Paul Morandi

Valerie Flaud

Sophie Tingry

David Cornu

Yaovi Holade
email: yaovi.holade@enscm.fr

Tartaric acid regulated the advanced synthesis of bismuth-based materials with tunable performance towards the electrocatalytic production of hydrogen peroxide

Tartaric acid regulated the advanced synthesis of bismuth-based materials with tunable performance towards the electrocatalytic production of hydrogen peroxide

Introduction

Historically, the synthesis of diverse morphologies and sizes of micro-and/or nano-crystals for different biomedical and catalytic applications was guided by the use of a variety of organic molecules, prevailingly citric acid, polyvinylpyrrolidone (PVP), the cetyltrimethylammonium bromide (CTAB). 1,2 The modification of the physico-chemistry of the reaction mixture results into chemical species acting as surfactants or capping agents. Furthermore, the tartaric acid was investigated as another natural carboxylic acid from plants to potentially substitute the citric acid in the preparation of colloidal metal nanoparticles, 3,4 a field referred to as "biogenic synthesis" to face the growing need of environmentally compatible technologies in materials synthesis. 3,5 The guiding principle behind the fabrication of colloidal particles when the tartaric acid is employed relies mainly on the coordination chemistry, [6][7][8] which is triggered by the presence of many OH groups. This organic acid has been used to successfully prepare monodispersed nanoparticles of gold and silver for their optical properties deriving from the surface plasmon resonance. 4,9 For their potential physico-chemical properties, bismuth-based materials such as Bi2X3 (X = S, Te, etc.) were also prepared in the presence of the tartaric acid. 10,11 Belonging to the semimetal group, the small overlap between the bottom of the conduction band and the top of the valence band provides interesting physicochemical and catalytic properties to bismuth. [12][13][14][15] For instance, bismuth-based composites gained a signification interest as active materials for the photo-/electrocatalytic synthesis of valuable chemicals (production of hydrogen peroxide, 16 ammonia 17 , formate 18,19 , hydrogen 15,[START_REF] Bera | Solar Energy Materials and Solar Cells[END_REF][START_REF] Bera | [END_REF] ), the control of the catalytic selectivity, [22][23][24][25] the sensing, 26 and energy conversion [27][28][29][30] applications.

For the above end-uses, various synthesis methods were developed to prepared different types of micro-and nano-structured materials ranging from metallic Bi to Bi2O3 oxides depending on the designed synthesis protocol. 8,18,19,27,[29][30][31] Since tartaric acid is a triacid and bismuth salts are usually trivalent, the coordination chemistry in an aqueous medium for the production of a variety of morphologies and sizes is of particular interest to study, especially the effect of the alkalinity.

Ma et al. 8 reported the preparation of bismuth nanoparticles and their subsequent self-assembly into nanobelts, arguing that the formation of nanobelts depends on the amount of NaOH. Authors hypothesized that NaOH determines the reduction rates of the bismuth-tartrate complex [Bi2(C4H2O6)2] 2-, which influences the shape of the bismuth products. 8 No in-deep physicochemical study was performed to correlate the composition of the produced materials to the reaction conditions. So, this study raised more questions about the role of alkalinity and especially the properties of materials that would result from such synthesis in the presence of tartaric acid since such organics can partially and even completely block the surface of the particles. A more comprehensive and focused study was therefore expected; in particular on how those materials would electrochemically react given the above applications of bismuth-based materials in electrosynthesis, and energy conversion/storage domains. For instance, photo-and electro-catalytic methods emerged in recent years as viable options to the high energy consumption anthraquinone process for the production of hydrogen peroxide, a chemical widely used in various industries. 16,[32][33][34][35][36][37][38][39][40][41] For electrocatalysts containing cost-effective non-noble metals, the alkaline medium is preferred to ensure chemical stability. Electrocatalytically, the two half-cell reactions and overall process describing the operation of an alkaline electrolyzer for the hydrogen peroxide production are resumed by Eqs. 1-3 since pKa(H2O2/HO2 -) = 11.75. This would mean that a low cell voltage is required to drive electrocatalytically the hydrogen peroxide production if highly selective catalysts are found; hence their better design is expected.

Cathode:

O2 + H2O + 2e -→ HO2 -+ HO -, E° = 0.76 V vs RHE (Eq. 1)

Anode: 4HO -→ O2 + 2H2O + 4e -, E° = 1.23 V vs RHE (Eq. 2)

Overall: O2 + 2HO -→ 2HO2 -, E°cell = 0.47 V (Eq. 3)

Bi-based materials were found to be effective anode for the two-electron photocatalytic waterto-H2O2 process. 16,42 However, to date, no electrocatalytic studies have been carried out on the potentiality of micro-and/or nano-structured bismuth materials derived from the above coordination chemistry with the tartaric acid. This would be worthwhile as the oxygen reduction reaction (ORR, Eq. 1) is highly sensitive to the surface state. Specifically, a high selectivity toward hydrogen peroxide is achieved when O2 is perpendicularly adsorbed by a single atom on the catalytic surface, which is enabled by the inter-distance between active sites and the heterogeneity of the catalytic surface. [43][44][45] It is therefore relevant to examine the potentiality towards ORR of the materials obtained by the coordination chemistry of tartaric acid. The direct hydrogen peroxide production from the oxygen is particularly important for the on-site treatment of organic pollutants through the electro-Fenton process. [46][47][48][49] To take up the above challenges, we aim in the present study to interrogate physico-chemically and electro-catalytically the effect of the molar ratio [sodium hydroxide]/[bismuth (III) nitrate] on the properties of the resulting bismuth-based materials. Specifically, we integrated various and complementary physical and electro-analytical techniques, namely the scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), cyclic voltammetry (CV), linear sweep voltammetry (LSV), rotating ring-disk electrode (RRDE), electrochemical impedance spectroscopy (EIS), bulk electrolysis and UV-Vis assays.

Our outcomes demonstrate that the different engineered types of bismuth-based micro-and nanostructured materials exhibit distinct surface and catalytic properties when the as-synthesized materials are employed in the specific application of electrochemical hydrogen peroxide production from oxygen. FAA-3-50) was purchased from Fuel Cell Store (USA) and activated according to the procedure freely available on the internet website of the supplier. The water as solvent and used during the entire study was ultrapure and was obtained from a Milli-Q Millipore source (MQ, 18.2 M cm at 20 °C).

Experimental Section

Synthesis of different types of bismuth-based materials

The used procedure to synthesize bismuth-based materials was derived from a published method, 8 but with some adjustments to be able to produce different types of materials. In a typical synthesis, a mass of 1.212 g of Bi(NO3)3•5H2O was dissolved by stirring in 5 mL of an aqueous solution of 1.8 M HNO3; all in a 500 mL three-neck round-bottom flask. Then, 0. [START_REF] Napporn | Fuel Cells and Hydrogen: From Fundamentals to Applied Research[END_REF] i.e., a combination of aqua regia (3 vol. of HNO3 and 1 vol. HCl) and hot water.

Procedures of physicochemical characterizations

The morphology of the as-fabricated materials was analyzed by SEM on a Hitachi S-4800 microscope. have been post-corrected for ohmic drop, as determined by EIS. To prepare a homogeneous catalytic ink, 130 µL of water and 50 µL iPrOH were subjected to a vortex mixing followed by the addition of 5 mg of a desired material to undergo a sonication in a water batch. Before any catalytic ink deposition on the GC disk, RRDE was polished with alumina slurry of 3, 1 and 0.05 μm successively and washed with water before a final step of sonication in water for 5 min in order to remove any traces of remaining alumina. To get a good thin film of the catalytic ink onto GC, the RRDE was mounted on an inverted rotator shaft set at 0 rpm. After pipetting 4 µL onto the GC tip (loading of 0.5 mgcat cm -2 ), the ink was subsequently dried under ambient conditions by increasing the rotator speed to 400 rpm and maintaining it until complete drying. [START_REF] Shinozaki | [END_REF]52 The electrochemical measurements were performed on a home-made PTFE cell to withstand the alkalinity, operated at room temperature (22 ± 2 °C) and was frequently and rigorously cleaned by the permanganate method. [START_REF] Napporn | Fuel Cells and Hydrogen: From Fundamentals to Applied Research[END_REF] For CV experiments, the electrochemical measurements were recorded in a conventional three-electrode setup. The GC disk coated with the catalytic ink was the working electrode, a 12.4 performed by fast potential cycling from 0.3 to 0.9 V vs RHE at 100 mV s -1 in O2-satured 1 M KOH for a total of 5000 cycles (1 cycle = forward and backward).

Procedures of electroanalytical measurements

Direct monitoring by the RRDE setup. The hydroxide peroxide in the form of HO2 -in alkaline media (pKa(H2O2/HO2 -) = 11.75) and the overall exchanged number of electrons nexp were determined online by Eqs. 4 and 5, respectively. [START_REF] Napporn | Fuel Cells and Hydrogen: From Fundamentals to Applied Research[END_REF]53,54 For those RRDE measurements, a fourelectrode potentiostat (also referred to as double-or bi-potentiostat) was utilized. RRDE was calibrated in N2-saturated 1 M KNO3 + 5 mM K3[Fe(CN)6] to determine its collection efficiency (N) defined as the fraction of the species formed at the disk that effectively arrives at the ring and reacts there, which was verified to be 24.5 ± 0.5% (Fig. S2), in agreement with the manufacturer's value of 24.9%.

R D I I N HO    1 200 (%) 2 (Eq. 4) D R ex I N I n   1 4 (Eq. 5)
ID is the current from O2 reduction at the disk and IR is the current from the intermediate HO2 - oxidation at the ring (note that current-potential curves were plotted according to IUPAC convention for which to a positive current is oxidation and a negative current is reduction 55 ).

Bulk electrolysis and UV-Vis assays. Bulk electrolysis was carried out in an H-type cell at room temperature (22 ± 2°C) for an applied potential of 0.6 V vs RHE (iR-uncorrected) and 2 h. The two compartments were separated by the anion exchange membrane. Anodic compartment contained 57 mL of outgassed 1 M KOH and a glassy carbon plate of 16.5 cm 2 as counter electrode.

The cathodic compartment contained 57 mL of O2-saturated 1 M KOH, the reference electrode (Hg|HgO|KOH 1 M) and the working electrode. The latter was made of a piece of GDE cut into a L-shape of 2 cm high and 1 cm wide (exposed area of 2 cm 2 , not taking into account its 3D structure) and enough space on the top for electrical wiring with gold. An aliquot of 41 µL of the catalytic ink was gradually deposed onto each side and left to dry at room temperature to reach the same loading as RRDE, i.e., 0.5 mgcat cm -2 . The solution was gently stirred during electrolysis and O2 flow (on surface) maintained. The samplings were firstly acidified toward pH 3.5, and the transformed H2O2 species (HO2 -(aq) + H + (aq) = H2O2, pKa = 11.75) were secondly quantified by UV-Vis assays using potassium titanium (IV) oxalate as a colored indicator. 40,[56][57][58] For that, the absorbance of the yellow pertitanic acid complex between hydrogen peroxide and potassium titanium oxalate was measured at λ = 390 nm on a JASCO V-570 UV-Vis-NIR spectrophotometer.

The faradaic efficiency (FE) also referred to as faradaic selectivity for the electrosynthesis of HO2 - was evaluated from Eq. 6.

100 100 (%) exp 2      Q CVF n y electricit total HO produce to used y electricit partial FE (Eq. 6)
C(mol L -1 ) is the molar concentration of HO2 -, V(L) is the volume of the electrolysis solution,

F(= 96485 C mol -1 ) is the Faraday constant, Q(C)
is the total electrical of charge passed through the reaction, and nexp is the exchanged number of electrons (nexp = 2 for O2-to-HO2 -).

Results and discussion

Preparation of various bismuth-based materials

The complexation between the tartaric acid and the bismuth (III) was used as a tool to design an advanced synthesis process that will allow us obtaining bismuth-based materials with different morphologies and subsequently modify their surface and structural properties. Scheme 1 resumes the designed methodology, which mainly relies on the change of the alkalinity of the reactor content that was adjusted by the sodium hydroxide base. Indeed, we first hypothesized that the change of the alkalinity in the starting colloidal solution should enable to regulate the complexation between the organic acid and the metallic cation. This would act as a basis to manipulate the seeding, growth and subsequent self-assembly into micro-and nano-structured materials. The latter will lay the foundation towards the preparation of composite and hybrid materials with advanced surface and electrocatalytic properties. Upon the addition of the tartaric acid to the reactor containing a dissolved bismuth (III) nitrate, a white temporary suspension attributed to the [Bi2(C4H3O6)2] organic complex was rapidly formed. [6][7][8] When sodium hydroxide was added, the previous white suspension disappeared for a certain amount (R higher than 12). This observation is explained by the deprotonation of the previous organic acid leading to a Bi2(C4H2O6)2] 2-ioncomplex, 7 as highlighted in Scheme 1. By combining the acidity of the native tartaric acid (pKa = 3.0 and 4.4) to the reduction kinetics of [Bi2(C4H3O6)2] and Bi2(C4H2O6)2] 2-by the sodium hypophosphite, different types of bismuth products are expected. To rationalize our synthesis methodology, the amount of bismuth precursor was fixed and that of sodium hydroxide was changed according to the molar ratio R = n(NaOH)/n(Bi(NO3)3•5H2O). This new design was highly motived by the possibility to produce various bismuth materials ranging from metallic to oxide by a simple change of one parameter, which could inspire and facilitate further scaling up.

We next carefully studied the produced materials by a set of physiochemical and electrochemical techniques before the evaluation of their ability towards the targeted electrocatalytic hydrogen peroxide production.

SEM and EDX analysis

We first utilized the microscopic techniques to determine whether the hypothesized production of different micro-and nano-structured bismuth-based materials by the change of the molar ratio R = n(NaOH)/n(Bi(NO3)3•5H2O) was successful or not. Fig. 1 shows the SEM images at different magnifications for the materials obtained for R = 0, 6, 12, 24 and 30. Additional SEM pictures are reported in Figs. S3-S9 to probe a wide spectrum of R = 0 to R = 30 and better see the effect of the alkalinity on the nature of the produced material. For R = 0 to R = 6, a brown color powder produced at a yield of about 100% (relative to the expected bismuth weight and the recovered one)

and consisting of nano-scale particles of the order of 100 nm, which undergo a self-assembly process to yield micro-particles. As it can be observed in Fig. 1 and Figs. S3-S4, the materials produced at R = 2-6 are more homogeneous with a well-defined morphology than the material fabricated without any addition of NaOH (R = 0). For R = 12, a white solid was recovered with a yield of 142% and mainly composed of fibers of about 100 nm width and micro-sized in length (see Fig. 1 and Fig. S5). This extra yield and the color of the recovered product would indicate that a composite material was obtained, which may likely involve the hydration, hydroxyl or oxyhydroxyl-based system. Further complementary studies by EDX, XRD and XPS will provide more information. The material obtained at R = 8 (Fig. S3) is a transition between the metallic state for R = 0-6 and the (oxy)hydroxyl one at R = 12. The ratio R = 18 was found to be another transition state towards the production of a black powder at a yield of 74%, which is mainly composed of microfibers and some microparticles (Fig. S6). Indeed, for R = 24, micro-and nanoscale particles materials were obtained at a chemical yield of 63% (Fig. 1 and Fig. S7). For R = 30 (Fig. 1 and Fig. S8), there is an overwhelming presence of micrometric particles of particular morphology (chemical yield of 60%) when we compared with the materials produced at R = 0-6.

Increasing excessively the amount of NaOH up to R = 36 results in the obtaining of crystals of various shapes from spherical to rods (chemical yield of 32%) as evidenced in Fig. S9. The raison behind a yield below 100% is simply due to the filtration process in which smaller particles are unavoidably passing through the pores of the membrane. Those SEM characterizations highlight, for the first time, a strong discrepancy leading to three types of materials, R = 0-6, R = 12, and R = 24-36. Indeed, the only reported synthesis method 8 did not result in the obtaining of the present materials. Those visual results confirm our postulated hypothesis and indicate that the growth and crystallization can be regulated by the modification of the alkalinity of the synthesis media. The detailed mechanism and driving force behind this synthesis will be discussed later in the manuscript when the materials will be well characterized. Furthermore, given the diverse morphologies and sizes of the as-synthesized materials, it is anticipated different trends in the electrocatalytic properties.

Having revealed and confirmed by SEM that we were able to prepare different composite and hybrid bismuth-based materials, we next sought to examine precisely the composition of those micro-and nano-structured materials. To address that, we utilized the EDX to focalize the study on the representative materials of R = 0, 6, 12, 24 and 30. The results are presented in terms of the overall mass composition (Fig. 2a), the overall atomic composition (Fig. 2b), and the atomic ratio of oxygen/bismuth (Fig. 2c). The bismuth content was evaluated to be 95.6 ± 0.5, 93.0 ± 2.1, 75.0 ± 2.0, 87.6 ± 2.0, and 88.7 ± 1.5wt%, for R = 0, 6, 12, 24 and 30, respectively. Control analysis by TGA-DSC of the as-synthesized material at R = 30 confirmed the trend about the presence of water/carbon species of ca. 2 wt% (Fig. S10). The high atomic percentages in Fig. 2b are simply the result of the low molecular weight of oxygen and carbon compared to bismuth. If the presence of carbon could be attributed to the used complexing agent (tartaric acid) and stabilizer (sodium oleate), the disparity suggests that the remaining amount depends strongly of the type of materials.

The oxygen-to-bismuth atomic ratio given in Fig. 2c provides insightful information about the type of material that was formed. In all cases, the ratio is below 6, which allow us to rule out the simple crystallization of the [Bi2(C4H3O6)2] molecular complex or the Bi2(C4H2O6)2] 2-ioncomplex (no sodium counter cation was detected by EDX) for which two atoms of bismuth correspond to 6 atoms of oxygen. Quantitatively, O/Bi atomic ratio is 0.17 ± 0.05, 0.32 ± 0.09,

PXRD screening

The previous discussion of the SEM and EDX results has pointed out the possible synthesis of bismuth-based materials with different crystallinity and composition. To unambiguously verify those potentialities, we next performed the powder X-Ray diffraction (PXRD) for the entire ten materials that were prepared through the regulator R = n(NaOH)/n(Bi(NO3)3•5H2O). Fig. 3 displays the obtained PXRD patterns of the as-synthesized bismuth-based materials at different molar ratio. The measurements were made under the same experimental conditions. From the Fig. 3a, the overall comparison of the ten types of materials for R = 0-36 already reveals a difference in intensity and the number of peaks. A separation (Fig. 3b, R = 6-36) and indexed patterns (Figs.

3b-c, R = 8-36 and R = 0-6) allow examining closely the different situations. For R = 0-8, the respectively. 8,[59][60][61] The observed peaks at 22.5 and 23.8° in Fig. 3a are assigned to the (003) and

(101) reflections of that metallic bismuth. 8,60,61 This bulk diffraction characterization clearly demonstrates that metallic micro-structured particles of bismuth are obtained when R is below 12, which is in full agreement with the previous EDX analysis where the O/Bi atomic ratio of 0.17 ± 0.05 suggested the presence of oxygen at metallic bismuth by a natural surface oxidation. Besides, a deeper look at the PXRD patterns of the samples obtained for R = 12 and R = 18 suggest that those materials are amorphous, which may be likely Bi(OH)3 or BiOOH and support again the EDX results. However, when R was further increased up to 36, diffraction peaks were observed in addition to those of metallic bismuth. Specifically, the diffraction peaks observed at 24.9, 27.8, 30.5, 33.1, 41.8, 49.2, 52.6, and 54.2° were indexed to the tetragonal structure of Bi2O3 with (210), ( 201), ( 211), ( 220), ( 212), ( 410), ( 411), (203) planes, respectively. 8,[27][28][29] This set of results by PXRD fully supports the successful design of a synthesis methodology to engineer different types of micro-and nano-structured bismuth-based materials with different shape, composition and crystallinity. Those characteristics will undoubtedly be a powerful tool to control the electrocatalytic selectivity towards the oxygen reduction to hydrogen peroxide. Considering that the targeted application for these materials in the present work concerns surface reactions, it is also important to examine their surface state in a sufficient detail. This will be the scope of the forthcoming section.

XPS probing

The through the electrons of the Bi-Bi bond has been assigned to the binding energy of the Bi 4f7/2 between 157.2 and 158.1 eV. [62][63][64][65][66] Herein, we observed a peak with a much lower binding energy of Bi 4f7/2 = 156.5 eV. Similar observation was made in literature and assigned to the metallic bismuth involved in organic molecules with Bi-O-bonds, 63 here resulting from the remaining trace of organics that were used as complexing agent or stabilizer. The presence of Bi(OH)3 with Bi-OH bonds either coming from the spontaneous surface oxidation or the direct synthesis was evidenced by the component with a peak at Bi 4f7/2 = 158.4 ± 0.4 eV. 62,64 The peak with the binding energy at Bi 4f7/2 = 158.8 ± 0.3 eV corresponds to the O=Bi-OH bonds in the oxyhydroxide BiOOH. 62,64 Finally, the Bi2O3 phase with Bi-O-Bi bonds was suggested for the peak a binding energy of Bi 4f7/2 at 161.0 ± 0.4 eV even if it normally appears around 160.3 eV. [62][63][64] This proposition was mainly supported by the fact that in EDX (Fig. 2c), O/Bi is much more closed to Bi2O3 species, which also appears in XRD patterns only when R ≥ 24 (Fig. 3). Furthermore, the quantitative data shown in Fig. 4f indicate that the surface is mainly composed of Bi(OH)3 for the entire materials, which results from the natural oxidation of bismuth and hides the trends observed by both PXRD and EDX. It can, however, be observed that more Bi 0 species are present at the surface for R below 12 while the amount of Bi2O3 species augments for R larger than 12. Fig. 5a illustrates the portion of the material that is typically analyzed by XPS with a depth of 1-5 nm, which can explained why Bi(OH)3 was predominantly observed in Fig. 4. Fig. 5b gives an overall surface atomic composition of the chemical species as-quantified from XPS, which are oxygen, carbon and bismuth. The trend is not the same in comparison to the results obtained by the EDX in Fig. 2, which is explained by the analyzed depth by both techniques. The high amount of carbon species is explained by the remaining trace of organics that could be easily eliminated by the electrochemical treatment with cyclic voltammetry, which is efficient in removing organic molecules without perturbing the shape or morphology of the particles. [67][68][69] The next section will aim to provide the electrochemical characteristics of those well-characterized materials by SEM, EDX, TGA-DSC, XRD, and XPS.

Electrochemical analysis

In a further attempt to simultaneously determine the electrochemical characteristics of the asfabricated materials while evaluating their catalytic performance towards the ORR, we next utilized the CV in 0.1 M KOH. To bring out a comprehensive understanding, we considered the four classes of materials obtained at R = 6, 12, 24 and 30 with a comparison to the control sample obtained for R = 0. The blank CVs at 100 mV s -1 were recorded in the outgassed electrolyte while those for ORR were obtained after saturation with O2. Figs 6a-e and Figs. S11-S14 show the obtained CVs. It should be pointed out that stable CVs were obtained after 30 cycles, which is common in electrochemistry to stabilize the surface. This also aims to potentially remove any remaining organic compounds as it was mentioned in the previous paragraph. Typical profiles of the successive CVs recorded at 100 mV s -1 in Ar-saturated 0.1 M KOH were reported Fig. S11.

The blank CV profile is different from one electrode to another one, which supports the idea of the preparation of different types of materials. During the positive scan, the anodic peaks were attributed to the oxidation of bismuth (0) to bismuth (III) to form BiOOH, Bi(OH)3, and Bi2O3 species. [27][28][29][30][70][71][72] It was reported that the pre-peak corresponds to a small fraction of Bi metal sites located near the bismuth-metal/micropore-solution interface while the main and the plateau are related to the oxidation of bismuth metal to bismuth (III). 70 During the reverse scan, the main cathodic peak corresponds to the reduction of the previously formed bismuth (III) to bismuth metal, which is considered to be composed of several steps and where the intermediate species BiO2 2-was used to explain to the presence of at least one cathodic peak. 70,73 The effects of the upper potential (Fig. 6f and Fig. S13) and the scan rate (Fig. S14) confirm the excellent reversibility of the oxidation and reduction processes observed in Figs. 6a-e. The present bismuthbased materials have a lower peak-to-peak gap in comparison to other reported bismuth materials. [27][28][29][30]71,[74][75][76][77] This high capacity of reversibility could therefore give rise to further application in electrochemical super-capacitor for the energy production and storage with an outstanding performance. [27][28][29][30]71,[74][75][76][77] The evaluation of the charging-discharging is beyond the scope of the present work.

In the presence of O2, the first cycle shows the appearance of a reduction wave starting at about 0.9 V vs RHE. As it is reported in Fig. S12, the reduction peak returns to its normal profile and position after the 2 nd cycle. The presence of the pre-peak for the electrode fabricated from the material R = 12 suggests that the electrocatalytic oxygen reduction reaction had induced a restructuring of the surface of the material or provided a better surface cleaning. Indeed, carbon monoxide oxidation at metallic surfaces had been employed as an electrochemical cleaning method when the synthesis method involved the use of organic molecules. 78 Overall, those preliminary tests by CV performed in the presence of O2 already suggest a good activity towards the electrocatalytic oxygen reduction reaction. It now remains to evaluate the depth of the reduction of this oxygen to know if it is a complete phenomenon with four transferred electrons (O2 + 2H2O + 4e -→ 4HO -) or partially two electrons to produce hydrogen peroxide (O2 + H2O + 2e -→ HO2 -+ HO -) as targeted herein.

Sketch of the developed synthesis process for various types of bismuth-based materials

The entire experimental results obtained from the physicochemical and electrochemical techniques were combined to make a scheme illustrating the different scenarios. Scheme 2 shows the proposed different steps. The step (a) is the dissolution of the Bi(NO3)3•5H2O precursor in the nitric acid solution in order to avoid precipitation in the hydroxide form. Seminal work showed that when sodium tartrate is added to an acetic acid solution containing bismuth nitrate, a white precipitate is formed, which contains 58.3% of bismuth, closed to the [Bi2(C4H3O6)2] compound (indicated in the step (b)). 7 This situation is therefore slightly similar to the material obtained in the case of R = 12 in which some traces of the complex may exist. It should be underscored that we unambiguously demonstrated by different set of techniques that it is not exactly the situation and the previous authors did not performed any reduction as we run herein. Furthermore, the extra amount of sodium hydroxide that was needed to dissolve this white suspension and form Bi2(C4H2O6)2] 2-is explained by the initial acidity of the solution and the requirement to deprotonate the native tartaric acid (pKa = 3.0 and 4.4) before complexation. Depending on the amount of the sodium hydroxide, either the [Bi2(C4H3O6)2] molecular complex or the Bi2(C4H2O6)2] 2-carboxylate is further reduced by the sodium hypophosphite to synthesize bismuth particles and release the tartrate as by-product. The step (c) accounts another situation, whereby the bimolecular complex undergoes a partial or full hydrolysis in the presence of a high amount of sodium hydroxide to generate different types of bismuthyl bismutho sodium tartrate. 7 Given the initial acidity of the medium and the findings from different characterizations, we conclusively hypothesized that: (i) the materials obtained for R = 0-6 correspond to the bismuth precursor described by Scheme 2a, (ii) the material obtained for R = 12 is better described by the bismuth precursors presented in Scheme 2b, and (iii) the materials obtained for R = 18-36 likely originate from the bismuth precursors drawn in Scheme 2c.

Catalytic performance towards electrochemical hydrogen peroxide production

We finally combined different electroanalytical methods to evaluate the electrocatalytic performance of the as-synthesized materials. The first setup was the rotating ring-disk electrode (RRDE), described by Eqs. 4, and 5. Fig. 7 shows the obtained results in 0.1 M KOH electrolyte for the different and representative materials obtained at R = 0, 6, 12, 24, and 36. The blank experiments were performed when no catalytic ink was deposited and referred to as GC (glassy carbon). We initially recorded the open circuit potential (OCP), which is a thermodynamic indicator of the reaction feasibility. OCP stabilizes to 0.93, 0.93, 0.85, 0.80, and 0.85 V vs RHE for R = 0, 6, 12, 24, and 36, respectively (0.85 V vs RHE for GC). Those values as slightly larger than the expected potential of the process O2 + H2O + 2e -→ HO2 -+ HO -, E° = 0.76 V vs RHE.

This could thus suggest that the starting process is O2 + 2H2O + 4e -→ 4HO -for which E° = 1.23

V vs RHE. It should however keep in mind that OCP is not a fair descriptor since unlike H2 oxidation, the oxygen reduction has low kinetic so that the onset potential does not always match with the OCP. So, we further recorded the LSV curves to simultaneously visualize the disk current density coming from the oxygen-to-hydrogen peroxide reduction process and the ring current density resulting from the oxidation the produced hydrogen peroxide. Fig. 7b displays the obtained LSV where the onset potential was found to be 0.75 ± 0.05 V vs RHE, thus in line with a twoelectron transferred process (Eq. 1). The quantitative results in Fig. 7c show that, at 0.6 V vs RHE, the couple of data associated to the hydrogen peroxide selectivity and real overall transferred number of electrons (%HO2 -, nex) for the different investigated materials is GC (88%, 2.2e-), R0(84%, 2.3e-), R6(63%, 2.7e-), R12(83%, 2.3e-), R24(53%, 2.9e-), R30(88%, 2.2e-). At 0.5 V vs RHE, the performance is GC(88%, 2.2e-), R0(77%, 2.5e-), R6(63%, 2.7e-), R12(88%, 2.2e-), R24(59%, 2.8e-), R30(92%, 2.1e-). It can clearly be observed that the material R = 0 is gradually losing its efficiency while R = 12 and R = 30 maintain a good performance in hydrogen production.

Even if GC showed a competitive selectivity, it should be pointed out that it has no real interest given its very high cost and the low current density obtained in the potential range of 0. The values of the Tafel slope, which is a mechanism and kinetic descriptor, support the previous choice. Before comparing our results to the existing literature with other types of electrocatalysts (no bismuth-based materials have been reported to date), [33][34][35]39,[79][80][81][82] we also utilized electrochemical impedance spectroscopy (EIS) for modeling the electrocatalytic interface as well as confirming the trends observed for the Tafel slope. The results in terms of the complex-plane Nyquist impedance at 0.67 V vs RHE are shown in Fig. 7e with inset the corresponding electrochemical equivalent circuit of R+QCPE//Rct, where R is the ohmic resistance, QCPE is the capacitance of the constant phase element and Rct is the charge transfer resistance (proportional to the rate constant). [START_REF] Lasia | Electrochemical Impedance Spectroscopy and its Applications[END_REF][START_REF] Orazem | Electrochemical Impedance Spectroscopy[END_REF] The extracted data are reported in Table S1 and the charge transfer resistance was used to draw the Tafel plots shown in Fig. 7f. The lowest charge transfer resistance was observed for R = 12 and R = 24, which was expected given the LSV profiles in Fig. 7b. Taking into account the selectivity for the targeted application, we concluded that R = 12 and R = 30 should be prioritized for further deep investigation. All the above results highlight that the best selectivity towards the electrochemical hydrogen peroxide production is when the starting materials contain oxygen species. The heterogeneous surface composed of bismuth and oxygen atoms consequently favors the perpendicular adsorption of the molecular oxygen. 43,44,[START_REF] Staszak-Jirkovský | [END_REF] To achieve this perpendicular adsorption of O2 by a single atom on the catalytic surface, the driving force is the spacing between adjacent active sites and the heterogeneity of the catalytic surface that modulates the electronic interactions. This explains the low performance of the metallic material ,d, f with the rotating speed is normal and is simply because the collection efficiency itself undergoes fluctuations (see Fig. S2a). The quantitative data gathered in Table 1 demonstrate that a better efficiency is obtained with the material R = 30 (95.5%, 2.1e-, and 30 A g -1 at 0.6 V vs RHE for 1600 rpm) in comparison to the material R12 (90.2%, 2.2e-, and 29 A g -1 at 0.6 V vs RHE for 1600 rpm). Since long-term stability matters for a practical application, we performed an accelerated stress test (AST) for the catalyst that gives the best performance (R = 30). This was carried out in O2-staurated 1 M KOH and was composed of 5000 cycles of CV in the potential window where the material is electrocatalytically active, i.e., 0.3-0.9 V vs RHE. The results in Fig. 8e,f, Table 1 and Fig. S15 clearly demonstrate that the catalytic material exhibits an outstanding stability and performance with a slight improvement of the mass-corrected kinetic current density after this accelerated ageing program (95%, 2.1e-, and 68 A g -1 at 0.6 V vs RHE for 1600 rpm). For the comparison with the recent and existing data, the obtained high kinetic current density, at 0.55 V vs RHE, of 977, 929, and 965 A g -1 for R12, R30 and R30-AST, respectively, outperform the reported values of 73-600 A g -1 . 34,79,80,82 voltammograms recorded in argon-saturated electrolyte and the coulometry experiment in the presence of oxygen. Fig. 9d demonstrates that the activity was maintained since the electrical charge is increasing. Furthermore, it can be also observed that the catalytic particles did not cover the entire surface of the microfibers within the 3D network of the carbon paper. Those observations rationally explain the slight decrease in faradaic efficiency that was observed during the bulk electrolysis experiment when compared to the value obtained by the RRDE method. This was expected since the carbon paper has a lower selectivity (Fig. 9d). Indeed, as it is impossible to cover all the microfibers of the carbon paper with the catalytic ink because of the 3D structure, a part of the input electricity will be used to drive O2 reduction at the carbon paper. In the case of the RRDE, a thin film has been obtained so that the observed activity can be entirely attributed to the bismuth material.

Conclusion

We presented in this contribution a work dealing with the preparation and characterization of advanced bismuth-based materials designed by the coordination chemistry and for the specific application of the direct electrosynthesis of hydrogen peroxide from oxygen. The successful preparation of different types of those catalytic materials was based on a strict control of a single experimental parameter, namely the molar ratio R = n(NaOH)/n(Bi(NO3)3•5H2O), which was varied from 0 to 36. The alkalinity and the tartaric acid as an eco-friendly organic molecule were combined together to regulate the formation of bismuth-based organic complexes as the initial building blocks. The latter further grow to yield bismuth particles with tunable properties and catalytic efficiency. From a set of physiochemical (SEM, EDX, XRD, and XPS) and electrochemical (CV) methods, the findings indicate that three classes of materials can be easily and conveniently produced depending on the value of the regulator R. Specifically, metallic bismuth (Bi), bismuth (oxy)hydroxide (BiOOH/Bi(OH)3), and bismuth oxide (Bi2O3) were obtained for R = 0-8, R = 12, and R = 24-36, respectively. These materials were applied to the electrosynthesis of hydrogen peroxide, which was qualitatively and quantitatively studied by the rotating ring-disk electrode (RRDE), electrochemical impedance spectroscopy (EIS), bulk electrolysis and UV-Vis. It was observed that the co-existence of both bismuth and oxygen atoms in the catalytic material is critical for targeting high selectivity. Specifically, at 0.5 V vs RHE in

cm 2

 2 glassy carbon plate acted as the counter electrode and a mercury-mercury oxide electrode (Hg|HgO|KOH 0.1 or 1 M, MOE, RE-61AP Reference Electrode for alkaline solution, BAS Inc.) was used as the reference electrode. The potentials were then converted versus the reversible hydrogen electrode (RHE) according to the calibration relationship E(V vs RHE) = E(V vs MOE) + ΔE. ΔE = 0.922 V (0.1 M KOH) and 0.946 V (1 M KOH) at 25 °C in 0.1 and 1 M according to the calibrating curves shown in Fig. S1. Given the used electrolyte, a corrosion-resistant material was used for the solution outgassing and oxygen saturation, for instance the Tygon ® XL-60 Clear, Peristaltic Pump Tubing (Veber Caoutchouc, France). The accelerated stress test (AST) was

2. 55

 55 ± 0.25, 1.21 ± 0.3, 1.18 ± 0.19 for R = 0, 6, 12, 24 and 30, respectively. This would mean that three categories of materials were prepared. First, metallic state (Bi 0 , O/Bi = 0) was obtained for R = 0-6 where the observed oxygen results from the natural oxidation of bismuth when exposed to ambient air. Second, a hydroxide or oxyhydroxide state (Bi(OH)3 or BiOOH, O/Bi = 2-3) was obtained for R = 12. Third, an oxide state (Bi2O3, O/Bi = 1.5) was obtained for R = 30 with substantial presence of metallic bismuth. Metallic and oxide materials are basically crystalline while the (oxy)hydroxide can exhibit an amorphous state. At this stage, it can thus be concluded that EDX has confirmed the observations made with the SEM. It consequently remains to confirm the crystallinity and phase identification by a more specialized technique, which is the purpose of the next section.

  preceding outcomes by SEM, EDX, and PXRD demonstrated that the regulation of the synthesis by the molar ratio R = n(NaOH)/n(Bi(NO3)3•5H2O) has enabled the development of three classes of bismuth-based materials. Given the sensitivity of bismuth to natural oxidation and the targeted application in heterogeneous catalysis, we further used XPS to probe the surface state and composition more closely. Figs. 4a-e show the high-resolution spectra of the Bi 4f region for the different type of materials. The presence of metallic bismuth at an oxidation state zero (Bi 0 )

  4-0.6 V vs RHE, which are expected for the implementation in practical electrolyzers. By taking the disk current density at ca. 0.45 V vs RHE as the limiting current density, the mass-corrected kinetic current density normalized to the amount of the catalyst (jk) was determined to compare the different catalysts. The results are plotted as Tafel plots in Fig.7d. At 0.55 V vs RHE, jk = 3.4, 8.8, 44.9, 3.7, and 9.7 A g -1 for R = 0, 6, 12, 24, and 30, respectively. Those results of selectivity and kinetic current density show that the materials obtained for R = 12 and R = 30 are the best performing catalysts.

  in comparison to the oxides and the oxy-hydroxides materials. The results obtained in 1 M KOH electrolyte are shown in Fig. 8 and Fig. S15 at the different speeds of the RRDE. The slight fluctuation in Figs. 8b

± 3 Scheme 1 .

 31 Scheme 1. Illustration of the used strategy to produce a library of bismuth-based materials by regulating the coordination between tartaric acid and bismuth (III).

Fig. 1 .

 1 Fig. 1. SEM characterization of the as-synthesized bismuth-based materials at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). Left to right, materials for R = 0, 6, 12, 24 and 30. Top to down: from an overview to a closer-view.

Fig. 2 .

 2 Fig. 2. EDX characterization of the as-synthesized bismuth-based materials at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a) Overall mass composition. (b) Overall atomic composition. (c) Atomic ratio of oxygen/bismuth.

Fig. 3 .

 3 Fig. 3. Powder XRD patterns of the as-synthesized bismuth-based materials at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a, b) Overall comparison of R = 0-36 (a) and R = 6-36 (b). (c, d) Indexed patterns for R = 8-36 (c) and R = 0-6 (d).

Fig. 4 .

 4 Fig. 4. XPS analysis the as-synthesized bismuth-based materials at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a-e) High-resolution of the Bi 4f region of: (a) R = 0, (b) R = 6, (c) R = 12, (d) R = 24, and (e) R = 30. (f) Surface atomic composition of bismuth-based species asdetermined from XPS.

Fig. 5 .

 5 Fig. 5. XPS analysis the as-synthesized bismuth-based materials at different molar ratio: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a) Picture showing the portion of the analyzed part by XPS. (b) Overall surface atomic composition of the chemical species as-determined from XPS.

Fig. 6 .

 6 Fig. 6. Characterization by CV of the materials obtained at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a-e) CVs at 100 mV s -1 in 0.1 M KOH in the presence of Ar (bleu curves, 30 th cycle) and in O2 (red curves: 1 st cycle): (a) R = 0, (b) R = 6, (c) R = 12, (d) R = 24, and (e) R = 30. (f) CVs at 100 mV s -1 in Ar-saturated 0.1 M KOH for different upper potential limits for the material obtained for R = 0.

Fig. 7 .

 7 Fig. 7. ORR in 0.1 M KOH for the materials obtained at different molar ratios: R = n(NaOH)/n(Bi(NO3)3•5H2O). (a) OCP vs time. (b) LSV curves recorded from a RRDE setup at 5 mV s -1 for the ring (top, normalized by the geometry surface area of the ring (0.11 cm 2 )) and disk (bottom, normalized by the geometric surface are of the disk (0.196 cm 2 )): the rink was set at 1.2 V vs RHE. (c) Quantitative results in terms of HO2 -(left y-axis) and transferred number of electrons (right y-axis). (d) Tafel plots by the kinetic current density normalized by the catalyst loading. (e) Complex-plane Nyquist impedance at Eappl = 0.672 V vs RHE (inset shows the corresponding electrochemical equivalent circuit of R+QCPE//Rct). (f) Tafel plots by Rct.

Fig. 8 .

 8 Fig. 8. RRDE measurements during ORR in 1 M KOH for the materials obtained at R = 12 (a, b), R = 30 (b-f) before AST (a-d) and after AST (e, f). (a, c, e) LSV curves recorded at 5 mV s -1 for ring (top, normalized by the geometry surface area of the ring (0.11 cm 2 )) and disk (bottom, normalized by the geometric surface are of the disk (0.196 cm 2 )): the rink was set at 1.2 V vs RHE. (b, d, f) Quantitative results in terms of HO2 -(left y-axis) and transferred number of electrons (right y-axis).

Fig. 9 .

 9 Fig. 9. Bulk electrolysis. (a) Control CVs at 100 mV s -1 in the H-type cell in 1 M KOH for the bare CP electrode and the modified CP electrode by catalytic ink of the material obtained for R = 30: in the presence of Ar (30 th cycle) and in O2-saturated electrolyte (1 st cycle starting at OCP = 0.98 V vs RHE and 2 nd cycle). (b) Electrical charge passed during the potentiostatic electrolysis at an applied potential of 0.6 V vs RHE (iR-uncorrected). (c) UV-Vis spectra of a sulfuric acid + potassium titanium oxalate solution in the absence and presence of the hydrogen peroxide from the electrolysis (after stabilization to pH = 3.5). (d) Quantified hydrogen peroxide by the UV-Vis assays in terms of productivity (left y-axis) and faradaic efficiency (right y-axis). (e-g) Postmortem SEM and HRSEM images of the CP-R30 electrode after bulk electrolysis: The scale bar = 15 µm for panels (e, f) and 0.15 µm for panel (g). Error bars represent 1 SD (n ≥ 3).

Table 1 .

 1 Results at 0.6 V vs RHE from RRDE measurement and bulk electrolysis in 1 M KOH.

	Entry	RRDE(1600 rpm) HO2 -1 (%) nex jk(A g -1 )	Electrolysis and UV-Vis Assays HO2 -1 (mol kg -1 cm -2 ) FE(%)
	R30	95.5	2.1	30	69 ± 3	92 ± 4
	R12	90.2	2.2	29	44 ± 3	62 ± 5
	CP	-	-	-	38 ± 4	37

Acknowledgment

This work was funded by the European Institute of Membranes of Montpellier through the Exploratory Project COGENFC (PAT-Axis-Energy-2018). We thank Didier Cot, Bertrand Rebiere, and Arie van der Lee of IEM for assistance during SEM, EDX and XRD measurements.

To confirm the efficiency observed with the RRDE method, hydrogen peroxide was spectrophotometrically tracked following bulk electrolysis, which was done with the catalyst deriving from R = 30. To this end, 2 hours electrolysis at an applied potential of 0.6 V vs RHE (iRuncorrected) was carried out in an H-type cell, wherein the two compartments were separated with an anion-exchange membrane. The entire results are shown in Fig. 9. Before the bulk electrolysis, control CVs were taken at 100 mV s -1 in the H-type cell for the bare carbon paper (CP) electrode and the ink-modified CP electrode (CP-R30). Fig. 9a shows the CV profiles in the absence and presence of oxygen, which are similar to the findings of Fig. 6 (note that the three dimensional (3D) structure of the carbon paper can slightly alters the voltammogram). This result justifies the choice of 0.6 V vs RHE for the electrolysis. Furthermore, the bare carbon paper has activity towards the electrocatalytic oxygen reduction at the applied potential of 0.6 V vs RHE. This explains the high value of the recorded electrical charge that was passed during the potentiostatic electrolysis. Fig. 9c depicts the UV-Vis spectra of a sulfuric acid + potassium titanium oxalate solution in the absence and presence of the hydrogen peroxide resulting from the electrolysis (after stabilization to pH = 3.5). This control experiment confirms that the wavelength of λ = 390 nm resulting from the pertitanic acid complex between and potassium titanium oxalate can be utilized to selectively quantify any produced hydrogen peroxide as previously reported. 40,[56][57][58] Fig. 9d and Table 1s report the quantified hydrogen peroxide by the UV-Vis assays by two metrics. Those findings in terms of the produced hydrogen peroxide and faradaic efficiency (FE, %) after 2 hours of electrolysis are: (i) 69 ± 3 mol kg -1 cm -2 and 92 ± 4 % for CP-R30, (ii) 44 ± 3 mol kg -1 cm -2 and 62 ± 5% for CP-R12, and (iii) 38 ± 4 mol kg -1 cm -2 and 37 ± 3% for CP. Those quantitative analytical results support the high performance for the catalyst fabricated from the materials obtained by R = 30. The production rate are comparable to up-to-date data gathered in ref. 34 In additional to that, it is important to mention that the faradaic yield of 92 ± 4% is higher than the reported efficiency of 70-87% 39,79 and approaches the benchmarked record values of 94-95%. 33,35 We finally performed post-mortem SEM and HRSEM of the CP-R30 electrode after the bulk electrolysis to reveal the localization and morphology of the particles within the 3D network of the carbon paper support. Figs. 9e,f present the recorded pictures, taken at different places. It can be seen that the particles are well attached to the surface of the microfibers as it is indicated in Fig. 9f. However, a slight modification of the morphology is observed in the high-resolution image, which could be attributed to the electrochemical treatments consisting of preliminary thirty cyclic Supplementary Information † Electronic supplementary information (ESI) available.

Conflicts of interest

The authors declare no competing financial interest.


Member, IEEE Chongwen Huang

Member, IEEE Sha Hu

Senior Member, IEEE George C Alexandropoulos

Senior Member, IEEE Chau Yuen

Fellow, IEEE Rui Zhang

Fellow, IEEE Marco Di Renzo

Fellow, IEEE Mérouane Debbah

Holographic MIMO Surfaces for 6G Wireless Networks: Opportunities, Challenges, and Trends

Future wireless networks are expected to evolve towards an intelligent and software reconfigurable paradigm enabling ubiquitous communications between humans and mobile devices. They will be also capable of sensing, controlling, and optimizing the wireless environment to fulfill the visions of lowpower, high-throughput, massively-connected, and low-latency communications. A key conceptual enabler that is recently gaining increasing popularity is the Holographic Multiple Input Multiple Output Surface (HMIMOS) that refers to a low-cost transformative wireless planar structure comprising of subwavelength metallic or dielectric scattering particles, which is capable of impacting electromagnetic waves according to desired objectives. In this article, we provide an overview of HMIMOS communications by introducing the available hardware architectures for reconfigurable such metasurfaces and their main characteristics, as well as highlighting the opportunities and key challenges in designing HMIMOS-enabled communications.

I. INTRODUCTION

Future wireless networks, namely beyond fifth Generation (5G) and sixth Generation (6G), are required to support massive numbers of end-users with increasingly demanding Spectral Efficiency (SE) and Energy Efficiency (EE) requirements [START_REF] Akyildiz | Combating the distance problem in the millimeter wave and terahertz frequency bands[END_REF]- [START_REF] Hu | Beyond massive MIMO: The potential of positioning with large intelligent surfaces[END_REF]. In recent years, research in wireless communications has witnessed rising interests in massive Multiple Input Multiple Output (MIMO) systems, where Base Stations (BSs) are equipped with large antenna arrays, as a way to address the 5G throughput requirements. However, it is still a very challenging task to realize massive MIMO BSs with truly large-scale antenna arrays (i.e., with few hundreds or more antennas) mainly due to the high fabrication and operational costs, as well as due to the increased power consumption.

Future 6G wireless communication systems are expected to realize an intelligent and software reconfigurable paradigm, where all parts of device hardware will adapt to the changes of the wireless environment [START_REF] Akyildiz | Combating the distance problem in the millimeter wave and terahertz frequency bands[END_REF], [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF], [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF]. Beamforming-enabled antenna arrays, cognitive spectrum usage, as well as adaptive modulation and coding are a few of the transceiver aspects that are currently tunable in order to optimize the communication efficiency. However, in this optimization process, the wireless environment remains an unmanageable factor; it remains unaware of the communication process undergoing within it [START_REF] Akyildiz | Combating the distance problem in the millimeter wave and terahertz frequency bands[END_REF], [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF]- [START_REF] Tang | Wireless communications with reconfigurable intelligent surface: Path loss modeling and experimental measurement[END_REF]. Furthermore, the wireless environment has in general a harmful effect on the efficiency of wireless links. The signal attenuation limits the connectivity radius of nodes, while multipath propagation resulting in fading phenomena is a wellstudied physical factor introducing drastic fluctuations in the received signal power. The signal deterioration is perhaps one of the major concerns in millimeter wave and in the forthcoming TeraHertz (THz) communications [START_REF] Akyildiz | Combating the distance problem in the millimeter wave and terahertz frequency bands[END_REF].

Although massive MIMO, three-Dimensional (3D) beamforming, and their hardware efficient hybrid analog and digital counterparts [START_REF] Abu-Surra | Spatially sparse precoding in millimeter wave MIMO systems[END_REF] provide remarkable approaches to conquer signal attenuation due to wireless propagation via softwarebased control of the directivity of transmissions, they impose mobility and hardware scalability issues. More importantly, the intelligent manipulation of the ElectroMagnetic (EM) propagation is only partially feasible since the objects in the deployment area, other than the transceivers, are passive and uncontrollable. As a result, the wireless environment as a whole remains unaware of the ongoing communications within it, and the channel model continues to be treated as a probabilistic process, rather than a nearly deterministic one enabled through software-controlled techniques.

Following the recent breakthrough on the fabrication of programmable metamaterials, reconfigurable intelligent surfaces have the potential to fulfill the challenging vision for 6G networks, and materialize seamless connections and intelligent software-based control of the environment in wireless communication systems when coated on the otherwise passive surfaces of various objects [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF]- [START_REF] Wu | Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[END_REF]. Holographic MIMO Surfaces (HMIMOS) aim at going beyond massive MIMO, being based on low cost, size, weight, and low power consumption hardware architectures that provide a transformative means of the wireless environment into a programmable smart entity [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF], [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF], [START_REF] Wu | Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[END_REF], [START_REF] Tang | Wireless communications with reconfigurable intelligent surface: Path loss modeling and experimental measurement[END_REF], [START_REF] Han | Large intelligent surfaceassisted wireless communication exploiting statistical CSI[END_REF], [START_REF] Taha | Enabling large intelligent surfaces with compressive sensing and deep learning[END_REF]. In this article, we overview the different emerging HMIMOS architectures and their core functionalities, and discuss their currently considered communication applications as well as their future networking challenges.

II. HMIMOS DESIGN MODELS

In this section, we present available hardware architectures, fabrication methodologies, and operation modes of HMIMOS systems that render them a flexibly integrable concept for diverse wireless communication applications.

A. Categorization based on the Power Consumption 1) Active HMIMOS: To realize reconfigurable wireless environments, HMIMOS can serve as a transmitter, receiver, or reflector. When the transceiver role is considered, and thus energy-intensive Radio Frequency (RF) circuits and signal processing units are embedded in the surface, the term active HMIMOS is adopted [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF], [START_REF] Pizzo | Spatial characterization of holographic MIMO channels[END_REF]. On another note, active HMIMOS systems comprise a natural evolution of conventional massive MIMO systems, by packing more and more software-controlled antenna elements onto a two-Dimensional (2D) surface of finite size. In [START_REF] Hu | Beyond massive MIMO: The potential of positioning with large intelligent surfaces[END_REF], where the spacing between adjacent surface elements reduces when their number increase, Figure 1: The two generic steps of holographic training and holographic communication [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF].

an active HMIMOS is also termed as Large Intelligent Surface (LIS). A practical implementation of active HMIMOS can be a compact integration of an infinite number of tiny antenna elements with reconfigurable processing networks realizing a continuous antenna aperture. This structure can be used to transmit and receive communication signals across the entire surface by leveraging the hologram principle [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF], [START_REF] Pizzo | Spatial characterization of holographic MIMO channels[END_REF]. Another active HMIMOS implementation is based on discrete photonic antenna arrays that integrate active optical-electrical detectors, converters, and modulators for performing transmission, reception, and conversion of optical or RF signals [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF].

2) Passive HMIMOS: Passive HMIMOS, also known as Reconfigurable Intelligent Surface (RIS) [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF], [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF]- [START_REF] Kaina | Shaping complex microwave fields in reverberating media with binary tunable metasurfaces[END_REF], [START_REF] Tang | Wireless communications with reconfigurable intelligent surface: Path loss modeling and experimental measurement[END_REF], or Intelligent Reflecting Surface (IRS) [START_REF] Wu | Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[END_REF], [START_REF] Wu | Towards smart and reconfigurable environment: Intelligent reflecting surface aided wireless network[END_REF], acts like a passive metal mirror or 'wave collector,' and can be programmed to change an impinging EM field in a customizable way [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF], [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF]. Compared with its active counterpart, a passive HMIMOS is usually composed of low cost passive elements that do not require dedicated power sources. Their circuitry and embedded sensors can be powered with energy harvesting modules, an approach that has the potential of making them truly energy neutral. Regardless of their specific implementations, what makes the passive HMIMOS technology attractive from an energy consumption standpoint, is their capability to shape radio waves impinging upon them and forwarding the incoming signal without employing any power amplifier nor RF chain, and also without applying sophisticated signal processing. Moreover, passive HMIMOS can work in full duplex mode without significant self interference or increased noise level, and require only low rate control link or backhaul connections. Finally, passive HMIMOS structures can be easily integrated into the wireless communication environment, since their extremely low power consumption and hardware costs allow them to be deployed into building facades, room and factory ceilings, laptop cases, or even human clothing [START_REF] Di Renzo | Smart radio environments empowered by reconfigurable AI meta-surfaces: An idea whose time has come[END_REF], [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF].

B. Categorization based on the Hardware Structure 1) Contiguous HMIMOS: A contiguous HMIMOS integrates a virtually uncountably infinite number of elements into a limited surface area in order to form a spatially continuous transceiver aperture [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF], [START_REF] Pizzo | Spatial characterization of holographic MIMO channels[END_REF]. For the better understanding of the operation of contiguous surfaces and their communication models, we commence with a brief description of the physical operation of the optical holography concept. Holography is a technique that enables an EM field, which is generally the result of a signal source scattered off objects, to be recorded based on the interference principle of the EM wave. The recorded EM field can be then utilized for reconstructing the initial field based on the diffraction principle. It should be noted that wireless communications over a continuous aperture is inspired by the optical holography, which is sketched in Fig. 1. In the training phase, the generated training signals from an RF source are split via a beamsplitter into two waves, the object and reference waves. The object wave is directed to the object, and some of the reflected wave mixed together with the reference wave beam that does not impinge on the object, are fed to the HMIMOS. In the communication phase, the transmitted signal is transformed into the desired beam to the object user over the spatially continuous aperture of the HMIMOS. Since the continuous aperture benefits from the integrated infinite number of antennas that is the asymptotic limit of Massive MIMO, its potential advantages are to achieve higher spatial resolution, and enable the creation and detection of EM waves with arbitrary spatial frequency components, without undesired side lobes.

2) Discrete HMIMOS: The discrete HMIMOS is usually composed of many discrete unit cells made of low power software-tunable metamaterials. The means to electronically modify the EM properties of the unit cells range from off the shelves electronic components to using liquid crystals, microelectromechanical systems or even electromechanical switches, and other reconfigurable metamaterials. This structure is substantially different from the conventional MIMO antenna array. One embodiment of a discrete surface is based on discrete 'meta-atoms' with electronically steerable reflection properties [START_REF] Liaskos | A new wireless communication paradigm through softwarecontrolled metasurfaces[END_REF]. As mentioned earlier, another type of discrete surface is the active one based on photonic antenna arrays. Compared with contiguous HMIMOS, discrete HMIMOS have

I I I I I C C C C C C C C C C S A S A S S A S A S S A S A S Figure 2:
The two operation modes of HMIMOS systems along with their implementation and hardware structures. A schematic view of the HMIMOS functions of EM field polarization, scattering, focusing, and absorption control is provided. some essential differences from the perspectives of implementation and hardware, as will be described in the sequel.

C. Fabrication Methodologies

There are various fabrication techniques for HMIMOS including electron beam lithography at optical frequencies, focused-ion beam milling, interference and nanoimprint lithography, as well as direct laser writing or printed circuit board processes at microwaves. Usually, these fabrication techniques will be ascribed to produce two typical apertures, continuous or discrete apertures, as shown in Fig. 2. A fabrication approach leveraging programmable metamaterials for approximately realizing a continuous microwave aperture [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF], [START_REF] Pizzo | Spatial characterization of holographic MIMO channels[END_REF] is depicted in Fig. 2(a). This meta-particle structure uses the varactor loading technique to broaden its frequency response range, and achieves continuous aperture and controllable reflection phase. It is a continuous monolayer metallic structure, and comprises an infinite number of meta-particles. Each metaparticle contains two metallic trapezoid patches, a central continuous strip, and varactor diodes. By independently and continuously controlling the bias voltage of the varactors, the surface impedance of continuous HMIMOS can be dynamically programmed, and thus manipulate the reflection phase, amplitude states, and the phase distribution at a wide range of frequency bands [START_REF] Akyildiz | Combating the distance problem in the millimeter wave and terahertz frequency bands[END_REF]. It should be highlighted that this impedance pattern is a map of the hologram, and can be calculated directly from the field distribution of the provided reference wave and reflected object wave, as discussed in Fig. 1. Exploiting intelligent control algorithms, beamforming can be accomplished by using the hologram principle.

In contrast to continuous aperture, another instance of HMIMOS is the discrete aperture that is usually realized with software-defined metasurface antennas. A general logical structure (regardless of its physical characteristics) was proposed in [START_REF] Liaskos | A new wireless communication paradigm through softwarecontrolled metasurfaces[END_REF], as shown in Fig. 2(b). Its general unit cell structure contains a metamaterial layer, sensing and actuation layers, shielding layer, computing layer, as well as an interface and communications layer with different objectives. Specifically, the meta-material layer is implemented by graphene materials for delivering the desired EM behavior through a reconfigurable pattern, while the objective of sensing and actuation layer is to modify the behavior of the meta-material layer. The shielding layer is made of a simple metallic layer for decoupling the EM behavior of the top and bottom layers to avoid mutual interferences. The computing layer is used to execute external commands from the interface layer or sensors. Finally, the interface and communications layer aims at coordinating the actions of the computing layer and updating other external wireless entities via the reconfigurable interface.

While HMIMOS is in its infancy, basic prototyping work on different kinds of this technology is going on in the world. A discrete HMIMOS was developed by the start-up company named "Greenerwave", and which shows the basic feasibility and effectiveness of the HMIMOS concept using the discrete metasurface antennas. In contrast, another startup company called "Pivotalcommware" with the investment of Bill Gates capital is developing the initial commercial products of the contiguous HMIMOS based on the lowcost and contiguous metasurfaces, which further verifies the feasibility of the HMIMOS concept as well as advancement of holographic technologies. Continued prototyping development is highly desired to both prove the HMIMOS concept with even brand new holographic beamforming technologies and discover potentially new issues that urgently need research.

D. Operation Modes

The following four operation modes for HMIMOS are usually considered: 1) continuous HMIMOS as an active transceiver; 2) discrete HMIMOS as a passive reflector; 3) discrete HMIMOS as an active transceiver; and 4) continuous HMIMOS as a passive reflector. Given the recent research interests and due to space limitations, we next elaborate on the first two representative modes of operation, which are also sketched within Fig. 2.

1) Continuous HMIMOS as Active Transceivers: According to this mode of operation, the continuous HMIMOS performs as an active transceiver. The RF signal is generated at its backside and propagates through a steerable distribution network to the contiguous surface constituted by the infinite software-defined and electronically steerable elements that generate multiple beams to the intended users. A distinct difference between active continuous HMIMOS and passively reconfigurable HMIMOS is that the beamforming process of the former is accomplished based on the holographic concept, which is a new dynamic beamforming technique based on software-defined antennas with low cost/weight, compact size, and a low-power hardware architecture.

2) Discrete HMIMOS as Passive Reflectors: Another operation mode of HMIMOS is the mirror or 'wave collector,' where the HMIMOS is considered to be discrete and passive. In this case, the HMIMOS include reconfigurable unit cells, as previously described, which makes their beamforming mode resembling that of conventional beamforming [START_REF] Abu-Surra | Spatially sparse precoding in millimeter wave MIMO systems[END_REF], unlike continuous transceiver HMIMOS systems. It is worth noting that most of the existing works (e.g., [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF], [START_REF] Kaina | Shaping complex microwave fields in reverberating media with binary tunable metasurfaces[END_REF], [START_REF] Wu | Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[END_REF]) focus on this HMIMOS operation mode which is simpler to implement and analyze.

III. FUNCTIONALITY, CHARACTERISTICS, AND COMMUNICATION APPLICATIONS

Different fabrication methods of HMIMOS systems result in a variety of functionalities and characteristics, with most of them being very relevant to late expectations for future 6G wireless systems (e.g., Tbps peak rates). In this section, we highlight the HMIMOS functions and key characteristics, and discuss their diverse wireless communications applications.

A. Functionality Types

Intelligent surfaces can support a wide range of EM interactions, termed hereinafter as functions. Ascribing to their programmable features and depending on whether they are realized via structures with discrete or continuous elements, HMIMOS have four common function types as illustrated in the bottom part of Fig. 2: • F1: EM Field Polarization, which refers to the reconfigurable setting of the oscillation orientation of the wave's electric and magnetic fields. • F2: EM Field Scattering, where the surface redirects an impinging wave with a given direction of arrival towards a desired or multiple concurrent desired directions. • F3: Pencile-like Focusing, which takes place when a HMIMOS acts as lens to focus an EM wave to a given point in the near or far field. The collimation (i.e., the reverse functionality) also belongs in this general mode of beamforming operation. • F4: EM Field Absorption, which implements minimal reflected and/or refracted power of the incoming EM field.

B. Characteristics

Compared with currently used technologies in wireless networks, the most distinctive characteristics of the HMIMOS concept lie in making the environment controllable by providing the possibility of fully shaping and controlling the EM response of the environmental objects that are distributed throughout the network. An HMIMOS structure is usually intended to perform as a signal source or 'wave collector' with reconfigurable characteristics, especially for application scenarios where it is used as a passive reflector with the objective of improving the EE. The fundamental properties of HMIMOS systems 1 and their core differences with massive MIMO and conventional multi-antenna relaying systems are summarized as follows:

• C1: HMIMOS can be nearly passive. One significant merit of passive HMIMOS is that they do not require any internally dedicated energy source to process the incoming information-carrying EM field.

C. Communications Applications

The unique features of HMIMOS enabling intelligent and rapidly reconfigurable wireless environments make them an emerging candidate technology for the low-power, highthroughput, and low-latency vision of 6G wireless networks. We next discuss representative communications applications of HMIMOS for outdoor and indoor environments. 1) Outdoor Applications: Consider the discrete passive HMIMOS as an indicative example that comprises a finite number of unit elements, and intended for forwarding suitably phase-shifted versions of its impinging signals to users over different outdoor scenarios, such as typical urban, shopping malls, and international airports, as illustrated in the upper part of Fig. 3. We assume that HMIMOS are planar structures of few centimeters thickness and variable sizes that can be easily deployed onto nearly all environmental objects.

• A1: Building connections. HMIMOS can extend the coverage from outdoor BSs to indoor users, especially in cases where there is no direct link between the users and BS, or the link is severely blocked by obstacles. • A2: Energy-efficient beamforming. HMIMOS are capable of recycling ambient EM waves and focusing them to their intended users via effective tuning of their unit elements. In such cases, surfaces are deployed as relays to forward the information bearing EM field to desired locations via efficient beamforming that compensates for the signal attenuation from the BS or co-channel interference from neighboring BSs. • A3: Physical-layer security. HMIMOS can be deployed for physical layer security in order to cancel out reflections of the BS signals to eavesdroppers. • A4: Wireless power transfer. HMIMOS can collect ambient EM waves and direct them to power-hungry IoT devices and sensors enabling also simultaneous wireless information and power transfer. 2) Indoor Applications: Indoor wireless communication is subject to rich multipath propagation due to the presence of multiple scatters and signal blocking by walls and furniture, as well as RF pollution due to the highly probable densification of electronic devices in confined spaces. As such, providing ubiquitous high throughput indoor coverage and localization is a challenging task. HMIMOS has the potential of being highly beneficial in indoor environments, leveraging from its inherit capability to reconfigure EM waves towards various Table I: Some recent research results on HMIMOS-based wireless communication systems.

Related Works Applications Functions Characteristics Main Contributions

[1] A1, A2, A5 F2, F3 C1, C3-C6 Presented a HMIMOS-based approach to combat the distance limitation in millimeter wave and THz systems; simulation results for an indoor set up corroborated the merits of proposed approach.

Indoor [START_REF] Hu | Beyond massive MIMO: The potential of positioning with large intelligent surfaces[END_REF] A2, A5, A6 F2, F3 C2-C6 Introduced an indoor signal propagation model and presented information theoretical results for active and continuous HMIMOS systems.

[6] A1-A3, A5 F1-F4 C1, C3-C6

Introduced the concept of programmable indoor wireless environments offering simultaneous communication and security; an indoor model and a simulation set up for HMIMOS communication were presented.

[7] A1, A2 F2, F3 C1, C3-C6 Designed a 0.4m 2 and 1.5mm thick planar metasurface consisting of 102 controllable unit cells operating at 2.45GHz; demonstrated increased received signal strength when deployed indoors.

[9] A1, A2, A5 F2, F3 C3-C6

Proposed free space pathloss models using the EM and physical properties of a reconfigurable surface; indoor field experiments validated the proposed models.

[5] A1, A2 F2, F3 C1, C3-C6

Proposed HMIMOS for outdoor MIMO communications and presented EE maximization algorithms; studied the fundamental differences between HMIMOS and conventional multi-antenna relays.

[8] A2, A4 F2, F3 C1, C3-C6

Presented jointly active and passive beamforming algorithms for HMIMOS-assisted MIMO communication; analyzed the interference distribution and studied the power scaling law.

Outdoor [START_REF] Han | Large intelligent surfaceassisted wireless communication exploiting statistical CSI[END_REF] A2 F2, F3 C1, C3-C6

Derived the optimal HMIMOS phase matrix for the case of available statistical channel information and presented a tight approximation for the ergodic capacity.

[12] A1, A2 F1-F4 C1, C3-C6 Studied compressive sensing and deep learning approaches for HMI-MOS channel estimation and online configuration.

communication objectives. An illustrative general example is sketched in the lower part of Fig. 3. In the left corner of this example where a HMIMOS is absent, the signal experiences pathloss and multipath fading due to refraction, reflection, and diffusion, which deteriorates its sufficient propagation to the target user. However, in the right corner of Fig. 3, signal propagation can be boosted using HMIMOS coated in the wall so as to assist the signal from the access point to reach the intended user with the desired power level.

• A5: Enhanced in-building coverage: As previously discussed, indoor environments can be coated with HMI-MOS to increase the throughput offered by conventional Wi-Fi access points. • A6: High accurate indoor positioning: HMIMOS has increased potential for indoor positioning and localization, where the conventional Global Positioning System (GPS) fails. Large surfaces offer large, and possibly continuous, apertures that enable increased spatial resolution. There has been lately increasing research interest in wireless communication systems incorporating HMIMOS. In Table I, we list some of the recent works dealing with different combinations among the functionalities of HMIMOS, their characteristics, and communication applications.

IV. DESIGN CHALLENGES AND OPPORTUNITIES

In this section, we present some main theoretical and practical challenges with HMIMOS-based communication systems.

A. Fundamental Limits

It is natural to expect that wireless communication systems incorporating HMIMOS will exhibit different features compared with traditional communications based on conventional multi-antenna transceivers. Recall that current communication systems operate over uncontrollable wireless environments, whereas HMIMOS-based systems will be capable of reconfiguring their EM propagation impact. This fact witnesses the need for new mathematical methodologies to characterize the physical channels in HMIMOS-based systems and analyze their ultimate capacity gains over a given volume [START_REF] Pizzo | Spatial characterization of holographic MIMO channels[END_REF], as well as for new signal processing algorithms and networking schemes for realizing HMIMOS-assisted communication. For example, continuous HMIMOS is used for the reception and transmission of the impinging EM field over its continuous aperture using the hologram concept. Differently from the currently considered massive MIMO systems, HMIMOS operation can be described by the Fresnel-Kirchhoff integral that is based on the Huygens-Fresnel principle [START_REF] Yurduseven | Dynamically reconfigurable holographic metasurface aperture for a mills-cross monochromatic microwave camera[END_REF].

B. HMIMOS Channel Estimation

The estimation of possibly very large MIMO channels in HMIMOS-based communication systems is another critical challenge due to the various constraints accompanying the available HMIMOS hardware architectures. Most of the few currently available approaches mainly consider large time periods for training all HMIMOS unit elements via pilots sent from the BS and received at the user equipment via generic reflection. Another family of techniques employs compressive sensing and deep learning via online beam/reflection training for channel estimation and design of the phase matrix [START_REF] Taha | Enabling large intelligent surfaces with compressive sensing and deep learning[END_REF]. However, this mode of operation requires large amounts of training data, and employs fully digital or hybrid analog and digital transceiver architectures for HMIMOS, which will results in increased hardware complexity and operational power consumption.

C. Robust Channel-Aware Beamforming

Channel dependent beamforming has been extensively considered in massive MIMO systems. However, realizing environment-aware designs in HMIMOS-based communication systems is extremely challenging, since the HMIMOS unit cells that are fabricated from metamaterials impose demanding tuning constraints. The latest HMIMOS design formulations (e.g., [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF], [START_REF] Wu | Intelligent reflecting surface enhanced wireless network via joint active and passive beamforming[END_REF]) include large numbers of reconfigurable parameters with non-convex constraints rendering their optimal solution highly non-trivial. For the case of continuous HMI-MOS, intelligent holographic beamforming is the objective in order to smartly target and follow individual or small clusters of devices, and provide them with high-fidelity beams and smart radio management. However, self-optimizing holographic beamforming technologies that depend on complex aperture synthesis and low level modulation are not available yet.

D. Distributed Configuration and Resource Allocation

Consider a communication system comprising multiple multi-antenna BSs, multiple HMIMOS, and massive number of where each user is equipped with a or multiple antennas. The centralized configuration of HMIMOS will require massive amounts of control mation be communicated to a central controller, which is prohibitive both in terms of computational overhead and energy consumption. Hence, distributed algorithms for the EE-optimal resource allocation and beamforming, HMIMOS configurations, and users' scheduling need to be developed. Additional optimization parameters complicating the network optimization are anticipated to be the power allocation and spectrum usage, as well as the users' assignment to BSs and distributed HMIMOS. Naturally, the more HMIMOS are incorporated in the network, the more challenging the algorithmic design will becomes.

V. CASE STUDIES

In this section, we summarize the performance of HMIMOS in two typical application scenarios: indoor positioning with an active continuous HMIMOS and outdoor downlink communication assisted by a passive discrete HMIMOS.

A. Indoor Positioning with an Active Continuous HMIMOS

We assume an active HMIMOS where the distance between any of each two adjacent unit elements is λ/2, with λ being the carrier wavelength. In such a discretized manner, traditional MIMO, massive MIMO, and HMIMOS are unified, and the differences lie in the number of antenna elements used, i.e., the surface area. It was shown in [START_REF] Hu | Beyond massive MIMO: The potential of positioning with large intelligent surfaces[END_REF] that the number of antennas in a traditional massive MIMO system for a given surface area πR 2 is equal to πR 2 /(λ 2 /4) = πτ z 2 /(λ 2 /4) ∼ = 20106τ , when z = 4m, λ = 0.1m, and τ (R/z) 2 (the normalized surface area). A typical massive MIMO array comprising of N = 200 antennas results in τ ≈ 0.01, while an active HMIMOS typically increases the surface-area (so as τ ) by 10 ∼ 20 [START_REF] Hu | Beyond massive MIMO: The potential of positioning with large intelligent surfaces[END_REF]. In Fig. 4a, the Cramér-Rao Lower Bounds (CLRBs) of user positioning in the presence of phase uncertainty are sketched. As depicted, the CRLB of positioning decreases linearly with τ for traditional MIMO, while massive MIMO falls short in reaching the cubic decreasing slope that is achieved by the active HMIMOS, yielding significant gains in user positioning.

B. EE Maximization with a Passive Discrete HMIMOS

We consider an outdoor 16-antenna BS simultaneously serving 16 single-antenna users in the downlink communication using a discrete passive HMIMOS with 32 unit elements that is attached to a surrounding building's facade [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF], and the simulation parameters are shown in Table II [START_REF] Huang | Reconfigurable intelligent surfaces for energy efficiency in wireless communication[END_REF]. The obtained EE performance using an approach based on Sequential Fractional Programming (SFP), as well as a gradient descent approach, to tune the HMIMOS system is sketched in Fig. 4b as a function of the maximum BS transmit power P max .

We have also numerically evaluated the EE of conventional Amplify-and-Forward (AF) relaying. It is shown that the HMIMOS-assisted system achieves a three-fold increase in EE compared to the AF relaying case when P max ≥ 32dBm.

When P max ≥ 32dBm, EE performance saturates, which reveals that the excess BS transmit power should not be used because it would decrease EE.

VI. CONCLUSION

In this article, we investigated the emerging concept of HMIMOS wireless communication, and in particular the available HMIMOS hardware architectures, their functionalities and characteristics, as well as their recent communication applications. We highlighted their great potential as a key enabling technology for the physical layer of future 6G wireless networks. HMIMOS technology offers huge advantages in terms of SE and EE, yielding smart and reconfigurable wireless environments. It reduces the cost, size, and energy consumption of network devices, providing ubiquitous coverage and intelligent communication in both indoor and outdoor scenarios. Benefiting from its merits, HMIMOS can be compactly and easily integrated into a wide variety of applications. Representative use cases are the extension of coverage, physical-layer security, wireless power transfer, and positioning. However, there are still challenges ahead to achieve the full potential of this emerging technology. Among them belong the realistic modeling of metasurfaces, the analysis of the fundamental limits of wireless communications with multiple HMIMOS, the implementation of intelligent environmentaware adaptation, and the channel estimation with nearly passive surfaces. These challenges provide academic and industrial researchers with a gold mine of new problems and challenges to tackle.

Figure 3 :

 3 Figure 3: Wireless communications applications of HMIMOS in outdoor and indoor environments.

Figure 4 :

 4 Figure 4: (a) CRLBs of positioning with an active HMIMOS of a radius R for the case where single user is located z = 4m away from center of surface. The wavelength λ is 0.1m, τ represents the normalized surface-area [4]. (b) Average EE with HMIMOS-assisted communication versus the maximum BS transmit power P max in dB.

Table II :

 II Simulation Parameters for the Average EE Performance Results in Fig. 4b. Circuit dissipated power coefficients at BS and AF relay: 1.2 Small scale fading model: Rayleigh Maximum transmit power at BS and AF relay Pmax: 20dBW Large scale fading model at distance d: 10 -3.53 d -3.76 Dissipated power at each user:

	Parameters	Values	Parameters	Values
	HMIMOS central element placement:	(100m, 100m)	Circuit dissipated power at BS:	9dBW
	BS central element placement:	(0m, 0m)
				10dBm
	Transmission bandwidth:	180KHz	Dissipated power at each HMIMOS element:	10dBm
	Algorithmic convergence parameter:	= 10 -3	Dissipated power at each AF relay transmit-receive antenna:	10dBm

It should be noted that not all HMIMOS architectures have all listed attributes. Few of them are inherent to passive HMIMOS, but not to active ones, and vice versa. However, we discuss HMIMOS properties here in a broad scope, including all available types up to date.


M Sulc

P Pugnat

R Ballou

G Deferne

J Hosek

S Kunc

A Siemko

OSQAR chameleon afterglow search experiment

Keywords: Chameleon particle, Magneto-optical experiment, LHC dipole magnet PACS: 12.60.Cn, 14.70.Bh, 14.80.Va, 95.36.+x

published or not. The documents may come

Introduction

The existence of dark energy, confirmed by astrophysical observation, can be explained by chameleon theories [START_REF] Khoury | Chameleon cosmology[END_REF]. These scalar-tensor theories describe chameleons as particles with effective mass growing sharply at the proximity of higher density matter [START_REF] Khoury | Chameleon fields: awaiting surprises for tests of gravity in space[END_REF]. The chameleons can couple with photons in the presence of an external magnetic field. This field allows photon oscillation into chameleon and back again. The chameleons can manifest through an afterglow from the vacuum as they change to photons. This effect can be studied by magneto-optical laboratory experiments [START_REF] Upadhye | Designing dark energy afterglow experiments[END_REF].

The first chameleon search laser experiments GammeV and GammeV-CHASE were performed with one Tevatron dipole magnet at Fermilab [START_REF] Chou | Search for Chameleon Particles Using a Photon-Regeneration Technique[END_REF][5] [START_REF] Steffen | Laboratory Constraints on Chameleon Dark Energy and Power-Law Fields[END_REF]. The 3.5 W second harmonic (532 nm) beam from Nd:YAG laser was shining through a vacuum chamber closed by transparent windows, placed inside a 6 m long magnet with a 5 T magnetic field. The production time was over 5 hours, the detection time was 1 hour, and the time interval between production and detection phases was longer than 300 s. No afterglow signal, induced by chameleon regeneration, was observed in the 1 Hz range of sensitivity. Only a signal in the orange and red portion of the spectrum (about 650-700 nm) with decay time around 130 s was observed. It also appeared when the magnetic field was off. Its exact origin is still unknown [START_REF] Steffen | Anomalous afterglow seen in a chameleon afterglow search[END_REF].

OSQAR-CHASE has been proposed to extend the pioneering work of the GammeV using a stronger 9 T magnetic field, a 14.3 m long magnets and more sensitive detection systems [START_REF] Pugnat | OSQAR-CHASE Proposal[END_REF]. The quick switching system between production and detection phases was also applied to observe possible strong chameleonphoton interactions.

OSQAR-CHASE experimental set-up

The OSQAR-CHASE experiment has been performed at CERN. A single spare LHC dipole producing 9 T magnetic field over 14.3 m has been used. The dipole is connected to its cryogenic feedbox and vacuum pumps. The dipole apertures have been equipped with an anticryostat serving as vacuum chambers. The anticryostat was closed by two BK7 glass windows with antireflexion coating. This experimental set-up was very similar to GammeV. The system was tested at preparatory phase in 2016 and fine-tuned to experimental run in 2017. The Verdi V18 CW laser with maximal power 18.5 W was used in production phase as a source of 2.33 eV (532 nm) photons. The laser beam was extended by beam expander and collimated to reach the center of the output window. Both input and output parts of anticryostat are equipped by light-tight valves. The valves 1 and 2 on input and output are open at production phase whereas the valve 3 before detector is closed to protect the photon detector. On the contrary the valve 1 and 2 are light-tightly closed and the valve 3 is open during afterglow measurement. This valve system allows to switch very quickly (in 10-20 s) between production and afterglow phases. The optical focusing system, consisting from two converging lenses, has been optimized to bring a maximal number of photons from the anticryostat to the region of interest at detector. ROI was almost centered circle on the CCD with a diameter of about 540 pixels. A CCD detector was used for photon detection. The thermoelectric cooled CCD-Andor iKon-DU934P-BV is composed of an array of 1024 x 1024 square pixels of 13 µm size with the used maximum binning of 16 x 16. The quantum efficiency, given by Andor iKon, is equal to 95% at 550 nm. In comparison the sensitivity of the overall detection system was experimentally found to be (0.65 ± 0.03) detected events per one incoming photon. The readout noise measured at 50 kHz is equal to 3.1 e/pixel for 16x16 binning whereas the dark current values were smaller than 0.00011 e/pixel/s at 90 • C for 16x16 binning.

Preliminary experimental results of OSQAR-CHASE

The set of runs was performed. Long runs consisted of 4 hours of production time followed by 4 hours of afterglow measurement and was divided to 14 intervals of 1000 s long CCD exposition time. The exponential decrease with starting amplitude above 4.3 Hz was observed. Decay time was (4300±700) s for all long runs, see figure1. But this signal was surprisingly higher outside ROI. The decay was inhomogeneous on the CCD image sensor. Detected signal count rate was lower inside the ROI than outside the ROI. It decreases more quickly in ROI. The difference between inside ROI and outside ROI signals was stabilized to 2.9 ± 0.4 Hz after 10 000 s.

The origin of false signal with long decay time about 4300 s is under investigation. This long time leads to a hypotheses privileging thermal origin for this false signal. The 18 W laser was shining during 4 hours through the output window to the beam dumper. Although this dumper collects almost all the beam energy, only a small amount of this one can be accumulated by the output window and by some parts close to the CCD. Emission of thermal IR photons from these parts together with conduction of heat are in equilibrium after 4 hours of chameleon production and dark current distribution on CCD is homogeneous. The slow cooling starts after laser switching-off and opening of valve 3. It can disturb this equilibrium especially on the Peltier cooled CCD chip. The dark current is more suppressed in the central region i.e in the ROI, where the cooling Peltier element is located. The exponential decay afterglow signal was observed at all types of our runs. But it disappears once the difference between data taken with and without magnetic field is performed.

Conclusion

Experimental results were preliminary analyzed and no magnetic afterglow signal was observed at a frequency higher than 0.7 Hz. Increasing of the sensitivity of the OSQAR-CHASE experiment over the previous GammeV reference experimentmagnetic field from 5 T to 9 T, power from 3.5 W to 18.5 W, detector efficiency from 0.29 to 0.65, decay rate sensitivity from 1.35 Hz to 0.65 Hz -assumes that the present coupling constant limit will be lowered by a factor of 4.

Figure 1 :

 1 Figure 1: Preliminary results of the OSQAR-CHASE 2017 experimental run for scalar Chameleon search recorded with the magnetic field and after subtraction of the background determined without magnetic field.

Acknowledgments

This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic by the Projects No. LM 2015058 and No. LTT17018.