GB2530827A

GB2530827A - Method of characterising surface chemistry

Info

Publication number: GB2530827A
Application number: GB1505779.7A
Authority: GB
Inventors: Kris Seunarine; Chris Spacie
Original assignee: Haydale Graphene Industries PLC
Current assignee: Haydale Graphene Industries PLC
Priority date: 2014-04-02
Filing date: 2015-04-02
Publication date: 2016-04-06
Anticipated expiration: 2035-04-02
Also published as: GB201405973D0; WO2015150830A1; GB2530827B; GB201505779D0

Abstract

A method of characterising the surface chemistry of particulate material, comprising (i) a sample preparation step, involving obtaining a test sample comprising a dispersion of the particulate material in a liquid; (ii) a monitoring step, involving monitoring changes in the dispersion over time to generate sample data; (iii) a comparison step, involving comparing the sample data to reference data; and (iv) a characterising step, involving determining surface characteristics of the particulate material based on the results of the comparison step. Preferably the monitoring step involves spectroscopically monitoring changes in the dispersion over time, especially by monitoring changes in transmission of light through the dispersion. The method can be used to determine whether particulate materials, preferably carbon nanoparticles such as carbon nanotubes or graphene nanoplatelets, have been functionalised. The method can also be used to determine the type and degree of functionalisation.

Description

METHOD OF CHARACTERISING SURFACE CHEMISTRY

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for characterising the surface chemistry of particulate materials, in particular, the surface chemistry of carbon particles, such as graphitic and graphene platelets, and carbon nanotubes.

INTRODUCTION

Various forms of particulate carbon show great promise as materials for use in a wide-range of technologies. For example, carbon nanotubes (CNT5), their remarkable properties and proposed properties, and methods of making them have been known for many years. More recently, research has focussed on making and exploiting graphene, a material corresponding to a single layer of the graphite structure but with properties exceeding those of graphite's because of the absence of neighbouring layers. For both types of material, however, their industrial uses are still very limited, largely because of processing and handling difficulties.

The surface properties of carbon particles play a key role in determining the ease with which they can be processed and handled. To this end, there has been interest in developing methods for modifying or introducing chemical groups at the particle surface, i.e., functionalising the particles, to alter the surface properties.

For example, FCT applications WO 2010/142953 and WO 2012/076853, belonging to the present applicants, describe methods for producing high-quality functionalised carbon particles by means of plasma treatment. The methods allow a range of functional groups to be reliably introduced to the surface of carbon particles, including fluorine, alkyl, hydroxyl, carboxylic acid, amine, amino (i.e., groups containing an amine and carboxylic acid group), amide, and oxo groups (C=O).

In order to relate particle performance to particle surface properties, it is important to have suitable techniques for characterising the particle surface. X-ray photoelectron spectroscopy (XPS) is a widely used technique which can be used to characterise the surface composition to a depth of about 0-10 nm in a sample of carbon particles, including the type and extent of chemical functionalisation. However, XPS experiments involve the use of expensive, complicated and highly-specialised equipment. Secondary Ion Mass Spectroscopy (SIMS) can provide information about the specific chemical groups at the surface of particles to a depth of up to 2 nm, and is often used to complement data provided by XPS. However, as with XPS, the equipment required to conduct such experiments is expensive and complex.

FuR and Raman spectroscopy are also well known techniques used for the analysis of functionalisation and associated chemical groups. However, in the case of FTIR, it is difficult to detect chemical signatures of surface groups on bulk materials, such as CNJTs and GNPs, because of the relatively large sampling depths (-1-2 pm) of the technique. In addition, both FTIR and Raman spectroscopy require the use of sophisticated equipment and skilled interpretation of results. Accordingly these techniques are most appropriate for use by academics and research institutions, but less appropriate in industrial settings Thus, in summary, conventional analysis techniques for characterising the surface of particles require expensive and complex equipment, produce results which can be difficult to interpret, and do not necessarily allow detection of low levels of functionalisation at a particle's surface. Thus, there is a need to develop quick, inexpensive and simple methods for characterising the surface chemistry of particles.

SUMMARY OF THE INVENTION

At its most general, the present invention provides a method of characterising the surface chemistry of a particulate material by monitoring changes in a dispersion of the material.

Accordingly, in a first aspect, the present invention provides a method of characterising the surface chemistry of a particulate material, comprising obtaining a dispersion of the particulate material in a liquid, monitoring changes in the dispersion over time by collecting sample data, and comparing the sample data to reference data.

The surface chemistry of the particulate material affects the stability of the dispersion i.e., the ability of the particles to stay dispersed in the liquid. In particular, the stability of a dispersion of particles depends on the particles' zeta potential, which is a measure of the magnitude of the electrostatic attraction or repulsion between particles. In dispersions formed from particles having a low zeta-potential the particles will flocculate overtime, i.e., aggregate and settle (sediment) or float. In stable dispersions formed from particles having a relatively high zeta-potential the particles may be stably dispersed in the liquid, and will undergo phase separation at a relatively slow rate. However, the use of zeta potential is not applicable to all functional groups commonly applied to particulate material.

In the present invention, the stability of the sample dispersion is monitored through the collection of sample data over time. Comparison of changes in the sample data with changes in reference data over time allows information about the surface chemistry of the particulate material to be determined. For example, such comparisons may allow the identification of (i) whether the particles are functionalised (ii) the type of functionalisation (e.g., nature and/or identity of chemical groups) at the particle surface and/or (iH) the degree of functionalisation at the particle surface. The comparison may involve correlating changes in the sample data to changes in the reference data over time e.g., looking for a similar rate of change in the data, or checking whether the sample data matches the reference data.

Alternatively, the comparison may involve identifying differences in the sample data compared to the reference data.

Advantageously, monitoring changes in dispersions of particulate materials provides a simple, generally applicable method of characterising the surface chemistry of such materials Furthermore, the methods are non-destrLlctive, meaning that particles may be recovered (e.g., by filtration) and re-used after measurement. Recovered particles can be re-examined using the methods of the invention (e.g., to build up additional data to improve characterisation of the sample); can be examined using other methods (e.g., XPS) to provide a cross-check of the results generated by the invention or to provide additional information about the particles; or can be used for other purposes (e.g., end products).

The method may be used to determine whether the surface of the particulate material is functionalised. By "functionalised", "functionalisation" etc., we mean that the particles have undergone a treatment step which results in chemical modification of the surface of the particles. This may involve the modification of existing chemical groups at the particle surface, or the introduction of new chemical groups. The treatment step may, or may not, affect the core of a particle, i.e., the bulk composition or structure of the particle, away from the particle's surface.

Such methods may be used, for example, to determine/confirm whether a particular treatment has resulted in surface-functionalisation. This is potentially useful in industrial and academic settings where a simple confirmation of the success of a surface-functionalisation step is required (e.g., for quality control purposes), without the need for more detailed and sometimes prohibitively expensive investigations (e.g., XPS investigations).

In preferred embodiments, the comparison step involves comparing the sample data to reference data which relates to particles which are not functionalised. If the sample data matches the reference data for non-functionalised particles this indicates that the sample particles are non-functionalised. Advantageously, this provides a quick and simple test for determining whether the sample particles have been functionalised.

Additionally, or alternatively, the comparison step may involve comparing the sample data to reference data which relates to particles which are functionalised. In such embodiments, a match between sample data and reference data for functionalised particles indicates that the sample particles are functionalised.

For example, the first aspect may be a method of determining whether the surface of a particulate material (e.g., carbon particles) is functionalised, comprising the steps of: -dispersing the particulate material in a liquid; -monitoring (e.g., spectroscopically, such as through transmission measurements) changes in the dispersion over time to generate sample data; -comparing the sample data to reference data; and -identifying whether the sample data matches reference data relating to a non-functionalised reference sample and/or matches reference data relating to a functionalised reference sample.

Additionally, or alternatively, the method may be used to characterise the chemical groups at the surface of the particulate material. In other words, the method is not only used to determine whether particles have been functionalised, but is also used to gain information about the chemical groups with which the particles have been functionalised.

In some embodiments, the method is used to determine the treatment which has been used to introduce the chemical groups at the surface of the particulate material. For example, it the particulate material has been functionalised using a plasma technique, such as one of those described in WO 2012/076853, the method may be used to determine the type of plasma-forming gas used, e.g., oxygen, water, hydrogen peroxide, alcohol, nitrogen, ammonia, amino-bearing organic compound, halogen (e.g., fluorine), halohydrocarbon. This provides an easy method for determining/confirming whether the particles have been subiected to a particular treatment method, e.g., for product or process quality control monitoring.

The method may be used to distinguish between different types of chemical group, e.g. halogen-containing surface chemical groups (surface chemical groups which are or contain F, Cl, Br or I), nitrogen-containing chemical groups (surface chemical groups which contain nitrogen, such as amine groups) and oxygen-containing surface chemical groups (surface chemical groups which are or contain oxygen, such as oxide and carboxylate). For example, the method may be used to determine whether a particulate material has been subjected to treatment with a fluorine-containing, nitrogen-containing or oxygen-containing plasma gas.

In other embodiments, the method is used to determine the specific type of chemical groups at the surface of the particulate materiaL For example, the method is used to determine the chemical composition of chemical groups at the surface of the particle, and/or the identity of chemical groups at the surface of the particle. In such embodiments, the comparison step involves comparing the sample data to reference data which relates to particles which are functionalised with known chemical groups.

For example, the first aspect may be a method of characterising the chemical groups (e.g., the nature or specific type) at the surface of a particulate material (e.g., carbon particles), comprising the steps of: -dispersing the particulate material in a liquid; -monitoring (e.g., spectroscopically, such as through transmission measurements) changes in the dispersion over time to generate sample data; -comparing the sample data to reference data; and -identifying whether the sample data matches reference data relating to particulate material with known surface chemistry.

Additionally, the method may be used to determine the degree (i.e., amount) of surface-functionalisation of the particulate material.

In some embodiments, the method is used to provide qualitative information about the degree of functionalisation. For example, the sample data may be compared to reference data to see whether the data satisfies one or more pre-defined criteria relative to the reference data, e.g., the sample may be designated as having a high' degree of functionalisation if the sample data indicates that the dispersion is more stable than the reference data, and a "low" degree of functionalisation if the sample data indicates that the dispersion is less stable than the reference data. This method can be used to quickly and easily verify whether a given sample has a desired degree of functionalisation, e.g., for quality control purposes.

In some embodiments, the method is used to provide quantitative information about the degree of functionalisation. In such embodiments, the amount of functionalisation may be determined by comparing the sample data to reference data relating to one or more reference samples (reference dispersions") having a known amount of functionalisation (e.g., determined through XPS). For example, the reference data may relate to several reference dispersions where the particulate material is functionalised with different amounts of the same chemical groups (e.g., 1%, 2%, 3% and 4% of the particle surface occupied by a particular chemical group, as determined by XPS), so that comparing the sample data to the reference data can be used to determine the amount of such chemical groups present on the sample particles. Advantageously, with suitably calibrated reference data, these methods provide a quick, simple and cheap method of providing quantitative information about the degree of functionalisation.

For example, the first aspect may be a method of determining the degree of functionalisation of a particulate material (e.g., carbon particles) functionalised with chemical groups, comprising the steps of: -dispersing the particulate material in a liquid; -monitoring (e.g., spectroscopically, such as through transmission measurements) changes in the dispersion over time to generate sample data; -comparing the sample data to reference data relating to particulate material functionalised with the same chemical groups.

In embodiments of the invention, the surface chemistry of the particles in the dispersion is determined by comparing the sample data to reference data. Preferably, the reference data relates to particles having a known surface chemistry.

The reference data may comprise or consist of theoretical data i.e., data obtained through calculations or theoretical modelling. Such theoretical data may be based on modelling the rate of aggregation of particles with a particular surface chemistry.

In preferred embodiments, the reference data comprises or consists of experimental data relating to one or more reference dispersions. In such embodiments, the reference data may relate to one or more reference dispersions comprising or consisting of particles having a known surface chemistry. In such instances, the surface chemistry may be known" because the particles were prepared using a particular technique, or because the surface chemistry is independently verified by another technique (e.g., XPS).

Typically, the reference data relates to one or more reference dispersions prepared under analogous (preferably identical) conditions to the sample dispersion. For example, the reference data preferably relates to particles having the same size, shape and core composition as the sample particles, dispersed in the same liquid (e.g., same salt concentration, pH, additives) under the same conditions (e.g., same temperature, dispersal method, etc.).

In some embodiments, the possible types of chemical groups which may be at the surface of the particles are known in advance. For example, it may be known that a particular sample of particles is functionalised using a particular technique, and that it is only possible to introduce certain types of chemical groups using that technique, and hence the reference data may only relate to particles having those types of chemical groups. Alternatively, it may be known that a particular sample of particles has been prepared by a particular manufacturer who only introduces certain types of chemical groups, and hence, again, the reference data may only relate to particles having those types of chemical groups.

In preferred embodiments, changes in the dispersion over time are monitored spectroscopically (i.e., the sample data comprises or consists of spectroscopic data). In such embodiments, it is preferred that the monitoring step involves measuring absorption or transmission of light or other electromagnetic radiation (preferably light, preferably visible light, most preferably visible white light) by/through the dispersion.

Advantageously, monitoring changes in the absorption (e.g., absorbance) or transmission (e.g., transmittance) of light or other electromagnetic radiation provides a simple, non-destructive, way of gaining information about the surface chemistry of particles in the dispersion. Furthermore, the equipment required to carry out the experiments is much simpler and cheaper than the equipment required for XPS measurements.

In particularly preferred embodiments, the monitoring step involves monitoring light transmission through the dispersion overtime, e.g., monitoring changes in the percentage of UV or visible light transmitted through the sample over time.

Advantageously, transmission measurements provide a general method for monitoring changes in the dispersion, e.g., it does not rely on particles absorbing radiation at a particular wavelength. In addition, the equipment required to carry out transmission measurements is relatively cheap and simple to construct.

Suitable equipment for carrying out transmission or absorption measurements may comprise, for example, a sample receptacle (e.g., a cuvette) that is transparent to the relevant light or other electromagnetic radiation, placed between a light source and a light detector (e.g., a photodiode).

In preferred embodiments, the fight source used for carrying out transmission measurements emits over a range of wavelengths, e.g., over a wavelength range of at least 300 nm, a wavelength range of at least 200 nm, a wavelength range of at least 100 nm, ora wavelength range of at least 50 nm. The upper limit for the wavelength range may be, for example, a range of 1000 nm, a range of 800 nm, a range of 500 nm, or a range of 400 nm.

Advantageously, using light over a wide range of wavelengths minimises artefacts which may arise from certain types of functionalised particles absorbing certain wavelengths. In some cases, the lower limit for the wavelength may be, for example, 400 nm, 450 nni, 500 nm or 600 nm. In some cases, the upper limit for the wavelength may be, for example, 500 nm, 600 nm or 700 nm. In especially preferred embodiments, the light source is a white-light source (i.e. emits over the whole visible spectrum, e.g., at wavelengths between 400 nm and 700 nm).

In some embodiments the measurements may comprise a parallel reference sample or control sample (e.g. the same liquid and other components but without the particulate matter; or the same liquid and other components but using unmodified particulate matter) for comparison with the test sample.

In some embodiments the illumination may be intermittent or (more preferably) continuous.

Preferably, the illumination intensity is constant throughout the duration of the monitoring step. In preferred embodiments, the illumination intensity is measured during illumination, since this allows any variations in intensity to be detected.

Monitoring of several sample dispersions andlor reference dispersions may take place simultaneously. The methods of the present invention are particularly suited to multiple simultaneous measurements, due to the simplicity of the measurement equipment (e.g., light source plus a photodiode). Advantageously, this allows the functionalisation of multiple samples to be characterised in a time-efficient manner. Furthermore, monitoring multiple dispersions simultaneously may mean that any changes in measurement conditions (e.g., temperature, light intensity etc.) affect all dispersions equally, minimising the chance of artefacts. Optionally, multiple sample dispersions containing the same particulate material are monitored simultaneously and the sample data for each dispersion is averaged to produce average sample data (e.g. mean) which is compared to reference data and/or is fitted to a model to produce fitted sample data which is compared to reference data.

The particulate material in the present invention is dispersed in a liquid (i.e., the dispersion is a sol or suspension). Advantageously, particulate material dispersed in a liquid can move relatively freely, decreasing the amount of time required to monitor changes in the dispersion.

The liquid may be non-polar or polar, and may be protic or aprotic. Preferably, the liquid is a polar protic or polar aprotic liquid. The liquid may be, for example, water, acetone, chloroform or hexane. Preferably, the liquid is acetone or (more preferably) water, e.g., deionised water.

The liquid may contain one or more additives ncluding salts, surfactants, acids, or bases.

The stability of the dispersion will depend in part on the particular liquid used. Thus, in instances in which characterising the surface chemistry relies on matching sample data to reference data, the sample dispersions and reference dispersions preferably have essentially identical (most preferably, identical) liquid. Furthermore, any additional components in the liquid are also preferably the same.

Preferably the liquid and any additives are chosen to be as transparent as possible to the wavelength of electromagnetic radiation used for the spectroscopy.

In some embodiments, the different behaviour of particles when dispersed in different liquids is used to characterise the surface chemistry of the particles. For example, the method may be carried out in a first liquid, and compared to reference data relating to reference dispersions in the same liquid. The method may be repeated with the same particulate material in a second liquid, and the results in the different liquids compared to determine or confirm whether the particulate material is functionalised and, if it is, to characterise the chemical groups at the surface of the particulate material.

In one embodiment, the method involves obtaining a dispersion of the particulate material in a first liquid, monitoring changes in the dispersion over time by collecting a set of sample data, and comparing the sample data to reference data, wherein the reference data relates to the same particulate material in a second liquid.

For example, functionalised particles may be more stable than non4unctionalised particles in a first liquid, and less stable in a second liquid. Thus, if a comparison of the sample data and reference data shows that the sample dispersion is more stable than the reference dispersion, this may indicate that the particles are functionalised. Likewise, if such a comparison shows that the sample dispersion is less stable than the reference dispersion, this may indicate that the particles are non-functionalised.

Suitably, the method is used to characterise the surface chemistry of carbon particles. The carbon particles may be, for example, non-functionalised or functionalised carbon nanotubes (e.g., single-wall carbon nanotubes (SWCNT5) or multi-wall carbon nanotubes (MWCNTs)), fullerenes, graphite (e.g., graphitic platelets, including graphitic nanoplatelets (GNPs), such as exfoliated graphite nanoplatelets (xGNPs)) or graphene (e.g., graphene platelets, including graphene nanoplatelets). Particularly advantageous results are obtained when the methods are applied to carbon nanotubes or GNPs, most preferably GNPs.

More generally, the method may be used to characterise the surface chemistry of functionalised nanomaterials (nanoparticles), in particular, inorganic nanomaterials which have been functionalised (e.g., ZnO, BN nanostructures etc.).

Preferably the method includes a step of preparing a sample dispersion by addition of a solid sample of the particular test material to the liquid (optionally also incorporating any additional components); and shaking the sample vigorously (e.g. by hand or using an agitation apparatus) or dispersing the sample using an ultrasonic probe. Preferably, the sample to liquid mass ratio, the agitation, and the temperature, are the same for all sample dispersions and reference dispersions. In instances where multiple dispersions are monitored simultaneously, the agitation step for each dispersion preferably takes place simultaneously.

Following the shaking or sonication step, the sample is transferred (preferably transferred immediately) to a spectrometer for testing. Preferably this is done as soon as possible following the shaking or sonication step (e.g. within 1 minute, preferably within 30 seconds, more preferably within 10 seconds, most preferably within 5 seconds) so that the changes in the dispersion can be monitored as soon as possible after dispersion of the particulate material.

In some cases, the shaking or sonication step may occur in the spectroscopy apparatus to allow essentially immediate recordal of the spectrum following dispersion.

The skilled person will be able to determine a suitable concentration of particles in the dispersion. Suitable concentrations will vary depending on the particular dispersion being created. For example, higher particle concentrations may be used for dispersions which are relatively stable, in order to increase the rate of aggregation. Similarly, lower particle concentrations may be used for dispersions which are relatively unstable, in order to decrease the rate of aggregation. The lower limit for the particle concentrations in the dispersion may be, for example, 5 mg/mI, 10 mg/mI, 25 mg/mI, 50 mg/mI, 100 mg/mI, or 150 mg/mI. The upper limit for the particle concentration in the dispersion may be, for example, mg/mI, 100 mg/mI, 150 mg/mI, 250 mg/mI, 500 mg/mI, or 1000 mg/mI. For example, the particle concentration in the dispersion may be between 5 mg/mI to 500 mg/mI.

In addition, suitable particle concentrations for the dispersion may be adjusted depending upon the type of particulate material being investigated, e.g., GNPs, CNTs etc. For example, it may be more appropriate to use lower concentrations for CNJTs than GNPs.

The amount of time over which changes in the dispersion are monitored will vary depending on the particular sample being studied. The skilled person will be able to determine suitable monitoring times for a particular sample. Suitable monitoring times may be, for example, at least 100 seconds, at least 200 seconds, at least 500 seconds, at least 1000 seconds, at least 2000 seconds, or at least 5000 seconds. The upper limit for the monitoring time may be, for example, 500 seconds, 1000 seconds, 2000 seconds, 5000 seconds, 10000 seconds, 20000 seconds or 50000 seconds. Preferably, the monitoring time is less than 1000 seconds.

Preferably, the dispersion is maintained at a set temperature during the monitoring step.

The lower limit for the temperature may be, for example, 1°C, 5°C, 10°C, 15°C or 20°C. The upper limit for the temperature may be, for example, 20°C, 25°C, 30°C, 50°C or 75°C. In preferred embodiments, the dispersion is maintained at room temperature e.g., between 20 and 24°C. In certain embodiments, it may be advantageous to perform measurements at elevated temperatures, since flocculation and sedimentation of the particulate material occurs faster in warmer dispersions.

In some embodiments, the temperature is controlled by apparatus used for the monitoring step. For example, in instances where the monitoring is carried out using a spectrometer, the spectrometer may include a temperature control module. Preferably, the dispersion has a uniform temperature throughout, to minimise convection currents within the liquid which might affect the sedimentation process.

In embodiments in which the particles settle/sediment over time, the dispersion may be subjected to centrifugation in order to increase the rate of sedimentation. This may be useful in instances where the dispersion is particularly stable. Alternatively, the particles are not subjected to centrifugation, e.g., particles are allowed to settle/sediment by gravity. The latter case allows simple equipment to be used to carry out the methods of the present invention (since there is no need for centrifugation equipment), and avoids any differences which might be introduced through different centrifugation protocols.

In a further aspect, the present invention provides apparatus for carrying out the methods of the first aspect. For example, the apparatus may comprise a spectrometer, comprising a light source, a light detector, and a sample-holding part between the light source and light detector, for receiving a sample receptacle (e.g., a cuvette). The spectrometer may include a temperature control module, for maintaining the same receptacle at a set temperature when the receptacle is positioned in the sample-holding part. The spectrometer may also include agitating means (e.g., an ultrasonic probe and/or a module for shaking the sample-holding) for agitating the sample before measurement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained further with reference to the accompanying drawings, in which: Fig. 1 is a plot showing transmission of light through dispersions of non-functionalised and functionalised GNPs in water over time; Fig. 2 is a plot showing fitted data for the transmission of light through multiple dispersions of non-functionalised and functionalised GNPs in water over time; Fig. 3 shows transmission of light through dispersions of non-functionalised and functionalised GNPs in acetone over time; Fig. 4 shows the correlation between the nitrogen content of GNP-NH2 particles and the particles' zeta potential (c); Fig. 5 shows transmission of light through dispersions of functionalised GNPs with different nitrogen atom contents at their surface, and different zeta-potentials; and Figs. 6 and 7 show transmission of light through dispersions of functionalised GNPs with different oxygen atom contents at their surface.

EXAMPLES

The stability of dispersions of non-functionalised and functionalised graphitic nanoplatelets (GNP5) was monitored, to confirm that the methods of the present invention can be used to characterise the surface chemistry of such materials.

Various functionalised GNPs were prepared using the methods described in WO 2012/076853. Non-functionalised GNPs (denoted raw GNP") were functionalised with different types of plasma precursor gas, including oxygen (samples denoted "GNP-02"), ammonia (samples denoted "GNP-NH2"), nitrogen (samples denoted "GNIP-N"), acrylic acid and argon (samples denoted "GNP-000H"), and fluorine (samples denoted "GNP-F").

Dispersions were then produced by adding 150 mg of the GNPs to a glass vial, filling the vial with water, and shaking to disperse the GNPs in the water. The vial was subsequently placed between an LED white-light source and a photodetector, and the transmission through the sample, as determined by the photodetector voltage, was monitored over time.

As shown in Figure 1, the light transmission through dispersions of non-functionalised GNPs in water consistently increased more rapidly than for functionalised GNPs, indicating that dispersions of non-functionalised GNPs in water were consistently less stable than dispersions of functionalised GNPs. This provided a simple way of testing whether GNPs were functionalised. The results also showed that GNPs having different types of functionalisation could be distinguished from one another. For example, GNP-02 were more stable than GNF-F, which in turn were more stable than non-functionalised GNPs.

Figure 2 shows transmission values for a separate set of experiments measuring dispersions of raw GNP, GNP-F, GNP-COOH and GNP-NH2 in water. These results also show a clear difference between raw GNPs and functionalised GNPs, and that GNP5 with different chemical groups at their surfaces have measurably different stabilities.

The experiments were repeated using acetone as the liquid, instead of water. The results, shown in Figure 3, again allow non-functionalised and functionalised GNPs, and different types of functionalisation, to be distinguished. These results also demonstrate that the stability of non4unctionalised and functionalised GNPs varies depending on the type of liquid. For example, in contrast to the results in water, GNP-02 in acetone were less stable than nori-functionalised GNPs in acetone.

The results also demonstrate that the variation of dispersion stability for GNPs in different liquids can be used to determine/confirm the type of surface functionalisation. For example, the results show that it is possible to discriminate between non-functionalised GNPs and GNF'-02 by comparing results for dispersions in both water and acetone: if the dispersion is more stable in acetone than water, this indicates that the GNPs are non-functionalised (conversely, if the dispersion is more stable in water than acetone, this indicates that the sample is GNP-02).

Taken together, these results show that the rate of change in transmission is dependent on the chemical species present at the surface of the GNP5, meaning that the rate of change in transmission can be used to determine the type of functionalisation.

To demonstrate that the amount of chemical groups introduced to the surface of GNPs can be distinguished, the experiments were repeated for dispersions of GNP-NH2 particles having different amounts of surface functionalisation.

GNP-NH2 particles were prepared as above, with the preparations adjusted so as to introduce different levels of surface functionalisation. The percentage nitrogen content of each of GNP-NH2 sample was determined to give an indication of the amount of functionalisation introduced at the surface. In addition, the zeta-potential of each GNP-NH2 sample was determined, and found to correlate with the measured nitrogen content (see Figure 4). The particles were dispersed in water at a pH 3 and monitored as described above. The results showed that the stability of the dispersion increased with the amount of functionalisation of the surface, and with increasing zeta-potential (see Figure 5). This demonstrates that the methods of the present invention can be used to distinguish between different levels of surface functionalisation.

Similar experiments were conducted for oxygen-functionalised GNPs. These experiments showed that the stability of the dispersion in water increased with the amount of oxygen functionalisation at the surface, as quantified using x-ray photoelectron spectroscopy (see Figure 6). These differences were measurable even at quite short times, as can be seen in Figure 7 (which shows best fit data across multiple test samples). Again, this demonstrates that the methods of the invention can be used to distinguish between different levels of oxygen functionalisation.

To demonstrate that the technique is applicable to other types of particles, the experiments were repeated for MWCNTs. Functionalised MWCNTs were prepared in the same manner as for the functionalised GNPs (i.e., using the methods described in WO 2012/076853), and were dispersed in water. The results (not shown) indicated that the technique allows non-functionalised and functionalised MWCNTs to be distinguished, and that different types of surface functionalisation can also be distinguished.

For the avoidance of doubt it is confirmed that in the general description above, in the usual way the proposal of general preferences and options in respect of different features and embodiments of the methods and apparatus constitutes the proposal of general combinations of those general preferences and options for the different features and embodiments, insofar as they are combinable and compatible and are put forward in the same context.

In respect of numerical ranges disclosed in the present description it will of course be understood that in the normal way the technical criterion for the upper limit is different from the technical criterion for the lower limit, i.e. the upper and lower limits are intrinsically dislinct proposals.

Claims

CLAIMS: 1. A method of characterising the surface chemistry of a particulate material, comprising: -a sample preparation step, involving obtaining a test sample comprising a dispersion of the particulate material in a liquid; -a monitoring step, involving monitoring changes in the dispersion over time to generate sample data; -a comparison step, involving comparing the sample data to reference data; and -a characterising step, involving determining surface characteristics of the particulate material based on the results of the comparison step.
2. A method according to claim 1, wherein the characterising step involves determining whether the particulate material is functionalised.
3. A method according to claim 2, wherein the comparison step involves comparing the sample data to reference data which relates to non-functionalised particulate material.
4. A method according to claim 2 or 3, wherein the comparison step involves comparing the sample data to reference data which relates to functionalised particulate material.
5. A method according to claim 2, wherein the characterising step involves determining whether the sample data matches reference data relating to non-functionalised particulate material and/or matches reference data relating to functionalised particulate material.
6. A method according to any one of the preceding claims, wherein the characterising step involves determining the type of functionalisation at the surface of the particulate material.
7. A method according to claim 6, wherein the characterising step involves determining the chemical groups at the surface of the particulate material.
8. A method according to claim 7, wherein the characterising step involves determining whether the sample data matches reference data relating to particulate material with known surface chemical groups.
9. A method according to any one of the preceding claims, wherein the characterising step involves determining the degree of functionalisation at the surface of the particulate material.
10. A method according to any of the preceding claims, wherein: -the sample preparation step further involves obtaining a reference sample, comprising a dispersion of a reference particulate material in a liquid; and -the monitoring step further involves monitoring changes in the reference sample over time to generate reference data.
11. A method according to claim 10, wherein the monitoring step involves monitoring the test sample and reference sample simultaneously.
12. A method according to any one of claims 10 or 11, wherein the sample preparation step involves preparing the test sample and reference sample simultaneously.
13. A method according to any one of claims 10 to 12, wherein the particulate material in the test sample and the reference particulate material in the reference sample are the same.
14. A method according to any one of claims 10 to 13, wherein the test sample comprises a dispersion of the particulate material in a first liquid, and the reference sample comprises a dispersion of the reference particulate material in a second liquid, and the characterising step involves determining surface characteristics based on the behaviour of the particulate materials in the different liquids.
15. A method according to claim 14, wherein one of the first and second liquids is a polar protic liquid and the other liquid is a polar aprotic liquid.
16. A method according to any one of the preceding claims, wherein the particulate material is carbon nanoparticles.
11. A method according to any one of claims 10 to 15, wherein the particulate material and reference particulate material are carbon nanoparticles.
18. A method according to claim 16 or 17, wherein the characterising step involves determining whether the carbon nanoparticles are non-functionalised or have been functionalised with oxygencontaining, nitrogen-containing, or halogen groups.
19. A method according to claim 16 or 17, wherein the characterising step involves determining whether the carbon nanoparticles have been functionalised by plasma processing.
20. A method according to claim 16 or 17, wherein the carbon nanoparticles have been functionalised by plasma processing, and the characterising step involves determining the plasma-forming gas used in the plasma processing.
21. A method according to claim 20, wherein the plasma-forming gas is selected from oxygen, water, hydrogen peroxide, alcohol, nitrogen, ammonia, an amino-bearing organic compound, halogen or halohycirocarbon.
22. A method according to claim 20, wherein the plasma-forming gas is selected from oxygen, nitrogen, ammonia, or fluorine.
23. A method according to any one of claims 16 to 22, wherein the carbon nanoparticles are carbon nanotubes or graphene nanoplatelets.
24. A method according to any one of claims 16 to 22, wherein the carbon nanoparticles are graphene nanoplatelets.
25. A method according to any one of the preceding claims, wherein: -the sample preparation step involves obtaining multiple test samples of the particulate material; and -the monitoring step involves monitoring changes in the multiple test samples over time to generate sample data.
26. A method according to any one of the preceding claims, wherein the monitoring step involves spectroscopically monitoring changes in the dispersion(s) over time.
27. A method according to claim 26, wherein the monitoring step involves monitoring changes in the transmission of light through the dispersion(s) over time.
28. A method according to any one of the preceding claims, wherein the particles are not subjected to centrifugation.
29. A method of preparing a particulate material, comprising: -a functionalisation step, involving subjecting the particulate material to a functionalisation process to produce treated particulate material; -a sample preparation step, involving obtaining a test sample comprising a dispersion of the treated particulate material in a liquid; -a monitoring step, involving monitoring changes in the dispersion over time to generate sample data; -a comparison step, involving comparing the sample data to reference data; and -a characterising step, involving determining surface characteristics of the particulate material based on the results of the comparison step.
A method according to claim 29, having any of the features of claims 5 to 28.
31 A method of characterising the efficacy of a particle functionalisation process, comprising; -a functionalisation step, involving subjecting a particulate material to a functionalisation process to produce treated particulate material; -a sample preparation step, involving obtaining a test sample comprising a dispersion of the treated particulate material in a liquid; -a monitoring step, involving monitoring changes in the dispersion over time to generate sample data; -a comparison step, involving comparing the sample data to reference data; and -a characterising step, involving determining the efficacy of the particle functionalisation process based on the results of the comparison step.
32. A method according to claim 31, wherein the reference data relates to untreated particulate material.
33. A method according to claim 31 or 32, having any of the features of claims 5 to 28.