Yuan Zhang

Ning Cao

Min Li

Sabine Szunerits

Ahmed Addad

Pascal Roussel

Rabah Boukherroub
email: rabah.boukherroub@univ-lille.fr

Self-template synthesis of ZnS/Ni3S2 as advanced electrode material for hybrid supercapacitors

Keywords: ZnS/Ni3S2, Self-template, Chemical precipitation, Hybrid supercapacitor

In this work, ZnS/Ni3S2 electrode material was synthesized by a simple self-template approach.

The synthetic route includes chemical precipitation of Zn-Ni precursor in a polyvinylpyrrolidone (PVP)/Milli-Q water solution by dropping ammonia solution and subsequent conversion of the Zn-Ni precursor to ZnS/Ni3S2 via chemical reaction with thioacetamide (TAA), as the sulfur source. Here, Zn-Ni precursor can be considered as a template that is converted to ZnS/Ni3S2 during the sulfidation process. The synthesized ZnS/Ni3S2 material displays a porous texture and a high specific surface area. These properties are beneficial for the investigation of the developed material as electrode in supercapacitors. Indeed, the obtained ZnS/Ni3S2 material exhibits improved specific capacity of 890.1 C g -1 at 1 A g -1 along with a good rate capability (70% retention of its initial value from 1 A g -1 to 20 A g -1 ). Furthermore, the ZnS/Ni3S2 electrode achieves a good cycling stability (~82% retention of its capacitance capacity at 10 A g -1 after 6000 charging-discharging cycles). Moreover, ZnS/Ni3S2 is used as a positive electrode and porous reduced graphene oxide (PrGO) as a negative electrode to assemble a hybrid supercapacitor. The fabricated hybrid supercapacitor offers a specific capacity of ~112.5 C g -1 at 0.5 A g -1 , and delivers an energy density of 30.6 W h kg -1 at a power density of 880 W kg -1 . This work not only demonstrates that ZnS/Ni3S2 material can serve as an efficient electrode material for supercapacitors, but also provides an easy operation way for preparing high performance metal-sulfide electrodes for energy storage application.

Introduction

As convenient energy storage devices, electrochemical supercapacitors have attracted substantial attention, because they represent the most promising devices to bridge function of dielectric capacitors with high power output, and batteries/fuel cells with high energy storage [START_REF] Wang | A review of electrode materials for electrochemical supercapacitors[END_REF].

Other remarkable benefits of supercapacitors are related to their ultra-long lifecycle, fast charge/discharge process and higher power density, which are all necessary for applications in digital products, hybrid electric vehicles and other portable electronic devices [START_REF] Min | One-pot Hydrothermal Synthesis of 3D Flower-like RGO/Co3O4/Ni(OH)2 Composite Film on Nickel Foam for High-performance Supercapacitors[END_REF][START_REF] Liu | Unconventional supercapacitors from nanocarbon-based electrode materials to device configurations[END_REF][START_REF] Liu | Advanced materials for energy storage[END_REF][START_REF] Zhu | Controllable preparation of highly uniform CuCo2S4 materials as battery electrode for energy storage with enhanced electrochemical performances[END_REF].

Usually, supercapacitors could be classified into either symmetric, asymmetric or hybrid supercapacitors. Generally, a symmetric supercapacitor is made up of two same electrode materials (the same material, having the same mass, thickness and so on). An asymmetric supercapacitor comprises a positive electrode and a negative electrode whenever the weight, thickness or the material is different, while a hybrid supercapacitor consists of a non-faradaic and a faradaic type electrode material such as carbonaceous and metal oxides (metal sulfides or metal hydroxides) electrode materials, respectively [START_REF] Zhang | Progress of electrochemical capacitor electrode materials: A review[END_REF][START_REF] Masikhwa | High performance asymmetric supercapacitor based on molybdenum disulphide/graphene foam and activated carbon from expanded graphite[END_REF][START_REF] Jagadale | Performance evaluation of symmetric supercapacitor based on cobalt hydroxide [Co(OH)2] thin film electrodes[END_REF]. In previous reports [START_REF] Jin | Hybrid supercapacitor based on MnO2 and columned FeOOH using Li2SO4 electrolyte solution[END_REF][START_REF] Lukatskaya | Multidimensional materials and device architectures for future hybrid energy storage[END_REF], hybrid supercapacitors usually provide wide operation voltage range and improved energy density.

Generally, the working voltage and energy density of hybrid supercapacitors are much larger than symmetric supercapacitors. However, the practical application of hybrid supercapacitors remains limited because of the lack of suitable electrode materials with good electrochemical performance.

Recently, a huge attention has been devoted to metal sulfides electrode materials in hybrid supercapacitors, because of their appreciable stability, wide range of morphological properties and good electrical conductivity [START_REF] Wan | Hydrothermal synthesis of cobalt sulfide nanotubes: The size control and its application in supercapacitors[END_REF][START_REF] Javed | Flexible full-solid state supercapacitors based on zinc sulfide spheres growing on carbon textile with superior charge storage[END_REF][START_REF] Wang | Controlled synthesis of nickel sulfide/graphene oxide nanocomposite for high-performance supercapacitor[END_REF]. The usage of ZnS in energy storage devices has also been extensively investigated [START_REF] Ramachandran | Solvothermal synthesis of Zinc sulfide decorated Graphene (ZnS/G) nanocomposites for novel Supercapacitor electrodes[END_REF][START_REF] Yan | Templatefree synthesis of ordered ZnO@ZnS core-shell arrays for high performance supercapacitors[END_REF]. However, like other mono-metal sulfides [START_REF] Wang | Controlled synthesis of nickel sulfide/graphene oxide nanocomposite for high-performance supercapacitor[END_REF][START_REF] Jiang | Construction of cobalt sulfide/graphitic carbon nitride hybrid nanosheet composites for high performance supercapacitor electrodes[END_REF][START_REF] Chen | Cobalt sulfide/N,S codoped porous carbon core-shell nanocomposites as superior bifunctional electrocatalysts for oxygen reduction and evolution reactions[END_REF],

ZnS experiences volume expansion and low conductivity during the cycling process, leading to poor electrochemical performance. Bimetal sulfides are found to offer suitable performance because of enhanced electrochemical properties, better thermal and mechanical stability than that of mono-metal sulfides [START_REF] Zhu | Controllable preparation of highly uniform CuCo2S4 materials as battery electrode for energy storage with enhanced electrochemical performances[END_REF][START_REF] Wei | Self-template synthesis of double shelled ZnS-NiS1.97 hollow spheres for electrochemical energy storage[END_REF][START_REF] Sui | High Energy Density Asymmetric Supercapacitor Based ZnS/NiCo2S4/Co9S8 Nanotube Composites Materials[END_REF]. Ni-based sulfide materials have received extensive attention, because of their high theoretical specific capacitance values, rich oxidation states and cost effectiveness [START_REF] Liu | Facilely constructing 3D porous NiCo2S4 nanonetworks for high-performance supercapacitors[END_REF][START_REF] Miao | Hydrothermal Synthesis of ZnxNi1-xS nanosheets for hybrid supercapacitor applications[END_REF][START_REF] Nandhini | Facile microwave-hydrothermal synthesis of NiS nanostructures for supercapacitor applications[END_REF][START_REF] Yu | Interconnected NiS nanosheets supported by nickel foam: Soaking fabrication and supercapacitors application[END_REF][START_REF] Chen | Facile Growth of Caterpillar-like NiCo2S4 Nanocrystal Arrays on Nickle Foam for High-Performance Supercapacitors[END_REF][START_REF] Zhao | In situ synthesis of novel urchin-like ZnS/Ni3S2@Ni composite with a core-shell structure for efficient electromagnetic absorption[END_REF]. Additionally, the preparation of nickel-based bimetal sulfides/selenides has proven to enhance their electrochemical performance as electrode materials for supercapacitors.

Thus, various materials such as Ni3S2@Co9S8 and NiS@NiSe2 [START_REF] Wei | Partial Ion-Exchange of Nickel-Sulfide-Derived Electrodes for High Performance Supercapacitors[END_REF], Ni3S2@β-NiS [START_REF] Li | Single-crystal β-NiS nanorod arrays with a hollowstructured Ni3S2 framework for supercapacitor applications[END_REF],

Ni3S2-NiS nanowires [START_REF] Zang | Template-Assisted Synthesis of Nickel Sulfide Nanowires: Tuning the Compositions for Supercapacitors with Improved Electrochemical Stability[END_REF], Ni3S2-Cu1.8S nanosheets [START_REF] Liu | In situ formation of Ni3S2-Cu1.8S nanosheets to promote hybrid supercapacitor performance[END_REF], hollow Ni3S2/NiS@Ni3S4 core/shell submicrospheres [START_REF] Cheng | One-step controlled synthesis of hierarchical hollow Ni3S2/NiS@Ni3S4 core/shell submicrospheres for high-performance supercapacitors[END_REF], Ni3S2-NiS/N-doped graphene [START_REF] Luo | Nitrogen-doped graphene/multiphase nickel sulfides obtained by Ni-C3N3S3 (metallopolymer) assisted synthesis for high-performance hybrid supercapacitors[END_REF] have been prepared and investigated as electrodes for supercapacitors.

Unlike nickel sulfides, the preparation of Ni-Zn electrodes has been reported only in a few reports [START_REF] Zhao | In situ synthesis of novel urchin-like ZnS/Ni3S2@Ni composite with a core-shell structure for efficient electromagnetic absorption[END_REF][START_REF] Li | Cactus-like ZnS/Ni3S2 hybrid with high electrochemical performance for supercapacitors[END_REF]. Zhao et al. were the first to report the synthesis of ZnS/Ni3S2@Ni composite with a core-shell structure using a two-step process consisting of solution reduction and template method, and applied the resulting material for electromagnetic absorption [START_REF] Zhao | In situ synthesis of novel urchin-like ZnS/Ni3S2@Ni composite with a core-shell structure for efficient electromagnetic absorption[END_REF]. In a recent report, Li et al. adopted a two-step hydrothermal method to produce cactus-like ZnS/Ni3S2 hybrid with high electrochemical performance for supercapacitors [START_REF] Li | Cactus-like ZnS/Ni3S2 hybrid with high electrochemical performance for supercapacitors[END_REF]. The ZnS/Ni3S2 electrode displayed a specific capacitance of 2093 F g -1 at 1 A g -1 and a cycling stability of 64% after 5000 cycles at a current density of 3 A g -1 .

Compared with various synthetic routes such as hydrothermal [START_REF] Li | Cactus-like ZnS/Ni3S2 hybrid with high electrochemical performance for supercapacitors[END_REF][START_REF] Tong | Zinc cobalt sulfide nanosheets grown on nitrogen-doped graphene/carbon nanotube film as a highperformance electrode for supercapacitors[END_REF], solvothermal [START_REF] Parveen | Solid-state symmetrical supercapacitor based on hierarchical flower-like nickel sulfide with shape-controlled morphological evolution[END_REF], electrodeposition [START_REF] Yu | Nanohoneycomb-like manganese cobalt sulfide/three dimensional graphene-nickel foam hybid electrodes for high-rate capability supercapacitors[END_REF], microwave-assisted approach [START_REF] Nandhini | Facile microwave-hydrothermal synthesis of NiS nanostructures for supercapacitor applications[END_REF] or other methods [START_REF] Yang | Controlled synthesis of zinc cobalt sulfide nanostructures in oil phase and their potential applications in electrochemical energy storage[END_REF][START_REF] Zhang | Phase controllable fabrication of zinc cobalt sulfide hollow polyhedra as high-performance electrocatalysts for the hydrogen evolution reaction[END_REF], self-template route [START_REF] Cheng | Self-template synthesis of hollow ellipsoid Ni-Mn sulfides for supercapacitors, electrocatalytic oxidation of glucose and water treatment[END_REF] is expected to be an effective operation method to prepare metal sulfides, because both structure and morphology of the final product are controllable and the step of template removal is avoided. Indeed, by using this technique, we were able to successfully synthesize ZnS/Ni3S2 sulfides with a large specific capacity of 890.1 C g -1 at 1 A g -1 , good rate capability (70% retention from 1 to 20 A g -1 ), and good cycling stability (~82% of capacitance capacity retention at 10 A g -1 ) after 6000 charge-discharge cycles. Moreover, a hybrid supercapacitor, comprising ZnS/Ni3S2 as a positive electrode and porous reduced graphene oxide as a negative electrode, achieved a specific capacity of ~112.5 C g -1 at 0.5A g -1 and an energy density of 30.6 W h kg -1 at a power density of 880 W kg -1 , making it a suitable electrode material for electrochemical energy storage devices.

Experimental

Preparation of ZnS/Ni3S2 electrode materials

ZnS/Ni3S2 was prepared following the method in Figure 1. Briefly, a solution A was prepared by dissolution of polyvinylpyrrolidone (1 g) in Milli-Q water (15 mL) at room temperature under magnetic stirring. Separately, a solution B was prepared by successive dissolution of Zn(CH3COO)2•2H2O (1 mmol) and NiCl2•6H2O (2 mmol) in 5 mL Milli-Q water. The solutions A and B were mixed together and 286.5 μL of ammonia (35%) were added dropwise to this mixture under vigorous magnetic stirring at room temperature. The reaction was kept overnight at 50 °C in an oven. After cooling to room temperature, the formed precipitate was separated by centrifugation, rinsed copiously with ethanol and Milli-Q water until the pH of the rinsing solution is ~7. The resulting precipitate was re-dispersed in Milli-Q water (30 mL) for further use. An aqueous solution of thioacetamide (100 mg in 10 mL Milli-Q water) was mixed with the above solution (10 mL) at room temperature under magnetic stirring. After 30 min of stirring, the solution was heated for 5 h at 130 °C. After natural cooling to room temperature, a black solid precipitate was formed. It was separated by centrifugation, rinsed sequentially with Milli-Q water and ethanol, and dried overnight at 60 °C to give ZnS/Ni3S2.

Results and discussion

Characterization of the electrode materials

Information on the chemical composition and oxidation states of surface elements of the synthesized samples were assessed using X-ray photoelectron spectroscopy (XPS). The XPS full spectrum of the obtained ZnS/Ni3S2 is depicted in Figure 2a, revealing the presence of Zn, Ni, and S. The presence of C, O, N in the XPS spectrum originates from residual PVP stabilizer.

The high resolution XPS spectra of the Ni, Zn and S elements are additionally provided (Fig. 2b-d). The Ni2p XPS high resolution spectrum of the obtained sample (Fig. 2b) comprises Ni2p3/2 and Ni2p1/2 spin-orbit doublets along with two shakeup satellite peaks. The Ni2p3/2 at ~855.0 eV, Ni2p1/2 at ~872.9 eV, and the two shake-up satellite peaks at ~859.6 and ~878.5 eV are characteristic of Ni 2+ [START_REF] Zhu | In situ chemical etching of tunable 3D Ni3S2 superstructures for bifunctional electrocatalysts for overall water splitting[END_REF][START_REF] Li | In situ growth of nickel-cobalt oxyhydroxide/oxide on carbon nanotubes for high performance supercapacitors[END_REF]. In the Zn2p XPS high resolution spectrum (Fig. 2c), two strong peaks at ~1021.3 and ~1044.3 eV attributed to Zn 2+ are visible, suggesting the presence of Zn 2+ in the synthesized sample. The S2p high resolution XPS spectrum (Fig. 2d) can be fitted with two peaks at ~160.9 and 162.1 eV assigned to S2p3/2 and S2p1/2, respectively [START_REF] Chen | Facile Growth of Caterpillar-like NiCo2S4 Nanocrystal Arrays on Nickle Foam for High-Performance Supercapacitors[END_REF][START_REF] Hou | Hollow mesoporous hetero-NiCo2S4/Co9S8 submicro-spindles: unusual formation and excellent pseudocapacitance towards hybrid supercapacitors[END_REF]. The crystalline phase of the as-synthesized ZnS/Ni3S2 was investigated by XRD analysis (Fig. 3a). Two different phases could be observed. The diffraction peaks at 28.6°, 47.6° and 56.5°, attributed to the (1 1 1), (2 2 0) and (3 1 1) crystal planes, match the diffraction peaks of the face centered cubic sphalerite ZnS structure (JCPDS 05-0566) [START_REF] Javed | Flexible full-solid state supercapacitors based on zinc sulfide spheres growing on carbon textile with superior charge storage[END_REF]. Additionally, the diffraction peaks at 2θ values of 21.8°, 31.0°, 49.4° and 55.1° are assigned to the (1 0 1), (1 1 0), (1 1 3) and

(1 2 2) crystal planes of Ni3S2 (JCPDS 44-1418) [START_REF] Zhu | In situ chemical etching of tunable 3D Ni3S2 superstructures for bifunctional electrocatalysts for overall water splitting[END_REF][START_REF] Xiog | One-step synthesis of architectural Ni3S2 nanosheet-on-nanorods array for use as high-performance electrodes for supercapacitors[END_REF][START_REF] Chen | High-performance all-solid-state asymmetric supercapacitors based on sponge-like NiS/Ni3S2 hybrid nanosheets[END_REF]. The observed diffraction peaks indicate that the product is a mixture of ZnS and Ni3S2 phases.

Using the Bragg's law (Eq. 1), the interplanar distance d can be determined:

Where d corresponds to the interplanar distance, θ represents the scattering angle, n is a positive integer, and λ=1.54056 Å.

Using The crystallite size of the synthesized sample was determined using the Scherrer formula [START_REF] Ramachandran | Solvothermal synthesis of Zinc sulfide decorated Graphene (ZnS/G) nanocomposites for novel Supercapacitor electrodes[END_REF]:

Where β corresponds to the full width half maximum (FWHM) intensity, and λ=1.54056 Å.

Using the FWHM of the (1 1 1) peak, an average crystallite size of 8 nm was estimated. phase, while the lattice fringe spacing of 0.28 nm is in good accordance with the (1 1 0) lattice plane of Ni3S2 phase (Fig. 3d). The results corroborate the XRD analysis. SEM images of ZnS/Ni3S2 are depicted in Figure 3e-g. Unlike the smooth surface of bare ZnS sample (Fig. S1), the SEM image of ZnS/Ni3S2 sample, at a lower magnification, reveals that the surface is very rough, suggesting that addition of Ni enhances the surface roughness (Fig. 3e).

Furthermore, it could be clearly observed that this rough structure is composed of numerous loose particles and porous structures (Fig. 3f-g). This roughness combined with a loose and porous structure is expected to provide more electroactive sites between active material and electrolyte [START_REF] Wang | Template-free synthesis of hierarchical hollow NiSx microspheres for supercapacitor[END_REF][START_REF] Park | Sodium-ion storage properties of nickel sulfide hollow nanospheres/reduced graphene oxide composite powders prepared by a spray drying process and the nanoscale Kirkendall effect[END_REF].

The synthesized ZnS/Ni3S2 is expected to exhibit a large specific surface area (SSA), owing to its unique roughness and loose structure. Nitrogen adsorption-desorption isotherms were used to assess the SSA and porous texture of ZnS/Ni3S2 (Fig. 4), which are identified as the typical type IV, according to the IUPAC classification. Additionally, hysteresis loops in the 0.6-0.9 relative pressure region are apparent (Fig. 4a). The results indicate the presence of mesoporous structures in the sample, as evidenced by the pore diameter distribution profile (Fig. 4b), determined from the desorption isotherm using the BJH model. The presence of a pore diameter peak at ~10 nm is a good indication of the presence of mesopores in ZnS/Ni3S2. The BET SSA of ZnS/Ni3S2 is 147.9 m 2 g -1 . The SSA of ZnS/Ni3S2 sample, prepared through our method, is found to be larger than that reported for metal sulfides such as ZnS-NiS1.97 (105.3 m 2 g -1 ) [START_REF] Wei | Self-template synthesis of double shelled ZnS-NiS1.97 hollow spheres for electrochemical energy storage[END_REF], ZnxCo1-xS (77.2 m 2 g -1 ) [START_REF] Yang | Controlled synthesis of zinc cobalt sulfide nanostructures in oil phase and their potential applications in electrochemical energy storage[END_REF], Zn0.76Co0.24S@Ni3S2 (89.1 m 2 g -1 ) [START_REF] Lv | Construction of hierarchical zinc cobalt sulfide@nickel sulfide core-shell nanosheet arrays for high-performance asymmetric solidstate supercapacitors[END_REF], ZnCo2S4 core-shell nanospheres (117.3 m 2 g -1 )

[47] and flowerlike Sb2S3 (14.7 m 2 g -1 ) [START_REF] Li | High Performance of 3D Symmetric Flowerlike Sb2S3 Nanostructures in Dye-Sensitized Solar Cells[END_REF]. Accordingly, the porous structure and large specific surface area are necessary to achieve electrode materials with good electrochemical energy storage performance [START_REF] Yan | High-performance supercapacitor electrodes based on highly corrugated graphene sheets[END_REF][START_REF] Wang | Interconnected porous and nitrogen-doped carbon network for supercapacitors with high rate capability and energy density[END_REF]. Therefore, it is quite reasonable to hypothesize that the as-synthesized ZnS/Ni3S2 with such roughness and loose structure may have promising applications in electrochemical supercapacitors.

Electrochemical properties

In order to investigate the potential window of ZnS/Ni3S2 electrode, cyclic voltammetry (CV) curves along with cathodic and anodic peak current densities vs. the square root of sweep rate at various scan rates (10 -100 mV s -1 ) in the 0 to 0.7 V potential window are recorded (Fig. S2).

Obviously, the CV plots exhibit obvious redox peaks (Fig. S2a), indicating a faradaic batterytype characteristic of the ZnS/Ni3S2 electrodes [START_REF] Lukatskaya | Multidimensional materials and device architectures for future hybrid energy storage[END_REF]. Additionally, when the scan rate increases (Fig. S2a), the current density was enhanced without affecting the shape of the CV curves. The results suggest a good rate capability of the ZnS/Ni3S2 electrode. Furthermore, when the scan rate increases, the oxidation peak shifts to high potential and the reduction peak moves towards low potential. This phenomenon is most likely due to electrochemical polarization during the surface redox reaction of the electrodes [START_REF] Yu | Nanohoneycomb-like manganese cobalt sulfide/three dimensional graphene-nickel foam hybid electrodes for high-rate capability supercapacitors[END_REF]. The relationship between the cathodic and anodic peak current densities vs. the square root of sweep rate (Fig. S2b) is also recorded to determine if the capacity is related to surface redox processes or bulk diffusion. The linear relationship confirms that the redox reactions of ZnS/Ni3S2 were under OH -diffusion control rather than surface redox processes [START_REF] Tian | Microwaveassisted non-aqueous homogenous precipitation of nanoball-like mesoporous α-Ni(OH)2 as a precursor for NiOx and its application as a pseudocapacitor[END_REF].

CV curves of ZnS/Ni3S2 electrodes over different potential windows are also recorded at a scan rate of 50 mV s -1 (Fig. S2c). Redox peaks, observed in all CV curves, are attributed to the electrochemical redox reactions of M (Zn, Ni) species at the interfaces between electrolyte and electrode. However, when the potential window is extended above +0.6 V, a significant increase of the current attributed to oxygen evolution reaction (OER) is observed. A potential window (0 to +0.55 V) and a scan rate ranging from 5 to 50 mV s -1 were chosen to avoid the polarization and OER of the prepared ZnS/Ni3S2 electrode. Furthermore, the CV curves of ZnS/Ni3S2 electrode and bare nickel foam are measured at the scan rate of 10 mV s -1 in the potential range of 0 to +0.55 V (Fig. S3). The CV curve of bare nickel foam is almost a straight line, indicating that the capacity of bare nickel foam is almost negligible under our experimental conditions. The redox peaks observed in the CV curve of ZnS/Ni3S2 electrode, are likely due to the reversible faradaic redox reaction of M (Zn, Ni)S-OH. Therefore, redox peaks seen in the CV curve of ZnS/Ni3S2 electrode are attributed to the following faradaic redox reactions [START_REF] Ramachandran | Solvothermal synthesis of Zinc sulfide decorated Graphene (ZnS/G) nanocomposites for novel Supercapacitor electrodes[END_REF][START_REF] Lv | Construction of hierarchical zinc cobalt sulfide@nickel sulfide core-shell nanosheet arrays for high-performance asymmetric solidstate supercapacitors[END_REF][START_REF] Wang | A Ni1-xZnxS/Ni foam composite electrode with multi-layers: one-step synthesis and high supercapacitor performance[END_REF][START_REF] Wang | High-performance NiCo2O4@Ni3S2 core/shell mesoporous nanothorn arrays on Ni foam for supercapacitors[END_REF]:

+ ↔ + + ↔ + + + 3 ↔ ( ) + 3
Faradaic battery-type behavior of the ZnS/Ni3S2 electrode was investigated by recording CV plots at different scan rates (5 -50 mV s -1 ) (Fig. 5a). Redox peaks could also be seen in the CV curves in the potential window of 0 -+0.55 V, indicating faradaic battery-type behavior of the electrode. The position of anodic and cathodic peaks for ZnS/Ni3S2 electrode are given in Table S1. It is well known that electrochemical impedance spectroscopy (EIS) is an extremely informative method for electrochemical nature (the charge transfer and the capacity) of electrode materials used in electrochemical supercapacitors. Therefore, EIS analysis was used to investigate the frequency-dependent behavior of ZnS/Ni3S2 electrode materials. Figure 5d depicts the Nyquist plots with frequencies of some selected points and the corresponding fitted spectra of ZnS/Ni3S2 electrode. An equivalent circuit, based on reported works [START_REF] He | Self-assembled CoS2 nanoparticles wrapped by CoS2-quantum-dots-anchored graphene nanosheets as superior-capability anode for lithium-ion batteries[END_REF][START_REF] Ivanishchev | Study of structural and electrochemical characteristics of LiNi0.33Mn0.33Co0.33O2 electrode at lithium content variation[END_REF][START_REF] Ivanishchev | Models of lithium transport as applied to determination of diffusion characteristics of intercalation electrodes[END_REF][START_REF] Tao | Zinc cobalt sulfide nanosheets array derived from 2D bimetallic metal-organic frameworks for high-performance supercapacitor[END_REF], and the S2, respectively. In the fitted equivalent circuit, Rs denotes the series resistance, including the resistance of inherent active material, electrolyte resistance and the contact resistance. Rct, and CPE are assigned to the faradaic charge transfer resistance and constant phase element accounting for the faradaic pseudocapacitor capacitance capacity. CF and W represent the faradaic capacitance and Warburg resistance (diffusion resistance), respectively. According to the fitted equivalent circuit, the obtained series resistance (Rs), related to the high frequency region >10 4 Hz (intercept at the real axis), is about 1.03 Ω/cm 2 ; the charge transfer resistance (Rct), related to the depressed-semicircle in the high-to-medium frequency region (10 4 to 1 Hz), is about 1.01 Ω/cm 2 . The appearance of CPE may be attributed to the porosity of the prepared ZnS/Ni3S2 electrode material and the calculated value is about 0.26 sec 1/n /cm 2 (n=0.47) and the faradaic capacitance is found to be 0.13 F/cm 2 . Moreover, in the lower frequency region (< 1 Hz), the inclined line of the Warburg line, which is related to the electrolyte ion (OH -) diffusion from the electrolyte (2 M KOH) into the loose and porous ZnS/Ni3S2 electrode surface [START_REF] Min | One-pot hydrothermal synthesis of reduced graphene oxide/Ni(OH)2 films on nickel foam for high performance supercapacitors[END_REF], represents a low diffusion resistance of 0.0046 sec 1/2 /cm 2 . The slope of the Warburg line is higher than 45°, indicating the easy access of OH -electrolyte for fast and reversible faradaic redox reaction and more ideal capacitor behavior [START_REF] Zhu | Mesoporous NiCo2S4 nanoparticles as high-performance electrode materials for supercapacitors[END_REF][START_REF] Luo | Rapid synthesis of three-dimensional flower-like cobalt sulfide hierarchitectures by microwave assisted heating method for high-performance supercapacitors[END_REF], which further confirms that the ZnS/Ni3S2 electrode can retain good capacitive performance. (2k cycles at 5.0 A g -1 ) [START_REF] Yang | Controlled synthesis of zinc cobalt sulfide nanostructures in oil phase and their potential applications in electrochemical energy storage[END_REF] rGO-Ni3S2 -616.0

(1.0 A g -1 )
92.7

(5k cycles at 5.0 A g -1 ) [START_REF] Namdarian | One step synthesis of rGO-Ni3S2 nano-cubes composite for high-performance supercapacitor electrodes[END_REF] Ni3S2/carbon nanotube 800.0

(3.2 A g -1 ) - 80 (1k cycles at 3.2 A g -1 ) [62] ZnCo2S4 1045.3 (2.0 A g -1 ) - 95.5
(5k cycles at 4.0 A g -1 ) [START_REF] Cheng | Mesoporous hollow ZnCo2S4 core-shell nanospheres for high performance supercapacitors[END_REF] ZnS-NiS2 970.0

(2.0 A g -1 ) -87

(1k cycles at 5.0 A g -1 ) [START_REF] Li | MOF-derived self-sacrificing route to hollow NiS2/ ZnS nanospheres for high performance supercapacitors[END_REF] NiS2 1725.0

(5 A g -1 ) -123.0%

(10k cycles at 20.0 Ag -1 ) [START_REF] Dai | Template Synthesis of Shape-Tailorable NiS2 Hollow Prisms as High-Performance Supercapacitor Materials[END_REF] ZnxNi1-xS/NF 1412.0

(1.0 A g -1 ) -81.5

(4k cycles at 2.0 A g -1 ) [START_REF] Mao | Hydrothermal Synthesis of ZnxNi1-xS nanosheets for Hybrid Supercapacitor Applications[END_REF] ZnS-NiS1.97 -696.8

(5.0 A g -1 )

Less 5.5% loss (6k cycles at 10.0 A g -1 ) [START_REF] Wei | Self-template synthesis of double shelled ZnS-NiS1.97 hollow spheres for electrochemical energy storage[END_REF] Co-S-80 285.0 (0.5 A g -1 ) -86.5

(1k cycles at 0.5 A g -1 ) [START_REF] Wan | Hydrothermal synthesis of cobalt sulfide nanotubes: The size control and its application in supercapacitors[END_REF] ZnS/Ni3S2

ZnS/Ni3S2 2093 (1.0 A g -1 )
1618.4

(1.0 A g -1 ) -890.1

(1.0 A g -1 )

64

(5k cycles at 3.0 A g -1 )

81.7

(6k cycles at 10.0 A g -1 )

[32]

Our work

The cycling stability of ZnS/Ni3S2 electrode was established by repeated 6000 charge/discharge cycles at a current density of 10 A g -1 (Fig. 6a). Encouragingly, the retention of the ZnS/Ni3S2 electrode is about 82% of its initial capacitance capacity after 6000 charge/discharge cycles, which is better than 64% achieved by cactus-like ZnS/Ni3S2 at 3 A g -1 [START_REF] Li | Cactus-like ZnS/Ni3S2 hybrid with high electrochemical performance for supercapacitors[END_REF].

XPS analysis was conducted to assess the chemical changes that have occurred during the stability test. Hybrid supercapacitors are believed to combine the advantage of faradaic battery-type electrode material as a positive electrode and double-layer capacitor electrode material as a negative electrode; the combination of these two types of electrode materials is supposed to be able to enlarge the cell's operating voltage greatly and thus improve its energy density [START_REF] Jin | Hybrid supercapacitor based on MnO2 and columned FeOOH using Li2SO4 electrolyte solution[END_REF][START_REF] Lukatskaya | Multidimensional materials and device architectures for future hybrid energy storage[END_REF][START_REF] Brousse | To Be or Not To Be Pseudocapacitive?[END_REF].

To further assess the practical utilization of the prepared ZnS/Ni3S2 electrode, a hybrid supercapacitor was assembled using ZnS/Ni3S2 electrode material as a positive electrode and porous reduced graphene oxide (PrGO) electrode material as a negative electrode. The prepared PrGO electrode material has interesting features such as a porous structure (see TEM image in Fig. S5), mandatory to achieve efficient negative electrode materials. The electrochemical performance of PrGO electrode material is also depicted in Figure S6. The absence of redox peaks in the CV curves along with the triangle shape of the charge and discharge curves reveal that the PrGO electrode store energy through double-layer capacitance.

A 3-electrode system was used to determine the optimal operating potential window of the hybrid supercapacitor based on the ZnS/Ni3S2 and PrGO working potential range. Figure 7a depicts the working potential window vs. Hg/HgO of ZnS/Ni3S2 (0 to +0.55 V) and PrGO (-0.9 to +0.1 V) at a scan rate of 10 mV s -1 . Based on these values, the cell voltage of the fabricated hybrid supercapacitor was fixed at 1.5 V.

Figure 7b

displays a series of CV curves of the hybrid supercapacitor at different cell voltages at a scan rate=50 mV s -1 . Interestingly, increasing the cell voltage does not seem to affect the shape of the CV curves. Additionally, redox peaks are obvious for all investigated potential windows. However, the polarization becomes much more obvious as the cell voltage was extended to 1.8 V. GCD curves were also recorded at various cell voltage values (current density=5 A g -1 ) to determine the power density (P) an energy density (E) (Fig. S7). It could be could offer (30.9 W h kg -1 at 1.8 V, 26.7 W h kg -1 at 1.6 V and 18.2 W h kg -1 at 1.5 V). Both CV and GCD analysis indicated that the PrGO//ZnS/Ni3S2 hybrid supercapacitor operating potential window could be extended up to 1.8 V. However, a cell voltage of 1.6 V was used for further investigation of the electrochemical properties of the PrGO//ZnS/Ni3S2 hybrid supercapacitor, considering the energy density and the polarization of assembled hybrid supercapacitor. A g -1 , respectively. The performance of the hybrid supercapacitor was also good even at a high current density of 5 A g -1 ; indeed, the hybrid supercapacitor attained an energy density of ~26.7

W h kg -1 at a high power density of ~5659 W h kg -1 . These values are comparable to and even better than those reported in the literature for many other supercapacitor devices and slightly lower than 51.2 Wh kg -1 at 849.4 W kg -1 recorded using cactus-like ZnS/Ni3S2 (Table 2). The practical application of the asymmetric supercapacitor is further demonstrated through powering yellow, red and green LEDs using two asymmetric supercapacitors connected in series.

As can be seen in Figure 8, the yellow, red and green LEDs can be sequentially illuminated upon connection to fully charged supercapacitors.

Conclusion

In summary, ZnS/Ni3S2 electrode material with high specific surface area was synthesized using a self-template method. The ZnS/Ni3S2 electrode exhibited a high specific capacity of 890.1 C g -1 at 1 A g -1 , with good cycling stability (~82% of capacitance capacity retention) at a current density of 10 A g -1 after 6000 cycles in 2M KOH aqueous electrolyte. Furthermore, a

Figure 1 .

 1 Figure 1. Schematic of the synthetic route of ZnS/Ni3S2 electrode materials.

Fig. 2 .

 2 Fig. 2. XPS analysis of ZnS/Ni3S2: (a) full spectrum; (b), (c) and (d) are respectively the Ni2p, Zn2p and S2p high resolution spectra. "Sat" in Fig. 2b denotes satellite peaks.

Figure 3b -

 3b Figure 3b-d depicts typical TEM images of as-prepared ZnS/Ni3S2. Figure3bshows a low resolution TEM image, from which a quasi-sheet-like structure can be observed.

Figure 3 .

 3 Figure 3. (a) XRD patterns, (b, c, d) TEM images, and (e, f, g) SEM images of ZnS/Ni3S2; the insets in (c), (d) are the HRTEM images.

Figure 4 .

 4 Figure 4. (a) Nitrogen adsorption-desorption isotherm and (b) pore size distribution curve of ZnS/Ni3S2.

Figure 5b depicts

  Figure 5b depicts the GCD plots of ZnS/Ni3S2 electrode within the current density span of 1 to 20 A g -1 . Clear potential plateaus, due to the electrochemical redox reactions of M (Zn, Ni) species at the interfaces between electrolyte and electrode, are visible in the charge and discharge processes of all GCD curves. The potential plateaus in GCD curves match the redox characteristics observed in the CV curves, indicating strong faradaic battery-type characteristics of the electrode. All GCD plots provide almost symmetric characteristics, revealing the good charge-discharge coulombic efficiency during electrochemical capacitive and reversible redox processes of the prepared ZnS/Ni3S2 electrode.

Figure 5 .

 5 Figure 5. Electrochemical properties of ZnS/Ni3S2 electrode in 2 M KOH: (a) CV curves at different scan rates (5 to 50 mV s -1 ), potential=0 -0.55V; (b) GCD profiles at a current density of 1, 2, 3, 5, 7, 10, 15 and 20 A g -1 ; (c) the corresponding specific capacities within the current density span of 1 to 20 A g -1 ; (d) Nyquist plots.

  corresponding calculated circuit parameters are also depicted in FigureS4 and Table

  Figure 6b depicts the XPS high resolution spectrum of the Ni2p of the ZnS/Ni3S2 electrode after 6000 charge-discharge cycles. The spectrum can be deconvoluted with two spinorbit peaks at 856.4 and 874.3 eV assigned to Ni 2+ 2p3/2 and Ni 2+ 2p1/2, respectively. The presence of two satellite peaks (~862.1 and 880.1 eV), characteristic of Ni 2+ , reveals the excellent reversible redox process during charge-discharge cycling.

Figure 6 .

 6 Figure 6. (a) Stability test of ZnS/Ni3S2 electrode at 10 A g -1 for 6000 cycles; (b) Ni2p high resolution XPS spectrum after 6000 cycles. "Sat" in Figure 6b denotes satellite peaks.

Figure 7 . 2 The

 72 Figure 7. (a) CV curves of the positive and negative electrodes at a scan rate of 10 mV s -1 ; (b) CV curves recorded at different cell voltages; (c) CV curves at different scan rates; (d) GCD curves recorded as a function of current density; (e) the corresponding specific capacity as a function of current density; (f) typical Ragone plot of the hybrid supercapacitor.

Figure 8 :

 8 Figure 8: Photographs of LEDs (yellow, red and green) powered by two asymmetric supercapacitors connected in series.

Table 1 )

 1 . Such a desirable performance of the ZnS/Ni3S2 electrode indicates its potential usage for energy storage devices.

	1, 742.0, 726.5, 700.8, 677.8, 653.1, 632.2, and 620.1 C g -1
	corresponding to 1618.4, 1349, 1320.9, 1274.2, 1232.3, 1187.4, 1149.4, and 1127.5 F g -1 at
	current densities of 1, 2, 3, 5, 7, 10, 15, 20 A g -1 , respectively. When the current density increases
	from 1 to 20 A g -1 , the ZnS/Ni3S2 electrode retained about 70% of its initial specific capacitance
	capacity value, revealing a good rate capability. Moreover, the specific capacity (capacitance) of
	ZnS/Ni3S2 electrode is superior to that of many metal sulfide-based electrode materials and
	slightly lower to that reported for cactus-like ZnS/Ni3S2 prepared by a two-step hydrothermal

Table 1 .

 1 Comparison of the specific capacity values of metal-sulfide-based electrode materials for energy storage applications.

	Electrode material	Specific capacitance (F g -1 )	Specific capacity (C g -1 )	Capacity retention (%)	Ref
	Zn0.76Co0.24S	486.2 (2.0 A g -1 )	-	86.4

Table 2 .

 2 Comparison with other metal sulfide hybrid supercapacitors.

	Electrode material	Working voltage / V	Energy density / Wh kg -1	Power density / W kg -1	Reference
	Active carbon (AC)//CoS/graphene	1.6	29	800	[67]
	AC//NiSx	1.6	4.1	53.5	[44]

Acknowledgements

Financial support is acknowledged from the Centre National de la Recherche Scientifique (CNRS), the University of Lille, and the Hauts-de-France region. N.C. thanks the National Natural Science Foundation of China (grants No.51701240), Key Research and Development Program of Shandong Province (grant No.2017GGX20123) and the Fundamental Research Funds for the Central Universities (grant No.19CX05001A) for financial support. Y.Z. thanks the Chinese government for the China Scholarship Council Award.


Mohamed C Hafik

B Akkay • M Atthieu Pizenberg •

Axel C Arlier

Eithne Balavoine

Géraldine Morin •

Vincent Charvillat

Protocols and Software for Simplified Educational Video Capture and Editing

Keywords: MOOC, Educational Videos, Educational Specification Standards, Video Acquisition, Video Acquisition, Video Editing

Recently, educational videos have become important parts of e-learning systems which have in turn become widely used due to their flexibility. These videos should be of high quality since higher production values lead to superior learning outcomes. However, creating high quality video is a difficult task for teachers since it needs technical knowledge that includes video recording and timeline usage. Hence, creating educational video production software, that is at the same time easy-to-use and able to produce high quality educational videos, is very advantageous. In this paper, we developed protocols for an easy-to-use piece of software that enables teachers that have little technological background to produce their own educational videos autonomously. In fact, our contribution is to reduce the complexity of the whole video production process by introducing a preparation step based on micro-teaching and upstream specification. An evaluation of the software with six teachers is performed. This evaluation, based on think-aloud protocol and quantitative measurements, showed that the introduction of the preparation step allowed the participant teachers to produce high quality educational videos in less than three hours.

Introduction

Over the past few years, a real revolution in e-learning has been happening. It consists in moving lots of content and learning materials online in the form of Massive Open Online Courses (MOOCs) [START_REF] Li | The moderating effects of discipline on the relationship between asyn-chronous discussion and satisfaction with moocs[END_REF]).

University of Toulouse, IRIT, France

One thing that these systems have in common is their focus on delivering content to the learner through educational video sequences. These videos should be attractive and illustrative for students since higher production values lead to superior learning outcomes. However, there was consensus that the video production step is the most expensive component of creating a MOOC, in terms of both time and money [START_REF] Hansch | Video and online learning: Critical reflections and findings from the field[END_REF]. Indeed, it is a challenging task for teachers because it needs technical knowledge (video recording, timeline usage...). In fact, there are three educational video production gaps for a teacher. First, there is the generational skills gap consisting in a digital fluency which is divided between students and the post-secondary education faculty. There is also a technological skills gap between the digital skills that educators currently possess and those they should possess. Finally, there is a gap which consists in the existence of a range of digital skill levels between all faculty members, known as skills. In fact, the aim of the proposed software is to bridge the above mentioned gaps by simplifying educational video production for teachers (acquisition and editing steps). Our contribution is to introduce a preparation step at the beginning of the video production process. This step brings teachers into their comfort zone, which is the learning environment where they can decompose and organize their lessons. Indeed, this preparation allows teachers that are not familiarized with new technologies to be guided during the acquisition and editing steps and then to be able to create autonomously their own high quality educational videos.

In this paper, we developed software that simplify educational video production in a mini-studio while producing high quality videos. In fact, the proposed software contains three stages. The first stage consists in a preparation step which enables the acquisition of short periods chunks (micro-lessons). This allows to teach content in a most efficient and effective manner possible and to simplify video design, production and update. This step is based on upstream specification for educational video case (green screen acquisition, prompter, video resources, structuring, editing...). The second stage consists of a guided acquisition on a technical environment (a green screen mini-studio). During the acquisition, the teacher, standing in front of the camera, can view her/his comments and annotated resources displayed on a screen. The third stage contains the final video edition step. It is made up of an online web service that has a reduced set of interactive tools using a simplified timeline. A result can be seen in Fig. 1.

The remaining part of this paper is structured as follows: Section 2 is devoted to a brief synthesis of the most relevant works on video production systems. In Section 3, we are going to present the proposed system. An evaluation of the proposed solution is presented in Section 4 in order to demonstrate its effectiveness. Section 5 concludes the paper and presents some directions for future works.

Related work

The video production process consists of three phases: planning (pre-production), capture (production) and processing (post-production). It depends on its interaction with users (it can be automatic, semi-automatic, or manual) and its processing time. In this section, we first talk about professional software, their limits and why they should be simplified. Then, we describe how some complexities are reduced by simplifying the software used in both the capture and editing steps. Then, we talk about video production software that are adapted for educational purpose.

Professional video production software

There are many video editing systems that provide the user with full control over the editing process such as Adobe Premiere (Adobe 2018), Speed Razor (In-Sync 2003) and iMovie (Apple 2018). These professional software offer users a manual video editing tool while including many effects. They allow the creation of highquality videos. However, they are expensive and complicated to use especially for novices and they need technical knowledge and expertise. In fact, they are based on sophisticated timeline usage which needs special training for a non-professional user. Indeed, they use a frame-based video editing which is very timeintensive since users will operate at a very low level of detail. They are typically targeted at professionals users and require extensive training, planning and human coordination. For example, when using professional software tool such as OpenShot [START_REF] Thomas | Openshot (version 2.4.2)[END_REF] to make a video such as shown in Fig. 1, the user has many tasks to do. In fact, she/he needs to place, in order, all recorded videos, images slides, additional resources and the background inside the timeline (Fig. 2). She/he needs also to rescale image slides and additional resources and chooses their coordinates in the screen. Then, she/he needs to choose the chroma key color with a sophisticated configuration of lighting parameters in order to avoid artifacts on foreground and background, etc. These tasks make editing video a difficult, tedious, and error-prone activity for a non-professional user. That is why this type of software should be simplified to allow a novice user such as a teacher to create her/his own educational videos by herself/himself.

Simplified video production software

Contrary to professional software, simplified software are automatic, easy-to-use and real-time. However, they generally produce low quality videos that are not attractive and illustrative for students. In these software, the simplification could be integrated in different ways. It can be in the acquisition step or in the editing step.

Simplifying video acquisition step

Some works [START_REF] Uehara | Video shooting navigation system by real-time useful shot discrimination based on video grammar[END_REF][START_REF] Kumano | Online training-oriented video shooting navigation system based on real-time camerawork evaluation[END_REF] proposed an online training-oriented video shooting navigation system to train users how to shoot nice shots for later editing work. [START_REF] Winkler | Automatic camera control for tracking a presenter during a talk[END_REF]) presented a solution that uses a depth camera for automatic camera control for tracking a presenter during a talk. [START_REF] Wulff | Lecturesight -an open source system for automatic camera control in lecture recordings[END_REF] developed a cost-effective solution for automatic camera control for lecture recordings. [START_REF] Hulens | Autonomous lecture recording with a ptz camera while complying with cinematographic rules[END_REF] proposed an even more advanced system that tracks the lecturer while taking cinematographic rules into account. Some software such as Camstudio (Nickthegeek et al. 2013) record all screen and audio activity on the computer and combine them with web camera stream to create video tutorials for school or college class.

Simplifying video editing step

Some software simplified the editing step. For example, [START_REF] Shrestha | Automatic mashup generation from multiple-camera concert recordings[END_REF][START_REF] Bano | Vicomp: composition of user-generated videos[END_REF] automatically combined the most desirable segments from different recordings resulting in a single video stream. [START_REF] Casares | Simplifying video editing using metadata[END_REF]) used metadata to make digital video editing more accessible to novices. [START_REF] Lu | Timeline editing of objects in video[END_REF]) simplified the timeline. In fact, they presented a video editing technique based on changing the timelines of individual objects in video in order to leave them in their original places while putting them at different times. LACES [START_REF] Freeman | LACES: Live Authoring through Compositing and Editing of Streaming Video[END_REF]) is a tablet-based system enabling simple video editing by simplifying interaction with users.

Educational video production software

To produce educational videos, both acquisition and editing steps should be adapted for pedagogic content [START_REF] Savov | Multimedia applications in education[END_REF]. Thus, lesson slides, prompter and some additional resources can be included. Indeed, teachers can view, comment on and annotate resources displayed on a screen. Some software focused on the simplification of the acquisition step. For example, T-Cube [START_REF] Volker | T-cube: A multimedia authoring system for elearning[END_REF]) is an easy-to-use multimedia authoring system designed for eLearning. In this software, video, audio and screenshot, are recorded and encoded in the classroom and simultaneously published on the Internet. [START_REF] Tabuenca | Mobile authoring of open educational resources for authentic learning scenarios[END_REF]) presented a mobile authoring tool for open educational resources to foster universal access to educational resources. [START_REF] Chen | A tabletop lecture recording system based on gesture control[END_REF] presented a lecture recording system that employs gestures and digital cameras to facilitate remote-distance teaching. The lecturer can control the pan-tilt-zoom camera (PTZ camera) to record the teaching process by the gestures using Kinect. Computer vision algorithms, like face tracking, are used to offer real-time feedback during such directed actions. Some other methods, used specification standards to simplify the editing step. In fact, there are many specification standards for educational content (SCORM: Sharable Content Object Reference Model, NSL: Narrative Structure Language, ...). These technical standards allow e-Learning platforms to operate together. They allow also instructional designers to distribute their content on a wide variety of elearning platforms without rewriting efforts. [START_REF] Zsombori | Automatic generation of video narratives from shared ugc[END_REF]) introduced an approach to the automatic generation of video narratives from user generated content gathered in a shared repository using the stan-dard NSL. [START_REF] Sharda | Authoring educational multimedia content using learning styles and story telling principles[END_REF]) presented a methodology for authoring Educational Multimedia content using story telling as the underlying pedagogical technique. [START_REF] Wang | Teaching-material design center: An ontology-based system for customizing reusable e-materials[END_REF] described a system, which follows the standard SCORM, to separate e-material for use (as teaching templates/learning objects) and to label the material with use of semantic metadata for searching. Finally, the advantage of these educational video production software is that they are adapted for pedagogical content and produce high quality videos. However, they focus either on the simplification of the acquisition step or the simplification of the editing step i.e. they don't simplify the whole video production process.

The research questions of this paper are: "How could we simplify the whole process of educational video production for teachers while producing high quality videos?" (see also [START_REF] Chew | Exploring challenges faced by different stakeholders while implementing educational technology in classrooms through expert interviews[END_REF][START_REF] Storme | The emerging pedagogy of moocs, the educational design of technology and practices of study[END_REF])) and "Does the preparation step reduce the complexity of educational video production process?" (see also [START_REF] Zsombori | Automatic generation of video narratives from shared ugc[END_REF][START_REF] Sharda | Authoring educational multimedia content using learning styles and story telling principles[END_REF]). In fact, the idea of this paper is to reduce the complexity of educational video production by introducing an upstream specification (preparation step) that will result in some constraints related to the educational case (slides, additional video resources, lesson structuring, ...). This preparation step brings teachers into their comfort zone, which is the learning environment where they can decompose and organize their lessons. Indeed, it allows teachers that are not familiarized with new technologies to be guided during the whole video production process. Finally, it allows the teacher to be able to create autonomously their high quality educational videos.

Proposed Method

In this paper, we propose an educational video production software that is semi-automatic, simplified and cost-effective. In fact, the main idea of simplification is to introduce a preparation step which is based on micro-learning method (the modularization of learning) to simplify both acquisition and editing steps. This micro-learning method takes place in short byte size unit (micro-lessons) and aims to teach content in a most efficient and effective manner possible. Thus, video content will be easy to design, produce and update in the future. As shown in Fig. 3, the proposed software is composed of three main parts:

-Preparation step: making effort here, thinking "micro" and design "granular" -Video acquisition step: shooting video chunks autonomously -Video editing step: video editing as web-service using simplified timeline

Preparation Step

The teacher, helped possibly by an educational engineer, designs a story, decomposes it in micro-lessons and projects this decomposition on slides. For every slide, she/he can add a text file that will be shown in the prompter. For each group of slides, the teacher also can add some resources (video, audio, image, text). In the example of Fig. 4a, Teacher1 prepared a 5-6 minute video clip broken down into 11 slides and 5 groups of slides. She added video to the first group of slide. The goal of the video chunk was to link the class content to industry standard, in terms of geometric model. The construction of this video chunk was to proceed in five steps:

-Giving the context and illustrating the use of NURBS standard in a industrial software: using additional video -Recalling the class content and competences of the students: knowledge on polynomial and piecewise polynomial models on one hand, homogeneous coordinates on the other hand; -Stating the requirements for using the industry standard, and point out its interest (modeling circular shapes) -Closing the gap between the class content and this requirements: simply express the piecewise polynomial models with homogeneous coordinates leads to rational models, able to model circular shapes; -Concluding (one slide) to encourage the students to feel competent for the industry standard models.

After preparing micro-lessons, the user should provide to the software a yaml file (specification file) which contains five main sections as described in Fig. 4b: -Author section: contains user information (first name, last name, role). -Project section: contains the title of the video, the slides attribute which represents the path to a pdf file containing the slides to use, the date attribute and the description attribute. -Graphical charter section: provides information used to giving a graphic identity to the video. The logo attribute is the path of the image to use as a logo in the graphic charter. The background attribute stores the path of the image to use as a background in the graphic charter instead of the green screen background.

Video Acquisition Step

The proposed software allows a simple way acquisition of video chunks, for teachers. Thus, many process are automated.

Studio

The video is recorded in a green-screen studio. Thus, the green color will be replaced with a chosen background through a chroma keying process. This allows to add features to the video such as the logo of the organization producing the video or some images relevant to the video. As shown in Figure 5 (a) the studio contains:

-User interface display -Camera -Spotlights -Users controls -Green background

Checkers

To avoid frustrating the user during acquisition, the proposed software contains a group of checkers, each one focusing on the audio, video, sources and lighting. Thus, if something does not work with the material or the files provided by the user, these checkers prevent the user from recording:

-Resources checker : verifies the syntax of specification file. It also verifies that the resources described above are loaded in the correct format and are ready for usage.

-Video checker : verifies if the camera is connected to the computer and if the video stream is received through the camera Software Development Kit. -Audio checker : verifies if an audio stream is received.

This feature also allows the calibration of the microphone in order to avoid a saturated or too quiet audio channel. When the check is started the program starts recording the user, then playback what have been recorded. Then, the system asks the user if the sound is heard and satisfying. -Lighting checker : verifies the lighting of the studio room, in order to facilitate the substitution of green screen with a chosen background. The camera takes a picture of the user in front of the green screen, then replace that background with a picture. The result is displayed to the user which can improve the lighting of the background by turning on or off the different lights of the studio room.

Group of Slides Manager

The user is able to select the group of slides (described in the specification file) that she/he wants to use for her/his video before recording it. Those groups of slides are displayed through a list with their page numbers below. When a group of slides is selected, the software allows the user to switch between the slides inside this group, while recording the video.

Video Recorder

The video recorder adapts its user interface according to the contents actually being used. It contains a slider widget, a prompter widget, a recording widget and an additional resource widget which are displayed according to the specification file. The slider widget contains buttons to navigate in the current group of slides. The video recorder stores the time when the switching between slides event is happened in an events yaml file that will be used later in the video editing process.

The prompter widget contains speed and zoom buttons.

The recording widget contains controls to start and stop recording and replay. These events are also stored in the events yaml file. After recording, the user can validate her/his video chunk or she/he can record another one and choose the best between the new and the last one.

The video recorder compresses the produced videos in a lighter format for video editing purposes.

Video Editing Step

After recording video chunks, recorded video and generated events are used in the video editing step. In this step, the user will choose her/his favorite compositing for each video chunk using a simplified timeline. This timeline allows the user to generate the final video while previewing the result in real-time.

Editing Interface

The software provides a user-friendly editing interface using Web development technologies. This interface is organized as on the picture above (Fig. 6). The user is able to select a group of slides using the menu on the right. When she/he chooses the group of slide, the corresponding slides are shown in the editing area. This area contains a preview zone displaying the result of the editing. It contains also a selection of buttons, allowing the user to choose templates. Each template has one particular color. When the user chooses a template for a part of the video, this part appears in the timeline with the color of the chosen template (see the timeline in Fig. 6). The user is able to drag the cursor on the timeline in order to change the current video time. She/he can also click on the timeline areas to change the template type. She/he have also the possibility to drag and drop the timeline delimitation to adjust the video editing. The "Validate" button allows the user to signal that she/he has finished to edit the group of slides. A "Validate all video" button is displayed when all groups of slides have been confirmed. Clicking on this button downloads the video editing file. The proposed software provides also a template recommendation system. In fact, for each selected template, the software recommends the next template that is most suitable (see the red lines in Fig. 6). This helps users to make their videos more dynamic and more attractive for the students.

Template Engine

The proposed system provides a template system to simplify the video editing. The proposed system uses EDL (Edit decision list) template files to generate video editing files from the resources characteristics. These files describes the position, the scale, the rotation, the effects to apply (keying for most cases) and opacity of each element. The video editing engine describes the global video editing file using all the files provided by the template engine. This description is done according to the JSON (JavaScript Object Notation) format.

Live Preview

The proposed software provides a web video rendering engine to display a real time preview of the video edit- ing. This video engine internally uses a timeline representation of the editing step. In fact, it is able to play gaplessly sequences of videos, to combine image and video with a chroma key effect and to combine two video into one ("picture in picture" effect). It also provide external control for the playback (playing, pausing, muting, seeking at specified time).

The compositor

The compositor uses the files provided by the video editing engine to generate the full resolution video. First, the audio is generated by a Python script that uses FFmpeg library. Then, the video is generated by a C++ program that uses OpenCV and Qt libraries to parse EDL files. Finally, a shell script merges the generated audio and video using FFmpeg library. In fact, the proposed software is organized as a client-server application: it processes the final video on the server side and displays a low-resolution version of the compositing on the client side.

Experimental Study

We conducted a user study to evaluate the impact of the preparation step on the simplification of the whole video production process, and the quality of the produced videos. Participants were asked to use the software to perform preparation, acquisition and editing steps. Our participants are six teachers (see Table 1) of different ages and with different levels of familiarity with new technology.

Procedure

The experiment had three parts: tutorial, experimental tasks, and post-experiment questionnaire. In the first part, participants read the software tutorial. They learned how to use the software. In the second part, each participant performed the experimental tasks. At the end, participants filled out a paper questionnaire about the study in which they were asked about the system in general, the preparation step, acquisition step, the editing step and the tutorial. During the experimental task, we used both quantitative evaluation and think-aloud protocol. Think-aloud protocol consists in asking participants to say whatever comes into their minds as they complete the task. This makes thought processes as explicit as possible and then gives us an idea about the participant's cognitive processes.

Metrics for quantitative evaluation

In the preparation step, we measured:

Preparation time: time needed to prepare the slides and the specification.

In the video acquisition step, we measured:

-Acquisition time: time needed for the acquisition; -User assistance: number of interventions of an assistant when user faces a difficulty; -Average of capture repetition number : number of time the user repeat the capture when she/he makes a mistake.

In the video editing step, we measured:

-Editing time: time needed for editing; -User assistance: number of interventions of an assistant when user faces a difficulty; -Difficulty when using the timeline: number of clicks on undo button when she/he makes a mistake; -Difficulty when choosing templates: number of clicks on template buttons divided by the number of selected templates; As we mentioned in section 3, the proposed software provides a template recommendation system. Thus, this measurement gives an idea whether the user chooses directly the recommended templates or not. -Template usefulness: number of selected templates divided by number of available templates in the interface. This measurement gives an idea whether the templates available to the user are all useful or not.

Results and Analysis

-Think-aloud protocol : First, most users like the idea of simplifying video production since they were used to having problems with creating their own educational videos: they may spend all the day and the final product was not very satisfying. They affirm also that videos are generally produced by expensive production companies in their universities. We find that nearly all participants made a positive comment about the software. -Quantitative evaluation: We were very encouraged by the fact that all of the study participants were able to succeed in their acquisition and editing tasks, especially given the brevity of the tutorial. In fact, on average, experimental tasks took participants 137.16 minutes. As we can see in Fig. 7a and Fig. 7b, the learning curves for acquisition and editing steps decrease rapidly and they are comparable to the learning curves of simplified software. (We note that the size of chunks are close and vary between 1.1mn and 1.4mn; For example, for teacher1, the size of chunks 1 to 5 are respectively 1.10mn, 1.3mn, 1.40mn, 1.32mn and 1.12mn). At the same time, the quality of the produced videos is comparable to professional videos (Fig. 1).

Preparation

-Think-aloud protocol : Users understand quickly the structure of the specification file, they think that the specification file was intuitive. They affirm also that micro-learning aspect of this tool is definitely a plus for any teacher for the organization of their lessons. However, they imagine that maybe a web page and a clearly defined set of slides and text options would help the novices with the extended version of having more degrees of freedom for experts. -Quantitative evaluation: On average, preparation step took participants 85.83 minutes (see Table 2). This means that users spent more than one hour on preparing their lessons before starting the video production. However, the time spent on preparation has an important impact on reducing the time of acquisition and editing. In fact, this preparation reduces the production time from one day on average to one hour on average.

Acquisition

-Think-aloud protocol : Users find that checkers are very useful and have practical ergonomics. They think that the ergonomics of the group of slide selection window and the record interface are practical. They also think that the video production is very user friendly. They affirm that recording micro-lessons separately helps them to avoid errors. However, they suggest to use a pointer to draw inside the slide. They suggest also to use a clicker to change slides instead of backspace because clicking on the back visually interrupt and distract the user. -Quantitative evaluation: On average, acquisition took participants 32 minutes. The participants repeated their recording at most twice (see Table 2). As we can see in the table, the acquisition time, user assistance and capture repetition number are low for all participants.

Editing

-Think-aloud protocol : Users understood quickly the interface. They note that templates are intuitive, easy to understand and numerous enough. However, some users suggest that it would be good to have the ability to move slide transitions. They suggest also to put the voice histogram to know when they can crop the end of the video. Some users suggest the use of a touch screen instead of the mouse. -Quantitative evaluation: On average, preparation step took participants 19.33 minutes. The user assistance, difficulty of using timeline, difficulty of choosing templates scores are low for all participants. However, the template usefulnesses varies according to the user (Teacher 4 and 5 used only few templates).

Evaluation of the video production with alternative tools

One year after creating her educational video with our software, we called teacher1 to produce the same video with an alternative tool Apowersoft (Apowersoft 2017) for acquisition and MPEG-streamclip [START_REF] Wondershare | Mpeg-streamclip[END_REF] for editing. These simplified video production software are bought by our university which puts them at the disposal of the teachers. First, we showed Teacher1 the tutorials of Apowersoft and MPEG-streamclip. Then, we asked her to produce two videos: with and without using the preparation step.

Video acquisition without preparation

Teacher1 repeated capture several times. Then, she faced difficulties to exploit the additional video resources in order to make the video-in-video effect. Finally, she asked for her preparation because it simplifies the capture and facilitates the exploitation of the additional videos. Quantitatively, Teacher1 spent 65mn trying to capture and she produced a video with low quality.

Video production with preparation

Thanks to the decomposition in chunk of the preparation step, Teacher1 found the possibility of exploitation of the additional video (video-in-video effect). However, she still has synchronization problem between the recording and additional video starting. She also asked for the possibility of using the prompter. Finally, she produced a video with low quality, but better than the first one. Quantitatively, Teacher1 spend 28mn in acquisition with Apowersoft. The learning curve decreases rapidly and is comparable to the learning curve presented in Fig. 7a. She spent 35mn in editing with MPEG-streamclip. The learning curve decreases less rapidly than Apowersoft curve.

Comparison between our software and alternative tools

Teacher1 affirmed that she preferred using our software for the following reasons. First, although MPEGstreamclip and Apowersoft were easy to use, they produced low quality videos and they didn't offer a lot of possibilities. Indeed, for acquisition, our software provides chroma keying, picture-in-picture effect and prompter. However, Apowersoft provides only screen recorder facilities. For editing, our software provides multiple template, template recommendation system and live preview. However, MPEG-streamclip provides only simple timeline editor. Furthermore, the preparation step of our software allowed her to simplify the acquisition step. In fact, each chunk was recorded separately which reduced the possibility of mistakes. Besides our software allowed her to resolve synchronization problem between the recording and additional video starting. Moreover, the acquisition step of our software is more adapted for educational content, and the editing step is simplified using predefined templates. Finally, the quality of the video produced by our software was higher and comparable with professional video editing software (see Fig. 9).

Conclusion

In this paper, we developed an easy-to-use software that enables teachers that have little technological background to produce their high quality educational videos autonomously. Our contribution is the introduction of a preparation step that simplify the whole educational video production process while producing high quality videos. In fact, the preparation step bridges the gaps described in Section I. First, it bridges generational skills gap by bringing teachers into their comfort zone, which is the learning environment where they can decompose and organize their lessons. Indeed, this step bridges also technological skills and inequality skills gaps since it allows teachers that are not familiar with new technologies to be guided during the acquisition and editing steps and then to be able to create autonomously their high quality educational videos. An evaluation of the software with six teachers is performed. This evaluation, based on think-aloud protocol and quantitative measurements, showed that the introduction of the preparation step allowed the participant teachers to produce their videos in less than three hours. It shows also that the learning curves of the acquisition and editing steps decrease rapidly, while the quality of the produced videos is still higher and comparable with the quality of professional video production software.

For future works, we will first improve each step of the proposed software, using the feedback of the different users. Second, we will propose an interactive web interface that will help teachers to generate their preparation files. Finally, we will introduce augmented reality techniques to assist users during the acquisition step.

Fig. 1

 1 Fig. 1 An example of educational video produced by the proposed software including video-in-video effect.

Fig. 2

 2 Fig. 2 An example of OpenShot project.

Fig. 3

 3 Fig. 3 Flowchart of the proposed software.

Fig. 4

 4 Fig. 4 Example of preparation step: (a) micro-lessons preparation and (b) structure of the specification file.

Fig. 5

 5 Fig. 5 Video acquisition: (a) green-screen studio (b) checkers interface, (c) group of slides manager interface and (d) recorder interface.

Fig. 6

 6 Fig. 6 Editing interface.

Fig. 7

 7 Fig. 7 Learning curves of all participants for (a) acquisition step and (b) editing step.

Fig. 8

 8 Fig. 8 Evaluation of the video production with Apowersoft (Apowersoft 2017).

Fig. 9

 9 Fig. 9 Videos produced by Teacher1 using (a) the proposed software and (b) Apowersoft.

Table 1

 1 Participant teachers.

		Teacher1	Teacher2	Teacher3	Teacher4	Teacher5	Teacher6
	Lesson topic	3D modeling	Machine learning	English	Algebra	Computer network	Image processing
	Age	46	48	49	52	48	61
	Familiarity with new technology	medium	high	low	medium	high	low
	Slides number	11	19	9	75	51	10
	Groups of slides number	5	10	4	14	10	6
	Prompter usage	yes	no	no	yes	no	yes
	Additional resources	yes	yes	no	no	no	no

Table 2

 2 Evaluation of the proposed software.

		Participants	Teacher1 Teacher2 Teacher3 Teacher4 Teacher5 Teacher6
	Preparation metrics	Preparation time	60min	75min	45min	120min	90min	125min
		Acquisition time (mn)	22	34	19	47	39	31
	Acquisition metrics	User assistance	2	1	3	3	1	4
		Average of capture repetition number	1.8	1.6	2	1.2	1.3	2
		Editing time (mn)	16	19	11	26	21	23
		User assistance	1	1	2	2	1	3
	Video editing metrics	Difficulty of using timeline	3	2	2	3	2	3
		Difficulty of choosing templates	2	1.5	2.5	2	2.5	2.5
		Templates usefulness	0.5	0.6	0.6	0.1	0.2	0.5
	Total Video production time (mn)	98	128	75	193	150	179