Hydrogen production method using acidified nanodiamond as a photocatalyst

Abstract

The present invention relates to a method for producing hydrogen by photolysis of water, which includes at least a step of bringing an aqueous solution into contact with nanodiamond under sunlight, natural light or artificial light (or illumination).

Description

Technical Field

[0001] The present invention relates to the general technical field of nanomaterials, particularly photocatalysts, and more particularly to nanomaterials for hydrogen (H2) production by photoinduced water splitting, known by the expression "water splitting".

[0002] In fact, the present invention proposes the use of diamond oxide nanoparticles or nanodiamonds as photocatalysts. In other words, the present invention proposes a method for producing hydrogen from water, which includes the step of contacting an aqueous solution with diamond oxide nanoparticles under light irradiation. The present invention also relates to a specific photocatalyst composition comprising diamond oxide nanoparticles and at least one other (photo)catalyst.

Background Art

[0003] While energy demand is increasing, due to the depletion of fossil fuels such as coal, oil and natural gas, particular interest is being shown in hydrogen (H2), which has several advantages over fossil fuels.

[0004] First of all, the combustion of fossil fuels releases carbon dioxide (CO2) and promotes global warming, while the combustion of H2 produces only water (H2O).

[0005] Furthermore, in order to avoid H2 production from natural gas, which results in the generation of CO2, alternative renewable solutions with less environmental impact, such as the photocatalytic production of H2 from water, have been proposed. In this method, the photoinduced dissociation of water, i.e., under light irradiation, produces H2, which can be used directly for combustion, in fuel cells, in chemical or petrochemical processes, or particularly for storage in liquid form.

[0006] Methods for the photocatalytic production of H2 are known from the prior art.

[0007] Photocatalysts are based on the principle of activation of a semiconductor or a series of semiconductors as photocatalysts using the energy supplied by irradiation.

[0008] Semiconductors are characterized by their unique bandgap (the difference between the valence band and the conduction band). Any photon with an energy greater than the bandgap can be absorbed by the semiconductor. Conversely, any photon with an energy less than the bandgap cannot be absorbed by the semiconductor.

[0009] A photocatalyst can be defined as the absorption of photons with energies greater than the bandgap width in the case of semiconductors, which induces the formation of electron-hole pairs. Thus, there is excitation of electrons in the conduction band and formation of holes in the valence band. This electron-hole pair reacts with compounds present in a medium such as H2O to initiate the generation of H2 or enables the formation of free radicals that recombine according to various mechanisms.

[0010] Therefore, it is important to have high-performance photocatalysts, especially for generating H2 by dissociating water using sunlight, and they must be as follows.

[0011] a) Having a band structure suitable for generating charge carriers that enable oxidation and reduction reactions for H2 generation on the surface, where protons are derived from the oxidation of water by photo-generated holes, b) Having low recombination of photo-generated charge carriers to ensure the best sunlight absorption and optimal efficiency of the photocatalytic reaction, c) Being available in large quantities, at low cost, and environmentally friendly (which greatly limits the use of metal particles such as platinum particles and rare earth oxide particles), and d) Continuing to be effective during operation.

[0012] To take these constraints into account, on the one hand, it is a matter of concern to develop photocatalysts that are nanometer-sized to access a larger total surface area and, on the other hand, are semiconductors that generate charge upon illumination, with conduction and valence band positions suitable for the oxidation of water and the reduction of protons, and having a bandgap energy compatible with the wavelengths of visible light.

[0013] The literature already contains numerous materials that have been identified as photocatalysts in the photocatalytic decomposition of water for H2 production. Titanium dioxide (TiO2) with anatase structure has certain characteristics advantageous for this reaction, particularly, (i) photo-stability in water, (ii) an appropriate position of the valence band that can easily initiate the first oxidation step of water (H2O -> 2H + + 1 / 2O2 + 2e - ), (iii) acceptable charge carrier behavior, (iv) non-toxicity, and (v) relatively moderate cost compared to other types of photocatalysts, and is thus undoubtedly the most studied semiconductor.

[0014] However, the main limitation of TiO2 lies in its large bandgap (3.1 - 3.2 eV) that requires activation by wavelengths less than 400 nm (UV range), which severely restricts the use of natural sunlight composed of visible photons, about 40% of which do not have sufficient energy to effectively activate TiO2. Furthermore, due to the difficulty of carrying out the half-reaction of reduction to hydrogen (2H + + 2e - -> H2), rare and expensive noble metals are often added as co-catalysts to carry out the catalytic reduction of hydrogen.

[0015] Already, numerous strategies have been implemented to overcome these limitations, such as modification of composition, morphology, chemical structure, size, surface area, deposition of metal nanoparticles exhibiting surface plasmon characteristics, and combination with other semiconductors (formation of heterojunctions), which can induce electronically, optically, or chemically advantageous effects.

[0016] Nanomaterials containing diamond have also been proposed as photocatalysts. They are often used in the form of hybrids or composites.

[0017] Thus, in 2016, Lin et al. proposed p-type cuprous oxide nanocrystals integrated into nanodiamond for broadband photocatalytic hydrogen generation (Non-Patent Document 1).

[0018] International Publication WO2016 / 193464 describes the use of a photocatalytic composite material comprising at least one semiconductor compound having a band gap of 2 to 5 eV and preferably diamond nanoparticles with a hydrogenated surface (Patent Document 1).

[0019] In other composite materials of the prior art that are not useful for photocatalysis of water but are useful for water purification, optionally boron-doped nanodiamonds are implemented in combination with graphitic carbon nitride (or g-C3N4) (Non-Patent Document 2) and optionally silver nitrate (AgNO3) (Patent Document 2).

[0020] Among the literature, there is one literature reporting the effect of hydrogen production by water splitting using nanodiamonds alone (Non-Patent Document 3). However, in this study, a pulsed laser having a wavelength of 532 nm with a high output (80 mJ / pulse) is used. There is nothing suggesting that such materials can be used for water dissociation using a much lower output solar spectrum. Furthermore, as is clear from Non-Patent Document 3, it is suggested that hydrogen-terminated sites function as electron reservoirs and hydrogenation significantly increases the quantum efficiency, and the use of nanodiamonds with a hydrogenated surface is preferred.

[0021] The inventors aimed to propose a photocatalyst capable of producing hydrogen from water using sunlight as a photon source without the need for complex processes such as the use of co-catalysts such as noble metals or hydrogenation.

Prior Art Documents

Patent Documents

[0022]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0023]

Non-Patent Document 1

[0024] The present invention enables the inventors to achieve the goals they set. In fact, the inventors have shown that it is possible to decompose water and produce hydrogen using diamond nanoparticles as photocatalysts under light irradiation.

[0025] On the one hand, oxidized nanodiamond can be used without adding other photocatalysts or cocatalysts such as metal particles. In other words, in the method according to the present invention, oxidized nanodiamond can be used as the sole photocatalyst.

[0026] For the sake of caution, diamond is a wide-band semiconductor (5.5 eV) and theoretically not suitable for the absorption of visible light. However, in the form of nanoparticles, its light absorption characteristics are modified either by the optical effects associated with the nanometer size or by the introduction of structural defects during their synthesis.

[0027] Furthermore, the inventors' research shows that the effect exists for nanodiamond with an oxidized surface. Depending on its surface chemistry, the band diagram of nanodiamond changes. The diamond surface saturated with hydrogen has a more negative conduction band and valence band edge electrochemical potential than the diamond surface saturated with oxidation functions. This is shown in FIG. 1 of bulk diamond.

[0028] Therefore, the most reducing structure (i.e., hydrogenated diamond) is not the most advantageous for enabling the production of H2. This is counterintuitive to those skilled in the art. On the contrary, the inventors have shown that oxidized nanodiamond must be used to maximize hydrogen production.

[0029] More specifically, the present invention relates to the use of oxidized nanodiamond as a photocatalyst in the production of hydrogen. This production of hydrogen is obtained under sunlight, natural light or artificial light (or illumination).

[0030] In other words, the present invention relates to a method for producing hydrogen by photolysis, which at least includes the step of bringing an aqueous solution into contact with oxidized nanodiamond under sunlight, natural light or artificial light (or illumination), that is, under sunlight irradiation, natural light irradiation or artificial light irradiation.

[0031] The expressions "photolysis of water", "photolysis (photo) of water under light irradiation", "photocatalyst of water" and "photocatalyst (photo) of water under light irradiation" are equivalent and can be used interchangeably in this specification.

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

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

[0034] More specifically, they can be obtained by (i) grinding natural or synthetic bulk diamonds, (ii) detonation as described in particular in WO 2016 / 193464 (Patent Document 2), (iii) direct HPHT growth or CVD.

[0035] The average size of the diamond nanoparticles implemented in the present invention is 1 to 500 nm, i.e., 1 to 200 nm, particularly 2 to 100 nm, more specifically 5 to 50 nm.

[0036] In a specific embodiment, the diamond nanoparticles implemented in the present invention can be doped particularly with nitrogen or phosphorus (n-type doping) or with boron (p-type doping).

[0037] As described above, the nanodiamonds implemented in the present invention are oxidized, i.e., the surface is oxidized. In other words, the surface of the oxidized nanodiamond has more oxygen atoms than the surface of the non-oxidized nanodiamond.

[0038] Typically, the oxidized nanodiamonds implemented in the present invention have an oxygen / carbon ratio of at least 5% atoms, as determined by XPS (X-ray photoelectron spectroscopy), without performing pretreatment of the oxidized nanodiamonds for dehydration or the like.

[0039] To obtain oxidized nanodiamonds, these nanodiamonds must be subjected to an oxidation treatment.

[0040] The oxidation treatment aims to oxidize the surface of the nanodiamond by fixing and / or introducing groups rich in oxygen, i.e., the same or different groups containing at least one oxygen atom. In the present invention, the group containing at least one oxygen atom is particularly selected from the group consisting of a carboxy group (-C(=O)OH), a hydroxy group (-OH), a carbonyl group (-C(=O)-), and a percarboxylic acid (-C(=O)-O-OH).

[0041] Such an oxidation treatment is based on two broad types of surface modification as follows.

[0042] - Physical treatment: Plasma treatment (high frequency, microwave), UV treatment, X-ray or gamma-ray treatment, electron and heavy ion irradiation treatment, etc., which can be carried out under CO2 or in an oxygen-containing atmosphere such as air, O2, O2 / argon, or ozone. For example, UV treatment under ozone.

[0043] - Chemical treatment: For example, alcoholic potassium hydroxide treatment, high-temperature or low-temperature treatment with a mixture of sulfuric acid (H2SO4) and nitric acid (HNO3), treatment with a mixture of sulfuric acid (H2SO4) and hydrogen peroxide (H2O2), also known as the "piranha mixture", treatment with a mixture of H2O2 and iron, also known as the "Fenton reagent", treatment with strong acids (HCl, H2SO4, HNO3, HClO4), soda treatment, treatment with strong oxidants (KMnO4, K2Cr207, KClO3, or CrO3 in hydrochloric acid, sulfuric acid, or nitric acid), molten salt (KNO3) treatment, ozone treatment, and heat treatment under CO2 or in an oxygen-containing atmosphere such as air, O2, O2 / argon, or ozone.

[0044] The above oxidation treatment can be carried out at atmospheric pressure and / or room temperature (Tamb), or at a pressure greater than atmospheric pressure and / or a temperature greater than room temperature (especially in the case of heat treatment). "Room temperature" means a temperature of about 23°C (i.e., 23°C ± 5°C).

[0045] Advantageously, the oxidation treatment carried out is a heat treatment under an oxygen-containing atmosphere such as, for example, air, O2, O2 / argon or ozone. The latter advantageously includes annealing at a temperature of 500°C ± 50°C for 1 to 5 hours, especially 1 to 3 hours, for example under air, O2, O2 / argon or ozone, especially under air. The following experimental part shows such a heat treatment. Thus, the method according to the invention may have a preliminary step for the preparation of oxidized nanodiamonds, which comprises supplying nanodiamonds to the oxidation treatment defined above.

[0046] Typically, when the oxidation treatment is carried out, the oxidized nanodiamonds are resuspended. This includes subjecting the oxidized nanodiamonds to ultrasonic treatment and then centrifuging, thereby obtaining a colloidal suspension of the oxidized nanodiamonds.

[0047] Prior to ultrasonic treatment, the oxidized nanodiamonds are brought into contact with an aqueous solution. Such a solution is called an "aqueous solution" because it has an aqueous solvent as the solvent. "Water" in the context of the present invention means tap water, deionized water, distilled water or ultrapure water (18.2 MΩ·cm at 25°C).

[0048] Typically, the amount of oxidized nanodiamonds used in the contact with the aqueous solution before ultrasonic treatment is between 1 g and 50 g per liter of aqueous solution, i.e., between 10 g and 40 g per liter of aqueous solution, especially about 30 g per liter of aqueous solution (i.e., 30 g / L ± 5 g / L).

[0049] The ultrasonic treatment step is carried out at a temperature between 4°C and 20°C, i.e., between 6°C and 15°C, in particular at a temperature of about 10°C (i.e., 10°C ± 2°C). Advantageously, a thermostat is used during the ultrasonic treatment step. Furthermore, the ultrasonic treatment step lasts between 15 minutes and 3 hours, specifically between 30 minutes and 2 hours, in particular about 1 hour (i.e., 1 hour ± 15 minutes).

[0050] The centrifugation step is carried out to separate the colloidal suspension of nano-diamond oxide corresponding to the supernatant obtained after the centrifugation step from the clusters of nano-diamond oxide that form the pellet obtained after the centrifugation step. For this purpose, the centrifugation step is carried out at a value between 1500 g and 4000 g, i.e., a value between 2000 g and 3000 g, in particular about 2400 g (i.e., 2400 g ± 200 g).

[0051] The centrifugation step is carried out at a temperature between 4°C and 20°C, i.e., between 6°C and 15°C, in particular at a temperature of about 10°C (i.e., 10°C ± 2°C). Furthermore, the centrifugation step lasts between 15 minutes and 2 hours, i.e., between 30 minutes and 1 hour, in particular about 40 minutes (i.e., 40 minutes ± 5 minutes).

[0052] In step a), the nano-diamond oxide is brought into contact with the aqueous solution defined above. In a specific embodiment, the solvent of the aqueous solution used during step a) contains only water, i.e., this solvent consists of water, in particular ultrapure water (18.2 MΩ·cm at 25°C).

[0053] Typically, the amount of nano-diamond oxide used during the contact with the aqueous solution is between 1 mg and 1 g per liter of aqueous solution, i.e., between 5 mg and 500 mg per liter of aqueous solution, in particular between 10 mg and 50 mg per liter of aqueous solution, and more specifically about 12.5 mg per liter of aqueous solution (i.e., 12.5 mg / L ± 1 mg / L).

[0054] The contact between the aqueous solution and the oxidized nanodiamond can be carried out continuously or discontinuously under stirring and / or under an inert gas such as argon, nitrogen, helium or a mixture thereof. Advantageously, this contact is carried out under stirring and under a continuous flow of nitrogen.

[0055] Typically, the contact between the aqueous solution and the oxidized nanodiamond is carried out between 5°C and 80°C, particularly between 15°C and 50°C, and more specifically at room temperature.

[0056] During the contact, the oxidized nanodiamond can be in the form of a suspension. Alternatively, during the contact, the oxidized nanodiamond may be supported.

[0057] Any type of support conventionally used to hold a photocatalyst in a method for water decomposition under light irradiation can be used in the context of the present invention. Exemplary examples include two-dimensional supports such as a fabric sheet made of optical fibers, surface coatings such as paints, and dense or porous three-dimensional supports such as foams or honeycombs.

[0058] The contact between the oxidized nanodiamond and the aqueous solution is carried out under light irradiation. This light irradiation can be carried out using natural light (sunlight) or artificial light, in particular irradiation devices such as lamps, UV lamps, visible light lamps, UV-visible light lamps, IR lamps, excimer lamps, LEDs, lasers, laser diodes or ultra-wideband fiber sources. In the context of artificial solar irradiation, it is clear that the irradiation device used reproduces the spectrum and the output of sunlight.

[0059] Advantageously, the irradiation device has an irradiance between 25 mW / cm 2 ~150 mW / cm 2 and in particular of about 53.5 mW / cm 2 ±5 mW / cm 2 In other words, "under light irradiation" means under natural or artificial sunlight (or illumination).

[0060] The radiation carried out during this light irradiation can be UV radiation (wavelength 200 - 400 nm), visible radiation (wavelength 400 - 800 nm) or near-infrared radiation (wavelength 800 - 1200 nm) and combinations thereof.

[0061] Advantageously, the light irradiation carried out by the method of the present invention is natural light irradiation, i.e., natural sunlight (or illumination).

[0062] In the context of the present invention, the aqueous solution may further contain a sacrificial agent. Typically, the sacrificial agent used in the present invention is an electron donor that can be oxidized by the oxygen formed during the photolysis reaction of water, making it possible to effectively improve the yield of H2 generation.

[0063] The sacrificial agent used in the method according to the present invention is typically selected from the group consisting of amines and alcohols, more specifically from the group consisting of methanol, ethanol, triethanolamine (TEOA) and mixtures thereof. Advantageously, the sacrificial agent used in the context of the method of the present invention is TEOA or methanol.

[0064] When a sacrificial agent is present, the sacrificial agent is present in a volume of 0.05% - 50% by volume, particularly 0.1% - 1% by volume, based on the volume of the aqueous solution.

[0065] In a specific embodiment, the oxidized nanodiamond is the only photocatalyst used in the method.

[0066] Alternatively, in another specific embodiment, the oxidized nanodiamond is used with at least one other element selected from the group consisting of photocatalysts, catalysts, adsorbents, and combinations thereof. Thus, the oxidized nanodiamond can be combined with at least one other photocatalyst of the type of inorganic, molecular (quantum dots and clusters), or organic (dyes) photocatalysts in a heterojunction, and / or at least one catalyst of the type of metal, inorganic, or molecular (quantum dots and clusters) catalysts, and / or at least one adsorbent that improves adsorption and charge transfer of species such as activated carbon or MOF (Metal Organic Frameworks), and these adsorption materials can also have semiconductor properties such as those of a specific MOF. In another embodiment, the oxidized nanodiamond forms a catalyst composition with other elements selected from the group consisting of photocatalysts, catalysts, adsorbents, and combinations thereof. Preferably, this catalyst composition does not contain platinum or other noble metals.

[0067] As is known to those skilled in the art, any semiconductor-type or molecular-type photocatalyst used in the method for photocatalytic decomposition of water to generate H2 can be used in the context of the present invention.

[0068] However, in a specific embodiment of the method according to the present invention, the photocatalyst combined with the oxidized nanodiamond does not contain graphitic carbon nitride.

[0069] Advantageously, the photocatalyst implemented with the oxidized nanodiamond is selected from the group consisting of transition metals, transition metal derivatives, metal carbides, metal nitrides, metal oxides, and metal sulfides. In particular, the photocatalyst implemented with the oxidized nanodiamond is selected from the group consisting of transition metal oxides and transition metal sulfides. In particular, the photocatalyst implemented with the oxidized nanodiamond is selected from the group consisting of TiO2, TiO2-B (titanate sheet form), ZnO, WO3, and Fe2O3.

[0070] When used together with nanodiamond oxide, the photocatalyst, catalyst and / or adsorbent can be in a dispersed form, particularly in the form of nanoparticles. The average size of these nanoparticles is between 1 and 1000 nm, i.e., between 2 and 200 nm, particularly between 3 and 100 nm, more specifically between 4 and 50 nm, particularly between 5 and 20 nm.

[0071] Alternatively, the photocatalyst, catalyst and / or adsorbent can be used in an aggregated form.

[0072] The present invention also relates to a photocatalyst composition used in the method according to the present invention. This photocatalyst composition contains at least one other element selected from the group consisting of nanodiamond oxide and a photocatalyst, catalyst, adsorbent and combinations thereof, and the photocatalyst composition does not contain graphitic carbon nitride.

[0073] In the photocatalyst composition according to the present invention, the nanodiamond oxide is present in an amount of 0.1 to 80% by weight, i.e., 1 to 50% by weight, particularly 2 to 30% by weight, based on the amount of the at least one other element selected from the group consisting of a photocatalyst, catalyst, adsorbent and combinations thereof.

[0074] Other features and advantages of the present invention will also become apparent by reading the following examples given for illustrative and non-limiting purposes with reference to the accompanying drawings.

Brief Description of the Drawings

[0075]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Mode for Carrying Out the Invention

[0076] I. Preparation of Oxidized Nanodiamond The diamond nanoparticles used were synthesized by the detonation method and obtained from PlasmaChem (Germany).

[0077] The diamond nanoparticles were first oxidized by annealing in air at atmospheric pressure according to the following protocol.

[0078] - Place two crucibles filled with 200 mg of untreated nanoparticles at the center of the tube furnace. - Raise the temperature of the furnace from room temperature (Tamb) to 200 °C at a heating rate of 20 °C / min, and then maintain the nanoparticles at 200 °C for 15 minutes. - Raise the temperature of the furnace from 200 °C to 500 °C at a heating rate of 30 °C / min, and then maintain the nanoparticles at 500 °C for 1 hour and 30 minutes. - Next, take out the two crucibles "while hot" using tongs, place them at room temperature (Tamb), and cool the nanoparticles as quickly as possible to stop annealing. - When the nanoparticles have returned to room temperature (Tamb), weigh them. The loss generally observed is about 50% of the initial mass.

[0079] Next, make the nanoparticles into a suspension according to the following protocol. - Put 100 mg of the oxidized nanoparticles into a Falcon 15 mL centrifuge tube. - Add 3 mL of ultrapure water (18.2 MΩ·cm). - Set the temperature of the solution to 10 °C in a thermostat. - Then, while maintaining the temperature of the solution at 10 °C, sonicate it using a Cup Horn device (Bioblock Scientific 750 W, amplitude 60%, cycle 1 s ON / 1 s OFF, duration 60 min). - After sonication, centrifuge the solution (2400 g, 40 min) to remove the largest clusters. - Immediately after centrifugation, pipette out the supernatant containing the nanoparticles in the colloidal suspension (about 2.5 mL out of the first 3 mL). - Then, put the suspension into a plastic bottle and store it shielded from light at room temperature (Tamb). This storage can be carried out over a long period exceeding one year.

[0080] The concentration of the nanoparticles in the suspension is determined by drying 100 μL of the suspension at Tamb overnight and measuring the mass of the dry residue.

[0081] II. Characterization of Oxidized Nanodiamonds II.1. Method - Measurement of the hydrodynamic diameter of the suspension in 1 mL of a 1 mg / mL suspension of nanoparticles by dynamic light scattering (DLS) (HORIBA SZ-100 Nanopartica series). - Measurement of the zeta potential of the suspension with 700 μL of a 1 mg / mL suspension of nanoparticles by electrokinetic light scattering (HORIBA SZ-100 Nanopartica series). - Characterization of the surface chemistry of nanodiamonds in suspension by Fourier transform infrared spectroscopy: 2 μL of a 5 mg / mL suspension of nanoparticles is evaporated on the ATR crystal of a Bruker Alpha II spectrometer. - Chemical characterization of nanodiamonds in suspension by XPS and determination of the O / C ratio: 20 μL of a 1 mg / mL suspension of nanoparticles is evaporated on a silicon substrate coated with 50 nm of gold. The analysis is performed on a Kratos Analytical Axis Ultra DLD (monochromatic Al Kα source).

[0082] II.2. Results As shown in Figure 2, oxidized nanodiamonds have a hydrodynamic diameter of 48 nm ± 10 nm and a zeta potential of 57 mV ± 5 mV in suspension.

[0083] The infrared absorption spectrum of the oxidized nanodiamonds shown in Figure 3 highlights the presence of a C=O bond (1750 cm -1 ) probably involved in carboxylic acids, as well as the presence of a C-O bond (1000 - 1300 cm -1 ) related to surface alcohol functions or esters. The shoulder between 3000 and 3500 cm -1 also reflects the presence of an O-H bond related to carboxylic acids. In fact, since the spectrum is recorded under a dry nitrogen flow, there is no contribution from OH due to ambient humidity. Finally, it is important to note the absence of absorption between 2800 and 3000 cm -1 , which means that there are no C-H bonds on the particle surface.

[0084] XPS analysis highlights the presence of three elements in oxidized nanodiamonds, namely carbon, oxygen, and nitrogen (Figure 4). The latter is mainly located at the center of the nanoparticles and is derived from the nitrogen explosives used during detonation synthesis. The atomic ratios of each element are shown in Table 1 below.

[0085]

Table 1

[0086] III. Photocatalytic Behavior of Nanodiamond Oxide III.1. Operating Procedures To study the photocatalytic behavior of diamond oxide nanoparticles, a suspension of nanodiamonds (NDs) was continuously irradiated under a continuous flow of inert gas (N2) using a 150 W lamp (Spatite Hit 150 G12 8800 K (Art-nr 226224)) that reproduces the solar spectrum. An experiment was conducted to make the suspension contain a small proportion of antioxidant / sacrificial agent (triethanolamine, TEOA). The experiment was carried out at room temperature (Tamb) while controlling the temperature of the suspension throughout the photocatalytic test.

[0087] The quantification of the generated hydrogen was performed by on-line measurement using gas chromatography over a period of 2 hours when the hydrogen generation was stabilized. In this way, it is possible to track the formation rate of the generated H2.

[0088] The experimental conditions are as follows. The photocatalyst (10 or 20 mg) was suspended in 800 mL of ultrapure H2O (mQ), TEOA (0.1 - 1 vol.% TEOA), magnetic stirring at 700 rpm, continuous flow of N2 at 100 cm 3 / min, analysis time of 2 hours after stabilization, and analysis values were obtained every 2.5 minutes.

[0089] III.2. Results Figure 5 shows that at the same concentration, nanodiamond oxide (Plasma Chem) exhibits good performance similar to that of the reference commercial photocatalyst TiO2 P25 (Evonik).

[0090] It is also observed that the generation of H2 is related to the concentration of the sacrificial agent but is non-proportional (Figure 6). This is because in fact, TEOA mainly acts as a trap for O2, and the generation of H2 by the photocatalytic decomposition of water by sunlight was confirmed.

[0091] Furthermore, the optimal concentration of the oxidized nanodiamond seems to be around 10 mg, that is, around 12.5 mg / L (Fig. 7).

Claims

**Claim 1** Use of nano-diamond oxide as a photocatalyst in the production of hydrogen under sunlight, natural light or artificial light (or illumination). **Claim 2** A method for producing hydrogen by photocatalytic decomposition of water, comprising at least the step of bringing an aqueous solution into contact with nano-diamond oxide under sunlight, natural light or artificial light (or illumination). **Claim 3** The method according to claim 2, wherein the nano-diamond oxide is determined by XPS (X-ray photoelectron spectroscopy) without pretreatment of the nano-diamond oxide and has an oxygen / carbon ratio of at least 5 atomic %. **Claim 4** The method according to claim 2 or 3, having a preliminary stage of preparing nano-diamond oxide, including subjecting the nano-diamond to an oxidation treatment. **Claim 5** The method according to claim 4, wherein the oxidation treatment is annealing at a temperature of 500 °C ± 50 °C for 1 to 5 hours in an oxygen-containing atmosphere. **Claim 6** The method according to any one of claims 2 to 5, wherein the contact between the aqueous solution and the nano-diamond oxide is carried out under stirring and / or under an inert gas, particularly under stirring and under a continuous nitrogen flow. **Claim 7** The method according to any one of claims 2 to 6, wherein the light irradiation is natural light irradiation. **Claim 8** The method according to any one of claims 2 to 7, wherein the aqueous solution contains a sacrificial agent, particularly triethanolamine (TEOA) or methanol. **Claim 9** The method according to any one of claims 2 to 8, wherein the nano-diamond oxide is the sole photocatalyst. **Claim 10** The method according to any one of claims 2 to 8, wherein the nano-diamond oxide is used together with at least one other element selected from the group consisting of a photocatalyst, a catalyst, an adsorbent and combinations thereof.

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