Lotfi Khezami
email: lhmkhezami@imamu.edu.sa

M A Ben Aissa

Abueliz Modwi

Mokhtar Ismail

Ahlem Guesmi

Faisal K Algethami

M Ben Ticha

Aymen Amine Assadi

Phuong Nguyen-Tri
email: phuong.nguyen-tri@uqtr.ca

Mohamed Ali

Ben Aissa

Manel Ben Ticha

Harmonizing the

Keywords: ZrO 2 @g-C 3 N 4 photocomposite, XPS, IC dye photodegradation: pH efficacy

come

1-Introduction

Lately, researchers have concentrated on two-dimensional (2D) nanomaterials because of their large specific surface area, the high amount of existing active sites, and unique optical and electrical characteristics [START_REF] Ouyang | Visible-light-response g-C3N4@ N, S-TiO2 nanocomposites for superior photocatalysis and photoelectrochemical performance[END_REF][START_REF] Ji | Ultrathin Co3O4 nanosheets anchored on multi-heteroatom doped porous carbon derived from biowaste for high performance solid-state supercapacitors[END_REF]. Amongst them, g-C 3 N 4 (graphite-carbon nitride) has attracted expanding interest in the photocatalysis domain in an environment due to its 2D graphene-like construction along with its features of non-poisonous, stable, plentiful [START_REF] Luo | Three-dimensional network structure assembled by g-C3N4 nanorods for improving visible-light photocatalytic performance[END_REF][START_REF] Chen | Solar-driven efficient degradation of emerging contaminants by g-C3N4-shielding polyester fiber/TiO2 composites[END_REF] and lowering bandgap width near ≈2.75 eV [START_REF] Ismael | A review on graphitic carbon nitride (g-C3N4) based nanocomposites: synthesis, categories, and their application in photocatalysis[END_REF][START_REF] Yan | Photodegradation performance of g-C3N4 fabricated by directly heating melamine[END_REF][START_REF] Hu | Simultaneous photocatalytic Cr (VI) reduction and 2, 4, 6-TCP oxidation over g-C3N4 under visible light irradiation[END_REF]. Notwithstanding, the g-C 3 N 4 received using the conventional thermal decomposition process regularly manifests bulk structure; the low specific surface areas and quick recombination of photo-generated charge carriers considerably restrict its more usage [START_REF] Zhang | Nonmetal group doped g-C3N4 combining with BiF3: Yb3+, Er3+ upconversion nanoparticles for photocatalysis in UV-Vis-NIR region[END_REF][START_REF] Liu | Fabrication of ultra-thin g-C3N4 nanoplates for efficient visible-light photocatalytic H2O2 production via two-electron oxygen reduction[END_REF][START_REF] Li | Z-scheme BiVO4/g-C3N4 heterojunction: an efficient, stable and heterogeneous catalyst with highly enhanced photocatalytic activity towards Malachite Green assisted by H2O2 under visible light[END_REF].

These different important characteristics of g-C 3 N 4 encourage its application for visible illuminated photocatalytic purposes employing solar energy. Besides, g-C 3 N 4 is abundant and simplistic fabricated through one-stage polymerization of affordable feedstocks similar to urea [START_REF] Toghan | Boosting unprecedented indigo carmine dye photodegradation via mesoporous MgO@ g-C3N4 nanocomposite[END_REF][START_REF] Toghan | Mesoporous TiO2@ g-C3N4 composite: Construction, characterization, and boosting indigo carmine dye destruction[END_REF], dicyandiamide [START_REF] Zhang | A facile band alignment of polymeric carbon nitride semiconductors to construct isotype heterojunctions[END_REF], thiourea [START_REF] Hong | Mesoporous carbon nitride with in situ sulfur doping for enhanced photocatalytic hydrogen evolution from water under visible light[END_REF], melamine [START_REF] Wang | Graphite-like C 3 N 4 hybridized ZnWO 4 nanorods: Synthesis and its enhanced photocatalysis in visible light[END_REF], and cyanamide [START_REF] Takanabe | Photocatalytic hydrogen evolution on dye-sensitized mesoporous carbon nitride photocatalyst with magnesium phthalocyanine[END_REF][START_REF] Wang | A metal-free polymeric photocatalyst for hydrogen production from water under visible light[END_REF]. Although, purely g-C 3 N 4 suffers from deficiencies such as accelerated recombination of photo-created electron-hole pairs, a low specific surface area, and a lowering visible light application performance [START_REF] Zhang | A facile band alignment of polymeric carbon nitride semiconductors to construct isotype heterojunctions[END_REF][START_REF] Wang | Graphite-like C 3 N 4 hybridized ZnWO 4 nanorods: Synthesis and its enhanced photocatalysis in visible light[END_REF][START_REF] Zhang | Polycondensation of thiourea into carbon nitride semiconductors as visible light photocatalysts[END_REF][START_REF] Zhao | Graphitic carbon nitride based nanocomposites: a review[END_REF]. Then, investigate easy and reliable approaches to manufacturing the g-C 3 N 4 nanomaterials based on application as photocatalysts with enhanced physicochemical features and great photocatalytic performance.

Newly, various efforts and strategies have been manipulated to magnify the visible light photocatalytic activity of g-C 3 N 4 , for instance, the development of heterojunction employing such as TiO 2 @g-C 3 N 4 [START_REF] Tang | Tuning dimensionality TiO2/g-C3N4 heterostructure for enhanced elemental mercury removal performance under visible-light[END_REF], ZnO@g-C 3 N 4 [START_REF] Neena | Hierarchical Sr-ZnO/g-C3N4 heterojunction with enhanced photocatalytic activities[END_REF], Bi 2 WO 3 @g-C 3 N 4 [START_REF] Wang | Enhancement of photocatalytic activity of Bi 2 WO 6 hybridized with graphite-like C 3 N 4[END_REF], CdS@g-C 3 N 4 [START_REF] Fu | Novel C 3 N 4-CdS composite photocatalysts with organic-inorganic heterojunctions: in situ synthesis, exceptional activity, high stability and photocatalytic mechanism[END_REF], which can comprehensively grow the utilization of the g-C 3 N 4 as nanocomposites. Furthermore, J o u r n a l P r e -p r o o f limited nanomaterials can couple with g-C 3 N 4 to form a heterojunction composition. The critical requirement to develop an adequate visible-light-induced g-C 3 N 4 based nanocomposite is that the nanocomposites should have a suitable band construction that is profitable to generate a coupling hybridization structurally.

Depending on the advantages mentioned earlier, the ZrO2@g-C3N4 photocomposite was prepared from its precursors via ultrasonic irradiation. The features of the prepared nanocomposite were investigated, and its possible utilization as a ZrO 2 @g-C 3 N 4 was examined.

A study on the photocatalytic degeneration capability of the IC dye using ZrO 2 @g-C 3 N 4 nanocomposite at various pH conditions was monitored.

2-Experimental part

g-C 3 N 4 nanosheets fabrication

ZrO 2 @g-C 3 N 4 photocatalyst preparation

ZrO 2 @g-C 3 N 4 photocatalyst was manufactured via a simplistic step ultra-sonication procedure.

At first, 920 mg of g-C 3 N 4 nanosheet was weighted and sonicated in 0.150 L of methanol solvent for 15min. A 400 mg of ZrO 2 nanomaterials were combined with the methanolic blend, and the resulting mixture was subjected to sonication for 15 min. The yellow collected suspension was subjected to sonication for an extra 45 min and drained in an oven at 363 K for 1440 min. At last, the obtained materials were milled and annealed in a programmed furnace at 453 K for 120 min.

Characterization

The morphology construction of the specimens was examined through X-ray diffraction (XRD), Bruker AXS, Ger-man, by CuKa radiation source (1.5406 Å) at a scan rate of 0.02.s -1 . A stimulating voltage and selection current of about 40 kV and 20 mA, respectively, were employed. The TEM photographs of pure ZrO 2 , g-C 3 N 4 , and ZrO 2 @g-C 3 N 4 were performed J o u r n a l P r e -p r o o f using a transmission electron microscope, with accelerating voltage 200 kV, via a Tecnai G20 (USA) device. The absorbance in the diffuse reflection method, utilizing barium sulphate as reference, was measured employing a UV-2550 spectrophotometer (Japan). A Nicolet 5700 FT-IR spectrophotometer has served to report the FT-IR spectra of the fabricated specimens on a wavenumber interval ranging from 400 to 4000 cm -1 . In order to investigate the chemical bonding and the interaction features of ZrO 2 @g-C 3 N 4 catalyst with an Al Ka X-ray source (1486.68 eV, VG ESCALAB 220i-XL, UK), XPS (X-ray photoelectron spectroscopy) was operated. The nanocomposites' surface areas and average pore size distributions were studied using the Brunauer-Emmett-Teller (BET) method using a Micromeritics ASAP 2020 apparatus, where samples are outgassed for 120 min, at 150 oC, then weighed and analyzed. The photoluminescence (PL) spectra were measured on the ZrO 2 @g-C 3 N 4 nanocomposites samples using a fluorescence spectrophotometer (Perkin Elmer LS55) equipped with a xenon lamp, and the selected excitation wavelength is λ =340 nm.

Photocatalytic degradation experiment details

Indigo carmine (IC) dye has been chosen as a pollutant organic compound to assess the photocatalytic capability for the fabricated ZrO 2 @g-C 3 N 4 photocomposite. In the conventional photocatalytic trial, 0.050 g of the specimens were suspended separately in 0.100 L IC liquid dye with a fixed concentration of 30 ppm. The degradation of the IC dye was studied in the visible light region at ambient requirements. Before the photocatalytic activity trial, specimen suspension was put in darkness for 30 min and continuously agitated till achieving the equilibrium between the nanomaterials and the IC dye. After adsorption dye equilibrium, 0.05 L of the mixture solution was taken as an original IC concentration (Co). The remaining mixture was then subjected to visible-light radiation (OSRAM lamp 58 IM/W). Then with fixed time J o u r n a l P r e -p r o o f intervals, 0.05 L from the suspension was taken and centrifuged for 10 min in order to separate IC dye and estimate its photodegradation. The effectiveness of the photodegradation treatment was evaluated by measuring the difference in the dye concentration via the absorbance measurement at 𝛌 max = 610 nm using SHIMADZU-UV-1650 PC spectrophotometer.

Results and discussions

XRD confirmation

g-C 3 N 4 , ZrO 2 , and ZrO 2 @g-C 3 N 4 accumulated XRD models are reported in figure 1. For real g-C 3 N 4 , characteristic diffraction peaks are located at 2θ =13.0° and 27.58° with interlayer stacking plane (002) of the hexagonal crystal for JCPDS 87-1526 and the in-plane structural sorting motif (100) [START_REF] Yu | Surface engineering for extremely enhanced charge separation and photocatalytic hydrogen evolution on g-C3N4[END_REF]. The powder x-ray diffraction design of pure ZrO2 nanomaterials is manifested in Fig. 1. The peaks located at two theta angles of 24.00 (110), [START_REF] Li | Seed-induced growing various TiO2 nanostructures on g-C3N4 nanosheets with much enhanced photocatalytic activity under visible light[END_REF] [START_REF] Xavier | Electrochemical and Mechanical Investigation of Newly Synthesized NiO-ZrO 2 Nanoparticle-Grafted Polyurethane Nanocomposite Coating on Mild Steel in Chloride Media[END_REF][START_REF] Channu | Synthesis and characterization of ZrO2 nanoparticles for optical and electrochemical applications[END_REF]. Besides, the ZrO 2 @g-C 3 N 4 bands' pattern shows both g-C 3 N 4 and ZrO 2 peaks, establishing the precursors rapprochement of in the nanocomposite crystalline composition. The confirmation of the nanocomposite pureness was based on the nonappearance of other crystalline state. Although, the ZrO 2 principal intensity height has changed towards fewer 2θ values from 28.1924 to 28.0514°; meanwhile the FWHM also increased from β = 0.3731° to 0.6128°. These findings may be attributed to the reduction of the crystallite size (D) which was assessed using the Scherrer formula [START_REF] Gonçalves | Size-strain study of NiO nanoparticles by Xray powder diffraction line broadening[END_REF]:

Where: θ corresponds to the diffraction angle, β corresponds to the line expanding at half the maximum intensity (FWHM), J o u r n a l P r e -p r o o f ʎ corresponds to the wavelength of the Cu Ka radiation, 0.9 corresponds to the dimensionless configuration factor.

Selecting the most intense reflection (-111), the ZrO 2 @g-C 3 N 4 crystallite was found to be 13.95 nm, while the pure ZrO 2 has D = 23.03nm and absolute g-C 3 N 4 equal 7.94 nm.

Morphological study

The TEM photographs of the real g-C 3 N 4 nanosheets, ZrO 2 nanoparticles, and ZrO 2 @g-C 3 N hybrid nanostructures are depicted in Figs. 2a,c, and e. The primary g-C 3 N 4 TEM picture exhibits soft surface sheets' layers as a common graphitic nanostructure one [START_REF] Li | Seed-induced growing various TiO2 nanostructures on g-C3N4 nanosheets with much enhanced photocatalytic activity under visible light[END_REF]. On the other hand, the ZrO 2 TEM image (Fig. 2c) presents like-spherical shape nanoparticles with regular size, agreeing with the XRD findings. Furthermore, the TEM photograph (Fig. 2e) of the ZrO 2 @g-C 3 N 4 photocomposite manifests ZrO 2 nanoparticles separated over the g-C 3 N nanomaterials. Therefore, an interface and efficient solid interplay that attaches g-C 3 N 4 to ZrO However, the ZrO 2 @g-C 3 N 4 reveals the presence of Zr, O, C, and N elements. These results successfully proved the coupling of the ZrO 2 with the g-C 3 N 4 nanosheets to develop the photocomposite, as noted in (Fig. 2f).

Optical properties and DRS calculations

The UV-vis and DRS results of pure g-C 3 N 4 , ZrO 2 , and ZrO 2 @g-C 3 N 4 photocomposite are presented in Fig. 3. The optical specimen characteristics' findings revealed that the highest absorption wavelength of pure g-C 3 N 4 for radiation is 420 nm and for ZrO 2 is about 390.8 nm.

Besides, the maximum absorption line edge of ZrO 2 @g-C 3 N 4 photocomposite is 451.7 nm, and the absorption of visible light is notably improved. Furthermore, these obtained results designate that combining ZrO 2 develops the visible radiation absorption appearance of pure g-C 3 N 4 . The energy gap estimation was computed employing the Kubelka-Munk equation [START_REF] Bi | Nano-zirconia supported by graphitic carbon nitride for enhanced visible light photocatalytic activity[END_REF][START_REF] Feng | Applied Catalysis B: environmental performance for enhanced full-spectrum-light photocatalytic activities[END_REF]:

αhν = A (hν -Eg) n/2
Where:

h corresponds to the Plank's constant, ν corresponds to the light frequency, α corresponds to the absorption coefficient Eg is the band-gap and A is a constant, n is a constituent based on the semiconductor optical transition features (i.e., for direct n = 1 transition, for indirect transition n = 4).

J o u r n a l P r e -p r o o f The energy band gaps of ZrO 2 nanoparticles, g-C 3 N 4 nanosheets, and ZrO 2 @g-C 3 N 4 were counted as 3.64, 2.89 and 2.86 eV, severally. These obtained results were confined to the similar distinguished values in earlier statements [START_REF] Wang | Synthesis and characterization of a ZrO 2/gC 3 N 4 composite with enhanced visible-light photoactivity for rhodamine degradation[END_REF][START_REF] Muhmood | Efficient and stable ZrO 2/Fe modified hollow-C 3 N 4 for photodegradation of the herbicide MTSM[END_REF]. The enhancement of the optical characteristics of photocomposite than purely g-C 3 N 4 nanosheets may be owing to the interaction of ZrO 2 nanoparticles with graphitic carbon nitride. In addition to the formation of chemical bonds within the ZrO 2 and g-C 3 N 4 also increased visible absorption characteristics.

J o u r n a l P r e -p r o o f 10

FTIR and functional groups determination

The large absorption lines in the FTIR spectrum of purely g-C 3 N 4 , in the wavenumber range (3500-3000 cm -1 ) may be attributed to N-H's stretching vibration [START_REF] Niu | Graphene-like carbon nitride nanosheets for improved photocatalytic activities[END_REF].

J o u r n a l P r e -p r o o f

In the interval ranging from 1600 to 1200 cm -1 , the detected set of bands could be accredited to the common C-N bonds stretching [START_REF] Li | Solvent-free and metal-free oxidation of toluene using O2 and g-C3N4 with nanopores: nanostructure boosts the catalytic selectivity[END_REF]. The presence of a peak located at 810 cm -1 is appropriate for the bending vibration of the s-triazine ring, suggesting the existence of hexazine structure in the chemical formation of the as-fabricated g-C 3 N 4 nanosheets materials consequently [START_REF] Zhu | Surface imprinting of a gC 3 N 4 photocatalyst for enhanced photocatalytic activity and selectivity towards photodegradation of 2-mercaptobenzothiazole[END_REF].

As shown in (Fig. 4), peaks detected at 1232 cm -1 and 1444 cm -1 in the FTIR spectra of the ZrO 2 nanopowders are consecutively related to the twisting oscillation of an absorbed water (H 2 O) compound and a surface hydroxyl group (OH), [START_REF] Singh | Microwave synthesis, characterization, and photoluminescence properties of nanocrystalline zirconia[END_REF] A wide vibration band is detected in the wavenumber variety 3330 -3456 cm -1 due to surface hydroxyl group and the absorbed water molecule [START_REF] Rezaei | Synthesis of high surface area nanocrystalline MgO by pluronic P123 triblock copolymer surfactant[END_REF]. The band recognized at 412 cm -1 designates the appearance of pure ZrO 2

nanoparticles [START_REF] Sidhu | Photoluminescence quenching of Zirconia nanoparticle by surface modification[END_REF]. The results of pure g-C 3 N 4 are similar with FT-IR spectra of ZrO 2 @g-C 3 N 4 , as presented in

XPS analysis

The chemical bonding and configuration of the ZrO 2 @g-C 3 N 4 surface were observed using the XPS technique, as displayed in Fig. 5(a-d). The XPS chemical outline spectra of the ZrO 2 @g-C 3 N 4 photocomposite catalyst exhibition just Zr, O, C, and N components revealed the creation amid zirconium oxides (ZrO 2 ) and graphitic carbon nitride (g-C 3 N 4 ) as in Fig. 5(a-d).

J o u r n a l P r e -p r o o f data reported for the ZrO 2 @g-C 3 N 4 construction [START_REF] Muhmood | Efficient and stable ZrO 2/Fe modified hollow-C 3 N 4 for photodegradation of the herbicide MTSM[END_REF][START_REF] Ismael | Photocatalytic activity of ZrO 2 composites with graphitic carbon nitride for hydrogen production under visible light[END_REF].

PL analysis

Photoluminescence spectra were used to examine the effectiveness of photogenerated. Fig. 6 displays the photoluminescence spectra of graphitic carbon nitride (g-C 3 N 4 ) and the ZrO 2 @g-C 3 N 4 photocomposite at a 360 nm excitation wavelength. As illustrated in Fig. 6, the PL spectra of pure g-C 3 N 4 and the ZrO 2 @g-C 3 N 4 photocomposite exhibit a prominent main emission peak centered at approximately 440 nm, which may be due to electron-hole recombination in g-C 3 N 4 [START_REF] Chen | In-situ synthesis of AgNbO3/g-C3N4 photocatalyst via microwave heating method for efficiently photocatalytic H2 generation[END_REF]. Additionally, the PL intensity of the ZrO 2 @g-C 3 N 4 photocomposite is lower than that of pure g-C 3 N 4 , implying that the ZrO 2 @g-C 3 N 4 photocomposite has a reduced electron-hole recombination rate of electrons and holes when exposed to visible light [START_REF] Chen | Rapid fabrication of KTa0. 75Nb0. 25/g-C3N4 composite via microwave heating for efficient photocatalytic H2 evolution[END_REF].

Nanomaterials surface area Assessment

The nitrogen adsorption-desorption isotherms of pure g-C 3 N 4 and ZrO 2 @g-C 3 N 4 nanocomposite are shown in Fig. 7a andb. The isotherms of the ZrO 2 @g-C 3 N 4 nanostructures are type IV, indicating the presence of mesopores during sample formation due to the weak hysteresis loop generation. Pure g-C 3 N 4 specimen display also type IV behavior, explained by the weak hysteresis loop generation [START_REF] Wang | Synthesis and photocatalytic activity of SiO2/g-C3N4 composite photocatalyst[END_REF][START_REF] Modwi | Effect of annealing on physicochemical and photocatalytic activity of Cu5% loading on ZnO synthesized by sol-gel method[END_REF]. The specific surface area of ZrO 2 @g-C 3 N 4 nanostructures was 95.685 m 2 /g, characterized by a pore volume of 0.216 cm 3 /g, which is less than the specific surface area of pure g-C3N4 (154.055 m 2 /g and pore volume of 0.912 cm 3 /g). Thus, while the large specific surface area of pure and nanocomposite is shown to boost the photocatalytic activity for IC dye degradation, the surface area is not the only factor contributing to the nanocomposite's activity. The charge separation between the two semiconductors is the determining factor in composite production. The photodegradation kinetics of the IC dye obeys the Lagergren kinetics model (Fig. 8b) as designated using the linear correlation of In(C 0 /C t ) incremented by the time [START_REF] Yu | Enhanced photocatalytic performance of direct Z-scheme gC 3 N 4-TiO 2 photocatalysts for the decomposition of formaldehyde in air[END_REF], which is established through the acceptable regression coefficient as shown in Table 1. The developed photocatalytic performance of the ZrO 2 @g-C 3 N 4 photocomposite is indicated through the high J o u r n a l P r e -p r o o f degradation rate constant (k) and lower half-life time values recorded in Table 1. The degradation impact of the photocomposite fabricated is five folds of pure g-C 3 N 4 nanosheets, proving that the hetero-structured junction within ZrO 2 and g-C 3 N 4 is essential for accomplishing enhanced action ZrO 2 @g-C 3 N 4 photocomposite [START_REF] Muhmood | Efficient and stable ZrO 2/Fe modified hollow-C 3 N 4 for photodegradation of the herbicide MTSM[END_REF].

pH impact on The IC dye degradation

The organic dye degradation takes place on the surface of nanomaterials, so the dye adsorption phenomena are an essential step in the photocatalytic rule. Earlier attempts stated that organic dye compounds with significant adsorption capability decompose quickly [START_REF] Gouvea | Semiconductor-assisted photocatalytic degradation of reactive dyes in aqueous solution[END_REF][START_REF] Anas | Photocatalytic degradation of organic dye using titanium dioxide modified with metal and non-metal deposition[END_REF]. Also, a greater pH value can provide a larger hydroxyl ions (OH -) concentration to respond with holes and generate hydroxyl radicals ( • OH), magnifying the dye photodegradation rate. Besides at below pH value, the before-mentioned IC dye will deliver a higher photodegradation appearance owing to the convenience of holes that will respond with water to create • OH. Therefore, the amount of adsorbed dye on the nanomaterial's surface and • OH concentration will define the quantity of photodegradation [START_REF] Gutteridge | Superoxide-dependent formation of hydroxyl radicals in the presence of iron salts. Detection of 'free'iron in biological systems by using bleomycin-dependent degradation of DNA[END_REF]. The pH of the dye solution is a disturbing parameter in photodegradation experiments due to its influence on the ionization state, the surface phase This behavior is because, at low pH, ZrO 2 present with ZrO 2 @g-C 3 N 4 nanostructures responds to the free H + in the water and is thoroughly destroyed in the solution. Besides, the H + complex with the negatively charged IC dye diminishes the electrostatic interactions force with active sites ZrO 2 @g-C 3 N 4 nanostructures [START_REF] Devi | Influence of physicochemical-electronic properties of transition metal ion doped polycrystalline titania on the photocatalytic degradation of Indigo Carmine and 4-nitrophenol under UV/solar light[END_REF]. Accordingly, the maximum pH necessary to destroy IC dye molecules is the neutral conditions when the best competence occurs (i.e., at pH 7).

Radical Scavengers Assessment

The radical trapping factors were examined to determine the effectiveness of radicals that contribute to dye degradation. Fig. 10 illustrates the IC dye photodegradation percentage rate versus t using a different scavenger. The results demonstrated that dye degradation yields are 82, 39, 90, 20, and 93%, which quenched with the interest of AgNO 3 (e -) [START_REF] Modwi | Fabrication of (Y 2 O 3) n-ZnO nanocomposites by high-energy milling as potential photocatalysts[END_REF], EDTA (h + ) [START_REF] Zhang | Photocorrosion inhibition and photoactivity enhancement for zinc oxide via hybridization with monolayer polyaniline[END_REF],

IPA ( • OH) [57], ASC ( • O 2 -
) [START_REF] Lv | Facile constructing novel 2D porous g-C3N4/BiOBr hybrid with enhanced visible-light-driven photocatalytic activity[END_REF], and free scavenger. The findings show a large reduction in the of the EDTA on the IC dye photodegradation [START_REF] Tang | Novel spindle-shaped nanoporous TiO2 coupled graphitic g-C3N4 nanosheets with enhanced visible-light photocatalytic activity[END_REF]. Alternatively, a second part is performed during the • OH radicals [START_REF] Tong | Biomimetic fabrication of g-C3N4/TiO2 nanosheets with enhanced photocatalytic activity toward organic pollutant degradation[END_REF] as designated by the weaker influence of its scavenger, the IPL, whereas there is no impact of nitric acid on the IC dye decolorization rule intimates no direct relationship electrons in the IC dye degradation [START_REF] Toghan | Mesoporous TiO2@ g-C3N4 composite: Construction, characterization, and boosting indigo carmine dye destruction[END_REF]. This impact of scavenger results demonstrated that • O 2 -and h + are playing principal reactive species in the IC dye catalytic degradation by ZrO 2 @gC 3 N 4 photocomposite.

The IC dye photodegradation Mechanism

Determined CB and VB values of ZrO 2 @gC 3 N 4 are obtained using the expressions [START_REF] Senthil | Synthesis and characterization of lowcost g-C3N4/TiO2 composite with enhanced photocatalytic performance under visible-light irradiation[END_REF]: their sorption on the nanocomposite surface to perform the oxidation [START_REF] Palma-Goyes | Comparative degradation of indigo carmine by electrochemical oxidation and advanced oxidation processes[END_REF]. The significant difference between the migration rate of the photogenerated e -and h + may rationalize the • O 2 production. The following reaction with the dye molecules would be more expeditious than its direct oxidation by holes [START_REF] Li | Seed-induced growing various TiO2 nanostructures on g-C3N4 nanosheets with much enhanced photocatalytic activity under visible light[END_REF].

E CB = X -E C -0.5 E g J o u
Consequently, this mechanism concurs adequately with the scavengers' processes imply that • O 2 -and h + are the effective species for the dye photodegradation utilizing the ZrO 2 @g-C 3 N 4 nanocomposite. This mechanism is in agreement with a similar study that investigated the generation of • OH on the TiO 2 /C 3 N 4 surface through C 6 H 4 (CO 2 H) 2 (TA) where no • OH was identified. This phenomenon was attributed to the lower edges potential VB of g-C 3 N 4 compared to that of OH -/ • OH couple potentials [START_REF] Yu | Enhanced photocatalytic performance of direct Z-scheme gC 3 N 4-TiO 2 photocatalysts for the decomposition of formaldehyde in air[END_REF]. Therefore, the proposed photodegradation Z-scheme mechanism's model advocates the formation of excess • O 2 -along with additional powerful oxidizing agents' h + than in the 'type II-heterojunction' one. This assumption is consistent with the experiments of scavenger to support the presumed Z-scheme mechanism path in the photodegradation of the tested dye [START_REF] Omrani | A ternary Cu2O/BiVO4/WO3 nano-composite: Scavenging agents and the mechanism pathways in the photodegradation of sulfasalazine[END_REF]. Fig. 11 exemplifies the plausible mechanism of the dye molecule degradation process. J o u r n a l P r e -p r o o f

Repeatability and reusability studies

The photocatalyst must be reusable and exhibit high repeatability to be effective in photocatalytic application. The photodegradation process was done three times under the same conditions to determine the repeatability of the IC dye degradation on the ZrO 2 @g-C 3 N 4 photocomposite. As illustrated in Fig. 12 a, the photodegradation rates of IC dyes stay nearly constant throughout the degradation processes, indicating high repeatability in photodegradation measurement. The reusability of the utilized photocatalyst is critical for meeting financial requirements. For this reason, the photodegradation and regeneration processes must be repeated to validate the photocatalyst application. The utilized ZrO 2 @g-C 3 N 4 photocomposite was regenerated by calcining for one hour at 776 K under the same optimized experimental conditions. The suspension was filtered after each experiment, and the photocatalyst was washed several times with distilled water, calcined at 776 K, and reused. Fig. 12 b illustrates the regeneration process. The photodegradation percentage of IC is found to be near 90% after three cycles, indicating that the ZrO 2 @g-C 3 N 4 photocomposite has the potential to be used efficiently for IC photodegradation.

J o u r n a l P r e -p r o o f

Fig. 1 .

 1 Fig. 1. XRD outlines of g-C 3 N 4 , ZrO 2 , and ZrO 2 @g-C 3 N 4 photocomposite

J o u r

  n a l P r e -p r o o f is performed to facilitate the photo-excited charge carriers' detachment for lowering the energy band-gap and improving the photodegradation process. The elemental composition results from the EDS spectrum (Fig.2b) confirm only C and N for the crumpled layered nanosheet of pure g-C 3 N 4 . Besides Fig.2d, the elemental composition of ZrO 2 consists of only Zr and O elements.

Fig. 2 .

 2 Fig. 2. TEM pictures and EDX (a, b), (c, d), and (e, f) for g-C 3 N 4 , ZrO 2 and ZrO 2 @g-C 3 N 4

Fig. 3 .

 3 Fig. 3. (a, c, and e) UV-Visible spectra, and (b, d, and f) band-gaps of g-C 3 N 4 , ZrO 2 , and

Fig 4 . 4 Fig. 4 .

 444 Fig. 4. FTIR spectra results for g-C 3 N 4 , ZrO 2 , and ZrO 2 @g-C 3 N 4 nanomaterials

Fig. 5 .

 5 Fig. 5. XPS spectra, (a) C 1s, (b) N 1s, (c) Zr, and (d) O 1s for ZrO 2 @g-C 3 N 4 photocomposites.

Fig. 6 .

 6 Fig. 6. PL spectra of pure g-C 3 N 4 , and ZrO 2 @g-C 3 N 4 photocomposite

Fig 7 : 3 . 7 . Photodegradation analysis 3 . 7 . 1 .

 737371 Fig 7: Surface area isotherms characteristics of pure g-C 3 N 4 (a), and ZrO 2 @g-C 3 N 4 photocatalyst

Fig. 8 .

 8 Fig. 8. (a) nanomaterials for IC dye degradation against time, and (b) Lagergren kinetics model.

J o u r

  photodegradation as a function of time and photocatalytic rate % in the pH range from 2 to 11.

Fig 9 :

 9 Fig 9: influence of pH variations on IC dye (a) photodegradation time and (b) photodegradation

J o u rFig 10 :

 10 photodegradation %

  photocatalytic dye elimination was accomplished by direct oxidation with the generated holes (h + ) on the g-C 3 N 4 VB and the • O 2 -inducted from photo-excited e -s. This process is possibly

Fig 11 :

 11 Fig 11: Scheme of IC dye photodegradation mechanism using ZrO 2 @g-C 3 N 4 nanocomposite

Figs. 12 :

 12 Figs. 12: Photocatalytic repeatability (a) irradiated and reusability (b) for IC degradation

Table 1 :

 1 photocatalysis parameters of IC dye decomposition

	Samples	k (min -1 )	t 1/2 (min)	R 2	RSS
	Pure g-C 3 N 4	0.00512	135.4	0.99	0.00100
	Pure ZrO 2	0.00041	1690	0.89	0.00006
	ZrO 2 @g-C 3 N 4	0.02790	24.84	0.98	0.03500

Table 2 :

 2 A comparison of the ZrO 2 @g-C 3 N 4 photodegradation efficiency with other composites from literature.

	Photocatalyst	IC Photodegradation time	k (min -1 )	Ref.
	Cobalt doped ZrO 2	90 min	0.0024	[70]
	ZnO/CuO/Si nanocomposite	85 min	0.0220	[71]
	Mg doping ZnO nanoparticles	90 min	0.0087	[72]
	TiO 2 quantum dots	120 min	0.0230	[73]
	Mg/ZnO nanomaterials	90 min	0.0180	[72]
	Multi-element doped ZrO 2	150 min	0.0100	[74]
	(CuO + NiO)/AMSNC	60 min	0.0148	[75]
	CaO nanomaterials	70 min	0.0149	[76]
	alkali-activated magnesium	60 min	0.0124	[75]
	ZrO 2 @g-C 3 N 4 photocomposite	60 min	0.0279	This work
	A comparison of IC dye photocatalytic degradation with several nanomaterials' catalysts found
	in the literature is displayed in

Table 2 .

 2 The findings demonstrate the type of nanomaterials, rate of photodegradation, and IC Photodegradation time for various photocatalysts. The existing activity of ZrO 2 @g-C 3 N 4 photocomposite utilizes visible radiation for photodegradation of IC dye associated with additional activities related. The contemporary ZrO 2 @g-C 3 N 4 nanomaterial has the better photocatalytic ability for IC dye destruction.

findings, it can be concluded that the ZrO2@g-C3N4 photocomposite, fabricated through a simplistic step ultra-sonication procedure, can be used as an effective catalyst to eliminate organic compounds from wastewater.

5-Acknowledgements

The authors extend their appreciation to the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University for funding this work through Research Group no. RG-21-09-70.

Author contribution statement

L. Khezami: Conceptualization, writing original draft preparation, supervision; M. Ali Ben Aissa: writing, review , A Modwi: writing, review, Mokhtar Ismail: writing, review, Ahlem Guesmi: writing, review, F. K. Algethami¨: writing, review, Manel Ben Ticha: writing, review, Aymen Amine Assadi : writing, validation, review the final version, P. Nguyen-Tri: writing, validation, supervision, review the final version

Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper J o u r n a l P r e -p r o o f


Nahyun Shin

Moonsu Kim

Jaeyun Ha

Yong-Tae Kim
email: yongtaekim@inha.ac.kr

Jinsub Choi
email: jinsub@inha.ac.kr

Flexible anodic SnO 2 nanoporous structures uniformly coated with polyaniline as a binder-free anode for lithium ion batteries

Keywords: SnO 2, polyaniline, volume expansion, lithium diffusion, lithium-ion batteries

Herein we report a wet-chemical process to obtain binder-free anodes for lithium-ion batteries (LIBs) using a straightforward and easily scalable approach. Electrochemical deposition, anodization, and electropolymerization are used to firmly attach a nanoporous and flexible SnO 2 film, uniformly coated with polyaniline (PANI), onto a Cu substrate. The threedimensional, flexible, and nanoporous SnO 2 @PANI electrodes exhibit good specific capacity, capacity retention, and charge transfer resistance. Its favorable electrochemical properties are attributed to the large surface area, high electrical conductivity, and effective reduction of volume expansion. These results demonstrate that the proposed approach to forming SnO 2 @PANI electrodes is a promising strategy for constructing high-performance binder-free anodes for emerging flexible LIBs.

Introduction

There is a growing need to develop renewable energy technologies and effective energy storage systems to address environmental pollution and the energy crisis. Consequentially, there is an increased demand for stable lithium-ion batteries (LIBs) with high energy density and long lifespan that can be used in portable electronic devices, electric vehicles (EVs), and massive energy storage system (ESS). [START_REF] Nishi | Lithium ion secondary batteries; past 10 years and the future[END_REF][START_REF] Kang | Battery materials for ultrafast charging and discharging[END_REF] Although graphite is currently used in commercial LIBs, its low specific capacity (372 mA h g -1 ) renders it ill-suited to meet the high energy and power density required by next-generation LIBs. [START_REF] Cabana | Beyond intercalation-based Li-ion batteries: the state of the art and challenges of electrode materials reacting through conversion reactions[END_REF] Among various anode materials that show promise as alternatives to graphite, SnO 2 is considered a possible candidate due to its ultrahigh theoretical specific capacity of 1494 mA h g -1 and low electrode potential compared with Li/Li + (0-1 V). In addition, SnO 2 is highly abundant, inexpensive, and environmentally benign. [START_REF] Chen | SnO2-based nanomaterials: synthesis and application in lithium-ion batteries[END_REF][START_REF] Zhao | SnO2-based nanomaterials: synthesis and application in lithium-ion batteries and supercapacitors[END_REF][START_REF] Li | Extra storage capacity in transition metal oxide lithium-ion batteries revealed by in situ magnetometry[END_REF][START_REF] Hu | Light-Motivated SnO2/TiO2 Heterojunctions Enabling the Breakthrough in Energy Density for Lithium-Ion Batteries[END_REF] However, the large volume expansion (300%) and low conductivity of SnO 2 cause several challenging issues relating to mechanical stress and pulverization of the electrode during lithiation; these critical drawbacks must be addressed before LIBs made with SnO 2 anodes can be commericialized. [START_REF] Huang | In situ observation of the electrochemical lithiation of a single SnO2 nanowire electrode[END_REF][START_REF] Yesibolati | SnO2 Anode Surface Passivation by Atomic Layer Deposited HfO2 Improves Li-Ion Battery Performance[END_REF][START_REF] Liu | Reversible formation of coordination bonds in Sn-based metal-organic frameworks for high-performance lithium storage[END_REF] There have been several attempts to reduce the volume expansion of SnO 2 and shorten the Li ion diffusion transport length through electrode micro-/nanoization and using a buffered matrix. [START_REF] Etacheri | Ordered network of interconnected SnO2 nanoparticles for excellent lithium-ion storage[END_REF][START_REF] Wang | Hierarchical SnO2 nanostructures: recent advances in design, synthesis, and applications[END_REF][START_REF] Li | Operando magnetometry probing the charge storage mechanism of CoO lithium-ion batteries[END_REF] Recently, self-aligned electrodes have been grown directly onto current collectors without using conductive additives or binders, and their ability to increase battery energy density and prevent electrochemical side reactions between binder and electrolyte has been investigated. 14- 16 These studies have shown that anodization is a straightforward technique to grow layers of self-aligned metal oxide with varied structures onto valve metals like Al, Ti, etc. [START_REF] Prakasam | A new benchmark for TiO2 nanotube array growth by anodization[END_REF][START_REF] Piao | Fabrication of nanostructured materials using porous alumina template and their applications for sensing and electrocatalysis[END_REF] Anodic porous alumina is widely used as a template to prepare many functional materials for optical, electronic, and electrochemical applications, and anodic tubular titania is used for gas-sensing or catalytic applications. SnO 2 nanostructures prepared by anodization can be similarly useful as favorable binder-free anode materials for LIBs owing to their improved capacity to accommodate volume expansion during lithiation. Furthermore, coating the SnO 2 electrode with a conductive polymer can efficiently suppress volume expansion and also improve electron transport by enhancing electrical conductivity. [START_REF] Wu | Supercapacitors based on flexible graphene/polyaniline nanofiber composite films[END_REF][START_REF] Zhang | Synthesis of a novel polyanilineintercalated layered manganese oxide nanocomposite as electrode material for electrochemical capacitor[END_REF][START_REF] Sun | Bi-Metal (Zn, Mn) Metal-Organic Framework-Derived ZnMnO3 Micro-Sheets Wrapped Uniformly with Polypyrrole Conductive Network toward High-Performance Li-Ion Batteries[END_REF] In particular, polyaniline (PANI) as one of the nitrogen-rich conducting polymers, is excellent candidate due to its outstanding electrical conductivity from the conjugated p-electron system, good structural stability, flexibility, and ease of synthesis properties. [START_REF] Pimphor | The optimization and comparison of the electrochemical performances for the polyaniline and melamine doping onto activated porous carbon material derived from pineapple leaf fiber as anode material for lithium-ion batteries[END_REF][START_REF] Shi | A Mott-Schottky Heterogeneous Layer for Li-S Batteries: Enabling Both High Stability and Commercial-Sulfur Utilization[END_REF] Herein, we demonstrate a simple and easily scalable approach for fabricating nanoporous polyaniline-coated SnO 2 (SnO 2 @PANI) anodes for LIBs via electrochemical deposition of Sn onto a Cu substrate, anodization, and subsequent PANI electropolymerization. We employed the cost-effective and scalable approach (compared with conventional coating technology) of optimizing anodizing conditions and controlling electropolymerization to achieve nanoporous SnO 2 @PANI electrodes with high surface areas, shorter Li ion diffusion lengths, low resistances, and effective reduction in volume expansion.

Experimental

Preparation of SnO 2 electrode on Cu foil

Prior to electrochemical deposition, Cu foil (0.127 mm thick, 99.9%, Alfa Aesar) was cut into dimensions of 1.5 cm × 1.5 cm and degreased in sequence with acetone, ethanol, and deionized (DI) water by ultrasonication for 10 min. The electrochemical deposition of Sn on the Cu substrate was performed using a two-electrode system composed of Cu foil and Pt mesh as the working and counter electrodes, respectively. A current density of 10 mA cm -2 was applied for 10 min in an aqueous electrolyte consisting of 0.22 M SnCl 2 and 0.31 M sodium citrate tribasic dihydrate. The anodization of Sn deposited onto the Cu foil substrate was carried out in a 0.1% oxalic acid electrolyte at different voltages and for different times. Anodized samples were subsequently rinsed with DI water and dried in an oven at 60 °C.

Preparation of SnO 2 @PANI electrode

Electropolymerization of PANI onto the SnO 2 electrode was performed in an electrolyte composed of 0.2 M oxalic acid and 0.1 M aniline at 25 °C. A standard three-electrode cell of SnO 2 on Cu foil, Pt mesh, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. The SnO 2 @PANI electrodes were obtained by applying a potential from -0.5 to 1.5 V (vs. Ag/AgCl) at different scan rates ranging from 60 to 120 mV s -1 . for three cycles. The obtained electrodes were rinsed with DI water and dried in an oven at 60 °C.

Material Characterization

The morphologies of the prepared electrodes were characterized using a field-emission scanning electron microscope (FE-SEM, S-4300, Hitachi, instrument at Core-Facility Center for Sustainable Energy Materials of Korea Basic Science Institute (KBSI)) and a field-emission transmission electron microscope (FE-TEM; JEM-2100F, JEOL, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). The crystallinity of the samples was analyzed using an X-ray diffractometer (XRD; DMAX 2200V PC, Rigaku, Japan) with Cu Kα radiation (λ = 1.54056 Å). X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific, USA) was employed to determine the elemental binding states of the samples.

Electrochemical Characterization

The SnO 2 @PANI electrodes as a binder-free electrode for LIBs were characterized using CR2032 coin-type half-cell, which was assembled in an Ar-filled glove box with less than 0.3 ppm O 2 and H 2 O. The as-prepared samples had a diameter of 1 cm and were used as the working electrode, with pure Li foil used as the counter electrode. A glass fiber (GF/A, Whatman) with a diameter of 1.6 cm was used as the separator, and a 1 M solution of LiPF 6 dissolved in a 50:50 (w/w) mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) was employed as the electrolyte. The galvanostatic charge and discharge cycling tests were performed in the potential range of 0.01-3.0 V (vs. Li/Li + ) using a battery test cycler (CT-4009T-5V50mA-164, Sinopro). Cyclic voltammetry (CV) measurements were conducted using an electrochemical workstation (SP-150, Bio-Logic, Seyssinet-Pariset, France) between 0.01 and 3 V at a scan rate of 0.1 mV s -1 . Electrochemical impedance spectroscopy (EIS) was performed using an electrochemical workstation (PGSTAT302N Autolab, Metrohm) in the frequency range from 100 kHz to 0.01 Hz with a voltage amplitude of 5 mV.

Results and Discussion

Material characterization

SnO 2 @PANI electrodes are fabricated by electrochemical deposition, followed by anodization, and subsequent electropolymerization, as illustrated in Scheme 1. As shown in Figure S1, after a short transient response, the current density becomes stable, resulting in uniform deposition of Sn metal on the entire Cu foil substrate. [START_REF] Heubner | Scalable fabrication of nanostructured tin oxide anodes for high-energy lithium-ion batteries[END_REF] The surface structure of Sn is rough, primarily due to Sn (the condensed species) having a greater surface energy than the substrate (Cu foil), as explained by the Volmer-Weber growth model. [START_REF] Asseli | Electrochemical nucleation and growth of Zn-Ni alloys from chloride citrate-based electrolyte[END_REF] The current density vs. time transient plot recorded during anodization (Figure 1(a)) demonstrates that this process can be divided into three steps. In step I, the current density drops sharply owing to the formation of an oxide barrier layer. Within a few oscillations, the current density reaches a steady state (step II), indicating growth of a porous oxide structure composed of discontinuous nanochannels with thin gaps separating each layer (Figure 1(b), where the anodization was performed at 10 V for 15 min). The formation of these longitudinally discontinuous layers is attributed to vigorous oxygen evolution during anodization, which disturbs stable growth of the oxide layer. [START_REF] Zaraska | Synthesis of nanoporous tin oxide layers by electrochemical anodization[END_REF] Fortunately, a discontinuous structure is desirous because it can alleviate electrode pulverization due to the large volume expansion that results from nanochannels filling with conductive soft materials during battery cycle testing. As the electrochemically deposited Sn layer is over-etched to reach the Cu substrate, the current density is gradually reduced, exfoliating the oxide layer from the substrate (Step III).

The effect of applied potential and time during the anodization of Sn on Cu foil are shown in Figure 1(c)-(k), demonstrating that the nanochannel pore size increases as a function of voltage (from 5-10 V) and time (from 10-20 min). In particular, more uniform porous structures were formed at higher voltages. However, an applied voltage above 10 V and time more than 20 min caused the SnO 2 film to peel off from the substrate owing to heavy oxygen evolution and complete consumption of the electrodeposited Sn metal layer (Figure S2). [START_REF] Cao | Morphology evolution of the anodized tin oxide film during early formation stages at relatively high constant potential[END_REF] The surface area of anodized samples prepared under diverse conditions was investigated using electrochemical active surface area (ECSA) evaluation, as shown in Figure S3. The ECSA was estimated using a potential range of 0.6-0.9 V (vs. RHE) with scan rates ranging from 0 to 100 mV s -1 without Faradaic currents. Samples prepared at 10 V have higher surface areas (219.69 μF cm 2 ) than those prepared at 5 V (32.04 μF cm -2 ) and 7 V (53.29 μF cm -2 ), indicating that surface area increases with increasing anodizing potential. Based on these results, the optimized anodization conditions were determined to be 10 V and 15 min. In agreement with a previous report [START_REF] Liu | Reversible formation of coordination bonds in Sn-based metal-organic frameworks for high-performance lithium storage[END_REF] , the anodized layers (which changed color from white to black) are highly flexible, and do not crack or detach from the Cu substrate. This flexibility results from the formation of an amorphous SnO 2 layer onto the Cu substrate, firmly gluing it in place (Figure S4). Therefore, anodized SnO 2 has promising potential in future applications of LIBs with flexible electrodes. [START_REF] Zhang | SnO2 nanoflower arrays on an amorphous buffer layer as binder-free electrodes for flexible lithium-ion batteries[END_REF] confirms that SnO 2 was formed. [START_REF] Hong | Nitrogen-doped carbon coated SnO2 nanoparticles embedded in a hierarchical porous carbon framework for high-performance lithium-ion battery anodes[END_REF] In addition, the XPS spectrum in the O 1s region for anodic SnO 2 (Figure 3(c)) shows the expected SnO 2 peaks at 530.9, 532.1, and 533.0 eV. The peaks at 530.9 and 532.1 eV are associated with O 2-ions in the SnO 2 lattice (O L ) and oxygen vacancy defects (O V ), respectively. The peak at 533.0 eV is attributed to chemisorbed and dissociated oxygen species on the surface (O S ). [START_REF] Motsoeneng | Designing SnO2 nanostructure-based sensors with tailored selectivity toward propanol and ethanol vapors[END_REF][START_REF] Supothina | Characterization of SnO2 thin films grown from aqueous solutions[END_REF] The XRD and XPS results indicate that anodic porous SnO 2 layer was amorphous.

CV was used to study electropolymerization of PANI (from suspensions of aniline) onto anodic SnO 2 electrodes. Data was recorded in the potential region from -0.5 to 1.5 V (vs. Ag/AgCl) for three cycles with scan rates ranging from 60 to 120 mV s -1 (Figure S5). The anodic peak at approximately 0.72 V originates from the polymerization of aniline. This peak's intensity increases as a function of the cycle number, indicating that the amount of PANI deposited onto the electrode increases with more cycles. [START_REF] Nasr-Esfahani | Fabrication of a highly sensitive and selective modified electrode for imidacloprid determination based on designed nanocomposite graphene quantum dots/ionic liquid/multiwall carbon nanotubes/polyaniline[END_REF] The SEM images of SnO 2 @PANI prepared at scan rates of 60, 80, 100, and 120 mV s -1 (denoted as SnO 2 @PANI-60, 80, 100, and 120, respectively) are shown in Figure 3(a)-(d), and verify that the porous structure of the SnO 2 coating is maintained after electrodeposition of polyaniline. However, lower scan rates led to the formation of thicker PANI layers, causing a decrease in the pore size of the SnO 2 @PANI anodes: the pore size of SnO 2 @PANI-60 is the smallest, while the pore size of SnO 2 @PANI-120 is almost identical to that of the uncoated SnO 2 @PANI electrode. The TEM images of SnO 2 @PANI-100 (Figure 3(e) and 3(f)) confirm that the porous structure of SnO 2 remains intact and is uniformly coated with a polymerized layer of PANI with a thickness of approximately 3 nm. Furthermore, the TEM-EDS mapping analysis (Figure 3

(g)) indicates

that the distribution of Sn, O, C, and N throughout the composite is uniform.

The existence of electropolymerized PANI is confirmed by the FT-IR spectra of SnO 2 @PANI anodes (Figure 4(a)). The spectra exhibit characteristic bands due to PANI at 1593, 1503, 1305, 1155, and 827 cm -1 , proving that the CV approach successively electropolymerized PANI onto porous SnO 2 structures. Among these characteristic bands of SnO 2 @PANI, those at 1593 and 1503 cm -1 are attributed to the C=C stretch of the quinonoid and benzenoid rings, respectively. The peak at 1305 cm -1 corresponds to the C-N stretch of the secondary aromatic amine and that at 1155 cm -1 corresponds to the aromatic C-H in-plane bending. The peak at 827 cm -1 is associated with the out-of-plane deformation of C-H in the 1,4-disubstituted benzene ring. [START_REF] Chen | Nanostructured Polyaniline-Decorated Pt/C@PANI Core-Shell Catalyst with Enhanced Durability and Activity[END_REF] The successful formation of PANI onto the porous SnO 2 electrode was further confirmed by the XPS spectra of SnO 2 and SnO 2 @PANI, as shown in Figure 4

Electrochemical analysis

The anode performance of SnO 2 and SnO 2 @PANI in lithium half-cells were evaluated to investigate the effect of the applied voltage during anodization and the thickness of the PANI layer on the anodic SnO 2 electrode. The cycling performance of the SnO 2 electrode grown directly on the Cu substrate was measured at a current density of 0.1 A g -1 . As shown in Figure S6, the nanoporous SnO 2 prepared at a higher applied anodization potential exhibits a higher discharge capacity than those prepared at lower applied potentials. This is owing to the resulting larger electrochemical surface area where Li + ions can be stored. However, after the 50 th cycle, the capacity retention of the SnO 2 electrode prepared with an applied potential of 10 V was only 28.82% compared with that after the 5 th cycle. This indicates that the introduction of a buffer layer to suppress volume expansion is necessary. Figure 5(a) shows the initial four CV curves of the SnO 2 @PANI electrode obtained at a scan rate of 0.1 mV s -1 over the potential range of 0.01-3.0 V (vs. Li/Li + ). During the first cathode scan, the reduction peak at ~0.9 V is associated with the formation of solid electrolyte interphase (SEI), accompanied with the irreversible reduction of SnO 2 to metallic Sn. [START_REF] Zhang | SnO2 nanoflower arrays on an amorphous buffer layer as binder-free electrodes for flexible lithium-ion batteries[END_REF][START_REF] Di Lupo | Novel SnO2/mesoporous carbon spheres composite anode for Li-ion batteries[END_REF][START_REF] Sun | Facile construction of ultrathin SnOx nanosheets decorated MXene (Ti3C2) nanocomposite towards Li-ion batteries as high performance anode materials[END_REF] The reduction peak at ~0.1 V and the oxidation peak at ~0.6 V correspond to the two-step Li-Sn alloying/dealloying process of the Li-Sn alloy (Equations 1 and 2) that predominantly contributes to the overall capacity. [START_REF] Wen | In situ growth of mesoporous SnO2 on multiwalled carbon nanotubes: A novel composite with porous-tube structure as anode for lithium batteries[END_REF] In the following cycles, the reduction peak at 0.1 V shifts to a higher potential of 0.31 V due to the formation of intermediate products of Li-Sn alloy during the multi-step alloying process. [START_REF] Liu | Reversible formation of coordination bonds in Sn-based metal-organic frameworks for high-performance lithium storage[END_REF] The other redox peak pairs at potentials of 0.58 V, 1.25 V and 1.20 V, 1.78 V are attributed to the partially reversible reactions of Sn to SnO and SnO 2 . [START_REF] Huang | Rapid microwave-assisted synthesis of SnO2 quantum dots/reduced graphene oxide composite with its application in lithium-ion battery[END_REF][START_REF] Shao | Unique interconnected graphene/SnO 2 nanoparticle spherical multilayers for lithiumion battery applications[END_REF] The CV curves after the first cycle almost overlap, showing good cycling reversibility of SnO 2 @PANI electrode. There is no apparent peak corresponding to PANI in the CV curve, implying that the PANI coating does not contribute to the capacity of the overall lithium alloying/dealloying reactions.

SnO 2 + 4Li + + 4e -→ Sn + 2Li 2 O (1) Sn + xLi + + xe -→ Li x Sn (0 ≤ x ≤ 4.4) (2)
Figure 5(b) shows the charge/discharge profiles of SnO 2 @PANI-100 for selective cycles at a current density of 0.1 A g -1 . The initial capacity loss from the discharge capacity of 1644 to 1002 mA h g -1 is mainly attributed to the irreversible reduction of SnO 2 to metallic Sn and electrolyte decomposition caused by the formation of an SEI layer. As the cycle progresses, the plateau at 1.2 V disappears, meaning that the reduction reaction of SnO 2 to metallic Sn (Equation 1) is irreversible. It exhibits a specific capacity of ~440 mA h g -1 after 50 cycles at a current density of 0.1 A g -1 , which is comparable to the other SnO2@PANI-based electrodes (Table S1). The cycling performance of the SnO 2 @PANI anodes prepared at scan rates ranging from 60 to 120 mV s -1 was tested at a current density of 0.1 A g -1 . As shown in Figure 5(c), despite the increase in the total mass due to the addition of PANI onto the porous SnO 2 electrode, the capacity after 50 cycles is clearly greater than that of the uncoated nanoporous SnO 2 electrode. This is attributed to the larger lithium diffusion coefficient of PANI than SnO 2 , favoring the lithium diffusion process. [START_REF] Zhang | SnO2@ PANI core-shell nanorod arrays on 3D graphite foam: a highperformance integrated electrode for lithium-ion batteries[END_REF][START_REF] Hou | Synthesis of ultralong ZnFe2O4@ polypyrrole nanowires with enhanced electrochemical Li-storage behaviors for lithium-ion batteries[END_REF] An increased capacity retention, ranging from 45.34 to 52.63%, after 50 cycles compared with five cycles was measured. Figure 5(d) shows that capacity retention increases when the scan rate is increased from 60 to 100 mV s -1 , but then decreases when the scan rate is increased to 120 mV s -1 . These results demonstrate that the SnO 2 @PANI-100 electrode effectively reduces SnO 2 volume expansion, while maintaining pores of sufficient size to allow for the diffusion of lithium ions into the nanoporous SnO 2 structures. Figure 5(e) shows the rate performance of the uncoated nanoporous SnO 2 and SnO 2 @PANI-100 electrode at different current densities. The specific capacities of the SnO 2 @PANI-100 electrode at all current densities were higher than that of the uncoated SnO 2 , indicating that the rate capability of the SnO 2 @PANI-100 electrode is better than that of SnO 2 owing to the PANI buffer layer. EIS measurements were conducted to further investigate the charge-transfer kinetics at the interface between the electrode and electrolyte. The Nyquist plots of SnO 2 and the SnO 2 @PANI electrodes after 50 cycles at a current density of 0.1 A g -1 are shown in Figure 6.

From the inset of the equivalent electrical circuit, the semicircles in the high-and midfrequency regions are attributed to the resistance to SEI formation (R sf ) and charge transfer resistance at the interface between the electrode and electrolyte (R ct ), respectively. The straight line at low frequency is related to the Warburg impedance of lithium ion diffusion. As summarized in Table 1, owing to the conductive properties of PANI the SnO 2 @PANI electrodes exhibit much lower charge transfer resistance compared with the uncoated nanoporous SnO 2 electrode. The lowest charge transfer resistance was recorded using SnO 2 @PANI-100 (20.69 Ω), resulting in the facile transfer of lithium ions and electrons at the interface between this electrode and the electrolyte. In addition, in the low-frequency region, the straight line graph of imaginary impedance against real impedance has a steeper slope for the SnO 2 @PANI electrodes than for the uncoated SnO 2 electrode, indicating faster Li ion diffusion, which is consistent with the cycling performance observed in Figure 5(c). [START_REF] Jeong | Hierarchical hollow spheres of Fe2O3@ polyaniline for lithium ion battery anodes[END_REF]

Conclusions

In summary, SnO 2 @PANI anodes were successfully synthesized through the electrochemical deposition of Sn onto a Cu substrate, anodization of Sn, and the electropolymerization of PANI as a buffer layer to reduce volume expansion and improve diffusion kinetics of Li + ions during battery charging and discharging. The anodic SnO 2 binderfree anode grown directly onto a Cu substrate has 3D nanoporous structures with a large surface area, high porosity, and excellent structural flexibility with good adhesion on Cu foil. This leads to shortened lithium ion diffusion lengths and improved electron transport, resulting in improved electrochemical properties. In addition, uniformly deposited PANI onto nanoporous SnO 2 electrodes serves as an effective buffer layer. Optimizing the thickness of PANI by controlling the scan rate offered a high electrical conductivity and relieved the severe volume expansion caused by lithiation of the SnO 2 electrode. The SnO 2 @PANI-100 electrode exhibits a specific capacity of ~440 mA h g -1 after 50 cycles at a current density of 0.1 A g -1 , with a low charge transfer resistance and high lithium diffusion kinetics. We believe that the proposed fabrication process is a potentially cost-effective, simple, and easily scalable approach for designing SnO 2 @PANI binder-free electrodes with high performance in flexible LIBs.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Figure 2 (

 2 Figure 2(a) shows the XRD patterns of electrochemically deposited Sn metal and anodized

  (b), where peaks due to C 1s and N 1s spectra appear only after electropolymerization. The high-resolution C 1s spectrum (Figure 4(c)) can be peak-fitted at 284.9 eV, 286.1 eV, and 288.0 eV, which are attributed to the C-C/C-H, C-N, and C-O bonds, respectively. The four peaks in Figure 4(d) indicate thatthe N 1s spectrum is in the form of benzenoid amine (-NH-) with a binding energy centered at 399.8 eV and quinoid imine (-N=) at 398.8 eV. Positively charged nitrogen cationic radicals (N + ) are centered at 400.6 eV and 401.8 eV.[START_REF] Gao | Electrochemical synthesis of layer-by-layer reduced graphene oxide sheets/polyaniline nanofibers composite and its electrochemical performance[END_REF][START_REF] Zhang | Grapheneencapsulated selenium/polyaniline core-shell nanowires with enhanced electrochemical performance for Li-Se batteries[END_REF]

Figure 1 .

 1 Figure 1. (a) I-t curve of electrodeposited Sn anodization. SEM images of (b) SnO 2 nanochannels and (c-k) porous surfaces of SnO 2 fabricated by anodization using different voltages and times.

Figure 2 .

 2 Figure 2. (a) XRD of electrodeposited Sn and anodized SnO 2 . The (b) Sn 3d XPS spectrum of electrodeposited Sn (bottom) and anodized SnO 2 (top) and (c) O 1s XPS spectrum of SnO 2 .

Figure 3 .

 3 Figure 3. SEM images of aniline electropolymerized on SnO 2 at cyclic voltammetry scan rates of (a) 60 mV s -1 , (b) 80 mV s -1 , (c) 100 mV s -1 , (d) 120 mV s -1 . SnO 2 @PANI-100 (e), (f) TEM images and (g) EDS-mapping.

Figure 4 .

 4 Figure 4. (a) FT-IR spectra of SnO 2 and SnO 2 @PANI at various electropolymerization cyclic voltammetry scan rates. (b) XPS survey spectra of SnO 2 and SnO 2 @PANI. High-resolution XPS spectra of (c) C 1s and (d) N 1s for SnO 2 @PANI.

Figure 5 .

 5 Figure 5. Electrochemical performance of SnO 2 @PANI 100: (a) Cyclic voltammetry, (b) galvanostatic charge/discharge profiles. (c) Cycling performance of SnO 2 and various SnO 2 @PANI electrodes at a current density of 100 mA g -1 . (d) Diagram of SnO 2 @PANI 100 specific capacity and capacity retention after 50 cycles, (e) rate capability at different discharge current densities of SnO 2 @PANI 100 compared with SnO 2 .

Figure 6 .

 6 Figure 6. Electrochemical impedance spectra of SnO 2 , SnO 2 @PANI 80, SnO 2 @PANI 100, and SnO 2 @PANI 120.

Table 1 .

 1 Impedance parameters for SnO 2 , SnO 2 @PANI 80, SnO 2 @PANI 100, and SnO 2 @PANI 120 after 50 cycles as measured by an equivalent electric circuit.

	Materials	R e [ ] 𝛀	R sf [ ] 𝛀	R ct [ ] 𝛀
	SnO 2	6.91	23.53	40.20
	SnO 2 @PANI 80	8.44	15.40	25.17
	SnO 2 @PANI 100	4.66	14.85	20.69
	SnO 2 @PANI 120	3.75	16.65	32.57
	CRediT author statement

Nahyun Shin: Conceptualization, Investigation, Data curation, Visualization, Writing -Original draft. Moonsu Kim: Formal analysis, Investigation. Jaeyun Ha: Formal analysis, Investigation. Yong-Tae Kim: Conceptualization, Visualization, Writing -review & editing, Supervision. Jinsub Choi: Conceptualization, Writing -review & editing, Supervision, Funding acquisition.

Appendix A. Supplementary data

Supplementary data related to this article can be found at online.

Acknowledgement

Conflict of interest:

The authors declare no conflicts of interest. Scheme 1. Illustration of the SnO 2 @PANI electrode fabrication process.

Highlights

 SnO 2 @PANI anodes were successfully synthesized through wet-chemical approach.

 Anodic nanoporous SnO 2 leads to shortened lithium ion diffusion lengths.

 Uniformly deposited PANI onto nanoporous SnO 2 serves as an effective buffer layer.

 Optimized thickness of PANI layer is controlled by adjusting the scan rate.