Method for the production of dihydrogen using oxidized nanodiamonds as photocatalysts

Abstract

The present invention relates to a method for producing dihydrogen by photodissociation of water, comprising at least a step of bringing an aqueous solution in contact with oxidized nanodiamonds under solar, natural or artificial illumination (or light).

Description

[0001] PROCESS FOR PRODUCING DIHYDROGEN USING OXIDIZED NANODIAMONDS

[0002] AS PHOTOCATALYSTS

[0003] DESCRIPTION

[0004] TECHNICAL FIELD

[0005] The present invention relates to the general technical field of nanomaterials and, more particularly, to nanomaterials for photocatalysis and in particular for the production of dihydrogen (Hz) by photo-induced dissociation of water, a process known by the English expression "water splitting".

[0006] Indeed, the present invention proposes to use as photocatalysts, diamond nanoparticles or oxidized nanodiamonds. In other words, the present invention proposes a method for producing dihydrogen from water, comprising a step of bringing an aqueous solution into contact with oxidized diamond nanoparticles under light irradiation. The present invention also relates to a particular photocatalytic composition comprising oxidized nanodiamonds and at least one other (photo)catalyst.

[0007] STATE OF THE PRIOR ART

[0008] Due to the increasing demand for energy on the one hand and the decrease in fossil fuels such as coal, oil and natural gas on the other hand, particular interest has been paid to dihydrogen (Hz) which has several advantages over fossil fuels.

[0009] First of all, the combustion of H2O only produces water (H2O) unlike fossil fuels whose combustion emits carbon dioxide (CO2), thus contributing to global warming.

[0010] Furthermore, to avoid the production of H2 from natural gas, which also leads to the production of CO2, alternative, sustainable and less environmentally harmful solutions have been proposed, including the production of H2 by photocatalysis of water. In this process, the dissociation of water induced by photons, i.e. under light irradiation, produces H2 which can be used directly for combustion, in fuel cells, for applications in chemical or petrochemical processes or even stored, particularly in liquid form.

[0011] Photocatalytic H? production processes are known in the state of the art.

[0012] Photocatalysis is based on the principle of activating a semiconductor or a set of semiconductors such as a photocatalyst, using the energy provided by irradiation.

[0013] A semiconductor is characterized by its band gap (or, in English terminology, "band-gap") between the valence band and its own conduction band. Any photon with energy greater than its band gap can be absorbed by the semiconductor. Conversely, any photon with energy lower than its band gap cannot be absorbed by the semiconductor.

[0014] Photocatalysis can be defined as the absorption of a photon, whose energy is greater than the band gap, which induces the formation of an electron-hole pair in the case of a semiconductor. We therefore have the excitation of an electron at the conduction band and the formation of a hole on the valence band. This electron-hole pair allows the formation of free radicals which will either react with compounds present in the medium such as H2O, in order to initiate oxidation-reduction reactions, or recombine according to various mechanisms.

[0015] It is therefore necessary to have efficient photocatalysts for the production of H2 by dissociation of water, in particular using sunlight, which must: a) have an adequate band structure for the generation of charge carriers that will allow oxidation and reduction reactions on its surface aimed at the production of H2 from protons resulting from the oxidation of water by photogenerated holes, b) present the best possible absorption of sunlight and a low recombination of the photogenerated charge carriers to guarantee optimal efficiency of the photocatalytic reaction, c) be available in large quantities, at a low cost and environmentally friendly, which greatly limits the use of metallic particles such as platinum particles, rare earth oxides, etc., and d) remain efficient in operation.

[0016] To take these constraints into account, it is therefore of interest to develop photocatalysts, on the one hand, nanometric to access a large developed surface, and, on the other hand, semi-conductors to photo-generate charges, with conduction and valence band positions whose energy levels are adapted to the oxidation of water and the reduction of protons, and whose band gap energy is adapted to visible wavelengths.

[0017] In the literature, a number of materials have already been identified as photocatalysts for the production of H? by water dissociation. Titanium dioxide (TiO?) with anatase structure is certainly the most studied semiconductor, due to certain properties favorable to this reaction, in particular, (i) its photostability in water, (ii) the adequate position of its valence band allowing easy initiation of the first step of water oxidation (H2O -> 2H ++ % O2 + 2 e , (iii) acceptable charge carrier dynamics, (iv) its non-toxicity and (v) its relatively moderate cost, compared to other types of photocatalysts.

[0018] However, the main limitation of TiO2 lies in its large band gap (3.1-3.2 eV), requiring activation by wavelengths below 400 nm (UV range), which considerably restricts the use of natural sunlight which consists of about 40% visible photons, too low in energy and therefore ineffective to activate TiO2. In addition, due to the difficulty in carrying out the reduction half-reaction to hydrogen (2H + + 2nd _-> H2), rare and expensive noble metals are often added as co-catalysts to carry out the catalytic reduction to dihydrogen. Many strategies have already been implemented to overcome these limitations such as modifications of composition, morphology, chemical structure, size, surface, deposition of metal nanoparticles developing surface plasmonic properties, as well as coupling (formation of heterojunctions) with other semiconductors, allowing the induction of favorable electronic, optical or chemical effects.

[0019] Diamond-based nanomaterials have also been proposed as photocatalysts. They are often used in the form of hybrids or composites.

[0020] Thus, in 2016, Lin et al proposed p-type copper(l) oxide nanocrystals integrated with nanodiamonds for broad-spectrum photocatalytic hydrogen evolution [1].

[0021] International application WO 2016 / 193464 A1 describes the use of a photocatalytic composite comprising at least one semiconductor compound with a band gap ranging from 2 to 5 eV and diamond nanoparticles whose surface is advantageously hydrogenated [2].

[0022] In other state-of-the-art composites useful not for water photocatalysis but for water decontamination, nanodiamonds possibly doped with boron are implemented with graphitic carbon nitride (or g-C3N4) [3], possibly in association with silver nitrate (AgNOs) [4].

[0023] Only one study in the literature reports a dihydrogen production effect by water dissociation with nanodiamonds alone [5]. However, in this work, a pulsed laser of wavelength 532 nm, with high power (80 mJ / pulse), is used. There is no evidence that such a material can be used for water dissociation using the spectrum of sunlight which has a much lower power. Moreover, it is clear from [5] that the use of nanodiamonds with hydrogenated surfaces is preferred since hydrogenation significantly increases the quantum yield, suggesting that the hydrogen-terminated sites function as electron reservoirs.The inventors set themselves the goal of proposing photocatalysts capable of producing H? from water using sunlight as a source of photons, without requiring the use of co-catalysts such as noble metals or complicated treatments such as hydrogenation.

[0024] STATEMENT OF THE INVENTION

[0025] The present invention achieves the inventors' goal. Indeed, they have shown that it is possible to dissociate water under light irradiation to produce dihydrogen using oxygenated diamond nanoparticles as photocatalysts.

[0026] On the one hand, the oxidized nanodiamonds can be used without the addition of other photocatalysts or co-catalysts such as metal particles. In other words, in the method according to the invention, the oxidized nanodiamonds can be used as the sole photocatalysts.

[0027] As a reminder, diamond is a wide-bandgap semiconductor (5.5 eV) and is therefore theoretically not suitable for absorbing visible light. However, in the form of nanoparticles, its light absorption properties are modified either by optical effects linked to the nanometric dimensions, or by the incorporation of structural defects during their synthesis.

[0028] Furthermore, the inventors' work shows that the effect is present for a nanodiamond with an oxidized surface. Depending on its surface chemistry, the nanodiamond's band diagram evolves. A hydrogen-saturated diamond surface will have more negative conduction and valence band edge electrochemical potentials than a diamond surface saturated with oxidized functions. This is illustrated in Figure 1 for bulk diamond.

[0029] Thus, the most reducing structure (i.e. hydrogenated diamond) is not the most favorable for enabling the production of H2. This is counterintuitive for those skilled in the art. On the contrary, the inventors have shown that it is necessary to use oxidized nanodiamonds to maximize the production of dihydrogen. More particularly, the present invention relates to the use of oxidized nanodiamonds as photocatalysts for the production of dihydrogen. This production of dihydrogen is obtained under solar illumination (or light), natural or artificial.

[0030] In other words, the present invention relates to a process for producing dihydrogen by photodissociation of water, comprising at least one step of bringing an aqueous solution into contact with oxidized nanodiamonds under light irradiation, i.e. under natural or artificial solar illumination (or light).

[0031] The terms "photodissociation of water", "(photo)dissociation of water under light irradiation", "photocatalysis of water" and "(photo)catalysis of water under light irradiation" are equivalent and can be used interchangeably in this manuscript.

[0032] Similarly, the expressions "under solar illumination, natural or artificial", "under sunlight, natural or artificial" and "under solar irradiation, natural or artificial" are equivalent and can be used interchangeably in this manuscript.

[0033] The present invention uses nanodiamonds, i.e., diamond in the form of nanoparticles. These nanodiamonds can be obtained from natural diamond or synthetic diamond. A synthetic diamond is typically obtained by high-pressure high-temperature (HPHT) synthesis or by chemical vapor deposition (CVD).

[0034] They can more particularly be obtained (i) by grinding solid diamond, natural or synthetic; (ii) by detonation in particular as described in international application WO 2016 / 193464 A1 [2], (iii) directly by HPHT or CVD growth.

[0035] The average size of the diamond nanoparticles used in the present invention is between 1 and 500 nm, in particular between 1 and 200 nm, in particular between 2 and 100 nm and, more particularly, between 5 and 50 nm. In a particular embodiment, the diamond nanoparticles used in the present invention may be doped, in particular doped with nitrogen or phosphorus (n-type doping) or doped with boron (p-type doping).

[0036] As previously explained, the nanodiamonds implemented in the present invention are oxidized, i.e., surface oxidized. In other words, the surface of the oxidized nanodiamonds has more oxygen atoms than the surface of the non-oxidized nanodiamonds.

[0037] Typically, the oxidized nanodiamonds used in the invention have an oxygen / carbon ratio of at least 5 atomic% determined by XPS (photoemission spectroscopy) without prior treatment of the oxidized nanodiamonds such as annealing to dehydrate.

[0038] To obtain oxidized nanodiamonds, these nanodiamonds must be subjected to an oxidizing treatment.

[0039] The oxidizing treatment aims to oxidize the surface of the nanodiamonds by fixing and / or introducing, onto the latter, groups, identical or different, rich in oxygen, i.e. groups, identical or different, comprising at least one oxygen atom. In the present invention, a group comprising at least one oxygen atom is in particular chosen from the group consisting of a carboxylic group (-C(=O)OH), a hydroxyl group (-OH), a carbonyl group (-C(=O)-) and a percarbonic group (-C(=O)-O-OH).

[0040] Such oxidizing treatment is based on two main types of surface modifications based on:

[0041] - physical treatments such as plasma treatment (radio frequencies, microwaves), UV treatment, X-ray or gamma ray treatment, electron and heavy ion irradiation treatment, these different physical treatments being able to be carried out under CO2 or under an oxygenated atmosphere such as, for example, under air, under O2, under O2 / Argon or under ozone such as, for example, UV treatment under ozone;

[0042] - chemical treatments such as treatment with alcoholic potash, treatment with a mixture of sulfuric acid (H2SO4) and nitric acid (HNO3) hot or cold, treatment with a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) also known as "piranha mixture", treatment with a mixture of H2O2 and iron also known as "Fenton's reagent", treatment with a strong acid (HCl, H2SO4, HNO3, HCIO4), treatment with soda, treatment with a strong oxidant (KMnO4, C^C , KCIO3 or Crus in hydrochloric acid, sulfuric acid or nitric acid), treatment with molten salts (KNO3), ozone treatment and heat treatment under CO2 or in an oxygenated atmosphere such as, for example, under air, under O2, under 02 / Argon or under ozone.

[0043] Oxidizing treatments as described above can be carried out either at atmospheric pressure and / or at room temperature (Tamb) or at a pressure higher than atmospheric pressure and / or at a temperature higher than room temperature (particularly in the case of heat treatments). By "room temperature" is meant a temperature of the order of 23°C (i.e. 23°C ± 5°C).

[0044] Advantageously, the oxidizing treatment implemented is a heat treatment under an oxygenated atmosphere such as, for example, under air, under O2, under O2 / Argon or under ozone. The latter advantageously consists of annealing at a temperature of 500°C ± 50°C for a duration of between 1 h and 5 h and in particular between 1 h and 3 h, under an oxygenated atmosphere such as, for example, under air, under O2, under O2 / Argon or under ozone, and in particular under air. The experimental part below illustrates such a heat treatment. Consequently, the method according to the invention may have a prior step of preparing the oxidized nanodiamonds consisting of subjecting nanodiamonds to an oxidizing treatment as previously defined.

[0045] Typically, once the oxidative treatment is carried out, the oxidized nanodiamonds are resuspended. This consists of subjecting the oxidized nanodiamonds to sonication and then to centrifugation, whereby a colloidal suspension of oxidized nanodiamonds is obtained.

[0046] Prior to sonication, the oxidized nanodiamonds are brought into contact with an aqueous solution. Such a solution has, as solvent, a water-based solvent, thus justifying the name aqueous solution. By "water", is meant, in the context of the present invention, tap water, deionized water, distilled water or even ultra-pure water (18.2 MQ.cm at 25°C).

[0047] Typically, the quantity of oxidized nanodiamonds used when contacting with the aqueous solution prior to sonication is between 1 g / l of aqueous solution and 50 g / l of aqueous solution, in particular between 10 g / l of aqueous solution and 40 g / l of aqueous solution and, in particular, of the order of 30 g / l of aqueous solution (i.e. 30 g / l ± 5 g / l).

[0048] The sonication step is carried out at a temperature between 4°C and 20°C, in particular between 6°C and 15°C and, in particular, of the order of 10°C (i.e. 10°C ± 2°C). Advantageously, a thermostatically controlled bath is used during the sonication step. In addition, the sonication step lasts between 15 min and 3 h, in particular between 30 min and 2 h and, in particular, of the order of 1 h (i.e. 1 h ± 15 min).

[0049] The centrifugation step is implemented to separate the colloidal suspension of oxidized nanodiamonds corresponding to the supernatant obtained at the end of the centrifugation step from the aggregates of oxidized nanodiamonds forming the pellet obtained at the end of the centrifugation step. To do this, the centrifugation step is carried out at a value between 1500 g and 4000 g, in particular between 2000 g and 3000 g and, in particular, of the order of 2400 g (i.e. 2400 g ± 200 g).

[0050] The centrifugation step is carried out at a temperature between 4°C and 20°C, in particular between 6°C and 15°C and, in particular, of the order of 10°C (i.e. 10°C ± 2°C). In addition, the centrifugation step lasts between 15 min and 2 h, in particular between 30 min and 1 h and, in particular, of the order of 40 min (i.e. 40 min ± 5 min).

[0051] The oxidized nanodiamonds are brought into contact during step a) with an aqueous solution as previously defined.

[0052] In a particular embodiment, the solvent of the aqueous solution used during step a) comprises only water, i.e. this solvent is made up of water and in particular ultra-pure water (18.2 MΩ.cm at 25°C).

[0053] Typically, the quantity of oxidized nanodiamonds used when contacting with the aqueous solution is between 1 mg / l of aqueous solution and 1 g / l of aqueous solution, in particular between 5 mg / l of aqueous solution and 500 mg / l of aqueous solution, in particular, between 10 mg / l of aqueous solution and 50 mg / l of aqueous solution and, more particularly, of the order of 12.5 mg / l of aqueous solution (i.e.

[0054] 12.5 mg / l ± 1 mg / l).

[0055] The contact between the aqueous solution and the oxidized nanodiamonds can be carried out under stirring and / or under inert gas such as argon, nitrogen, helium or one of their mixtures, in continuous or discontinuous flow. Advantageously, this contact is carried out under stirring and under a continuous flow of nitrogen.

[0056] Typically, the contact between the aqueous solution and the oxidized nanodiamonds is carried out at a temperature between 5°C and 80°C, in particular between 15°C and 50°C and, more particularly, at room temperature.

[0057] Upon contact, the oxidized nanodiamonds may be in the form of a suspension. Alternatively, upon contact, the oxidized nanodiamonds may be supported.

[0058] Any type of support conventionally used to retain photocatalysts in a process for dissociating water under light irradiation can be used in the context of the present invention. Illustrative examples include 2D supports such as, for example, textile webs and in particular textile webs made of optical fibers; surface coatings such as, for example, paints; and dense or porous 3D supports such as, for example, foams or honeycombs.

[0059] The contact between oxidized nanodiamonds and aqueous solution is done under light irradiation. This light irradiation can be natural (sunlight) or artificial, in particular by means of an irradiation device such as a lamp, a UV lamp, a visible lamp, a UV-visible lamp, an IR lamp, an excimer lamp, an LED, a laser, a laser diode or a supercontinuum fiber source. It is obvious that, in the context of "artificial solar irradiation", the irradiation device used reproduces the spectrum of sunlight and its power.

[0060] Advantageously, the irradiation device has an irradiance of between 25 mW / cm 2 at 150 mW / cm 2 and in particular of the order of 53.5 mW / cm 2 ± 5 mW / cm 2 In other words, "under light irradiation" means under natural or artificial solar illumination (or light).

[0061] The radiation used during this light irradiation can be UV radiation (wavelength ranging from 200 to 400 nm), visible radiation (wavelength ranging from 400 to 800 nm) or near IR radiation (wavelength ranging from 800 to 1200 nm) and one of their combinations.

[0062] Advantageously, the light irradiation used in the method of the invention is natural light irradiation, i.e. natural solar illumination (or light).

[0063] In the context of the present invention, the aqueous solution may further contain a sacrificial agent. Typically, the sacrificial agent used in the invention is an electron donor agent, capable of being oxidized by the oxygen formed during the water photodissociation reaction and, in fact, makes it possible to improve the yield of H2 production.

[0064] The sacrificial agent used in the process according to the invention is typically chosen from the group consisting of amines and alcohols and more particularly from the group consisting of methanol, ethanol, triethanolamine (TEOA) and one of their mixtures. Advantageously, the sacrificial agent used in the context of the process according to the invention is TEOA or methanol.

[0065] When present, the sacrificial agent is present in an amount of between 0.05% and 50% by volume relative to the volume of aqueous solution and, in particular, between 0.1% and 1% by volume relative to the volume of aqueous solution.

[0066] In a particular form of implementation, oxidized nanodiamonds are the only photocatalysts used in the process.

[0067] Alternatively, in another particular embodiment, the oxidized nanodiamonds are used with at least one other element selected from the group consisting of photocatalysts, catalysts, adsorbents and combinations thereof. Thus, the oxidized nanodiamonds can be combined with at least one other photocatalyst of the inorganic, molecular (quantum dots and clusters) or organic (dyes) photocatalyst type in a heterojunction, and / or with at least one catalyst of the metallic, inorganic or molecular (quantum dots and clusters) catalyst type and / or with at least one adsorbent such as activated carbon or MOFs (for "Metal Organic Frameworks") which improve the adsorption of species and the transfer of charges, these adsorbent materials also being able to have semiconducting properties like certain MOFs.In this variant, the oxidized nanodiamonds and the other element(s) selected from the group consisting of photocatalysts, catalysts, adsorbents and their combinations form a catalytic composition. Advantageously, this catalytic composition does not comprise platinum or other noble metal.

[0068] Any photocatalyst of the semiconductor or molecular type, known to those skilled in the art and used in water photodissociation processes to produce H? can be used in the context of the present invention.

[0069] However, in a particular embodiment of the method according to the invention, the photocatalyst associated with the oxidized nanodiamonds does not contain graphitic carbon nitride.

[0070] Advantageously, the photocatalyst(s) implemented with the oxidized nanodiamonds is / are chosen from the group consisting of transition metals, transition metal derivatives, metal carbides, metal nitrides, metal oxides, metal sulfides. In particular, the photocatalyst(s) implemented with the oxidized nanodiamonds is / are chosen from the group consisting of transition metal oxides and transition metal sulfides. In particular, the photocatalyst(s) implemented with the oxidized nanodiamonds is / are chosen from the group consisting of TiOz, TiOz-B (in the form of a titanate sheet), ZnO, WO3 and FezOs.

[0071] When used with oxidized nanodiamonds, the (photo)catalyst(s), catalyst(s) and / or adsorbent(s) may be in dispersed form and in particular in the form of nanoparticles. The average size of these nanoparticles is between 1 and 1000 nm, in particular between 2 and 200 nm, in particular between 3 and 100 nm, more particularly between 4 and 50 nm and, most particularly, between 5 and 20 nm. Alternatively, the (photo)catalyst(s), catalyst(s) and / or adsorbent(s) may be used in aggregated form.

[0072] The present invention also relates to a photocatalytic composition used in the method according to the invention. This photocatalytic composition comprises oxidized nanodiamonds and at least one other element selected from the group consisting of photocatalysts, catalysts, adsorbents and combinations thereof, said photocatalytic composition not comprising graphitic carbon nitride.

[0073] In the photocatalytic composition according to the invention, the oxidized nanodiamonds are present in an amount of between 0.1 and 80% by mass, in particular between 1 and 50% by mass, in particular between 2 and 30% by mass relative to the amount of said at least one other element chosen from the group consisting of photocatalysts, catalysts, adsorbents and combinations thereof.

[0074] Other characteristics and advantages of the present invention will become apparent upon reading the examples below given for illustrative and non-limiting purposes and referring to the appended figures.

[0075] BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 shows the energy band diagram of hydrogenated and oxidized diamond compared to other semiconductors [6].

[0077] Figure 2 presents the dynamic light scattering (DLS) size distribution of oxidized nanodiamonds prepared as described in point I below.

[0078] Figure 3 shows the Fourier transform infrared absorption spectrum of oxidized nanodiamonds prepared as described in point I below.

[0079] Figure 4 shows the XPS analysis of oxidized nanodiamonds prepared as described in point I below.

[0080] Figure 5 shows the comparison of the average H? production rate by photocatalytic splitting of water under artificial solar irradiation in the presence of 10 mg of oxidized nanodiamonds “Plasma Chem” (2 repeatability tests) or TiO? P25 (Evonik), with 1 vol. % of TEOA (sacrificial agent). Figure 6 shows the average H? production rate by photocatalytic splitting of water under artificial solar irradiation in the presence of 10 mg of oxidized nanodiamonds “Plasma Chem”, as a function of the TEOA content (sacrificial agent).

[0081] Figure 7 shows the average rate of H? production by photocatalytic dissociation of water under artificial solar irradiation in the presence of 1 vol. % TEOA (sacrificial agent), as a function of the content of oxidized nanodiamonds “Plasma Chem”.

[0082] DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0083] I. Preparation of oxidized nanodiamonds.

[0084] The diamond nanoparticles used were synthesized by detonation and obtained from the company PlasmaChem (Germany).

[0085] The diamond nanoparticles were first oxidized by air annealing under atmospheric pressure according to the following protocol:

[0086] - 2 crucibles each filled with 200 mg of untreated nanoparticles are placed in the center of a tubular furnace;

[0087] - the oven temperature is brought from Tamb to 200°C with a temperature ramp of 20°C / min then the nanoparticles are maintained at 200°C for 15 min;

[0088] - the oven temperature is brought from 200°C to 500°C with a temperature ramp of 30°C / min then the nanoparticles are maintained at 500°C for 1h30;

[0089] - the two crucibles are then removed “hot” using pliers and placed in Tamb to cool the nanoparticles as quickly as possible and thus stop the annealing;

[0090] - when the nanoparticles are returned to Tamb, they are weighed. The loss generally observed is about 50% of the initial mass.

[0091] The nanoparticles are then suspended according to the following protocol: - 100 mg of oxidized nanoparticles are placed in a 15 mL Falcon centrifuge tube;

[0092] - 3 mL of ultra-pure water (18.2 MQ.cm) are added;

[0093] - the temperature of the solution is brought to 10°C in a thermostatic bath;

[0094] - the solution is then sonicated using a Cup Horn device (Bioblock Scientific 750 W, amplitude 60%, cycle 1sec ON / 1sec OFF, duration 60 min), maintaining the temperature at 10°C,

[0095] - following sonication, the solution is then centrifuged (2400g, 40 min) in order to remove the largest aggregates,

[0096] - immediately after centrifugation, the supernatant containing the nanoparticles in colloidal suspension is removed by pipetting (approximately 2.5 mL of the initial 3 mL),

[0097] - the suspension is then kept in a plastic bottle at room temperature, away from light. This storage can be carried out over long periods, i.e. more than 1 year.

[0098] The nanoparticle concentration of the suspension is determined by drying, at Tamb, overnight, 100 pL of suspension and measuring the mass of the dry residue.

[0099] II. Characterization of oxidized nanodiamonds.

[0100] It .1. Methods.

[0101] - Measurement of the hydrodynamic diameter of suspended objects by dynamic light scattering (DLS) on 1 mL of suspension at 1 mg of nanoparticles / mL (HORIBA SZ-100 Nanopartica Series).

[0102] - Measurement of the Zeta potential of suspended objects by electrophoretic light scattering on 700 pL of suspension at 1 mg of nanoparticles / mL (HORIBA SZ-100 Nanopartica Series).

[0103] - Characterization of the surface chemistry of suspended nanodiamonds by Fourier transform infrared spectroscopy: 2 pL of the suspension at 5 mg nanoparticles / mL are evaporated on the ATR crystal of a Bruker Alpha II spectrometer.

[0104] - Chemical characterization of suspended nanodiamonds by XPS and determination of the O / C ratio: 20 pL of the suspension at 1 mg of nanoparticles / mL are evaporated on a silicon substrate coated with 50 nm of gold. The analysis is carried out on a Kratos Analytical Axis Ultra DLD (monochromatized Al Kct source).

[0105] 11.2. Results.

[0106] As illustrated in Figure 2, the oxidized nanodiamonds exhibit, in suspension, a hydrodynamic diameter of 48 nm ± 10 nm and a Zeta potential of 57 mV ± 5 mV.

[0107] The infrared absorption spectrum of oxidized nanodiamonds provided in Figure 3 highlights the presence of C=O bonds (1750 cm 1 ) probably involved in carboxylic acids, as well as the presence of C-O bonds (1000-1300 cm 4 ) linked to alcohol functions or surface esters. We also note a shoulder between 3000 and 3500 cm 1reflecting the presence of OH bonds, linked to carboxylic acids. Indeed, the spectrum is recorded under dry nitrogen flow, so there is no contribution of OH from ambient humidity. Finally, it is important to note the absence of absorption between 2800 and 3000 cm 4 , which means the absence of C-H bonds on the surface of the particles.

[0108] XPS analysis highlights the presence of three elements in the oxidized nanodiamonds, namely carbon, oxygen and nitrogen (Figure 4). The latter is located mainly in the core of the nanoparticle and comes from the nitrogen explosives used during the detonation synthesis. The atomic proportions of each element are given in Table 1 below.

[0109] Table 1

[0110] III. Photocatalytic behavior of oxidized nanodiamonds.

[0111] 111.1. Procedure. To study the photocatalytic behavior of oxidized diamond nanoparticles, our experiments consisted of continuously illuminating the suspension of nanodiamonds (NDs), placed under a continuous flow of inert gas (Nz) with a lamp reproducing the solar spectrum and with a power of 150 W corresponding to an irradiance of 53.5 W / cm 2 (Spatite Hit 150 G12 8800 K (Art-nr 226224)) nanodiamond suspensions in the presence of a low proportion of antioxidant / sacrificial agent (triethanolamine or TEOA). The experiments were carried out at Tamb by controlling the suspension temperature throughout the photocatalytic test.

[0112] The quantification of the dihydrogen produced was carried out, at the stabilization of hydrogen production, by an online measurement by gas chromatography for 2 h. It is thus possible to follow the kinetics of formation of the Hz produced.

[0113] The experimental conditions are as follows: photocatalyst (10 or 20 mg) suspended in 800 mL ultrapure (mQ) H2O, TEOA (0.1 to 1 vol. % TEOA), magnetic stirring 700 rpm, continuous flow N2 100 cm 3 / min, analysis time after stabilization 2 h, analytical acquisition every 2.5 min.

[0114] III.2. Results.

[0115] Figure 5 shows that at the same concentration, the oxidized nanodiamonds (Plasma Chem) are just as efficient as the commercial reference photocatalyst TiO2 P25 (Evonik).

[0116] We also observe that the production of H2 is linked to the sacrificial agent content (Figure 6), however in a non-proportional manner, which confirms a production of H2 by solar photodissociation of water, the TEOA mainly playing the role of trap for O2.

[0117] Furthermore, the optimal content of oxidized nanodiamonds appears to be around 10 mg, or 12.5 mg / L (Figure 7).

[0118] Bibliographic references

[0119] [1] Lin et al, 2016, "Nanodiamond-Embedded p-Type Copper(l) Oxide Nanocrystals for Broad-Spectrum Photocatalytic Hydrogen Evolution", Adv. Energy Mater., vol. 6, 1501865. [2] International application WO 2016 / 193464 in the name of CNRS et al, published on December 8, 2016.

[0120] [3] Su et al, 2019, “Heterostructured boron doped nanodiamonds@g-C3N4 nanocomposites with enhanced photocatalytic capability under visible light irradiation”, Int J of Hydrogen Energy, vol. 44, 19805.

[0121] [4] Patent application CN 110639595 A in the name of Henan Inst Engineering, published on January 3, 2020.

[0122] [5] Jang et al, 2012, "Nanodiamonds as photocatalysts for reduction of water and graphene oxide", Chem. Comm., vol. 48, pages 696-698. [6] Nebel, 2013, "Photocatalysis: A source of energetic electrons", Nat.

[0123] Mater., vol. 12, pages 780-781.

Claims

CLAIMS 1) Use of oxidized nanodiamonds as photocatalysts in the production of dihydrogen, under natural or artificial solar illumination (or light). 2) Process for producing dihydrogen by photodissociation of water, comprising at least one step of bringing an aqueous solution into contact with oxidized nanodiamonds under natural or artificial solar illumination (or light). 3) Method according to claim 2, characterized in that said oxidized nanodiamonds have an oxygen / carbon ratio of at least 5 atomic% determined by XPS (photoemission spectroscopy) without prior treatment of the oxidized nanodiamonds. 4) Method according to claim 2 or 3, characterized in that said method has a prior step of preparing the oxidized nanodiamonds consisting of subjecting nanodiamonds to an oxidizing treatment. 5) Method according to claim 4, characterized in that said oxidizing treatment consists of annealing at a temperature of 500°C ± 50°C for a period of between 1 h and 5 h under an oxygenated atmosphere. 6) Method according to any one of claims 2 to 5, characterized in that said contacting between said aqueous solution and said oxidized nanodiamonds is carried out with stirring and / or under inert gas and in particular, with stirring and under a continuous flow of nitrogen. 7) Method according to any one of claims 2 to 6, characterized in that said light irradiation is natural light irradiation. 8) Method according to any one of claims 2 to 7, characterized in that said aqueous solution contains a sacrificial agent and, in particular, triethanolamine (TEOA) or methanol. 9) Method according to any one of claims 2 to 8, characterized in that said oxidized nanodiamonds are the only photocatalysts used. 10) Method according to any one of claims 2 to 8, characterized in that said oxidized nanodiamonds are used with at least one other element chosen from the group consisting of photocatalysts, catalysts, adsorbents and their combinations.

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