Marie Garric

Xavier Boumaza

Kim Paricaud

Pauline Lansalot-Matras

Lydie Porte

Leonardo Astudillo

Grégory Pugnet

Laurent Sailler

Guillaume Moulis

Keywords: Myalgies fébriles révélant une endocardite infectieuse à Propionibacterium acnes Febrile myalgia revealing Propionibacterium acnes endocarditis endocardite infectieuse, Propionibacterium acnes, fasciite infectious endocarditis, Propionibacterium acnes, fasciitis

Résumé

Introduction

Propionibacterium acnes endocarditis is rare and difficult to diagnose. We report a case of Propioniacterium acnes endocarditis revealed by a lower limb fasciitis.

Introduction

L'interniste est souvent confronté à des plaintes musculaires atypiques [START_REF] Chauvet | Diagnostic des maladies musculaires dans un service de médecine interne[END_REF]. Face à des myalgies fébriles, une myosite autoimmune [START_REF] Hervier | Phénotypes cliniques et pronostic du syndrome des antisynthétases[END_REF], une sarcoidose [START_REF] Esteves | Is there any association between Sarcoidosis and infectious agents?: a systematic review and meta-analysis[END_REF] ou bien une fasciite non infectieuse [START_REF] Hachulla | Les fasciites inflammatoires non infectieuses: un syndrome-frontière[END_REF] peuvent être évoquées.

Propionibacterium acnes est une bactérie Gram positif anaérobie commensale de la peau. De pousse longue [START_REF] Achermann | Propionibacterium acnes: from commensal to opportunistic biofilm-associated implant pathogen[END_REF], elle peut être responsable d'infection invasive torpide notamment sur matériel étranger. Les endocardites infectieuses à Propionibacterium acnes sont rares et de diagnostic difficile. Nous rapportons le cas d'une infection d'endoprothèse aortique à Propioniacterium acnes mimant une fasciite.

Observation

Un patient de 54 ans présentait une fois par mois depuis cinq mois des myalgies fébriles des membres inférieurs, cédant spontanément en 24 heures. Il avait comme seul antécédent un anévrysme de l'aorte ascendante diagnostiqué fortuitement en 2014, traité par réimplantation de la valve native aortique et pose d'endoprothèse de l'aorte ascendante en aval de la valve. . Des hémocultures avaient été réalisées au cours d'un épisode de myalgies fébriles rendues stériles. Le patient était finalement hospitalisé en Médecine Interne pour des myalgies hyperalgiques morphino-réquérantes. Il était apyrétique et présentait uniquement à l'examen physique un souffle d'insuffisance aortique connu et non modifié. Le bilan biologique montrait un syndrome inflammatoire avec une CRP à 100 mg/L et une hyperleucocytose à polynucléaires neutrophiles. Les enzymes musculaires étaient normales. Les hémocultures restaient stériles après cinq jours de culture. Devant la majoration des douleurs associée à un pic fébrile unique à 39.5°C, un scanner thoraco-abdomino-pelvien et des membres inférieurs avec injection de produit de contraste était réalisé, montrant un aspect de fasciite des membres inférieurs isolée (Figure 1). Une antibiothérapie probabiliste associant amoxicilline-acide clavulanique, clindamycine et gentamycine par voie intraveineuse était alors initiée permettant d'obtenir l'amélioration des myalgies. L'échocardiographie transthoracique puis transoesophagienne révélait une insuffisance aortique de grade 2/4, sans végétation. La Tomographie par Emission de Positron au 18 Fluorodesoxyglucose ( 18 FDG-TEP-scanner) montrait l'apparition de foyers hypermétaboliques intramusculaires des membres inférieurs prédominants à gauche ainsi qu'un foyer hypermétabolique localisé en regard de la prothèse aortique sur sa paroi antérieure gauche (Figure 2). L'Imagerie par Résonance Magnétique (IRM) musculaire des membres inférieurs mettait en évidence de multiples plages inflammatoires intramusculaires compatibles avec des emboles septiques, ainsi qu'un épaississement des fascias intermusculaires entre les loges superficielles et profondes (Figure 3). Cet examen était complété par une échographie des membres inférieurs montrant des thromboses artérielles étagées prédominant sur la jambe gauche. L'IRM cérébrale était normale. Finalement, Propionibacterium acnes était identifié sur tous les flacons anaérobies des hémocultures, dix jours après leur prélèvement. L'ensemble de ces résultats permettait de retenir le diagnostic d'endocardite infectieuse sur valve native avec infection de la prothèse aortique associée à des emboles septiques. L'antibiothérapie était alors modifiée par de l'amoxicilline à la posologie de 12 g/jour par voie intraveineuse et le matériel aortique était enlevé deux semaines plus tard, avec pose d'une prothèse de type Bentall. La culture du matériel prothétique retiré montrait une pousse de Propionibacterium acnes. La valve native resta stérile. Une antibiothérapie par amoxicilline et rifampicine a été poursuivie pendant trois mois avec un relais per os deux semaines après la chirurgie. Aucune rechute n'est survenue après un an de suivi.

Discussion

Propionibacterium acnes, récemment renommée Cutibacterium [START_REF] Scholz | The natural history of cutaneous propionibacteria, and reclassification of selected species within the genus Propionibacterium to the proposed novel genera Acidipropionibacterium gen. nov., Cutibacterium gen. nov. and Pseudopropionibacterium gen. nov[END_REF], est une bactérie Gram positif anaérobie commensale de la peau, également retrouvée dans la flore digestive et ORL. Elle est impliquée dans la physiopathologie de l'acné et a été soupçonnée comme pouvant jouer un rôle dans la pathogénie de certaines maladies auto-immunes comme la sarcoïdose [START_REF] Esteves | Is there any association between Sarcoidosis and infectious agents?: a systematic review and meta-analysis[END_REF]. Propionibacterium acnes est rarement responsable d'infections invasives. De croissance lente, elle est alors généralement responsable d'infections torpides et parfois très tardives sur du matériel étranger grâce à sa capacité de formation de biofilm, au niveau cardiaque, endovasculaire, articulaire, et même de prothèses mammaires ou oculaires [START_REF] Portillo | Propionibacterium acnes: an underestimated pathogen in implant-associated infections[END_REF]. intervention chirurgicale de hernie inguinale, retenue comme porte d'entrée de la bactériémie, sans mention sur la présence ou non de matériel étranger mis en place lors de cette chirurgie [START_REF] Park | Clinical significance of Propionibacterium acnes recovered from blood cultures: analysis of 524 episodes[END_REF]. Le délai médian de positivité des hémocultures est de 6,1 jours sur les flacons aérobies et de 6,4 jours sur les flacons anaérobies. Ce délai peut aller jusqu'à quinze jours [START_REF] Achermann | Propionibacterium acnes: from commensal to opportunistic biofilm-associated implant pathogen[END_REF]. Cette pousse très lente explique que chez notre patient, les hémocultures réalisées en ambulatoire aient été rendues stériles car conservées seulement cinq jours.

Les endocardites infectieuses à Propionibacterium acnes sont rares, représentant moins de 1% des endocardites infectieuses [START_REF] Kestler | Endocarditis caused by anaerobic bacteria[END_REF]. Celles-ci prédominent chez l'homme (dans plus de 80% des cas) et surviennent majoritairement sur du matériel prothétique, le plus souvent des années après l'implantation (avec un délai médian de 26,5 mois) [START_REF] Van Valen | Prosthetic valve endocarditis due to Propionibacterium acnes[END_REF]. On identifie dans la littérature une forte prévalence des complications cardiaques (volumineuses végétations, abcès), probablement en lien avec le retard diagnostique évoqué plus haut. Dans la série rétrospective de Banzon et al., 21 patients sur 24 (87,5%) se sont présentés avec de la fièvre associée à des frissons au moment du diagnostic. Tous les patients décrits dans cette série avaient une atteinte cardiaque gauche, majoritairement localisée au niveau de la valve aortique (79,1%), et 23 (95,8%) étaient porteurs de matériel étranger. L'échographie cardiaque montrait le plus souvent de volumineuses végétations (12,5%), des abcès périvalvulaires (45,8%) et une déhiscence valvulaire (33,3%) [START_REF] Banzon | Propionibacterium acnes endocarditis: a case series[END_REF]. Dans la série de van Valen et al., onze patients sur treize se sont présentés avec une nouvelle fuite paravalvulaire de la valve prothétique, responsable chez dix d'entre eux d'une dyspnée de grade III ou IV sur l'échelle de la New-York Heart Association (NYHA) [START_REF] Van Valen | Prosthetic valve endocarditis due to Propionibacterium acnes[END_REF]. Ceci n'est toutefois pas la règle : dans notre observation, les échocardiographies transthoracique et transoesophagienne ne mettaient pas en évidence de lésions d'endocardite mais seulement la fuite aortique déjà connue. En revanche, le 18 FDG-TEP-scanner a contribué au diagnostic en montrant un hypermétabolisme de la prothèse. Selon une étude récente, le 18 FDG-TEPscanner a montré une sensibilité de 80,5% et une spécificité de 73,1% pour le diagnostic d'endocardite infectieuse sur prothèse [START_REF] Mahmood | Meta-analysis of 18F-FDG PET/CT in the diagnosis of infective endocarditis[END_REF]. Cependant, cette étude n'était pas stratifiée par germes et l'apport du 18 FDG-TEP-scanner pour le diagnostic d'infection invasive à Propionibacterium acnes sur matériel prothétique n'est pas connu. En tous les cas, il convient de proposer cet examen à distance du geste chirurgical (au moins trois mois après, mais des études sont nécessaires pour définir le délai exact) afin d'éviter de nombreux faux positifs en lien avec l'inflammation résiduelle post-opératoire [START_REF] Swart | 18Ffluorodeoxyglucose positron emission/computed tomography and computed tomography angiography in prosthetic heart valve endocarditis: from guidelines to clinical practice[END_REF]. La scintigraphie aux leucocytes marqués par un isotope radioactif ( 99m Tc-Hexamethyl Porpylene Amine Oxime) serait plus spécifique que le 18 FDG-TEPscanner pour le diagnostic d'endocardite infectieuse au prix d'une moindre sensibilité [START_REF] Rouzet | Respective performance of 18F-FDG PET and radiolabeled leukocyte scintigraphy for the diagnosis of prosthetic valve endocarditis[END_REF]. Selon les recommandations de l'European Society of Cardiology, un hypermétabolisme au TEP-scanner autour d'une valve prothétique implantée depuis plus de 3 mois ou une augmentation d'activité à la scintigraphie aux leucocytes marqués est considéré comme un critère majeur du diagnostic d'endocardite [START_REF] Habib | ESC Guidelines for the management of infective endocarditis. The Task Force for the Management of Infective Endocarditis of the European Society of Cardiology (ESC) Endorsed by: European Association for Cardio-Thoracic Surgery (EACTS), the European Association of Nuclear Medicine (EANM)[END_REF].

Les complications emboliques des endocardites à Propionibacterium acnes sont présentes dans 15,5 à 29,1% des observations [START_REF] Banzon | Propionibacterium acnes endocarditis: a case series[END_REF][START_REF] Sohail | Infective endocarditis due to Propionibacterium species[END_REF]. Dans la série de Banzon et al., 7 patients sur 24 ont développé des emboliques.

Parmi eux, quatre ont présenté des emboles cérébraux et deux des infarctus rénaux ou spléniques. Dans la revue de littérature de Sohail et al., 8 patients sur 58 ont présenté des complications emboliques, principalement au niveau de la rate, des reins et du cerveau. Quatre patients avaient des anévrysmes mycotiques périphériques. On peut souligner l'atypie de notre présentation dans laquelle le seul symptôme clinique initial était dû aux emboles périphériques, responsables de myalgies mimant une fasciite, sans symptomatologie cardiaque associée. Ainsi, si le 18 FDG-TEP-scanner est prescrit, il mérite d'être étendu aux membres pour mettre en évidence des localisations septiques secondaires comme dans notre observation.

Les localisations cérébrales doivent par contre être recherchées par l'angio-IRM [START_REF] Habib | ESC Guidelines for the management of infective endocarditis. The Task Force for the Management of Infective Endocarditis of the European Society of Cardiology (ESC) Endorsed by: European Association for Cardio-Thoracic Surgery (EACTS), the European Association of Nuclear Medicine (EANM)[END_REF]. En pratique clinique, l'antibiothérapie de choix des endocardites à Propionibacterium acnes n'est pas consensuelle, faute d'étude prospective. De récentes études rétrospectives ont montré une efficacité de la vancomycine, de la ceftriaxone [START_REF] Banzon | Propionibacterium acnes endocarditis: a case series[END_REF] ainsi que de la benzylpénicilline intraveineuse plus ou moins associée à la rifampicine durant au moins 4 semaines, sans récidive après 38 mois de suivi suivant l'exérèse du matériel infecté [START_REF] Van Valen | Prosthetic valve endocarditis due to Propionibacterium acnes[END_REF]. Notons que la rifampicine est également utilisée pour sa pénétration dans le biofilm, mais des études prospectives sont nécessaires pour valider son utilisation [START_REF] Furustrand Tafin | Role of rifampin against Propionibacterium acnes biofilm in vitro and in an experimental foreign-body infection model[END_REF].

En raison de la présence de biofilm, les infections invasives à Propionibacterium acnes requièrent généralement une chirurgie d'exérèse du matériel infecté. Dans la série de Banzon et al. de 24 patients, un seul patient a été traité par antibiothérapie seule (ceftriaxone durant 6 semaines), et a récidivé 11 mois après la fin de l'antibiothérapie [START_REF] Banzon | Propionibacterium acnes endocarditis: a case series[END_REF]. Dans la série de Cependant, aucune résistance n'était mise en évidence pour la vancomycine, les pénicillines et le linézolide [START_REF] Oprica | ESCMID Study Group on Antimicrobial Resistance in Anaerobic Bacteria. European surveillance study on the antibiotic susceptibility of Propionibacterium acnes[END_REF].

Conclusion

Introduction

  Les endocardites infectieuses àPropionibacterium acnes sont rares et de diagnostic difficile. Nous rapportons le cas d'une infection d'endoprothèse aortique à Propioniacterium acnes responsable d'une fasciite.

  Du fait de son caractère commensal cutané, les prélèvements bactériologiques positifs à Propionibacterium acnes sont souvent considérés comme une simple contamination, rendant le diagnostic d'infections invasives difficile. Une étude rétrospective monocentrique coréenne a recensé 522 patients ayant une ou plusieurs hémocultures positives à Propionibacterium acnes entre 1997 et 2009. Dans cette étude, seulement 18 patients avaient une bactériémie cliniquement significative, parmi lesquels un seul cas d'endocardite infectieuse a été diagnostiqué (chez un patient porteur d'une malformation cardiaque du septum interventriculaire). Ce diagnostic d'endocardite infectieuse a été posé deux mois après une

  Sohail et al., 47 patients (81,0%) ont bénéficié d'une chirurgie d'exérèse. Six d'entre eux (12,7%) sont décédés durant le suivi, contre 27,2% parmi les patients traités exclusivement par antibiothérapie. Par ailleurs, les auteurs rapportent deux récidives, toutes deux chez des patients traités pas antibiotiques seuls. La mortalité secondaire aux endocardites à Propionibacterium acnes est variable en fonction des études, estimée entre 1,8 et 15,5 % [10,16]. Propionibacterium acnes est sensible à de multiples classes d'antibiotiques notamment aux β-lactamines, aux quinolones, aux glycopeptides, au cotrimoxazole, aux cyclines ainsi qu'à la clindamycine et à la rifampicine. Des résistances ont récemment émergé aux macrolides, à la clindamycine, au cotrimoxazole ainsi qu'aux cyclines [18], selon les valeurs de concentration minimale d'inhibition (CMI) fixées par l'European committee on antimicrobial susceptibiliy testing (EUCAST) [19]. D'après une étude européenne menée entre 1996 et 2002, 15,1% des souches de Propionibacterium acnes étaient résistantes à la clindamycine et 17,1% à l'érythromycine, à des degrés variables selon les pays.

  Devant une symptomatologie inflammatoire atypique chez un homme porteur d'une prothèse endovasculaire, une infection systémique à Propionibacterium acnes doit être évoquée avec réalisation d'hémocultures de culture prolongée, jusqu'à 15 jours. En cas d'échographie cardiaque non contributive, les examens de médecine nucléaire peuvent être utiles au diagnostic. Conflit d'intérêts : aucun. Remerciements : Les auteurs remercient le Dr Anne Julian du Service de Médecine nucléaire, le Dr Hubert Basselerie et Mme Morwenn le Boulc'h du Service d'Imagerie médicale du CHU de Toulouse pour leur aide diagnostique et les illustrations. Légendes des Figures :

Figure 1 :

 1 Figure 1 : Scanner injecté au temps veineux de la jambe gauche en coupe axiale : aspect oedématié du membre inférieur avec une infiltration de la graisse sous-cutanée (a) et périvasculaire (b) associée à un épaississement des fascias (c).

Figure 2 :

 2 Figure 2 : Imagerie de fusion 18 FDG-tomographie par émission de positrons/scanner en coupes axiales des jambes du patient : foyers hypermétaboliques en regard des trois loges musculaires de la jambe gauche (flèches) et dans une moindre mesure au niveau des loges musculaires antérieure et postérieure de la jambe droite.

Figure 3 :

 3 Figure 3 : IRM des membres inférieurs en coupe coronale en pondération T2 avec injection de Gadolinium. Multiples plages inflammatoires intramusculaires avec hypersignal T2 des fascias musculaires témoignant d'une fasciite (flèches).


Houda Ennaceri
email: h.ennaceri@aui.ma

Mourad Boujnah

Darja Erfurt

Jörg Rappich

Xi Lifei

Asmae Khaldoun

Abdelilah Benyoussef

Ahmed Ennaoui

Abdelhafed Taleb
email: abdelhafed.taleb@upmc.fr

Influence of Stress on the Photocatalytic Properties of Sprayed ZnO Thin Films

Keywords: ZnO, Ultrasolic Spray Pyrolysis, Density Functional Theory, compressive strain, photoelectrochemical performance

Zinc oxide (ZnO) thin films grown on glass substrates are prepared by means of the Ion Layer Gas Reaction process. The XRD pattern revealed that the ZnO lattice parameters decrease continuously, with the increasing of the film thickness, indicating a continuous variation in the compressive strain. The influence of the thickness on the optical, morphological and structural properties of deposited ZnO thin films was investigated. The UV-Vis absorption spectra of deposited thin films were shown to exhibit a blue-shift, resulting in an increase in the optical band gap from 3.13 eV to 3.24 eV. Structural parameters such as crystallite size, lattice parameters, Zn-O bond length, and residual stress have been determined, and the compressive strain (tensile stress) is found to increase with the increasing of spray time and in turn deposited thin film thickness. The PL spectra of deposited ZnO films, show stronger PL intensity with increasing deposited thin film thickness, which confirm the influence of deposited film thickness on the recombination process of charge carriers and thus on its optical properties. These results, were confirmed by the photo-electrochemical experiments. In addition, a wettability alteration

Introduction

This last century photovoltaic and photocatalytic approaches appear to be serious alternatives to replace fossil fuels, which is responsible for most of the environmental pollution. These approaches allow respectively the conversion of sunlight energy into electrical and chemical energy (green energy). This energy transition is becoming credible, meanly due to tremendous progress and development in material science. Different metal oxides such as TiO2, ZnO, In2O3, SnO2 and Nb2O5 have been used as photo-anodes to convert sunlight energy into green energy. Among them ZnO is considered as one of the most studied semiconductors because of its various properties such as, conductivity change under illumination, high transparency in the visible spectral region [START_REF] Reynolds | Valenceband ordering in ZnO[END_REF] and photocatalytic capability [START_REF] Srikant | On the optical band gap of zinc oxide[END_REF]. Furthermore, ZnO is basically hydrophilic [START_REF] Li | Growth mechanism and growth habit of oxide crystals[END_REF][START_REF] Carraway | Photocatalytic oxidation of organic acids on quantum-sized semiconductor colloids[END_REF], nevertheless, hydrophobic surfaces can be prepared by creating dual-scale rough structures [START_REF] Herminghaus | Roughness-induced non-wetting[END_REF]. ZnO crystallizes on three main forms: the hexagonal wurtzite, the cubic zinc blende, and the cubic rocksalt. The hexagonal wurtzite crystal structure (space group C6V = P63mc) is known to be the most stable and the most common phase of ZnO. The zinc blende is a metastable structure (space group 4 3 ), which can only be stabilized by heteroepitaxial growth on cubic substrates. High pressure induces a transition from the wurtzite structure to the rocksalt structure (space group 3 ) at around 10 GPa [START_REF] Alvarado | Thermoelectric properties of rocksalt ZnO from first-principles calculations[END_REF].

Due to its properties, ZnO is a potential material candidate for various applications including, gas sensors [START_REF] Zhu | Room-temperature gas sensing of ZnO-based gas sensor: A review[END_REF], anti-reflective-coatings [START_REF] Hussain | Zinc oxide as an active n-layer and antireflection coating for silicon based heterojunction solar cell[END_REF], LED [START_REF] Pearton | Advances in ZnO-based materials for light emitting diodes[END_REF], dye sensitized solar cells [START_REF] Nayeri | Low temperature carving of ZnO nanorods into nanotubes for dye-sensitized solar cell application[END_REF] and UV photo catalysis [START_REF] Lee | Recent developments of zinc oxide based photocatalyst in water treatment technology: A review[END_REF]. Different applications share the same required properties such as high specific surface, absorption band extension to visible and infrared spectral region. For photovoltaic and photocatalytic water splitting application, the same mentioned properties are required and ZnO is considered to have a promising potential as photo-anode for these applications. For water splitting, photo catalytic process induces the production of hydrogen and oxygen within the NIR, Visible and UV spectral range. Due to the recombination process of excitons and the limited light absorption of semiconductors material, the sunlight energy conversion is low. To improve this conversion efficiency, the deep understanding of different semiconductor properties is needed.

To prepare ZnO thin films, different preparation techniques have been developed such as sol gel [START_REF] Dutta | Effect of sol concentration on the properties of ZnO thin films prepared by sol-gel technique[END_REF], RF magnetron sputtering [START_REF] Alfaro Cruz | ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production[END_REF], chemical vapor deposition [START_REF] Fay | Low pressure chemical vapour deposition of ZnO layers for thin-film solar cells: temperature-induced morphological changes[END_REF], ink-jet printing [START_REF] Chang | Selective growth of ZnO nanorods for gas sensors using ink-jet printing and hydrothermal processes[END_REF], electrochemical deposition [START_REF] Ennaceri | Water-resistant surfaces using zinc oxide structured nanorod arrays with switchable wetting property[END_REF] and spray pyrolysis [17]. Among them, the spray pyrolysis technique has several advantages, such as simplicity and low cost, control of the films' chemical uniformity and homogeneity, and the scaling-up possibility to produce large surface thin films.

Depending on the ZnO preparation method and conditions, powder or thin film could be obtained. Although ZnO powders have higher photocatalytic performance compared to thin films, the major advantage of using ZnO thin films in photo-catalytic applications rather than ZnO powders is the better recovery for reusability. In fact, for ZnO thin films, the catalysts and reaction products do not have to be separated, as it is the case for ZnO powders or colloidal suspensions [START_REF] Rajeshwar | Heterogeneous photocatalysis treatment of organic dyes in air and aqueous media[END_REF]. Moreover, using ZnO in thin films form can prevent or decrease light scattering, which increases the transmittance of light, and would result in an enhanced photocatalytic activity [START_REF] Otsuka | An application of transparent mesoporous bulk silica to a titanium dioxide photocatalyst with adsorption and decomposition functions[END_REF]. However, thin films photo-catalysts generally have lower surface areas compared to powders, leading to surface area constraints, and inferior intrinsic photocatalytic performance in comparison to powders [START_REF] Sopyan | An efficient TiO2 thinfilm photocatalyst: photocatalytic properties in gas-phase acetaldehyde degradation[END_REF][START_REF] Gan | Photoelectrocatalytic activity of mesoporous TiO2 films prepared using the sol-gel method with tri-block copolymer as structure directing agent[END_REF]. The foremost impact of surface morphology, crystal structure, compactness and surface defects on the photocatalytic and photovoltaic performance of semiconductors is largely discussed in the literature [START_REF] Wang | Nanostructured VO2 photocatalysts for hydrogen production[END_REF]. In fact, thin films exhibiting smaller grain size are reported to have higher surface area, which facilitates and increases the interaction between the photo catalyst's surface and the adsorbed species or sunlight [START_REF] Yu | The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO2 thin films prepared by liquid phase deposition[END_REF].

Many work in the literature established the effect of UV light exposure on the wettability and therefore the photocatalytic activity of ZnO. It has been reported that hydrophilic property of ZnO coated surfaces containing photo-catalytic particles can be triggered by the ultraviolet irradiation. Upon exposure to UV light, electron/hole pairs ( /ℎ ) are generated in the surface of ZnO thin films, which results in a switching in the wetting behavior from hydrophobic to hydrophilic according to the following process [START_REF] Khranovskyy | Surface morphology effects on the light-controlled wettability of ZnO nanostructures[END_REF]:

ZnO + 2ɦѵ → 2ɦ + + 2 e - (1) O 2-+ ɦ + → O 1 -(Surface trapped hole) Zn 2+ + e -→ Zns + (Surface trapped electron) (2) (3) O 1-+ ɦ + → ½ O2 + Vo (Oxygen vacancy) (4)
The reaction of photo generated holes with lattice oxygen (Eq. 2) results in the formation of oxygen vacancies, and the reaction of electrons with lattice Zinc ion leads to the formation of defective (

) sites (Eq. 3). The presence of oxygen vacancies at the surface of ZnO films increases the water adsorption at the film's surface when irradiated with UV light, making the surface more hydrophilic. The photo catalytic performance of thin films is deeply affected by several factors such as the crystal structure, surface area, crystallite size, chemical composition and surface defects and film thickness [START_REF] Sheng | Thickness dependence of photocatalytic activity of anatase film by magnetron sputtering[END_REF][START_REF] Kumar | Zinc oxide based photocatalysis: tailoring surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications[END_REF]. All these factors are entangled and their effects on the photocatalytic performance of the films are difficult to untangle for immobilized thin films. They are also very sensitive to the deposition technique and the deposition parameters.

Recent work by Ramirez-Canon et al. [START_REF] Ramirez-Canon | Multiscale design of ZnO nanostructured photocatalysts[END_REF] show that the parameters that affect photocatalytic activity of immobilized photo catalysts are light absorbance capacity and crystal size, rather than surface area. Despite the fact that the photo catalytic reaction is fundamentally a surface reaction, the dependence of the photo catalytic activity on the film thickness has been observed and reported for different materials such as titanium dioxide [START_REF] Tada | Dependence of TiO2 Photocatalytic Activity upon Its Film Thickness[END_REF], indium tin oxide [START_REF] Kumar | Thickness Dependent Physical and Photocatalytic Properties of ITO Thin Films Prepared by Reactive DC Magnetron Sputtering[END_REF], hematite [START_REF] Dias | Extremely stable bare hematite photoanode for solar water splitting[END_REF], and zinc oxide [START_REF] Kenanakis | Photocatalytic degradation of stearic acid by ZnO thin films and nanostructures deposited by different chemical routes[END_REF]. In most cases, the photo catalytic performance is shown to strongly depend on the film thickness.

In this work, transparent wurtzite ZnO thin films with nano-petal structures are prepared by the Ion Layer Gas Reaction technique. The effect of thickness and spray time on the optical, structural and morphological properties is investigated. Also, the photocatalytic activity is assessed by photoluminescence measurements as well as photo-electrochemical testing. This work attempts to correlate the photocatalytic activity with the surface morphology, crystallinity, structure and defects of the ZnO thin films, which are very important parameters to assess the performance of ZnO electrodes for photovoltaic and water splitting applications.

Experimental

Materials and Methods

The ZnO thin films were deposited by the Spray-ILGAR technique, following the procedure explained in our previous work [START_REF] Ennaceri | Deposition of multifunctional TiO2 and ZnO top-protective coatings for CSP application[END_REF]. Low iron soda lime glass (provided by Berliner Glas KGaA Herbert Kubatz GmbH & Co) was used as substrate. The precursor solution with 20 mM concentration was prepared by dissolving (C10H14O4Zn.H2O) in pure Ethanol using an ultrasonic device. Prior to the deposition, the glass substrates heated to a temperature of 400°C and the deposition was performed using Nitrogen as a carrier gas, with a flow rate of 5 liters per minute, and a spraying rate of 25 ml per 15 minutes.

Thin Film Characterization

The structural characterization and chemical composition of deposited thin films was achieved by X-ray diffraction using a Siemens D5000 diffractometer. The phase and crystal structure were identified using Cu-Kα radiation source with λ= 1.5406Å and a secondary monochromator in the range 2θ from 20 • to 80 • (cathode voltage: 40 kV, current: 40 mA).

The surface morphology of deposited thin film was examined by means of scanning electron microscopy on a Zeiss Ultra 55 microscope (acceleration voltage: 10 kV) and the atomic force microscopy (AFM) measurements were performed using a XE-70 SPM with a silicon tip (PPP-NCHR). The film thickness was also confirmed using SEM cross sections. The spectroscopic analysis (UV-VIS transmission and absorption spectra) was achieved using a Perkin Elmer Lambda 950 spectrophotometer. The photoluminescence measurements were conducted in a system consisting of a pulsed dye laser pumped by a nitrogen laser from Lasertechnik Berlin, with a prism monochromator from Zeiss, and a Si-photodiode detector which is time integrating over about 200mue sec.

The photo-electrochemical (PEC) measurements were accomplished in a Teflon cell equipped with a quartz window. Ag/AgCl and platinum were used as a reference and a counter electrodes, respectively. The photocurrents were conducted using an AM1.

Results and Discussion

Structural Properties

ZnO thin films with different thicknesses were deposited, by varying the deposition time, the spray times were fixed as 30 min, 45 min, 1 hour and 2 hours, achieving thin film thicknesses of 100 nm, 230 nm, 305 nm, and 720 nm, respectively. As it can be observed from Fig. 1 ZnO thin film thickness versus deposition time show a quasi-linear behavior. The XRD analysis (Fig. 2) shows the presence of the ZnO wurtzite phase according to the powder diffraction card of ZnO (JCPDS 36-1451). All deposited thin films with different thicknesses are oriented along (002) crystallographic plane, and the growth takes place along the c-axis direction, which is explained by the fact that the ZnO (0001) crystallographic plane is characterized by a thermal stability as well as a lower surface energy. It is shown from the XRD pattern (Fig. 2) that the increase of the ZnO thin film thickness results in more intense and narrower diffraction peaks. The full width at half maximum (FWHM) decreases with increasing thickness, indicating larger grain size and better crystallinity of deposited thin films. Close inspection of the XRD spectra reveals that all the XRD peaks shift towards higher angle with increasing thickness of ZnO, which indicates the occurrence of residual stress in the thin films. The shifting towards higher angles indicates that the thin films are under compressive strain [START_REF] Myoung | Effects of Thickness Variation on Properties of ZnO Thin Films Grown by Pulsed Laser Deposition[END_REF]. The 2θ values for the (002) crystallographic plane of all deposited ZnO thin films are larger compared to the bulk ZnO (2θ = 34.40°) [START_REF] Vinodkumar | Effect of cadmium oxide incorporation on the microstructural and optical properties of pulsed laser deposited nanostructured zinc oxide thin films[END_REF], which implies that the prepared thin films are exposed to compressive strain in the c-axis direction. Higher shifting in the diffraction angles is observed for thicker ZnO thin films compared to the standard reference value, which indicates higher compressive strain in the sample corresponding to the thickness of about 720nm.

The grain size can be estimated from the FWHM of the high intense peaks, namely the one corresponding to (002) crystallographic plane, using the Scherrer formula [START_REF] Cullity | Elements of X-Ray Diffraction[END_REF][START_REF] Scherrer | Bestimmung der Grösse und der inneren Struktur von Kolloidteilchen mittels Röntgenstrahlen[END_REF]:

= cos (5)
Where is the grain size, the constant K is the shape factor (K = 0.9), λ is the wavelength (λ = 1.54 Å), is the line broadening at half-maximum intensity (FWHM), and is the Bragg diffraction angle. It can be observed from Table 2 that the average crystallite size steadily increases with increasing thin film thickness, which is attributed to the fact that thicker thin films undergo longer heat treatment. The lattice constants for the ZnO hexagonal structure have been calculated using Bragg's law [START_REF] Cullity | Elements of X-ray Diffraction[END_REF]:

2 ! sin = (6)
Where ( ! ) is the inter-planar spacing given in Eq. ( 3) [START_REF] Rao | Thickness effect on structural, optical and electrical properties of ZnO thin films by Spray Pyrolysis[END_REF][START_REF] Gedamu | Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors[END_REF], is the Bragg diffraction angle, n is the diffraction order (n = 1), and λ is the $% & radiation wavelength (λ = 1.54 Å)

! = 1 ' 4 3 ( ℎ ) + + ) + ℎ+ , ) -+ . ) / ) (7)
The a-lattice constant of the ZnO thin films are estimated from the XRD peak corresponding to (100) crystallographic plane using the following equation [START_REF] Lupan | Effects of annealing on properties of ZnO thin films prepared by electrochemical deposition in chloride medium[END_REF]:

, = 2 sin (122) 4 4 3 (ℎ ) + ℎ. + + ) ) + 5 , / 6
) . )

, = √3 sin (122) (8)
The c-lattice constant of the ZnO thin films are estimated from the XRD peak corresponding to (002) crystallographic plane using the following equation:

/ = 2 sin (22)) 4 4 3 5 / , 6 ) (ℎ ) + ℎ. + + ) ) + . ) (10)
/ = sin (22)) (11)
The lattice constants of the bulk ZnO are (a = b = 3.2498 Å) and (c = 5.2066 Å) [START_REF] Morkoç | Zinc Oxide: Fundamentals, Materials and Device Technology[END_REF]. The calculated lattice constants of the ZnO thin films with different thicknesses are presented in Table 2.

In order to identify the nature of strain along the c-axis perpendicular to the film's substrate surface, a comparison between these deposited ZnO thin films and the lattice constant of the bulk ZnO (c = 5.2066 Å) is conducted. ZnO thin films exhibiting lower values compared to the bulk are subject to compressive strain along the c-axis, while thin films exhibiting higher values compared to the ZnO bulk are subject to tensile strain along the c-axis. In the present work, all deposited ZnO thin films exhibited lattice c values lower than that of the bulk ZnO, which implies that the ZnO thin films with different thicknesses are under the state of compressive strain along the c-axis perpendicular to the substrate surface, which implies a unit cell compression along the c-axis direction. Moreover, from the values presented in Table 2, it can be observed that the lattice parameters a and c decrease with increasing thin film thickness. Malek et al. expalined this phenomenon by the presence of dangling bonds, possessing unpaired electrons, and forming electric dipoles [START_REF] Malek | Thermal annealing-induced formation of ZnO nanoparticles: Minimum strain and stress ameliorate preferred c-axis orientation and crystal-growth properties[END_REF]. This will lead to the increase in the electrostatic attraction between the zinc and oxygen ions ( ) and 8 ) ) ions, and hence resulting in lattice compression. From the XRD patterns (Fig. 2), it can be observed also that the peaks are shifting towards higher angles compared to the bulk as the thin film thickness increases, indicating the presence of compressive strain. The unit cell volume of hexagonal structure of ZnO thin films is calculated from its lattice constants using Eq. 12:

9 = √3 2 , ) / (12)
The unit cell volume values of different prepared samples are presented in Table 3. It is apparent that the unit cell volume decreases with increasing thin film thickness and that the smallest unit cell volume is observed for the highest deposited thin film thickness of 720 nm. The decrease in the unit cell volume is a consequence of higher residual stress, which is enhanced with thin film thickness. The density of the deposited thin films is also computed using Eq. 13:

: = 1.6609 × ? × , ) / ' 3 4 ( 13
)
Where : is the density of different prepared thin films, M is the molecular weight of ZnO (M=81.408 g/mol for ZnO), and n is the number of molecules per unit cell (n=2), a and c are the lattice parameters of ZnO. The estimated density values are presented in Table 2, and show that the thin film density increases with the increasing of its thickness. This result is in line with the scanning electron microscopy analysis presented in Fig. 3. Additionally, the Zn-O bond length in the c-direction is also calculated from the lattice constants using the following equation:

@ = A( 1 3 -B , ) / ) C + 1 4 D / (14)
As shown in Table 3, a decrease in the Zn-O bond length in the c-direction is observed, with the increase in thin film thickness, which is in agreement with the previous results. For all deposited ZnO thin films, the calculated values of the lattice constant c are smaller than the value of the bulk ZnO. Therefore, the films are subject to compressive strain and unit cell compression along the c-axis direction, as shown from the lattice constant and strain values (Table 2 and Fig. 2). More evidence that all ZnO deposited thin films are under compressive strain in the c-direction is shown in Table 1. It can be seen also that the diffraction angles of (002) crystallographic plane for all deposited thin films are larger than that of the bulk ZnO. The impact of unit cell compression in the c-axis direction results in a tension force acting in the plane of the deposited thin film, and therefore, an increasing of tensile stress in the lateral direction is generated in deposited thin films. This can be explained by the fact that with the increasing deposition time, the adsorbing atoms have more energy to assemble to the lowest energy levels, and the atomic mobility increases to reduce the structural defects. Therefore, the grains grow bigger, and the density of the films increases (Fig. 3). This results are consistent with the crystallinity improving when deposited thin films thickness increases. The morphology of deposited ZnO thin films was studied by using filed emission gun scanning electron microscopy (FEGSEM) and the results are depicted in Fig. 3. It can be observed that the morphology of deposited ZnO thin films is flower like and cover the whole substrate.

Futhermore, ZnO deposited thin films cross section shows dense films with no distinguished porosity. In addition, deposited ZnO thin films keep the same morphology with the sheet section increasing faster than its thickness versus the deposition time, which is a signature of different steps of the same growth mechanism. From the XRD characterisation, it can be observed that the intense peaks correpond to the (100), ( 002) and (101) crystallographic planes, which could be assigned to the sheet surface of ZnO. However, closer analysis of the FEGSEM shows that most of the ZnO sheets are more or less perpendicular to the substrate (Fig. 3(e), (f), (g) and (h)). In addition, these XRD and FEGSEM results indicate that ZnO deposited thin films is of polycrystalline Wurtzite structure with random growth orientations. This later ZnO thin film characteristic could be attributed to the amorphous nature of used glass substrates for deposition.

The AFM investigations allowed the determination of the root mean square (RMS) roughness as 64nm, 118nm and 143nm, for film thickness of 100nm, 230nm and 305nm respectively. It can observed that the RMS roughness increases with deposited film thickness, whereas, a subsequent decreases of the RMS roughness (127 nm) is observed for highest thickness of 720nm.

Furthermore, it is well known that the substrate temperature has a strong influence on the properties of deposited ZnO thin films, according to large literature dealing with this issue. In fact, the increase of substrate temperature enhances the atomic mobility on the surface, reduces the structural defects, and favors a relaxation of the stress in deposited ZnO thin films. For example, it is reported in the literature that deposited ZnO thin films at a lower substrate temperature shows a strong compressive stress, whereas at a high temperature, the strain quickly relaxes due to the high kinetic energy of growing ZnO [START_REF] Benger | Role of strain and growth conditions on the growth front profile of InxGa1-xAs on GaAs during the pseudomorphic growth regime[END_REF]. Prasada Rao et al [START_REF] Rao | Physical properties of ZnO thin films deposited at various substrate temperatures using spray pyrolysis[END_REF] reported that ZnO thin film deposited at substrate temperature of 723 K shows low stress, which they explain by thin film thickness or morphology variation.

The average uniform strain (E FF ) in the lattice along the c-axis perpendicular to the substrate surface is estimated from the lattice parameters using the following:

E FF = / G / 2 / 2 × 100 (%) (15)
Where c denotes the lattice parameter of the strained hexagonal cell of ZnO thin films, calculated from the (002) XRD peak, and c0 denotes the unstrained lattice parameter of the bulk ZnO (c0 = 5.2066 Å [START_REF] Morkoç | Zinc Oxide: Fundamentals, Materials and Device Technology[END_REF]. Positive values of the strain (tensile strain) indicate a stretching of the material.

On the other hand, negative values of the strain (compressive strain) indicate that the material is being compressed, which is the case for the deposited ZnO films.

The impact of unit cell compression along the c-axis direction results in a tension force acting in the plane of the deposited thin film. Consequently, an increasing tensile stress is generated in deposited ZnO thin films. Therefore, even under compressive strain in the film plane, axial tensile stress is generated by Poisson's effect in the lateral direction (normal to the film plane).

The residual stress of the ZnO deposited thin films (: I ) is calculated using the following equation:

: I = 2$ 1J ) G $ JJ ($ 11 + $ 1) ) 2$ 1J × E FF (16)
Where cij denote the elastic stiffness constants (c11 = 208.8 GPa, c12 = 119.7 GPa, c13 = 104.2

GPa, c33 = 213.8 GPa [START_REF] Barret | Structure of Metals[END_REF]. Using the indicated values of the elastic stiffness constants, the residual stress is calculated using the following formula:

: I = G233 × E FF KLM,N (17)
Positive values of the stress indicate that the stress is tensile, whereas negative values of the stress indicate that the stress is compressive [START_REF] Li | Research on the properties of ZnO thin films deposited by using filtered cathodic arc plasma technique on glass substrate under different flow rate of O2[END_REF]. The calculated values of strain and stress are indicated in Table 3. It can be observed that the residual stress increases with increasing ZnO deposited thin film thickness (Table 3), which is due to the Zn-O bond length decreasing.

Generally, the total stress in thin films can be either intrinsic or extrinsic stress. The first, being introduced by imperfections during the growth process such as structural defects, distortions in the lattice constants or impurities, and the second being introduced by the difference in the thermal expansion coefficient between the ZnO films and the glass substrate than that of the glass substrate; therefore, a compressive in-plane stress is exerted by the substrate on the ZnO thin film as it cools down from the deposition temperature of 400 °$ to room temperature. The intrinsic stress in the ZnO thin films with different thicknesses is tensile, and its magnitude is higher compared to the compressive thermal component. Moreover, the thermal strain resulting from the difference in the linear thermal expansion coefficient of the ZnO deposited thin films and the glass substrate is much smaller compared to the measured strain [START_REF] Kumar | Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering[END_REF], showing that the stress in the thin films originates form the growth process rather than from thermal expansion. Hence, the stress in the films is intrinsic. Additionally, as it has been reported in the literature, extrinsic stress relaxes with increasing film thickness [START_REF] Kumar | Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering[END_REF]. In the present study, and since the ZnO deposited thin film thicknesses range from 100 nm to 720 nm, the extrinsic stress is ignored and the total estimated stress is predominantly intrinsic.

In the present work, the intrinsic strain of all the prepared samples is compressive in nature. As shown in Table 3, the strain increases with deposited ZnO thin films thickness. As a consequence, thicker ZnO thin film has the highest stress, implying that the degree of crystal imperfections is increases with deposited thin film thickness. It can also be noted that the increase of thin film thickness enhances its crystallite size and increases the residual compressive strain. Furthermore, the positive tensile stress in deposited thin films results from the stretching crystal size, indicating a decreasing lattice constant compared to the bulk ZnO. The tensile stress values increase from 0.22 GPA to 0.70 GPA, when the thin film thickness moves from 100 nm to 720 nm respectively (Table 3).

Optical Properties

The optical transmittance of deposited ZnO thin films decreases with increasing film thickness as shown in Fig. 4. (from left to right). It can be observed that all deposited thin films are highly absorbing at wavelength higher than 370 nm, which is the UV cut-off for ZnO absorption.

The optical band gap of the different deposited thin films was calculated by plotting the graph of the incident photon energy (hv) versus (&ℎR) ) , where & is the absorption coefficient that can be spectroscopically calculated from the measured optical transmittance using Eq. 18, where d is the thin film thickness and T is the optical transmission.

(

) 18
The absorption coefficient is related to the optical band gap as follows: It can also be observed that the optical band gaps of prepared ZnO thin films are lower than that of the bulk ZnO (3.37 eV). However, the blue-shift of the band gap becomes more evident with the increasing deposited thin film thickness, which is due to the improved crystalline structure with the increasing of spray time. Because of stress increasing (Table 3), the band gap was found to be increasing with increasing film thickness (as illustrated in Fig. 5). This is due to the decrease in the lattice parameter, which implies that the atoms get closer. It can be observed that the energy band gap increases as the compressive strain/tensile stress increases, which is in good agreement with previous studies that show that the strain affects the inter-atomic spacing of semiconducting materials, and that the band gap increases with compressive strain along the c-direction [START_REF] Srikant | Optical absorption edge of ZnO thin films: The effect of substrate[END_REF]. When the lattice parameter becomes smaller (compressive strain), atoms get closer, therefore their valence electrons get closer, which increases the electrostatic potential energy (U), and makes it more difficult for the electrons to move to the conduction band, which consequently increases the band gap.

In order to validate the experimental results, theoretical calculations using the density functional theory (DFT) are conducted in order to investigate the effect of compressive strain on the optical properties of ZnO thin films. The theoretical study was conducted through ab initio first principle calculations within the WIEN2k package [START_REF] Blaha | WIEN2k, Augmented Plane Wave + Local Orbitals Program for Calculating Crystal Properties[END_REF]. The PBE approximation (Perdew Burke

Ernzerhof) [START_REF] Perdew | Generalized gradient approximation made simple[END_REF] was used to calculate the exchange and correlation contributions, and the new modified Becke-Johnson (mBJ) potential [START_REF] Tran | Accurate band gaps of semiconductors and insulators with a semi local exchange-correlation potential[END_REF] was used:

( ) ( ) ( ) ( ) ( ) , , 2 1 5 3 2 12 mBJ BR t r r c r c n r σ χ σ χ σ σ υ υ π = + - (20)
Where:

• , BR χ σ υ : the Becke Roussel (PB) potential
• t σ : the spin dependent kinetic energy density

• n σ : the spin dependent electron density

• C: a system dependent parameter

The PB potential was initially suggested to imitate the Slater potential. The system dependent parameter c can be determined according to Tran and Blaha work using the following equation for bulk crystalline materials [START_REF] Tran | Accurate band gaps of semiconductors and insulators with a semi local exchange-correlation potential[END_REF]:

( ) ( )

1 2 3 ' 1 d ' ' n cell cell r c r V n r α β   ∇ = +       ∫ (21)
The Monkhorst-Pack division was used to create a set of k-points within the Brillouin zone (lmax = 10, Rmt*Kmax = 7.0). The experimental optical results are compared to the theoretical calculations performed using the mBJ approximation. It can be observed from Fig. 6 that the mBJ approximation corroborates with the experimental results.

In order to compare with first-principle calcuations, we considered four sizes of hexagonal ZnO thickness ranging from 100 nm to 720 nm by using the experimental lattice parameters as indicated above. According to what was proved experimentally (Table 2), we found that the reduction of the lattice parameters comes by increasing the size of the thickness, which points out that there is a compressive strain on the structure. Taking into consideration the lattice parameters of the bulk ZnO as reference, we apply the compression strain for thicknesses of 100nm, 230 nm, 305 nm, and 720 nm, respectively. Afterward, we relax our structure to find the stable parameters. Then the geometry relaxation of all the considered systems maintain an almost unchanged hexagonal structure. As clearly shown in Fig. 6, the band gap from the total density of state (DOS) increases, when thin films thickness increases from 100 nm to 720 nm under a compressive strain, which is the case if compared to our experiemantal results. To confirm this behavior, we calculate the optical band gap as it is shown in Fig. 6(b) and as it can be clearly observed, the band gap increases with the thickness under a compressive strain, which is in agreement with our experiemantal results.

The calculated values of the optical band gap estimated with the simulation are 3.14 eV and 3.24

eV, for thicknesses of 100 nm and 720 nm respectively.

Chemical Characterization

X-ray photoelectron spectroscopy (XPS) measurements were performed in order to characterize and investigate the chemical state of deposited ZnO thin films with different thicknesses. The XPS spectra were recorded using Mg-Kα radiation (1256.6 eV), and the extended scan is shown in Fig. 7. The XPS spectra corresponding to the Zn 2p shows sharp peaks, indicating the presence of Zn 2+ lattice ions. [ [START_REF] Mosquera | Exciton and core-level electron confinement effects in transparent ZnO thin films[END_REF], showing that a shift of core-level energies to lower binding energies is possible when the grain size increases, and leading to quantum confinement effect.

As checking the stoichiometric formation of deposited ZnO thin films, the Zn/O ratio was determined to be 1.12, 1.16, 1.39, and 1.59 for thicknesses of 100 nm, 230 nm, 305 nm, and 720 nm, respectively. It can be observed that the atomic ratio of Zn to is higher than 1 for all samples, which suggests the presence of oxygen vacancies at the surface of the prepared samples [START_REF] Ennaceri | Thickness effect on the optical properties of TiO2-anatase thin films prepared by ultrasonic spray pyrolysis[END_REF]. Oxygen vacancies act as active sites, and therefore, can easily combine with other reactive species. The atomic ratio of Zn to O increases with the thickness, suggesting an increase in the concentration of oxygen vacancies as the deposition time increases. Also, there is a direct relation between oxygen vacancies and visible light absorption [START_REF] Han | Controlling Morphologies and Tuning the Related Properties of Nano/Microstructured ZnO Crystallites[END_REF], which explains the increase of optical absorption with increasing film thickness.

Wetting Properties and Photoelechtrochemical Test

The water contact angles measurements show an inverse relation between the contact angle and

On the other hand, the Cassie-Baxter model is expressed as follows:

(

) 23
Where (T) is the Cassie-Baxter roughness factor, which is the fraction of the liquid-solid interface. Both the Wenzel model and the Cassie-Baxter models show that the hydrophobicity increases with surface roughness. However, an inverse relationship between the surface roughness and the contact angle is observed for the ZnO films with different thicknesses. The water contact angles are 98°, 90°, 84°, and 82° for 100 nm, 230 nm, 305 nm, and 720 nm, respectively (Fig. 9).

According to the Cassie-Baxter model, an increase of the surface fraction (T) would lead to a decrease in the water contact angle ( S ). The roughness can increase if the supporting nanopetals under the water droplet become closer to each other and roughly perpendicular to the substrate. Since in most cases, the superhydrophobic behavior is due to the air trapped within the surface nanostructure, the distance between nanopetals in the present deposited nanostructured must be as small as possible [START_REF] Shirtcliffe | Superhydrophobic textures for microfluidicsAdv[END_REF]. However, the size and the orientation of nanopetals in deposited ZnO thin film will influence strongly the surface roughness, which can influence in turn the pinning effect of the surface roughness. The water contact angles of prepared samples were recorded under exposure to UV irradiation for different times ranging from 30 minutes to 210 minutes. The contact angle measurements were taken every 30 minutes time interval. A decrease in the contact angle is observed for all samples and a surface transition between hydrophobic and hydrophilic is perceived. After 3.5 hours of UV exposure, the lowest contact angle is observed for the highest thickness of 720 nm (Fig. 10). This is explained by the increase in the water adsorption on the film's surface with increasing film thickness. In order to investigate the influence of annealing treatment on the surface of deposited ZnO thin films, the roughness and the surface tension were measured before and after the treatment. By using the AFM, the roughness of thinner (100nm) and thicker (720nm) deposited ZnO thin film was measured before and after annealing treatment at temperature of 500°C and over 60 min.

The obtained results show that the roughness decreases for both samples. It was observed that the RMS roughness decreases from 64 nm to 30 nm for thinner deposited ZnO thin film (100 nm)

and from 127 nm to 69 nm for thicker film (720 nm) (Fig. 10). As a consequence, a water contact angle was observed to decrease from 98° to 91° and from 82° to 79° for deposited ZnO thin film with thickness of 100 nm and 720 nm respectively. These results are in good agreement with the fact that the heat treatment induces the roughness decreasing and the appearance of surface hydrophilic behavior.

The PL spectra of the sprayed ZnO thin films are depicted on Fig. 11 and the measurements were performed in the range from 370 nm to 800 nm, with 365 nm excitation wavelength. Two main PL emissions are observed: the first peak is a UV-emission peak at 381 nm and the second is a broad green emission peak at 518 nm. The UV emission peak is attributed to free exciting recombination, whereas the broad green emission at 518 nm corresponds to the structural defect, such as oxygen vacancies. As observed from the close inspection in Fig. 11, the PL intensity peak near 518 nm of ZnO samples increases with the increasing deposition time, which indicates the presence of more surface oxygen vacancies, while increasing the thickness of deposited thin films. This result corroborates with the XPS measurements discussed in the previous section. These surface defects can drastically affect the photocatalytic activity of deposited ZnO thin films.

The intensity of the PL signal is an index that reveals the recombination rate of the photogenerated charge carriers. Lower thicknesses of deposited ZnO thin films exhibit lower PL intensities compared to higher thicknesses, which suggest lower recombination rate of the charge carriers, which can be beneficial for improving its photocatalytic properties.

In addition, and in order to investigate the effect of temperature on the PL emissions, post annealing treatment of the thicker deposited ZnO thin film (720 nm) was performed at 500 ℃ and 600 ℃ during 60 minutes. It can be observed that the PL intensity of the deposited thin films increases after annealing treatment (Fig. 11), therefore, it can be concluded that exposing deposited thin films with higher deposition time at elevated temperatures, exhibits the same behavior as performing post-deposition annealing treatment. Moreover, the scattered light from the samples surface (stronger offset) increases due to the change in the surface roughness. A PL shift towards shorter wavelength is also observed (Fig. 11) after post annealing treatment, which can be a result of the scattered light of the excitation beam.

The PL measurements suggest that the photocatalytic activity decreases with increasing spray time and in turn deposited thin films thickness. This can be explained by the increase in the grain size with the spray time, and that thinner thin films have higher surface area (also observed form SEM images). Hence, we get greater interfacial contact with the chemical species in the solution, promoting redox reactions. Also, and as can be observed form the SEM images ( The band gap energy was measured from PL spectra, and the results showed that it increases from 3.23 eV to 3.26 eV with increasing the thickness of deposited ZnO thin films. These results are in good agreement with those of theoretical calculations of DFT and calculations from the optical absorption spectra. In order to further confirm these results, the effect of annealing temperature on the bad gap energy was investigated. Deposited ZnO thin films were annealed in the air atmosphere at temperature of 500 °C and 600 °C, during 60min. The band gap energy was calculated from the PL spectra, and the results showed an increasing with the annealing temperature from 3.27 eV at temperature of 500 °C to 3.275 eV at temperature of 600 °C.

The photocatalytic activity of deposited ZnO thin films was assessed by measuring the degradation of methylene blue under UV irradiation. The obtained results show a decrease of MB degradation rate, with the increase of the film thickness of deposited ZnO thin films (insert Fig. 12).

As shown in Fig. 11, the highest and lowest UV-emission PL peak intensity (381 nm) are observed for deposited ZnO thin film with thicknesses of 305 nm and 100 nm respectively.

These results are in good agreement with those of PEC measurement (Fig. 12), which show high photocurrent for deposited ZnO thin film with thickness of 100 nm and its decreases with increasing of thickness. Both the photoluminescence and photo-electrochemical measurements suggest that the photocatalytic activity decreases with increasing the thickness of deposited ZnO thin films.

The photocatalytic reaction is directly related to the grain size of deposited thin films as to the surface area and therefore the recombination rate of the photo-generated electron/hole pairs. The average carrier diffusion time (U) from the bulk to the surface can be calculated by the following equation:

U = V ) W ) (24)
Where (V) is the grain's radius, and ( ) is the diffusion coefficient of the carrier [START_REF] Shang | Bi2WO6 Nanocrystals with High Photocatalytic Activities under Visible Light[END_REF][START_REF] Bell | The Impact of Nanoscience on Heterogeneous Catalysis[END_REF].

Therefore, a decrease in the grain radius would reduce the recombination of the photo-generated electron/hole pairs moving to the surface, and therefore improves the photocatalytic performance. Moreover, a decrease in the grain's radius increases the surface area, which leads to the reaction sites increasing.

From the water contact measurements (Fig. 9), a surface wettability transition between hydrophobic and hydrophilic can be observed, which shows that the hydrophilic conversion is favorable with the presence of tensile stress in the films. The effect of internal residual stress on the sprayed ZnO thin films has been shown to increase the photo-induced wetting sensitivity, but not the photocatalytic activity of the films, which is in good agreement with the literature [START_REF] Kenanakis | Photocatalytic degradation of stearic acid by ZnO thin films and nanostructures deposited by different chemical routes[END_REF].

In summary, from the photoluminescence measurement, which provides information about the charges separation and recombination in the photo-catalysts, it can be observed that the intensity of the PL peak increases with increasing the thickness of deposited ZnO thin films. The higher the intensity of PL signal, the higher the recombination rate of the photo generated electron/holes, and therefore, the photocatalytic efficiency decreases. This conclusion is supported by the photocatalytic tests conducted on deposited ZnO thin films of different thicknesses, which shows the best MB degradation with ZnO thin film thickness of about 100 nm. Furthermore, PEC measurements provide additional support to this conclusion in terms of higher photocurrent observed with ZnO thin film thickness of about 100 nm, in comparison to thicker films. On the other hand, further support is provided by the bang gap energy increasing with thickness.

Conclusion

The ZnO thin films deposited by the spray-ILGAR method were demonstrated to exhibit improved photocatalytic performance with decreasing spray time/film thickness. The XRD data was used to determine the structural parameters such as the crystallite size, lattice parameters, Zn-O bond length, and residual stress, which revealed a decrease in the lattice parameters and a continuous variation in the compressive strain with the increasing spray time and in turn deposited ZnO thin film thickness. Furthermore, it was demonstrated by different techniques that the energy band gap increases as the compressive strain increases, which was confirmed by theoretical first principle calculations using the density functional theory (DFT). These results

show that the optical band gap of deposited ZnO thin films can be tuned by the application of compressive strain.

The photocatalytic efficiency is shown to decrease with increasing deposition time, which is explained by the increase in the structural and surface defects with the deposited ZnO thin film thickness. This favor the recombination process of electron/hole pairs.

The wettability of deposited thin films was examined and a wetting behavior switching from hydrophobic to hydrophilic was observed under exposure to UV irradiation. Furthermore, the effect of internal residual stress on the optical properties of sprayed ZnO thin films has been shown to increase the photo-induced wetting sensitivity, but not the photocatalytic activity of deposited thin films.

Figure 1 .

 1 Figure 1. Variation of the film Thickness with the deposition time.

Figure 2 .

 2 Figure 2. XRD pattern of ZnO thin films prepared with different spray times.

Figure 3 .

 3 Figure 3. SEM cross section and surface micrographs, and AFM images of deposited ZnO thin films with different thicknesses (a-c) 100nm, (d-f) 230nm, (g-i) 305nm and (j-l)720nm.

  . The linear thermal expansion coefficients of ZnO (& 11 = 6.05 × 10 O °$ 1 , & JJ = 3.53 × 10 O °$ 1 ) and soda lime glass (& JJ = 9 × 10 O °$ 1 ) [47]. The linear thermal expansion coefficient of ZnO is less

Figure 4 .

 4 Figure 4. Optical Transmittance Spectra of the ZnO thin films with different thicknesses. The blue shift can be observed from the inset image.

Fig. 4

 4 Fig. 4 shows the optical transmittance and Fig. 5(a) the optical absorption (A=1-T-R) spectra of deposited ZnO thin films with different thicknesses of 100 nm, 230 nm, 305 nm and 720 nm

( 19 )

 19 The optical band gap is obtained by the extrapolation of the straight dotted lines of Fig 5 (b) to (&ℎR) ) = 0. The calculated experimental optical band gap is 3.13 eV, 3.18 eV, 3.20 eV, and 3.24 eV for ZnO deposited thin film thicknesses of 100 nm, 230 nm, 305 nm and 720 nm, respectively.

Figure 5 .

 5 Figure 5. (a) UV-Vis Absorption Specta (b) Direct band gap of deposited ZnO thin films prepared with different spray times.

Figure 6 .

 6 Figure 6. (a) Total density of state of ZnO thin films with different thicknesses (b) Optical band gap of ZnO thin films with different thicknesses using the mBJ approximation

Figure 7 .

 7 Figure 7. Extended scan of XPS Spectra for the ZnO thin films with different thicknesses.

Figure 8 .

 8 Figure 8. (a) O 1s XPS spectra of ZnO thin films (b) Zn 2p 3/2 XPS spectra of ZnO thin films.

Fig 8 (

 8 Fig 8 (a) shows the O1s core level spectra, which is resolved in two peaks. The first peak (OL) centered at binding energy of 531.57 eV is ascribed to lattice oxygen in the ZnO crystal[START_REF] Chen | X-ray photoelectron spectroscopy and auger electron spectroscopy studies of Al-doped ZnO films[END_REF]. On the other hand, the second peak (OH) located at a higher binding energy of 533.25 eV, is attributed to the presence of oxygen vacancies in the ZnO matrix, which is in agreement with XPS results reported in the literature[START_REF] Zheng | XPS studies and photocurrent applications of alkali-metalsdoped ZnO nanoparticles under visible illumination conditions[END_REF]. As represented in Fig.8(a), deposited ZnO thin films by the ILGAR technique exhibit a strong dependency between the O/OH -ratio and the deposition time. By comparing the peak intensities (Fig 8 (a)), it can be observed that, while increasing the deposition time and film thickness, the main lattice oxygen peak (peak OL) increases whereas the hydroxyl peak (peak OH) decreases. This means that the quantity of the adsorbed OH -decreases with increasing film thickness, and can be explained by the reaction of Zn-based precursor on the surface during the deposition procedure. Therefore, the transformation of the Zn-based precursor (zinc hydroxide) to ZnO is dependent on the substrate temperature and the heating time. Hence, for a longer deposition time we have more oxide than hydroxide. Also, it can be observed form Fig 8 (b) that the Zn 2p 3/2 binding energy is shifting towards lower binding energy as the thickness increases. The binding energy for Zn 1p 3/2 is centered at 1022.9 eV for the lowest thickness of 100 nm (Fig 8 (b)), and is shifted to 1022.7 eV for the 720 nm thick ZnO thin film. These results are in good agreement with the work of Mosquera et al.

  the surface roughness, with the highest contact angle obtained from the ZnO thin film with the lowest thickness of 100 nm (WCA=98°). The surface morphology and wettability are related according to the Wenzel model and Cassie-Baxter model. The Wenzel model is expressed as follows: where r is the roughness factor, and ( S ) and ( ) are the apparent contact angles on rough and smooth surfaces, respectively:

Figure 9 .

 9 Figure 9. Water contact angle measurements under UV light exposition of different ZnO thin film thicknesses, the inserts are the corresponding contact angle before UV exposure.

Figure 10 .

 10 Figure 10.Surface morphology change of different deposited ZnO thin films, (a, b) 100 nm thickness (c, d) 720 nm thickness before (left) and after (right) annealing treatment at temperature of 500°C and over 60min.

Figure 11 .

 11 Figure 11. Photoluminescence spectra of sprayed ZnO thin films with different thicknesses

Fig 3 )

 3 , the ZnO nano-petals get larger in dimensions, and that the films become denser. Therefore, increasing the spray time leads to grain coalescence, which is shown by the decrease in the water contact angle (Fig9).

Figure 12 .

 12 Figure 12. Linear sweep voltammograms of sprayed ZnO thin films with different thicknesses. The insert plot show the MB degradation versus the irradiation time.

Table 1 .

 1 Comparison between the 2 θ values for the (002) plane with the standard value for ZnO value.

		2 values for the (002) plane
	Sample	Observed	Standard [34]
	100 nm	34.443°	34.40°
	230 nm	34.463°	34.40°
	305 nm	34.501°	34.40°
	720 nm	34.516°	34.40°

Table 2 .

 2 Lattice constants, density, and crystallite size of ZnO thin films deposited with different spray times.

	Lattice

a=b(Ȧ) constant Lattice c(Ȧ) constant Density (g/cm 3 ) c/a Ratio Crystallite Size (nm) Sample Calculated Standard Calculated Standart Calculated Standard

	100nm	3.2480		5.2015		5.6883		1.6014 24.86
	230nm	3.2432	3.2498	5.1985	5.2066	5.7083	5.675	1.6028 25.73
	305nm	3.2431		5.1930		5.7150		1.6012 26.11
	720nm	3.2425		5.1902		5.72		1.6	27.04

Table 3 .

 3 Unit cell volume, Zn-O bond length, stress and strain of the ZnO thin films deposited with different spray times

		Unit cell	Volume V(Ȧ 3 )	Bond length	Stress (GPA)	Strain of c-axis
		Calculated	Standard	(Ȧ)		(%)
	100nm	47.5239		1.9765	0.22	-0.10%
	230nm	47.3565	47.62	1.9741	0.36	-0.15%
	305nm	47.3014		1.9733	0.61	-0.26%
	720nm	47.2598		1.9728	0.70	-0.32%

Funding Acknowledgment

The authors gratefully acknowledge the DAAD (Deutscher Akademischer Austauschdienst) for providing financial support to the first author to conduct this work.