Wen-Ting Wang

Zhuang-Hao Zheng

Fu Li
email: lifu@szu.edu.cn

Cao Li

Ping Fan
email: fanping@szu.edu.cn

Jing-Ting Luo

Bo Li

Synthesis Process and Thermoelectric Properties of n-type Tin Selenide Thin Films

Keywords: tin selenide, thermoelectric property, thermal evaporation

Tin selenide (SnSe) is a very promising thermoelectric material, but there are few reports related to the thermoelectric properties of its thin films, especially for n-type SnSe. In this work, n-type SnSe thin films were synthesized via thermal evaporation using powdered SnSe that was prepared directly by mechanical alloying. It was found that the crystallinity of the films gradually improved and the closer the elemental ratio of Sn and Se was to a ratio of 1:1 with increasing the current through the tungsten boat during the thermal evaporation of the SnSe precursor from 100 to 130 A. Further, the thin films obtained at 120 A show the highest power factor (0.13 µW/mK 2 ) of all the samples fabricated in this study. The samples were then annealed, and the thermoelectric properties of the films were enhanced as this step improved their crystallinities. A Seebeck coefficient of over 100 µV/K could be achieved with an annealing time of 4 h. Combined with a recorded electrical conductivity of 47 S/cm, a maximum power factor of 120 µW/mK 2 at 473 K has been obtained for the annealed film with n-type conductive characteristics; this is nearly six times higher than that reported for polycrystalline n-type bulk SnSe at the same temperature.

Introduction

Thermoelectric (TE) materials are attracting increasing attention owing to their promising applications in waste heat recovery and electronic cooling [START_REF] Bell | Cooling, heating, generating power, and recovering waste heat with thermoelectric systems[END_REF][START_REF] Dehkordi | Thermoelectric power factor: Enhancement mechanisms and strategies for higher performance thermoelectric materials[END_REF][START_REF] Li | Ag-doped SnSe 2 as a promising mid-temperature thermoelectric material[END_REF]. The performance of TE materials can be evaluated by the dimensionless figure of merit, ZT = S 2 σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. A larger ZT value means a higher TE conversion efficiency of the corresponding device. Thus, to obtain a good device, excellent electrical transport properties, along with a low thermal conductivity to achieve a high ZT for the material, are required. As there is a constraint relationship between the electrical and thermal transport properties, the search for practical materials with large ZT values is challenging. Thin films with two dimensional structures have been received significant attention in the field of TE materials, because their thermal and electrical transport properties can be controlled independently [START_REF] Cai | Thermoelectric properties and micro-structure characteristics of annealed N-type bismuth telluride thin film[END_REF]. In addition, flexible generators made using thin films have considerable potential for applications in the microelectronics industry [START_REF] Giani | Growth of Bi 2 Te 3 and Sb 2 Te 3 thin films by MOCVD[END_REF][START_REF] Nishino | Flexible thermoelectric generator with efficient vertical to lateral heat path films[END_REF].

Therefore, research into preparing high performance thin films has become important within the field of thermoelectricity [START_REF] Yadav | Fiber-based flexible thermoelectric power generator[END_REF].

Tin selenide (SnSe) crystallized with a layered crystal structure is known to be a narrow band gap semiconductor with an indirect band gap of 0.90 eV at room temperature and direct band gap of 1.30 eV at high temperatures (~750 K) [START_REF] Bicer | Electrodeposition and growth mechanism of SnSe thin films[END_REF]. It has been intensively investigated already as it is an earth abundant and environmentally benign component of photovoltaic devices such as thin film solar cells, where it is used because its band gap falls within the optimum range for photovoltaic applications [START_REF] Minnam Reddy | Perspectives on SnSe-based thin film solar cells: a comprehensive review[END_REF][START_REF] Inoue | Heteroepitaxial growth of SnSe films by pulsed laser deposition using Se-rich targets[END_REF]. Recently, SnSe has been investigated for its suitability for TE applications because of its environmentally friendly composition and the high ZT value of single crystalline SnSe, which is attributed to its extremely low thermal conductivity and moderate electrical transport properties [START_REF] Zhao | Ultralow thermal conductivity and high thermoelectric figure of merit in SnSe crystals[END_REF][START_REF] Peng | Broad temperature plateau for high ZTs in heavily doped p-type SnSe single crystals[END_REF]. However, the layered crystal structure causes the poor mechanical properties of the single crystalline SnSe, since it can easily cleave along the (001) plane [START_REF] Tang | Realizing high figure of merit in phase-separated polycrystalline Sn 1-x Pb x Se[END_REF]. Therefore, researchers have focused their attentions on polycrystalline SnSe owing to its good

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
3 mechanical properties and easily controlled production conditions. To date, most studies on polycrystalline SnSe have focused on p-type SnSe, with only few reports related to n-type one [START_REF] Zhao | SnSe: a remarkable new thermoelectric material[END_REF][START_REF] Zhang | Studies on thermoelectric properties of n-type polycrystalline SnSe 1-x S x by iodine doping[END_REF]. In particular, reports on the TE properties of n-type SnSe thin films are scarce.

In this work, n-type SnSe thin films were prepared by thermal evaporation. The precursor for the thermal evaporation step was synthesized by mechanical alloying using commercial Sn and Se powders; this is a simple and cost-effective approach for fabricating high-performance TE materials that has emerged in recent years [START_REF] Li | Polycrystalline BiCuSeO oxide as a potential thermoelectric material[END_REF][START_REF] Pan | Thermoelectric performance enhancement in n-type Bi 2 (TeSe) 3 alloys owing to nanoscale inhomogeneity combined with a spark plasma-textured microstructure[END_REF].

The physical properties of thin films are extremely sensitive to the growth conditions and the type of deposition technique employed for their growth. Thus, we investigated the composition, microstructure, and TE properties with an emphasis on the effect of the synthesis conditions, including the applied current to the crucible holding the SnSe precursor during thermal evaporation as well as the annealing temperature.

Experimental details

SnSe thin films were prepared by thermal evaporation using powders that were directly prepared by mechanical alloying with commercial Sn (99.999%) and Se (99.999%) powders as the raw materials according to the nominal composition of SnSe. The thin films were fabricated on 20 mm × 20 mm × 2 mm BK7 glass substrates under 4 × 10 -3 Pa by using a FJL-520 ultra-high vacuum system with different current of 100, 110, 120, and 130 A on tungsten boat. The substrate glass slides were cleaned using air-laid paper after being cleaned in an ultrasonic bath of (sequentially) acetone, alcohol, and deionized water for 20 min each prior to thermal evaporation. After the thermal evaporation step, the films were annealed at 473, 573, and 673 K for 1 h under an argon atmosphere at 440 Pa.

The phase structures of the SnSe thin films were probed using X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) with the conventional θ -2θ mode. The microstructures were observed via scanning electron microscopy (SEM, Supra 55 Sapphire, Zeiss, Germany), and the composition was analyzed by energy dispersive spectroscopy (EDS). The thicknesses of all the films were measured by a Dektak3 ST

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4
surface-profile measurement system (Ultima4, Rigaku, Japan). The Seebeck coefficient and electrical conductivity were recorded simultaneously as a function of temperature (from 300 to 473 K) using a Seebeck coefficient/electrical resistance measuring system (SBA458, Netzsch, Germany). The Hall mobility and Hall concentration were measured using a Hall effect measurement system (HL5500, Nanometric, USA). The uncertainty of Seebeck coefficient and electrical conductivity measurements is within 5%. Therefore, the combined uncertainty for all measurements involved in power factor (PF=S 2 σ) determination shown in the plot is estimated to be 10%. The standard deviation of the measured power factor below 450 K is within 5% for three different samples with the same composition and preparation technology.

Results and discussion

The phase structure and microstructure of SnSe powder prepared by mechanical alloying before thermal evaporation were firstly observed using XRD and SEM. The results indicated that all the locations of diffraction peaks of the powder corresponded well to those of the standard SnSe (JCPDS#48-1224), indicating that a single, orthorhombic SnSe phase was synthesized. The synthesized powder, with nonuniform particle size of 100-500 nm in diameter, are aggregated to a certain extent. The average ratio of Sn to Se detected by EDS are close to the stoichiometry of SnSe. Fig. 1 shows the XRD patterns of unannealed SnSe thin films deposited at different crucible currents during the thermal evaporation. It shows that the samples are amorphous when the applied current is below 110 A. As the current was increased, the as-deposited thin films started to show some characteristic diffraction peaks related to the (111), (311), and (411) plane of SnSe, indicating that the crystallinity of the films gradually enhanced. The SEM images of all samples are shown in Fig. 2. There are no obvious crystalline grains for the films deposited with currents of 100 and 110 A.

However, when the current was increased, more and more crystalline grains appear in the films [START_REF] Indirajith | Synthesis, deposition and characterization of tin selenide thin films by evaporation technique[END_REF]. The ratios of Sn to Se of the as-deposited films obtained under different currents were detected by EDS which are presented in Table 1. We found

M A N U S C R I P T A C C E P T E D

that the ratio of Sn to Se of all the films deviated from 1:1, although the ratio in the precursor was nearly 1:1. The content of Sn is obviously greater than that of Se. With increasing the applied current, the Se content increased and the ratio gradually started approaching 1:1. This means that the evaporation of Se is sensitive to the current during the thermal evaporation, which is likely caused by the different saturated vapor pressures of Sn and Se [START_REF] Inoue | Heteroepitaxial growth of SnSe films by pulsed laser deposition using Se-rich targets[END_REF].

Fig. 3(a) shows the temperature dependence of the electrical conductivity for all unannealed films. The data show that the electrical conductivity increases along with the temperature above 350 K owing to thermal excitation, which indicates semiconducting behavior. With increasing temperature, the lattice is expanded and the orbital overlap between atoms is enhanced. This makes the energy band dispersion stronger, which consequently reduces the band gap. Thus, the smaller band-gap at a higher temperature made it easier for the carriers to be excited and thereby the carrier concentration would be increased. As a result, the electrical conductivity, which can be expressed as ne σ µ = (where n is the carrier concentration, µ is the carrier mobility and e is the electron charge), increases with increasing temperature. In addition, the electrical conductivities show a weak dependence on the thermal evaporation current, and the values for the films obtained at different conditions do not differ significantly. Around room temperature, the electrical conductivities are about 0.25 S/cm. When the temperature increases, the values rise to about 1.5 S/cm at 450 K. Compared with the reported results for bulk polycrystalline samples, the electrical conductivities of our films are lower, which is mainly owing to their lower carrier concentrations and mobility [START_REF] Li | Enhanced thermoelectric performance in n-type polycrystalline SnSe by PbBr 2 doping[END_REF]. In fact, the Hall carrier concentration and mobility at room temperature of a representative film obtained at 130 A were measured to determine its transport properties. The results indicate that the carrier concentration was 2.56×10 17 cm -3 , and the carrier mobility was only 2.80 cm 2 /Vs which is nearly one order of magnitude lower than that reported for a comparative bulk sample [START_REF] Chen | Thermoelectric properties of p-type polycrystalline SnSe doped with Ag[END_REF]. As discussed above, the crystallinity of that film sample appears to be poor. Therefore, we attribute the lower carrier concentration and mobility to the

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 6 poor crystallinity.
The temperature dependence of the Seebeck coefficient of the fabricated films is shown in Fig. 3(b). It can be seen that all the values are negative, which indicates that these samples are n-type conductors with electrons as the dominate carriers. As mentioned above, the ratio of Sn to Se of all the films deviated from 1:1, and the films exhibited a lack of Se. Thus, Se vacancies will introduce more electrons, leading to an n-type conductive mechanism. This result is in agreement with a recent report in which a bulk sample with extra Sn was determined to be n-type with a negative

Seebeck coefficient [START_REF] Zhang | The chemistry and structural thermal stability of hole-doped single crystalline SnSe[END_REF]. Moreover, similar to the electrical conductivity results as mentioned above, the Seebeck coefficient also shows a weak dependence on the applied current. As shown in Fig. 3(b), the absolute Seebeck coefficient for all the films roughly increased as the temperature was increased. Around room temperature, the absolute value was about 10 µV/K; when the temperature increased to 450 K, it improved to 27 µV/K. However, because of poor crystallinity, all the values were still much lower than those reported for bulk samples [START_REF] Tang | Realizing high figure of merit in phase-separated polycrystalline Sn 1-x Pb x Se[END_REF]. Nevertheless, the Seebeck coefficient was greatly increased after annealing, which would be discussed below.

Combing the electrical conductivity and Seebeck coefficient, the power factors of the unannealed films were calculated as shown in Fig. 3(c). It can be seen that the power factor increases as the temperature increases, and that it rises sharply from 350 to 425 K. As a result, a slightly higher power factor of 0.13 µW/mK 2 was obtained for the film prepared at 120 A (as-deposited) because of its slightly higher absolute planes of SnSe (JCPDS#48-1224). It should also be noted that some characteristic diffraction peaks in the range of 2<theta> = 45° to 70° appeared in the films after annealing, indicating that the crystallinity had improved. The surface morphologies for the annealed films are shown in Fig. 5. The images show that the grains grew after annealing. The average grain sizes for the films annealed at 473, 573 and 673 K were approximately 27, 50 and 71 nm in diameter respectively, which increased with increasing the annealing temperature. Furthermore, the porosity of the films increased along with the temperature. It is reported that thermal induced elements segregation and evaporation took place in the sample and lead to phase separation as well as formation of large voids in the materials [START_REF] Zhang | The chemistry and structural thermal stability of hole-doped single crystalline SnSe[END_REF]. After determining the composition by EDS, we found that the Sn content decreased with increasing the annealing temperature. Since the film before annealing was Sn-rich sample as mentioned above, thus, it is deduced that the Sn elements might segregate preferentially and evaporate during thermal treatment, resulting in the formation of voids and reduction of the density of the film. The Sn to Se ratio was nearly 1:1 when the annealing temperature was either 573 or 673 K. Moreover, the thickness of the film without annealing is around 550 nm. After the films were annealed at 473, 573 and 673 K, they have gradually reduced to 520, 500 and 100 nm, respectively. Notably, the decrease in thickness for the film annealed at 673 K is very pronounced, indicating that the films volatilized dramatically. In fact, when the annealing temperature was raised to over 673 K, no or a few products can be found on the glass substrates because of the volatilization, thus practically limiting the annealing temperature to 673 K.

Fig. 6 shows the electrical transport properties as a function of temperature for the films after heat treatment. The electrical conductivity as shown in Fig. 6(a) increased after annealing, and generally enhanced along with the annealing temperature. After the film was annealed at 673 K for 1 h, the electrical conductivity improved to 1.0 S/cm at room temperature, and then increased to 5.0 S/cm as the temperature increased to 450 K, which is twice higher than that of the unannealed films. Notably, this conductivity is also much larger than that reported for n-type bulk

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

polycrystalline SnSe 0.8 at the same temperature [START_REF] Lv | Thermoelectric properties of polycrystalline SnSe 1-x prepared by mechanical alloying and spark plasma sintering[END_REF]. This enhanced electrical conductivity might be caused by the increased carrier concentration and carrier mobility owing to the heat treatment (which causes a recrystallization), since the annealed films have a better crystallinity and larger grain sizes [START_REF] Kim | Effect of rapid thermal annealing on thermoelectric properties of bismuth telluride films grown by co-sputtering[END_REF][START_REF] Li | The influence of heat treatment on the thermoelectric properties of copper selenide thin films prepared by ion beam sputtering deposition[END_REF]. Actually, the measured carrier concentration and carrier mobility of the film annealed at 573 K at room temperature were 1.53 × 10 18 cm -3 and 3.72 cm 2 /Vs, and those of the film annealed at 673 K were 1.69×10 18 cm -3 and 4.21 cm 2 /Vs. These values are larger than those of the unannealed film, which were 2.56×10 17 cm -3 and 2.80 cm 2 /Vs, respectively. Additionally, it also shows that both of the carrier concentration and carrier mobility for the sample annealed at 673 K are slightly higher than those of the sample annealed at 573 K. According to the XRD and SEM as mentioned above, the annealed films have a better crystallinity and larger grain sizes, which means that the grain size has increased and some types of crystal defects gradually disappear during the annealing process. Thus, better crystallinity, larger grain size, fewer boundaries and defects would all result in the improvement of the carrier concentration and mobility because of weak scattering [START_REF] Huang | Influence of annealing on thermoelectric properties of bismuth telluride films grown via radio frequency magnetron sputtering[END_REF][START_REF] Cappelli | Nano-crystalline Ag-PbTe thermoelectric thin films by a multi-target PLD system[END_REF].

The Seebeck coefficients of the films after annealing are shown in Fig. 6(b). All the values are negative, suggesting the samples are n-type conductors. The absolute Seebeck coefficients are significantly increased compared with that before annealing; this was particularly the case for the samples annealed at 573 and 673 K. As discussed above, the absolute value for the unannealed film was below 27 µV/K across the entire measured temperature range. However, it has improved to 200-250 µV/K after the sample was annealed at over 573 K, thus increasing the Seebeck coefficient by one order of magnitude compared to the unannealed film. Furthermore, the Seebeck coefficient displayed weak temperature dependence, which is similar to the behavior reported for n-type bulk polycrystalline SnSe 0.8 [START_REF] Lv | Thermoelectric properties of polycrystalline SnSe 1-x prepared by mechanical alloying and spark plasma sintering[END_REF]. Combined with the improved electrical conductivity of over 5 S/cm as mentioned above, the power factor is significantly enhanced, as shown in Fig. 6(c). A maximum power factor of 21 µW/mK 2 at 450 K was achieved for the film that was annealed at 673 K, which displays n-type characteristics. It is worth noting that the power factor in this work is

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT
comparable with that reported for n-type bulk polycrystalline SnSe 0.8 [START_REF] Lv | Thermoelectric properties of polycrystalline SnSe 1-x prepared by mechanical alloying and spark plasma sintering[END_REF].

These results indicate that annealing is efficient for improving the TE properties of SnSe thin films. Thus we decided to conduct an additional experiment and increase the annealing time further to 4 h. We found that the electrical transport properties were further enhanced so that they were even comparable to those reported for p-type bulk polycrystalline SnSe at the same temperature [START_REF] Chen | Thermoelectric properties of p-type polycrystalline SnSe doped with Ag[END_REF]. As shown in Fig. 7, the electrical conductivity was around 25 S/cm at room temperature. As we increased the measurement temperature to 473 K, the conductivity increased further to 47 S/cm, which is nearly one order of magnitude greater than that of the film annealed for 1 h.

The measurement results for the carrier concentration and mobility indicate that the significant enhancement of the electrical conductivity can be attributed to both the increased carrier concentration and mobility. The value of the carrier concentration at room temperature is 8.09 × 10 18 cm 3 , while that of the mobility is 19.31 cm 2 /Vs; these values are comparable to those reported for bulk samples. After observing the phase structure of the annealed films by XRD, it is found that a small amount of impure SnSe 2 phase appeared after annealing the films for 4 h. Thus, we propose that the enhanced carrier concentration is caused by the SnSe 2 impurities, since SnSe 2 is an n-type conductor [START_REF] Li | Ag-doped SnSe 2 as a promising mid-temperature thermoelectric material[END_REF]. Additionally, the absolute Seebeck coefficient remained at a high value (around 140-160 µV/K) across the whole measured temperature range, even though it decreased slightly because of the increased carrier concentration. Thus, a large power factor of 120 µW/mK 2 was achieved at 473 K by annealing the thin SnSe film for 4 h owing to the combination of an enhanced electrical conductivity and a moderate Seebeck coefficient. This power factor is nearly six times higher than that of n-type bulk polycrystalline SnSe 0.8 at the same temperature [START_REF] Lv | Thermoelectric properties of polycrystalline SnSe 1-x prepared by mechanical alloying and spark plasma sintering[END_REF]. Usually, the thermal conductivity of thin films is lower than that of the corresponding bulk material because of the increased scattering introduced by boundaries. Therefore, a higher ZT value than that reported for n-type bulk polycrystalline samples would be obtained in this work via a larger power factor and a lower thermal conductivity. This suggests that using low dimensional films is actually a promising way of achieving good properties in n-type SnSe. Moreover, better TE properties should be achievable Seebeck coefficient [START_REF] Peng | Broad temperature plateau for high ZTs in heavily doped p-type SnSe single crystals[END_REF]. Since Na doping can lower the Fermi level and increase the number of carrier pockets, therefore, the TE properties of the film can be optimizing by Na doping. Furthermore, compared with the SnSe thin films by pulsed laser deposition [START_REF] Inoue | Heteroepitaxial growth of SnSe films by pulsed laser deposition using Se-rich targets[END_REF][START_REF] Suen | Enhanced thermoelectric properties of SnSe thin films grown by pulsed glancing-angle deposition[END_REF], the present results are comparable to those of the film prepared in normal angle deposition, while lower than that prepared in glancing angle deposition.

It is reported that the glancing angle deposition can greatly reduce the grain size of the thin film and improve the Seebeck coefficient and power factor [START_REF] Suen | Enhanced thermoelectric properties of SnSe thin films grown by pulsed glancing-angle deposition[END_REF]. Thus, combining with doping and regulating the microstructure, better TE properties should be realizable.

Conclusions

A series of n-type SnSe thin films were fabricated via thermal evaporation using a powder that was directly prepared by mechanical alloying. The results indicate that the crystallinity of the film is gradually enhanced when the current used in a thermal evaporator to supply the evaporation crucible with power is increased from 100 to 130

A. The evaporation of Se is sensitive to the current applied to heat the crucible, leading to the deviation from the ideal 1:1 ratio for Sn to Se. The TE performance shows a weak dependence on the thermal evaporator current. However, after annealing the films, their electrical transport properties improved significantly. The combination of the high Seebeck coefficient of 209 µV/K together with the enhanced electrical conductivity of over 5 S/cm results in a maximum power factor of 21 µW/mK 2 for an n-type conductive film prepared with a thermal evaporation current of 130 A and an annealing temperature of 673 K. Additionally, the electrical conductivity was further improved to 47 S/cm by increasing the annealing time. Thus, an even higher power factor of 120 µW/mK 2 was achieved when the annealing time was increased to 4 h, indicating that annealing is an efficient way to improve the TE properties of SnSe films. 2. The electrical transport properties of the films after annealing have greatly improved.

3. A maximum power factor of 120 µW/mK 2 was achieved at 473 K for the annealed film.

  Fig.4shows the XRD patterns of the annealed films. All the samples display

  conductivity were further increased by doping the films. In fact, the electrical transport property in the present work is much lower than that of the SnSe single crystal with Na doping because of the lower electrical conductivity and

Fig. 1 .

 1 Figure captionsFig. 1. XRD patterns of the as-deposited films obtained at different crucible currents during the thermal evaporation of the powders.

Fig. 2 .

 2 Fig. 2. SEM images of the as-deposited films obtained at different crucible currents during the thermal evaporation of the powders, namely at (a) 100 A, (b) 110 A, (c) 120 A, and (d) 130 A.

Fig. 3 .

 3 Fig. 3. Electrical transport properties as a function of temperature for the films obtained at different thermal evaporation current intensities. Panel (a) shows the electrical conductivities, panels (b) shows the Seebeck coefficients, and panel (c) shows the power factors.

Fig. 4 .Fig. 5 .

 45 Fig. 4. XRD patterns of the as-deposited films after heat treatment at different temperatures.Fig. 5. SEM images of the as-deposited films after heat treatment at different temperatures, namely at (a) 473 K, (b) 573 K, and (c) 673 K.

Fig. 6 .

 6 Fig. 6. Electrical transport properties as a function of temperature for the SnSe thin films annealed at different temperatures. Panel (a) shows the electrical conductivities, panel (b) shows the Seebeck coefficients, and panel (c) shows the power factors.

Fig. 7 .

 7 Fig. 7. Electrical transport properties as a function of temperature for the films after annealing for either 1 or 4 h. Panel (a) shows the electrical conductivities and the Seebeck coefficients, and panel (b) shows the power factors.

Figure

  Figure 2

2

 2 Figure 2

18 Figure 4

 184 Figure 3

Figure 5 M

 5 Figure 5

Figure 6 M

 6 Figure 6

Figure 7 M

 7 Figure 7

Table 1

 1 The composition of Sn to Se of the as-deposited films obtained under different thermal evaporation current as detected via EDS.

	Current intensity (A)	Sn (at% % % %)	Se (at% % % %)	Sn/Se ratio
	100	99.4	0.6	165.5
	110	71.7	28.3	2.5
	120	62.7	37.3	1.7
	130	57.2	42.8	1.3

Acknowledgements

Wen-ting Wang and Zhuang-hao Zheng contributed equally. This work was supported by the National Nature Science Foundation (Grant No. 51302140), the Shenzhen Science and Technology Plan Project (Nos. JCYJ20170818142740568, JCYJ20160422102622085 and JCYJ20150827165038323), and by the Natural Science Foundation of SZU (No. 2016016). We thank Jim Bailey, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT


Aymen Assadi
email: aymen.assadi@ensc-rennes.fr

Amine

Loganathan Sivachandiran

Tri Phuong Nguyen

Gharib-Abou Ghaida

Bouzaza Abdelkrim

Tuan Anh Nguyen

Wolbert Dominique

Pilot scale degradation of mono and multi volatile organic compounds by surface discharge plasma/TiO2 reactor: investigation of competition and synergism

Keywords: Nonthermal plasma surface discharge, photocatalysis, VOC binary mixture, synergetic effect

This paper mainly deals with the isovaleraldehyde degradation with the help of a nonthermal plasma surface discharge (NPSD) coupled with photocatalysis. The efficiency of NPSD reactor, for gas treatment, was studied for different binary mixtures: (1) mixture of aldehydes (Isovaleraldehyde and Butyraldehyde) and (2) mixture of aldehyde and amine (Isovaleraldehyde and Trimethylamine). A planar continuous reactor is used to investigate the effect of addition of another pollutant on the performance of oxidation process.

A synergetic effect was observed by combining NPSD and photocatalysis for the degradation of mixture of pollutants. In addition, combined NPSD / photocatalysis has significantly enhanced the CO2 selectivity, as compared to NPSD alone. This is attributed to the formation of more reactive species due to the presence of TiO2 in the plasma discharge zone. Moreover, ozone and UV light on TiO2, produced by plasma, have activated the surface leading to enhanced mineralization. In addition, the byproducts of each binary mixture were identified and evaluated.

Graphical abstract

A nonthermal plasma surface discharge (NPSD) coupled with photocatalysis Highlights  Removal of pollutant in air mixture by photocatalysis and plasma is investigated  Synergetic effect of combined system is explored.

 The by-products of each binary mixture were identified and evaluated.

 Monitoring of mineralization and ozone has been done at pilot scale.

Introduction

Throughout the past century industrial, agricultural, and domestic activities have been responsible for the emission of many undesirable volatile organic compounds (VOCs) to the atmosphere. These compounds have drawn considerable attention in the last two decades as they were and still are major contributors to air pollution [START_REF] Quoc An | Application of atmospheric non-thermal plasma-catalysis hybrid system for air pollution control: Toluene removal[END_REF]. Therefore, the abatement of VOCs has become a major global challenge as its emission aggravated the problems of climate change and global warming. Consequently, different strategies are being developed to minimize, if not completely eliminated, the risks associated with VOCs emissions [2]. Indeed, the prevailing processes for emissions control such as incineration, absorption, adsorption, condensation, and biofiltration do not provide fully acceptable results [START_REF] Parmar | Emerging Control Technologies for Volatile Organic Compounds[END_REF].

As an outcome, advanced oxidation processes (AOPs) have emerged as suitable alternative technologies to effectively remove VOCs from gaseous effluents [START_REF] Van Durme | Combining non-thermal plasma with heterogeneous catalysis in waste gas treatment: A review[END_REF][START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Musialik-Piotrowska | Catalytic oxidation of trichloroethylene in twocomponent mixtures with selected volatile organic compounds[END_REF][START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-synergetic effect and byproducts identification[END_REF]. These AOPs are especially characterized by the in situ production of hydroxyl radical (°OH) at room temperature [START_REF] Aouadi | MnOx/TiO2 Catalysts for VOCs Abatement by Coupling Non-thermal Plasma and Photocatalysis[END_REF]. Among the various AOPs, the photocatalytic and non-thermal plasma (NTP)

A C C E P T E D M

A N U S C R I P T catalytic processes have been widely investigated. Photocatalysis is based on the use of low energy UV photons to excite an electron from the conduction band to the valence band in a semiconductor catalyst (commonly TiO2) leading to the formation of electron-hole pairs [START_REF] Neyts | Plasma-Surface Interactions in Plasma Catalysis[END_REF].

The formed electrons and holes lead to the formation of very reactive hydroxyl radicals in the presence of moisture/water on photocatalytic surface or in the gas phase [START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: Systematic experimental evaluation[END_REF]. The OH produced on the catalyst surface effectively decomposes many toxic pollutants [START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: Systematic experimental evaluation[END_REF].

Nonetheless, photocatalytic oxidation has disadvantages like catalyst deactivation and slow process, which restricts its commercial application for depollution purposes.

In order to enhance the photodegradation performance, photocatalyst is usually combined with plasma discharge [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Klett | Acetaldehyde removal using an atmospheric non-thermal plasma combined with a packed bed: Role of the adsorption process[END_REF]. Most commonly, for air depollution, electrical discharges are used to produce NTP-favorable for the removal of various pollutants (e.g., NOx, odors, SOx and VOCs)under moderate conditions (i.e. room temperature and atmospheric pressure), which is intriguing towards industrialization of the process [START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF]. It is widely reported that the coupling of photocatalyst with NTP has shown considerable enhancement in the performance of the process [START_REF] Karuppiah | Abatement of mixture of volatile organic compounds (VOCs) in a catalytic non-thermal plasma reactor[END_REF][START_REF] Subrahmanyam | Nonthermal plasma abatement of trichloroethylene enhanced by photocatalysis[END_REF][START_REF] Assadi | Isovaleraldehyde elimination by UV/TiO2 photocatalysis: comparative study of the process at different reactors configurations and scales[END_REF][START_REF] Sivachandiran | Isopropanol saturated TiO2 surface regeneration by non-thermal plasma: Influence of air relative humidity[END_REF][START_REF] Sivachandiran | Non-Thermal Plasma Assisted Regeneration of Acetone Adsorbed TiO2 Surface[END_REF][START_REF] Zhu | Plasma-catalytic removal of formaldehyde over Cu-Ce catalysts in a dielectric barrier discharge reactor[END_REF].

In atmospheric NTP, highly energetic electrons and chemically reactive species (e.g., free radicals, excited atoms, ions, and molecules) are generated and implied into of both physical and chemical reactions. Nonthermal plasma treatment has a distinct non-equilibrium character, meaning that the gas temperature is near at room temperature, whilst the electrons are highly energetic with a typical mean temperature of 1-10 eV [START_REF] Wang | Removal of gas phase dimethylamine and N,N-dimethylformamide using non-thermal plasma[END_REF]. As a result, NTP can easily break most of the chemical bonds and triggers the thermodynamically unfavorable chemical reactions at room temperature. Therefore, the oxidation of VOCs using photocatalytic/NTP system, at room temperature, atmospheric pressure, and with molecular oxygen as the only oxidant, is promising [START_REF] Wang | Removal of gas phase dimethylamine and N,N-dimethylformamide using non-thermal plasma[END_REF]. The combined NPSD / photocatalysis system has been investigated previously, mostly for single pollutant removal, under UV irradiation [START_REF] Aouadi | MnOx/TiO2 Catalysts for VOCs Abatement by Coupling Non-thermal Plasma and Photocatalysis[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][START_REF] Sivachandiran | Non-Thermal Plasma Assisted Regeneration of Acetone Adsorbed TiO2 Surface[END_REF]. Therefore, it can be proposed that the integration of solid catalysts in plasma discharge area could have a great potential for generating a synergistic effect, improving their activity and resulting in a remarkable enhancement of pollutant oxidation and less by-products formation.

In this work, a pilot scale planar dielectric barrier discharges (DBD) reactor has been developed for the plasma/photocatalytic oxidation of two binary mixtures of pollutants. The effect of photocatalysts on the oxidative removal of pollutants has been evaluated. Moreover, the mutual effect between pollutants and the influence of concentration on oxidation pathway have also been studied. Indeed, the synergistic effect between NTP and photocatalyst for various pollutants is explored in pilot scale process.

A C C E P T E D

M A N U S C R I P T

Materials and methods

Chemicals and analysis

Chemicals are purchased from Sigma-Aldrich (USA) at various purity grades: Isovaleraldehyde (Isoval) (≥98%), Trimethylamine (TMA) in water (50%) and Butyraldehyde (Buty) (≥99%). All chemicals are used as received without any further purification.

The gas products are analyzed by a dual-channel gas chromatography Fisons GC9000 equipped with a flame ionization detector (FID). The separations of Isoval, Buty and by-products are performed by a Chrompact FFAP-CB column and nitrogen gas is used as the mobile phase. The temperature of oven, the injector and the detector are, 100, 120 and 200 °C, respectively. The details of the analysis system were given elsewhere [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-synergetic effect and byproducts identification[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF]. The by-products produced by the combined system, during the degradation of mixture of VOCs, were identified using a Gas chromatography-mass spectrometry (GC-MS, Perkin Elmer Clarus 500). The GC was equipped with Chrompac TM capillary column FFAP-CB (60 m × 0.32 mm × 1 µm). This column was connected to Mass Spectrometer (MS) devoted to compounds identification. Due to very low concentrations, the sample was focused on a multi-bed cartridge (Carbopack B).

After heating, it was refocused on a trap filled with Carbopack B at 173 K, so-called Cooled Injection System (CIS). The splitless sample Injection method was opted to inject the refocused sample to GC-MS.

Catalyst synthesis

The photocatalytic material, which is provided by Ahlstrom [21], consists of a mixture of TiO2 and SiO2 deposited on Glass Fiber Tissue (GFT) with 2 mm thickness. Indeed, 13 g/m 2 of colloidal silica ensure the fixation of 13 g/m 2 of titanium dioxide nanoparticles (PC500 Millennium). As prepared colloidal TiO2/SiO2 nanoparticle is deposited on GFT by wet impregnation method in pure water. The total specific surface area of the TiO2/SiO2/GFT is 20.6 m 2 .g -1 [21].

Gas stream preparation

As shown in Fig. 1, the TMA, Buty and Isoval were directly injected in liquid state into dry air flow, supplied by the compressed air system, using two syringe pumps (Fig. 1). The heating

A C C E P T E D M A N U S C R I P T
tapes were wrapped on stainless steel pipe at the injection zones to ensure good evaporation of each pollutant. The static mixer, placed upstream the reactor, was used for the homogenization of the mixture. The air flow was controlled by a mass flow controller (MFC, Bronkhorst) calibrated with normal cubic meter per hour on the range 0-10 m 3 .h -1 (Fig. 1).

Non Thermal plasma/photocatalytic reactor

As can be seen in Fig. 2, the rectangular planar glass reactor consists of 1000 mm length and 135 mm width. Inside the glass reactor, a rectangular shape stainless steel plates with 2 mm thickness, were inserted and used as high voltage electrodes. The copper sheet, with 1 mm thickness, wrapped around the planar reactor was employed as a ground electrode. The discharge length was fixed at 800 mm.In planar reactor, TiO2/SiO2/GFT is used as a photocatalyst and eight lamps (Philips PL-S 9 W / 10 / 4 P) were used as a UV source. The details of the high voltage power source and discharge characteristics of the quartz reactor are reported elsewhere [START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF][START_REF] Assadi | Abatement of 3-methylbutanal and trimethylamine with combined plasma and photocatalysis in a continuous planar reactor[END_REF][START_REF] Costa | Study of butyraldehyde degradation and by-products formation by using a surface plasma discharge in pilot scale: Process modeling and simulation of relative humidity effect[END_REF][START_REF] Assadi | A continuous air reactor for photocatalytic degradation of Isovaleraldehyde: Effect of different operating parameters and Chemical degradation pathway[END_REF]. Briefly, in planar DBD reactor, plasma surface discharge was ignited using an AC high voltage power supply with a fixed voltage of 20 kV (peak-to-peak) and 50 Hz frequency. The applied and discharge voltage were measured by a four-channel digital oscilloscope (Lecroy).

To evaluate the performance of the NTP/photocatalyst combined system, the specific energy density (SED), the removal efficiency, and the elimination capacity are calculated using the following equations:

SED ( J/L) = 𝑃( 𝑊) 1000 × 𝑄 (𝑚 3 /ℎ) 3600 (1)
Where SED is the specific energy density (J/L) and Q is the flowrate (m 3 /h).

Removal Efficiency (RE), is calculated by the following equation:

RE (%) = C in -C out C in × 100 (2)
Elimination Capacity (EC), is determined using the following equation:

EC (mg h ⁄ ) = (𝐶 𝑖𝑛 -𝐶 𝑜𝑢𝑡 ) × Q air (3)
Where Cin and Cout are the inlet and outlet pollutant concentrations (mg.m -3 ), respectively.

The synergistic effect (F) of plasma surface discharge and photocatalysis for VOC removal is defined as follows:

F (%) = 〈RE (%)〉 𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑𝑠𝑦𝑠𝑡𝑒𝑚 -(〈RE (%)〉 𝑃𝑙𝑎𝑠𝑚𝑎 + 〈RE (%)〉 𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠 ) (4) A C C E P T E D M A N U S C R I P T
Where 〈RE (%)〉 𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑𝑠𝑦𝑠𝑡𝑒𝑚 is VOC's removal efficiency of photo and plasma catalysis; 〈RE (%)〉 𝑃𝑙𝑎𝑠𝑚𝑎 is VOC's removal efficiency of plasma catalysis alone (%); 〈RE (%)〉 𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑖𝑠 is VOC's removal efficiency of photocatalysis alone (%).

The selectivity of CO and CO2 is calculated as follows:

SCOX (%) = [COX] / (∑[nCi, COV × (Cin,i -Cout,i)] ( 5
)
Where n is number of carbon atoms in the VOCs, COx is the total concentration of CO and CO2.

Results and Discussion

This study is aimed at the removal of different types of VOCs in mixture. However, in order to understand the oxidation behavior of VOCs in a mixture, initial experiments were carried out with single VOC/air mixture in TiO2/SiO2/GFT packed planar reactor with an external UV light and the results are discussed in the following section.

Photocatalytic removal

Single VOC removal

The photocatalytic single VOC removal efficiency of the reactor has been studied for various inlet concentrations. For all model VOCs, the inlet concentrations were varied from 0.6 to 2.45 mmol.m -3 . The elimination capacity of the photocatalytic process, for three different VOCs with various inlet concentrations, is shown in Fig. 3.

For all pollutants, the degradation rate increases with increasing the inlet concentration. It is observed that with low inlet concentration, the degradation rate obeys first-order kinetics, i.e.

the degradation rate is proportional to the inlet concentration. In this case, the limiting steps are the adsorption of pollutants onto the catalyst surface and desorption of the product to the gas phase. It can be suggested that initially not all the photoactive sites are occupied. Indeed an increase in the bulk concentration increases the surface coverage thus increases the VOC degradation efficiency. It is observed that at higher inlet concentration, more than 2 mmol.m -3 , the elimination capacity reaches steady state. This can be explained by the fact that, under higher inlet concentration, the photoactive sites are saturated in the first few minutes, thereafter increase in the inlet concentration does not show any significant changes in the pollutant It is worth to mention that the oxidation kinetic of TMA is much lower than that of other aldehydes. This fact can be explained by the following hypotheses: i) the reactive adsorption of aldehydes on TiO2 surface, and ii) the influence of the byproducts formation. It is widely reported that the aldehydes and ketones are strongly adsorbed on TiO2 surface, even under dark conditions, and lead to aldol reaction [START_REF] Sivachandiran | Non-Thermal Plasma Assisted Regeneration of Acetone Adsorbed TiO2 Surface[END_REF]. In turn, these produced species by photocatalytic process will effectively react with the newly formed species, leading to CO2 formation in the gas phase. It can be suggested that the degradation pathway of TMA is more complicated than aldehydes. Consequently, the degradation of TMA probably produces more by-products which could accumulate in the bulk and on the surface. These by-products, containing mainly nitrogenous and carbonaceous derivatives, could competitively adsorb with initial molecules on the same active sites [START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: Systematic experimental evaluation[END_REF]. Therefore, it can be proposed that the competitive adsorption and by-products formation have significantly affected the TMA removal efficiency as compared to Isoval and Buty.

Multi VOCs removal

Fig. 4 shows the elimination capacity of the mixture of Isoval and Buty with various inlet concentrations like: 25, 50, and 100% of each pollutant.

As can be seen in Fig. 4 (a) and (b), it is observed that the elimination capacity of the individual pollutant is enhanced when mixed together. Interestingly, from monocompound to multicomponent systems, the EC has increased from 92 to 125 mg/h for Isoval when the inlet concentration is about 1.11 mmol/m 3 . It is evidenced that Isoval highly oxidized in multicomponent system than when it was in monocomponent system. This trend can be due to the chemical affinity of pollutants towards the active sites. A better affinity of Isoval allows it to occupy more active sites and then to be more degraded. As shown in Figure 4 a & b, Buty conversion decreased, in the order of VOC mixture with 50 % Buty + 50 % Isoval > VOC mixture with 75 % Buty + 25 % Isoval. For two levels of inlet concentration, Buty conversion in multicomponent system is higher than single component at the same conditions, indicating that there is a synergetic effect of multicomponent system. This effect will be confirmed later by the identification of the byproducts formed in multicomponent system.

Plasma/photocatalytic process

A C C E P T E D M A N U S C R I P T
Here we summarize the results of isovaleraldehyde and butyraldehyde treatment with plasma alone and plasma/TiO2 catalytic reactor in multicomponent and monocomponent systems.

Moreover, to evaluate the mutual effects of mixture removal efficiency, the Trimethylamine (TMA), another VOC of amine group, has also been mixed with Isoval and tested under the same conditions as like Buty/Isoval mixture.

Photo/plasma catalytic removal of binary mixture of 50% Buty + 50% Isoval

Fig. 5 shows the removal efficiencies of Isoval and Buty in single and multi VOC systems using NPSD plasma, photocatalysis and a combined process. It is noticed that the mixing of pollutants has decreased the removal efficiency as compared to the treatment of a single component.

Mingling of Isoval and Buty leads to reduce their maximum removal efficiencies by 30 and 20%, respectively.

Therefore, it can be suggested that the treating of mixed VOCs could produce more byproducts, thus the consuming electrons or radicals produced by the silent discharge.

In one hand, as reported in Fig. 4, the photocatalytic treatment of mixture of pollutant significantly improves the removal efficiency. On the other hand, the removal efficiency of multicomponent system has been decreased by 50% in NPSD on TiO2/SiO2/GFT as compared to a single VOC. This can be explained by the fact that the competition occurs between the pollutants with the active species such O° and HO°.

It is interesting to note that a synergetic effect is observed, irrespective of the mixture, when NPSD and photocatalyst is combined, At lab and in pilot scales, previous studies have shown that combining plasma with photocatalysis has induced a synergetic effect on the oxidation efficiency of VOC in monocomponent system with flow rate varied from 1 to 500 m 3 /h [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-synergetic effect and byproducts identification[END_REF][START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF]. Indeed, in this study, the synergetic effect is observed for multicomponent system. Moreover, the factor of synergetic effect (F %) for each VOC in mixture is about 8-10 % under the same conditions when plasma and photocatalysis are performed individually. This result is in good agreement with the report in the literature [START_REF] Karuppiah | Abatement of mixture of volatile organic compounds (VOCs) in a catalytic non-thermal plasma reactor[END_REF]. And also, in accordance with the observations of Piotrowska et al. It is reported that the elimination of n-butyl acetate was better in a mixture of aromatics and alcohols [START_REF] Musialik-Piotrowska | Catalytic oxidation of trichloroethylene in twocomponent mixtures with selected volatile organic compounds[END_REF].

The synergetic effect of NPSD/photocatalytic process can be explained by the following hypotheses:

A C C E P T E D M A N U S C R I P T
• Action of free radicals (N • , O • , H • , OH • ) produced by the dissociation of molecules like N2, O2, H2O. These actives species can interact directly with Buty and Isoval molecules in multicomponent system [START_REF] Fridman | Non-thermal atmospheric pressure discharges[END_REF][START_REF] Chen | Oxidation of toluene by dielectric barrier discharge with photo-catalytic electrode[END_REF][START_REF] Kim | Advanced oxidation of aromatic VOCs using a pilot system with electron beam-catalyst coupling[END_REF].

• Regeneration of TiO2 surface by DBD surface discharge [START_REF] Maxime | Removal of gas-phase ammonia and hydrogen sulfide using photocatalysis, nonthermal plasma, and combined plasma and photocatalysis at pilot scale[END_REF][START_REF] Neyts | Plasma-Surface Interactions in Plasma Catalysis[END_REF] due to ionic winds. This would lead to the more active sites available for VOCs adsorption and subsequent enhanced conversion.

• Decomposition of ozone on the surface of TiO2 which gives more active species such as O •

for Buty and Isoval oxidation [START_REF] Klett | Acetaldehyde removal using an atmospheric non-thermal plasma combined with a packed bed: Role of the adsorption process[END_REF][START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF].

Removal of Isoval and TMA in single component and in binary mixture (50% TMA+ 50% Isoval)

In this section, in order to understand the influence of TMA addition on Isoval removal process and vice-versa, the binary mixture of 50% TMA + 50% Isoval has been prepared and treated using plasma, photocatalytic and combined process. The concentration of TMA and Isoval have been varied between 0.5 to 1.7 mmol/Nm 3 . Fig. 6 shows the removal efficiencies (%) of Isoval and TMA in mono and multicomponent systems. Interestingly, the removal efficiencies of TMA decrease with increasing the Isoval concentration. This observation is contrary to the result observed with binary mixture of 50% Isoval+ 50% Buty. This can be attributed the chemical bond strength and molecular stability of the TMA [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-synergetic effect and byproducts identification[END_REF]. As can be seen in Fig. 6, when

Isoval is mixed with TMA the removal efficiency decreased from 30 to 13% as compared to single component.

As seen from the data presented in previous publication [START_REF] Assadi | Use of DBD plasma, photocatalysis, and combined DBD plasma/photocatalysis in a continuous annular reactor for isovaleraldehyde elimination-synergetic effect and byproducts identification[END_REF] and in the present study, the elimination capacity of any pollutant with combined system was higher than the sum of elimination capacities with plasma NPSD and photocatalysis performed separately.

As reported in Fig. 6, synergistic effect is observed in combined system. Indeed, factor of synergistic effect (F%) is about 10% for Isoval and TMA of single and binary mixture.

End products analysis

It is observed that the CO formation in plasma and photocatalytic combined system, irrespective of the inlet concentration, is negligible. Similar results have also been reported for the photocatalytic removal of cyclohexane [START_REF] Assadi | A study of pollution removal in exhaust gases from animal quartering centers by combining photocatalysis with surface discharge plasma: from pilot to industrial scale[END_REF], trichloroethylene [START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: Systematic experimental evaluation[END_REF], and formaldehyde [START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF].

A C C E P T E D M A N U S C R I P T

CO2 selectivity

The CO2 selectivity of each tested pollutant was shown in figure 7. It is widely reported that the photocatalytic process was a series of successive reactions; the competition towards the active sites of the catalyst becomes more important.

The CO2 selectivity of Isoval (with DBD plasma alone) was about 43% and this value was about 62% for the combined DBD/photo catalysis processes. Consequently, this behavior is due to the photocatalytic activity of TiO2 in the presence of UV radiation. In fact, when the two processes are combined, carbon dioxide selectivity was improved as compared to DBD plasma alone, irrespective of the pollutant tested [START_REF] Chen | Oxidation of toluene by dielectric barrier discharge with photo-catalytic electrode[END_REF][START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF] Using DBD plasma alone, the low values of overall CO2 selectivity were explained by the fact that the quantities of generated reactive species become insufficient, leading to a limited chemical reaction. These results are in line with those in the literature related to formaldehyde [START_REF] Zhu | Plasma-catalytic removal of formaldehyde over Cu-Ce catalysts in a dielectric barrier discharge reactor[END_REF] and isovaleraldehyde removal [START_REF] Palau | Isovaleraldehyde degradation using UV photocatalytic and dielectric barrier discharge reactors, and their combinations[END_REF].

On the other hand, combined DBD plasma/photocatalysis significantly enhances the CO2 selectivity, as compared to DBD plasma alone. This is attributed to the formation of more reactive species due to the presence of TiO2 in the plasma discharge zone. Moreover, UV radiation is able to activate ozone on the surface of TiO2 to improve mineralization.

Consequently, possible pathways for pollutant removal in combined DBD plasma/photocatalysis mainly include gas-phase radical attacks due to various other species such as high-energy electrons, excited molecules or radicals.

In fact, from single component to multicomponent system, an inhibition effect for the collectivity of three processes tested is observed. On the other hand, the mixture (TMA and Isoval) seems to be more affected in multicomponent systems. This behavior may be attributed to the mineralization of TMA which is less than that for Buty due to the less conversion of TMA to intermediate species compared to Buty. Thus, the Buty intermediates have more probability to be attacked by electrons and active species.

Ozone formation

Ozone is well known, as an unwanted and inevitable, by-product during plasma oxidation.

Moreover, it plays an important role during VOC oxidation. The atomic oxygen is formed by in-situ decomposition of ozone and reacts with the pollutants. The residual ozone concentration in the reactor exhaust for each process (plasma alone and the combined process) is reported in Figure 8. It is can be seen that, from mono to multicomponent system, with plasma/TiO2 reactor, ozone concentration is decreased as compared to plasma alone. This can be explained by the fact that TiO2 surface is activated by UV radiation, which can reduce the formation of ozone or favor its destruction [START_REF] Sivachandiran | Isopropanol saturated TiO2 surface regeneration by non-thermal plasma: Influence of air relative humidity[END_REF]. On the other hand, results with multicomponent systems illustrate that the active species (O•) generated by plasma, are able to oxidize by-products and to react with oxygen molecules to form ozone [START_REF] Aouadi | MnOx/TiO2 Catalysts for VOCs Abatement by Coupling Non-thermal Plasma and Photocatalysis[END_REF].

By-products formation

Isoval and Buty in multicomponent systems

The formation of by-products in plasma/TiO2 reactor, for the selected VOCs, was investigated.

Figure 9 shows the by-products detected when the treated gas concentration was around 1 mmol/m 3 . As can be seen in Fig. 9, about seven by-products have been formed when the plasma and the photocatalysis is combined which is in line with the finding of Gharib et al., [START_REF] Gharib-Abou Ghaida | Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF]. When only Buty is treated by plasma/photocatalysis system along with non-treated Buty, the following intermediates have been identified: acetone (1), ethyl acetate (2), acetic acid (4), propionic acid (5) and butyric acid (6) are detected. It is worth mentioning that the similar kind of intermediates, as in plasma/TiO2 reactor, have also been detected when plasma was used alone. Interestingly, at the reactor downstream, ethanol (3), isobutyric acid (7) and residual

Isoval have been identified when Isoval is added to the mixture.

Isoval and TMA multicomponent system

In our previous study, a special attention was devoted to the analysis of reaction products during TMA degradation in a single component system [START_REF] Gharib-Abou Ghaida | Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF] and the following by-products are identified: (Dimethylamino) acetonitrile, N, N-Dimethyl formamide, nitromethane, acetone, acetic acid, methanol, and ethanol [START_REF] Gharib-Abou Ghaida | Association of surface dielectric barrier discharge and photocatalysis in continuous reactor at pilot scale: Butyraldehyde oxidation, by-products identification and ozone valorization[END_REF]. Moreover, the oxidation of TMA leads to the formation of NOx. On the contrary, as reported in Fig. 10, when the TMA is incorporated with Isoval, the detected by-products are: acetone, isobutyric acid, methanol, acetic acid and CO2. Importantly, carbon-nitrogen compounds are not identified as noticed in single component system. This can be attributed to the recombination of by-products (TMA or/and Isoval). The main findings can be summarized as follows:

(1) The degradation of the two compounds in the mixture was inhibited in comparison to the degradation of the pure compounds. The photocatalytic oxidation step of the two aldehydes was found to be competitive. The highly -volatile compound (Isoval) was less adsorbed on the catalyst surface however more conversion is observed, probably due to the influence of less by-products formation.

(2)

The nature of all by-products from VOCs mixture decomposition were successfully identified using GC-MS.

(3) For each operating mixture of pollutants, we have unambiguously shown that there was a synergetic effect for the pollutants oxidation and the combination of plasma and TiO2

shown promising results for the removal of low concentrated volatile organic compounds from air.

A C C E P T E D

M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T

  elimination capacity. The similar behavior was also observed for Isoval and TMA, these results are in line with those reported previously[START_REF] Assadi | A continuous air reactor for photocatalytic degradation of Isovaleraldehyde: Effect of different operating parameters and Chemical degradation pathway[END_REF].

  In this study, removal of volatile organic compounds, in single component and multicomponent systems by surface discharge plasma/TiO2 combination, were systematically investigated in a continuous flow reactor. Aldehydes (Isoval and Buty) and trimethylamine could be effectively removed by combined systems.

Fig 1 :

 1 Fig 1: Experimental set-up.

Fig. 2 :Figure 3 :Figure 4 :Figure 5 :

 2345 Fig. 2: Schema (a) and cross sectional drawing (b) of the planar reactor. A C C E P T E D M A N U S C R I P T

Figure 6 :

 6 Figure 6: Removal efficiencies of Isoval and TMA in mono and multicomponent systems using plasma NPSD, photocatalysis and a combined processes (SDE = 14 J L -1 , UV intensity = 20 Wm -2 ).

Figure 7 :

 7 Figure 7: CO2 selectivity (%) of three pollutants in single component and multicomponent systems using NPSD, photocatalysis and a combined process (SDE = 14 J L -1 , UV intensity = 20 Wm -2 ).

Figure 8 :

 8 Figure 8: Ozone concentration produced (ppm) during the degradation of three pollutants in mono and multicomponent systems (50% Isoval+50% Buty & 50% Isoval+ 50% TMA) using NPSD, and a plasma/TiO2 reactor (SDE = 14 J L -1 , UV intensity = 20 Wm -2 ).

Figure 9 :

 9 Figure 9: By-products identified during the oxidation of a mixture of 50% Buty and 50% Isoval in the combined system.

Figure 10 :

 10 Figure 10: By-products generated during the oxidation of a mixture of 50% TMA and 50% Isoval, using in the combined system. A C C E P T E D M A N U S C R I P T

Acknowledgements

The authors would like to thank the Ecole Nationale Supérieure de Chimie de Rennes (ENSCR France) for the financial support of this work.


Chao Li

Guoxiang Wang
email: wangguoxiang@nbu.edu.cn

Xianghua Zhang

Yawen Zhang

Investigation of laser-irradiated structure evolution and surface modification by in situ Micro-Raman spectroscopy

Keywords: Thin films, Raman spectra, Laser irradiation, Metastable phases

The structure evolution and surface modification of the ZnSb, ZnSb-Bi and ZnSb-NiO phase-change films are detected by in situ Raman laser irradiation with the different laser power and irradiation time. The results show that undoped ZnSb films exhibit a phase transformation from amorphous to metastable phases at 250mW. The Bi-doped ZnSb films exhibit two different crystallization behaviors as the laser power increases. The films containing low Bi-doping concentration exhibit an amorphous-to-metastable phase transition, while the films containing high Bi-doping concentration directly crystallize into a mixture of new Bi-related crystalline phases and metastable ZnSb phases. As for ZnO-doped ZnSb films, the structure evolution encounters an amorphous phase, metastable ZnSb phase or mixture of both.

Identifying the structure evolution in the metastable phase by in-situ Raman measurement method is of great significance for phase change memory application.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

2 Phase-change memory (PCM) has the two-level data storage [START_REF] Wuttig | Phase-change materials for rewriteable data storage[END_REF], which is due to the rapid and reversible switching of phase-change materials [START_REF] Loke | Breaking the speed limits of phase-change memory[END_REF] between amorphous phase of high resistance state and crystalline phase of low resistance state upon heating [START_REF] Burr | Phase change memory technology[END_REF]. The data storage capacity of two-level is low, which has become the major obstacle in the development of multi-level data storage. It is well known that the conventional germanium-antimony-tellurium (Ge-Sb-Te) phase-change films have three-state, such as amorphous, cubic and trigonal phases, which can lead to multi-level data storage properties [START_REF] Lee | Microscopic mechanism of doping-induced kinetically constrained crystallization in phase-change materials[END_REF]. However, the poor thermal stability and low crystalline resistance of GST is currently still a big challenge to apply in multi-level data storage.

In comparison, the novel phase change materials based on Te-free Zn-Sb ternary system have been extensively used as phase-change layer in non-volatile memory device due to its better thermal stability and higher crystalline resistance. Among the Zn-Sb phase change materials, the crystal structure of ZnSb (the ratio of Zn to Sb is 1:1) alloy has been widely used and investigated [START_REF] Park | Phase transition characteristics and nonvolatile memory device performance of Zn x Sb 100-x alloys[END_REF][START_REF] Chen | Crystallization behaviors of Zn x Sb 100-x thin films for ultralong data retention phase change memory applications[END_REF]. It exhibits a metastable orthorhombic structure and a stable hexagonal ZnSb structure confirmed d by X-ray diffraction and transmission electron microscopy (TEM) in order to distinguish the distinct state [START_REF] Chen | Crystallization behaviors of Zn x Sb 100-x thin films for ultralong data retention phase change memory applications[END_REF]. However, the temperature span of intermediate resistance state for ZnSb films is rather narrow, which make the control of the resistance state being very difficult. Wang et al. [START_REF] Wang | Influence of Bi doping upon the phase change characteristics of Ge 2 Sb 2 Te 5[END_REF] reported that the addition of Bi into the GeTe-Sb 2 Te 3 could result in the decreased crystallization temperature, which was correlated with the reduction in the crystalline activation energy. Thus, we introduce the Bi into ZnSb to decrease the phase transformation temperature so that we adopt in situ continuous Raman irradiation to observe the vibration modes in the metastable phases.

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

3

The Raman spectra can record information based on atomic arrangement in amorphous marks in a crystalline layer of a phase-change material [8]. Due to the large difference in the optical reflectivity between the amorphous and crystalline state, structural information can be obtained based on the change in Raman mode, position and intensity [START_REF] Fu | Structural evolution of Ge 2 Sb 2 Te 5 films under the 488nm laser irradiation[END_REF]. When the laser irradiation power is relatively low, no obvious compositional change and no Raman vibration mode can be observed in an exposed region. Then, a bright spot may appear and become larger in size as the laser power increases beyond its crystallization temperature. However, the ablation may happen in the case of exposed samples with a higher laser power, because the laser irradiation induces temperature above its melting point [START_REF] Gawelda | MANUSCRIPT 9 Dynamics of laser-induced phase switching in GeTe films[END_REF].

Thus, we adjust laser power to irradiate the ZnSb-based films in order to observe the structure evolution in terms of its Raman mode vibration. The paper is devoted to the in situ Raman studies of laser irradiation effect on amorphous materials intending to show and analyze phase changes in these materials with the evidence for sequential transitions from one phase to metastable phase with increasing laser power. Through systematic investigations on the relation between Raman mode variation and laser power, we confirm the existence of the intermediate phase.

The Bi-doped and NiO-doped ZnSb films with a thickness of 200nm were deposited on Si substrate by using magnetron co-sputtering method with separate Bi/NiO and ZnSb targets, respectively. The chamber was evacuated to 2 × 10 -5 Pa, and then Ar gas was introduced at 0.3 Pa. Undoped ZnSb films with the same thickness were also prepared for comparison. The composition of the ZnSb-based films was determined by energy dispersive spectroscopy (EDS). The structure evolution was monitored and measured by a laser confocal Raman spectra (Renishaw inVia,

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 4
Gloucestershire, UK). The irradiation source for inducing phase change was an Ar + laser with an wavelength of 785nm and maximum laser power of 500mW while selecting a grating (~1200 l/mm) corresponding to the laser source. We altered scan conditions, the exposure time and laser power, by changing the acquisition tab of spectral acquisition setup. The exposure time is 15s for laser irradiation. The 8 different laser powers (~0.25, 0.5, 2.5, 5, 25, 50, 250, 500mW) were set available for laser irradiation. Since the initial laser power is very small, resulting in a poor signal-noise ratio (SNR) of the spectra, it is difficult to obtain real smooth Raman curve. The morphological characteristics of the films after Raman laser irradiation was observed by using a super-long depth-of-view optical microscope (VHX-1000E, Keyence) in a 2-D mode.

Fig. 1 (a) shows the Raman spectra of ZnSb films irradiated by different laser power for 15s. It is found that a broad peak G at ~155.7 cm -1 is reasonably assigned to the vibration of numerous wrong bonds formed in the as-deposited state. With the laser power increasing from 0.25 to 50mW, the amorphous nature can be kept. The higher laser power irradiation results in a reorganization of the amorphous network with the rupture of the wrong bonds and formation of energetically more stable heteropolar Zn-Sb bonds. Subsequently, the two crystallization peaks A (108.6 cm -1 ) and B (144.6 cm -1 ) appear at the laser power of 250 mW. The two peaks shift towards a low wavenumber at 140.6 cm -1 and 104.3 cm -1 at 500 mW, respectively. Comparing the peak positions with that in the Raman spectra for thermal-annealed ZnSb samples [START_REF] Li | Suppression for an intermediate phase in ZnSb films by NiO-doping[END_REF], we confirm that the Raman peaks attribution roughly coincides with the precipitation metastable ZnSb phases.

The structure evolution from amorphous to crystalline is observed clearly by the change in the surface morphology as shown in Fig. 1

(b). It records the laser-induced

M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT

5 surface modification after in situ laser irradiation on the film surface. We observe the smooth surface morphologies of amorphous ZnSb films irradiated at 0.25, 0.5, 2.5, 5, 25 and 50mW. As the laser power increases to 250mW, the ZnSb film startes to crystallize, corresponding to the metastable phase. The surface morphology has an obvious long crystallization area (see dotted yellow circle). With further irradiating up to 500mW, it is noteworthy that the surface morphology of ZnSb film changes significantly and a dark center appeared, implying that the element ablation may happen so that the Si substrate can be exposed.

For Bi-doped ZnSb films, the Raman spectra with different laser powers from 0.25 to 500mW for 15s were shown in Figs. 2(a)-(c). In the case of a lower Bi-doping concentration as shown in Fig. 2(a), it is found that the amorphous broad peak G is also kept until 50mW, while the position of the Raman peaks (A and B) in the crystalline state is the similar as that in undoped ZnSb, indicating that the metastable phase is also precipitated in (ZnSb) 96.84 (Bi) 3.16 . Noteworthy, there are two new peaks C and D appeared. This may be related to the vibration of Bi-related crystalline phases. From the results in Fig. 2(b) and (c), the Raman peaks of ZnSb films containing high Bi-doping concentration are very different from that in ZnSb films containing low Bi-doping concentration. Obviously, the (ZnSb) 82.75 Bi 17.25 and (ZnSb) 75.93 Bi 24.07 films exhibit crystallization behavior at the initial low laser power of 0.25mW. The result predicts that the films have been crystallized at room temperature with doping excess Bi, which would deteriorate the thermal stability and cause crystallization. This is well agreement with the role of Bi investigated in previous reports on Bi-GST [START_REF] Wang | Influence of Bi doping upon the phase change characteristics of Ge 2 Sb 2 Te 5[END_REF] and Bi-Sb-Te [START_REF] Simpson | Reduction in crystallization time of Sb:Te films through addition of Bi[END_REF]. With the laser power increasing from 0.25 to 50 mW gradually, we can determine that the intensity of Raman peaks A and B is decreased compared with that in low Bi-doping concentration ZnSb. Different from 2(c) exhibits a sharper and stronger intensity while peaks A and B become weaker and broader. This illustrates that the initial metastable ZnSb phase is suppressed and new crystalline phases become dominated due to higher Bi-doping concentration. The Bi, which lies in the same group as Sb, has larger atomic radius (γ Bi =0.163nm, γ Sb =0.163nm) [START_REF] Wang | Te-based chalcogenide films with high thermal stability for phase change memory[END_REF], which is easier to enter the network structure of ZnSb film and replace Sb position. Thus, it may result in the generation of new Raman peaks originated from the vibrational modes of (Bi,Sb) and/or ZnSbBi crystalline phases. Fig. 3 (a) shows the Raman spectra of NiO-doped ZnSb films irradiated at different laser powers for 15s, respectively. The broad Raman peak G of the NiO-ZnSb films irradiated at 0.25mW is observed, which implies the presence of an amorphous phase. With the laser power increasing, the amorphous nature of (ZnSb) 94 (NiO) 6 films can be kept until 50mW. When the film is irradiated at 250mW, a mixture of amorphous and crystalline state has been appeared, just as the small peak A' and broad peak B'. Then, it is completely transferred into the metastable ZnSb phase at 500mW. The variation is very different from Fig. 1 and Fig. 2, illustrating that the addition of NiO into ZnSb improve the thermal stability, but the metastable phase can still be detected at higher 500mW power. The corresponding surface modification has been depicted in Fig. 3(b). The amorphous smooth surface is kept until 50mW. A small crystallization area is observed at 250mW, implying partial crystallization. With the laser power increasing, the crystallization area becomes large obviously.

In summary, the Raman mode vibration of the ZnSb-based films exposure to the different laser powers from 0.25mW to 500mW has been studied by using Renishaw Raman Spectrometer. The results show that the amorphous and crystallization states

6 Fig. 2 (

 62 Fig.2(b), the Raman peaks C and D of ZnSb films containing 24.07at% Bi-doping

Fig. 1 (Fig. 2

 12 Figure captions

  Fig.3

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Grant No. 61604083), the Natural Science Foundation of Ningbo City, China (Grant No. 2017A610094) and was sponsored by the K. C. Wong Magna Fund in Ningbo University.


Viatcheslav Jouikov
email: vjouikov@univ-rennes1.fr

Jacques Simonet

An innovative anodic approach for efficient immobilization of an acetylenic triple bond via silicon-carbon bond scission

Keywords: Si-C bond, propynyl, anodic cleavage of organosilanes, grafting to gold, paired anodic/cathodic grafting

The anodic scission of silicon-carbon bonds is successfully applied for the efficient covalent coverage of conducting solid surfaces with propynyl groups under quite simple conditions.

The anodic cleavage of propynyl trimethylsilanes (Me) 3 Si-C≡C-CH 2 R in acetonitrile containing tetraalkylammonium salts leads to a dense grafting of the propyne C≡C-CH 2 R units onto glassy carbon, gold, and platinum surfaces. When R is a good leaving group (e.g. bromide), versatile electrodes might be prepared upon their polarization alternately (i) anodically and (ii) cathodically. A first example in which the anodic acetylation is immediately followed by cathodic immobilization of graphene and fullerene is reported.

Introduction

Glassy carbon is still considered as an efficient electrode material within the cathodic domain [START_REF] Lund | Organic Electrochemistry[END_REF][START_REF] Mccreery | Advanced carbon electrode materials for molecular electrochemistry[END_REF]. At the same time, most of the processes of fixation of organic moieties at the GC interface have been reported to occur through the addition of electrogenerated free radicals, mainly within the cathodic range [START_REF] Pinson | Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts[END_REF][START_REF] Podvorica | Aryl Diazonium Salts[END_REF][START_REF] Belanger | Electrografting: a powerful method for surface modification[END_REF]. Additionally, grafting processes with the radicals generated from the oxidation of alcohols [START_REF] Maeda | Direct covalent modification of glassy carbon surfaces with 1-alkanols by electrochemical oxidation[END_REF], amines [START_REF] Barbier | Electrochemical bonding of amines to carbon fiber surfaces toward improved carbon-epoxy composites[END_REF], and carboxylates [START_REF] Andrieux | Derivatization of carbon surfaces by anodic oxidation of arylacetates. Electrochemical manipulation of the grafted films[END_REF] have been reported. On the other hand, the intrinsic cathodic charge of various carbon materials gives them a polynucleophlic character [START_REF] Simonet | Electrochemical behaviour of graphite cathodes in the presence of tetraalkylammonium cations[END_REF][START_REF] Jouikov | Doping of cathodically polarized glassy carbon by natural graphite. A simple procedure for overlaying different carbons with electrochemically modifiable graphene layers[END_REF][START_REF] Jouikov | Novel method for grafting alkyl chains onto glassy carbon. Application to the easy immobilization of ferrocene used as redox probe[END_REF], making possible many efficient grafting processes reviewed recently [START_REF] Jouikov | Electrochemical conversion of glassy carbon into a polynucleophilic reactive material. Applications for carbon chemical functionalization. A mini review[END_REF].

The fact that different forms of carbon can be anodically charged in the presence of suitable reactive anions [START_REF] Simonet | Electro-catalysis at chemically modified solid surfaces[END_REF][START_REF] Jouikov | [END_REF] has been much less exploited. For instance, graphite can efficiently incorporate bromine in a manner obviously similar to the insertion of Br -into

A C C E P T E D

M A N U S C R I P T 2 anodically charged carbon [START_REF] Simonet | Electro-catalysis at chemically modified solid surfaces[END_REF]. From this work emerges the idea of anodic generation of electrophilic centers at the carbon surface. Indeed, the capability of a wide variety of carbons (e.g. glassy carbon, graphites and fullerenes) to be anodically charged at potentials more positive than +1.5 V vs. Ag/AgCl/KCl sat enables the formation of electrophilic centers on their surface which, in the presence of trimethylsilyl (TMS) substituted derivatives R-TMS, opens up new possibilities for modifying carbonaceous interfaces. This concept was considered in a recent work [START_REF] Jouikov | [END_REF] originally developed to enable allylation and benzylation of different carbon materials. This process stresses the elevated kinetic instability of silicenium cations, in particular TMS + , due to their extreme electrophilicity. Very efficient reactions of such species with a large variety of both πand -electron donors have been reported.

Experimental

The present contribution focuses on the oxidation of trimethylsilylated compounds TMS-R with the R substituent being 1-propyne (1), 3-bromo-1-propyne (2) and 4-iodophenyl-1-propyne (3). All products are commercially available from Alfa Aesar. Anhydrous polar solventsacetonitrile (ACN) for oxidation and N,N-dimethylformamide (DMF) for cathodic processeswere used to prepare 0.1 M solutions of electrochemical grade tetramethylammonium and tetrabutylammonium salts (essentially with BF 4 -, ClO 4 -and PF 6 anions). Fullerene C 60 (> 99.0 % purity) was purchased from TGI. Graphene was obtained in the laboratory by cathodic exfoliation of natural graphite [START_REF] Simonet | Electro-catalysis at chemically modified solid surfaces[END_REF].

In this first contribution on the use of ethynylic TMS derivatives for the efficient decoration of solid electrodes, only smooth glassy carbon and gold surfaces (0.8 mm 2 ) are described. The electrodes were polished with silicon carbide paper (Struers 500 and 1200) and consecutively rinsed with water, ethyl alcohol and acetone.

All potentials are referred to Ag/AgCl/KCl sat system in water.

A C C E P T E D

M A N U S C R I P T 3

Results

Anodic scission at gold and carbon

It is established hereafter that organotrimethylsilanes incorporating the TMSsubstituted ethynyl unit are anodically oxidizable at GC and gold in acetonitrile containing TMABF 4 (Scheme 1A). This process leads to the immobilization of 1-propynyl groups on gold to produce a very stable and compact surface coverage as shown by a sharp first peak (Figure 1 B, E p = +1.54 V) while the second scan does not exhibit any noticeable oxidation step. FTIR spectra of the propynyl-modified interfaces obtained in this way are shown in Figure 2. A rough estimation of the coverage based on current integration during the first scan leads to a global radical production of (10 ± 5)10 -9 mol cm -2 that could be associated with partial immobilization of the propynyl radical on gold, resulting in strong inhibition of its surface. This large amount does not exclude partial addition of transient radicals onto the primarily grafted unsaturated organic layer. By contrast, the oxidation of 1 at a carbon electrode was found to occur through a diffusion-controlled process, since the oxidation peak remains proportional to both the substrate concentration and the square root of the scan rate.

The large difference in behaviour between gold and glassy carbon as anode materials could be explained by the catalytic effect of gold producing a large number of free radicals at the surface.

Grafting of a carbon-carbon triple bond onto Au and carbon is supported by the fact that the added unsaturated layers chemically bind iodine and bromine. Cathodic reduction of the resulting diiodo form (observed at -0.49 V exclusively during the first scan) attests to the efficiency of immobilization. EDS mapping of the surfaces modified by chemical addition of halogens shows a dense coverage of the carbon surface with iodine and bromine atoms.

Additionally, Figure 1 (C andE) shows the FTIR and EIS responses of the grafted layer supporting an efficient coverage of the anodes.

Coupled reduction/oxidation processes of versatile TMS-C≡C-CH 2 X series

The second approach concerns the behaviour of ethynyl structures possessing two electroactive groups that can be activated at opposite polarizations of the working electrode By contrast, the anodic oxidation of the TMS-capped propargylic layer covalently attached to the Au surface by cathodic grafting described above (Scheme 1 C, D) was shown to be successful (Figure 3, A2) as evidenced by the symmetric anodic peak (+1.54 V) assigned to scission of the Si-C bond, as shown in 3.1. It is thought that a fairly thick layer was formed after 3 cathodic scans (up to -2 V). Under these conditions, the total elimination of the TMS groups from the modified interface was achieved at +1.54 V after several anodic scans. The nature of the resulting layer has not yet been assessed.

Extension of this grafting mode using TMS group

Quite importantly, the easy and selective grafting of the propynyl moiety described above can be extended to the immobilization of many non-oxidizable groups connected to the {-C≡C-CH 2 -} motif. One such example is the oxidation of compound 3 at smooth glassy carbon, performed at E 1/2 [START_REF] Pinson | Attachment of organic layers to conductive or semiconductive surfaces by reduction of diazonium salts[END_REF]. The surface obtained after repetitive scanning up to 2.2 V was found to contain iodine in the grafted layer. Cathodic voltammetry on the surface modified in this way shows an irreversible step (E 1/2 = -1.22 V) observed during the first scan only.

Integration of this cathodic peak permitted us to estimate that the surface coverage with Ph-I amounts to (3±0.5)  10 -9 mol cm -2 . Additionally, EDS mapping of this interface before the reduction confirms the presence of a large number of iodine atoms in the grafted layer.

Conclusion

Anodic electrochemistry of organotrimethylsilanes R-TMS either at carbon (exploiting its positive doping) or at gold (direct oxidation) surfaces may offer a wide palette of new possibilities for modifying solid interfaces. Due to the high versatility of TMS units as

ACCEPTED MANUSCRIPT

A C C E P T E D M A N U S C R I P T 5 excellent cationic leaving groups at positively charged solid surfaces, the efficient anodic immobilization of the acetylenic triple bond should be emphasised. Additionally, using the same process, the grafting of activated propynyl moieties, eventually decorated with an efficient anionic leaving group, was revealed to be successful, allowing, through facile anodic/cathodic sequential processes, efficient and innovative decoration of gold, and has been applied to different forms of carbon as well. These preliminary results show that other metals like platinum, palladium and titanium seem to present similar properties. Therefore, the present contribution can be seen as an introduction to the further development of promising paired processes of double immobilization using an ambifunctional reagent to build layers out of electron-rich chains in tight contact between two conducting solid materials (e.g. carbon vs. carbon, or gold vs. carbon as explained above).

A C C E P T E D M A N U S C R I P T

Legends for Figures

(

  Scheme 1B. The latter can react in situ with radicalophiles deposited on the layer or present in the solution. The reactions with graphene and fullerene C 60 are presented in Figure3(B1 and

  Figure 1. Anodic modification of solid surfaces with TMS-R compounds. (A) Propynation at gold through a radical process. (B) Anodic oxidation of TMS-propyne (6 mmol L -1 ) at a gold microelectrode (0.8 mm 2 ) in AN/0.1 M TMABF 4 . Two first scans. Scan rate 50 mV s -1 . (C) FTIR spectrum of the layer obtained from the anodic oxidation of TMS-propyne 1 (12 mmol L -1 ) at +1.54 V at smooth gold. (D) For comparison, FTIR response of Au surface before addition. (E) Nyquist plots of the reduction of chloranil (3 mmol L -1 ) in ACN/0.1 mmol L -1 TBAPF 6 at a smooth Au electrode with freshly immobilized propyne layer. (F) Same as in (E), at the non-modified surface. E= -1.16 V, ΔE = 10 mV. Frequency range from 0.306 MHz to 43 mHz.

Figure 2 .

 2 Figure 2. FTIR spectra of GC interface: (A) bare, (B) GC with grafted -C≡CCH 2 Br, (C) GC with grafted -CH 2 C≡CMe. (D) GC with grafted -CH 2 C≡CTMS. (a) 2850-2960 cm -1 ( CH 3 ) and 2960-3000 cm -1  CH 2 ), (b) 2180--2210 cm -1  CC ). (c) 1450, 1380 cm -1 ( CH 2 , CH 3 ) and 1410, 1320 cm -1 ( C-Si ). (d) 1150 cm -1 ( CH 2 -Br ).

Figure 3 .Scheme 1 .

 31 Figure 3. Principle of a double cleavage/immobilization at smooth gold with a typical TMS-RX compound: reduction step (A1) followed by oxidation (A2) of the primarily grafted layer. After its sonication and rinsing, cathodic reduction of an acceptor (or any radicalophile) deposited at the modified interface can be performed. (A1) Three consecutive scans of the reduction of TMS-C≡C-CH 2 Br (13 mmol L -1 in DMF/0.1 M TMABF 4 ) at a smooth gold electrode (0.8 mm 2 ). Scan rate 50 mV s -1 . (A2) The immobilized TMS-C≡C-CH 2 -layer is now anodized in ACN/TMABF 4 . The anodic signal assigned to oxidation of the TMS-carbon bond progressively vanishes upon scanning. (B1), (B2) The electrodes prepared according to A1 are covered and rubbed with C 60 or graphene and cathodically reduced in DMF leading to an external addition of radicalophilic particles. See text.

  Figure 1

  The Chemistry of Organic Silicon Compounds, Eds. Z. Rappoport, Y. Apeloig, John Wiley and Sons, Chichester, UK, 1998, V. 2, 575. [16] M. Ochiai, E. Fujita, M. Arimoto, H. Yamaguchi, A new synthesis of allyl nitrates from allylmetal (group IVb) compounds and thallium (III) nitrate, Chem. Pharm. Bull.

	6
	[15] P.D. Lickiss, In: 32 (1984) 887.
	[17] M.V. Sheridan, K. Lam, W.E. Geiger, An anodic method for covalent attachment of
	molecules to electrodes through an ethynyl linkage, J. Am. Chem. Soc. 135 (2013)
	2939.
	[18] M.V. Sheridan, K. Lam, W.E. Geiger, Covalent attachment of porphyrins and
	ferrocenes to electrode surfaces through direct anodic oxidation of terminal ethynyl
	groups, Angew. Chem. Int. Ed. 52 (2013) 12897.
	[19] J.-I. Yoshida, T. Murata, S. Isoe, Oxidative cleavage of carbon-silicon bond in
	allylsilanes and benzylsilanes, Tetrahedron Lett. 27/29 (1986) 3373.

Graphical abstract

ACCEPTED MANUSCRIPT

A C C E P T E D M A N U S C R I P T

13

Highlights

 The anodic scission of silicon-carbon bonds was successfully realized at solid interfaces at potentials above +1.5 V vs. Ag/AgCl electrode.  The propynyl moiety was covalently immobilized onto gold, platinum and glassy carbon surfaces.  With gold, particularly dense and compact coverages were obtained.  This new method of direct grafting of ethynyl groups to solid surfaces is easy and requires simple conditions (acetonitriletetraalkylammonium salts).