Samuel Hybois
email: samuel.hybois@ensam.eu

Pierre Puchaud

Maxime Bourgain

Antoine Lombart

Joseph Bascou

François Lavaste

Pascale Fodé

Hélène Pillet

Christophe Sauret

Comparison of shoulder kinematic chain models and their influence on kinematics and kinetics in the study of manual wheelchair propulsion

Keywords:

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Manual wheelchairs (MWC) can help people with physical impairments to regain independent mobility. This is a primary factor for autonomy, improving social connection, active participation and self-reliance [START_REF] Finlayson | Experiencing the loss of mobility: perspectives of older adults with MS[END_REF] . However, due to the repetitive and demanding motion of the upper limbs, MWC propulsion can induce overuse pain and injuries [START_REF] Boninger | Shoulder magnetic resonance imaging abnormalities, wheelchair propulsion, and gender[END_REF][START_REF] Curtis | Shoulder pain in wheelchair users with tetraplegia and paraplegia[END_REF] . To overcome such issues, some studies have investigated the influence of MWC configuration on propulsion effort [START_REF] Sprigle | Impact of mass and weight distribution on manual wheelchair propulsion torque[END_REF][START_REF] Boninger | Manual wheelchair pushrim biomechanics and axle position[END_REF][START_REF] Kotajarvi | The effect of seat position on wheelchair propulsion biomechanics[END_REF][START_REF] Samuelsson | The effect of rear-wheel position on seating ergonomics and mobility efficiency in wheelchair users with spinal cord injuries: a pilot study[END_REF][START_REF] Richter | The effect of seat position on manual wheelchair propulsion biomechanics: a quasi-static model-based approach[END_REF][START_REF] Hughes | Biomechanics of wheelchair propulsion as a function of seat position and user-to-chair interface[END_REF][START_REF] Gutierrez | Effect of fore-aft seat position on shoulder demands during wheelchair propulsion: part 2. An electromyographic analysis[END_REF] . These studies showed that settings such as axle position, seat height and anterior-posterior position influenced upper limbs biomechanics (muscle forces and kinematics) and handrim biomechanics (push and release angles, handrim forces). Another branch of MWC propulsion optimization was investigated with the biomechanical effect of various propulsion techniques [START_REF] Slowik | The influence of wheelchair propulsion hand pattern on upper extremity muscle power and stress[END_REF][START_REF] Boninger | Propulsion patterns and pushrim biomechanics in manual wheelchair propulsion[END_REF][START_REF] De Groot | Effect of wheelchair stroke pattern on mechanical efficiency[END_REF][START_REF] Qi | Patterns of shoulder muscle coordination vary between wheelchair propulsion techniques[END_REF][START_REF] Kwarciak | The effects of four different stroke patterns on manual wheelchair propulsion and upper limb muscle strain[END_REF] . It showed that MWC propulsion patterns (arcing, single-loop, doubleloop and semi-circular) impacted muscle forces, articular loading and energy expenditure of the upper limbs, along with spatiotemporal parameters like cadence. These studies have allowed to introduce general clinical guidelines during the prescription of a MWC [START_REF] Requejo | Evidence-based strategies for preserving mobility for elderly and aging manual wheelchair users[END_REF][START_REF]Paralyzed Veterans of America Consortium for Spinal Cord Medicine Preservation of upper limb function following spinal cord injury -a clinical practice guideline for health-care professionals[END_REF] . However, some questions remain unanswered when investigating the subject-specific effects of certain MWC configurations or propulsion techniques, with the purpose of reducing both muscular and articular demands.

To achieve such a goal, a first step is to understand the mechanical behavior of the upper limbs during MWC activities. The quantification of upper limb kinematics is challenging because soft-tissue artifacts (STA) limit the accuracy of motion capturebased methods [START_REF] Leardini | Human movement analysis using stereophotogrammetry. Part 3. Soft tissue artifact assessment and compensation[END_REF] and particularly for the scapula [START_REF] Šenk | A new method for motion capture of the scapula using an optoelectronic tracking device: a feasibility study[END_REF] . Technical marker clusters positioned on the acromion or on the spine of the scapula have been proposed to non-invasively capture the motion of this bone during dynamic activities [START_REF] Van Andel | Recording scapular motion using an acromion marker cluster[END_REF][START_REF] Duprey | Three-dimensional rotations of the scapula during arm abduction: evaluation of the acromion marker cluster method in comparison with a model-based approach using biplanar radiograph images[END_REF][START_REF] Lempereur | Validity and reliability of 3D marker based scapular motion analysis: a systematic review[END_REF][START_REF] Warner | Measuring scapular kinematics during arm lowering using the acromion marker cluster[END_REF][START_REF] Morrow | Scapula kinematics and associated impingement risk in manual wheelchair users during propulsion and a weight relief lift[END_REF] . Kinematics of the upper limb can then be combined with the measure of external forces applied by the user on the MWC to assess net joint moments from inverse dynamics computational methods. To limit STA-related inaccuracies, multibody kinematic optimization (MKO), which relies on the definition of a kinematic chain, seemed to be a promising approach [START_REF] Lu | Bone position estimation from skin marker co-ordinates using global optimisation with joint constraints[END_REF][START_REF] Roux | Evaluation of the global optimisation method within the upper limb kinematics analysis[END_REF] and recent publications reported an accuracy improvement reaching 40-50% for scapulohumeral rotation using this technique [START_REF] Begon | Multibody kinematics optimization with marker projection improves the accuracy of the humerus rotational kinematics[END_REF][START_REF] Michaud | Scapular kinematic reconstruction -segmental optimization, multibody optimization with open-loop or closed-loop chains: which one should be preferred?[END_REF] . However, several formulations have been developed when modeling the kinematic chain of the shoulder complex with the connection between the thorax and the scapula being an ambitious challenge. One of the first musculoskeletal models used to investigate MWC propulsion was the Delft Shoulder and Elbow model [START_REF] Van Der Helm | A finite element musculoskeletal model of the shoulder mechanism[END_REF][START_REF] Blana | A musculoskeletal model of the upper extremity for use in the development of neuroprosthetic systems[END_REF] . It described the scapulothoracic joint as a tangent gliding plane on an ellipsoid [START_REF] Van Der Helm | A finite element musculoskeletal model of the shoulder mechanism[END_REF][START_REF] Blana | A musculoskeletal model of the upper extremity for use in the development of neuroprosthetic systems[END_REF][START_REF] Garner | A kinematic model of the upper limb based on the visible human project (VHP) image dataset[END_REF][START_REF] Naaim | Effect of various upper limb multibody models on soft tissue artefact correction: a case study[END_REF] and allocated three degrees of freedom (DoF) to the glenohumeral and acromioclavicular joints. However, this model was generally not scaled to fit the anthropometrics of each subject even if the benefit of such a procedure on the kinematic accuracy was shown a few years later [START_REF] Bolsterlee | Modelling clavicular and scapular kinematics: from measurement to simulation[END_REF] . Moreover, scaling the model remained difficult to perform properly. An upper extremity model available with the OpenSim software [START_REF] Holzbaur | A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control[END_REF][START_REF] Saul | Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model[END_REF] has also been used to investigate MWC propulsion [START_REF] Slowik | The influence of wheelchair propulsion hand pattern on upper extremity muscle power and stress[END_REF][START_REF] Rankin | Individual muscle contributions to push and recovery subtasks during wheelchair propulsion[END_REF][START_REF] Morrow | A comparison of static and dynamic optimization muscle force predictions during wheelchair propulsion[END_REF][START_REF] Slowik | Compensatory strategies during manual wheelchair propulsion in response to weakness in individual muscle groups: a simulation study[END_REF] . It had the benefit of being open-source, thus available to the whole research community, but was characterized by several coupling regression equations [START_REF] De Groot | A three-dimensional regression model of the shoulder rhythm[END_REF] to describe the 3D motion of both the clavicle and the scapula with respect to humeral elevation, namely the scapulohumeral rhythm. This model was built to allow the scaling of geometrical and inertial properties, but regression equations were believed to limit the use of this model in complex movement configurations. Previous studies already showed that the scapulothoracic joint can be modeled by a contact ellipsoid [START_REF] Pronk | The shoulder girdle: analysed and modelled kinematically[END_REF][START_REF] Van Der Helm | Geometry parameters for musculoskeletal modelling of the shoulder system[END_REF] and an ellipsoid mobilizer formulation (i.e. without kinematic constraints) was recently implemented in OpenSim [START_REF] Seth | Minimal formulation of joint motion for biomechanisms[END_REF][START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] . This model was assumed to be more physiological when describing complex motions of the shoulder and was proven to be computationally efficient. However, it has only been used to investigate single-joint arm movements and not functionally relevant tasks.

The aim of this study was therefore to compare the effect of an ellipsoid mobilizer formulation, with respect to a regressionbased model and a fixed-glenohumeral joint model, on shoulder kinematics (glenohumeral joint center displacement, joint angles, marker reconstruction errors), shoulder kinetics (glenohumeral net joint moments) and computer processing time, during the analysis of MWC propulsion tasks (start-ups and straight steady-state propulsion) collected from human subjects. We hypothesized that the ellipsoid mobilizer formulation would allow a better kinematic reconstruction of the wheelchair propulsion, with a subsequent impact on net joint moments at the shoulder.

Material and methods

Population

Following approval by the relevant ethics committee (CPP Paris VI Pitié Salpêtrière, France no. 2014-A01203-44), ten subjects with various levels of disability were involved in this study, to increase variability in upper limb kinematics and propulsion techniques. They were previously informed of the protocol and gave their written informed consent before the beginning of the experiments. The inclusion criterion imposed that subjects were experienced with MWC propulsion and did not present any shoulder pain or injury at the time and within the six months before the beginning of the experiments. The population included people with the following levels of disability and MWC expertise: 2 able-bodied who received a 3-weeks MWC practice training, 1 poliomyelitis, 1 with spinal amyotrophy, 1 with congenital malformation, 3 with paraplegia and 2 with lower limb amputation who were elite wheelchair sports athletes. The characteristics of the subjects were as follows: age: 32.9 years old (SD: 6.9 y.o.; range: 24-46 y.o); height: 1.70 m (SD: 0.09 m; range: 1.48-1.80 m); mass: 69.8 kg (SD: 7.8 kg; range: 48-80 kg).

Experiments

During the experiments, each subject was equipped with a total of 38 skin reflective markers placed on the torso, the head and on both upper limbs. The motion of the scapula was also tracked using a technical cluster composed of 3 reflective markers, placed on the spine of the scapula (as in the work by Morrow et al. [START_REF] Morrow | Scapula kinematics and associated impingement risk in manual wheelchair users during propulsion and a weight relief lift[END_REF] ; see Fig. 1 ). Markers locations were recorded with an 8-cameras optoelectronic motion capture system (Vicon system, Oxford Metrics Inc., UK) at a rate of 100 Hz. Before the experiments, the location of the scapula spine cluster on the musculoskeletal model was calibrated with a scapula locator device during a static pose ( Fig. 1 (a)). Each subject then propelled a dedicated wireless field instrumented MWC over 10 m in a motion analysis laboratory covered with linoleum. Forces and torques applied by the hands on the handrims, seat, backrest and footrest were recorded at a 100 Hz frequency with a wireless field instrumented MWC (FRET-2, Fig. 1 (b)) and synchronized with the motion capture system. This instrumented wheelchair (TSR-mesures, France [START_REF] Dabonneville | A self-contained wireless wheelchair ergometer designed for biomechanical measures in real life conditions[END_REF][START_REF] Hybois | Shoulder kinetics during start-up and propulsion with a manual wheelchair within the initial phase of uninstructed training[END_REF][START_REF] Sauret | Handrim mechanical power during wheelchair propulsion on level and cross-slope surfaces: a preliminary study[END_REF] ) was adjusted with standard settings that remained unchanged between participants (weight: 38 kg; wheelbase: 430 mm). All participants completed the entire acquisition sessions. One start-up cycle and one steady-state propulsion cycle were processed for each subject. For start-up, subjects were asked to start from the center of the motion capture system calibrated volume and to perform 3 to 4 pushes. In this task, only the first push was analyzed. For steadystate propulsion, subjects started outside the calibrated volume, were pushed by an assistant to favor the ignition of linear velocity, and were asked to perform 6 to 7 pushes, with one cycle (the third or fourth) entirely captured within the calibrated volume of the motion capture system.

Model

A custom-made musculoskeletal model of the thorax and both upper limbs (referred to as M ellips ) was designed in OpenSim. The definition of the kinematic chain was based on the unilateral scapulothoracic joint model developed by Seth et al. [START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] . Since this model was limited to the thorax and right-side shoulder, it was extended to a full shoulder-to-hand kinematic chain and symmetrized to result in a bilateral model ( Fig. 1 (c)). The resulting M ellips model displayed 2 DoF at the clavicle, 4 DoF between the scapula and thorax (ellipsoid joint), 3 DoF at the glenohumeral joint and 2 DoF at both the elbow and the wrist. A geometrical contact constraint between the clavicle and scapula was added at the acromion. For comparison purposes, this M ellips model was modified to result in a second model ( M regr ) with an identical kinematic chain except for the shoulder joint description, which was chosen identical to the upper extremity model from Holzbaur et al. [START_REF] Holzbaur | A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control[END_REF] and Saul et al. [START_REF] Saul | Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model[END_REF]. This M regr model did not display independent scapula DoF with respect to the thorax, but involved several coupling regression equations. As opposed to M ellips , this second model thus inferred sternoclavicular motion from scapular orientations via regression methods. Finally, a third model ( M fix ) was derived from M ellips , with the scapula and clavicle locked, resulting in the 3-DoF glenohumeral joint to represent the whole shoulder. Aside from the shoulder joint definition, all parameters (segment mass and inertia, other joint definitions) were identical for the three models. The 3 versions of the models were generic, but a homothetic scaling was applied to each bony segment independently, based on distances between markers. This scaling step of the models was performed for each subject of the population. Kinematic data (marker trajectories) and kinetic data (handrim and seat forces) collected for each subject were fed into the model, with a process detailed below.

Data processing

Markers trajectories were smoothed with an average sliding window (5 values) with 2-passes in reverse direction to minimize the shifting effect. Gaps in trajectories were filled using a C2-spline interpolation (gaps shorter than 15 frames, i.e. 0.15 s) or using a rigid registration method [START_REF] Söderkvist | Determining the movements of the skeleton using well-configured markers[END_REF] based on the other markers of the same segment (gaps longer than 15 frames). Data processing was performed with OpenSim 3.3 [START_REF] Delp | OpenSim: open-source software to create and analyze dynamic simulations of movement[END_REF] , identically for the three models. First, model geometries and inertial parameters were scaled to the anthropometry of each subject, based on anatomical landmarks located with markers (or palpated with the scapula locator device in the case of the scapula). Afterwards, MKO [START_REF] Lu | Bone position estimation from skin marker co-ordinates using global optimisation with joint constraints[END_REF] was performed to compute the generalized coordinates of the models using the inverse kinematics algorithm implemented in OpenSim, with both anatomical and technical markers trajectories as inputs. This was performed during steady-state propulsion and start-up cycles. Net joint moments were obtained using a Newton-Euler recursive inverse dynamics algorithm, expressed in the thorax orthonormal coordinate system, centered on the humeral head center [START_REF] Cooper | Glenohumeral joint kinematics and kinetics for three coordinate system representations during wheelchair propulsion[END_REF][START_REF] Desroches | Effect of system tilt and seat-to-backrest angles on load sustained by shoulder during wheelchair propulsion[END_REF] , to favor clinical interpretation and comparison between models.

Data analysis

To compare the influence of the different shoulder models on marker reconstruction, root mean squared errors (RMSE) between experimental and reconstructed markers were computed and then averaged by segment. The evaluation of multibody kinematic optimization was made with the assumption that a lower RMSE denoted a better kinematic reconstruction. The comparison in kinematics also included the displacement of the glenohumeral joint center in the thorax reference frame (computed following ISB recommendations [START_REF] Wu | ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion-Part II: shoulder, elbow, wrist and hand[END_REF] ) and joint angles which are the DoF of the model (i.e. clavicle protraction/retraction and clavicle elevation/depression). Mean values of the net shoulder moments (i.e. flexion/extension, internal/external rotation and total) along each cycle were computed and compared between models, as well as peak values over the entire cycle, that were reached during the push phase. Peak values of the flexion and internal rotation components were also reported.

The computer processing time with the different models was estimated for each subject on a conventional desktop computer (Windows 7, Intel® Xeon® CPU 2.80 GHz, RAM: 6 GB). It was defined as the time needed to execute a workflow composed of multibody kinematics optimization (3D motion capture file of a steady-state propulsion cycle as input) and inverse dynamics (generalized coordinates and external forces as inputs), using the scaled model of the corresponding subject.

Statistics

For each calculated variable, mean values and standard deviations were computed and reported over the whole population for each task (start-up and steady-state propulsion).

Results

Kinematics

The acromion marker translation in the thorax reference frame, averaged over the 10 subjects was greater in the anterior-posterior direction than in the vertical direction (see Table 1 ). Subsequently, to track this motion in the frontal and transversal planes, musculoskeletal models that enabled the mobility of the clavicle (i.e. models M ellips and M regr ) exhibited variations of their sternoclavicular generalized coordinates (i.e. joint angles) during both steadystate propulsion and start-up ( Fig. 2 ). The inter-individual variability of sternoclavicular angles, which can be assessed by the standard deviation corridor, was also higher with M ellips than with M regr ( Fig. 2 ). Because of this movement of the clavicle, M ellips displayed the highest amplitude for the glenohumeral joint center displacement in the thorax reference frame ( Table 1 ), in both anterior-posterior and vertical directions ( Fig. 3 ). Model M regr showed only a narrow range of motion for the glenohumeral joint center, while it remained fixed for M fix, by definition.

Finally, the evaluation of the reconstructed kinematics, performed by computing the RMSE per segment between experimental and reconstructed markers, showed that the M ellips model resulted in a limited error compared to M regr and M fix ( Table 1 ) for both tasks (steady-state propulsion and start-up). Comparison between M regr and M fix showed that M regr resulted in lower RMSE than M fix for proximal segments (thorax, clavicle, and scapula) but not for distal segments (arm, forearm). The distribution of the overall reconstruction error (i.e. RMSE averaged over the whole marker set) along the cycle showed that for the M ellips model, the reconstruction was slightly better during the recovery phase than during the push phase ( Fig. 5 ). Conversely, the timing of the peak error for M regr occurred at the transition between the push and recovery phases, while M fix displayed its lowest reconstruction error at the end of the push phase.

Kinetics

Resulting net shoulder moments varied depending on the version of the musculoskeletal model used to perform the inverse dynamics process ( Table 1 , Fig. 4 ). Peak internal rotation and peak total shoulder moments displayed similar values between models, as well as the mean resulting shoulder moment. However, the flexion component (in the sagittal plane) varied noticeably among models.

Computer processing time

Finally, in terms of computation performance, the computer processing time was the lowest for M ellips (14.0 ± 1.0 s). Similar values were obtained for M fix (15.5 ± 1.1 s) but M regr resulted in higher computation time (55.5 ± 12.3 s).

Discussion

Experimental data showed a displacement of the acromial marker with respect to the thorax, especially in the fore-aft direc-tion. This result underlined the necessity to allow sternoclavicular mobility when analyzing MWC propulsion. The inter-individual variability of the sternoclavicular angles, which were greater with M ellips than with M regr , proved that model M regr favored a particular motion of the clavicle due to the prediction equation based on the arm elevation. Hence, model M regr hindered to reproduce the propulsion technique for individuals of the population who spontaneously engaged their clavicle in protraction/retraction. This is emphasized for this activity because MWC propulsion mainly occurs in the sagittal plane with low humerus elevation angles. Therefore, an advantage of the M ellips model is its flexibility to potentially account for differences due to the level of disability, which is not possible with the other models.

The M ellips model showed an improved RMSE for each segment of the upper limbs kinematic chain, with respect to other models. The reconstruction errors obtained were consistent with those reported by Blache and Begon [START_REF] Blache | Influence of shoulder kinematic estimate on joint and muscle mechanics predicted by musculoskeletal model[END_REF] , which ranged from 7 to 23 mm for analytical, sports-related and daily life movements. The ability of M ellips to reproduce the motion of the upper limbs should be related to the scapula mobility on the ellipsoid, which resulted in a larger glenohumeral joint center displacement ( Fig. 3 ). Conversely, the M regr model, despite its theoretical ability to describe the glenohumeral motion, resulted in a quasi-fixed position of the glenohumeral joint center during the cycle ( Fig. 3 and Table 1 ) due to the low arm elevation during MWC propulsion.

In addition, the differences in the glenohumeral joint center location between models impacted the lever arms of handrim forces, which mainly explain the differences in the resulting net shoulder moments. This could lead to a change in the sign of the joint moment and consequently to differences in muscles recruitment.

The average of peak flexion moment during the push phase obtained with model M ellips was consistent with values already reported in the literature [START_REF] Hybois | Shoulder kinetics during start-up and propulsion with a manual wheelchair within the initial phase of uninstructed training[END_REF][START_REF] Van Drongelen | Mechanical load on the upper extremity during wheelchair activities[END_REF] . The resulting glenohumeral joint moment displayed similar values to those reported in the study of Vegter et al. [START_REF] Vegter | Early motor learning changes in upper-limb dynamics and shoulder complex loading during handrim wheelchair propulsion[END_REF] . It is important to note, however, that such comparisons should be made cautiously because the coordinate systems in which moments were expressed largely varied in the literature investigating MWC shoulder kinetics [START_REF] Cooper | Glenohumeral joint kinematics and kinetics for three coordinate system representations during wheelchair propulsion[END_REF][START_REF] Morrow | Upper-limb joint kinetics expression during wheelchair propulsion[END_REF][START_REF] Collinger | Shoulder biomechanics during the push phase of wheelchair propulsion: a multisite study of persons with paraplegia[END_REF][START_REF] Mercer | Shoulder joint kinetics and pathology in manual wheelchair users[END_REF][START_REF] Van Drongelen | Mechanical load on the upper extremity during wheelchair activities[END_REF][START_REF] Sabick | A new method to quantify demand on the upper extremity during manual wheelchair propulsion[END_REF][START_REF] Veeger | Load on the shoulder in low intensity wheelchair propulsion[END_REF][START_REF] Koontz | Shoulder kinematics and kinetics during two speeds of wheelchair propulsion[END_REF][START_REF] O'reilly | On representations for joint moments using a joint coordinate system[END_REF][START_REF] Desroches | Expression of joint moment in the joint coordinate system[END_REF] .

As mentioned in the paper introducing the scapulothoracic joint model [START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] , in which the computational speeds were reportedly faster than real time, the ellipsoid mobilizer approach enabled the computer processing time to be drastically reduced with M ellips compared to M regr . Owing to the higher number of segments used in the present study than in the work of Seth et al. [START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] , however, it was not possible to achieve real-time computation with a 100 Hz frame rate. Another explanation for the differences between the present study and that of Seth et al. [START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] may lie in the measurement protocols, namely the use of intracortical bone pins for markers on the scapula and the subject-specificity of the ellipsoid parameters and clavicle length.

This study does have certain limitations, however. First of all, since this work aimed at embracing a modeling approach, participants were recruited from a convenience sample. This was a limit when drawing subject-specific conclusions about the effects of the model but allowed to apply the method on multiple individuals with various propulsion techniques. A subsequent limitation was that the small size of population questioned the relevance of a statistical analysis, since only one cycle was analyzed for each participant. Apart from mean and standard deviations, the authors decided not to include any statistics. However, on a set of ten subjects with different levels of disability, this study demonstrates the forces and weaknesses of three upper-limb kinematic chains when studying MWC propulsion biomechanics. Due to the multiple embedded sensors, the instrumented MWC used for the experiments was also significantly heavier than most of the conventional MWC. This may have led some participants to modify the biomechanics of their upper limbs during propulsion when compared to their own MWC. However, this should not challenge the application of results since MWC propulsion parameters (linear speed, contact and release angles, push and recovery phases temporal parameters, handrim forces and torques etc.) were in accordance with previously reported results from experiments performed with lightweight wheelchairs [START_REF] Hybois | Shoulder kinetics during start-up and propulsion with a manual wheelchair within the initial phase of uninstructed training[END_REF][START_REF] Eydieux | Changes in wheelchair biomechanics within the first 120 minutes of practice: spatiotemporal parameters, handrim forces, motor force, rolling resistance and foreaft stability[END_REF] . Finally, another limitation of this study is that no gold-standard is provided to directly evaluate the accuracy of joint angles with the different models. Such a comparison would have required the use of intracortical bone pins, which is a highly invasive technique whose regular use would be ethically questionable. In addition, intracortical bone pins require anesthesia that would limit the motion and their fixations can also cause discomfort or pain modifying the studied motion.

However, the results of this study are not challenged by the absence of gold standard for kinematics because even if slight differences can exist between real scapula motion and scapula cluster [START_REF] Karduna | Dynamic measurements of three-dimensional scapular kinematics: a validation study[END_REF] , only the ellipsoid model is able to track the motion of the scapula during MWC propulsion.

Conclusion

This study aimed at comparing the shoulder kinematics and kinetics during MWC propulsion, computed with musculoskeletal models displaying different kinematic chains: either implemented with an ellipsoid mobilizer, regression equations, or with no scapula movement in the thorax coordinate system. The results showed the relevance of modeling the scapula as gliding on an ellipsoid [START_REF] Van Der Helm | A finite element musculoskeletal model of the shoulder mechanism[END_REF][START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] , which is a more physiological description, in comparison with musculoskeletal models based on regression equations [START_REF] Holzbaur | A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control[END_REF][START_REF] Saul | Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model[END_REF] . The model derived from the work of Seth et al. [START_REF] Seth | A biomechanical model of the scapulothoracic joint to accurately capture scapular kinematics during shoulder movements[END_REF] displayed the best markers reconstruction, and was able to both capture subject-specific propulsion techniques and provide shoulder moments consistent with values reported with current models such as the one from Holzbaur et al. [START_REF] Holzbaur | A model of the upper extremity for simulating musculoskeletal surgery and analyzing neuromuscular control[END_REF] and Saul et al. [START_REF] Saul | Benchmarking of dynamic simulation predictions in two software platforms using an upper limb musculoskeletal model[END_REF]. Another benefit of the mobilizer approach is its ability to drastically reduce the computer processing time. To pursue investigation, subject-specific methods should be developed to define the kinematic chain, especially the scapulothoracic ellipsoid parameters (center, orientation and radii) and the clavicle length. This is a crucial step before computing muscle forces and drawing clinical conclusions.

Fig. 1 .

 1 Fig. 1. (a) Calibration of the right scapular spine cluster position with a scapula locator. The left scapula spine cluster can be seen on the left shoulder of the subject. (b) Experimental protocol with the instrumented field wheelchair. (c) Musculoskeletal model developed ( M ellips ), with the ellipsoid mobilizer in red.

Fig. 2 .

 2 Fig. 2. Sternoclavicular generalized coordinates averaged over the 10 subjects (mean in bold line ± 1 standard deviation in shaded). Red: computed with M ellips model; Blue: computed with M regr ; Left: clavicle protraction; Right: clavicle elevation; Top: propulsion; Bottom: start-up.

Fig. 3 .

 3 Fig. 3. Successive positions of the glenohumeral joint center computed with the three models in the thorax reference frame along a propulsion cycle for one subject.

Fig. 4 .

 4 Fig. 4. Illustration of typical shoulder flexion (negative)/extension and internal (positive)/external rotation moments for one specific subject of the cohort during a steadystate propulsion cycle.

Fig. 5 .

 5 Fig. 5. Evolution of the overall marker reconstruction errors along a typical locomotion cycle (push phase + recovery phase) for one specific subject of the cohort.

Table 1

 1 Summary of the biomechanical outputs of the study, averaged over the 10 subjects (standard deviations between brackets). For comparison, steady-state propulsion and start-up cycles are separated, as well as the model versions ( M ellips , M fix and M regr ).

	Steady-state propulsion	Start-up

Funding

None.

Conflicts of interest

The authors declare they have no financial or personal relationship with other people or organization that could inappropriately influence their work.

Ethical approval

The study protocol was approved by the relevant ethics committee CPP Paris VI Pitié Salpêtrière, France (Ref. no. 2014-A01203-44).


Among the wide variety of existing carbons materials, activated carbons (ACs) exhibit, on average, the highest hydrogen adsorption capacities (up to 7 wt. % at 4 MPa and 77 K) [1,[START_REF] Sdanghi | Hydrog. Storage Technol[END_REF]. Hydrogen adsorption on ACs is really advantageous at moderate pressures, lower than 40MPa. Thus, the use of ACs can reduce by 23% the volume needed for hydrogen storage when compared to pure compression at 20 MPa and room temperature [START_REF] De La Casa-Lillo | [END_REF]. Hydrogen adsorption at supercritical conditions on ACs has been adequately fitted with the modified Dubinin-Astakhov (MDA) equation. However, the physical meaning of the MDA parameters is not completely understood [4] whereas it is of primary importance for designing hydrogen storage systems [5,6,7]. In order to analyze the relationship between the textural properties and the hydrogen adsorption capacity of these materials, we first fitted the excess hydrogen adsorption with the MDA equation.

METHODOLOGY

MODIFIED DUBININ-ASTAKHOV Equation

A high percentage of mesopores was observed for MSC-30, SA1500 and SA20 carbons while MSP20X and TH90I were essentially microporous.

𝑛 𝑒𝑥𝑐 = 𝑛 𝑎𝑏𝑠 -𝜌 𝑔 𝑉 𝑎 n exc = RT +T 2 ln 2 P -ρ g ∆𝐻 𝑎 0 = -𝛼 -𝑙𝑛 𝑛 𝑎 𝑛 𝑚𝑎𝑥 ∆𝑆 𝑎 = 𝑛 𝑚𝑎𝑥 𝛽 𝜋 2 1 -𝐸𝑟𝑓 -ln 𝑛 𝑎 𝑛 𝑚𝑎𝑥 + 𝑛 𝑎 𝛽 -ln 𝑛 𝑎 𝑛 𝑚𝑎𝑥 + 𝑛 𝑎 𝑅 𝐿𝑛(𝑃 0 /𝑃 0 )
n abs [mol kg -1 ]: absolute adsorbed amount of H 2 ρ g [kg m -3 ] : adsorbed phase density V a [m 3 kg -1 ] : adsorbed phase volume n max [mol kg -1 ]: maximal hydrogen amount P°[MPa] : saturation pression α [J mol -1 ] : enthalpie factor β [mol J -1 K -1 ] : entropie factor ∆H a 0 [kJ mol -1 ] : adsorption enthalpy ∆S a [kJ mol -1 ] : adsorption entropy P 0 [MPa] : pseudo saturation pressure P 0 [MPa] : saturation pressure R : 8,314 [J mol -1 K -1 ] Θ : hydrogen fractional filling

DUBININ-ASTAKHOV Model

The modified Dubinin-Astakhov (MDA) equation proved to be a good analytical tool for understanding experimental hydrogen adsorption data over a wide range of pressures and temperatures. The evolution of the MDA parameters was correlated to the textural properties of the studied CACs. The entropy factor showed a linear increase with the average micropore size while the enthalpy factor showed the opposite trend. Enthalpie and entropie variation with the hydrogen fractional filling were determined for all the CACs. Figure 5 shows that V a and n max increased with S NLDFT , highlighting the paramount importance of surface area in hydrogen adsorption. Figure 6 shows the β and α parameters as a function of the average micropore size. a, related to the adsorption enthalpy, decreased as the average micropore size increased, due to the stronger interactions between the hydrogen molecules and the walls within narrow pores. β increased with pore size due to the lower ordering of hydrogen molecules achieved. Figures 7 and8 show adsorption enthalpy and entropy as a function of Θ, while the first one decreases the second one increases with fractional filling.

1

  Institut Jean Lamour, UMR 7198 CNRS and Université de Lorraine, Épinal, France 2 Laboratoire d'Energétique et de Mécanique Théorique et Appliquée, UMR 7563 CNRS and Université de Lorraine, Vandoeuvre-lès-Nancy, France * Presenting author's e-mail: vanessa.fierro@univ-lorraine.fr

Figure 4 .Figure 6 .Figure 2 .Figure 3 .Figure 8 .

 46238 Figure 4. MDA fitting of excess H 2 Isotherms on MSC30

ACKNOWLEDGEMENTS

Financial support from CONACYT-SENER scholarship 708651/ 739301 is thankfully acknowledged. This study was partly supported by the French PIA project "Lorraine Université d'Excellence", reference ANR-15-IDEX-04-LUE and TALiSMAN project, funded by FEDER (2019-000214). CACs free supply from Ingevity ™ and Silcarbon Aktivkohle ™ is gratefully aknowledged.


Nawal Berrada

Alexandre Desforges

Christine Bellouard

Emmanuel Flahaut

Jeŕome Gleize

Jaafar Ghanbaja

Brigitte Vigolo

Protecting Carbon Nanotubes from Oxidation for Selective Carbon Impurity Elimination

Purity of carbon nanotubes (CNTs) is essential to avoid a dramatic decrease in their performances. In addition to metallic impurities, carbonaceous impurities have been shown to be responsible for pronounced effects. However, they are highly difficult to be selectively removed from CNT samples because of the similar chemical reactivity of these two kinds of carbon species. The existing purification methods often lead to high CNT consumption (>90 wt %). The proposed method consists of a one-pot gas-phase treatment combining chlorine and oxygen. The CNT powder maintained in a chlorine stream is submitted to oxygen at moderate temperature [350 and 500 °C for single-walled CNTs (SWCNTs) and double-walled CNTs (DWCNTs), respectively], and the thermal treatment is then pursued at 900-1000 °C under chlorine alone. Our work reveals that this approach is able to significantly improve the selectivity of elimination of carbonaceous impurities. Thanks to the proposed purification treatment, only 19 and 11 wt % of carbon species (mainly carbon impurities) are lost for DWCNTs and SWCNTs, respectively. The mechanism proposed involves a protective effect by grafting of chlorine favored to the CNT walls. Because our simple one-pot purification method is also versatile and scalable, it opens new perspectives for CNT applications in high-added value fields.

INTRODUCTION

Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene, are exciting materials showing a combination of superior properties: lightness, thermal and electrical conductivity, and optical and mechanical properties. 1 The technology of CNT synthesis, although in a continuous state of improvement, cannot avoid the use of catalysts to increase the conversion rate from carbon precursors to the production of carbon nanomaterials, as this rate is still far below 100%. [2][3][START_REF] Jia | Advances in Production and Applications of Carbon Nanotubes[END_REF][START_REF] Cui | Formation Mechanism of Carbon Encapsulated Fe Nanoparticles in the Growth of Single-/Double-Walled Carbon Nanotubes[END_REF][START_REF] Yadav | Kinetic study of single-walled carbon nanotube synthesis by thermocatalytic decomposition of methane using floating catalyst chemical vapour deposition[END_REF] The presence of both metal and carbon impurities has negative effects on the CNT properties. [START_REF] Pumera | Voltammetry of Carbon Nanotubes and Graphenes: Excitement, Disappointment, and Reality[END_REF][START_REF] Pan | Carbon Nanotubes for Supercapacitor[END_REF] It is widely known that magnetic properties of CNTs are dominated by catalyst residues even at low content. [START_REF] Vejpravova | Magnetic Impurities in Single-Walled Carbon Nanotubes and Graphene: A Review[END_REF] Thermal and electrical conductivity and mechanical properties (tensile strength) are dramatically decreased because of carbon impurities in CNT samples. [START_REF] Matsumoto | Quantitative Assessment of the Effect of Purity on the Properties of Single Wall Carbon Nanotubes[END_REF] Although it was first attributed to the CNTs themselves, the observed electrocatalytic activity has been shown to be due to metallic catalyst impurities [START_REF] Kruusma | Single Walled Carbon Nanotubes Contain Residual Iron Oxide Impurities Which Can Dominate Their Electrochemical Activity[END_REF][START_REF] Banks | Iron Oxide Particles Are the Active Sites for Hydrogen Peroxide Sensing at Multiwalled Carbon Nanotube Modified Electrodes[END_REF] or carbonaceous by-products. [START_REF] Wang | Carbonaceous Impurities in Carbon Nanotubes Are Responsible for Accelerated Electrochemistry of Cytochrome c[END_REF][START_REF] Stuart | Nanographite Impurities within Carbon Nanotubes Are Responsible for Their Stable and Sensitive Response Toward Electrochemical Oxidation of Phenols[END_REF][START_REF] Wang | Carbonaceous Impurities in Carbon Nanotubes Are Responsible for Accelerated Electrochemistry of Acetaminophen[END_REF] Similar mistrusted effects on electrochemical properties from carbon impurities in graphene samples have been reported by Pumera et al. [START_REF] Pumera | Impurities in Graphenes and Carbon Nanotubes and Their Influence on the Redox Properties[END_REF] The purification of CNTs has been extensively desired, and various methods allow to efficiently remove metallic contamination. [START_REF] Hou | Purification of Carbon Nanotubes[END_REF][START_REF] Makama | Recent Developments in Purification of Single Wall Carbon Nanotubes[END_REF] Some methods to remove only catalyst residues consist of annealing the CNT powder at a temperature in the 2000-2500 °C range under N 2 , Ar, or vacuum. [START_REF] Andrews | Purification and Structural Annealing of Multiwalled Carbon Nanotubes at Graphitization Temperatures[END_REF][START_REF] Zhang | Purification of Multiwalled Carbon Nanotubes by Annealing and Extraction Based on the Difference in van Der Waals Potential[END_REF][START_REF] Huang | 9% Purity Multi-Walled Carbon Nanotubes by Vacuum High-Temperature Annealing[END_REF] However, even if high-quality CNTs can be prepared from the standard methods, they still fail in selectively removing carbonaceous impurities without excessive loss of CNTs, leading to low sample yield. [START_REF] Cho | A Simple and Highly Effective Process for the Purification of Single-Walled Carbon Nanotubes Synthesized with Arc-Discharge[END_REF] The main reason explaining this difficulty of eliminating carbon impurities without attacking the CNTs is their too close chemical reactivity. [START_REF] Mercier | Soft Oxidation of Single-Walled Carbon Nanotube Samples[END_REF][START_REF] Vigolo | A Comprehensive Scenario for Commonly Used Purification Procedures of Arc-Discharge as-Produced Single-Walled Carbon Nanotubes[END_REF] Standard approaches use strong acids, which are known to damage the CNTs and lead to weak sample yield; they also create functional groups at the CNT surface and produce large amounts of amorphous carbon debris. [START_REF] Salzmann | The Role of Carboxylated Carbonaceous Fragments in the Functionalization and Spectroscopy of a Single-Walled Carbon-Nanotube Material[END_REF] These surface groups and debris are prejudicial for further CNT applications, for which surface and interfacial phenomena play the major role. [START_REF] Pan | Carbon Nanotubes for Supercapacitor[END_REF][START_REF] An | Inherent Carbonaceous Impurities on Arc-Discharge Multiwalled Carbon Nanotubes and Their Implications for Nanoscale Interfaces[END_REF] Gas-phase routes using air and/or oxygen, hydrogen, carbon dioxide, and ammonia have been also reported as efficient purification methods to selectively remove carbonaceous impurities from CNT samples. [START_REF] Ismail | A Review of Purification Techniques for Carbon Nanotubes[END_REF][START_REF] Park | Purification Strategies and Purity Visualization Techniques for Single-Walled Carbon Nanotubes[END_REF] Thermal air or oxygen oxidation at moderate temperature was able to selectively attack amorphous carbon or carbonaceous impurities of lower oxidation resistance than CNTs. [START_REF] Li | Oxidation of Multiwalled Carbon Nanotubes by Air: Benefits for Electric Double Layer Capacitors[END_REF][START_REF] Shaffer | Dispersion and Packing of Carbon Nanotubes[END_REF][START_REF] Bortolamiol | Double-Walled Carbon Nanotubes: Quantitative Purification Assessment, Balance between Purification and Degradation and Solution Filling as an Evidence of Opening[END_REF][START_REF] Ando | Structure Analysis of Purified Multiwalled Carbon Nanotubes[END_REF] Hightemperature heating in hydrogen or in ammonia has been proposed as an effective purification treatment. [START_REF] Vivekchand | New Method of Purification of Carbon Nanotubes Based on Hydrogen Treatment[END_REF][START_REF] Wang | An Integrated Route for Purification, Cutting and Dispersion of Single-Walled Carbon Nanotubes[END_REF] Carbon dioxide was also used as a milder oxidant with successful attack of carbon impurities and less damaging of CNTs. [START_REF] Smith | Selective Oxidation of Single-Walled Carbon Nanotubes Using Carbon Dioxide[END_REF][START_REF] Delpeux | An efficient two-step process for producing opened multi-walled carbon nanotubes of high purity[END_REF] For these gas-phase methods, a subsequent treatment usually in a liquid medium, such as nitric or hydrochloric acid, is required to remove the metallic impurities. Interestingly, halogens, such as bromine [START_REF] Kulawik | A Review of Procedures of Purification and Chemical Modification of Carbon Nanotubes with Bromine[END_REF] and chlorine, under various forms (hydrogen chloride, [START_REF] Zimmerman | Gas-Phase Purification of Single-Wall Carbon Nanotubes[END_REF] tetrachloromethane, [START_REF] Barkauskas | A Novel Purification Method of Carbon Nanotubes by High-Temperature Treatment with Tetrachloromethane[END_REF] or chlorine [START_REF] Chng | Purification of Carbon Nanotubes by High Temperature Chlorine Gas Treatment[END_REF][START_REF] Mercier | Selective Removal of Metal Impurities from Single Walled Carbon Nanotube Samples[END_REF] ) are able to effectively remove metallic impurity from CNTs, carbon species (including CNTs and carbonaceous impurities) being untouched. In a previous work, [START_REF] Desforges | Dramatic Enhancement of Double-Walled Carbon Nanotube Quality through a One-Pot Tunable Purification Method[END_REF] we had shown that the addition of oxygen in chlorine could lead to the removal of carbon impurities from double-walled CNTs (DWCNTs). Unfortunately, the combustion rate at the high temperature used (950 °C) was too rapid and not possible to be controlled to reduce the CNT loss, which was as high as 95%. In spite of the extensive research in the field, efficient elimination of carbonaceous impurities from CNT samples has not been achieved without damaging and consuming the majority of CNTs; the missing key is to reach a high degree of selectivity of carbonaceous impurity attack.

We present here experimental results that evidence a high selectivity in the elimination of carbonaceous impurities with respect to CNTs occurring in particular working conditions. Moreover, in contrast with the few available reported nondamaging methods, our approach allows to prepare purified CNTs while limiting the number of chemical treatments. [START_REF] Ma | Purification of Single-Walled Carbon Nanotubes by a Highly Efficient and Nondestructive Approach[END_REF][START_REF] Wang | A Highly Selective, One-Pot Purification Method for Single-Walled Carbon Nanotubes[END_REF][START_REF] Ballesteros | Steam Purification for the Removal of Graphitic Shells Coating Catalytic Particles and the Shortening of Single-Walled Carbon Nanotubes[END_REF] Our entirely gas-phase treatment consists of a concomitant thermal activation of a protection/combustion effect of the carbon species before the spontaneous elimination of the formed metallic chlorides in a one-pot treatment. The favored oxidation of carbonaceous impurities is demonstrated by applying this treatment to both single-walled CNTs (SWCNTs) and DWCNTs. The results show that the combination of chlorine and oxygen at temperatures as low as 350-500 °C can be a powerful asset for an effective carbon impurity elimination with a much higher carbon yield than previously reported 81 wt % instead of 5 wt %. [START_REF] Desforges | Dramatic Enhancement of Double-Walled Carbon Nanotube Quality through a One-Pot Tunable Purification Method[END_REF] These results have been achieved thanks to a strong protecting effect of CNTs against combustion occurring in a quite low-temperature range (350-500 °C) when oxygen is introduced in chlorine while carbonaceous impurities remain susceptible to be vaporized. The proposed reaction mechanism, unprecedented in the literature, which allows to explain this finding, is based on the identified tunable reactivity between oxygen and chlorine toward carbon species, that is, CNTs and carbon impurities. The method we propose here hence fulfills all of the requirements to purify large volumes of CNTs with high sample yields, which is of great interest for their practical applications.

MATERIALS AND METHODS

2.1. CNT Purification. The DWCNT sample was synthesized by catalytic chemical vapor deposition under optimized operating conditions; [START_REF] Flahaut | Gram-Scale CCVD Synthesis of Double-Walled Carbon Nanotubes[END_REF] the raw DWCNT sample is referred to as rD. The raw HiPco SWCNT samples used for this work were supplied by NanoIntegris Inc. For the purification of the HiPco sample, because of the numerous conditions tested, we had to use several raw batches of SWCNTs. Each one has been distinguished: rS1-2 is the starting raw SWCNT sample, which has been purified to prepare S1 and S2; rS3-5 corresponds to the raw SWCNTs used to obtain S3, S4, and S5; and rS6 for S6. The experimental conditions used for the purification of both SWCNTs and DWCNTs are given in the Supporting Information, Table S1. Moreover, the treatment of a DWCNT sample was intentionally stopped after the O 2 dwell under Cl 2 at 500 °C; this sample is referred to as mD.

For the purification treatment (Figure 1), the CNT powder (∼150 mg) was placed in a silica boat (length 10 cm) in a tubular oven (diameter 2 cm), which was first flushed by nitrogen (nitrogen being also the carrier gas for the process) to carefully remove air; purified chlorine at around 200 mL/min was incorporated into the set-up while the temperature was increased (10 °C/min). When the temperature reached the chosen value, T O 2 (T O 2 variation range: 250-950 °C), O 2 with a flow rate of 2-4 mL/min was injected for the desired duration (t 1 -t 2 in the 20-60 min range). After the Cl 2 /O 2 treatment stage, the sample was heated at T Cl 2 for 1 h under chlorine alone before natural cooling (see also Table S1).

With the used conditions regarding the O 2 flow rate and duration of its injection, the used O 2 volume is around 200 mL (9 × 10 -3 mol). The amount of carbon for the experiments is at most 8 × 10 -3 mol. That means that for every experiment carried out, the amount of oxygen content used is sufficient enough to burn all the unwanted carbon-based species.

2.2. Characterization Techniques. Transmission electron microscopy (TEM) observations were performed using a JEM-ARM 200F apparatus at an accelerating voltage of 80 kV. At least 30-40 images taken at different places were analyzed for each sample in order to guarantee a representative description of the samples. Thermogravimetric analysis (TGA) was performed with a Setaram Setsys evolution 1750 by using dry air as the carrier gas and a temperature ramp of 5 °C/min from room temperature to 900 °C. The metal oxide content evaluated by TGA was directly used to calculate the yield of removal of metallic impurity (Y m ) after purification; that means that the oxygen part corresponding to the respective stoichiometric amount in the oxidized form of each metal present in the (raw or purified) CNT samples is included in Y m .

Micro-Raman spectroscopy was carried out with a LabRAM HR 800 micro-Raman spectrometer with two incident wavelengths: 632.8 and 514.5 nm. The incident laser beam, focused on the sample with an ×50 objective, was reduced with an optic filter to avoid any damage due to overheating during the spectrum recording. The recorded number of spectra (at least 3) depended on the observed data dispersion. After subtraction of a baseline, the I D /I G intensity ratio was determined by dividing the height of the D band by that of the G band. As only one representative spectrum is shown for each sample, the D/G ratio was close to the average of I D /I G of the sample. The given I D /I G evolutions correspond to the evolution of the average value of the I D /I G ratios obtained for each spectrum for the sample, after the applied chemical treatment. For Auger electron spectroscopy (AES), the CNT powder is fixed with a copper scotch on a molybdenum sample holder. A primary electron energy of 2500 eV is focused on the sample, and a cylindrical detector with a resolution of 1 eV is used for data collection.

X-ray photoelectron spectroscopy (XPS) spectra were collected on a Kratos Axis Ultra (Kratos Analytical, UK) spectrometer equipped with a monochromatic Al Kα source (1486.6 eV). All spectra were recorded at a 90°takeoff angle, with the analyzed area being about 0.7 × 0.3 mm. Survey spectra were acquired with 1.0 eV step and 160 eV analyzer pass energy and the high-resolution regions with 0.1 eV step and 20 eV pass energy (instrumental resolution better than 0.5 eV). Curve fitting was performed using a Gaussian/Lorentzian (70/30) peak shape after Shirley's background subtraction and using the X-vision 2.2.11 software.

RESULTS AND DISCUSSION

3.1. Purification Approach, Carbonaceous Impurity Removal, and CNT Quality. From the synthesis method, catalyst residues remain in the CNT samples, including DWCNTs and SWCNTs. DWCNTs contain cobalt-and molybdenum-based impurities. The latter represent about 10% of the sample weight. The as-produced HiPco SWCNT sample contains iron-based impurities at a non-negligible amount, reaching a third or more of the sample weight. The used DWCNTs have an internal and external average diameter of 1.35 and 2.05 nm, respectively, with a length between 1 and 10 μm. The diameter of the SWCNTs varies from 0.7 to 2 nm, and their length is similar to that of the DWCNTs. Nonnanotube carbon species are also present in both CNT samples, inherent to the chemical vapor deposition (CVD) synthesis. The close reactivity between these carbonaceous impurities and the CNTs is clearly put into evidence from their resistance against oxidation by means of TGA. The combustion of CNT samples commonly appears as one single weight loss, meaning that CNTs and non-nanotube carbon species burn off in the same temperature range, as is the case also for the DWCNTs and SWCNTs used here (cf. Figure 2). The as-produced DWCNT and SWCNT powders were submitted to our one-pot thermal treatment under chlorine (until T max ), in which oxygen is introduced at a chosen temperature, T O 2 (T O 2 being ≤T max ), the carrier gas being nitrogen, N 2 (Figure 1 and Table 1). With the aim of investigating the mechanisms involved in the Cl 2 /O 2 -based treatment, we have performed experiments with different approaches: (i) O 2 was introduced in the reactor at a temperature lower than that used for the final dwell in chlorine (T O 2 < T max ) (D1, S1, and S2); (ii) O 2 was introduced during the Cl 2 dwell, T O 2 = T max (D2, S3, and S4); (iii) treatments under Cl 2 only (D3 and S5) or O 2 only (D4 and S6) were also conducted (Table 1 and Supporting Information, Table S1). Among all of the performed treatments, for the sake of clarity, we present those showing significant differences in metallic removal and/or carbon impurity elimination, consistency and reproducibility having been carefully tested.

The typical behavior from TGA of the raw and treated CNT samples after treatment with Cl 2 alone and with a mixture Cl 2 / O 2 is shown in Figure 2. For the raw DWCNT sample, the shape of the TGA curves and the observed combustion temperatures are quite similar (around 500 °C) before and after purification (Figure 2a). Raw SWCNTs burn off around 350 °C, and the combustion temperature is upshifted to around 550 °C after purification (Figure 2b). The pronounced upshift (∼+200 °C) of the combustion temperature for SWCNTs may be due to the greater metal content in this sample compared to that in DWCNT; the upshift of the burnoff temperature of CNTs is indeed commonly observed after the elimination of the metallic impurities [START_REF] Chiang | Purification and Characterization of Single-Wall Carbon Nanotubes[END_REF] (Supporting Information, Figure S1).

Figure 3 shows TEM images of the used DWCNT and SWCNT samples before (Figure 3, rD and rS) and after different treatment conditions. Metallic and carbon impurities can be easily noticed in both SWCNT and DWCNT samples on the TEM images of the starting raw samples. As normally observed, these impurities widely cover the CNT interlacing. After heating the powdered CNT sample under chlorine, carbon impurities were still present and could be clearly imaged (Figure 3, D3 and S5). These non-nanotube species are present in various shapes, more or less spherical or crooked particles, with a size in the 5-10 nm range or higher. On the nanometer scale, their structure is quite well ordered with graphitic multi-layer walls observable at high magnification (not shown). Quantifying the carbonaceous impurities within a CNT sample is complex. [START_REF] Sen | Controlled Purification of Single-Walled Carbon Nanotube Films by Use of Selective Oxidation and Near-IR Spectroscopy[END_REF][START_REF] Dillon | Employing Raman Spectroscopy to Qualitatively Evaluate the Purity of Carbon Single-Wall Nanotube Materials[END_REF][START_REF] Itkis | Comparison of Analytical Techniques for Purity Evaluation of Single-Walled Carbon Nanotubes[END_REF][START_REF] Arepalli | Protocol for the Characterization of Single-Wall Carbon Nanotube Material Quality[END_REF] Even if they remain very rough, their content can be estimated from TEM, 28 about 10-30% in these samples. As expected and already reported by applying chlorine alone, [START_REF] Chng | Purification of Carbon Nanotubes by High Temperature Chlorine Gas Treatment[END_REF][START_REF] Mercier | Selective Removal of Metal Impurities from Single Walled Carbon Nanotube Samples[END_REF] carbon impurities were not eliminated from the CNT samples (D3 and S5). However, they were completely destroyed as oxygen was added to chlorine (Figure 3, D1 and S1). In the Cl 2 /O 2 -treated samples (D1, D2, S1, S2, S3, and S4), CNTs appear very clean, without any other impurity visible on most of the TEM images (Table 1). From time to time, either metallic or carbon impurities could still be observed, but the efficiency of their removal is really obvious. In the presence of chlorine, the carbon impurities could be selectively eliminated from the CNT samples when O 2 was introduced at 500 °C or higher and at 350 °C or higher for DWCNTs and SWCNTs, respectively. Absorbance spectroscopy, performed on S1 (corresponding to the SWCNTs purified by using the optimized conditions), has shown that carbon impurity content was significantly reduced in these purified SWCNTs compared to the raw SWCNTs (cf. Supporting Information, Figure S2). If O 2 is introduced at lower temperature, the amount of carbon impurities with respect to CNTs is similar to what is observed in the respective raw sample. When only oxygen was used (D4 and S6), carbon impurities were not eliminated from the samples (not shown) and combustion of both CNTs and carbon impurities occurred simultaneously, as further discussed from TGA.

Raman spectroscopy is a powerful technique to probe any modification (improvement or damaging) of the CNT structure after a chemical treatment. [START_REF] Dillon | Systematic Inclusion of Defects in Pure Carbon Single-Wall Nanotubes and Their Effect on the Raman D-Band[END_REF] In a Raman spectrum, the G band (around 1590 cm -1 ) is characteristic of the sp 2 network formed by the double-bonded carbon atoms and the D band (around 1330 cm -1 ) is related to "disorder" or defects when the hybridization of carbon atoms turns to sp 3 through damaging or functionalization. The evolution of the I D /I G intensity ratio is hence a signature of the structural quality modification of CNTs. Figure 4 gives the variation of the I D /I G intensity ratio for the studied samples, including Cl 2 -, O 2 -, and Cl 2 /O 2 -purified CNTs for both DWCNT and SWCNT.

For our treatment conditions, except for S2 with the red incident wavelength (λ = 632.8 nm), for which the temperature (600 °C) for oxygen introduction can be prejudicial to some part of CNTs, I D /I G was decreased for all the treated samples, meaning that the CNTs were not damaged by any of the applied treatments (Figure 4). The observed decrease in the D band might be due to various effects, which can be also combined: (i) curing of CNT defects due to the high temperature used, and (ii) elimination of the most defective CNTs, (iii) removal of amorphous carbon layers deposited on CNTs. 53,54 3.2. Impurity Removal Yield, Sample Yield, and Overall Efficiency of the Purification Method. TGA in air is commonly used to determine oxidized mineral impurity content in CNT samples. With increasing temperature, complete combustion of the carbon species occurs, and the remaining weight corresponds to non-carbon impurities: that is, oxidized metallic impurities. As usually encountered, the oxidized metallic impurities, M r , are about 10 wt % (∼2 at. %) and 30-45 wt % (∼7-15 at. %) for the used raw DWCNT and SWCNT samples, respectively (Table 1). The atomic content of metal-based impurities, n m , was roughly estimated by dividing the metal oxide weight (from TGA) by the molar mass of the corresponding catalyst(s).

Whatever the CNT purification conditions, in the presence of chlorine, the content of the metal residues (n m ) falls down to values close to 1 at. % for the two types of samples (Table 1). Removal of metal impurities certainly occurs in the hightemperature range when either Cl 2 or Cl 2 /O 2 is used. At these temperatures, metal chlorides are easily formed and their sublimation is favored, [START_REF] Mercier | Selective Removal of Metal Impurities from Single Walled Carbon Nanotube Samples[END_REF] leading to their elimination from the CNT samples.

The metal removal yield, Y m , is defined as

= - Y M M M m r p r ,
where M r and M p correspond to the mineral residue content from TGA, that is, the oxidized metallic impurities of the raw and the purified CNT, respectively. Y m is relatively good (80-90%), especially for SWCNTs because the raw SWCNT sample contains a large amount of catalyst impurities. That part of remaining metal residues is much more difficult to remove from the samples without destroying the majority of CNTs. It certainly corresponds to the metal nanoparticles already described to be highly protected in the CNT inner channels, their encapsulation occurring during CNT growth by CVD. [START_REF] Cui | Formation Mechanism of Carbon Encapsulated Fe Nanoparticles in the Growth of Single-/Double-Walled Carbon Nanotubes[END_REF][START_REF] Pumera | Carbon Nanotubes Contain Residual Metal Catalyst Nanoparticles Even after Washing with Nitric Acid at Elevated Temperature Because These Metal Nanoparticles Are Sheathed by Several Graphene Sheets[END_REF][START_REF] Kolodiazhnyi | Towards an Ultrasensitive Method for the Determination of Metal Impurities in Carbon Nanotubes[END_REF][START_REF] Jin | Residual metals present in ″metal-free″ N-doped carbons[END_REF] For both DWCNT and SWCNT samples, the carbon impurities could be efficiently eliminated when Cl 2 /O 2 was used instead of Cl 2 alone (Figure 2 and Table 1).

The second relevant parameter to assess a CNT purification process is the carbon consumption (C c ) because it eventually allows to appreciate the CNTs lost through the treatment. C c corresponds to the carbon loss through purification: is the overall sample yield determined from the ratio between the weight of the purified sample m p over the mass of the raw one m r . We point out that C c includes the consumption of both CNTs and carbon impurities. As expected, increasing T O 2 , which allows a better catalyst removal, also induces high carbon consumption (>50%) (D2, S4, and S5). Importantly, this consumption is well below 50% for D1 and S1, samples in which metallic and carbon impurities have been removed with good yields for both of them.

In the literature, it is problematic to retrieve the overall sample loss or carbon impurity loss because of the complexity of the applied treatments and the multi-step nature of the methods involved. Regarding DWCNTs, Flahaut et al. have reported an effective elimination of disordered carbon species by air [START_REF] Osswald | In Situ Raman Spectroscopy Study of Oxidation of Double-and Single-Wall Carbon Nanotubes[END_REF] with a sample yield lower than 20%. [START_REF] Bortolamiol | Double-Walled Carbon Nanotubes: Quantitative Purification Assessment, Balance between Purification and Degradation and Solution Filling as an Evidence of Opening[END_REF] Selective elimination of carbon impurities from SWCNT samples is found in a few studies. The work by Dementev et al. reported the selective removal of carbon impurities from (arc dischargeproduced) SWCNTs by dynamic air oxidation, but metal impurities were not removed. [START_REF] Dementev | Purification of Carbon Nanotubes by Dynamic Oxidation in Air[END_REF] Rosario-Castro et al. used a standard multi-step purification method for purifying HiPco SWCNTs, including a nitric acid treatment followed by wet air oxidation and Soxhlet extraction in HCl, and finally, the sample was annealed at 425 °C. [START_REF] Rosario-Castro | Combined Electron Microscopy and Spectroscopy Characterization of As-Received, Acid Purified, and Oxidized HiPCO Single-Wall Carbon Nanotubes[END_REF] Iron content was reduced by 89%, and carbon loss subsequent to the applied treatments was not determined. Wang et al. applied a wet combined treatment using H 2 O 2 and HCl. They could decrease the iron content in HiPco SWCNTs by 86% while keeping a carbon yield of 75%, but "little damage" of the CNT sidewalls was reported. [START_REF] Wang | A Highly Selective, One-Pot Purification Method for Single-Walled Carbon Nanotubes[END_REF] (3) t 1 ≤ t ≤ t 2 ; (4) t = t f ; t x is the time sequence as defined in Figure 1. Also, typical Raman spectroscopy spectrum of DWCNTs in the raw state (b), DWCNTs collected just after the Cl 2 /O 2 step (c), and DWCNTs at the end of the purification procedure (d). λ incident = 632.8 nm.

Among the purification procedures involved in halogen-based compounds, after the oxidation of iron impurities by oxygen, Xu et al. used a mixture of fluorinated compounds (SF 6 and C 2 H 2 F 4 ) to form metal fluorides eliminated by subsequent Soxhlet extraction (multi-step method). [START_REF] Xu | Controlled Multistep Purification of Single-Walled Carbon Nanotubes[END_REF] The authors reported a SWCNT yield of 68% with "little sidewall damage". Zimmerman et al. have developed a chlorine-based method consisting of bubbling Cl 2 in HCl containing SWCNTs, and they lost 96 wt % of the starting HiPco SWCNTs sample. [START_REF] Zimmerman | Gas-Phase Purification of Single-Wall Carbon Nanotubes[END_REF] To the best of our knowledge, purification showing a selective elimination of carbon impurities, with low carbon consumption (<50%) and without any CNT damage, has not been reported yet. Remarkably, our results show that the method we propose is able to successfully combine metal elimination and selective attack of carbon impurities among carbon species in a one-pot treatment. The structural quality of the purified CNTs is preserved and even improved without an excessive consumption of the nanotubes.

3.3. Proposed Mechanism. We propose a mechanism explaining the combustion selectivity toward the carbonaceous impurities based on the known specific reactivity between halogens and carbon nanomaterials, including CNTs. This mechanism also originates from a deep analysis of the behavior of carbon nanomaterials in an oxidative medium.

The mechanism proposed in Figure 5a is based on several consistent aspects. Figure 5a(1) schematizes the CNTs and the carbonaceous impurities. In this work, from combustion selectivity of the carbon impurities observed with and without chlorine (see, for instance, D1 and D3), it is obvious that chlorine plays the major role in preventing CNT from combustion by oxygen. The reactivity of CNTs with halogens or halogenated compounds has shown remarkable effects in carbon chemistry. [START_REF] Tasis | Chemistry of Carbon Nanotubes[END_REF] For example, regarding fluorine, the easiness [START_REF] Claves | An Unusual Weak Bonding Mode of Fluorine to Single-Walled Carbon Nanotubes[END_REF][START_REF] Chamssedine | Fluorination of Single Walled Carbon Nanotubes at Low Temperature: Towards the Reversible Fluorine Storage into Nanotubes[END_REF] and reversibility [START_REF] Pehrsson | Thermal Fluorination and Annealing of Single-Wall Carbon Nanotubes[END_REF][START_REF] Zhao | Thermal Recovery Behavior of Fluorinated Single-Walled Carbon Nanotubes[END_REF] of the fluorination of CNTs account for its extensive interest especially because fluorocarbons could be used in a lot of applications. Grafting of chlorine to large sp 2 -based carbon materials or carbon impurities has been shown to be less favorable than the formation of covalent C-Cl bonds on smaller and high-quality CNTs. [START_REF] Erbahar | Chlorination of Carbon Nanotubes[END_REF][START_REF] Ijas | Interaction of Chlorine with Stone-Wales Defects in Graphene and Carbon Nanotubes and Thermodynamical Prospects of Chlorine-Induced Nanotube Unzipping[END_REF] In our samples, both DWCNTs and SWCNTs have a lower curvature radius (<3 nm) compared to that within the non-nanotube species (>5 nm), as observed by TEM. CNTs with a smaller diameter are thus expected to have a better affinity with chlorine (Figure 5a(2)). Compared to the raw CNTs (Figure 5b), Raman spectroscopy performed on the CNT sample for which the treatment was stopped just after the Cl 2 /O 2 dwell (mD), on purpose, shows an increase in the D/G ratio compared to the raw CNTs (Figure 5c). The intensity of the D band is decreased at the end of the Cl 2 /O 2 -process, as shown in Figure 5d and in agreement with Figure 4. We have not observed any G-band shift probably because of the too small amount of chlorine-containing functional groups present on the CNT walls. Such G-band shift because of doping effect by halogen was reported to be weakly pronounced or not visible after fluorine grafting. [START_REF] Chamssedine | Fluorination of Single Walled Carbon Nanotubes at Low Temperature: Towards the Reversible Fluorine Storage into Nanotubes[END_REF][START_REF] Fedoseeva | Effect of Hydrogen Fluoride Addition and Synthesis Temperature on the Structure of Double-Walled Carbon Nanotubes Fluorinated by Molecular Fluorine[END_REF] However, the observed D/G ratio for mD may be attributed to covalent functionalization of the CNT walls by chlorine (cf. also Supporting Information, Figure S3). This hypothesis is also supported by the AES of this CNT sample for which chlorine could be well detected [START_REF] Prodromides | The Characterisation of Non-Evaporable Getters by Auger Electron Spectroscopy: Analytical Potential and Artefacts[END_REF] [Figure 6a and Fourier transform infrared (FTIR) results, Supporting Information, Figure S4]. XPS analysis also revealed the presence of chlorine species at the surface of mD (Figure 6b). Two components (each consisting of the 3/2 and 1/2 level with a spin-orbit splitting of 1.6 eV) were required to obtain a satisfactory fit of the Cl 2p core-level spectrum. The low-energy contribution with the 2p 3/2 component at ca. 197.5 eV corresponds to more electronegative chlorine atoms probably coming from metal. The high-energy contribution with the 2p 3/2 component at ca. 199.9 eV is attributed to chlorine bonded to carbon [START_REF] Pelech | Simultaneous Purification and Functionalization of Carbon Nanotubes Using Chlorination[END_REF] in agreement with Raman spectroscopy. From XPS, chlorine atomic content was decreased by a factor of almost 2 between mD and D1, being as low as around 0.83 at. % in D1 (Supporting Information, Figure S5).

Even if chlorine is only sparsely attached to the CNTs, its known power as a flame retardant leads to a favored carbon impurity combustion when oxygen is introduced in the reactor. This protective shield against combustion, which has been already suggested without any evidence, [START_REF] Zimmerman | Gas-Phase Purification of Single-Wall Carbon Nanotubes[END_REF][START_REF] Desforges | Dramatic Enhancement of Double-Walled Carbon Nanotube Quality through a One-Pot Tunable Purification Method[END_REF] leads to the combustion of carbonaceous impurities by O 2 at T O 2 whereas the CNTs are protected by Cl 2 (Figure 5a(3)). This protection induces a substantial improvement of removal selectivity between carbonaceous impurities and CNTs close to the combustion temperature (S1, D1). This beneficial effect disappears when T O 2 is increased, because carbon combustion becomes highly favorable and very fast (S2, S3, and S4). The well-known lability of the C-Cl bond certainly favors the detachment of chlorine from the CNTs by heating (Figure 5a(4),d). On the basis of the proposed mechanism, our method is versatile because purification occurs with optimum efficiency as oxygen is introduced at the combustion temperature of the samples for both DWCNT and SWCNT.

CONCLUSIONS

This work proposes a solution to the long-standing issue of carbon impurities removal while avoiding excessive attack and consumption of CNTs. The challenge comes from the widely known too close chemical reactivity of carbon impurities and CNTs. This is an important finding because CNT purification cannot be avoided for a lot of applications and the existing purification methods are time and sample consuming. Our one-pot treatment consists of heating the sample under chlorine up to T max (around 1000 °C) and introducing oxygen at a given temperature, T O 2 ≤ T max . The best treatment is then obtained when T O 2 corresponds to the combustion temperature range of the CNT sample, as determined by TGA. We have shown that chlorine plays an essential role by combining the removal of metallic impurities and protection of the CNTs from combustion. Moreover, the efficiency of the proposed method has been demonstrated to be due to a substantial increase in the combustion selectivity of the carbonaceous impurities with respect to CNTs. The proposed mechanism appears then to be a powerful tool because it could be easily adapted to other source of carbon nanostructures with other kind of metal impurities (Ni, Al, ...) also. Moreover, the present approach has the enormous advantage to produce, in a one-pot treatment, large volumes of purified CNTs. The developed treatment process hence has a high scale-up potential, and it could open new opportunities for CNT applications.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b12554.

Experimental conditions for the performed treatments for both DWCNTs and SWCNTs, stability against combustion from TGA under dry air after applying an annealing program under helium, carbon impurity in SWCNT samples investigated by UV-NIR absorbance spectroscopy, chlorine grafting to CNT walls evidenced by RBM analysis from Raman spectroscopy and with FTIR, and XPS wide scans for rD, D1, and mD (PDF)

■ AUTHOR INFORMATION

Corresponding Author *E-mail: Brigitte.Vigolo@univ-lorraine.fr. Phone: +33 372 742 533.

ORCID Brigitte Vigolo: 0000-0002-1463-0121

Notes

The authors declare no competing financial interest.

Figure 1 .

 1 Figure 1. Schematic of the applied one-pot purification method displaying the nature of the gas used in each step, and the temperature sequence (dwells and ramps).

Figure 2 .

 2 Figure 2. TGA curves (in dry air, temperature ramp 5 °C/min) for the raw and selected CNTs treated with Cl 2 and Cl 2 /O 2 (a): raw DWCNT rD (blue); Cl 2 -purified DWCNT D3 (red); Cl 2 /O 2 -purified DWCNT D1 (green) and for SWCNTs (b): raw SWCNT (blue) rS1-2; Cl 2 -purified SWCNT S5 (red); Cl 2 /O 2 -purified SWCNT S1 (green).

aY

  m and C c are defined in the text. ¥ From an earlier work.[START_REF] Desforges | Dramatic Enhancement of Double-Walled Carbon Nanotube Quality through a One-Pot Tunable Purification Method[END_REF]

Figure 3 .

 3 Figure 3. Typical TEM bright-field micrographs of the raw and treated DWCNT and SWCNT samples: raw DWCNT (rD), Cl 2purified DWCNT (D3), Cl 2 /O 2 -purified DWCNT (D1), raw SWCNT (rS1-2), Cl 2 -purified SWCNT (S5), Cl 2 /O 2 -purified SWCNT (S1).

Figure 4 .

 4 Figure 4. Variation of I D /I G (%) of each treated sample compared to its respective raw CNT sample. ¥ From a preceding work (ref 42).

Figure 5 .

 5 Figure 5. (a) Proposed mechanism for the selective removal of carbon impurities. (1) t = t i ; (2) t ≤ t 1 ; (3) t 1 ≤ t ≤ t 2 ; (4) t = t f ; t x is the time sequence as defined in Figure 1. Also, typical Raman spectroscopy spectrum of DWCNTs in the raw state (b), DWCNTs collected just after the Cl 2 /O 2 step (c), and DWCNTs at the end of the purification procedure (d). λ incident = 632.8 nm.

Figure 6 .

 6 Figure 6. Auger (a) and XPS (b) analysis of mD.

Table 1 .

 1 Temperature at Which O 2 Is Introduced in the Purification Reactor, Temperature of Cl 2 Dwell (T max ), Occurrence of Carbon Impurity Removal, Resulting Oxidized Metallic Impurity Content M p in wt % and Their Content n m in at. %, Metal Removal Yield Y m , and Carbon Consumption after Purification C c a sample O 2 introduction temperature (T O2 )/°C Cl 2 dwell temperature (T max )/°C carbon impurity removal M p (M r )/wt % n m /at. % Y m /% C c /%

	rD

■ ACKNOWLEDGMENTS

The authors would like to thank L. Aranda and P. Franchetti for their help for TGA and Raman spectroscopy experiment, respectively. They also thank Prof. Dr. M. Dubois for the fruitful discussions about fluorine chemistry. P. Lonchambon is acknowledged for help with sample preparation. The authors thank the University of Lorraine for its financial support, especially for N.B.'s Doctoral Fellowship. Auger experiments were performed using equipment from the TUBEDavm funded by FEDER (EU) at IJL, ANR, the Region Lorraine and Grand Nancy. We acknowledge L. Pasquier and Prof. Dr. S. Andrieu from Institut Jean Lamour for performing the Auger experiments and analysis. We would like to thank the platform "Spectroscopies et Microscopies des Interfaces" (Laboratory of Physical Chemistry and Microbiology for Materials and the Environment, LCPME, Nancy, France) and A. Renard, Dr. M. Mallet (LCPME) and Dr. M. Dossot for XPS and absorbance analyses. This work was partly supported by the PRC CNRS/ RFBR grant no. 1023.

Supporting Information

Protecting Carbon Nanotubes From Oxidation for Selective Carbon Impurity Elimination N. Berrada, A. Desforges, C. Bellouard, E. Flahaut, J. Gleize, J. Ghanbaja, B. Vigolo

1) Experimental treatment conditions

The treatment conditions used for purifying the DWCNT and SWCNT samples are gathered in Table S1.

Table S1. Experimental conditions used for purifying DWCNT and SWCNT samples. The used flow rates are 4 mL/min and 200 mL/min for O 2 and Cl 2 , respectively. ¥ from a preceding work

2) Stability against combustion highlighted by TGA under dry air

Both DWCNT and SWCNT samples have been annealed under He (O 2 < 0.1 ppm mole) by following the same heating steps than that used under Cl 2 /O 2 for D1 and S1, respectively. The prepared samples are referred to as a-rS and a-rD, respectively. The thermograms (using a Setaram TGA92, temperature ramp 5°C/min under air were recorded for the two thermally treated samples. The results are shown in Fig. S1.

For the DWCNT samples, we observe a small upshift of the combustion temperature (+ 35°C) between the annealed DWCNTs and the raw and the purified samples (rD and D1). The impurity content being quite similar for rD and a-rD, the observed upshift for a-rD treated under inert gas may be due to a healing effect: such effect is obviously not observed for the sample treated under chlorine D1.

For the SWCNT samples, we notice here a high increase (28.0 to 53.6 wt. %) of the remaining mineral impurity content despite the good quality of the inert gas used. The gain in weight starting from 200 °C, corresponding to the metal oxidation, is more pronounced after annealing (a-rS). We hypothesize that a carboreduction phenomenon could happen during the annealing process. Indeed, carbon can reduce the metal oxides into metal which is re-oxidized during TGA under air. In that case, carbon is gasified which could explain its relative reduction in weight in the sample. In a-rS, the metal content is higher than that of rS. If we only consider the catalysis effect due to these impurities, the combustion should occur at lower temperature than that of rD. Therefore, the better oxidation stability of a-rS observed after the thermal treatment under He is probably due an annealing effect. For S1, even if such annealing effect cannot be excluded, the much greater combustion temperature observed is probably mainly due to the strong reduction in the metallic impurities.

Figure S1 shows the thermograms of the raw, the purified and the thermally treated raw sample for both DWCNTs (Fig. S1a) and SWCNTs (Fig. S1b).

3) Carbon impurity investigated by UV-NIR absorbance spectroscopy

Near Intrared (NIR) spectroscopy was carried out by using a Cary 6000i UV-Vis-NIR apparatus from Agilent. The samples were prepared by following the method found in the work reported by Itkis et al. 2 Briefly, 10 mg of the SWCNT powder (rS or S1) was added to 20 mL of N,Ndimethylformamide (DMF). After 10 min of sonication, the solutions were diluted 10 times in order to obtain light grey solutions. Without centrifugation, the supernatant was used for analysis so that the analyzed part was representative of the sample. Figure S2 shows Vis-NIR results for rSWCNT (Fig. S2a) and S1 (Fig. S2b). A semi-quantitative analysis was conducted by using the S 22 transition as reported in the work from Itkis et al. 2 For our two (rS and S1) samples, after subtracting a baseline, the total area corresponding to the S 22 band , the contribution of SWCNTs (-plasmon) and the carbon impurities, i.e. AA(T), and the area of only the S 22 feature, i.e. AA(S), were calculated between the spectral cutoffs 12500 and 17050 cm -1 . The ratio AA(S)/AA(T) was of 0.0116 and 0.0365 for rS and S1, respectively. Even if an absolute estimation of the purity is difficult to obtain 2 , these results show that the carbon impurity contribution is significantly lower in the purified SWCNTs in agreement with TEM observations.

4) Evidence of chlorine grafting to CNT walls

Figure S3 shows the RBM of the raw DWCNTs (rD) (Fig. S3a), the DWCNTs for which the treatment was stopped just after the Cl 2 /O 2 dwell at 500°C (mD) (Fig. S3b) and the DWCNTs collected at the end of the Cl 2 /O 2 purification method (D1) (Fig. S3c). The CNT powder was mixed with potassium bromide (KBr) and compacted under pressure to obtain a pellet. Transmission IR spectra were recorded in the 400-4000 cm -1 range. The spectral resolution was 2 cm -1 and 50 scans were co-added for each spectrum.

Figure S4 shows IR spectra of the raw DWCNTs (rD) (Fig. S4c), the DWCNTs for which the treatment was stopped just after the Cl 2 /O 2 dwell at 500°C (mD) (Fig. S4a), and the DWCNTs collected at the end of the Cl 2 /O 2 purification method (D1) (Fig. S3b),. Compared to rD and D1, an additional peak of weak intensity but clearly visible appears at around 667 cm -1 . This feature is assigned to the C-Cl vibrations 3 detected only in mD. Figure S5 shows the XPS wide scans of rD (Fig. S5a), D1 (Fig. S5b), and mD (Fig. S3c)