comprises a cavity in the electrically non-conductive diamond plate that extends from the second surface of the electrically non-conductive diamond plate to the first surface of the electrically non-conductive diamond plate.
• the connection member extends from the second surface to the first surface, the connection member has a first end substantially flush with the first surface.
• the cavity in the electrically non-conductive diamond plate has a surface.
• the surface may be metallised, for example with one or more metal layers.
• An example of suitable combination of metal layers is a Ti base layer, a Pt barrier layer and an Au capping or bonding layer.
• the remainder of the cavity may be unfilled.
• the remainder of the cavity may be partially filled or completely filled, for example with an electrically conductive polymeric material, such as silver-loaded epoxy resin, or a solder material or a braze material.
• the choice of filling material may depend on the environment, for example the temperature, to which the microelectrode will be exposed when in use.
• the cavity may be partially filled with electrically conductive diamond material.
• connection member A means of connection to an external circuit may be provided as part of the connection member.
• a wire may be soldered or attached using silver- loaded epoxy resin into the cavity or a socket soldered into the cavity. Other methods of connection are not excluded.
• connection members have first ends substantially flush with the first surface of the electrically non-conductive diamond plate
• first ends of the connection members may be at least partially covered with a layer electrically non-conducting material.
• the first ends of the connection members may be completely covered with a layer of electrically non-conductive material.
• the thickness of electrically non-conductive material used to at least partially or completely cover the first ends of the connection members may be chosen according to the application. For some applications, they are advantageously thick enough to ensure that the layer is continuous and sufficiently robust, but not so thick as to disrupt the flow of fluid (e.g. an electrolyte) across the microelectrode when the microelectrode is in use.
• fluid e.g. an electrolyte
• the electrically non-conductive material that at least partially or completely covers the first ends of the connection members may have a thickness of 10 m or less, alternatively 3 ⁇ or less, alternatively, the thickness is 1 ⁇ or less, alternatively 300 nm or less, alternatively 100 nm or less, alternatively 30 nm or less, alternatively 10 nm or less.
• the electrically non-conductive material that at least partially or completely covers the first ends of the connection members may have a thickness of 3 nm or more, alternatively 10 nm or more, alternatively 30 nm or more, alternatively 100 nm or more, alternatively 300 nm or more, alternatively 1 ⁇ or more, alternatively 3 ⁇ or more.
• the electrically non-conductive material that at least partially or completely covers the first ends of the connection members may be any suitable material.
• the electrically non-conductive material that at least partially or completely covers the first ends of the connection members comprises electrically non-conductive diamond-like carbon (DLC).
• DLC electrically non-conductive diamond-like carbon
• connection members comprise electrically conductive diamond material
• electrically non-conductive DLC is chemically compatible with the diamond material of the connection members and also has a similar level chemical resistivity.
• the covering material is DLC
• the nature of the covering material does not significantly restrict the nature of the chemical environments in which the microelectrode can be used.
• the use of DLC is advantageous as it adheres well to diamond, being chemically very similar and its thermal expansion coefficient is also very similar to that of diamond such that there is a low possibility of delamination of the DLC layer from the underlying material.
• An example of a method for applying a DLC layer that is suitable for use in the current invention is disclosed in WO2008/099220.
• the compatibilities of DLC to diamond material of the plate and in some embodiments the connection members are particularly applicable to CVD diamond material which is used in some embodiments as the conductive diamond material in the recesses making up the working electrodes, and in the cavities comprising the connection members.
• the microelectrode of the current invention When the microelectrode of the current invention is in use for characterising an electrochemical reaction, the microelectrode may be arranged such the first ends of the connection members are not exposed to the electrolyte.
• the cavities in the diamond plate that are in communication with the working electrodes can have any suitable geometry.
• a cavity consists of a hole of approximately cylindrical or truncated cone shape extending from the second surface of the electrically non-conductive diamond plate.
• the diameter may be between about 5 ⁇ and 1 mm, alternatively between about 5 ⁇ and 100 ⁇ .
• the taper of the cone may be typically between 0.1 ° and 20°.
• the cavity may be approximately cylindrical or truncated cone shaped for its entire extent or a combination of the cylindrical for part of its extent or truncated cone for part of its extent.
• the cavity extends from the second surface to a working electrode, the cavity has a substantially circular cross-section parallel to the second surface of the electrically non-conductive diamond plate and a stepped profile perpendicular to the second surface.
• cavities are also envisaged, for example those of rectilinear profile or any other regular or irregular profile.
• the cavities may be formed by any suitable process.
• cavities are formed by a laser drilling process using laser operating at a wavelength of approximately 1 ⁇ (for example Nd:YAG). This process is also known as "laser trepanning".
• Laser trepanning This process is also known as "laser trepanning".
• the recesses in the first surface of the electrically non-conductive diamond plate may be formed, for example, by a plasma etching process.
• Inductively-coupled plasma etching is suitable for performing the plasma etching.
• the inductively-coupled plasma etching process utilises a gas mixture comprising argon and a halogen-containing gas.
• the halogen- containing gas is chlorine; the use of such a process and gas mixture for the etching of diamond surfaces is disclosed in WO2008/09051 1 .
• the recesses may be formed by a laser processing technique.
• An example of a laser processing technique that is capable of ablating diamond and is therefore suitable for forming the recesses of the invention is a deep ultra- violet (UV) argon-fluorine (Ar-F) excimer laser, which produces deep UV radiation at a wavelength of 193 nm.
• UV deep ultra- violet
• Ar-F argon-fluorine
• Electrically conductive diamond material may be deposited in the recesses by a chemical vapour deposition process. Suitable processes include microwave assisted plasma chemical vapour deposition and hot filament chemical vapour deposition. Other chemical vapour deposition processes are not excluded.
• the electrically conductive diamond material may be deposited from a process gas comprising hydrogen, methane (or other hydrocarbon) and diborane (or other gaseous boron containing gas).
• methods involve depositing electrically conductive material so that it not only lies in the recesses but also extends over at least part of the surface of the electrically non-conductive diamond plate around the recesses.
• the deposition step may also involve overfilling the recesses so that the electrically conductive material extends above the level of the electrically non-conductive diamond plate above the recess.
• the method may involve planarising the surface after electrically conductive diamond material has been deposited. Planarising may be performed so as to make the surface of the working electrode flush with the electrically non-conductive diamond plate on either side of the recesses. Planarisation may also generally expose the electrically non- conductive diamond plate on either side of the recess.
• Planarisation may in some embodiments be to the original first surface of the electrically non-conductive diamond plate. Planarisation may in other embodiments be further, and be to a depth below the original first surface of the electrically non-conductive diamond plate, i.e. into the thickness of the electrically non-conductive diamond plate.
• the planarisation process may be performed by any suitable means. Suitable processes include conventional lapidary processes such as lapping with diamond grit and diamond polishing using a diamond-loaded resin-bonded wheel.
• the electrically conductive diamond material is generally laid down to provide the working electrodes in a desired electrode pattern.
• the planarisation process may remove sufficient depth of material to expose that electrode pattern, but not so much that the desired electrode pattern is partially or completely removed.
• the initial thickness of the electrically non-conductive diamond plate is desirable. It is advantageous for the first and second surfaces of the initial electrically non-conductive diamond plate to be substantially parallel in order to facilitate precise and accurate planarisation.
• the angular separation between the normals to the first surface and the second surface of the electrically non-conductive diamond plate may be 3° or less, alternatively 2° or less, alternatively 1 ° or less, alternatively 0.8° or less, alternatively 0.5° or less, alternatively 0.2° or less, alternatively 0.1 ° or less, alternatively 0.05° or less.
• the variation in thickness of the initial electrically non-conductive diamond plate may be 52 m/mm or less, alternatively 35 m/mm or less, alternatively 17 m/mm or less, alternatively 14 m/mm or less, alternatively 9 m/mm or less, alternatively 3.5 m/mm or less, alternatively 1 .7 m/mm or less, alternatively 0.9 m/mm or less.
• the electrically non-conductive diamond plate may be cleaned to remove any debris from the recess preparation process.
• An example of a suitable cleaning process is a mixture of hot concentrated sulphuric acid and potassium nitrate; other cleaning processes are possible and may be used.
• any debris may be removed by a cleaning process, for example using a mixture of hot concentrated sulphuric acid and potassium nitrate; other cleaning processes are possible and may be used.
• the electrically conductive diamond material is deposited into one or more recesses using a CVD process.
• the method involves at least partially filling the cavities in the second surface of the electrically non-conductive diamond plate with electrically conductive material other than electrically conductive diamond material.
• the recesses, cavities in the electrically non- conductive diamond plate, electrically conductive diamond material and electrically non-conductive material and other features have the characteristics set out above with respect to the description of the microelectrode and its features.
• Some embodiments of the method may include the step of depositing a layer of electrically non-conductive material so as to at least partially cover the cover the first ends of the connection members.
• the electrically non-conductive material may be electrically non-conductive diamond-like carbon. Where the electrically non-conductive material is electrically non-conductive diamond-like carbon the thickness of the layer is 10 nm or greater. Where the electrically non-conductive material is electrically non-conductive diamond-like carbon the thickness of the layer is 2 ⁇ or less.
• Figure 1 a is a perspective view which shows an embodiment of a microelectrode for electrochemical use according to the invention having two working electrodes;
• Figure 1 b is a cross section through the microelectrode of figure 1a along AA';
• Figure 1c is a cross section through the microelectrode of figure 1a along BB';
• Figure 2 is a view showing the microelectrode figure 1 connected to an external circuit and immersed in an electrolyte;
• Figure 3a-e is a sequence of cross sectional views showing in schematic outline the steps for the fabrication of the microelectrode of figure 1. Detailed Description of Certain Embodiments
• Figures 1 a to c show an embodiment of microelectrode 1 .
• the microelectrode 1 for electrochemical use comprises a planar, rectangular, electrically non-conductive polycrystalline CVD diamond plate 2, having a first surface 4 and a second surface 6 that is substantially parallel to the first surface.
• Two discrete recesses 16, 18 are arranged in the first surface, 2, in the pattern of the working electrodes (also referred to herein as the "electrode pattern").
• the recesses are approximately 15 ⁇ deep, approximately 50 ⁇ in width and are separated from each other by approximately 1 mm.
• the side surfaces of the recesses are at an angle of 90° to the first surface.
• the recesses contain electrically conductive diamond material and this material thereby forms two working electrodes 8 and 10.
• the microelectrode 1 contains two connection members 12 within the diamond plate 2 that extend from the second surface 6 to a working electrode 8, 10.
• Each connection member comprises a cavity 14 in the second surface 6 and has a surface 16.
• cavity 14 having surface 16 consists of a series of concentric cylindrical sections and/or truncated conical sections of progressively smaller diameters extending from the second surface 6 to each working electrode 8, 10.
• microelectrode 1 of fig 1 may be used for monitoring electrochemical reactions. Such an embodiment is shown schematically in figure 2.
• microelectrode 1 is immersed in an electrolyte 42 and connected by means of electrical connections 44 to a suitable external circuit 48 that enables a potential difference to be imposed between the working electrodes 8, 10 (not shown in fig 2) and also the current that flows between the working electrodes to be monitored. This enables, for example, electrochemical oxidation and reduction reactions to be monitored.
• a method of making the microelectrode 1 of fig 1 is described making reference to fig 3. Any or all of the optional features described with reference to fig 3 may be used herein in any combination.
• the first step of the method shown in fig 3a comprises providing an electrically non-conductive diamond plate 50 having a first surface 52 and an opposed second surface 54.
• the first surface 52 and second surface 54 are substantially parallel.
• step 56 at least two separate recesses 58, 60 are formed in the first surface 52.
• the recesses are approximately 15 ⁇ deep, approximately 50 ⁇ wide when measured at the first surface 52 and the side surfaces of the recess are at an angle of 90° to the first surface 52.
• the at least two separate recesses 58, 60 are formed such that they are in a preselected electrode pattern.
• the recessed plate is shown in fig 3b.
• electrically conductive diamond material 70 is deposited in recesses 58, 60.
• the electrically conductive diamond material 70 is deposited by a CVD technique over the whole of the first surface 52 of the diamond plate 50 such that the whole surface is covered with electrically conductive diamond material to approximately the same.
• CVD processes the material is built up as a layer, the thickness of which increases with the duration of the deposition process.
• the layer of material that is deposited substantially conforms to the profile of the surface upon which it is being deposited.
• the plate with electrically conductive diamond deposited on its surface is shown fig 3c.
• step 72 the upper surface (in the orientation shown in Fig 3d) is planarised by removing deposited diamond material 70 down to the depth of the first surface 52 of the electrically non-conductive diamond plate 50, or further, thereby providing discrete exposed electrodes 74, 76 surrounded by electrically non-conductive diamond material on the planarised surface 92.
• the planarised surface 92 may correspond to the first surface 52 of the plate 50 or be lower (in the orientation shown) than that surface. Because the side surfaces of the recesses are at an angle of 90° to the first surface 52, the process can tolerate the planarised surface 92 being lower than the first surface 52 without the separation between the electrodes 77 changing.
• the planarisation process of step 72 may be performed by any suitable means. Suitable processes include conventional lapidary processes such as lapping with diamond grit and diamond polishing using a diamond-loaded resin-bonded wheel. Non-abrasive processes, for example plasma processes or laser processes, may also be used for planarisation.
• connection members between second surface 54 and working electrodes 74, 76 are formed in step 80.
• a cavity 82 having surface 86 is formed for each connection member extending between the second surface 54 and the working electrodes 74, 76.
• the shape of the cavity is a series of coaxial truncated conical sections of progressively smaller diameters.
• the cavity may be formed by laser drilling ("trepanning").
• Surfaces 86 of the cavities can be metallised with one or more metal layers. Such metal layers may be applied by sputtering and an example of a suitable combination of layers is a Ti base layer, a Pt barrier layer and an Au bonding layer.
• a wire (not shown) providing electrical connection to an external circuit may be affixed into the cavity using a silver-loaded epoxy resin (or by any other suitable means, such as soldering or brazing).
• the electrical connection may be encapsulated such that when the microelectrode is in use, access by the electrolyte to any conductive material that forms part of the electrical connection is not possible.

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Citations (8)

• 2012
  • 2012-09-17 WO PCT/EP2012/068213 patent/WO2014040650A1/fr not_active Ceased

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