Adrien Baillard

Estelle Appert

Matthieu Weber

Véronique Jacob

Hervé Roussel

Laetitia Rapenne

Odette Chaix-Pluchery

Vincent Consonni
email: vincent.consonni@grenoble-inp.fr

Single and Co-Doping of ZnO Nanowires with Al and Cl Using One Precursor by Chemical Bath Deposition

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

INTRODUCTION

The intentional doping of ZnO nanowires (NWs) grown by chemical bath deposition (CBD) [START_REF] Vayssieres | Purpose-Built Anisotropic Metal Oxide Material: 3D Highly Oriented Microrod Array of ZnO[END_REF] has emerged as a critical issue to carefully optimize their electronic structure properties driving their optical and electrical properties for many engineering devices. [START_REF] Faiz | Zinc Oxide Light-Emitting Diodes: A Review[END_REF][START_REF] Vanmaekelbergh | Nanowire Lasers[END_REF][START_REF] Ouyang | Self-Powered Uv Photodetectors Based on ZnO Nanomaterials[END_REF][START_REF] Consonni | Nanowires for Solar Cells: A Comprehensive Review[END_REF][START_REF] Briscoe | Piezoelectric Nanogenerators -a Review of Nanostructured Piezoelectric Energy Harvesters[END_REF][START_REF] Liu | A Review on Bidirectional Analogies between the Photocatalysis and Antibacterial Properties of ZnO[END_REF][START_REF] Mirzaei | Resistive Gas Sensors Based on Metal-Oxide Nanowires[END_REF][START_REF] Kang | Review of ZnO-Based Nanomaterials in Gas Sensors[END_REF][START_REF] Rackauskas | Nanowire Application in Chemoresistive Sensing: A Review[END_REF][START_REF] Theerthagiri | A Review on ZnO Nanostructured Materials: Energy, Environmental and Biological Applications[END_REF] While the growth of ZnO NWs by CBD relies on the introduction of a zinc salt [e.g. zinc nitrate (Zn(NO 3 ) 2 , zinc acetate, zinc sulphate] with a source of HO -ions [e.g. sodium hydroxide, ethanolamine, hexamethylenetetramine (HMTA)] in aqueous solution, [START_REF] Lincot | Solution Growth of Functional Zinc Oxide Films and Nanostructures[END_REF][START_REF] Xu | One-Dimensional ZnO Nanostructures: Solution Growth and Functional Properties[END_REF] the use of chemical additives has widely been developed over the past decade. Chemical additives including organic compounds (e.g. polyethylenimine, ethylenediamine, phenolphthalein) have been introduced in the chemical bath to engineer the dimensions and shape of ZnO nanostructures, for instance by promoting their axial or radial growth to form NWs or platelets, respectively. [START_REF] Greene | Solution-Grown Zinc Oxide Nanowires[END_REF][START_REF] Parize | Effects of Polyethylenimine and Its Molecular Weight on the Chemical Bath Deposition of ZnO Nanowires[END_REF][START_REF] Liu | Hydrothermal Synthesis of ZnO Nanorods in the Diameter Regime of 50 nm[END_REF][START_REF] Yahiro | Nanometric Morphological Variation of Zinc Oxide Crystals Using Organic Molecules with Carboxy and Sulfonic Groups[END_REF] Ammonia (NH 3 ) has been introduced to tune the pH of the chemical bath and hence to limit the supersaturation and favour the heterogeneous growth of ZnO NWs. [START_REF] Xu | Preferential Growth of Long ZnO Nanowire Array and Its Application in Dye-Sensitized Solar Cells[END_REF][START_REF] Syrrokostas | High-Quality, Reproducible ZnO Nanowire Arrays Obtained by a Multiparameter Optimization of Chemical Bath Deposition Growth[END_REF][START_REF] Villafuerte | Modulating the Growth of Chemically Deposited ZnO Nanowires and the Formation of Nitrogen-and Hydrogen-Related Defects Using pH Adjustment[END_REF] Joo et al. further reported the introduction of ancillary metal sulphate in the chemical bath, producing A-type (e.g., Cd, Cu, Mg, Ca) and B-type (e.g., Al, Ga, In) ion complexes acting as differential capping agents on the surfaces of ZnO NWs through attractive electrostatic interactions and hence adjusting their aspect ratio from 0.1 to 100. [START_REF] Joo | Face-Selective Electrostatic Control of Hydrothermal Zinc Oxide Nanowire Synthesis[END_REF] More recently, chemical additives containing the dopant such as metal nitrate [e.g. Al nitrate (Al(NO 3 ) 3 ), Ga nitrate (Ga(NO 3 ) 3 ), Cu nitrate (Cu(NO 3 ) 2 ), Ni nitrate (Ni(NO 3 ) 2 ), and Co nitrate (Co(NO 3 ) 2 )] 22- 27 or metal glycolate (e.g. Sb glycolate) [START_REF] Wang | An Aqueous Solution-Based Doping Strategy for Large-Scale Synthesis of Sb-Doped ZnO Nanowires[END_REF] have been introduced to control the physical properties of ZnO NWs.

The incorporation process of dopants relies on the adsorption process of ion complexes containing the dopant on the charged surfaces of ZnO NWs, which is mainly governed by electrostatic and chemical forces. The present approach has been developed for a number of dopants (e.g. Al, Ga, Cu, Sb, Ni, and Co), most of them substituting for the zinc sites in the lattice and acting as shallow donors or acceptors, respectively. [START_REF] T-Thienprasert | Compensation in Al-Doped ZnO by Al-Related Acceptor Complexes: Synchrotron X-Ray Absorption Spectroscopy and Theory[END_REF][START_REF] Demchenko | Impurity Complexes and Conductivity of Ga-Doped ZnO[END_REF][START_REF] Yan | Doping of ZnO by Group-IB Elements[END_REF][START_REF] Limpijumnong | Doping by Large-Size-Mismatched Impurities: The Microscopic Origin of Arsenic-or Antimony-Doped P-Type Zinc Oxide[END_REF][START_REF] Pei | Studies of Magnetic Interactions in Ni-Doped ZnO from First-Principles Calculations[END_REF][START_REF] Walsh | Theoretical Description of Carrier Mediated Magnetism in Cobalt Doped ZnO[END_REF] The use of Sb acetate and Mn acetate was also reported for the doping of ZnO NWs with Sb and Mn, respectively. [START_REF] Briscoe | In Situ Antimony Doping of Solution-Grown ZnO Nanorods[END_REF][START_REF] Aleinawi | Spectroscopic Probing of Mn-Doped ZnO Nanowires Synthesized Via a Microwave-Assisted Route[END_REF] Alternatively, the development of the intentional doping with dopants substituting for the oxygen sites in the lattice has been much less investigated. [START_REF] Janotti | Fundamentals of Zinc Oxide as a Semiconductor[END_REF][START_REF] Mccluskey | Defects in ZnO[END_REF] Among the halogen-related column, chlorine constitutes one of the most promising dopants. [START_REF] Liu | Defect Formation in Chlorine-Doped Zinc Oxide[END_REF][START_REF] Tsin | Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar[END_REF] Cl -ions have been used to engineer the shape of ZnO nanostructures to form platelets following its adsorption process on the polar c-planes in the low-pH region. [START_REF] Xu | Morphological Control of ZnO Nanostructures by Electrodeposition[END_REF] Zinc chloride is further a typical chemical precursor to grow ZnO NWs by electrodeposition with an optimized structural morphology [START_REF] Peulon | Cathodic Electrodeposition from Aqueous Solution of Dense or Open-Structured Zinc Oxide Films[END_REF] and a high density of free electrons above 10 19 cm -3 . [START_REF] Mora-Seró | Determination of Carrier Density of ZnO Nanowires by Electrochemical Techniques[END_REF] The incorporation process of Cl dopants into ZnO has therefore been reported using a number of chemical deposition techniques. [START_REF] Tchelidze | Perspectives of Chlorine Doping of ZnO[END_REF][START_REF] Chikoidze | Electrical Properties of Chlorine-Doped ZnO Thin Films Grown by MOCVD[END_REF][START_REF] Fan | Control of the Doping Concentration, Morphology and Optoelectronic Properties of Vertically Aligned Chlorine-Doped ZnO Nanowires[END_REF] And yet, the development of the single doping of ZnO NWs with Cl when using CBD is still unexplored largely. A few investigations have reported the use of ammonium chloride (NH 4 Cl) as a chemical additive, [START_REF] Wang | Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes[END_REF][START_REF] Boruah | Surface Photo-Charge Effect in Doped-ZnO Nanorods for High-Performance Self-Powered Ultraviolet Photodetectors[END_REF][START_REF] Sahoo | UV-Assisted Water Splitting of Stable Cl-Doped ZnO Nanorod Photoanodes Grown Via Facile Sol-Gel Hydrothermal Technique for Enhanced Solar Energy Harvesting Applications[END_REF] but the pH value has not been adjusted despite its critical importance in the adsorption and incorporation processes.

The simultaneous co-doping of ZnO NWs grown by CBD offers an additional, powerful way to tune their physical properties with a higher precision, but this approach is still limited. The only reports have consisted in using two metallic salts usually incorporating two different dopants substituting for the same zinc sites in the lattice. Recently, the use of Al(NO 3 ) 3 and Ga(NO 3 ) 3 in the same chemical bath has shown a great promise to incorporate both dopants into ZnO NWs. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] Alternatively, a promising and yet unexplored approach consists in developing the co-doping of ZnO NWs by using one single metallic salt incorporating two different dopants substituting for the zinc and oxygen sites, respectively. Among the possible candidates, the chemical additives incorporating Al and Cl are of high potential because the doping of ZnO NWs grown with Al by CBD is well documented. [START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF][START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Yun | Hydrothermal Synthesis of Al-Doped ZnO Nanorod Arrays on Si Substrate[END_REF][START_REF] Hsu | Synthesis and Conductivity Enhancement of Al-Doped ZnO Nanorod Array Thin Films[END_REF][START_REF] Synhaivskyi | Evidence of Piezoelectric Potential and Screening Effect in Single Highly Doped ZnO:Ga and ZnO:Al Nanowires by Advanced Scanning Probe Microscopy[END_REF] Aluminium chloride (AlCl 3 ) has already been used as a Cl chemical precursor in the growth of ZnO NWs by CBD. [START_REF] Wang | Cl-Doped ZnO Nanowires with Metallic Conductivity and Their Application for High-Performance Photoelectrochemical Electrodes[END_REF] However, the pH value was not adjusted, such that different electrostatic interactions are expected from Cl(I) or Al(III) species. In the low-pH region, all the faces of ZnO NWs are positively charged and it was proved that the incorporation of Al dopants cannot proceed. [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF] The different reports suggest that Cl dopants may be incorporated using AlCl 3 as a chemical precursor and that can be coupled with the ability to incorporate Al dopants in the high-pH region to obtain Al-and Cl-codoped ZnO NWs. Moreover, the aqueous medium incorporating metal nitrates contains a large amount of hydrogen and nitrogen, which are non-intentionally incorporated into ZnO NWs during their growth by CBD. [START_REF] Huang | Correlating the Enhancement of UV Luminescence from Solution-Grown ZnO Nanorods with Hydrogen Doping[END_REF][START_REF] Barbagiovanni | Radiative Mechanism and Surface Modification of Four Visible Deep Level Defect States in ZnO Nanorods[END_REF][START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF][START_REF] Villafuerte | Engineering Nitrogen-and Hydrogen-Related Defects in ZnO Nanowires Using Thermal Annealing[END_REF] The interactions between two anionic dopants as Cl and nitrogen, but also their interaction with hydrogen whose the presence has been detected in ZnO NWs by nuclear magnetic resonance, [START_REF] Huang | Correlating the Enhancement of UV Luminescence from Solution-Grown ZnO Nanorods with Hydrogen Doping[END_REF] elastic recoil detection analysis, [START_REF] Barbagiovanni | Radiative Mechanism and Surface Modification of Four Visible Deep Level Defect States in ZnO Nanorods[END_REF] and optical spectroscopy [START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF] have not been studied so far.

In this work, the single and simultaneous co-doping of ZnO NWs with Al and Cl grown by CBD is performed by focusing on the separate addition of several chemical precursors in the bath, including Al(NO 3 ) 3 , NH 4 Cl, and AlCl 3 , respectively. The role of Al(III) and Cl(I) species in the chemical bath along with the related physicochemical processes are studied using thermodynamic computations coupled to in situ pH measurements. The influence of the single and simultaneous co-doping on the morphological and structural properties of ZnO NWs, and on the Al and Cl incorporation mechanisms, is investigated by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS) using STEM, inductively coupled plasma-mass spectrometry (ICP-MS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and UV-Vis-NIR absorption measurements.

EXPERIMENTAL DETAILS

Deposition techniques.

(001) Silicon wafers and Corning borosilicate glass were used as substrates and cleaned in an ultrasonic bath with acetone and isopropyl alcohol. The polycrystalline ZnO seed layers were obtained by dip coating using a sol-gel process, as reported in Ref. [START_REF] Parize | Effects of Hexamethylenetetramine on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition[END_REF] . The growth of ZnO NWs was achieved by CBD. The substrates with the polycrystalline ZnO seed layers were placed in a sealed reactor containing a chemical precursor solution composed of zinc nitrate hexahydrate [Zn(NO 3 ) 2 ꞏ6H 2 O, Sigma-Aldrich] and HMTA [C 6 H 12 N 4 , Sigma-Aldrich] in an equimolar concentration of 30 mM mixed in deionised water. The addition of dopants in the chemical bath was intentionally achieved using aluminium nitrate nonahydrate

[Al(NO 3 ) 3 ꞏ9H 2 O, Sigma-Aldrich], ammonium chloride [NH 4 Cl, Sigma-Aldrich], or aluminium chloride hexahydrate [AlCl 3 ꞏ6H 2 O, Sigma-Aldrich]. The [Al]/[
Zn] ratios were adjusted to 0.3, 1 and 3%, while the [Cl]/[Zn] ratios were set to 1, 3 and 9%. Deionised water with a given volume was added to compensate for the dopant solution volumes. The initial pH (pH 0 ) at room temperature was adjusted to a value of around 10.9 by using ammonia [NH 3 , Sigma-Aldrich] with a concentration of 725 mM. The sealed reactor was placed for 3 h in an oven heated at 90 °C. Both the pH and temperature were measured in situ using an InLab Versatile Pro pH electrode from Mettler Toledo. Eventually, the ZnO NW arrays were annealed at 400 °C for 1 h in a tubular furnace under oxygen atmosphere.

Characterization techniques. The morphology of ZnO NWs on Si was determined with a ZEISS

Gemini 300 FESEM instrument equipped with an in-lens detector and working with an accelerating voltage of 5 kV. The XRD acquisitions were achieved with a Bruker D8 Advance diffractometer using the Cu Kα 1 radiation according to the Bragg-Brentano configuration. The θ/2θ XRD patterns were recorded in the range of 20-80° (in 2 Theta scale), calibrated with the (004) diffraction peak of Si pointing at 2θ = 69.131°, and refined with Celref 3 software using the 00-036-1451 ICDD file. STEM specimens were prepared by scratching the surface of ZnO NW arrays using a diamond tip and spread on a copper grid. STEM-EDS spectra and maps were collected with a JEOL SDD Centurio detector having a large solid angle of up to 0.98 steradians incorporated in the JEOL 2100F field-emission-gun scanning electron microscope operating at 200 kV and having a 0.2 nm resolution in the scanning mode. The chemical composition was determined with a NexION 1000 PerkinElmer ICP-MS instrument. The nebulizer and spray chamber were a MEINHARD plus Glass Type C and Glass Cyclonic at 2 °C, respectively. The KED mode was used with He (7% H 2 ) as collision gas. The ZnO NW arrays were previously dissolved in a 0.6% nitric acid solution [HNO 3 , Carlo Erba] by dipping a glass substrate. The RF power was 1450 W, the KED flow rate was 4 mL/min, and the integration time was set to 10 and 100 ms for detecting Zn and Al, respectively. The PerkinElmer reference solutions were diluted to obtain 0.01, 0.1, 0.2, 0.5, and 1 mg/L of Zn and Al standards. The analyzed solutions were diluted to lie in the calibration range. The solution containing only the glass substrate was previously assayed to make sure that no contaminants are released during the preparation. The XPS acquisitions were performed in ultrahigh vacuum (10 -8 mbar) at room temperature using a ThermoScientific K-Alpha spectrometer equipped with a monochromatic Al Kα radiation source (1486.6 eV). The spot size area was set to 400 µm in diameter. The angle between the sample surface and analyser was adjusted to 90°. The XPS spectra of the Al and Cl core levels were collected with a pass energy of 30 eV and a step size of 0.1 eV. The binding energy was calibrated with respect to the C-C bond component of the C 1s core level at 285.0 eV. The Raman scattering acquisitions of ZnO NWs on glass substrates were performed using a HORIBA/JOBIN YVON Labram spectrometer equipped with a liquid-nitrogen-cooled CCD detector. The Raman spectra were collected over the ranges of 70-800 cm -1 (3200-3700 cm -1 ) using the 488 nm (514.5 nm) Ar + laser line with a power on the sample surface close to 7 mW (4 mW) and an acquisition time of 600 s (3600 s), respectively. The laser was focused with a 100 X objective to a spot size of approximately 1µm². All of the Raman spectra were calibrated with a Si reference sample considering the theoretical Raman line set to 520.7cm -1 . A PerkinElmer Lambda 950 UV-Vis-NIR spectrophotometer equipped with an integrating sphere was used to perform total transmittance measurements over a wavelength range of 250-800 nm with a 1 nm step. The Tauc plot approach was used to determine the optical bandgap energy from the intercept of the linear portion of (αhυ) 2 vs hυ graph with the hυ-axis where α is the absorption coefficient. [START_REF] Viezbicke | Evaluation of the Tauc Method for Optical Absorption Edge Determination: ZnO Thin Films as a Model System[END_REF] 2.3. Thermodynamic computations. Thermodynamic simulations were achieved using Visual MINTEQ software to establish the speciation diagrams of Al(III), Cl(I), and Zn(II) species at 85 °C for each growth condition (i.e., with varying NH 3 concentrations, Al(NO 3 ) 3 , AlCl 3 or NH 4 Cl chemical precursors). The Zn 2+ single metallic cation in aqueous solution is capable of forming hydroxide, amine, and chloride complexes with HO -ions, NH 3 , and Cl -ions, respectively, acting as the three possible ligands. The Al 3+ single metallic cation in aqueous solution is able to form hydroxide and chloride complexes with HO -and Cl -ions, respectively, acting as the two possible ligands. The general set of chemical reactions is as follows: nM x+ + iL ↔ M L , where M L is the complex considered, i is the coordination number, L is the ligand, M is the metallic cation, and x is the cation charge. The related stability constants β associated with each reaction are given by: β . These constants were taken at 25 °C from NIST for Al(III), Cl(I), and Zn(II) species and deduced at 85 °C from Van't Hoff relation.

RESULTS AND DISCUSSION

Strategy for the Simultaneous Co-Doping of ZnO Nanowires with Al and Cl. The doping of

ZnO NWs by CBD has been investigated when using metal nitrate as a chemical additive such as Al(NO 3 ) 3 , 22- 23 Ga(NO 3 ) 3 , [START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF] and Cu(NO 3 ) 2 . [START_REF] Lausecker | Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping[END_REF] The adsorption process of ion complexes containing the dopant on the charged surfaces of ZnO NWs during CBD is required for the incorporation of dopants, but its nature and magnitude strongly depend on the pH value and dopant. When using column-IIIA dopants including Al and Ga, electrostatic forces dominate the adsorption process such that the incorporation process operates in the high-pH region, where the nonpolar m-plane sidewalls of ZnO NWs are positively charged. [START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF][START_REF] Gaffuri | The Path of Gallium from Chemical Bath into ZnO Nanowires: Mechanisms of Formation and Incorporation[END_REF] The co-doping of ZnO NWs with Al and Ga was found to incorporate a higher concentration of Ga dopants as Ga Zn than of Al dopants as Al Zn both of them acting as shallow donors owing to energetic considerations. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] However, this also showed that competitive physicochemical processes can proceed on the nonpolar m-plane sidewalls of ZnO NWs, favouring the incorporation of a given dopant at the expense of the other dopant. When using alternative column-IB dopants like Cu, additional chemical forces also play a significant role and can get predominant to drive the adsorption process in the low-and high-pH regions. [START_REF] Lausecker | Chemical Bath Deposition of ZnO Nanowires Using Copper Nitrate as an Additive for Compensating Doping[END_REF] In the present case, the adsorption and pencil-like tip. This specific shape as compared to the well-defined hexagonal section obtained using standard conditions in the low-pH region is explained by the higher axial growth rate resulting in the incomplete formation of the nonpolar m-planes on the sidewalls of ZnO NWs. In other words, the formation of the next polar c-plane on the tip of ZnO NWs starts before the total achievement of the previous nonpolar m-plane on their sidewalls. Additionally, the vertical alignment of ZnO NWs is driven by the c-axis oriented polycrystalline ZnO seed layer, [START_REF] Guillemin | Critical Nucleation Effects on the Structural Relationship between ZnO Seed Layer and Nanowires[END_REF][START_REF] Cossuet | Ordered ZnO Nanowires with Controllable Inclination on Semipolar Zno Surfaces by Chemical Bath Deposition[END_REF] but altered by a bundling process operating over a critical length where the gain of surface energy exceeds the cost of bending elastic energy. [START_REF] Kaganer | Growth, and Bundling of GaN Nanowires in Molecular Beam Epitaxy: Disentangling the Origin of Nanowire Coalescence[END_REF] The evolutions of the length, diameter, and aspect ratio as a function of the Cl -ions also play a significant role that is certainly less than the major role of Al(OH) 4 -complexes, but larger than the role of Ga(OH) 4 -complexes when using the co-doping of ZnO NWs grown by CBD with Al and Ga. 50 showing that Cl dopant substitutes for O atom to form Cl O with a relatively low formation energy, [START_REF] Liu | Defect Formation in Chlorine-Doped Zinc Oxide[END_REF] which is nevertheless higher than the formation energy of Al Zn . [START_REF] Tsin | Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar[END_REF] The concentration of Cl O is thus expected to be lower than the concentration of Al Zn , in turn affecting the wurtzite structure of ZnO NWs to a lesser extent by further considering that the Cl ionic radius is relatively close to the O ionic radius. Importantly, the a-and c-lattice dopants is more efficient when using Al(NO 3 ) 3 with a Al/Zn atomic ratio of 0.47% which is higher than 0.27% when using AlCl 3 . Interplay effects between Al(III) and Cl(I) species during CBD may account for the differential incorporation of Al dopants, notably through competitive adsorption processes of Al(III) and Cl(I) species on the surfaces of ZnO NWs as deduced from the effects on the structural morphology. However, it is worth noticing that although Cl(I) species reduce the incorporation of Al dopants, it is not fully hampered as Ga(III) species can do. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] In addition, the Al/Zn atomic ratios before and after thermal annealing are kept constant, such that the amount of Al dopants is not affected. The shift of the Al-related peak to a higher binding energy when AlCl 3 is used as chemical additive may be induced by the specific formation of a bond with a highly electronegative element as chlorine. [START_REF] Lee | Non-Metallic Element (Chlorine) Doped Zinc Oxide Grown by Pulsed Laser Deposition for Application in Transparent Electrode[END_REF] After thermal annealing, the relative intensity of the Al-related peak in Al-doped ZnO NWs significantly decreases when only Al(NO 3 ) 3 was used in the chemical bath, indicating a smaller amount of Al on the surface. Also, the Alrelated peak even vanishes in Al-and Cl-doped ZnO NWs when AlCl 3 was used, suggesting that the Al content is under the detection limit of around 0.5% even if the bulk composition is constant as observed by ICP-MS measurements. This indicates that the Al dopants migrate from the surfaces into the bulk of ZnO NWs during thermal annealing. XPS spectra of the Cl 2p core level are presented in Figure 6b. Before thermal annealing, the Cl-related peak associated with the 2p 3/2 core level occurs in both Cl-doped ZnO NWs and Al-and Clcodoped ZnO NWs when NH 4 Cl and AlCl 3 are respectively used as chemical additives. Despite the fact that the intensity of the Cl-related peaks is low, it should be noted that it is higher when NH 4 Cl was added to the chemical bath as compared to AlCl 3 . This shows that the surfaces of Cl-doped ZnO NWs are Cl-richer when using NH 4 Cl, which may indicate again interplay effects with Al(III) species in the adsorption process when using AlCl 3 . In contrast, after thermal annealing, the relative intensity of the Cl-related peak increases when using either NH 4 Cl or AlCl 3 as chemical additives. This indicates that Cl dopants migrate from the bulk to the surfaces of ZnO NWs during thermal annealing, namely in the opposite direction as Al dopants do. The Raman spectra of non-intentionally doped, Al-and Cl-doped ZnO NWs before and after thermal annealing are presented in Figure 7 by focusing on the low-wavenumber range from 70 to 800 cm -1 and in Figure 8 by focusing on the high-wavenumber range from 3200 to 3700 cm -1 . In the low-wavenumber range (Figure 7a ] ratio is increased up to 3%. The present increase originates from the Burstein-Moss shift [START_REF] Burstein | Anomalous Optical Absorption Limit in InSb[END_REF][START_REF] Moss | The Interpretation of the Properties of Indium Antimonide[END_REF] operating when the free electron density is higher than the critical concentration of 4.2 x 10 18 cm -3 in ZnO. [START_REF] Brochen | Non-Metal to Metal Transition in N-Type ZnO Single Crystal Materials[END_REF] In contrast, when using NH [START_REF] Walsh | Origins of Band-Gap Renormalization in Degenerately Doped Semiconductors[END_REF] and more specifically in the case of Ga-doped ZnO NWs. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] Interestingly, the use of AlCl is much less pronounced. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] Before thermal annealing as presented in Figure 10b, the incorporation of Al and Cl dopants within a respective cationic and anionic doping approach operates. When using Al(NO 3 ) 3 as chemical additive, onesixth of the initial Al(III)/Zn(II) species ratio in aqueous solution is incorporated into Al-doped ZnO NWs.

incorporation
parameters
Al Zn acting as shallow donor in the wurtzite structure of ZnO is preferentially formed owing to its low formation energy regardless of the Fermi level and oxygen chemical potential. [START_REF] T-Thienprasert | Compensation in Al-Doped ZnO by Al-Related Acceptor Complexes: Synchrotron X-Ray Absorption Spectroscopy and Theory[END_REF] The presence of Al dopants affects the formation of hydrogen-related defects by strongly reducing their concentration. The surfaces of Aldoped ZnO NWs are further Al-rich, again revealing the major role of Al(III) as capping agents. When using NH 4 Cl as chemical additive, the incorporation of Cl dopants into Cl-doped ZnO NWs is found to be less marked. Cl O acting as shallow donor in the wurtzite structure of ZnO is preferentially formed owing to its relatively low formation energy under O-rich conditions [START_REF] Liu | Defect Formation in Chlorine-Doped Zinc Oxide[END_REF][START_REF] Tsin | Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar[END_REF] corresponding to the high-pH region used. [START_REF] Villafuerte | Modulating the Growth of Chemically Deposited ZnO Nanowires and the Formation of Nitrogen-and Hydrogen-Related Defects Using pH Adjustment[END_REF] The formation of Cl interstitials (Cl i ) with a fairly low concentration is however not excluded. [START_REF] Tsin | Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar[END_REF] The presence of Cl dopants does not significantly influence the formation of hydrogen-related defects. The surfaces of Cl-doped ZnO NWs are further Cl-rich, again showing the significant role of Cl(I) as capping agents. When using AlCl 3 as chemical additive to grow Al-and Cl-codoped ZnO NWs, it is worth noticing that the incorporation of Al dopants is less pronounced than in Al-doped ZnO NWs and hence hampered to some extent by the Cl(I) species through competitive incorporation processes. Cl(I) species can nevertheless not fully hamper the incorporation of Al dopants as Ga(III) species do in Al and Ga-codoped ZnO NWs. [START_REF] Baillard | Interplay Effects in the Co-Doping of ZnO Nanowires with Al and Ga Using Chemical Bath Deposition[END_REF] The preferential incorporation of Al dopants over Cl dopants in the present CBD conditions is explained by the formation energy of Al Zn that is lower than the formation energy of Cl O . [START_REF] Tsin | Electrodeposition of ZnO Window Layer for an All-Atmospheric Fabrication Process of Chalcogenide Solar[END_REF] V Zn -assisted mechanism. [START_REF] Johansen | Aluminum Migration and Intrinsic Defect Interaction in Single-Crystal Zinc Oxide[END_REF] The appearance of standard AMs in the Raman spectra also reports the activation of Al doping. [START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF][START_REF] Verrier | Effects of the pH on the Formation and Doping Mechanisms of ZnO Nanowires Using Aluminum Nitrate and Ammonia[END_REF] In contrast, the surfaces of Cl-doped ZnO NWs and Al-and Cl-codoped ZnO NWs exhibit a larger amount of Cl. This suggests a migration process of Cl dopants from the bulk toward the surfaces, namely in the opposite direction as Al dopants do. The different localization of intrinsic defects and dopants is a typical observation in ZnO nanostructures and has been reported in the so-called core-shell model, where V Zn and V O are preferentially located in the bulk and on the surfaces of ZnO nanocrystals, respectively. [START_REF] Nadupalli | About Defect Phenomena in ZnO Nanocrystals[END_REF] Additionally, the hydrogen-related defects including H BC and V Zn -nH complexes are no longer formed significantly after thermal annealing, as expected from Ref. [START_REF] Villafuerte | Engineering Nitrogen-and Hydrogen-Related Defects in ZnO Nanowires Using Thermal Annealing[END_REF] . The present findings show that the simultaneous co-doping of ZnO NWs with Al and Cl acting as cationic and anionic dopants, respectively, can proceed during the CBD process when using AlCl 3 as the only chemical additive in the high-pH region. Interplay effects occur through competitive adsorption and incorporation processes between Al(III) and Cl(I) species opening perspectives to finely adjust the physical properties of ZnO NWs and to design synergistic approaches for many engineering devices.

CONCLUSIONS

The single and co-doping of ZnO NWs with Al and Cl using Al(NO 3 ) 3 , NH 4 Cl, and AlCl 3 as chemical additives have been investigated to elucidate the effects on their structural morphology and properties. The single and co-doping processes have been achieved in the high-pH region regardless of the chemical additive, where Al(OH) 4 -complexes and Cl -ions are the predominant Al(III) and Cl(I) species as determined from thermodynamic computations and hence readily adsorb on the positively-charged m-plane sidewalls through attractive electrostatic forces. The Al(III) species have been found to act as capping agents and to progressively inhibit the radial growth of ZnO NWs until a saturation is reached. The Cl(I) species have also been found to act as capping agents and to inhibit the radial growth of ZnO NWs, but this is accompanied by the promotion of their axial growth. Using the simultaneous co-doping approach, significant interplay effects between Al(III)

and Cl(I) species have been shown through competitive adsorption and incorporation processes. The Al(III) species predominantly affect the morphology of ZnO NWs as compared to Cl(I) species and its presence on their surfaces is more pronounced, but smaller when using AlCl 3 as compared to Al(NO 3 ) 3 following XPS measurements. The incorporation of Al dopants has been shown to be larger than the incorporation of Cl dopants owing to energetic considerations, but smaller when using AlCl 3 as compared to Al(NO 3 ) 3 following XRD, Raman scattering, and ICP-MS measurements. Interestingly, the thermal annealing under oxygen atmosphere usually performed to activate the Al doping of ZnO NWs results in the opposite migration processes of Al dopants toward the bulk and of Cl dopants toward their surfaces. Eventually, the Al doping more strongly affect the formation of hydrogen-related defects including H BC and V Zn -nH complexes than the Cl doping. The simultaneous cationic and anionic co-doping of ZnO NWs with Al and Cl using one single chemical additive appears as an additional way to tune their physical properties and facilitate their integration into engineering devices.

CONFLICTS OF INTEREST

There are no conflicts to declare.

ASSOCIATED CONTENT

In situ measurements of the pH and temperature during the CBD of non-intentionally doped, Al-and Cl-doped ZnO NWs (Figure S1); evolution of the length of non-intentionally doped, Al-and Cl-doped ZnO NWs as a function of the steady-state pH measured at the end of the CBD process (Figure S2); intensity ratio of the AM over E 2 High lines of non-intentionally doped, Al-and Cl-doped ZnO NWs before and after thermal annealing (Figure S3); concentration and dilution factor from ICP-MS measurements in non-intentionally doped, Al-and Cl-doped ZnO NWs before and after thermal annealing at 400 °C for 1 h under oxygen atmosphere (Table S1).

TOC Graphic

4 -

 4 processes of column-VIIB dopants including Cl are still open. To establish the strategy for the co-doping of ZnO NWs with Al and Cl using only one chemical precursor (i.e. AlCl 3 ), the speciation diagrams of Al(III) and Cl(I) species as a function of pH are plotted in Figure1ausing Visual MINTEQ software. In the low-pH region, the formation of Al(OH) 3 precipitates prevails, while, in the high-pH region, Al(OH) complexes constitute the dominant Al(III) species. The same speciation diagram of Al(III) species is found when Al(NO 3 ) 3 is used as a single chemical precursor. The transition from neutral to negatively charged Al(III) species proceeds at a pH value of about 6.6. Interestingly, Cl -ions constitute the dominant Cl(I) species in the low-and high-pH regions regardless of the value ranging from 5.2 to 9.2. Again, the same speciation diagram of Cl(I) species is found when NH 4 Cl is used as a single chemical precursor. Interestingly, the negatively charged Cl(I) species in the low-pH region may act as capping agents on the polar c-planes and hence hamper their development to form low aspect ratio ZnO nanostructures, as reported in Ref.[START_REF] Xu | Morphological Control of ZnO Nanostructures by Electrodeposition[END_REF] . The formation of additional complexes on the investigated range of pH is excluded as Al(III) and Cl(I) species do not preferentially interact with each other. The use of AlCl 3 in the high-pH region hence results in the formation of Cl -ions with a concentration that is three times larger than the concentration of Al(OH) 4 -complexes, in turn preserving the [Cl(I]/[Al(III] ratio introduced in the chemical bath. In contrast, the Zn(II) and Cl(I) species interact each other, but only alter slightly the speciation diagram of Zn(II) species specifically in the low-pH region, as presented in Figure 1b. In the low-pH region, Zn(II) species are predominantly stabilized as Zn 2+ ions, but ZnCl + and ZnNO 3 + complexes are also formed with a much smaller proportion of around 0.3 and 3.3% at a pH value lower than 6, respectively. In contrast, Zn(NH 3 ) 4 2+ complexes are predominantly formed in the high-pH region, reducing the homogeneous growth in the bulk solution, and no additional complexes with Cl(I) species are determined. Knowing that the point of zero charge (PZC) value is of 8.7 ± 0.2 and 10.2 ± 0.2 at room temperature for Zn-polar c-and nonpolar m-planes, respectively, 60-61 the adsorption process of Al(OH) 4 -complexes and Cl -ions is favourable through attractive electrostatic forces in the high-pH region, as represented in Figure 1c. The in situ measurements of temperature and pH are reported in Figure S1 of Supporting Information, delineating a grey-coloured rectangular area in Figure 1a,b.

Figure 1 . 61 3. 2 .

 1612 Figure 1. (a,b) Distribution of (a) Al(III), Cl(I) and( b) Zn(II) species at 85 °C as a function of pH, as computed by Visual MINTEQ software. The NH 3 concentration was varied in the range of 0-1000 mM, while the AlCl 3 and Zn(NO 3 ) 2 concentration were set to 0.9 and 30 mM, respectively. Each data point corresponds to the addition of 1 mM of NH 3 in the chemical bath. The grey rectangular area denotes the range of pH values experimentally measured at the end of the CBD process. (c) Schematic representing the adsorption processes of Al(OH) 4 -complexes and Cl -ions on the charged surfaces of ZnO NWs during CBD. The sign of the surface electrical charge is taken at the pH value used during the CBD process, considering the expected PZC values of both the top polar c-planes and nonpolar m-plane sidewalls of ZnO NWs around 8.7 ± 0.2 and 10.2 ± 0.2 atroom temperature, respectively.[START_REF] Valtiner | Stabilization and Acidic Dissolution Mechanism of Single-Crystalline ZnO(0001) Surfaces in Electrolytes Studied by in-Situ Afm Imaging and Ex-Situ Leed[END_REF][START_REF] Kunze | Self-Localization of Polyacrylic Acid Molecules on Polar ZnO(0001)-Zn Surfaces[END_REF]

  Interestingly, when using AlCl 3 as the only chemical additive for the co-doping process, the length of Aland Cl-doped ZnO NWs initially decreases to 2884 ± 138 nm as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 0.3% and then reaches a plateau around 3000 nm as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is further increased to 3% . The diameter of Al-and Cl-doped ZnO NWs follows a similar behaviour, first decreasing to 83 ± 20 nm as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 0.3% and then reaching a plateau around 75 nm as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is further increased to 3%. The evolutions of the length and diameter of Al-and Cldoped ZnO thus follow the evolution of the length and diameter of Al-doped ZnO NWs. Therefore, the dimensions of Al-and Cl-doped ZnO NWs are overall driven mainly by the Al(III) species in the chemical bath. Al(OH) 4 -complexes predominantly act on the sidewalls of Al-and Cl-doped ZnO NWs as compared to Cl -ions. Competitive adsorption processes between Al(OH) 4 -complexes and Cl -ions are expected on the nonpolar m-plane sidewalls of ZnO NWs. The aspect ratio of Al-and Cl-doped ZnO NWs continuously increases from 31.8 ± 7 to 37.1 ± 3 nm as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 3% where a plateau is reached and surprisingly follows the evolution of the aspect ratio of Cl-doped ZnO NWs. This indicates that

Figure 2 . 3 . 3 .

 233 Figure 2. (a) Top-view and cross-sectional view FESEM images of non-intentionally doped, Al-and Cl-doped ZnO NWs grown by CBD using Al(NO 3 ) 3 , AlCl 3 , and NH 4 Cl as chemical additives. The scale bar is the same for each top-view and cross-sectional view, respectively. Evolution of the (b) length, (c) diameter, and (d) aspect ratio of non-intentionally doped, Al-and Cl-doped ZnO NWs as a function of the [Al]/[Zn] and [Cl]/[Zn] ratios using Al(NO 3 ) 3 , AlCl 3 , and NH 4 Cl as chemical additives. The diameter was measured at half of the height of ZnO NWs.

  of Al-and Cl-doped ZnO NWs using AlCl 3 as the only chemical additive follow the same evolution as described for Al-doped ZnO NWs when using Al(NO 3 ) 3 as the only chemical additive. The a-lattice parameter initially decreases to 3.241 ± 0.004 Å as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 1%, and then increases to 3.248 ± 0.001 Å as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 3%. Correlatively, the c-lattice parameter of Al-and Cl-doped ZnO NWs increases from 5.205 ± 2.10 -4 to 5.208 ± 2.10 -4 Å as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 0.3% but saturates even as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 1%. Finally, the c-lattice parameter decreases to 5.207 ± 1.10 -4 Å as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is further increased to 3%. The distinct evolution confirms the major incorporation of Al dopants, and supports the incorporation of Cl dopants that likely proceeds to a lesser extent. Interplay effects are expected as well to form Al-and Cl-related defects in Al-and Cl-doped ZnO NWs.

Figure 3 .

 3 Figure 3. XRD patterns of non-intentionally doped, Al-and Cl-doped ZnO NWs grown by CBD using Al(NO 3 ) 3 , AlCl 3 , and NH 4 Cl as chemical additives (a) before and (b) after thermal annealing at 400 °C for 1 h under oxygen atmosphere. (c,d) a-lattice parameter and (e,f) c-lattice parameter (c,e) before and (d,f) after thermal annealing at 400 °C for 1 h under oxygen atmosphere as a function of the [Cl]/[Zn] and [Al]/[Zn] ratios in the chemical bath using Al(NO 3 ) 3 , AlCl 3 , and NH 4 Cl as chemical additives. The dashed lines indicate the reference values as obtained from the 00-036-1451 ICDD file.

3. 4 . 4 .

 44 Incorporation of Al and Cl Dopants into ZnO Nanowires. Non-intentionally doped, Al-and Cldoped ZnO NWs were diluted in nitric acid solution to perfom ICP-MS measurements and quantitatively determine the amount of incorporated Al dopants. Since Cl is considered as an interfering element using ICP-MS, it cannot be studied quantitatively here. The ICP-MS data are defined as the atomic ratio of Al dopants over Zn, such that the initial amount of ZnO NWs diluted in nitric acid solution has no influence on the chemical analysis. The Al/Zn atomic ratios in non-intentionally doped, Al-and Cl-doped ZnO NWs before and after thermal annealing are presented in Figure The mass concentration of Zn and Al dopants in nitric acid solution together with the dilution factor used are presented in Table S1 of Supporting Information. The amount of Al in non-intentionally doped and Cl-doped ZnO NWs using NH 4 Cl as the only chemical additive is measured in the range of 100-400 ppm, originating from the contamination of the chemical precursors with residual impurities. When the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] and [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratios are increased to 1%, the Al/Zn atomic ratio in Al-and Cl-doped ZnO NWs is around 0.13% regardless of the chemical additive. When the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] and [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratios are further increased to 3%, the incoporation of Al

Figure 4 . 5 .

 45 Figure 4. Evolution of the Al/Zn atomic ratio in non-intentionally doped, Al-and Cl-doped ZnO NWs before (full symbols) and after (empty symbols) thermal annealing at 400 °C for 1 h under oxygen atmosphere as a function of the [Al]/[Zn] and [Cl]/[Zn] ratios in the chemical bath as obtained from ICP-MS measurements.

Figure 5 .

 5 Figure 5. (a,e,i) Bright-field TEM images. Corresponding STEM-EDS elemental maps of the (b,f,j) Zn (green), (c,g) Al (orange), and (k) Cl (blue) elements. The Si signal comes from the substrate. (d,h,l)

Figure 6 .

 6 Figure 6. XPS spectra of the (a) Al 2p and (b) Cl 2p core levels of Al-and Cl-doped ZnO NWs before (solid lines) and after (dashed lines) thermal annealing at 400 °C for 1 h under oxygen atmosphere. The XPS data are illustrated for the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio of 3%, [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio of 3% and [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio of 9%.

2 Low 1 (

 21 ,b),the Raman lines at 101, 332, 437, and 574 cm -1 are respectively attributed to the E LO) phonon modes[START_REF] Cusco | Temperature Dependence of Raman Scattering in ZnO[END_REF] of the wurtzite structure of vertically aligned ZnO NWs along the polar c-axis. The evolution of the position of the E 2 High line in non-intentionally doped, Al-and Cl-doped ZnO NWs before and after thermal annealing is presented in Figures 7c,d, respectively. Before thermal annealing, a progressive and significant shift in the position of the E 2 High line from 438.1 to 438.8 and 438.7 cm -1 occurs as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] and [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratios are increased from 0 to 3 and 9%, respectively. 67 In contrast, a smaller shift of the position of the E 2 High line to a maximum value of 438.4 cm -1 occurs when the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 1%, which clearly reveals the role of Cl dopants when using AlCl 3 as chemical additive. After thermal annealing, the position of the E 2 High line of non-intentionally-doped ZnO NWs shifts to lower wavenumber from 438.1 to 437.4 cm -1 , suggesting a stress relaxation process in the ZnO NWs with a theoretical position expected at around 437-438 cm -1 . 66 All Al-and Cl-doped ZnO NWs exhibit the same trend with a significant decrease in the position of the E 2 High line. For Cl-doped ZnO NWs grown using NH 4 Cl as chemical additive, the position of the E 2 High line reaches a plateau at 437.8 cm -1 after thermal annealing, regardless of the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio. In contrast, for Al-doped ZnO NWs grown using Al(NO 3 ) 3 or AlCl 3 as chemical additives, the position of the E 2 High line after thermal annealing is close to the one of non-intentionally-doped ZnO NWs, apart from the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio of 1% where the line points at 437.7 cm -1 and can be related to the significant variation of the lattice parameters.

Figure 7 .

 7 Figure 7. Raman spectra in the low-wavenumber range for non-intentionally doped, Al-and Cl-doped ZnO NWs (a) before and (b) after thermal annealing at 400 °C for 1 h under oxygen atmosphere. The Raman

Figure 8 .

 8 Figure 8. Raman spectra in the high-wavenumber range for non-intentionally doped, Al-and Cl-doped ZnO NWs (a) before and (b) after thermal annealing at 400 °C for 1 h under oxygen atmosphere for [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio of 3%, [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio of 3% and [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio of 9%.

  3 as chemical additive to co-dope ZnO NWs with Al and Cl first increases significantly the optical band gap energy from 3.24 to 3.28 eV as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 0.3%, and then decreases to 3.25 eV as the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio is further increased to 3%. It is worth noticing that the variation of the optical band gap energy of Cl-doped ZnO NWs between the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio from 1 to 9% and of Al-and Cl-codoped ZnO NWs between the [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio from 0.3 to 3% is quite similar, indicating an incorporation process of Cl dopants in both cases. After thermal annealing, the optical band gap energy retains a rather constant value of around 3.26 eV in nonintentionally doped, Al-and Cl-doped ZnO NWs when using Al(NO 3 ) 3 and NH 4 Cl as chemical additives, respectively. In contrast, the optical band gap energy of Al-and Cl-codoped ZnO NWs when using AlCl 3 as chemical additive follows the same trend after thermal annealing, showing that the dopant-induced defects do not evolve in the same way and that Al and Cl dopants exhibit interplay effects.

Figure 9 .

 9 Figure 9. (a,b) Tauc plots in non-intentionally doped, Al-and Cl-doped ZnO NWs (a) before and (b) after thermal annealing at 400 °C for 1 h under oxygen atmosphere. The data are illustrated for the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio of 3%, [AlCl 3 ]/[Zn(NO 3 ) 2 ] ratio of 3% and [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio of 9%. The dashed lines denote the fit of the linear portion to deduce the optical band gap energy from its intercept with the hυ-axis. (c,d) Evolution of the optical band gap energy of non-intentionally doped, Al-and Cl-doped ZnO NWs (c) before and (d) after thermal annealing at 400 °C for 1 h under oxygen atmosphere as a function of the [Al]/[Zn] and [Cl]/[Zn] ratios in the chemical bath.

3. 5 10 .

 510 Al and Cl Doping Processes in ZnO Nanowires. From the experimental data collected with the different characterization techniques, a schematic diagram representing the single and co-doping processes of ZnO NWs with Al and Cl when using Al(NO 3 ) 3 , AlCl 3 and NH 4 Cl as chemical additives is illustrated in Figure The single and simultaneous co-doping of ZnO NWs with Al and Cl operates in the high-pH region obtained by adding ammonia in the chemical bath. The dominant Al(III) and Cl(I) species are stabilized in the form of Al(OH) 4 -complexes and Cl -ions, respectively, resulting in their efficient adsorption processes on the positively-charged m-plane sidewalls owing to attractive electrostatic forces, as presented in Figure 10a. The Al(III) and Cl(I) species both act as capping agents and strongly affect the morphology of ZnO NWs. When using Al(NO 3 ) 3 as chemical additive to grow Al-doped ZnO NWs, Al(OH) 4 -complexes are found to inhibit the radial growth by hampering the development of the nonpolar m-plane sidewalls as reported in Ref. 22 . The inhibition process is however influenced by the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio: its magnitude initially increases and then saturates with the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio, revealing its dependence on the surface coverage with Al(OH) 4 -complexes. When using NH 4 Cl as chemical additive to grow Cl-doped ZnO NWs, Cl -ions are shown to inhibit the radial growth while promoting the axial growth over the investigated range of [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio. When using AlCl 3 as chemical additive to grow Al-and Cl-codoped ZnO NWs, Al(OH) 4 -complexes predominantly act on their nonpolar m-plane sidewalls as compared to Cl -ions through competitive adsorption processes. Nevertheless, Cl -ions also play a significant role besides the major role of Al(OH) 4 -complexes, in contrast to Al-and Ga-codoped ZnO NWs for which the role of Ga(OH) 4 -complexes

  The formation of hydrogen-related defects is again affected by the incorporation of Al dopants. Interestingly, the surfaces of Al-and Cl-codoped ZnO NWs are less Al-and Cl-rich than the respective surfaces of Al-doped ZnO NWs and Cl-doped ZnO NWs, revealing interplay effects between Al(III) and Cl(I) species through competitive adsorption processes. After thermal annealing as presented in Figure 10c, the amount of Al dopants incorporated into Al-doped ZnO NWs and Al-and Cl-codoped ZnO NWs is similar, but the related defects are redistributed and their location is impacted. The surfaces of Al-doped ZnO NWs and Al-and Cl-codoped ZnO NWs exhibit a smaller amount of Al, indicating that a migration process of Al dopants toward the bulk operates for instance via a

Figure 10 .

 10 Figure 10. Schematic illustration of (a) the physicochemical processes at work in the chemical bath and of the Al and Cl co-doping processes of ZnO NWs grown by CBD (b) before and (c) after thermal annealing at 400 °C for 1 h under oxygen atmosphere.

  ± 95 nm as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 1% and then increases to 3224 ± 222 nm as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 3%. The diameter of Al-doped ZnO NWs undergoes a strong decrease from 124 ± 28 to 70 ± 13 nm as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 1% and then increases to 119 ± 24 nm as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 3%. The resulting aspect ratio of Al-doped ZnO NWs continuously decreases from 32 ± 7 to 26.5 ± 1 as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 1% and then very slightly increases to 27.1 ± 9 as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio reaches 3%. The evolution of the length, diameter, and aspect ratio of Al-doped ZnO NWs is in very good agreement with Ref.[START_REF] Verrier | Tunable Morphology and Doping of ZnO Nanowires by Chemical Bath Deposition Using Aluminum Nitrate[END_REF] , where a small variation of the steady-state pH was explained to significantly influence the supersaturation in the chemical bath and hence their dimensions. However, the change of the length of Al-doped ZnO NWs cannot here be The addition of NH 4 Cl in the chemical bath does not imply a significant variation of the diameter of Cl-doped ZnO NWs, which lies in the range of 110-140 nm, but follows, to a lesser extent, the same behaviour as the length of Cl-doped ZnO NWs. It should be noted that the variations of both the length and diameter of Cldoped ZnO NWs are an indication of the effect of Cl during the CBD. The resulting aspect ratio of Cl-doped around 8.75, as reported in FigureS1of Supporting Information. This indicates that the length and diameter of Cl-doped ZnO NWs is not explained by the small variation of the steady-state pH, but by the adsorption process of Cl -ions acting as capping agents. The evolution of the diameter and aspect ratio of Cldoped ZnO NWs suggests that Cl -ions do not influence significantly the radial growth, but instead promote the axial growth over the investigated range of [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio.

	ZnO NWs continuously increases from 31.8 ± 7 to 45.1 ± 3 as the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio is increased to
	9% where a plateau is reached. Generally, the evolutions of the length, diameter, and aspect ratio of Cl-doped

[Al]/[Zn] and [Cl]/[Zn] ratios when using Al(NO 3 ) 3 , AlCl 3 , and NH 4 Cl as chemical additives are presented in Figure 2b-d. The length of Al-doped ZnO NWs when using Al(NO 3 ) 3 as the only chemical additive initially decreases from 3923 ± 193 to 1844 related to a variation of the steady-state pH measured at the end of the CBD process, as presented in Figure S2 of Supporting Information. This indicates the predominant role of Al(III) species in the chemical bath in addition to the role of the pH value. The evolutions of the diameter and aspect ratio of Al-doped ZnO NWs suggest that Al(OH) 4 -complexes act as capping agents and progressively inhibit the radial growth as the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio is increased to 1%. However, the inhibition process of the radial growth of Aldoped ZnO NWs saturates when the [Al(NO 3 ) 3 ]/[Zn(NO 3 ) 2 ] ratio is further increased to 3%. This indicates that the magnitude of the surface coverage of the sidewalls of ZnO NWs with Al(OH) 4 -complexes may directly affect the efficiency of the inhibition process operating in the development of the nonpolar m-planes. In contrast, the length of Cl-doped ZnO NWs when using NH 4 Cl as the only chemical additive increases from 3923 ± 193 to 5752 ± 573 nm as the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio is increased to 1% and undergoes a subsequent decrease and an eventual increase to 5699 ± 204 nm as the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio reaches 9%. ZnO NWs are opposite to the evolutions of the length, diameter, and aspect ratio of Al-doped ZnO NWs. The evolution of the pH during CBD does not depend on the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio, resulting in a steady-state pH value of

  4 Cl as chemical additive, the optical band gap energy of Cl-doped ZnO NWs first increases slightly to 3.25 eV as the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio is increased to 1% and then decreases continuously to 3.22 eV as the [NH 4 Cl]/[Zn(NO 3 ) 2 ] ratio is further increased to 9%. The present decrease may come from the band gap renormalization usually reported in degenerate n-type semiconductors,

ACKNOWLEDGMENTS

The authors acknowledge the financial support from the French National Research Agency through the DOSETTE (ANR-17-CE24-0003) and IMINEN (ANR-22-CE09-0032) projects. This work has further benefited from some of the characterization equipments of Grenoble INP-CMTC platform and of IUT 1 Chemical Department platform. It has partially been supported by the French National Research Agency in the framework of the "Investissement d'Avenir" program (ANR-15-IDEX-02) through the project CDP ECO-SESA.

Furthermore, after thermal annealing, five additional modes (AM) point at 276 (AM 1 ), 509 (AM 2 ), 579 (AM 3 ), 623 (AM 4 ), and 643 cm -1 (AM 5 ), [START_REF] Bundesmann | Raman Scattering in ZnO Thin Films Doped with Fe[END_REF] and are assigned to the B [START_REF] Villafuerte | Modulating the Growth of Chemically Deposited ZnO Nanowires and the Formation of Nitrogen-and Hydrogen-Related Defects Using pH Adjustment[END_REF] This originates from the disordered-activated Raman scattering process due to the breakdown of the translational symmetry of the crystal lattice. [START_REF] Manjón | Silent Raman Modes in Zinc Oxide and Related Nitrides[END_REF] The presence of these AMs in the Raman spectra of non-intentionally-doped ZnO NWs was previously discussed in Ref. [START_REF] Villafuerte | Modulating the Growth of Chemically Deposited ZnO Nanowires and the Formation of Nitrogen-and Hydrogen-Related Defects Using pH Adjustment[END_REF] and is attributed to the incorporation of residual impurities provided by the different chemical precursors and/or the incorporation of nitrogen from HMTA molecules and NH 3 . Nevertheless, despite the fact that the AMs occur in the Raman spectra of non-intentionally-doped ZnO NWs after thermal annealing, the addition of the AlCl 3 chemical precursor in the bath induces a sharp increase in the intensity of AMs as seen in Figure S3 of Supporting Information. This is specifically revealed for AM 2 and AM 3 , supporting the significant incorporation of Al dopants into ZnO NWs. The intensity of AM 3 is further slightly higher when using AlCl 3 as chemical additive than when using Al(NO 3 ) 3 , suggesting the incorporation of Al and Cl dopants into ZnO NWs through the formation of additional related defects.

In the high-wavenumber range as seen in Figure 8a,b, several Raman lines and bands are associated with the incorporation of hydrogen-related defects into ZnO NWs. [START_REF] Villafuerte | Zinc Vacancy-Hydrogen Complexes as Major Defects in ZnO Nanowires Grown by Chemical Bath Deposition[END_REF] Before thermal annealing as presented in Figure 8a, the two most important Raman bands are located at around 3375 and 3575 cm -1 , and assigned to V Zn -nH defect complexes [START_REF] Lavrov | Hydrogen-Related Defects in ZnO Studied by Infrared Absorption Spectroscopy[END_REF][START_REF] Herklotz | Infrared Absorption on a Complex Comprising Three Equivalent Hydrogen Atoms in ZnO[END_REF] and interstitial hydrogen in the bond-centred sites (H BC ), [START_REF] Lavrov | Hydrogen-Related Defects in ZnO Studied by Infrared Absorption Spectroscopy[END_REF] respectively. The intensity of both Raman bands only decreases slightly as NH 4 Cl was added to the chemical bath, indicating that the incorporation of Cl dopants plays a limited role on the formation of hydrogen-related defects. In contrast, the use of either Al(NO 3 ) 3 or AlCl 3 as chemical additives in the bath results in a significant decrease in the intensity of both Raman bands, confirming that the incorporation of Al dopants strongly affects the formation of hydrogen-related defects. A shoulder around 3500 cm -1 is attributed to O-H bonds on the surfaces of ZnO NWs [START_REF] Lavrov | Hydrogen-Related Defects in ZnO Studied by Infrared Absorption Spectroscopy[END_REF][START_REF] Lavrov | Infrared Absorption Spectroscopy of Hydrogen-Related Defects in ZnO[END_REF] and usually observed when the growth operates in the low-pH region. [START_REF] Villafuerte | Modulating the Growth of Chemically Deposited ZnO Nanowires and the Formation of Nitrogen-and Hydrogen-Related Defects Using pH Adjustment[END_REF] Here, no related-peak is noted owing to the different growth process in the high-pH region. After thermal annealing as presented in Figure


D Della Valle

Anxionnaz Zoé

Philippe Carriere
email: philippe.carriere@ec-lyon.fr

Raynal Florence

EXTRAPOLATION OF MULTIFUNCTIONAL HEAT EXCHANGERS AT INDUSTRIAL SCALE -THE EXTRAPOLIS PROJECT

Keywords: extrapolation, micromixing, process intensification, vortex generators, static mixerreactor, multifunctional heat exchanger

This presentation is aimed to introduce the EXTRAPOLIS project, subsidised by the CNRS (France) in the frame of the " Energy 2006-2009" program (PR1-2-22). The general purpose of this project is to promote the use of multifunctional exchangers-reactor at industrial scale. The scientific results are the subject of 3 communications in this GPE congress.

INTRODUCTION

The concept of multifunctional exchanger-reactors is born from the development of the process intensification, consisting in the use of innovative geometries designed for the sake of : energy gain (intensified transfers) better productivity (less diluted reagents) better safety (local control of temperature) reduction of effluents (best selectivity) A previous CNRS research project on "Multifunctional Heat Exchangers (MHE)" ("Energy 2003-2006", AC PR2.1), has confirmed the feasibility and the benefits of such systems, consisting in the achievement of various highly exothermic chemical reactions in open loop reactors. An increase by a factor 100 to 10000 of the thermal power exchange with the secondary flow, compared to the batch processes, allows a sufficient temperature control to effectively prevent the runaway risk. They ensure a good and homogeneous mixing, which is the key parameter for the process efficiency. All this makes possible to increase the concentrations, and spare a part of the costly product separation. Actually, MHE are convenient for the objectives of sustainable development These features have been established at laboratory scale, for flow rates about 1L/h . In order to meet the industrial needs, the productivity has to be increased a factor 1000. This objective cannot be attained by simple homothetic geometry of the laboratory reactor, because of the degradation of all the performances (mixing efficiency, heat transfer) in the size increase. It is well known that the dimensionless numbers governing the physical mechanisms of (turbulence, macromixing, heat transfers, mass transfers, micromixing, and chemical reactions) cannot be in the whole maintained at the same level in a size up, while the transfers are systematically penalized. The general purpose of exchangers extrapolation in industrial situations needs geometrical modifications especially to retain the surface density factor. The practical skill in this domain of the CEA partner has been a food for though in this research program. In order to master and optimize the scaling operation, it arises in particular the need of fundamental investigations for a better understanding of the governing transfer mechanisms, in the idea of favoring systems that provide palliative solutions to counteract the unfavorable effects of the scale up. The broad outline of the EXTRAPOLIS project consists hence in a theoretical axis and an experimental one. The theoretical section is focused on fundamental aspects: the conceptual aspects of extrapolation, an innovative approach for the description of the fundamental mixing mechanisms, and the implementation in CFD programs of a micromixing model, to account for the small turbulence scales part in the molecular contact between the chemical species, especially for high Schmidt numbers. The experimental section consists in the study of selected geometries, a plate type exchanger DEANHEX, and a HEV exchanger, to understand and optimize their behavior in extrapolation. Finally, making the most of the previous investigations, the realization of a set of prototypes specifically designed for extrapolation purposes is planned to conclude this project. This presentation is aim to explain the general problematic of the project, and the scientific results achieved in this context are the topic of three communications in the present congress, as specified further.

GENERAL DESCRIPTION OF THE EXTRAPOLIS PROJECT

The main objective of the EXTRAPOLIS project is the contribute to the development of multifunctional exchangers-reactors in the chemical industries, and for that:

• Understand the evolutions of performances in the scale up of multifunctional exchangers-reactors • Explore solutions for the retain of laboratory performances at industrial scale, especially for rapid and exothermal reactions • Design and build-up of original prototypes able to maintain the mixing abilities in the scale up

Principle of multifunctional heat exchangers

Multifunctional heat exchangers are basically heat exchangers or static mixer providing a high level of transfers, especially suitable for the heat removal of highly exothermal reactions, and that could be classified in "intensified systems" 1,2,3. Intensification can be yielded by different contrivances that have in common to disrupt the flow. As shown in figure 1 for a wide variety of MHE geometries, these disturbances can be induced by obstacles, like fins, vortex generators, solid foams filling the flow section, but also by bends or flux recombination. Understanding the action mode of these different systems towards the mixing properties is quite important to propose firstly a good recommendation for extrapolation rules, and secondly to conceive a good design for an optimal behavior in the scale up. The first kind of flow disturbances are of dynamical origin, as far as they are the result of vortices and wake effects due to the flow derivation. In a good design, they considerably intensify the transfer without increasing substantially the pressure drop. To the contrary, a bad design will be in favor of "useless pressure drop" by the generation of excessive dead zones in the flow. Another trap would be the idea of saturating the flow with the insets: beyond a certain cluttering, the transfers are damped and the over energy consumption is in this case wasted. The other type of disturbances is of kinematical type: the mixing is ensured by the advection and the cross over of stream lines. This kind of mixing must be profitable for extrapolation because it can be achieved independently of the Reynolds number, so that the system is not tamed by the dynamic similitude. This is the fundamental idea of the DEANHEX mixer, conceived in the LGC laboratory, where the mixing is ensured by the Dean flow in the bends, and the intensity of the mixing is governed by the curvature radius and the bend angle. A variety geometries corresponding to this criterion were till now studied by our research team in the previous program, as well in laminar, inertial or turbulent regimes [START_REF] Ferrouillat | Open loop thermal control of exothermal chemical reactions in Multifunctional heat exchanger[END_REF] , examples are given in fig 1.

Good working knowledge of extrapolation for heat exchangers

The approach of the extrapolation of MHE can be transposed of the actual practice for heat exchanger as worked out in the CEA. In order to ensure the same thermal exchange power for large size exchanger, some compromises have to be done in the size of the primary and secondary flow sections, but the geometry of the components have to be chosen for maintaining the same flow features at laboratory scale and industrial scale, like illustrated in figure 3. In this approach, it clearly appears that the same geometry cannot be used with satisfactory results in the size up. Hence, following the method of the CEA 5 research department, the main idea that will be developed in the EXTRAPOLIS project is to target the best systems to make up for the loss of performances in the scale up. The main leads that will be explored for the intensification of the mixing are the following:

-the potentiality of kinematical mixing -the use of insets like vortex generators -the design of bends -the corrugation of walls -the use of nonstationary flow, like pulsated flows These topics will be investigated in the different steps of the project, either in a theoretical approach or in an experimental one.

Outline of the EXTRAPOLIS project

The research program takes place in the 2006-2009 period, hence comes in its last year. All the topics presented in figure 4 are nearly completed, except for the development of the pilot EXTRAPOLEX that needs the finalization of the other themes. The two sections of the project, the theoretical and experimental ones, involve the complementary competences of all the partners.

Mixing and local stirring

Mixing is considered through the kinematics of the local, instantaneous gradient of a passive tracer or scalar quantity, such as concentration. The rise of the tracer gradient, which is equivalent to the stretching of material surfaces, results in the enhancement of mixing through acceleration of molecular diffusion. This is caused by the mechanical action of straining, a matter of strain intensity and orientation of the gradient with respect to the local strain principal axes. In particular, increase of the tracer gradient is promoted by alignment with the most compressive strain direction. The question indeed comes to understanding how the tracer gradient behaves under the combined effects of molecular diffusion, strain and rotation. In addition, since stirring most often occurs in time varying flows, local stirring properties are determined by the dynamics of the tracer gradient, namely its response to any nonstationary mechanical actions or forcing 6 . The approach is based on universal mechanisms and is thus valid for explaining local stirring in different flow regimes. Through analytical and numerical studies, we specifically investigate the case of nonstationary forcing and its possible effect on promotion of stirring. This topic will be presented by in the present conference by Dr. Gonzalez (CORIA) in "Investigation of local stirring through the behavior of tracer gradient".

Micromixing in CDF

Micromixing is actually accounted for in numerous commercial CFD programs for the needs of combustion simulations, which occur for the most in turbulent regime in gas phase. In liquid phase, the available models are not adequate because the molecular contact takes place at much smaller scales than the Kolmogorov scale, and appears hence as the limiting factor in the chemical reaction. As recommended by Fox 7 , the transport of a passive tracer ands its variance are added to those of reactive species, in order to compute the reaction rate. This model will allow reliable numerical simulation in a wide scale range. It will be validated and tested by comparison with measurements carried out in the CEA. This aspect is developed in the LTN, in collaboration with the CORIA.

EXPERIMENTAL SECTION OF EXTRAPOLIS

The performances of the MHE are supported by some flow properties that have to be characterized: the macromixing (RTD), the global heat and mass transfer coefficient, the friction factor and Colburn factor, and the micromixing efficiency. This requires a set of methods that have been carried out partly in the "Multifunctional exchangers" program, as well as in the present program.

Extrapolation of a "good candidate": the DEANHEX reactor

A plate-type heat exchanger/reactor, namely DEANHEX, including one process plate and one utility plate, seems a promising configuration for operating an extrapolation. A wavy channel is mechanically Fig5. Extrapolation by flux division etched on each plate with a numerical robot. The present work aims at optimizing the geometry of the process channel by correlating its geometrical parameters (curvature radius and bend angle) with hydrodynamic criteria. An optimization of the straight length is made for the "relaxation" of the secondary flow, in the aim of minimizing the pressure drop. This work will be the subject of the communication in the actual congress of Z. Anxionnaz in "Hydrodynamic study and optimization of the geometry of a Heat Exchanger/Reactor", in the frame of a PhD thesis in the LGC and CEA.

Extrapolation of a "good candidate" : the HEV mixer

The HEV mixer has been studied for a few years in the LTN [START_REF] Mokrani | Experimental study of the influence of the rows of vortex generators on turbulence structure in a tube[END_REF][START_REF] Kaci | Enhancement of turbulent mixing by embedded longitudinal vorticity: a numerical study and experimental comparison[END_REF] . The tube equipped with vortex generators provides a very efficient mixing in turbulent flow for a small pressure drop. Typically, this device is difficult to extrapolate on several orders of magnitude. Based on LDA turbulence measurements, the purpose here is to investigation the evolution of the turbulent length scales for a 2.5 extrapolation factor. This will be the subject of a communication in the GPE-EPIC 2009 conference of T. Lemenand: "Evolution of length scales and turbulence spectra in the extrapolation of HEV exchanger size".

Figure 1 -

 1 Figure 1-Examples of multifunctional exchangers-reactors

Figure 2 -

 2 Figure 2-From the elementary scale to the industrial plant

Figure 4 -

 4 Figure 4-Structuration of the EXTRAPOLIS project

Design of a new EXTRAPOLIS component

The new pilot will be designed and constructed in the CEA (june-september 2009), with the collaboration of the LGC and LMFA. The operating principle will be based on a kinematical mixing, probably a flux recombination, although this issue has not been settled at the present time.

CONCLUSIONS AND FUTURE WORKS

The main issues of this project are to propose a practical answer based on scientific investigations. The EXTRAPOLEX prototype will be designed on the base of the flow properties considerations. The global performances in the scale up will be studied experimentally and tested for an industrial chemical reaction.