Introduction. Les théories naïves de la matière pendant l'enfance.

Florence Labrell

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, 1994) seront présentées comme des intuitions ou comme un savoir plus ou moins construit.

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. On peut donc se demander si les théories naïves de la biologie (TNB) et de la physique (TNP) sont éducables.

Des classes logiques aux théories naïves en passant par les catégories

Pour situer l'émergence de la notion de TN, il est utile de faire un détour par des modèles antérieurs de la pensée enfantine. Sous l'impulsion de Jean Piaget, le développement mental a

longtemps été envisagé comme une succession d'étapes au cours desquelles l'enfant progresserait vers une organisation logique pilotée par l'équilibration de ses structures mentales [START_REF] Piaget | La Genèse des Structures Logiques Elémentaires[END_REF]. Ainsi, au cours de la petite enfance, la pensée serait égocentrique comme en témoignent les réponses d'enfants de 7 ans à la question d'où proviennent les nuages ? : ceux-ci mentionnent en général que les nuages sont solides, entièrement fabriqués par les hommes ou par Dieu (Piaget, 1947(Piaget, /2005)). Ce type de réponse artificialiste rejoint certaines réponses animistes relatives aux éléments vivants : à 8 ans et demi, Vog rapporte qu'un poisson est vivant parce qu 'il nage (2005, p.166). Dans ce cadre explicatif du développement conceptuel l'enfant classerait donc d'abord les éléments de son environnement en fonction de leurs caractéristiques de surface sans conceptualiser une organisation taxonomique plus générale.

Les années 1990 ont ensuite vu la publication de plusieurs résultats de recherches sur les très jeunes enfants, grâce à la mise en place de méthodes d'études adaptées (habituation tactile et/ou visuelle, par exemple) et de choix d'objets plus valides écologiquement, correspondant à ceux utilisés par les enfants [START_REF] Bideaud | Le développement des catégorisations : « capture » logique ou « capture » écologique des propriétés des objets ?[END_REF]. Ces résultats ont mis en évidence des attributions de propriétés du vivant précoces puisqu'à 3 ans, les enfants attribuent plus de propriétés biologiques (grandit, manger, mourir, etc…) aux animaux qu'aux plantes et aux objets manufacturés [START_REF] Inagaki | Young children's recognition of commonalities between animals and plants[END_REF]. Selon [START_REF] Mandler | Drinking and driving don't mix: inductive generalization in infancy[END_REF], dès 9 mois, le bébé catégorise le monde environnant en distinguant les objets animés des objets non animés et les principes de base qui les organisent : le bébé fera ainsi la distinction entre des oiseaux et des avions qui se ressemblent perceptivement mais ne sont pas la même sorte de chose (kind). Par ailleurs, en 1991, Henry Wellman et Susan Gelman ont montré, au travers de cinq études réalisées sur des enfants de 3 à 5 ans, que les plus jeunes étaient non seulement capables de faire la distinction entre la surface des objets, c'est-à-dire leur apparence, et ce qu'ils possèdent en eux (insides), mais aussi d'allouer à l'« essence » des objets une importance plus grande plus grande que l'apparence pour définir l'identité d'un objet et sa fonction.

On est donc passé de la représentation d'un enfant capable de classer logiquement les objets selon des propriétés générales (forme, couleur, aspect, localisation, etc..) à celle d'un enfant qui organise son environnement selon sa théorie de classement. Les bases de l'émergence des TN (folk theories) sont alors posées [START_REF] Murphy | The role of theories in conceptual coherence[END_REF]. Il s'agit « comme les théories scientifiques [ …] (de) représentations abstraites, complexes et cohérentes, de la façon dont le monde fonctionne, c'est-à-dire de sa structure causale » (Bonthoux & Blaye, 2007, p.110).

Théories naïves : constitution, origines et fonctionnement.

Le cas particulier de la matière (biologie et physique)

Théories naïves : constitution, origines et développement

Il semble donc que même très jeunes, les enfants ne se contentent pas de regrouper des objets selon leurs apparences, mais sur la base de théories extraites de propriétés internes non observables (core properties) et de principes fondamentaux [START_REF] Mandler | How to build a baby: II. Conceptual primitives[END_REF]. Par ailleurs, une cohérence conceptuelle est repérable dans ces regroupements puisque les concepts ne se développent pas de façon isolée, mais dans des domaines spécifiques [START_REF] Carey | Conceptual Change in Childhood[END_REF][START_REF] Wellman | Knowledge Acquisition in Foundational Domains[END_REF]. Les enfants pourraient donc expliquer, interpréter et faire des prédictions dans trois domaines de connaissances particuliers : le domaine relatif à la matière comme la physique et la biologie, et le domaine des théories de l'esprit, relatif aux pensées, savoirs, désirs, et besoins. Notons que cette approche de la cognition par domaines de connaissances, en lien avec des recherches sur l'évolution des bases neurologiques [START_REF] Sperber | The modularity of thought and the epidemiology of representations[END_REF], s'est complètement dissociée d'une approche unitaire de la pensée dans laquelle les mêmes processus cognitifs sont mobilisés quel que soit le contenu.

On trouve l'origine des TN dans deux grands courants explicatifs : celui de la modularité [START_REF] Leslie | ToMM, ToBy and agency: core architecture and domain specificity in cognition and culture[END_REF][START_REF] Spelke | Physical knowledge in infancy: reflections on Piaget's theory[END_REF] et celui de l'expertise [START_REF] Carey | Conceptual Change in Childhood[END_REF][START_REF] Vosniadou | Mental models of the earth: A study of conceptual change in childhood[END_REF] ou de la théorie des théories (Gopnik & Melzoff, 1997). Dans le premier courant modulariste on considère que les bébés possèdent dès la naissance des compétences cognitives relatives aux caractéristiques physiques des objets qui les entourent, comme la gravité [START_REF] Spelke | Physical knowledge in infancy: reflections on Piaget's theory[END_REF]. Les distinctions ontologiques entre la matière et l'esprit seraient innées, selon [START_REF] Keil | The birth and nurturance of concepts by domains: The of concepts of linving thing[END_REF]. Ces connaissances en germe vont contraindre l'apprentissage de nouveaux concepts, comme la permanence de l'objet. De son côté, le courant basé sur l'expertise considère que l'enfant développe au contact des interactions avec son milieu des systèmes de connaissance reliées causalement, formant des réseaux hiérarchiques complexes [START_REF] Wellman | Knowledge Acquisition in Foundational Domains[END_REF]. Par exemple, dans ses recherches sur l'acquisition du concept d'héritage génétique, Springer a montré au travers de deux études sur les ressemblances physiques entre parents et enfants que dès 4 ans ceux-ci développent une TN de la parenté qui s'appuie sur des prémisses de type : les bébés se développent dans le ventre de leur mère auxquelles ils associent des inférences de type : les bébés partagent plus de propriétés avec leurs mères qu'avec d'autres animaux [START_REF] Springer | Acquiring a Naive Theory of Kinship[END_REF].

Ce deuxième courant explicatif insiste également sur le poids de la culture dans le développement des TN [START_REF] Atran | Cognitive foundations of natural history[END_REF][START_REF] Atran | Causal constraints on categories and categorical constraints on abiological reasoning across cultures[END_REF]. Un article récent sur les théories des origines de l'homme chez des participants de 7 à 77 ans a mis en évidence une majorité de conceptions théistes1 quel que soit l'âge, à Tanna, sur l'île de Vanuatu dans l'océan Pacifique où les théories de l'évolution ne sont pas enseignées, au contraire d'explications littérales de la Bible auprès d'une population majoritairement presbytérienne [START_REF] Busch | The coexistence of natural and supernatural explanations within and across domains and development[END_REF].

Dans ce sens, le rôle du langage en tant que pourvoyeur d'informations sur les catégories, et transmis par les autres personnes, a également été invoqué par [START_REF] Gelman | Learning from Others: Children's Construction of Concepts[END_REF], notamment comme mode de transmission d'inputs en termes d'essentialisme (voir plus bas pour ce type de raisonnement conceptuel).

Il existe deux options théoriques principales pour expliquer le développement des TN au cours de l'enfance et leur évolution progressive intra et inter domaine (psychologique, biologique et physique). D'une part, Susan [START_REF] Carey | Conceptual Change in Childhood[END_REF] considère qu'avant 7 ans, les enfants se basent surtout sur les similarités perceptives entre éléments pour décider de leur appartenance au domaine du vivant et s'appuieraient sur les mécanismes généraux des apprentissages, sans spécificité liée à la biologie. Les différences entre les connaissances des enfants et des adultes seraient incommensurables (sans aucune commune mesure) selon Carey du fait de l'existence de mécanismes généraux de développement. Il existerait toutefois une filiation entre les théories naïves biologiques (TNB) de l'enfance et celles de l'âge adulte, les enfants ayant déjà développé certains concepts biologiques. Les enfants posséderaient très tôt des TNB, vagues et imprécises avant que des modèles scientifiques leur aient été enseignés.

Certaines études ont suggéré que les enfants acquièrent les différents domaines de façon autonome, notamment celui de la biologie dès la petite enfance, même s'ils n'ont pas de connaissances précises [START_REF] Inagaki | Young children's recognition of commonalities between animals and plants[END_REF][START_REF] Keil | The birth and nurturance of concepts by domains: The of concepts of linving thing[END_REF][START_REF] Keil | Folkscience: coarse interpretations of a complex reality[END_REF]. Au contraire, selon [START_REF] Carey | Conceptual Change in Childhood[END_REF], le développement de chacun des domaines de connaissances ne serait pas indépendant.

Ainsi le domaine de la biologie se développerait à partir de TN de base sur l'intentionnalité (le coeur bat parce qu'il en a envie). Les enfants acquerraient des connaissances biologiques en prenant comme référence l'être humain : la probabilité qu'un enfant attribue une propriété biologique à un animal donné serait d'autant plus élevée que l'animal cible est un mammifère.

Selon Carey, les enfants possèderaient donc initialement une pensée anthropocentrique. Ça n'est que vers 10 ans qu'un domaine biologique autonome commencerait à se développer à la suite d'un accroissement des connaissances et d'un certain changement structurel, les théories antérieures fondées sur la psychologie évoluant vers des théories biologiques.

2.2.Le cas particulier des théories naïves de la matière (biologie et physique).

Les théories naïves en biologie (TNB) : propriétés spécifiques, raisonnements et limites

La maîtrise du domaine biologique requiert au préalable une activité de catégorisation basique permettant de distinguer les éléments vivants de ceux qui ne le sont pas, à savoir les objets naturels (soleil, montagnes, eau, pierres, etc.) et les objets fabriqués. Dans le sillage des travaux princeps de Piaget sur une première forme de connaissance du vivant en termes de mouvement, des résultats ont aussi montré que dès 3 ans, les jeunes enfants étaient capables de distinguer un mouvement produit par un élément vivant lui-même d'un mouvement activé de l'extérieur [START_REF] Poulin-Dubois | Le développement de la distinction animé-inanimé : Effet de l'apparence[END_REF].

Toutefois, les travaux sur la catégorisation précoce en termes de vivant ont mis en évidence que la distinction vivant/non vivant est contrainte par les taxonomies, les animaux étant par exemple de meilleurs candidats au domaine biologique que les plantes, notamment à cause de cette absence de mouvement [START_REF] Brulé | Children's Justifications of Plants as Living Things between 5 and 7 Years of Age[END_REF][START_REF] Fouquet | Children's Understanding of Animal, Plant and Artifact Properties Between 3 and 6 Years[END_REF][START_REF] Inagaki | Young children's recognition of commonalities between animals and plants[END_REF][START_REF] Meunier | La catégorie des plantes. Etude développementale de son organisation[END_REF][START_REF] Nguyen | Four and six-year olds' biological concept of death: The case of plants[END_REF][START_REF] Stavy | Children's conceptions of plants as living things[END_REF].

Au-delà des catégories qui le composent, le domaine biologique possède également selon Frank [START_REF] Keil | The birth and nurturance of concepts by domains: The of concepts of linving thing[END_REF] un statut particulier car les conceptions qui en émanent peuvent, plus que celles issues de la physique ou de la psychologie, s'appuyer sur nos intuitions quotidiennes. De plus, les éléments vivants sont caractérisés par des transformations telle la croissance, de même qu'ils possèdent une structure interne complexe que ne possèdent pas les éléments non vivant même naturels, comme l'or ou l'eau. Enfin, les éléments vivants possèdent tout un ensemble de propriétés intrinsèques (reproduction, alimentation, mort, par exemple) qui ne sont pas seulement liées aux intentions humaines.

Pour appréhender et expliquer les éléments vivants, enfants et adultes développent des raisonnements particuliers, induits par le domaine, comme l'essentialisme ou la téléologie [START_REF] Keil | The birth and nurturance of concepts by domains: The of concepts of linving thing[END_REF], ou encore le vitalisme [START_REF] Hatano | Young children's naive theory of biology[END_REF][START_REF] Inagaki | Young children's naive thinking about the biological world[END_REF][START_REF] Morris | Vitalism in Naive Biological Thinking[END_REF].

De façon très globale, on peut considérer que l'essentialisme correspond à un type d'explication causale du fonctionnement biologique en termes d'essence interne qui influerait sur certains comportements [START_REF] Gelman | The essential child: origins of essentialism in everyday thought[END_REF], en plus qu'elle définirait l'identité de l'objet [START_REF] Ahn | Why essences are essential in the psychology of concepts[END_REF].

Par exemple, les jeunes enfants peuvent affirmer que telle plante peut mourir, parce que quelque chose à l'intérieur la fait mourir. Ce mode de raisonnement est très général et ne permet pas d'expliquer véritablement des mécanismes biologiques précis. C'est pourquoi il a été considéré faisant office de « bouche trou » (placeholder), préalable à des explications plus structurées. Il existerait de fait deux types d'explications essentialistes pouvant correspondre, soit à un raisonnement peu élaboré (cela dépend de la nature de l'objet), soit à un raisonnement plus abstrait (« essentialisme interne », lié à l'hypothèse de l'existence d'une composante intrinsèque, comme l'ADN, par exemple), selon [START_REF] Simons | An abstract to concrete shift in the development of biological thought[END_REF].

De son côté, le raisonnement de type téléologique concerne la finalité des comportements (les tournesols se tournent vers le soleil pour pouvoir en profiter). Toutefois, d'un point de vue strictement ontologique, seuls les éléments vivants peuvent agir pour obtenir des effets qui leur seraient bénéfiques. Une étude sur l'évolution des justifications enfantines a mis en évidence que les plus jeunes attribuent une cause psychologique à des comportements téléologiques (la machine se déplace vers le mur parce qu'elle pense qu'il y a de l'électricité), et qu'avec le développement, grâce à des connaissances biologiques plus abstraites, ils passeraient d'un modèle téléologique basé sur la psychologie à un modèle téléologique basé sur la biologie [START_REF] Opfer | Children's and Adults' Models for Predicting Teleological Action: The Development of a Biology-Based Model[END_REF]. Ce type d'explication téléologique caractérise à la fois un manque de connaissances scientifiques précises, typique des enfants, mais aussi possiblement le raisonnement d'adultes face à de phénomènes complexes liés au domaine naturel non vivant, par exemple, dans l'explication de phénomènes géologiques : à quoi servent les montagnes ? [START_REF] Kelemen | Function, goals and intention: Children's teleological reasoning about objects[END_REF].

Le raisonnement vitaliste quant à lui, repose sur l'hypothèse d'une force indépendante du corps qui transmettrait intentionnellement une énergie vitale aux différents organes. Cette force vitale, prélevée dans l'eau et la nourriture, permettrait aux êtres vivants de se développer, d'être actifs et d'éviter les maladies [START_REF] Inagaki | Vitalistic causality in young children's biology[END_REF]. Ainsi, selon [START_REF] Hatano | Young children's naive theory of biology[END_REF], le vitalisme serait une causalité intermédiaire entre la causalité psychologique qu'est l'intentionnalité et la causalité biologique (de type mécaniste). Certains auteurs considèrent qu'à l'instar de l'essentialisme, chez les jeunes enfants, le vitalisme aurait aussi une fonction de placeholder par rapport aux explications biologiques. En se développant, les enfants substituent progressivement la notion de vitalisme à celle d'intentionnalité pour expliquer le fonctionnement des organes. Puis, grâce à l'acquisition de connaissances et de compréhension des systèmes biologiques, l'approche mécaniste remplace le vitalisme chez les adolescents et les adultes [START_REF] Morris | Vitalism in Naive Biological Thinking[END_REF].

Les théories naïves en physique (TNP) : dimensions concernées, structures des connaissances, origines

Contrairement aux TNB, les TNP reposent moins sur des intuitions humaines, car aucune expérimentation directe n'est facilement envisageable. C'est particulièrement le cas des conceptions sur la forme de la Terre, vis-à-vis desquelles des connaissances contre-intuitives sont attendues [START_REF] Frède | Acquisition et construction de connaissances contre-intuitives en sciences[END_REF] puisque la Terre, contrairement aux apparences, n'est pas plate mais ronde.

Les TNP ont concerné des dimensions très variées relatives soit aux objets fabriqués, soit aux éléments naturels non vivants (la Terre d'un point de vue spatial ou géologique, les montagnes) et à leurs manifestations (pluies, séismes, son, chaleur). Pour résumer il s'agit de toutes les dimensions physiques qui affectent les êtres vivants et les choses. Cependant, la chaleur, le son ou la lumière qui en émanent ne correspondent pas directement à la matière, mais à celui des processus qui l'affectent selon Chi, 1997 (cité par [START_REF] Lautrey | Is children naive knowledge consistent? A comparison of concepts of sound and heat[END_REF].

L'étude du développement des TNP a concerné les conceptions initiales en physique observables dès les premiers mois de la vie. Les travaux de Renée Baillargeon et d'Elisabeth Spelke ont notamment montré, grâce au paradigme de l'habituation visuelle, que dès 2 mois et demi, le bébé humain perçoit plusieurs régularités régissant le mouvement des objets (comme l'inertie, la gravité, la solidité), et réagit visuellement si leur mouvement est improbable par modification artificielle [START_REF] Spelke | Physical knowledge in infancy: reflections on Piaget's theory[END_REF]. Les intuitions ou présuppositions relatives au mouvement s'organiseront au sein de TNP initiales dont les jeunes enfants n'auront pas forcément conscience mais dont on peut faire l'hypothèse qu'elles guideront l'acquisition de leurs connaissances ultérieures. La question qui se pose est celle de l'origine de l'organisation de ces connaissances puisque l'acquisition d'une connaissance au temps T repose sur une structure préexistante au temps T-1, elle-même dépendante d'une autre structure au T-2 et ainsi de suite [START_REF] Lautrey | Is children naive knowledge consistent? A comparison of concepts of sound and heat[END_REF].

En résumé, deux approches théoriques différentes débattent de la structure des TNP. D'une part, le point de vue des modèles mentaux qui seraient basés sur des TN [START_REF] Vosniadou | Mental models of the earth: A study of conceptual change in childhood[END_REF], 1994) D'autre part, l'approche des connaissances fragmentées (« knowledge in pieces ») rend compte d'idées non systématiquement connectées, permettant à l'enfant de réduire la complexité du monde environnant grâce à des « primitives phénoménologiques » (diSessa, 1993). Il ne s'agit donc pas, au contraire des modèles mentaux, de connaissances organisées en théories, mais plutôt de connaissances inconsistantes qui paraissent évidentes (et donc « primitives »). Ces connaissances sont de nature phénoménologique, à savoir qu'elles ont simplement lieu, sans avoir à être expliquées. De nombreuses incohérences dans les explications de phénomènes physiques ont ainsi été mis en évidence chez des enfants comme chez des adultes, soutenant l'hypothèse que certains phénomènes physiques peuvent être expliqués par des connaissances « en morceaux ». Du point de vue du développement, les primitives deviendraient de plus en plus systématiques et abstraites avec réorganisation et enrichissement de principes initialement incompatibles. Avec le temps, les primitives se rapprocheraient finalement des conceptions scientifiques du fait des expérimentations actives réalisées par l'apprenant et grâce à l'enseignement. L'apprenant prendrait conscience de ses erreurs grâce à ses propres manipulations de primitives erronées [START_REF] Disessa | Toward an epistemology of physics[END_REF]pour une synthèse, voir Frède, 2019).

Ainsi, comme dans l'approche des modèles mentaux, les primitives sont basées sur les enseignements précoces. Le rôle des informations culturelles est particulièrement invoqué pour rendre compte de l'évolution des primitives dans les conceptions de l'astronomie, c'est-à-dire de phénomènes difficilement observables. Comme les intuitions sur la forme de la Terre sont souvent non congruentes avec les informations culturelles et /ou scientifiques, l'enfant va facilement abandonner les premières au profit des secondes [START_REF] Siegal | Culture and children's cosmology[END_REF]. Les artefacts comme le langage, les mythes mais aussi les livres et des outils (comme les globes terrestres, [START_REF] Schoultz | Heavenly Talk: Discourse, Artifacts, and Children's Understanding of Elementary Astronomy[END_REF], sont identifiées comme des sources culturelles du développement conceptuel. Ainsi, du point de vue des origines des TNP, les observations (même celles débouchant sur des erreurs conceptuelles par exemple sur la forme de la Terre), les enseignements scolaires (cours de physique) mais aussi des dimensions plus quotidiennes issues des témoignages ou des croyances culturelles, nourrissent tous la construction des TNP2 .

Les TN sont-elles éducables ? Exemples dans le domaine de la physique et dans le

domaine de la biologie.

Les TN sont-elles éducables ?

A travers la réponse à cette question se pose celle du changement conceptuel, autrement dit, de quelle manière les enfants transforment-ils leurs TN en théories scientifiques ?

A nouveau les points de vue divergent selon les courants théoriques. Dans l'approche des modèles mentaux, on considère que lors d'un changement conceptuel les conceptions initiales ne sont pas seulement enrichies mais aussi révisées et restructurées [START_REF] Wellman | Knowledge Acquisition in Foundational Domains[END_REF].

Selon [START_REF] Carey | Conceptual Change in Childhood[END_REF], en cas de contradiction entre les faits observables et les théories de l'enfant, celui-ci va les modifier en opérant des révisions proches des révolutions scientifiques issues des sciences [START_REF] Kuhn | Children and adults as intuitive scientists[END_REF]. Toutefois, ces restructurations pourraient être plus ou moins profondes : en cas d'enrichissement grâce à un apport d'informations (il n'y a ni eau ni air sur la Lune, Vosniadou & Mattews, 1992, cités par Frède, 2019) ou de changement radical, comme par exemple lors de la révolution copernicienne en astronomie. De plus, pour qu'il y ait révision, il faudrait que les TN des enfants soient en conflit avec des informations contredisant plusieurs traits de leurs théories [START_REF] Hayes | The role of diverse instruction in conceptual change[END_REF].

Dans l'approche des connaissances fragmentées, plusieurs études ont montré au contraire que les TN ne seraient pas restructurées par les apprentissages et connaissances scientifiques, mais coexisteraient avec elles [START_REF] Shtulman | Scientific knowledge suppresses but does not supplant earlier intuitions[END_REF][START_REF] Shtulman | Tensions Between Science and Intuition Across the Lifespan[END_REF]. Il y aurait à l'âge adulte comme pendant l'enfance, une tension entre les connaissances scientifiques et les TNP. Pour que les premières soient activées, il faut que les TNP soient inhibées, alors qu'elles correspondent à de riches explications basées sur l'expérience au contraire des raisonnements abstraits et complexes véhiculées par les connaissances scientifiques. Selon [START_REF] Shtulman | Tensions Between Science and Intuition Across the Lifespan[END_REF], les TN n'étant affectées ni par l'âge, ni par l'expertise, ni par le domaine de connaissances (TNP ou TNB) seraient à rapprocher des connaissances fragmentées [START_REF] Disessa | Toward an epistemology of physics[END_REF]. Leur résilience permettrait d'expliquer en partie le scepticisme des adultes non experts en connaissances scientifiques, la science étant beaucoup moins partagée que ce qu'on imagine.

Exemples de changement conceptuel dans le domaine de la physique et dans le domaine de la biologie.

Les recherches de Hayes, Goodhew, Heit et Gillan (2003) avaient pour objectif d'étudier la transformation des TNP d'enfants de 6 ans relativement à leurs modèles sur la forme de la Terre. A cet âge, les enfants ont un modèle non sphérique de la Terre alimentés par deux erreurs conceptuelles majeures (l'espace est organisé par rapport à un sol plat et les objets sans supports tombent vers le bas, voir plus haut). Dans leurs expérimentations, Hayes et collaborateurs se sont appuyés sur le principe de la diversité stipulant que les changements conceptuels des modèles naïfs ont plus de chances de se produire si les apprenants confrontent les faits à plusieurs dimensions de leurs modèles [START_REF] Heit | Properties of inductive reasoning[END_REF]. Dans ce sens, deux groupes composés pour chacun de 40 enfants de 6 ans ont été entraînés soit sur l'une des dimensions contraires à leur modèle non sphérique de la Terre (par exemple la taille relative ou les effets de la gravité) soit sur les deux dimensions. Un troisième groupe contrôle n'avait reçu aucune information supplémentaire. Les enfants de chacun des groupes avaient au préalable été interrogés grâce à une adaptation du questionnaire de Vosniadou et Brewer, 1992 Les résultats de l'étude ont montré que les enfants transféraient la propriété apprise sur les plantes et les animaux, mais pas sur les artefacts. De plus, les enfants réalisaient un reclassement au posttest seulement si l'entraînement avait porté sur c'est les actions orientées vers un but, validant ainsi leur hypothèse. [START_REF] Opfer | Revisiting preschoolers' living things concept: A microgenetic analysis of conceptual change in basic biology[END_REF] ont modélisé le changement conceptuel incluant les plantes au domaine du vivant selon cinq étapes :

1. Au début de l'expérimentation, la plupart des enfants de 5 ans ne pensent pas que les plantes peuvent avoir des comportements orientés vers un but, contrairement aux animaux.

2. De la même façon, ces enfants considèrent que les animaux ont ce type de comportements orientés pour assurer leur survie (les chats chassent des souris pour se nourrir).

3. Quand grâce à l'entraînement les enfants comprennent que les plantes peuvent aussi avoir ce type de comportement (les tournesols s'orientent vers le soleil), ils essayent de rapporter cette connaissance à d'autres connaissances proches sur une autre catégorie.

4. Ils comparent ainsi les plantes aux animaux qui chassent pour se nourrir. 5. Ils attribuent alors aux plantes cette même possibilité de comportement orienté vers un but qui leur confère un statut d'élément vivant, au même titre que les animaux.

Finalement, dans le domaine de la physique comme dans celui de la biologie, des changements conceptuels semblent possibles au cours de la petite enfance, même si ces changements varient en termes d'ampleur, d'orientation et de stabilité [START_REF] Carey | Sources of conceptual change[END_REF].

Pour ne pas conclure

L'approche du développement conceptuel en termes de TN a profondément renouvelé les théories classiques structuralistes, puisqu'au-delà des catégories, la question de l'organisation de celles-ci dans des réseaux fonctionnels plus larges a été posée : l'enfant intégrerait ainsi certaines caractéristiques des éléments liés à la matière dans un modèle de connaissances plus global, soit fragmenté (diSessa, 1993) soit organisé [START_REF] Vosniadou | Mental models of the earth: A study of conceptual change in childhood[END_REF], mais dans tous les cas susceptible d'évoluer grâce au développement mental [START_REF] Carey | Conceptual Change in Childhood[END_REF] et à l'environnement (social et culturel) [START_REF] Gelman | Learning from Others: Children's Construction of Concepts[END_REF][START_REF] Herrmann | Anthropocentrism is not the first step in children's reasoning about the natural World[END_REF]. En introduisant des objets et des caractéristiques plus écologiques dans ses paradigmes d'étude, comme les catégories des aliments ou encore un contexte animiste pour les conceptions de l'astronomie, le courant des TN pose la question de la signification de ces connaissances pour l'être en développement [START_REF] Keil | Folkscience: coarse interpretations of a complex reality[END_REF].

Le courant des TN a aussi permis d'envisager une proximité entre les conceptions adultes, en théorie plus matures, et celles des enfants destinées à s'améliorer : des travaux issus des sciences cognitives ont montré que certaines conceptions scientifiques erronées à l'âge adulte restent résistantes à long terme [START_REF] Shtulman | Tensions Between Science and Intuition Across the Lifespan[END_REF]. Les TN de l'enfance seraient-elles concernées ? Les travaux sur le changement conceptuel ont pourtant montré leur capacité à évoluer sous l'effet de divers dispositifs [START_REF] Hayes | The role of diverse instruction in conceptual change[END_REF][START_REF] Opfer | Revisiting preschoolers' living things concept: A microgenetic analysis of conceptual change in basic biology[END_REF], témoignant de l'éducabilité des TN. Dans le domaine du vivant, l'éducation à la santé devrait pouvoir par exemple profiter des études sur les TN du cancer dont les causes sont peu présentes au cours de l'enfance [START_REF] Deyra | Certitudes et doutes d'enfants d'école primaire sur le cancer[END_REF]. Plus généralement, l'appui sur les TN de la matière permettraient d'envisager des dispositifs plus efficaces pour l'amélioration des conditions de vie sur la planète, à l'heure où la jeunesse est en mesure de se mobiliser pour la mise en place de programmes environnementaux de protection de la nature (Bandura & Cherry, 2019).

  Même si le raisonnement de type vitaliste a été observé dans plusieurs cultures occidentales, sud-américaines ou Océaniques, son origine est asiatique et ancestrale. Le poids de la culture dans les différents types de raisonnements ne peut ainsi être négligé. Herrmann, Waxman et Medin (2010) ont d'ailleurs montré que, contrairement à l'hypothèse de Carey, la pensée anthropocentrique n'est une première étape du développement conceptuel que pour les enfants des villes, qui n'ont pas d'autres points de comparaison que les humains. De leur côté, les enfants vivant dans la ruralité sont capables d'attribuer des fonctions biologiques à tout un ensemble d'animaux et de plantes. On retrouve cet effet de la culture sur le raisonnement téléologique qui est plus répandu chez des adultes américains ayant bénéficié d'une instruction scientifique que chez des indigènes Quechua du Pérou qui interagissent au quotidien avec la nature dans un contexte de traditions religieuses de mise en relation des éléments vivants[START_REF] Tapia | Development of Teleological Explanations in Peruvian Quechua-Speaking and U.S. English-Speaking Preschoolers and Adults[END_REF].

  pour rendre compte notamment des conceptions sur la forme de la Terre et sur le cycle jour/nuit. Selon ces auteurs, au cours du développement des modèles naïfs de la Terre reposeraient entre autres sur deux présupposés (l'espace est organisé par rapport à un sol plat et les objets sans supports tombent vers le bas) basés sur des observations quotidiennes et des informations culturelles. Trois modèles sur la forme de la Terre se succéderaient au travers desquels l'enfant se baserait d'abord sur ses seules intuitions d'une Terre plate et soutenue (modèle « initial »), puis un modèle « synthétique », dans lequel l'enfant essaie aussi d'intégrer au modèle initial la notion de rotondité de la Terre, culturellement admise dans le monde occidental. Enfin, le modèle « scientifique » serait tardivement acquis dans l'enfance, vers 11 ans, à partir du moment où plusieurs connaissances sur la gravité et sur le système solaire sont acquises. Les réponses incohérentes des enfants, dans 20 % des cas, correspondraient à des modèles « mixtes ». Dans cette approche des modèles mentaux, le développement est envisagé comme linéaire et consiste au passage d'un modèle initial à un modèle scientifique en passant par un modèle synthétique. Les TN sur le son et la chaleur seraient elles-aussi organisées de façon cohérente, avec trois modèles mentaux successifs se développant graduellement (Lautrey & Mazens, 2004).

  sur la forme de la Terre permettant des réponses verbales et dessinées. L'entraînement consistait en la présentation de films scientifiques courts et adaptés sur chacune des dimensions. Les résultats de cette étude vont dans le sens des hypothèses de leurs auteurs puisque d'une part, les enfants du groupe contrôle avaient les mêmes modèles initiaux non sphériques au post test qu'au prétest. D'autre part, 66 % des enfants entraînés avaient changé de modèle au post-test en faveur d'un modèle sphérique. Autrement dit, ces auteurs ont montré la possibilité de stimuler le changement conceptuel de jeunes enfants relativement à la forme de la Terre en tenant compte de leurs présupposés initiaux. De leur côté, Opfer et Siegler (2004) ont étudié comment de jeunes enfants parvenaient à intégrer les plantes au domaine biologique du vivant, grâce à un entraînement basé sur leurs comportements téléologiques des plantes. L'hypothèse des auteurs était en effet qu'une des origines de l'intégration des plantes par l'enfant au domaine du vivant est la prise en compte de la capacité de celles-ci à développer, comme les animaux, des comportements orientés vers un but. Trois autres conditions devaient aussi être admises par l'enfant : les besoins vitaux des plantes, leur propriété de croissance, leur appartenance catégorielle au domaine du vivant comme les animaux et contrairement aux artefacts. Cent six enfants de 5 ans ont ainsi été inclus dans le design expérimental d'Opfer et Siegler (2004) et répartis dans 4 groupes. Lors d'un prétest, les enfants de chaque groupe devaient d'abord classer des animaux, des plantes et des artefacts en termes d'éléments vivants/non vivants. Le type d'entraînement dépendait ensuite du groupe d'attribution et pouvait s'appuyer sur l'une ou l'autre des quatre propriétés biologiques (comportement orienté vers un but, besoin de nourriture, croissance, catégorisation vivant/non vivant). Par exemple, l'enfant appartenant à un groupe basé sur la nourriture, bénéficiait d'apports sur les besoins en eau des items vivants, quel que soit le classement effectué au prétest. Lors du posttest, l'enfant retrouvant les mêmes items à trier que lors du prétest, pouvait le cas échéant les reclasser suite à l'entraînement.

  Cinq contributions à l'étude du développement des TNB et des TNP vont maintenant alimenter les points de vigilance précédemment énoncés. Au travers d'une revue de littérature sur le développement des propriétés de maladie et de mort, Florence Labrell interroge l'origine, la structure et l'éducabilité de ces concepts caractéristiques du domaine biologique. Elle envisage notamment le poids de la culture, des représentations et de l'expérience sur le développement des TNB, à l'heure où l'espèce humaine est concernée par une pandémie virale. La question de l'éducabilité et du rapport entre connaissances scientifiques et intuition sera posée. De son côté, Jean-Pierre Thibaut montre grâce à une synthèse sur les représentations des aliments, comment ceux-ci peuvent être classés dans un grand nombre de catégories différentes, taxonomiques, thématiques et scripts. Il décrit l'émergence progressive de catégories alimentaires, taxonomiques et thématiques de même qu'un raisonnement sur les propriétés des aliments, et leur insertion dans des théories explicatives de leurs effets biologiques. Dans une recherche plus empirique sur les raisonnements biologiques, Andrzej Tarlowski s'est intéressé au raisonnement d'enfants de 5 ans sur la ressemblance entre les parents et leur progéniture en termes d'innéité dans une tâche issue d'une nouvelle version du paradigme d'adoption inter-espèces. Ses résultats plaident en faveur d'un raisonnement essentialiste organisant la pensée précoce sur l'hérédité. Une autre revue de littérature réalisée par Mesut Sackès, Sinem Güçhan Özgül et Kerem Avci porte cette fois-ci sur les conceptions physiques du monde naturel, plus particulièrement sur les mécanismes des précipitations et des nuages, c'est-à-dire le cycle de l'eau. Les auteurs donneront des pistes éducatives de même que des perspectives de recherches dans ce domaine peu investigué.

Il s'agit de conceptions affirmant l'existence d'un dieu unique, créateur de l'univers. Le théisme est le contraire de l'athéisme.

Pour la distinction entre le développement des modèles mentaux et de la théorie fragmentée, voir l'article de Valérie Frède dans ce numéro thématique


domain: chim phys

text:
Fernando Plascencia-Hernández

G Valverde-Aguilar

M N Ghazzal

C Colbeau-Justin

H Remita

Heriberto Pfeiffer

M A Valenzuela

Unraveling the effect of low Cu 2 O loading on P25 TiO 2 and its self-reduction during methanol photoreforming

Keywords: Cu 2 O/TiO 2 composites, methanol photoreforming, copper oxides reduction, simulated solar light

The use of noble metal-free catalysts is one of the most attractive routes for the photocatalytic production of H 2 under sunlight. In that sense, recently, copper compounds have had a growing interest in this subject.

Particularly, Cu 2 O/semiconductor composites have shown enhanced photocatalytic activity in comparison with single compounds. In the present work, different Cu X O(X=1, 2)/TiO 2 composites, with Cu X O weight ratio from 1 to 0.05 respect to TiO 2 source, were obtained through aqueous mixture. Firstly, the Cu X O particles were obtained by chemical reduction method at room temperature, and after, mixed with commercial Degussa (Evonik) P25 TiO 2 , through the dispersion of each one in a water:ethanol solution.

All these materials were characterized by different techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS) and time resolved microwave conductivity (TRMC). Through the DRS technique, it was observed that composites possessed two separated optical absorption bands in UV and visible regions, confirming the physical interaction between both oxides and an extended optical activity. Charge carrier dynamics was studied by TRMC. Finally, the photocatalytic activity of the materials was evaluated for H 2 production by methanol photoreforming at batch approach, using simulated solar light. The best H 2 production was obtained with the 0.1-Cu/Ti composite, moreover its photoperformance was similar after three evaluation cycles, suggesting high stability under the reaction conditions. During irradiation of all the composites in the reactive system, the color of the solution changed from slight yellow to deep purple, which was attributed to the change of the copper oxidation states.

Introduction

The global population growth and the corresponding energy demand are leading to the excessive fossil fuel consumption, increasing the greenhouse gases emission. This situation is being a critical factor for the increment in the global planet-warming. Therefore, the transition toward carbon-free energy sources like hydrogen (H 2 ) could be a reasonable alternative to solve the energy demand and the associated environmental impact. Hydrogen possesses attractive properties that make it the one of best candidates to substitute the current fuels; it has the highest energy content of any fuel (141.88 MJ/kg as gaseous state), non-toxic and clean, since water is the only emission after its combustion [START_REF] Abe | Hydrogen energy, economy and storage: Review and recommendation[END_REF], [START_REF] Hosseini | Hydrogen production from renewable and sustainable energy resources: Promising green energy carrier for clean development[END_REF]. Concerning to H 2 production, there is a large amount of information devoted to conventional routes as thermochemical, biomass pyrolysis, and solar approaches [START_REF] Hosseini | Hydrogen production from renewable and sustainable energy resources: Promising green energy carrier for clean development[END_REF]- [START_REF] Wang | Controllable sonochemical synthesis of Cu 2 O/Cu 2 (OH) 3 NO 3 composites toward synergy of adsorption and photocatalysis[END_REF]. Especially, solar energy conversion to H 2 seems to be one of the most suitable ways in the near future, by using photochemical, photoelectrocatalytic or photocatalytic processes [START_REF] Nikolaidis | A comparative overview of hydrogen production processes[END_REF].

The photocatalytic H 2 generation can be obtained mostly by two different approaches: i) photocatalytic water splitting [START_REF] El-Khouly | Solar energy conversion: From natural to artificial photosynthesis[END_REF] and ii) photocatalytic reforming of organic compounds [START_REF] Christoforidis | Photocatalytic Hydrogen Production: A Rift into the Future Energy Supply[END_REF]. The first method relates to the capability of water to be reduced and oxidized, by reacting with photogenerated electrons and positive holes during semiconductor irradiation, which can posses selected co-catalysts. The second approach is based on the ability of some organic species, to donate electrons to the positive holes, after semiconductor excitation, then, generating protons that can be reduced to hydrogen by the photogenerated electrons.

The photoreforming of organic compounds like methanol, ethanol or glycerol in aqueous solution is an attractive process, which occurs coupling the organic compound oxidation and proton reduction, preventing the back-reaction between H 2 and O 2 [START_REF] Jung | Hydrogen evolution via glycerol photoreforming over Cu-Pt nanoalloys on TiO 2[END_REF]. Undoubtedly, the most important aspects to consider in the photoreforming are the selection of a suitable catalyst, the type of organic compound employed and the use of co-catalysts [START_REF] Christoforidis | Photocatalytic Hydrogen Production: A Rift into the Future Energy Supply[END_REF], [START_REF] Colón | Towards the hydrogen production by photocatalysis[END_REF]. Titanium dioxide (TiO 2 , n-type semiconductor) has been the most studied material for this reaction, due to its extraordinary properties: high oxidative power, chemical stability, cheap and friendly to the environment [START_REF] Ge | A review of TiO 2 nanostructured catalysts for sustainable H 2 generation[END_REF] [START_REF] Ibrahim | A critical review of metaldoped TiO 2 and its structure-physical properties-photocatalytic activity relationship in hydrogen production[END_REF]. Furthermore, it has been reported that the mixture of rutile and anatase phases of this oxide is more photoactive than the presence of single phase only. The disadvantages of TiO 2 are the high value of its band gap (generally reported as E g = 3.2 eV), which demands UV-light for its activation [START_REF] Christoforidis | Photocatalytic Hydrogen Production: A Rift into the Future Energy Supply[END_REF], [START_REF] Clarizia | Hydrogen generation through solar photocatalytic processes: A review of the configuration and the properties of effective metal-based semiconductor nanomaterials[END_REF] and its fast recombination rate of photogenerated electronhole pairs [START_REF] Colón | Towards the hydrogen production by photocatalysis[END_REF].

For this reason, several options have been considered to improve the photocatalytic activity of TiO 2 under visible light, among them: (i) surface modification via organic materials, (ii) band gap modification by nonmetals, (iii) metals doping and (iv) semiconductor coupling [START_REF] Christoforidis | Photocatalytic Hydrogen Production: A Rift into the Future Energy Supply[END_REF], [START_REF] Colón | Towards the hydrogen production by photocatalysis[END_REF], [START_REF] Clarizia | Hydrogen generation through solar photocatalytic processes: A review of the configuration and the properties of effective metal-based semiconductor nanomaterials[END_REF]. The combination of n-type semiconductor with a p-type semiconductor, as copper oxides (CuO, Cu 2 O), forms a heterojunction, considered as efficient method to separate the photoinduced electrons and holes due to the effect of the inner electric field [START_REF] Li | Facile preparation and size-dependent photocatalytic activity of Cu 2 O nanocrystals modified titania for hydrogen evolution[END_REF]. The suitable properties of Cu 2 O such as its band gap of 2.0 eV, a wide absorption coefficient in visible spectrum and virtual non-toxicity, has been widely explored by several research groups on applications as, solar cells, gas sensor and water splitting, among others [START_REF] Pai | Solar Energy Materials & Solar Cells A comprehensive study on sunlight driven photocatalytic hydrogen generation using low cost nanocrystalline Cu-Ti oxides[END_REF]. Indeed, the Cu 2 O/TiO 2 heterojunctions have shown promising photocatalytic results in several reactions [START_REF] Liu | Creation of Cu 2 O@TiO 2 composite photocatalysts with p -N heterojunctions on exposed Cu 2 O facets, their energy band alignment study, and their enhanced photocatalytic activity under illumination with visible light[END_REF], [START_REF] Geng | Incorporation of Cu2O nanocrystals into TiO2 photonic crystal for enhanced UV-visible light driven photocatalysis[END_REF].

Their staggered gap (type-II) enables the photogenerated charge separation in two distinct semiconductors [START_REF] Low | Heterojunction Photocatalysts[END_REF]. Thus, the photogenerated electrons can be transferred from the Cu 2 O conduction band (CB) to TiO 2 , and the holes can be transferred from the TiO 2 valence band (VB) to the Cu 2 O VB [START_REF] Liu | TiO 2 /Cu 2 O Core/Ultrathin Shell Nanorods as Efficient and Stable Photocatalysts for Water Reduction[END_REF], [START_REF] Li | Synthesis and characterization of Cu 2 O/TiO 2 photocatalysts for H 2 evolution from aqueous solution with different scavengers[END_REF].

Conversely, the reactor configuration represents an important factor in the photocatalytic performance.

The use of batch reactors in photocatalysis allows to exploring the evolution of the reactants and products concentration, as well as the reaction kinetics, in an easy way and with good temperature control, avoiding the influence of internal and external diffusion [19] [20].

In this work, Cu 2 O-CuO particles were combined to P25 TiO 2 with different weight ratio (Cu 2 O-CuO/TiO 2 = 1, 0.5, 0.1 and 0.05) via wet mixture, to study the synergic effect during their photocatalytic performance in the methanol photoreforming. Surprisingly, during the irradiation with solar simulator, simultaneously methanol photoreforming and self-reduction of copper oxides to metallic copper (Cu 0 ) were carried out, suggesting a high reductive environment, being positive effect over the H 2 generation. This self-reduced process was explained based on several combined factors of the system Cu X O-TiO 2 and reaction conditions.

Materials and Methods

Materials

Copper (II) chloride (CuCl 2 ; 97%), sodium hydroxide (NaOH; 98.2%), L-ascorbic acid (99.5%), Polivinilpilorridone (PVP; M W 40,000), titanium dioxide P25 Degussa (P25 TiO 2 ; 99.5%) were purchased from Aldrich and used without further purification. Aqueous solutions were prepared using deionized water (Milli-Q with 18.6 MΩ), ethanol reagent grade (denatured) and methanol reagent grade.

Synthesis of Cu X O (X=1, 2) polyhedral microparticles

Cu X O polyhedral particles were synthesized at room temperature under chemical reduction route with slight modifications [START_REF] Jiang | Synthesis of cuprous oxide with morphological evolution from truncated octahedral to spherical structures and their size and shape-dependent photocatalytic activities[END_REF]. Sodium hydroxide solution (1M, 40 mL) was prepared and stirred vigorously.

Copper chloride solution (0.15 M, 40 mL) was added dropwise to the sodium hydroxide solution, and stirred during 15 minutes. After that, a 60 mL of PVP and L-ascorbic acid solution (1.42 weight ratio) was added suddenly in the last solution, and it was stirred for 30 minutes. The obtained Cu X O polyhedral particles were washed three times with a 1:1 water:ethanol solution, and dry under air for storage.

Synthesis of Cu X O/TiO 2 composites

Cu X O polyhedral particles and P25 TiO 2 nanoparticles were dispersed separately in two ethanol:water 0.6:1 v:v solutions in both cases. P25 TiO 2 solution was added dropwise to the Cu X O solution under vigorous stirring until to obtain a homogeneous solution. Then, it was dried in air and the final powder was stored. Under this procedure, four composites were synthesized with the nominal weight ratio showed in Table 1.

Table 1. Nominal composition of the composites

Characterization techniques

The characterization of the composites was accomplished through several techniques. X-ray diffraction (XRD) were recorded from 15 to 80°, with a scan rate of 2° min -1 , using a Rigaku ULTIMA IV diffractometer in Bragg-Brentano geometry (Cu-K α1 radiation, λ = 1.540562 Å) and nickel filter. Then, the morphology and particle size were determined by scanning electron microscopy (SEM), which was carried out on a JEOL 7800F microscope without any previous treatment. X photoelectron spectroscopy (XPS) measurements were performed using a K-Alpha from Thermo Fisher Scientific X-ray photoelectron spectrometer with a monochromatic Al Kα X-ray source (1486.6 eV) and a base pressure of 1x10 -9 Torr in the analytical chamber. The binding energy values were calibrated by contaminant carbon using the C 1s position at 285 eV, and all the reference binding energies were obtained from NIST X-ray Photoelectron Spectroscopy Database. UV-vis diffuse reflectance spectroscopy (DRS) was measured on a GBC Cintra 20 spectrometer on the range of 200-800 nm. Finally, charge-carrier dynamic experiments were carried out through Time Resolved Microwave Conductivity (TRMC), using an incident microwave generated by a Gunn diode of the K band at 30 GHz. As a pulse light source, a laser (EKSPLA, NT342B)

Composite

Weight ratio (Cu X O/P25 TiO 2 ) 1-Cu/Ti 1 0.5-Cu/Ti 0.5 0.1-Cu/Ti 0.1 0.05-Cu/Ti 0.05 tunable in the range between 220 and 2000 nm, equipped with an optical parametric oscillator (OPO) was used, delivering 8-ns FWMH pulses with a frequency of 10 Hz. Three excitation wavelengths were selected 360, 470 and 520 nm with a light energy density of 2.6 mJ cm -2 .

Photocatalytic evaluation

The photocatalytic evaluation was carried out as follows. First, the P25 TiO 2 Degussa, Cu X O particles and Cu X O/TiO 2 composites were tested through a home-made batch reaction system, using a methanol water mixture (MeOH:H 2 O, 1:10; v:v) as a reaction medium. The concentration of the photocatalyst was fixed at 1 g L -1 in the reaction volume. At the beginning, oxygen inside of the reactor was removed by bubbling N 2 gas for 15 min. After that, the mixture was stirred in dark condition for 30 minutes, to reach adsorptiondesorption equilibrium between the catalyst and the methanol-water solution. Then, the reactor was irradiated using Hg-Xe lamp light source with a power of 150 W for 8 h. The H 2 production was followed each hour using a gas chromatograph Perkin Elmer Clarus 480 with a TCD detector. Each evaluation was performed three times. Under similar procedure, water splitting reaction and the effect of the concentration of the photocatalyst were studied with the composite that showed the higher photoactivity. Furthermore, to assess the stability of this composite, cyclic experiments were performed, in which the photocatalyst was recovered by centrifugation after the first evaluation and redissolved in a fresh methanol-water solution (MeOH:H 2 O, 1:10), and following with the following evaluation.

Results

The diffraction patterns of the synthesized materials are displayed in Figure 1. In the copper particles, the diffraction peaks located at 29.6, 36. After the structural characterization, microstructural properties were determined via SEM technique, and showed in the Figure 2. According to the first image (Figure 2A), the Cu X O particles present a polyhedral morphology, as it was expected, with some agglomeration regions. The average size of these particles was calculated measuring 100 particles, and determined as 196 nm. For the Cu/Ti composites, in general, it seems that the Cu X O particles are surrounded by P25 TiO 2 nanoparticles randomly. The micrograph of the 1-Cu/Ti composite (Figure 2B), containing the highest Cu X O content, enables to identify Cu X O particles and P25 TiO 2 , whose primary particle size has been reported as 21 nm. Figure 2C shows the 0.5-Cu/Ti particles, where the Cu X O particles remain visible, although their concentration is lower, compared to the previous case. On the contrary, 0.1-Cu/Ti and 0.05-Cu/Ti composites show in Figure 2D and2E, respectively, Cu X O polyhedral particles were not identified, due to their low proportion respect to the P25 TiO 2 . However, the same configuration is expected, as it was noted in the 0.1-Cu/Ti and 0.05-Cu/Ti composites, where Cu X O particles are surrounded by P25 TiO 2 nanoparticles. The XPS measurements were performed to determine the surface composition and Cu species presented in the 0.1-Cu/Ti and 0.05-Cu/Ti composites, as shown in Figure 3. The survey spectra (Figure 3A) displayed four main elements: C, Ti, O, and Cu. The presence of carbon could have resulted from hydrocarbons pollution from the XPS instrument itself or the adsorbed hydrocarbon from the air, and it was taken as standard signal reference to the other peaks. The binding energy (BE) at 458.2 and 464.0 eV were assigned to Ti 2p 3/2 and Ti 2p 1/2 (Figure 3B), respectively. The separation measured between the Ti 2p 3/2 and Ti 2p 1/2 peaks was 5.8 eV, which correspond to the BE separation observed for stoichiometric TiO 2 [START_REF] Ghazzal | Tailored refractive index of inorganic mesoporous mixed-oxide Bragg stacks with bio-inspired hygrochromic optical properties[END_REF]. On the Figure 3C, it was plotted the signals which corresponds to the oxygen O 1s . After deconvolution of this region, two peaks were observed at 529.6 and 530.9 eV in both composites, which were assigned as an interaction between Cu-O in CuO, and the presence of Cu(OH) 2 , respectively. The first peak corroborates the presence of CuO in the surface of the Cu X O particles, determined by the XRD characterization, while the second one should be remained secondary specie formed during the Cu X O synthesis process. Figure 3D shows the spectrum of the Cu 2p region in the 0.1-Cu/Ti composite. As in the previous case, after the deconvolution of the Cu 2p spectrum, several peaks were identified. The peaks located at 932.4 and 952.4 eV, corresponded to the Cu 2 O specie, and they were assigned to Cu2p 3/2 and Cu2p 1/2 , respectively. By other hand, the peak located at 953.7 eV, can be designated as Cu2p 1/2 of CuO.

Additionally, satellite peaks were observed at 934.2, 941.1 and 943.7 eV. These satellite peaks have been reported at the existence of fully oxidized copper, as CuO, and assigned to Cu2p 3/2 . In the 0.05-Cu/Ti composite (Figure 3E), the same set of peaks were found in a similar position than those detected in the 0.1-Cu/Ti composite. The peaks located at 932.3 and 952.4 eV were associated to Cu 2 O and assigned to Cu2p 3/2 and Cu2p 1/2 , respectively. The same satellite CuO peaks were found at 934.2, 941.0 and 943.6 eV, although the peak at 934.2 eV, associated to CuO 2p 3/2 , displayed a higher intensity, with respect at the observed one in the 0.1-Cu/Ti composite. Table 2 shows the experimental surface atomic composition percent of each element in both composites, compared with the theorical atomic composition, based on the weight ratio considered in the synthesis of the composites. For oxygen, the atomic composition is similar in both, theoretical and experimental values. However, theoretical titanium and copper percentages were different respect to the experimental results. For the experimental values, in the titanium case, the atomic percentage was lower, meanwhile the copper percentage was higher, both respected to the theoretical value, on each composite. However, if the experimental values of the atomic percentages are compared of each element between the composites, in all the cases, the values are essentially the same, which means that, at least on the surface, the composition of these composites are similar. The Figure 4 displayed the UV-vis absorption spectra of Cu X O particles, commercial P25 TiO 2 , and Cu X O/TiO 2 composites. As expected, the absorption edge of the Cu X O particles is located in the visible range, from 477 to 650 nm. In contrast, P25 TiO 2 possesses a well-defined UV absorption band starting from 325 to 410 nm. In the Cu X O/TiO 2 composites, they clearly showed two bands, in the UV region and in the visible region, suggesting the potential adsorption in both, UV and visible spectra. In general, for all the composites, the first band started from 325-330 nm, while the ending of this band finished between 366 and 396 nm, in agreement to the copper oxides content, being the highest copper content sample (1-Cu/Ti) the lower value where the band was detected. On the other hand, the second band was observed around 477 nm in all the composites, and, the end of the band changed depending of the copper oxides content as in the first band, from 641 nm in the 1-Cu/Ti composite, to 606 nm in the 0.05-Cu/Ti composite.

Furthermore, the intensity of this band decreased according to the weight ratio of the Cu X O particles in the composites. The higher copper oxides content, the more intense absorption band. Therefore, these observations suggest that the copper oxides content influences the absorption edge of both, UV and visible bands. The band in UV part, red-shifted when the copper oxides content increases; meanwhile, in the visible region, the second band blue-shifted with the decrement of the copper species, whose intensity depends of the copper content.

In order to determine the E g energy of the Cu X O particles and the commercial P25 TiO 2 , Kubelka-Munk and Tauc methods were considered. The Kubelka-Munk function F(R), (Equation 2), depends of experimental diffuse reflectance (R), and it is related to the absorption coefficient α and scattering coefficient S [START_REF] Apopei | Mixed-phase TiO 2 photocatalysts: Crystalline phase isolation and reconstruction, characterization and photocatalytic activity in the oxidation of 4-chlorophenol from aqueous effluents[END_REF][24] [START_REF] Sharma | Surface modification of TiO 2 with copper clusters for band gap narrowing[END_REF]. However, it is well-known that S is almost constant at any wavelength [START_REF] Gupta | Facile synthesis of Cu 2 O and CuO nanoparticles and study of their structural, optical and electronic properties[END_REF] [26], therefore the F(R) can be proportional to α, F(R)α [START_REF]Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures[END_REF]. Additionally, it should be noted that, through the Tauc model, the absorption coefficient can be related to the band gap energy as shown the Equation 3 [START_REF] Messaoudi | Correlation between optical and structural properties of copper oxide electrodeposited on ITO glass[END_REF][28] [START_REF] Hossain | A comparative study on the influence of pure anatase and Degussa-P25 TiO 2 nanomaterials on the structural and optical properties of dye sensitized solar cell (DSSC) photoanode[END_REF], where h is the Planck's constant, ν is the frequency of the wavelength of the photon, A is a constant and n is related to the band gap transition in the material. Assuming the previous considerations, the experimental reflectance function can be associated to the band gap energy through the Equations 2 and 3, obtaining a function represented in the 4quation 4. Thus, Tauc graph is obtained plotting (F(R) * hν)

1 n versus the incident photon energy (hν) [START_REF] Nunes | Room Temperature Synthesis of Cu 2 O Nanospheres: Optical Properties and Thermal Behavior[END_REF].

F(R) = (1-R) 2 2 * R = α S (2)
(α * hν) 1 n = A(hν -E g ) (3)
(F(R) * hν) 1 n = A(hν -E g ) (4)
In this work, how Cu 2 O is the predominant copper oxide phase, it was selected to calculate the band gap copper oxide particles, taking into account a direct allowed transition (n=1/2), reported previously [START_REF] Gupta | Facile synthesis of Cu 2 O and CuO nanoparticles and study of their structural, optical and electronic properties[END_REF][30] [START_REF] Amekura | Fabrication of Oxide Nanoparticles by Ion Implantation and Thermal Oxidation[END_REF] . However, both direct allowed and forbidden transitions were compared, and the E g value was determined as 2.34 eV, almost the same in both cases (data not showed). The blue-shift of the band gap value respect to the E g reported in the bulk Cu 2 O (2.17 eV) [START_REF] Amekura | Fabrication of Oxide Nanoparticles by Ion Implantation and Thermal Oxidation[END_REF] [32][33] [START_REF]Cu 2 O as an emerging photocathode for solar water splitting -A status review[END_REF], can be explained by the polyhedral morphology of the particles, whose particular crystal facet structure determines its physical properties, as the optical behavior [START_REF] Sun | Recent advances in tuning crystal facets of polyhedral cuprous oxide architectures[END_REF][35] [36][37]. This phenomenon has been observed previously in similar Cu 2 O particles [24][38]. Following the same procedure, indirect allowed transition (n=2) was selected to calculate E g energy of the commercial P25 TiO 2 [START_REF] Nagy | Effect of Ag Co-catalyst on TiO 2 -Cu 2 O nanocomposites structure and apparent visible photocatalytic activity[END_REF] [39], being the Y axis assigned as (F(R)*hν) 1/2 . In this case, when F(R)=0, the calculated band gap is 3.31 eV. This value is in a good agreement with the previous literature reported [38][40]. assumed to rise from the high mobility electrons compared to positive holes. Thus, the signal intensity and decay pattern could be correlated to the photogenerated electrons at the surface of the photocatalysts.

All the curves exhibit typical TRMC signal, corresponding to the production of charge carriers right after the laser excitation, followed by a decay, which is due to surface trapping, recombination and surface reaction [START_REF] Colbeau-Justin | Structural influence on charge-carrier lifetimes in TiO 2 powders studied by microwave absorption[END_REF].

After irradiation, the Cu X O particles (Figure S1A) show an intense TRMC signal both, UV and visible range, enhancing in the visible range (520-580 nm), which it is in agreement with the spectral absorption observed in DRS (Figure 4). In the same line, Figure S1B shows commercial P25 TiO 2 also produces photogenerated electrons under UV irradiation. However, the TRMC signal is observed to be weaker in the visible range than the UV range, where it was reached a high value. Indeed, the band gap of P25 TiO 2 was estimated to be around 3.31 eV, a slightly higher value than that of pure TiO 2 anatase phase [START_REF] Jiang | Photoelectrochemical devices for solar water splitting -materials and challenges[END_REF], and absorbs only low fraction of the visible range. Coupling Cu X O and P25 TiO 2 in the composites, extend the ability to produce charge carriers in the UV-visible range as suggested by the TRMC signal, where the signal produced under visible wavelengths (470 and 520 nm) is related to the Cu X O proportion in these composites. In the sample 1-Cu/Ti, Figure S1C, the most intense signals were observed under wavelengths in the visible range, which is expected due to it has the highest Cu X O weight ratio. However, the observed signal decays faster to low values, in all the wavelengths proved. Similarly, in the Figure S1D, the signals produced under visible range in the composite 0.5-Cu/Ti were sharper compared to the signal in the UV range (360 nm), and as in the previous composite, fast decay is observed in all the measurements. For the less-copper oxides composites, 0.1-Cu/Ti and 0.05-Cu/Ti, Figures S1E andF, the intensity of the maximum signal decreased respect to the high-copper oxides composites, in all the wavelengths tested.

However, the decay stage was more stable, due to the intensity of the signal did not decrease at the same level observed in the high-copper oxides composites. In the composite 0.1-Cu/Ti, the generation of photoelectrons slightly decreased as a function of the wavelengths tested. The highest signal was detected under λ=520 nm, meanwhile the lowest it was observed when the material was irradiated at λ=360 nm.

Nevertheless, the decay stage was not remarkable, and several signals were observed after the most intense peak, suggesting the probable generation of photoelectrons. Finally, for the 0.05-Cu/Ti composite, the best generation of photoelectrons was produced at λ=470 nm, respect to the other wavelengths (520 and 360 nm). Likely, the coupled effect between copper oxides and titanium dioxide, enhances the generation of photoelectrons at this wavelength.

Photocatalytic activity in batch processes

The photocatalytic behavior was performed into a conventional batch system. It is important to mention that, after the initial 30 min of irradiation, a notorious color change from slight yellow to deep purple was observed (Figure 5A), which was kept throughout the subsequent irradiation time. Surprisingly, when the irradiation was stopped, the solution of reaction came back to its original slight yellow color, after around 10 h. This phenomenon was observed in all the experiments, and it will be analyzed later.

The evolved H 2 and CO 2 gases were measured every hour and up to 8 h of irradiation time. As shown in Figure 5B, the Cu X O polyhedral particles did not present photocatalytic activity throughout the evaluation period, due to the null H 2 production. For the commercial P25 TiO 2, low amount of H 2 was produced after 8 h under UV-vis irradiation. Conversely, the Cu X O/TiO 2 composites showed significant H 2 generation, compared to each single oxide, being the less-copper oxide composites, 0.1-Cu/Ti and 0.05-Cu/Ti, the most active materials. Furthermore, their H 2 production rates increased almost eight times in these composites, compared to the H 2 production rate of commercial P25 TiO 2 .

On the other hand, the CO 2 generation was measured to verify the stoichiometry of the methanol reforming reaction, and corroborated that, both H 2 and CO 2 , could be consumed or produced in secondary reactions as water splitting reaction, in the H 2 production case. Figure 5C revealed that the composite materials produced higher CO 2 , how in the H 2 generation case, respect to the single oxides. Furthermore, the CO 2 generation in the composites showed a similar tendency than the H 2 production, where the 0.1-Cu/Ti and 0.05-Cu/Ti composites produced the highest CO 2 amount. Note that, the CO 2 production rates with the 0.1-Cu/Ti and 0.05-Cu/Ti composites were fourteen times higher than the CO 2 production rate in commercial P25 TiO 2 . According to the results presented in

With the overall reaction being;

CH 3 OH + H 2 O hν+pcat ↔ K CO 2 + 3H 2 (8)
where, hν is the photon energy, pcat is the photocatalyst, k x the equilibrium constant of each reaction and K is the global equilibrium constant. Therefore, in agreement with Equation 8, the H 2 /CO 2 molar ratio must be 3. However, the average molar ratio of the experimental data showed different values for both, single oxides and the composites (Table 3). The H 2 /CO 2 molar ratio calculated in the Cu X O particles was 1.25, but, as the evolution of both gases was almost negligible, the photocatalytic activity is considered null. In contrast, the composites showed H 2 /CO 2 molar ratios between 2.39 and 2.9, indicating a variation of the H 2 production or CO 2 generation, respect to the theoretical values expected, by a secondary reaction.

Additionally, it is evident that this result has a relationship with the copper oxides weight ratio in each material, where the lower difference is observed in the 0.05-Cu/Ti composite. This result can be explained by the process represented in the following equation;

CH 3 OH + 2Cu 2 O + CuO hν (λ=UV) → CO 2 + 2H 2 O + 5Cu 0 (9)
In this secondary process carried out under the irradiation time, methanol is oxidized to carbon dioxide and water, meanwhile both copper oxides (Cu 2 O and CuO) are reduced to metallic copper (Cu 0 ) [START_REF] Fleisch | Reduction of copper oxides by UV radiation and atomic hydrogen studied by XPS[END_REF] [44]

[45] [START_REF] Polliotto | Copper-Modified TiO 2 and ZrTiO 4 : Cu Oxidation State Evolution during Photocatalytic Hydrogen Production[END_REF]. This reaction can be an additional way for the CO 2 generation, which can explain the H 2 /CO 2 molar ratio lower than 3, the theoretical expected value. Additionally, there is a direct dependence with the copper oxides content and the process, and it should be the reason because the H 2 /CO 2 molar ratio was lesser in the composites with the higher Cu X O content, that in the composites with lower Cu X O content.

Thus, the process proposed in Equation 9is favored in the composites with high copper oxides content, giving a lower H 2 /CO 2 molar ratio. In fact, this reaction explains the change of color of the solution during the irradiation time as well. When the composites are irradiated, the process represented in the Equation 9takes place, and the copper oxides in the composites are reduced to metallic copper, changing the color of the solution from slight yellow, characteristic color of copper oxides, to deep purple. After, when the irradiation is stopped, the oxidation environment and the presence of TiO 2 promotes the re-oxidation of the metallic copper, as it is explained in the discussion section.

On the other hand, in commercial P25 TiO 2 , the H 2 /CO 2 molar ratio was almost the double respect to the theoretical value, around 5.48. In this case, the increment of molar ratio is due to the increment of the H 2 production, respect to the expected stoichiometric amount from the methanol photoreforming. Therefore, the P25 TiO 2 is able to carry out methanol photoreforming reaction [START_REF] Cheng | TiO 2 e graphene nanocomposites for photocatalytic hydrogen production from splitting water[END_REF] [48] [START_REF] Gong | New insights into the photocatalytic activity of 3-D core-shell P25@silica nanocomposites: impact of mesoporous coating[END_REF], and the additional H 2 amount can be produced by water splitting, which has been reported for this material previously [START_REF] Filice | Laser processing of TiO 2 colloids for an enhanced photocatalytic water splitting activity[END_REF] [51]. For comparison purposes, the evaluation of 0.1-Cu/Ti composite was performed in the water splitting reaction, and the results are displayed in Figure 6A. Notoriously, no H 2 evolution was detected in this reaction (water + composite + irradiation), which means that, methanol photoreforming is the reaction carried out for H 2 production. Methanol is a hole collector and could inject electrons in the CB of the Cu X O/TiO 2 composite [START_REF] Gesesse | Enhanced Photogenerated Charge Carriers and Photocatalytic Activity of Biotemplated Mesoporous TiO 2 Films with a Chiral Nematic Structure[END_REF]. This dissociative adsorption of methanol is followed by the release of the hydrogen radicals. Thus, the produced hydrogen comes from to the released protons after methanol photooxidation, following the Equations 5-7. Furthermore, it is important to mention that, no color change was observed during water splitting evaluation, confirming the additional CO 2 generation process depicted in the Equation 9, where methanol is involved. Additionally, the photocatalyst concentration effect was studied with the same composite (0.1-Cu/Ti), using concentrations from 0.5 g L -1 to 2 g L -1 (Figure 6B). Clearly, methanol photoreforming is highly sensitive at the catalyst concentration, since the best photocatalytic performance was reached by using a 1 g L -1 catalyst concentration. Indeed, at low catalyst concentration (i.e. 0.5 g L -1 ), not enough proton conversion sites are available to be converted into molecular hydrogen. On the contrary, by increasing the catalyst load (1.5-2 g L -1 ), a significant decrease of the H 2 production rate is observed, which is attributed to light scattering of the material hindering the illumination in the reaction system, as well as agglomeration-sedimentation of the photocatalyst, despite of the stirring of the solution of reaction [START_REF] Akpan | Parameters affecting the photocatalytic degradation of dyes using TiO 2 -based photocatalysts : A review[END_REF] [54] [55] [START_REF] Lais | Experimental parameters affecting the photocatalytic reduction performance of CO 2 to methanol: A review[END_REF].

Finally, in the Figure 6C, the cyclic capacity results of the 0.1-Cu/Ti composite are presented. The photoactivity of the material was kept after three reaction cycles, being the maximum H 2 production on each cycle between 15 and 17 mmol of H 2 per gram of photocatalyst. This significative stability, despite of aqueous environment and irradiation for at least 24 hours, demonstrated that the Cu X O/TiO 2 composites are good candidates to reuse more than once for methanol photoreforming, without to lose its photocatalytic performance. Furthermore, it is important to point out that, the slight yellow color of the composite changed to deep purple on each reaction cycle, and then, after stopping the irradiation, washing and drying it, its color came back to slight yellow as in the beginning of the reaction, before the irradiation.

Thus, as a brief, the photoactivity was strongly susceptible to the Cu X O amount in the composites, obtaining the best results at low Cu X O concentrations (i.e., 0.1-Cu/Ti and 0.05-Cu/Ti composites). Then, an excess of the optimal amount of Cu X O can promote the recombination of the photo charge carriers, decreasing the photoactivity [11][18][57] [START_REF] Olivo | CO 2 photoreduction with water: Catalyst and process investigation[END_REF], moreover the feasibility of the secondary oxidation process of methanol, where CO 2 and H 2 O instead of H 2 are produced.

X-ray photoelectron spectroscopy after irradiation

To identify the copper species formed after irradiation, XPS characterization of 0.1-Cu/Ti composite was performed at the end of methanol photoreforming process (Figure 7). The deep purple color of the material after the evaluation was keeping partially for the XPS analysis. Specifically, oxygen and copper regions were analyzed, and the same regions of the initial spectra of the composite before evaluation were added as a comparison. As it is shown in the plots, the intensity of the signals after the evaluation decreased respect to the signal observed in the pristine sample. In Figure 7A, the O 1s spectra show a peak at 529.6

eV, attributed to CuO, and a shoulder at 530.9 eV, that it was mentioned to correspond to the presence of Cu(OH) 2 in the pristine samples. After irradiation, the signal at 529.6 eV was found well, and additional shoulder appeared at 531.3 eV, which was associated at the presence of no-stoichiometric titanium dioxide (TiO 1.55 or TiO 1.35 ). For the Cu 2p region (Figure 7B), the peaks localized at 932.5 and 952.4 eV, referred to Cu 2 O, correspond to Cu 2p 3/2 and Cu 2p 1/2 , respectively. Additionally, the presence of CuO was confirmed by the peaks at 934.4 and 954.3 eV, assigned as Cu 2p 3/2 and Cu 2p 1/2 , along with the satellite peaks at 941.4 and 943.7 eV, observed in both spectra. No additional peaks were found in this region, so that, no clear evidence about the metallic copper presence after the evaluation.

Discussion

From a global point of view, methanol photoreforming was fundamentally influenced by the spatial distribution, composition and electronic properties of the Cu X O/TiO 2 composites. According to XRD data and SEM micrographs showed in Figures 1 and2, both Cu X O and TiO 2 particles coexists as separated phases. However, they have close contact with each other randomly, without any specific configuration.

Due to this spatial distribution, both Cu X O and TiO 2 particles are in direct contact with the aqueous methanol solution. Therefore, during irradiation, the photogenerated charge carriers of both semiconductors can participate in redox reactions on the surface, since there is a high probability of charge transfer between the semiconductor-liquid interface, as it is illustrated in the Scheme 1A [START_REF] Clarizia | Hydrogen generation through solar photocatalytic processes: A review of the configuration and the properties of effective metal-based semiconductor nanomaterials[END_REF]. In fact, also, it is important to highlight the existence of four species in the composites, and consider their contribution in the electronic properties of the whole system. The first specie is Cu 2 O, as polyhedral particles, identified by several techniques such as XRD, SEM and XPS. Furthermore, in the same particles, CuO was detected by XRD patterns and it was corroborated in the 0.1-Cu/Ti and 0.05-Cu/Ti samples via XPS spectra, being part of the composites as well. Likely, this specie (CuO) can be presented over the surface oxidation of Cu 2 O particles, forming of a thin layer over the particles, as it has been reported elsewhere [START_REF] Leng | Polyhedral 50-Facet Cu 2 O Microcrystals Partially Enclosed by {311} High-Index Planes: Synthesis and Enhanced Catalytic CO Oxidation Activity[END_REF][59][60] [START_REF] Plascencia-Hernández | Cu 2 O cubic and polyhedral structures versus commercial powder: Shape effect on photocatalytic activity under visible light[END_REF]. The third and fourth components are the crystalline phases in commercial P25 TiO 2 , anatase and rutile, whose standard proportion has been defined as 80:20 [11][62]. These phases were corroborated by XRD patterns in all composites.

After the description of the presented species in the composites, it is important to consider the type of interactions between them. The most important interaction is resulted of the coexistence between Cu 2 O and TiO 2 , called p-n heterojunction, due to the nature of both semiconductors [START_REF] Muscetta | Hydrogen production through photoreforming processes over Cu 2 O/TiO 2 composite materials: A mini-review[END_REF]. In the interface of the semiconductors, and before the irradiation, the electrons from TiO 2 , n-type semiconductor, migrate toward p-type semiconductor, Cu 2 O. At the same time, holes from this p-type semiconductor travel to the opposite side to n-type semiconductor, due to the difference of the Fermi levels of the two oxides. Thus, when the charge equilibrium is reached by the difference of their Fermi levels, an internal electric field is created [START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF], as it can see in the Scheme 1B. Under these premises, the spatial distribution of the particles and the existence of four species in the composites, it was proposed a band alignment in the photocatalysts, through the Eg energy and their band edge commonly reported (Scheme 1C). According to the diffuse reflectance spectrum and Tauc plot shown in Figure 4, the Cu 2 O band gap was calculated as 2.34 eV, slightly higher than the most common value reported in the literature, around 2-2.2 eV [START_REF] Liu | Creation of Cu 2 O@TiO 2 composite photocatalysts with p -N heterojunctions on exposed Cu 2 O facets, their energy band alignment study, and their enhanced photocatalytic activity under illumination with visible light[END_REF] [65] [START_REF] Kaviyarasan | Photocatalytic and photoelectrocatalytic performance of sonochemically synthesized Cu 2 O@TiO 2 heterojunction nanocomposites[END_REF]. Despite of the increment in the Eg in the synthesized sample, the valence band (VB) and conduction band (CB) position, reported at 1.07 and -1.13

V [START_REF] Jiang | Photoelectrochemical devices for solar water splitting -materials and challenges[END_REF] [67] respectively, are considered in the alignment proposed. In the case of CuO, even though its band gap was not determined experimentally, its E g aligment was taken into account in the composites (the VB and CB positions as 1.5 and -0.2 V respect to NHE, respectively, [START_REF] Leung | Hydrogen production over titania-based photocatalysts[END_REF] [68] [START_REF] Zhang | CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications[END_REF]), due to it is part of the Cu 2 O particles and can contribute in the electronic properties of the composites. Concerning to the commercial P25 TiO 2 , the E g was estimated as 3.31 eV. Now, it should be take into account that, this value is composed by the E g of pure anatase (3.2 eV) and pure rutile (3.02 eV) [11] [70]. In anatase phase, the positions of the VB and CB have been measured around 2.9 and -0.29 V respect to NHE [START_REF] Leung | Hydrogen production over titania-based photocatalysts[END_REF], meanwhile for rutile phase, the E g position has been identified underneath respect to anatase CB and VB [START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF]. Thus, the proposed E g position of each component in the composite are represented following these assumptions.

Accordingly, a reaction path can be proposed starting from the irradiation of the composites with a simulated solar light. The UV wavelength excites the P25 TiO 2 particles, meanwhile the visible light excites copper oxides (Cu 2 O, CuO), where the generation of electron-hole pair in the semiconductors happens. The photogenerated charge carriers can migrate from each semiconductor to the solid-liquid interphase directly, by the spatial distribution of the particles, as well as to the Cu X O-TiO 2 contact. In the last case, as an internal electric field was formed in the p-n heterojunction, the electrons migrate from Cu X O to the TiO 2 CB, first to anatase phase and after that to rutile phase, where it is possible an accumulation of the charge carriers [START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF]. At the same time, the photogenerated holes can remain in the Cu 2 O and CuO VB, while, the generated holes in the VB of P25 TiO 2 migrate toward the Cu 2 O VB, by the proposed band alignment, driving a spatial separation of the charge carriers through the composite.

Thus, the overall scheme of photoredox reactions leads to minimal charge recombination, supported by semiconductor thermodynamics [START_REF] Li | Synthesis and characterization of Cu 2 O/TiO 2 photocatalysts for H 2 evolution from aqueous solution with different scavengers[END_REF], [START_REF] Low | Surface modification and enhanced photocatalytic CO 2 reduction performance of TiO 2 : a review[END_REF], [START_REF] Shi | Adsorption and photocatalytic degradation of tetracycline hydrochloride using a palygorskite-supported Cu 2 O-TiO 2 composite[END_REF]- [START_REF] Wang | A unique Cu 2 O/TiO 2 nanocomposite with enhanced photocatalytic performance under visible light irradiation[END_REF]. Additionally, aqueous methanol plays an active role as hole scavenger at this stage, avoiding the charge recombination and donating protons and electrons required for the H 2 production [48] [75]. The general reaction was showed in the evaluation section (Equation 8), and it can rewrote as the Equation 10. In agreement of this Equation, the theoretical oxidation potential required is 0.02 V [START_REF] Iwasita | Electrocatalysis of methanol oxidation[END_REF].

CH 3 OH + H 2 O → CO 2 + 6H + + 6e -E 0 = 0.02 V (10)
Additionally, methanol could be able to reduce copper species, following the secondary reaction (Equation 9) proposed in the evaluation section. Initially, the photocatalyst-aqueous methanol solution presented a slight yellow color, due to the Cu X O-TiO 2 composites. After irradiation, the color of the solution changed progressively toward intense red-purple (Figure 6A). This deep red-purple color in copper species has been attributed to the existence of metallic copper (Cu 0 ) [START_REF] Dang | Synthesis and optical properties of copper nanoparticles prepared by a chemical reduction method[END_REF]- [START_REF] Venkatakrishnan | Aerobic synthesis of biocompatible copper nanoparticles: promising antibacterial agent and catalyst for nitroaromatic reduction and C-N cross coupling reaction[END_REF]. In order to prove this assumption, the 0.1-Cu/Ti sample was analyzed by XPS after irradiation (Figure 8), but any clear evidence was found about the Cu 0 presence. However, the color change of the photocatalyst-aqueous methanol system, the increment of CO 2 evolution calculated through the H 2 /CO 2 molar ratio in the experimental results, and null color change during water splitting test, can support this assumption.

Besides, the change in the oxidation state is feasible at the band alignment proposed. The potential required to reduce Cu 2 O to Cu 0 is -0.360 V [START_REF] Yu | Electrodeposition of submicron/nanoscale Cu 2 O/Cu junctions in an ultrathin CuSO 4 solution layer[END_REF], which is located inside of the band gap of the Cu 2 O. Therefore, an intense radiation over Cu 2 O particles can promote their self-reduction to Cu 0 , by the photogenerated electrons [START_REF] Xi | Synergistic effect of Cu 2 O/TiO 2 heterostructure nanoparticle and its high H 2 evolution activity[END_REF], further to the secondary oxidation of methanol proposed. Additionally, it should consider that the high flux of the charge carriers in the p-n heterojunction can increase the electron density in the P25 TiO 2 CB [START_REF] Szaniawska | Enhanced photoelectrochemical CO 2 -reduction system based on mixed Cu 2 Ononstoichiometric TiO 2 photocathode[END_REF], whose position is near at the potential required to form Cu 0 as well. Thereby, the combined effect of the secondary methanol oxidation described and the electrons from VB of Cu 2 O or even in the p-n heterojunction can reduce copper species from Cu +2 and Cu +1 to Cu 0 [START_REF] Polliotto | Copper-Modified TiO 2 and ZrTiO 4 : Cu Oxidation State Evolution during Photocatalytic Hydrogen Production[END_REF], which represented in the Scheme 2A. Thus, the Cu 0 /P25 TiO 2 system is proposed after irradiation of the evaluation medium, and the required reduction potential for this process is represented in the optical band alignment of the initial species (Scheme 2B).

The presence of the Cu 0 particles modifies the charge transfer mechanism in the composites. After the copper reduction process, a Schottky barrier is formed between Cu 0 and P25 TiO 2 (Scheme 2C). This

Schottky barrier is able to trap electrons, which decrease the recombination rate of the photocarriers.

Therefore, the electron transfer carries out from P25 TiO 2 to Cu 0 particles, due to the work function in the metal is higher than the semiconductor one. This electron migration continues until to reach the equilibrium, where the Fermi levels of Cu 0 , as well as P25 TiO 2 are aligned with CB of P25 TiO 2 . By this way, P25 TiO 2 and Cu 0 particles gain excess of positive and negative charges, respectively [11] [82].

The excess of negative charge in the Cu 0 particles, added to the efficient charge carries separation, promote the H + reduction, enhancing the H 2 production, according with the equation 10 (Scheme 2D).

2H + + 2e -→ H 2 (11)
Cu 0 + h + → Cu +1 + e - (12)
Remaining holes from P25 TiO 2 participate actively in the re-oxidation of Cu 0 to Cu X O, playing a critical role in the copper re-oxidation process after irradiation. Finally, a comparative table was performed between previous reports and the obtained results in the current work (Table S1). The system copper species-titanium dioxide has been widely studied, due to the significative performance of the combination of these species during the photocatalytic hydrogen production through alcohols reforming. Among the strategies used, have been reported the modification of the copper oxides morphology, size, and the ratio respect to titanium dioxide. For the last one, commercial P25 TiO 2 has been selected as a titanium source, although several groups have synthesized it, modifying its morphology and size. Also, different elements as silver have been added to this kind of systems. Besides, a variety of sacrificial agents have been used

for the alcohols reforming, although, in the most of the cases, methanol has shown the best results. Respect to the source light, the works reported a potency between 100 to 500 W, with high-energy wavelengths in the UV region. The maximum H 2 production rate of each case is mainly linked to the alcohol proportion in the reaction medium and the structural properties of the oxides. The obtained values are in the range from 0.009 to 286 mmol g -1 h -1 , however, the most of them are between 2.048 and 4.5 mmol g -1 h -1 . The best H 2 production rate of this work is in this range, being 2.86 g -1 h -1 . This result could be considered exceptional, taking into account the easy synthesis method selected, low copper oxide ratio respect to P25 TiO 2 (0.01,0.05 to1), methanol proportion used (1:10) and the low power of the irradiation source (150 W), due to that similar or slightly higher results have been reported under difficult synthesis methods, higher alcohol proportion and potency of irradiation source.

Conclusions

The Cu X O/TiO 2 composites with different weight ratios were synthesized successfully by a simple, room temperature wet-mixed method, where synthesized-copper particles, composed by Cu 2 O and CuO, and the commercial P25 TiO 2 were used as precursors. Through this method, the composites acquire specific spatial distribution, where both particles are in contact with the solution of reaction, being a convenient distribution to participate in the redox reactions. Besides, the composites exhibited two broad optical absorption bands, from visible to UV regions, demonstrating photoactivity under both spectral regions.

This assumption was corroborated through TRMC measurements, where the irradiation with different wavelengths showed a direct relationship between the amount of photocarriers produced, the dominant specie, and the wavelength used; more P25 TiO 2 more photocarriers under UV light, more Cu X O oxides, more photocarriers under visible light. The as-synthesized composites were evaluated in methanol photoreforming reaction, by batch system. During irradiation, the Cu 2 O/CuO species in the composites undergo a self-reduction to Cu 0 , which was verified by a change of color of the photocatalyst-aqueous methanol solution, from slight yellow to intense red-purple. This behavior was attributed of different reasons. Firstly, the secondary oxidation reaction of methanol, corroborated by the decrease of H 2 /CO 2 molar ratio during the evaluation, where the copper oxides participate being reduced to metallic copper; and secondly, the efficient spatial separation of the charge carries due to the formation of p-n heterojunction, which leads the reduction of copper oxides to metallic copper, by high electron density. In consequence, a Schottky barrier was created in the new Cu 0 /P25 TiO 2 composite, resulting in high density of negative charges, which enhances the H 2 production, by the protons availability in the aqueous reaction medium and the increment of the stability of the photocatalyst. Thus, the charge carriers are able to participate in the photocatalytic reactions and modify the photocatalyst itself. The importance of methanol was proved during water splitting test, where the null H 2 production and no change of color were observed.

Furthermore, the Cu X O content influences the photocatalytic performance, where the number of active sites and light scattering-sedimentation phenomena play a critical role. Therefore, the combination of spatial distribution, the key role of methanol presence, the p-n heterojunction, the Cu X O/TiO 2 weight ratio, the presence of four species, and P25 TiO 2 homojunction allow to self-improve the photocatalysts and to enhance the photoactivity towards H 2 production via methanol photoreforming.

Figure 2 .

 2 Figure 2. SEM images of Cu X O particles A) and the 1-Cu/Ti B), 0.5-Cu/Ti C), 0.1-Cu/Ti D) and 0.05-

Figure 3 .

 3 Figure 3. XPS spectra of the 0.1-Cu/Ti and 0.05-Cu/Ti composites A), titanium region B), oxygen region

Figure 4 .

 4 Figure 4. Optical absorption spectra of Cu X O particles, commercial P25 TiO 2 and Cu X O/TiO 2 composites.

Figure 5 .

 5 Figure 5. Color change of the solution of reaction after of 30 minutes of irradiation, and during the

Figure 6 .

 6 Figure 6. Methanol photoreforming vs water splitting processes A), effect of the photocatalyst

Figure 7 .

 7 Figure 7. XPS spectra of the 0.1-Cu/Ti composite in oxygen A), and copper B) regions, before and after

Scheme 1 .

 1 Scheme 1. Structure and proposed band alignment of the Cu X O/TiO 2 composites.

Scheme 2 . 2 hν→

 22 Scheme 2. Photocatalytic performance of the Cu X O/TiO 2 composites during irradiation.

Table 2 .

 2 Experimental percent of the surface atomic composition

	Element	0.1-Cu/Ti	0.05-Cu/Ti
		Theoretical	Experimental	Theoretical	Experimental
	O1s	64.9	64.1	65.8	64.5
	Ti2p	31.6	29.2	32.4	29.0
	Cu2p	3.5	6.7	1.8	6.5

Table 3

 3

	, the following set of reactions can

Table 3 .

 3 H 2 and CO 2 evolution

	Photocatalyst	H 2 mmol g -1	H 2 evolution rate mmol g -1 h -1	CO 2 mmol g -1	CO 2 evolution rate mmol g -1 h -1	Molar ratio H 2 /CO 2
	Cu X O	0.05	0.007	0.04	0.01	1.25
	1-Cu/Ti	6.21	0.69	2.59	0.29	2.39
	0.5-Cu/Ti	14.29	1.79	5.34	0.67	2.67
	0.1-Cu/Ti	22.72	2.84	8.32	1.04	2.73
	0.05-Cu/Ti	22.94	2.86	7.91	0.99	2.90
	Commercial P25 TiO 2	1.48	0.37	0.27	0.07	5.48

Acknowledgements

The authors acknowledge the financial supports of SIP-20180100 and SIP-20180893 projects. FPH is grateful for CONACyT and BEIFI fellowships. M.V. acknowledges Université Paris-Saclay for the financial support through the Chaire Jean d'Alembert program and the IRS MOMENTOM (Ininiative de Recherche Stratégique).