Lesya Baudoin
email: lesya.baudoin@hceres.fr

Anne Glanard
email: anne.glanard@hceres.fr

Abdelghani Maddi
email: abdelghani.maddi@hceres.fr

Frédérique Sachwald
email: frederique.sachwald@hceres.fr

Analyse scientométrique du domaine de l'infectiologie de 2000 à 2020

Keywords: Publications scientifiques, Analyse scientométrique, Indice de spécialisation, Indice d'impact, France, Cartographie thématique, Infectiologie, Covid-19 Scientometric Analysis of Research on Infectious diseases, 2000-2020 Scientific publications, scientometric analysis, specialisation index, impact index, France, topic mapping, infectious diseases, Covid-19

Le domaine de l'infectiologie constitue un champ de recherche transversal. Afin de l'identifier précisément, l'article construit un corpus global des publications en infectiologie en combinant les moyens offerts par le langage contrôlé du thésaurus MeSH de Medline et par la catégorisation des revues scientifiques du Web of Science. Ce corpus mondial permet de caractériser les publications des 20 premiers pays publiant dans le domaine et de retracer les évolutions entre 2000 à 2020. La construction de cartes thématiques permet d'identifier les thèmes de recherche au sein de l'infectiologie dans le monde et en France. L'explosion des publications sur le Covid-19 en 2020 a un impact très visible sur la carte mondiale des thématiques en infectiologie et modifie le positionnement de certains pays dans ce domaine de recherche. La conclusion identifie des pistes d'approfondissement de l'analyse du domaine qui pourraient être suivies à mesure que des données plus complètes seront disponibles sur la période de la pandémie de Covid-19.

Introduction

L'infectiologie constitue un domaine particulièrement intéressant à étudier avec les outils de la scientométrie au moment où la recherche y connaît une croissance extraordinaire du fait de la pandémie de Covid-19. Cet article construit un corpus mondial des publications en infectiologie sur la période 2000-2020 afin de mesurer la croissance des publications dans le domaine de l'infectiologie, d'analyser sa structure thématique interne et de comparer les principaux pays producteurs à partir de différents indicateurs.

Le cas de la France est plus particulièrement analysé. De Louis Pasteur aux découvreurs du VIH, la recherche française en infectiologie compte de nombreux chercheurs dont l'apport scientifique a été remarquable. Dotée d'institutions renommées renforcées par des investissements à travers des Labex et Equipex, ainsi que de réseaux de la recherche hospitalière développés, la France possède d'importantes ressources scientifiques dans le domaine. Ses publications dans le champ des recherches sur l'immunité et l'infection représentent une part relativement importante du total de ses publications scientifiques en comparaison de la part de ce domaine dans les publications mondiales [START_REF] Ost ; Hcéres | La position scientifique de la France dans le monde et en France[END_REF]. Il apparaît donc intéressant de comparer l'évolution des publications françaises à celles d'autres pays en réponse à la pandémie de Covid-19.

L'infectiologie est un domaine de recherche transversal : il se nourrit des recherches en microbiologie, virologie, immunologie et génétique. Dans les domaines cliniques, au-delà de la spécialité « maladies infectieuses et tropicales », l'infectiologie est notamment très présente dans les soins critiques, la chirurgie et l'oncologie. La pharmacologie et la vaccinologie sont également étroitement liées à l'infectiologie. La constitution d'un corpus du domaine de l'infectiologie présente ainsi un enjeu méthodologique qui est abordé dans la première partie consacrée aux données et aux méthodes déployées dans l'article. La deuxième partie cartographie les principales thématiques présentes au sein du domaine de l'infectiologie et leur évolution en 2020. Le profil thématique du corpus des publications françaises est plus particulièrement analysé. La troisième partie compare la dynamique et les caractéristiques des publications en infectiologie des principaux pays producteurs depuis 2000. La conclusion revient sur les principaux résultats et sur les approfondissements qui paraissent utiles.

Données et méthode

Données

Le corpus est constitué en combinant deux sources de données. La première source, PubMed/Medline, met en oeuvre un vocabulaire contrôlé organisé, le thésaurus MeSH (Medical Subject Headings). Les termes MeSH sont des mots-clés normalisés liés par des relations sémantiques ; ils sont attribués manuellement par les indexeurs pour décrire le contenu des articles1 . Les descripteurs MeSH sont organisés en 16 catégories2 , chacune étant divisée en sous-catégories. Au sein de chaque sous-catégorie, les descripteurs sont organisés hiérarchiquement du plus général au plus spécifique jusqu'à treize niveaux. Le moteur de recherche de PubMed dispose d'une fonctionnalité qui identifie automatiquement tous les termes situés plus bas dans la hiérarchie MeSH. Cette fonctionnalité, appelée « explosion », permet d'optimiser les requêtes. Il est cependant nécessaire de s'assurer que tous les termes plus fins entrent dans le champ d'analyse. Par ailleurs, la recherche PubMed permet de pondérer les termes MeSH. Elle distingue les sujets majeurs de l'article, généralement obtenus à partir du titre et/ou de l'énoncé des objectifs, des sujets ayant une moindre importance dans l'article.

La seconde source de données mobilisée est la base de publications OST-WoS, version enrichie du Web of Science de Clarivate Analytics. Cette base de données indexe les publications scientifiques et leurs citations à l'échelle mondiale ; il s'agit de la source de données la plus ancienne et l'une des plus utilisées en scientométrie. Les plus de 20 000 revues indexées sont classées en 254 spécialités (WoS categories), qui, comme toutes les classifications basées sur des revues, n'est pas particulièrement spécifique. En outre, cette base ne dispose pas d'un vocabulaire normalisé et doit être interrogée en langage naturel, ce qui rend difficile la constitution de corpus thématiques précis.

Constitution du corpus

Le domaine de l'infectiologie étant transversal, la construction d'un corpus de publications représentatif demande une méthode adaptée. Un équilibre doit être trouvé entre couverture du domaine d'une part et précision ou pertinence d'autre part. La méthode retenue ici combine les avantages des deux sources de données mobilisées. Les articles pertinents ont été extraits de Medline en utilisant l'équation de recherche (1).

Equation ( 1 Le repérage des publications est effectué sur l'ensemble de la base OST-WoS, c'est-à-dire pour tous les types de documents sur le périmètre des index suivants du WoS : SCI-Science Citation Index Expanded, SSCI-Social Sciences Citation Index, A&HCI-Arts & Humanities Citation Index, CPCI-Conference Proceedings Citation Index (S et SSH). L'analyse est ensuite réalisée en ne retenant que les contributions scientifiques représentées par les types de publication suivants : les articles originaux (Article) y compris ceux des actes de colloques (Proceedings Paper), les synthèses (Reviews) et les lettres (Letters). Les documents pour lesquels manque une partie des métadonnées (adresse, catégorie WoS) ne sont pas pris en compte ; les publications rétractées sont également exclues. L'année de publication la plus récente disponible dans la version d'avril 2021 de la base est 2020, date pour laquelle les données ne sont pas tout à fait complètes. De ce fait, les indicateurs pour 2020 ne sont pas définitifs. En plus du nombre de publications, deux indicateurs sont calculés : l'indice de spécialisation et l'indice d'impact normalisé4 . Le premier mesure l'activité des pays dans une discipline/thématique donnée ; il est obtenu en rapportant la part de la discipline dans l'ensemble des publications du pays à la même part au niveau du monde. Le deuxième mesure l'intérêt suscité par les publications de l'acteur au sein de la communauté scientifique. L'impact normalisé par domaine de recherche d'un pays est défini par le nombre moyen de citations des publications du pays d'une année donnée, normalisé par la moyenne mondiale des citations obtenues par les publications de ce domaine la même année. Ces indicateurs sont indépendants de la taille des pays et permettent des comparaisons internationales.

Cartes thématiques des publications en infectiologie dans le monde et en France

Un des enjeux majeurs de l'analyse scientométrique est la compréhension de l'évolution des sciences et la formation des spécialités disciplinaires. Cet enjeu est d'autant plus central que la science évolue de façon spectaculaire donnant naissance à de nouveaux objets de recherche, au point qu'il est désormais impossible pour un chercheur d'appréhender l'ensemble de la littérature au sein même de sa spécialité.

Vers la fin des années 2000, plusieurs outils permettant d'analyser les liens entre les publications scientifiques ont été développés, aussi bien au niveau thématique qu'institutionnel. Ces outils fournissent des représentations visuelles des réseaux des auteurs ou des termes utilisés dans les publications scientifiques (dans les titres résumés ou mots-clés).

L'analyse thématique du corpus « Infectiologie cartographie » utilise le logiciel VOSviewer qui permet de générer des cartes à partir de données bibliographiques ou textuelles. La construction de la carte mondiale a suivi les 4 grandes étapes d'une cartographie statique : indexation des termes, construction de la matrice de cooccurrence, mesure de la similarité et constitution des clusters [START_REF] Chavalarias | Phylomemetic Patterns in Science Evolution-The Rise and Fall of Scientific Fields[END_REF][START_REF] Ost ; Hcéres | La position scientifique de la France dans le monde et en France[END_REF]. La représentation des termes sur une carte dans un espace à deux dimensions tient compte de la proximité entre les termes d'un point de vue de leur cooccurrence. Ainsi, plus deux termes reviennent ensemble régulièrement dans la liste des mots clés des publications, plus ils seront proches sur la carte et feront partie d'un même cluster. La taille des bulles des termes est représentative de leur nombre d'occurrences dans le corpus.

Carte mondiale des thématiques en infectiologie

La carte mondiale (Fig. 1) est structurée en 7 clusters thématiques décrits par le tableau 2. Les thèmes identifiés tiennent compte de l'ensemble des termes sous-jacents et pas uniquement de ceux qui sont visibles sur l'image.

Caractérisation des publications en infectiologie des principaux pays producteurs Des dynamiques nationales variées

Le nombre de publications en infectiologie a été multiplié par plus de deux en vingt ans, avec une progression moindre que celle de l'ensemble des publications scientifiques entre 2000 et 20196 (Figure 3). L'année 2020 constitue donc une rupture : le nombre de publications mondiales en infectiologie fait un bond pour atteindre 118 148, alors que le recueil des publications parues en 2020 est encore incomplet. Entre 2019 et 2020, le nombre de publications en infectiologie a progressé de près de 40%, ce qui représente une augmentation spectaculaire dans la base de données pour un domaine de recherche.

Les publications dédiées au Covid-19 contribuent fortement à cette augmentation, mais ne l'expliquent pas à elles toutes seules (Figure 3).

Évolution de la spécialisation des principaux producteurs

Le profil thématique des pays au sein du domaine de l'infectiologie contribue à expliquer la mobilisation plus ou moins forte et rapide des systèmes de recherche sur le nouveau sujet qu'a été le Covid à partir de 2020.

L'indice de spécialisation scientifique rapporte la part d'un domaine dans le total des publications d'un pays à ce même ratio pour le monde. La valeur neutre de l'indice de spécialisation est 1 ; lorsque l'indice est supérieur à 1, le pays est spécialisé dans le domaine par rapport à la référence.

Le pays le plus spécialisé en infectiologie parmi les principaux producteurs est l'Afrique du Sud (Figure 5). Son nombre de publications est modeste, mais la part de l'infectiologie dans ses publications est près de 3 fois supérieure à la part du domaine dans les publications mondiales en 2015-19. Cette forte spécialisation s'explique notamment par l'importance des recherches menées en Afrique du Sud sur le Sida et la tuberculose. Le Brésil est le 2 ème pays le plus spécialisé dans le domaine. La Chine augmente sensiblement son degré de spécialisation sur la période, tout en conservant un indice inférieur à la moyenne mondiale.

Fig.5. Indice de spécialisation en infectiologie des 20 premiers pays publiant le plus dans le domaine

Source : Base OST, Web of Science, calculs OST Le degré de spécialisation de la France en infectiologie est assez élevé (1,3) parmi les pays à revenus élevés : proche de celui de la Suisse, de la Belgique et des Pays Bas sur l'ensemble de la période. La France est ainsi plus spécialisée en infectiologie que les Etats-Unis, le Royaume Uni ou l'Allemagne (non spécialisée). Les pays à hauts revenus, souvent les plus spécialisés en biologie fondamentale et en recherche médicale (OST 2021) ne sont donc pas toujours spécialisés en infectiologie.

Plusieurs pays augmentent légèrement leur spécialisation en infectiologie en 2020 par rapport à 2015-19 : la Chine, le Royaume Uni ou la France notamment. L'Italie connaît une augmentation beaucoup plus forte de sa spécialisation, avec un indice qui croît de 60% pour dépasser 1,6. Cette évolution est liée au doublement du nombre de publications italiennes en infectiologie (Figure 4).

La figure 6 souligne la mobilisation rapide de certains pays sur le thème du Covid-19. Le grand nombre de publications de l'Italie dès le début de la pandémie se lit dans l'indice de spécialisation sur le thème du Covid-19, près de deux fois plus élevé que son indice de spécialisation en infectiologie. A l'inverse, la recherche en Afrique du Sud, très spécialisée en infectiologie, n'apparaît pas s'être mobilisée sur le Covid-19.

Le cas de l'Italie peut s'expliquer en partie par le fait que le pays a été durement touché dès le début de la pandémie. A ce stade précoce, les nombreuses publications portaient surtout sur des observations cliniques et la prise en charge des malades [START_REF] Odone | The runaway science: a bibliometric analysis of the COVID-19 scientific literature[END_REF][START_REF] Turatto | A Systematic Review and Bibliometric Analysis of the Scientific Literature on the Early Phase of COVID-19 in Italy[END_REF]. Outre l'Italie, la Turquie et la Chine sont plus spécialisés sur le thème du Covid qu'en infectiologie en général. Impact scientifique des publications des principaux pays producteurs L'impact normalisé par domaine de recherche d'un pays est défini par le nombre moyen de citations des publications du pays d'une année donnée, normalisé par la moyenne mondiale des citations obtenues par les publications de ce domaine la même année. L'indice est normalisé : il est calculé au niveau de chaque spécialité composant les disciplines afin de tenir compte de la structure disciplinaire des publications des pays. La valeur neutre de l'indice est 1 et un indice supérieur à 1 signifie que les publications du pays ont un impact supérieur à la moyenne mondiale.

Conclusion et approfondissements

La mobilisation de plusieurs outils d'analyse scientométrique a permis d'étudier la dynamique des publications mondiales dans le domaine de l'infectiologie de 2000 à 2020. La prise en compte de l'année 2020 a permis d'inclure les premières publications sur le Covid-19 et d'analyser l'impact de ce thème émergent sur la recherche en infectiologie en général et dans certains pays en particulier.

Les principaux pays publiant en infectiologie ont différents profils scientifiques et sanitaires

L'analyse du corpus mondial a souligné que les principaux pays publiant en infectiologie ne sont pas tous les mêmes que les principaux pays publiant toutes disciplines confondues. En effet, outre plusieurs pays à hauts revenus spécialisés en recherche médicale et des nouvelles puissances scientifiques telles que la Chine, l'Inde ou le Brésil, y figurent l'Afrique du Sud, Taiwan et la Turquie. La particularité de la recherche en l'infectiologie est la forte demande sociétale dans les pays où les maladies infectieuses constituent un lourd fardeau économique et social. Les pays spécialisés en infectiologie connaissent des contextes de recherche très différents : recherche médicale importante et industrie pharmaceutique développée (Suisse, Royaume Uni, Etats-Unis), infections persistantes représentant des enjeux majeurs de santé publique (Afrique du Sud, Inde, Brésil), héritage historique avec des recherches développées en lien avec des pays fortement touchés par des maladies infectieuses (France, Belgique, Espagne, Pays Bas).

Très forte augmentation des publications en réaction à la pandémie de Covid-19

L'analyse des publications parues en 2020 a montré que la pandémie de Covid-19 a très rapidement bouleversé le paysage mondial de la recherche en infectiologie. L'intensité et la rapidité de la production scientifique sur le sujet ont été spectaculaires. La réactivité a été particulièrement forte dans certains pays, comme l'Italie dont les publications en infectiologie ont connu une augmentation inouïe. Dans le cas de la France, la spécialisation dans le domaine du Covid n'apparaît pas différente de la spécialisation en infectiologie en général. C'est le cas pour la majorité des principaux pays publiant dans le domaine, à l'exception de l'Italie et, dans une moindre mesure, de la Chine, de la Turquie et de l'Espagne.

La pandémie a suscité une réponse inédite des communautés scientifiques, mais aussi du monde de l'édition. L'accélération de la communication s'est accompagnée d'un grand nombre de publications de faible qualité scientifique, avec des résultats non confirmés, voire erronés (Whitmore, et al. 2021). L'analyse approfondie de ces phénomènes ne pourra être menée que progressivement en fonction des données qui deviendront disponibles. L'analyse de la réponse de la communauté scientifique à la pandémie devrait aussi s'appuyer sur l'examen des co-publications internationales afin de voir dans quelle mesure les collaborations internationales ont pu contribuer à accélérer les travaux de recherche et à renforcer leur contribution scientifique. En effet, si les premières descriptions cliniques ont donné lieu à des contributions par des équipes locales (Aviv-Reuven et Rosenfeld 2021), les études plus ambitieuses tendent à mobiliser des collaborations internationales se traduisant par des co-publications internationales (Haghani, Bliemer, 2021).

Les cartes thématiques permettent de distinguer le profil spécifique de la France au sein du domaine de l'infectiologie

Il existe deux grands modes de constitution de corpus thématiques : à l'aide des requêtes textuelles, en langage naturel ou avec un langage contrôlé (index, thésaurus)

  ) : ("Infections"[MH] OR "Bacterial Vaccines"[MH] OR "Fungal Vaccines"[MH] OR "Protozoan Vaccines"[MH] OR "Toxoids"[MH] OR "Viral Vaccines"[MH] OR "Disease Notification"[MH] OR "Disease Eradication"[MH] OR "Disease Transmission, Infectious"[MH] OR "Contact Tracing"[MH] OR "Carrier State"[MH] OR "Chain of Infection"[MH] OR "Disease Outbreaks"[MH] OR "Travel-Related Illness"[Mesh:NoExp] OR "Quarantine"[MH] OR "Reinfection"[MH]) AND 2000/01/01:2020/12/01[dp] Les articles identifiés ont ensuite été mis en correspondance 3 avec la base OST-WoS dans sa version d'avril 2021. Ce premier corpus issu de PubMed a été complété en ajoutant les articles des revues des catégories WoS INFECTIOUS DISEASES et TROPICAL MEDICINE.

L

  'étude de la cartographie des thématiques au sein du domaine de l'infectiologie nécessite des mots clés auteurs, ce qui limite les publications pouvant être retenues pour l'analyse. En effet, sur l'ensemble des publications identifiées et présentes dans le périmètre OST, 732 593 (60%) disposent de mots clés auteurs. Ces mots clefs sont traités à l'aide d'outils d'analyse textuelle afin d'harmoniser les orthographes et regrouper ceux qui portent la même racine (ex : pluriel et singulier). Après le nettoyage (publications sans mots-clés ou qui présentent de faux mots-clés (ex. caractères spéciaux) etc. , 3 447 publications ont été supprimées. Le sous-corpus « Infectiologie cartographie » comporte 729 146 publications parues entre 2000 et 2020.Par ailleurs, un sous-corpus Covid-19 a été constitué à l'aide de 32 mots clés 5 qui peuvent être résumés en 6 mots : « 2019 ncov », « coronavirus », « Covid », « sars cov 2 », « wuhan seafood market pneumonia virus » et « sars cov2 ». Cette sélection s'appuie sur l'hypothèse que les publications parues entre 2019 et 2020 font référence à la pandémie en cours. L'application de ces filtres a permis d'identifier 19 515 publications parues en 2019-2020 et comportant des mots clés auteurs.

Fig. 1 .

 1 Fig.1. Cartographie thématique du corpus infectiologie, monde

Fig. 2 .

 2 Fig.2. Cartographie des thématiques de spécialisation de la France en infectiologie, 2000-2020

Fig. 3 .

 3 Fig.3. Évolution du nombre de publications en infectiologie dans le monde, 2000-20*

Fig. 4 .

 4 Fig.4. Nombre de publications des 20 premiers producteurs en infectiologie, 2000-20

Fig. 6 .

 6 Fig.6. Indice de spécialisation en recherche sur le Covid-19 des 20 premiers producteurs, 2020*

La figure 7

 7 fournit les indices d'impact jusqu'à l'année 2019 avec une fenêtre de citation de 2 ans. Les publications sur le Covid-19 ne peuvent donc pas être prises en compte pour cet indicateur. La figure 7 indique qu'en 2015-19, la Suisse, les Pays Bas, les Etats-Unis, la Suède, l'Allemagne, l'Australie et le Royaume Uni ont des indices au moins 20% au-dessus de la moyenne mondiale. Ces mêmes pays ont aussi des indices d'impact parmi les plus élevés toutes disciplines confondues (OST 2021).

Fig. 7 .

 7 Fig.7. Indice d'impact en infectiologie des 20 premiers producteurs

AnnexesFig. A1 .

 A1 Fig.A1. Cartographie temporelle des liens du mot-clé Covid-19 avec le reste du corpus

Tableau 1 : Nombre de publications par origine des données Origine des données Nombre de publications identifiées

	Description générale du corpus
	L'équation de recherche (1) a permis d'identifier 1 495 000 publications parues entre 2000 et 2020. Sur
	l'ensemble de ces publications issues de PubMed, 16,6% sont absentes de la base OST-WoS et 18,6%
	correspondent aux publications des catégories WoS INFECTIOUS DISEASES et TROPICAL
	MEDICINE. Symétriquement, 39,2% des publications extraites de la base OST-WoS à partir des
	catégories INFECTIOUS DISEASES et TROPICAL MEDICINE ne figurent pas dans les publications
	obtenues à partir de PubMed.
	Publications issues de PubMed présentes dans la base OST-WoS	968 055
	Publications issues de PubMed absentes dans la base OST-WoS	248 578
	Publications issues de PubMed présentes dans liste des publications des 2 spécialités WoS	278 367
	Publications des 2 spécialités WoS absentes de la liste issue de PubMed	179 099
	Source : Medline, base OST, Web of Science, traitement OST

Les publications absentes de la base OST-WoS ne sont pas prises en compte. Le corpus est ainsi réduit à 1 426 270 publications. L'application des filtres indiqués précédemment (présence d'adresse d'affiliation et de spécialiaté WoS) réduit le corpus « Infectiologie » à 1 215 199 publications. Ce corpus est mobilisé dans la partie consacrée au positionnement des principaux pays publiant dans le domaine (voir plus bas).

  par sélection d'éléments à partir d'une classification prédéfinie. Le recours à une nomenclature existante est plus simple mais moins performant pour circonscrire des périmètres thématiques complexes. Les requêtes textuelles en langage naturel sont lourdes à construire et sont à la fois imprécises et non exhaustives. Mais les bases citationnelles utilisées pour les analyses scientométriques ne possèdent pas de vocabulaire contrôlé. Pour pallier l'absence d'un vocabulaire contrôlé dans la base WoS, cette étude s'est appuyée sur celui de Medline pour construire l'équation de recherche. L'analyse a montré que dans le domaine de l'infectiologie le recouvrement entre la base OST-WoS et Medline est de 83%. L'absence de 17% des publications de Medline dans la base OST-WoS peuvent être multiples : des revues non indexées dans le WoS, des défauts d'appariement entre les deux bases, la fraicheur de la mise à jour, etc. Cette question n'a pas été étudiée et pourrait constituer un approfondissement en vue d'une amélioration de la méthode.Les cartes thématiques construites à partir des cooccurrences de mots clés dans les publications ont permis d'identifier 7 clusters au sein du corpus global des publications en infectiologie. L'analyse a ensuite mis en évidence les thématiques qui distinguent les publications françaises. Elles sont plus concentrées sur deux des clusters du corpus mondial, Infections opportunistes et secondaires d'une part et Zoonoses, maladies à transmission vectorielle et maladies tropicales d'autre part. Symétriquement, les publications françaises traitent relativement moins que le monde les thématiques Mécanismes fondamentaux de l'infection et interactions hôte-pathogène d'une part et Maladies infectieuses à transmission directe et santé publique d'autre part. En 2020, la France est apparue presque aussi spécialisée sur le thème du Covid-19 que sur l'infectiologie en général, ce qui ne suggère pas une orientation forte des recherches en faveur de ce thème. Le profil thématique de la France au sein de l'infectiologie demandera à être confirmé sur une période d'observation plus longue. Avec plus de recul, il pourrait en outre être possible d'analyser d'éventuels liens entre les difficultés de coordination identifiées au sein du système de recherche dans les débuts de la pandémie et les publications scientifiques issues des travaux sur le Covid-19. Dans cette perspective plus générale, il sera aussi utile de confronter les données de publications avec celles des essais cliniques.

Voir, https://www.adbs.fr/langages-documentaires#th%C3%A9saurus

https://meshb.nlm.nih.gov/treeView

L'intégration récente des identifiants PMID de PubMed dans la base WoS facilite l'utilisation conjointe des deux bases. Cependant l'appariement entre identifiants PubMed et WoS n'est pas parfait et demande d'effectuer des contrôles.

L'indice d'impact normalisé est basé sur le nombre de citations de chaque article et non sur la moyenne des citations d'une revue, comme le facteur d'impact.

Voir liste des mots clefs en annexe (Tableau A1).

La croissance du total des publications de 2000 à 2019 a été de 136%, contre 111% pour l'infectiologie


Drs Vallier

Mely

PhD Mathieu Gruet
email: gruet@univ-tln.fr

■ METHODS: Participants performed an MST and a CPET on a cycle ergometer in random order. Oxygen (O 2 ) uptake ( V • O 2 ), carbon dioxide (CO 2 ) retention (end-tidal P CO 2 ; P ETCO 2 ), minute ventilation, heart rate, and peripheral O 2 saturation (Sp O 2 ) were continuously monitored. Whole blood lactate, dyspnea, and leg discomfort were recorded immediately after both exercises.

■ RESULTS: Twenty patients with CF (aged 33 ± 8 years; forced expiratory volume in 1 second = 48% ± 17%) completed both tests. Peak values for V • O 2 (27 ± 7 vs 24 ± 6 mL/kg/min), heart rate (169 ± 19 vs 163 ± 16 bpm) and end P ETCO 2 (42 ± 7 vs 39 ± 8 mm Hg), and O 2 desaturation (end Sp O 2 , 86% ± 7% vs 90% ± 6%) were significantly higher during the MST than during the CPET (all P s < .05). Leg discomfort and whole blood lactate were significantly higher after the CPET (both P s < .05). Thirty-five percent and 40% of patients showed significant O 2 desaturation and CO 2 retention, respectively, during the MST, which was not detected during the CPET. A strong relationship was found between MST peak V • O 2 and body weight walking distance product ( r = 0.90; P < .01).

■ CONCLUSIONS: The MST may provide a strong indicator of exercise tolerance in adults with CF as indicated by high peak V • O 2 values. In clinical practice, body weight walking distance should be considered as the primary outcome. This test is also better than cycle ergometry CPET for detecting O 2 desaturation and CO 2 retention, further emphasizing its clinical interest. Exercise has positive effects on many health outcomes in cystic fibrosis (CF), and higher levels of physical fitness are associated with better quality of life 1 and survival 2 in this population. A comprehensive and accurate assessment of exercise tolerance is thus fundamental in the clinical management of this population, including exercise rehabilitation. Maximal cardiopulmonary exercise test (CPET) performed on a cycle ergometer with gas exchange measurements is the gold standard exercise test. However, many centers do not have the resources to routinely include the CPET [START_REF] Stevens | A survey of exercise testing and training in UK cystic fibrosis clinics[END_REF] and performing the CPET using cycle ergometers may not reflect habitual physical activity of some patients. Peripheral muscle fatigue is often an important limiting factor during cycle CPET, [START_REF] Moorcroft | Symptoms, lactate and exercise limitation at peak cycle ergometry in adults with cystic fibrosis[END_REF] so patients with significant muscle weakness may not achieve maximal cardiopulmonary stress. Moreover, treadmill walking tests are not interchangeable with hallway walking in patients with chronic respiratory disorders [START_REF] Almeida | Hallway versus treadmill 6-minute-walk tests in patients with chronic obstructive pulmonary disease[END_REF] and thus may not be representative of real-life walking. Accordingly, it seems inappropriate to systematically generalize cycle ergometer CPET outcomes to everyday activities.

A number of field-based exercise tests have been used in CF, mainly because of their ease of use. The 6-minute walk test (6MWT) and the 3-minute step test are among the most popular. 3 , 6 However, each test has its own limitations. The 6MWT requires substantial space (ie, 30-40 m) and may be less responsive to treatment than other externally paced tests. [START_REF] Pepin | Six-minute walking versus shuttle walking: responsiveness to bronchodilation in chronic obstructive pulmonary disease[END_REF] The reliability of the 3-minute step test has not yet been established in adults with CF, and important information (eg, oxygen [O 2 ] desaturation) may not be detected by this test. [START_REF] Narang | Threeminute step test to assess exercise capacity in children with cystic fibrosis with mild lung disease[END_REF] Eventually, both tests elicit submaximal responses and thus may not reflect the integrated response of the organism to exercise, nor identify the factors limiting exercise tolerance. 8 , 9 The modified shuttle test (MST) is a field-based incremental walking/running test that is well-tolerated, reliable, and sensitive to antibiotic treatment. 10 , 11 This test is simple to perform, necessitates minimal space, and is externally paced. Despite being increasingly used in clinical practice, [START_REF] Stevens | A survey of exercise testing and training in UK cystic fibrosis clinics[END_REF] only limited data are available on the physiological demands imposed on adults with CF during the MST.

The aim of the present study was to investigate the physiological responses of adults with CF during the MST as compared with the gold standard cycle ergometer CPET. Because of the specific breathing pattern and important muscle mass involved during walking/ running, we hypothesized that the MST, as compared with the CPET, would (1) elicit at least similar peak O 2 uptake ( V

• O 2 peak ); and (2) induce greater O 2 desaturation and carbon dioxide (CO 2 ) retention.

METHODS

Twenty adults with CF were recruited from a regional cystic fibrosis unit (CRCM, Respiratory Rehabilitation Center, Giens, France). Diagnosis of CF was based on clinical features and genotyping, according to standard guidelines. The patients were in a clinically stable state and had neither contraindications for maximal exercise testing nor significant orthopedic limitations limiting their ability to exercise. All subjects gave their written informed consent for the study, which was conducted according to the Declaration of Helsinki and approved by the local institutional review committee. Patients performed a CPET and an MST in random order during 2 separate days within 1 week. The CPET was performed using an electrically braked cycle ergometer (Ergoselect 100; Ergoline GmbH, Bitz, Germany). Absolute workload increments (similar for all subjects) were chosen to permit optimal comparison with absolute increments imposed by the MST. After a 1-minute warm-up at 20 watts, the workload was increased by 6 watts per minute to obtain an exercise duration of at least 7 minutes in all patients, allowing optimal V

• O 2 peak determination according to recent recommendations. [START_REF] Midgley | Challenging a dogma of exercise physiology: does an incremental exercise test for valid VO 2 max determination really need to last between 8 and 12 minutes?[END_REF] Subjects were instructed to maintain a cadence of 60 to 65 rpm throughout the test. The MST was performed according to the 15-level protocol described by Bradley et al. [START_REF] Bradley | Reliability, repeatability, and sensitivity of the modified shuttle test in adult cystic fibrosis[END_REF] Briefly, patients were required to walk/run at increasing speeds back and forth along a 10-m corridor. The test started with a speed of 1.8 km/h (level 1), followed by an increment of 0.6 km/h at each level. Each minute, patients were told to go a little faster and reminded that they were allowed to run at any time. The test ended when they were unable to maintain the set pace. Total distance and body weight distance product were calculated from the MST, and peak workload was determined from the CPET. Expired gases were continuously measured during both tests using a breath-by-breath telemetric system (K4b2; Cosmed, Rome, Italy). CO 2 production, minute ventilation ( V Ventilatory threshold (VT) was visually determined using the modified V-slope method and ventilatory equivalents. Maximal predicted voluntary ventilation (MVV) was estimated as 37.5 × forced expiratory volume in 1 second (FEV 1 ). The ratio peak V • E /MVV was calculated as an index of ventilatory limitation at peak exercise. Whole blood lactate was measured immediately after exercise from an ear-prick capillary sample and immediately analyzed using an enzymatic method (Lactate Pro; Arkray, Japan). Leg discomfort and dyspnea sensations at peak exercise were measured immediately after both tests using a Borg 6-20 RPE scale. Significant O 2 desaturation was defined as a > 5% fall of the resting Sp O 2 during the last minute of the CPET or MST. Significant CO 2 retention was defined as a rise of ≥ 5 mm Hg of end-tidal P CO 2 (P ETCO 2 ) from the initial workload until the peak workload. [START_REF] Javadpour | Does carbon dioxide retention during exercise predict a more rapid decline in FEV 1 in cystic fibrosis?[END_REF] A test was considered to be at or near the maximal level if at least one of the following criteria was met: peak HR, > 85% predicted; peak respiratory exchange ratio, > 1.09; sensations of leg discomfort or dyspnea, > 18.

• E ), V • O 2 ,

Statistical Analysis

Normality of distribution and homogeneity of variances of the main variables were verified using a Kolmogorov-Smirnov test and a Levene test, respectively. Paired t tests and linear regression were

RESULTS

Participant characteristics and exercise data are summarized in the Table . Eight patients had severe lung dis-ease (defined as FEV 1 < 40% predicted), 8 had moderate lung disease (FEV 1 = 40-60% predicted), and 4 had mild lung disease (FEV 1 > 60% predicted).

Nineteen subjects had pancreatic insufficiency, and 5 had CF-related diabetes. According to the criteria previously stated, all patients performed both tests at or near maximal exercise. Fifteen patients ran during the MST, all starting between the seventh and ninth levels.

DISCUSSION

This study described physiological responses to the MST as compared with the gold standard CPET in adults with CF. The main results are as follows: (1) the MST elicits higher V • O 2 peak and ( 2) is better at detecting O 2 desaturation and CO 2 retention in CF, compared with the CPET. CPET V • O 2 peak was 1.6 ± 0.5 L/min (56% of predicted values), which is consistent with previous studies investigating patients with CF with a similar degree of airway obstruction. 14 , 15 The higher peak V

• O 2 observed
during the MST may be explained by the greater muscle mass involved in fast walking or running than in cycling on the ergometer. This assumption is supported by the higher peak HR values observed during the MST, resulting in increased muscle blood flow and the ability to match greater O 2 demand due to higher muscle recruitment. These results differ from previous results in chronic obstructive pulmonary disease (COPD), where V • O 2 peak and HR were similar between the CPET and the MST. [START_REF] Palange | Ventilatory and metabolic adaptations to walking and cycling in patients with COPD[END_REF] In that study, the patients did not reach a speed requiring them to run and the MST was probably submaximal as indicated by low respiratory exchange ratio at peak exercise (ie, 0.82 ± 0.01).

In the present study, 75% of patients reached speeds compatible with running, which may explain the greater cardiovascular demand of the MST in our study. As maximal exercise involves the integrated functioning of multiple organs in multisystem diseases such as CF, CPET V • O 2 peak has been demonstrated to be one of the strongest markers of mortality in this population. [START_REF] Pianosi | Peak oxygen uptake and mortality in children with cystic fibrosis[END_REF] In view of the high V • O 2 peak values obtained during the MST, it is likely that this test has a high prognostic value in CF, which remains to be evaluated. As body weight of the patients directly affects the walking/running efficiency, we calculated body weight distance product during the MST, as recommended during the 6MWT in COPD. [START_REF] Carter | 6-minute walk work for assessment of functional capacity in patients with COPD[END_REF] This parameter demonstrated a strong and better association with MST V

• O 2 peak than with distance alone, suggesting that body weight distance should be chosen as the primary outcome of the MST. Larger studies across the spectrum of CF severity should be conducted to establish and validate an equation permitting to predict V • O 2 peak from MST body weight distance.

V • E at peak exercise was not different between the MST and the CPET, despite slight differences in the breathing pattern (ie, similar tidal volume but slightly higher respiratory rate during the MST). Higher respiratory rate at peak exercise during the MST may be explained by a compensatory mechanism for the slightly higher sensation of dyspnea during this test.

The absence of a difference in V • E is probably due to [START_REF] Javadpour | Does carbon dioxide retention during exercise predict a more rapid decline in FEV 1 in cystic fibrosis?[END_REF] The arm muscles are active during fast walking and this may trigger reflex impulses to the respiratory centers, potentially leading to dyssynchronous breathing, and thus explains impairments in gas exchange during the MST. As fast walking and/or slow running are more representative of daily life activities than cycling, the MST should be a better alternative to the CPET to detect exercise-induced O 2 desaturation and CO 2 retention.

It was possible to determine a VT in 80% of the patients during the MST. Exercise at VT (eg, HR at VT) is one of the most popular and effective strategies to individualize endurance exercise intensity in patients with chronic respiratory disorders. Since VT for a given individual on exercise modality (eg, cycling vs running), determination of VT during the MST should be useful for individualizing the walking or running (ie, depending on patient disease severity) component of endurance rehabilitation program.

In conclusion, this study has demonstrated physiological adaptations peculiar to the MST in adults with CF. In a practical perspective, the body weight distance measured during the MST should be considered as the main outcome and measurement of O 2 desaturation should allow for better detection of patients with CF who may exhibit O 2 desaturation during ambulation. The MST performed with gas exchange measurements should be a strong indicator of physical fitness as indicated by higher O 2 peak values and may provide useful information about the ventilatory pattern and the likelihood of CO 2 retention during dayto-day activities.

-Acknowledgment-

This study was financially supported by the Association Régionale d'Assistance Respiratoire à Domicile (ARARD).

■

  PURPOSE:The modified shuttle test (MST) is increasingly used in clinical practice to assess functional capacity in patients with cystic fibrosis (CF). The purpose of this study was to evaluate the physiological responses of the MST in adults with CF as compared with the gold standard cardiopulmonary exercise test (CPET).

  the Modified Shuttle Test in Adults With Cystic Fibrosis Jean-Marc Vallier , MD, PhD ; Mehdi Rouissi , MSc ; Laurent Mely , MD ; Mathieu Gruet , PhD

Figure .

 . Figure. Physiological responses during the modified shuttle test at each stage. A, Individual data for oxygen uptake ( V • O 2 ). B, Individual data for minute ventilation ( V • E ).

Participant Baseline Characteristics and Exercise Data a Participant Characteristics

  It was possible to determine VT in 16 patients during the MST and 19 patients during the CPET. Eight patients showed significant O 2 desaturation during the CPET and 15 during the MST. Seven patients (35% of the total population) showed significant O 2 desaturation during the MST that was not detected during the CPET. Eight patients showed significant CO 2 retention during the CPET and 14 during the MST. Eight patients (40% of the total population) showed significant CO 2 retention during the MST that was not detected during the CPET. Peak V

				V • O 2
	Age, y Gender M/F, n during the MST (MST V • O 2 peak ) was strongly related to 32.6 ± 8.3 • O 2 V • O 2 peak measured during the CPET (CPET V • O 2 peak ; r = 17/3 BMI, kg/m 2 21.2 ± 3.6 FEV 1 , L 1.78 ± 0.68 0.94; P < .01). A stronger relationship was found between MST V • O 2 peak and body weight distance product ( r = 0.90; P < .01) than between MST V • O 2 peak and total
	FEV 1 , % predicted FVC, L distance alone ( r = 0.83; P < .01). Individual kinetics of 47.8 ± 17.0 3.15 ± 1.0 V • E and V • O 2 during the MST are shown in the Figure .
	FVC, % predicted	71.5 ± 20.5
	Genotype	9 × Δ F508del hom
		7 × Δ F508del het
		4 × other mutations
	Exercise Data	MST	CPET	P Value
	MST distance, m	863 ± 311	…	…
	MST body weight distance, kg/m	51 761 ± 20 006	…	…
	CPET peak workload, watts	…	105 ± 32
	V • O 2 peak , mL/min	1 614 ± 456	1 421 ± 375	< .01
	V • O 2 peak , % predicted	…	56.4 ± 14	…
	V • O 2 peak , mL/kg/min	26.6 ± 6.9	23.7 ± 5.8	< .01
	Peak respiratory exchange ratio	1.18 ± 0.15	1.22 ± 0.09	.29
	Ventilatory threshold, mL/min	1 065 ± 344	879 ± 221	< .01
	Peak tidal volume, mL	1.49 ± 0.36	1.58 ± 0.50	.20
	Peak respiratory rate, breaths/min	49.0 ± 8.4	45.8 ± 9.5	.02
	Peak V • E , L/min	66.9 ± 20.1	64.0 ± 17.7	.32
	Peak V • E /MVV, %	105 ± 23	101 ± 21	.37
	End P ETCO 2 , mm Hg	41.9 ± 7.3	38.5 ± 7.7	.03
	End Sp O 2 , %	85.8 ± 6.7	89.5 ± 6.1	< .01
	Peak heart rate, bpm	169 ± 19	163 ± 16	.03
	End-exercise lactate concentration, mmol/L	4.7 ± 1.5	5.9 ± 0.9	< .01
	Borg Scale scores
	Dyspnea	17.0 ± 1.5	15.6 ± 2.5	.10
	Leg discomfort	13.9 ± 3.0	16.0 ± 2.5	.01
	Abbreviations: BMI, body mass index; CPET, cardiopulmonary exercise test; FEV 1 , forced expiratory volume in 1 second; FVC, forced vital capacity; MST, modi-fied shuttle test; MVV, maximal predicted voluntary ventilation; P ETCO 2 , partial pressure of end-tidal carbon dioxide; Sp O 2 , peripheral oxygen saturation; V • E , minute ventilation; V •

( + 13.6%), respiratory rate ( + 7.0%), and HR ( + 3.7%) were significantly higher at peak exercise during the T a b l e • O 2 peak , peak oxygen consumption. a Data are reported as mean ± SD, unless otherwise stated. JCRP-D-15-00078_LR 3 JCRP-D-15-00078_LR 3 21/03/16 12:34 PM 12:34 PM www.jcrpjournal.com

MST than during the CPET (all P s < .05; Table

)

. Peak lactate concentration ( + 25.5%) and peak exercise leg discomfort ( + 15.1%) were significantly higher after the CPET than after the MST (both P s < .05; Table

)

.

/ Journal of Cardiopulmonary Rehabilitation and Prevention 2016;36:1-5

AUTHOR QUERIES TITLE: Physiological Responses of the Modified Shuttle Test in Adults With Cystic Fibrosis AUTHORS: Jean-Marc Vallier , Mehdi Rouissi , Laurent Mely , and Mathieu Gruet [AQ01]: Would it be better to change "gold standard" to "criterion standard" throughout the text? Please advise.

[AQ02]: Please provide the city location of Arkray.


Christos Sarantopoulos
email: christos.sarantopoulos@gmail.com

Alain N Gleizes
email: alain.gleizes@ensiacet.fr

Francis Maury
email: francis.maury@ensiacet.fr

Chemical vapor deposition and characterization of nitrogen doped TiO 2 thin films on glass substrates

Keywords:

Photocatalytically active, N-doped TiO 2 thin films were prepared by low pressure metalorganic chemical vapor deposition (MOCVD) using titanium tetra-iso-propoxide (TTIP) as a precursor and NH 3 as a reactive doping gas. We present the influence of the growth parameters (temperature, reactive gas phase composition) on the microstructural and physico-chemical characteristics of the films, as deduced from X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and ultra-violet and visible (UV/Vis) spectroscopy analysis. The N-doping level was controlled by the partial pressure ratio R = [NH 3 ]/[TTIP] at the entrance of the reactor and by the substrate temperature. For R = 2200, the N-doped TiO 2 layers are transparent and exhibit significant visible light photocatalytic activity (PA) in a narrow growth temperature range (375-400 °C). The optimum N-doping level is approximately 0.8 at.%. However, the PA activity of these N-doped films, under UV light radiation, is lower than that of undoped TiO 2 films of comparable thickness.

Introduction

Titanium dioxide (TiO 2 ) is extensively used in modern life applications, e.g. cosmetics, pigments, building materials, antibacterial surfaces, biocompatible material for prosthesis and solar cells [1][2][3][4]. Many research works of the past two decades focus on the photocatalytic properties of this semi-conducting transparent material. Under UV light (wavelength b 387 nm) TiO 2 -anatase leads to the degradation and total mineralization of a great number of pollutants dispersed in air or water [START_REF] Fujishima | TiO2 Photocatalysis: Fundaments and Applications[END_REF]. Moreover, the photo-induced superhydrophilicity of TiO 2 films has also been achieved under UV light [START_REF] Watanabe | [END_REF]. These two properties are very attractive: a thin film of TiO 2 can decompose atmospheric pollutants present on its surface and get selfcleaning from any dust and organic residues at the same time. Thus, the coating of large glass surfaces with TiO 2 (buildings, cars, etc.) is a smart way to decrease maintenance costs and contribute to the cleaning of urban air. Considering that the UV light is only 5% of the sun spectrum, real commercial applications will drastically develop if activation of these semi-conducting films by the visible light is enhanced.

A simple idea to activate the TiO 2 photocatalyst by the visible light is to reduce the band gap (initially 3.2 eV for TiO 2 -anatase) by inserting intermediate electronic states in the band gap of the electronic structure. This can be achieved by creating defects, i.e. oxygen vacancies (TiO 2 -x ) or atoms in substitution or insertion (cationic and anionic doping). Oxygen deficient TiO 2 -x films produced by MOCVD [7] and powders produced by TiC, TiN, TiO and Ti 2 O 3 oxidation [8], gave positive results. Doping of TiO 2 anatase powders with Fe [9], Cr [10] and Ag [11] led to visible photocatalytic decomposition of oxalic acid, yellow XRG dye and rhodamine B, respectively. The optimum doping level reported so far is quite low (e.g. Fe +3 = 0.09% or Fe/ Ti = 0.05 [9]). TiO 2 -x and cation-doped TiO 2 samples suffer from thermal stability and low photocatalytic activity because of the presence of uncontrolled structural defects which act as electron and hole traps [12].

Concerning anion doping, recently Asahi et al. [13] proposed an interesting approach for elaborating N-doped TiO 2 anatase films by substituting oxygen with nitrogen atoms. The small difference between nitrogen and oxygen ion radii facilitates nitrogen substitution and does not distort the crystal lattice much. Theoretical modeling was also used to demonstrate that the semiconductor band gap decreases when the N 2p states mix with the O 2p states, thus rendering the material active under visible light [13,14]. Still, the electronic structure of the efficient nitrogen doped anatase is under question [8,15,16]. Despite the great number of scientific reports about N-doped TiO 2 thin films, only few works mention successful results for supported thin layers. F.-D. Duminica et al. have proposed an atmospheric pressure CVD process (APCVD) for the growth of pure TiO 2 [17] and further, by using N 2 H 4 as doping reactive gas, they have reported significant photocatalytic activity in the visible light range but for films deposited in narrow growth conditions [18]. Working with APCVD but using NH 3 as a reactive gas instead, Yates et al. found no evidence of photocatalytic activity [19]. However Maeda et al. [20] reported visible light activity of films prepared by plasma enhanced CVD under low pressure and using NH 3 as reactive gas. Efficient photocatalysts under visible light were also produced using other deposition methods such as ion-assisted electron-beam evaporation [21] or magnetron sputtering [13,22].

This work reports the growth of photocatalytically active N-doped TiO 2 films by low pressure CVD on glass substrates, using titanium tetra-iso-propoxide (TTIP) as titanium and oxygen precursor and NH 3 as nitrogen source. The microstructural characteristics and the photocatalytic activity of the films are discussed in relation with the growth parameters.

Experimental

A horizontal hot-wall CVD quartz reactor 5 cm in diameter was used for the film deposition. Borosilicate flat glass and Si(100) wafers were placed on an inclined stainless steel sample holder at a distance 14 cm from the entrance. The main precursor decomposition zone, found from preliminary runs, is extended from 11 to 18 cm from the entrance of the reactor. The growth temperature was controlled by a thermocouple K plugged into the sample holder. Liquid TTIP was placed in a bubbler thermostated in a water bath at 298 K. Pure N 2 (99.9992%) was fed through two lines equipped with mass flow controllers, one to bubble through the TTIP precursor, the other to dilute it. The total pressure was maintained at 20 Torr. For the N-doping, NH 3 gas (99.999%) was fed through the reactor during the deposition. Table 1 presents the typical MOCVD conditions. The film microstructure was studied by X-ray diffraction (XRD) using θ-θ and grazing geometry (Cu Kα). The surface morphology was observed by scanning electron microscopy (SEM) and the film thickness was determined on cross sections and by optical methods [23]. The arithmetical average of the surface roughness profile (R a ) was measured using an optical interferometer. The film composition was analyzed by secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS). Optical properties were determined using a UV-Vis spectrophotometer equipped with an integrating sphere. The band gap (E g ) was determined by extrapolation of the absorption edge using Tauc's plot.

The photocatalytic activity of the one-side coated borosilicate flat glass (32 × 32 mm 2 ) was determined from the initial decomposition rate (r 0 ) of an aqueous Orange G solution (10 -5 mol L -1 ) under UV and visible irradiation. The sample immersed in 25.0 cm 3 of the dye solution was placed into a quartz vessel (28.8 cm 3 ) transparent to wavelengths N290 nm. All solutions were first agitated for 1 h in the dark in order to reach equilibrium with the photocatalyst. The concentration was determined by measuring the absorbance of Orange G at 480 nm and applying the Beer-Lambert law. The kinetic curves of Orange G decomposition were obtained by plotting the concentration as a function of irradiation time. The r 0 was determined by the slope of the linear fitting over the first 60 min for UV irradiation and over the first 360 min for the visible light irradiation. For UV tests the sample was irradiated with 1.05 mW cm -2 (365 nm) using a UV lamp (HPLN Philips 125 W). The tests under visible light were performed using two fluorescent lamps (FCE27 52 W) coupled with a UV cut filter (400 nm) providing a cumulative irradiance in the range 450-550 nm of 14.1 mW cm -2 . The results of the structural analysis (XRD), photocatalytic activity and optical properties refer to films deposited on borosilicate glass substrates while composition analysis (SIMS, XPS) and SEM observations were performed on films grown on Si (100) wafers.

Results and discussion

Growth rate

Fig. 1 shows the variation of the growth rate of undoped and N-doped TiO 2 films with the substrate temperature using a partial pressure ratio NH 3 /TTIP of R = 2220. Below 350 °C the growth rate is enhanced when NH 3 is added. Above 350 °C the growth rate increases slowly with the temperature both in presence of and without NH 3 . Above this temperature threshold, the growth rate of undoped and N-doped films cannot be compared directly because the TTIP mole fraction is different. However, we observed that above 350 °C the growth rate of undoped-TiO 2 is approximately 4 times higher than that of N-doped TiO 2 (2 and 8 nm min -1 , respectively) which is certainly due to the fact that the TTIP mole fraction was 4 times higher for the growth of undoped layers. As a result, we assume the growth rate would be probably of the same order at high temperature if the same TTIP mole fraction had been used. By contrast, for deposition temperatures lower than 350 °C, the growth rate of N-doped TiO 2 is of the same order (or even higher at 300 °C) despite the higher TTIP mole fraction used for undoped TiO 2 films. In fact, the apparent activation energy for the deposition of N-doped TiO 2 films is very low (13 kJ mol -1 ) comparatively to that of pure TiO 2 films (93 kJ mol -1 ). This suggests that in the presence of NH 3 the process is not kinetically controlled as a result of fast reactions between NH 3 and TTIP. This is consistent with the observations of Jung et al. who considered that the decomposition of TTIP was catalyzed by NH 3 (direct interaction of NH 3 with electron deficient Ti in TTIP) leading to higher decomposition rates at temperatures lower than 330 °C [24].

Morphology, structure and composition

From X-ray diffraction patterns, the N-TiO 2 films grown above 350 °C using R = 2220 are well crystallized while those prepared at temperatures equal to or lower than 350 °C contain a significant part of amorphous phase. All films exhibit the anatase structure and are constituted of nano-crystals of various sizes. For growth temperatures between 300 °C and 350 °C, the mean crystal size is small (20 ± 5 nm) and does not change much with the temperature (Fig. 2). Increasing the growth temperature up to 400 °C leads to significant increase of the crystal size (40 ± 5 nm). Surface morphology reveals that the small TiO 2 crystals are agglomerated to form grains. The grain size, as determined by surface SEM observations, increases with the film thickness, i.e. with the deposition time because of crystal growth and aggregation phenomena during the growth (Fig. 3). The oxide layers prepared at low temperature (300 and 350 °C) are made of large aggregates of small crystals (namely grains) and present a compact granular morphology. The films prepared above 350 °C exhibit a uniform columnar morphology. This change in film morphology leads to a decrease of the surface grain size when the temperature increases from 300 to 400 °C. Taylor et al. reported a similar grain size variation versus deposition temperature [25]. These last samples show texture in the (211) and (220) directions whereas no preferential orientation is found for the granular morphology grown at and below 350 °C (Fig. 4). This is correlated with the change of film morphology above 350 °C, followed by the improvement of crystallinity. The surface roughness of N-doped TiO 2 films decreases when the growth temperature increases: R a = 177 nm, 107 nm and 21 nm for films prepared at 300 °C, 350 °C and 400 °C, respectively.

The N-doping level of the films is controlled by varying the mole fraction ratio R as well as the growth temperature. Even for low values of R (555), SIMS analysis shows that nitrogen is incorporated into the film (Fig. 5). The N concentration profile is constant through the film thickness, suggesting homogeneous doping. Fig. 6 shows that for high R values (2200), the relative N content is maximized for a growth temperature of 350 °C and decreases near to the detection limit when the temperature is 400 °C. The relative N content was estimated by XPS at 4.8 and 0.8 at.% for the growth temperature of 350 and 400 °C, respectively. At low temperatures (equal or inferior to 350 °C) the catalytic reaction of TTIP with NH 3 , leads to the incorporation of high amounts of N species [24]. At higher growth temperatures (T N 375 °C) the pyrolytic self-decomposition of TTIP becomes dominant and faster than NH 3 decomposition leading to growth of TiO 2 films with a relatively low N incorporation. Additionally, competitive reactions as N is released from growing films may occur as observed during thermal treatment in air at 450 °C of TiO x N y [26].

In XPS spectra, the N 1s level is found at 396 eV for films grown below 400 °C. According to [13], it is attributed to N atoms substituting for O atoms. For N-TiO 2 films deposited at 400 °C and above, this N 1s peak at 396 eV is not observed maybe because of the very low N incorporation (0.8 at.%). In these conditions, a small N 1s peak is detected at 400 eV which corresponds to chemisorbed nitrogen [13,27,28].

Optical properties

Changes in composition are also reflected in the optical properties of the films. The absorption of light by the films is shifted to the visible range when the quantity of ammonia is increased in the gas phase (Fig. 7). Simultaneously, the aspect of the films changes from transparent and colorless (R = 0) to green colored (R = 2200). The shift to the visible light region is also observed for all films of comparable thickness grown between 300 °C and 425 °C, even if not systematically marked by the above mentioned color change. With R = 2200, the color depends on growth temperature. The films prepared at 300 °C are light yellow and not very transparent (23%). Those prepared at 350 °C are light green and not transparent (6%). Those prepared between 375 °C and 425 °C are colorless and transparent (50-85%). Raising the quantity of ammonia (R = 4000) in the gas phase, leads to dark brown films with a metallic lustre. For all N-doped TiO 2 films the fundamental absorption edge was shifted to the visible range. Fig. 8 shows that the optical band gap variation with the growth temperature is not continuous. For a growth temperature of 300 °C, the band gap is 2.85 eV and it decreases to 2.20 eV when the growth temperature is 350 °C. Samples prepared at 375 °C and above show larger optical band gaps increasing from 3.00 eV to 3.25 eV. The latter value corresponds to pure anatase. The semiconductor band gap varies also with the grain size of the film [29,30]. However for the grain size values of the films studied here (50-250 nm), no significant influence is considered. These results are more likely consistent with the variation of the N content with the growth temperature (Fig. 6): the more nitrogen the structure incorporates, the narrower the band gap gets.

Photocatalytic activity

Figs. 9 and 10 show the photocatalytic activity of N-doped TiO 2 samples under UV and visible light irradiation. All films were deposited on glass substrates of same dimensions (32 × 32 mm 2 ) at temperatures between 300 °C and 450 °C, using R = 2200. They have approximately the same thickness. In order to compare the photocatalytic activity of the samples, the initial decomposition rate r 0 was normalized by the mass of deposited photocatalyst (r 0 ′ in mol L -1 min -1 g -1 ). Non doped TiO 2 films of same thickness were also tested and compared with the N-doped ones. It is generally observed that raising the growth temperature leads to films with higher photocatalytic activity. Under UV irradiation the activity of all N-doped TiO 2 films is lower than that of pure TiO 2 films (Fig. 9). The most efficient samples under UV irradiation (N-doped and pure TiO 2 films) were obtained at 375-400 °C. The decrease of r 0 ' for the films elaborated at 425 °C and above is due to the poor crystallization of the film originating from the glass surface thermal softening.

Under visible light radiation, the overall decomposition rates of Orange G on N-doped TiO 2 films are lower than under UV irradiation. Comparing with the non doped films grown at 400 °C, it appears that N-doping enhances photocatalytic activity in the visible light by a factor of 10. The best visible activity is obtained for films prepared in the narrow window 375-400 °C. This temperature range is slightly lower than in our previous work using N 2 H 4 as nitrogen source [18]. This result is to be correlated with the composition modification of TiO 2 films containing a small quantity of nitrogen (0.8 at.%) leading to a slight band gap shrinkage (3.00-3.05 eV) with respect to that of pure anatase (3.25 eV). The films prepared at 300 °C and 350 °C are photocatalytically inactive both under UV and visible light. This is explained by the low crystallinity of the films and the high content in incorporated nitrogen which act as defects favoring carrier recombination rather than efficient doping.

Summary and conclusions

N-doped TiO 2 films, photocatalytically active under visible and UV light were grown by low pressure CVD process using TTIP as a precursor and NH 3 as a reactive gas. A narrow window of the growth parameters was found for a PA in the visible range. This results from a compromise between the crystallinity and the nitrogen content of these transparent TiO 2 films grown on glass substrate. The optimal N content was found below 1 at.% for well crystallized anatase prepared at 375-400 °C. Other works have reported effective doping level of substitutional nitrogen between 1 and 2% [21,22]. Higher N concen-trations result in a high level of structural defects that behave as charge (electron, holes) recombination centers.

Fig. 1 .

 1 Fig. 1. Arrhenius plot of the growth rate of N-doped and pure TiO 2 films grown on Si(100) wafers using TTIP mole fractions of 65 × 10 -6 (N-TiO 2 ) and 260 × 10 -6 (TiO 2 ).

Fig. 2 .

 2 Fig. 2. Influence of the growth temperature on the mean crystal size of N-doped TiO 2 films determined from the FWHM of the (101) and (200) anatase XRD peaks (R = 2200).

Fig. 3 .

 3 Fig. 3. Variation of the surface grain size of N-doped TiO 2 films with the thickness for various growth temperatures using R = 2200.

Fig. 4 .Fig. 5 .

 45 Fig. 4. Variation of texture coefficient in the (211) and (220) direction with the growth temperature for N-doped TiO 2 films grown with R = 2200.

Fig. 6 .

 6 Fig. 6. Variation of the nitrogen content of N-doped TiO 2 films with the growth temperature for R = 2200. Left-axis: N atomic percentage as determined by XPS analysis (□). Right-axis: SIMS N/Si intensity ratio (▲) determined after Ar + sputtering to eliminate the surface contamination (film thickness ~200 nm).

Fig. 7 .

 7 Fig. 7. Optical absorption spectra of N-doped TiO 2 films prepared at 350 °C using various R ratios (film thickness = 300-350 nm).

Fig. 8 .

 8 Fig. 8. Variation of the optical band gap with growth temperature for N-doped TiO 2 films grown with R = 2200 (the dotted line is for bulk anatase band gap).

Fig. 9 .

 9 Fig. 9. Variation of the normalized initial photocatalytic decomposition rate under UV light radiation with growth temperature for N-doped (R = 2200) and pure TiO 2 films.

Fig. 10 .

 10 Fig. 10. Variation of the normalized initial photocatalytic decomposition rate under visible light radiation with growth temperature for N-doped (R = 2200) and pure TiO 2 films.

Table 1

 1 Typical MOCVD conditions used for the growth of N-doped TiO 2 layers.

	Growth temperature (°C)	300-425
	Pressure (Torr)	20
	Carrier gas flow rate (N 2 , sccm)	20
	NH 3 gas flow rate (sccm)	25-100
	NH 3 /TTIP	555-2220
	Total gas flow rate (sccm)	600
	Bubbler temperature TTIP (°C)	25
	TTIP molar fraction (10 -6 )	6 5
	Deposition time (min)	180-360
	Substrate	Borosilicate glass, Si(100)


Gao-Ren Li
email: ligaoren@mail.sysu.edu.cn

Dun-Lin Qu

Laurent Arurault

Ye-Xiang Tong

Hierarchically Porous Gd 3+ -Doped CeO 2 Nanostructures for the Remarkable Enhancement of Optical and Magnetic Properties

Rare earth ion-doped CeO 2 has attracted more and more attention because of its special electrical, optical, magnetic, or catalytic properties. In this paper, a facile electrochemical deposition route was reported for the direct growth of the porous Gd-doped CeO 2 . The formation process of Gd-doped CeO 2 composites was investigated. The obtained deposits were characterized by SEM, EDS, XRD, and XPS. The porous Gd 3+doped CeO 2 (10 at% Gd) displays a typical type I adsorption isotherm and yields a large specific surface area of 135 m 2 /g. As Gd 3+ ions were doped into CeO 2 lattice, the absorption spectrum of Gd 3+ -doped CeO 2 nanocrystals exhibited a red shift compared with porous CeO 2 nanocrystals and bulk CeO 2 , and the luminescence of Gd 3+ -doped CeO 2 deposits was remarkably enhanced due to the presence of more oxygen vacancies. In addition, the strong magnetic properties of Gd-doped CeO 2 (10 at% Gd) were observed, which may be caused by Gd 3+ ions or more oxygen defects in deposits. In addition, the catalytic activity of porous Gd-doped CeO 2 toward CO oxidation was studied.

Introduction

At present, the synthesis of ceria (CeO 2 ) nanostructures has aroused much interest because of a wide range of applications such as oxygen gas sensors, solid electrolytes in solid oxide fuel cells, phosphor/luminescence, catalysts for three-way automobile exhaust systems, and ultraviolet absorbers. 1 For some practical applications, the dopants, such as transition and nontransition metal ions, may be introduced into CeO 2 lattices for enhancing its optical properties or increasing its temperature stability and ability to store and release oxygen. 2 Recently attention has been focused on the preparation of rare earth iondoped CeO 2 because of its special electrical, optical, magnetic or catalytic properties. 3 For instance, the Eu 3+ (Sm 3+ ,Tb 3+ )doped CeO 2 nanocrystals showed remarkably enhanced photoluminescence (PL) intensity with respect to the CeO 2 nanocrystals due to the increased concentration of oxygen vacancies in the CeO 2 nanocrystals. 4 Gd-doped CeO 2 has shown potential applications in the next generation of compact solid oxide fuel cells for their enhancement in oxygen-exchange processes and associated catalytic reactions. 5 It is well-known that the performance of ceria-based materials is largely due to the ease in generating Ce 3+ from Ce 4+ . 6 In addition, the innate properties of ceria-based materials may be further amplified by producing ceria in nanostructured forms. 7 Therefore, the synthesis of various nanostructures of ceria-based materials and their physical and chemical properties have been widely reported. [START_REF] Ahniyaz | Bergstro ¨m[END_REF] In the past decade, there has been the tendency toward the development of novel, well-structured, and porous nanostructures, which have the ability to enhance the material properties by increasing the surface-area or providing channels.

For example, in the field of solid oxide fuel cells, a porous electrode structure is favorable for a rapid flux of gases through the pores, fast transport of ionic and electronic carriers through the solid phase, and efficient electrochemical reactions at the interfaces. However, at present there are relatively few reports on the preparation and characterization of porous rare earth iondoped CeO 2 nanostructures.

Various methods, such as hydrothermal synthesis, spray pyrolysis, surfactant-templated synthesis, coprecipitation, and combustion synthesis, have been put forward for the synthesis of diverse CeO 2 -based nanomaterials. 9 However, the above methods often required relatively high reaction temperatures (100s1000 °C), special equipment, or multiple reaction steps. In addition, the prepared materials with high specific surface are prone to agglomeration and exhibit broad pore size distributions, including, in some cases, small 1D pores. 10 The process of sintering at high temperature will lead to the loss of surface area and oxygen storage capacity of CeO 2 -based materials. Therefore, the preparation of CeO 2 -based materials with high specific surface area by the sintering process is still not wellknown technology. 11 Here we investigated an electrochemical deposition route for the preparation of hierarchically porous Gddoped CeO 2 at room temperature, providing a facile and lowcost route for the synthesis of high-quality porous Gd-doped CeO 2 foam nanostructures in high yield. These as-prepared hierarchically porous Gd-doped CeO 2 nanostructures have shown a remarkable enhancement of optical and magnetic properties. In addition, we also have studied catalytic properties of Gd-doped CeO 2 .

Experimental Section

The electrochemical deposition was carried out in a solution of 0.01 M Ce(NO 3 ) 3 -0.01 M Gd(NO 3 ) 3 -0.1 M NH 4 NO 3 . In this experiment a simple three-electrode cell was used in our experiments. A highly pure Pt foil (99.99 wt %, 0.25 cm 2 ) was used as the auxiliary electrode. A saturated calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system. All the electrochemical deposition experiments were carried out in a configured glass cell at room temperature, in which a Cu plate (99.99 wt %, 0.5 cm 2 ) served as the substrate. Before electrodeposition, Cu substrate was cleaned ultrasonically in 0.1 M HCl, distilled water, and acetone and then rinsed in distilled water again. The structures of products were characterized by X-ray diffractometry (D/MAX 2200 VPC with Cu KR radiation). An Oxford Instrument's INCA energy-dispersive spectrometer (EDS) was employed to analyze chemical composition. Microstructures of the deposits were characterized by field emission scanning electron microscopy (FE-SEM; JSM-6330F). The X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to assess the chemical state and surface composition of the deposits.

The UV-vis spectra of the samples were recorded on a UV-vis-NIR spectrophotometer (UV-3150). The photoluminescence (PL) spectra were carried out by spectrofluorophotometer (RF-5301PC) at room temperature. The magnetic property measurement system (MPMS XL-7) was used to investigate the magnetic behaviors of Ce 0.9 Gd 0.1 O 2-δ composites. The samples were also characterized by Brunauer, Emmett, and Teller (BET) nitrogen sorption surface area measurements (Micromeritics ASAP 2010). Specific surface areas of the prepareddepositswerecalculatedbytheBrunauer-Emmett-Teller (BET) method, and pore sizes were calculated by using the Barrett, Joyner, and Halenda (BJH) method (for large pores) or density functional theory (DFT) method (for small pores) on the basis of the adsorption branch of nitrogen sorption isotherms. Catalytic tests were carried out in a conventional fixed-bed quartz microreactor (8 mm in outer diameter) at atmospheric pressure. The system was first purged with high-purity N 2 gas (40 mL min -1 ) and then a gas mixture of CO/O 2 /N 2 (1:20:79) was introduced into the reactor, which contained 40-mg samples at a flow rate of 40 mL min -1 . Gas samples were analyzed by an online gas chromatography, using a Porapak Q column for the separation of CO 2 and CO and a 5A Molecular Sieve column for the separation of N 2 and O 2 . The catalytic performance was evaluated at various temperatures.

Results and Discussion

Figure 1 shows the SEM images of the CeO 2 deposits prepared in a solution of 0.02 M Ce(NO 3 ) 3 -0.1 M NH 4 NO 3 with current density of 1.0 mA/cm 2 . The porous CeO 2 foam structures were successfully prepared. The thickness of walls is about 50 nm as shown in Figure 1b. The sizes of pores are between 3 and 8 μm. The porous structures of the deposits can be manipulated by adding Gd(NO 3 ) 3 into deposition solution. When the electrodeposition was carried out in a solution of 0.01 M Ce(NO 3 ) 3 -0.01 M Gd(NO 3 ) 3 -0.10 M NH 4 NO 3 with a current density of 1.0 mA/cm 2 , the porous Gd-doped CeO 2 foam structures were prepared, and the SEM images were shown in Figure 2. It can be easily found that the foam structures have a higher density of pores, and the sizes of pores are smaller, which are about 200-500 nm.

EDS measurement was carried out at a number of locations throughout the specimens, and a representative EDS pattern of porous Gd-doped CeO 2 foam structure was shown in Figure 3. An oxygen peak at about 0.5 keV and Ce signals at about 0.9, 4.9, and 6.0 keV were observed, respectively. The Gd peaks at 0.9, 6.0, and 8.1 keV were observed. The EDS results therefore demonstrated that O, Ce, and Gd were electrodeposited. The composition analysis showed an approximate atom ratio of Ce to Gd was about 9:1 in the Gd-doped CeO 2 composites. For the element Ce, there are two different oxidation states Ce 3+ and Ce 4+ . The Ce 4+ oxidation state is more stable than Ce 3+ in the presence of air. It is well-known that there coexists a small amount of Ce 3+ at the surface of CeO 2 . 12 The results of XPS analyses of Gd-doped CeO 2 deposits (10 atom % Gd) were shown in Figure 4. The presence of Ce 3+ ions in the CeO 2 nanocrystals has been confirmed by XPS analysis, with Ce 3+ peaks found at 883 and 901 eV in Figure 4a(1). The binding energies of Ce 3+ ions in Gd-doped CeO 2 deposits are red-shifted compared to bulk CeO 2 . 13 The XPS peaks centered at 1188 and 1219 eV in Figure 4b can be attributed to the Gd 3+ contribution. In addition, the XPS peak centered at 529.3 eV in Figure 4c can be attributed to the O 2-contribution. 14 Therefore, the XPS results demonstrated the obtained deposits were Gd 3+ -doped CeO 2 with a small quantity of Ce 3+ oxidation state. The XPS spectrum of porous Gd-doped CeO 2 deposits (12 atom % Gd) was shown in Figure 4a(2). Although the peaks of Ce 4+ in Figure 4a(2) are almost the same as those in Figure 4a(1), the peaks of Ce 3+ in Figure 4a(2) are obviously lower than those in Figure 4a(1), indicating the variation of Ce(III)/Ce(IV) ratios.

The formation process of Gd-doped CeO 2 composites (10 atom % Gd) was investigated as follows. During the electrodeposition, OH -ions were formed via the electroreduction of NO 3 -ions in deposition solution on the surface of the cathode. These produced OH -ions will result in the formation of CeO 2 and Gd 2 O 3 , and the formation processes of CeO 2 and Gd 2 O 3 can be expressed as eqs 1, 2, and 3. It is well-known that the electrochemical deposition allows mixing of the chemicals at the atomic level. Therefore, when the electrodeposition was carried out, the mixed CeO 2 and Gd 2 O 3 at the atomic level were always obtained, and accordingly the homogeneous Gd-doped CeO 2 composites were prepared (eq 4).

NO 3

-+ H 2 O + 2e f NO 2 -+ 2OH - (1) 2Gd 3+ + 6OH -f Gd 2 O 3 + 3H 2 O (2) 4Ce 3+ + 12OH -+ O 2 f 4CeO 2 + 6H 2 O (3) Gd 2 O 3 + 18CeO 2 f 20Ce 0.9 Gd 0.1 O 2-δ (4)
Further characterization by X-ray powder diffraction was carried out, and the XRD patterns of CeO 2 (a) and Gd 3+ -doped CeO 2 (10 atom % Gd) (b) porous structures were shown in Figure 5. The results of XRD indicate that the CeO 2 nanocrystals are well-crystallized. The peaks corresponding to the CeO 2 (111), ( 200), (220), and (311) planes were observed, and all these diffraction peaks of CeO 2 can be indexed as a facecentered cubic-phase as identified by using the standard data JCPDS 34-0394. Here, we should notice that the XRD peaks of CeO 2 in the 2θ range from 25°to 70°exhibit the broadened peaks with a little shift toward smaller angles. According to the Scherrer equation (i.e., D ) Kλ/(β cosθ) (where λ is the wavelength of the X-ray radiation, K is a constant taken as 0.89, θ is the diffraction angle, and β is the full width at halfmaximum), the strongest peak (111) at 2θ ) 28.8°and the peak (220) at 2θ ) 47.4°were used to calculate the average crystallite size of CeO 2 nanocrystals, determined to be around 5.0 nm. The cell parameter (a) is calculated as about 0.5441 nm, a little larger than that of bulk CeO 2 (0.5411 nm). The larger cell parameter may be attributed to the lattice expansion effect resulting from the increased Ce 3+ ions and oxygen vacancies. 15 For Gd 3+ -doped CeO 2 porous structures, the XRD pattern in Figure 5b was almost the same as that of CeO 2 porous structures, and all diffraction peaks can be indexed to the face-centered cubic-phase of CeO 2 . The average crystal size of Gd 3+ -doped CeO 2 nanocrystals was estimated as about 4.5 nm by the Scherrer equation. However, the diffraction peaks of Gd or Gd 2 O 3 were not observed in the XRD pattern, indicating Gd 3+ ions entering into CeO 2 lattices. The cell parameter values were calculated as about 0.5429 nm, which is a little smaller than that of porous CeO 2 . This may be due to the lattice constriction effect resulting from Gd 3+ ions. This can be attributed to the replacement of Ce 3+ by trivalent Gd 3+ ions. As we all know, the ionic radius of Gd 3+ (107.8 pm) is smaller than those of Ce 3+ (115 pm). Therefore, the lattice constricts upon Gd 3+ doping will happen.

The porous CeO 2 and Gd 3+ -doped CeO 2 (10 atom % Gd) have been studied by the Raman scattering method as shown in Figure 6. For the porous CeO 2 , one high and sharp peak at 461 cm -1 is clearly observed in Figure 6a, which can be assigned to F2g symmetry because of a symmetric breathing mode of the oxygen atoms around the cerium ions. 16,17 The Raman shift is about 3 cm -1 compared with the bulk CeO 2 (464 cm -1 ). It is well-known that the change of the Raman peak′s position can be attributed to the changes of bond length or lattice spacing. For porous CeO 2 , the shift of the Raman peak is related to the enlarged lattice space because of the smaller size of CeO 2 crystals (5.0 nm). [18][19][20] In addition, the Raman shift also can be calculated by the following equation: 16Δω ) -3γω 0 Δa/a 0 (5

)
where γ is the Gru ¨neisen constant, ω 0 is the Raman frequency of bulk CeO 2 , Δa is the change in lattice constant, and a 0 is the lattice constant of bulk CeO 2 (0.5411 nm). The Raman frequency shift Δω can be calculated as about -3.32 cm -1 , which is very close to the above experimental value. However, for the porous Gd 3+ -doped CeO 2 , the first-order peak was shifted to 456 cm -1 , and this peak became broad compared with that of porous CeO 2 . For porous Gd 3+ -doped CeO 2 , the shift of the Raman peak is also related to the change in lattice spacing caused by the constriction of the lattice upon Gd-doping and to the change in the atomic geometry arising from Gd-doping. These lead to a larger shift compared with the porous CeO 2 . In addition, one weak peak at about 602 cm -1 was observed, which could be associated with the presence of the Gd element. The porosity of the synthesized porous CeO 2 and Gd 3+ -doped CeO 2 (10 atom % Gd) were characterized by nitrogen sorption analysis, using standard Brunauer-Emmett-Teller (BET) techniques. The nitrogen adsorption and desorption isotherms and the resulting pore size distributions of porous CeO 2 and Gd 3+ -doped CeO 2 are shown in Figures 7 and8, respectively. The porous CeO 2 and Gd 3+ -doped CeO 2 both display a typical type I adsorption isotherm and yield specific surface areas of 93 and 135 m 2 /g, respectively. Thus, the specific surface area of porous Gd 3+ -doped CeO 2 was larger than that of porous CeO 2 . The pore diameters of porous Gd 3+ -doped CeO 2 and CeO 2 are For the porous CeO 2 , there is a strong absorption band below 400 nm in the spectrum as shown in Figure 9, which is due to the charge-transfer transition from O 2-(2p) to Ce 4+ (4f) orbitals in CeO 2 . 21 However, the absorption spectrum of Gd 3+ -doped CeO 2 (10 atom % Gd) nanocrystals exhibits a red shift compared with porous CeO 2 nanocrystals and bulk CeO 2 . For CeO 2 nanocrystals, it is well-known that there coexists a small amount of Ce 3+ at the surface of CeO 2 and the fraction of Ce 3+ ions increases with decreasing particle size. 22,23 The presence of Ce 3+ ions in the CeO 2 nanocrystals can be confirmed by XPS analysis as shown in Figure 4a. The valence change from Ce 4+ to Ce 3+ on the surface of nanocrystals might have some additional contributions to the absorption of CeO 2 nanocrystals because the change from +4 to +3 increases the charge-transfer gap between O 2p and Ce 4f bands, which leads to the blue-shift of absorption spectrum of CeO 2 nanocrystals compared with bulk CeO 2 . 24 In addition, the average size of the obtained Gd 3+ -doped CeO 2 nanocrystals is about 5.0 nm, which is smaller than the estimated exciton Bohr radius for CeO 2 (7-8 nm). 25 Therefore, a quantum confinement effect possibly takes place, which contributes to the blue-shift of the absorption edge of the porous CeO 2 . When Gd 3+ was doped into CeO 2 , the peaks of Ce 4+ 3d 3/2 and Ce 4+ 3d 5/2 become stronger than that of the undoped CeO 2 nanocrystals as shown in the XPS spectrum in the inset in Figure 4a, while those of Ce 3+ 3d 3/2 and Ce 3+ 3d 5/2 are relatively weak, indicating the decrease of the Ce 3+ fraction on the surface of CeO 2 . This may be attributed to the replacement of Ce 3+ by trivalent Gd 3+ ions. Accordingly, the contribution of blue-shifting arising from the Ce 4+ to Ce 3+ valence change will become small. Therefore, the red-shifting occurs in the absorption spectrum of Gd 3+ -doped CeO 2 (10 atom % Gd) nanocrystals compared with that of CeO 2 nanocrystals. According to eq 6, the optical band gaps (E g ) of porous CeO 2 and Gd 3+doped CeO 2 can be estimated by using the data for the absorption spectra:

ahυ ) C(hυ -E g ) n ( 6
)
Here hυ is photon energy, R is the absorption coefficient, C is the constant, n ) 2 for an indirectly allowed transition, and n ) 1 / 2 for a directly allowed transition. 26 The direct band gap values of porous CeO 2 and Gd 3+ -doped CeO 2 nanocrystals can be determined as about 3.10 and 2.48 eV, respectively. The photoluminescent (PL) properties of porous CeO 2 and Gd 3+ -doped CeO 2 (10 atom % Gd) were also investigated. Figure 10 shows the excitation (a) and emission (b) spectra of porous Gd 3+ -doped CeO 2 (red lines) and CeO 2 (black lines). The band emission of CeO 2 can be remarkably enhanced by doping Gd 3+ into CeO 2 lattices. According to the above results, the emissions of CeO 2 and Gd 3+ were not detected in the Gd 3+doped CeO 2 , and the luminescence spectra shown in Figure 10 are from oxygen vacancies. 4 The extrinsic oxygen vacancies are generated in the fluorite lattice of CeO 2 nanocrystals because of the existence of Ce 3+ . 4 When Gd 3+ ions are doped into CeO 2 nanocrystals, the fraction of trivalent Ce 3+ ions will be decreased because of the replacement of Ce 3+ by Gd 3+ ions; however, the oxygen vacancy concentration in CeO 2 :Gd 3+ nanocrystals must be much higher because the doping concentration of Gd 3+ is much larger than the fraction of Ce 3+ in undoped nanocrystals. 4 Therefore, when Gd 3+ ions were doped into CeO 2 lattice, the enhancement of luminescence of Gd 3+ -doped CeO 2 deposits is attributed to the increase of oxygen vacancies.

Figure 11 shows the magnetization hysteresis loops of porous CeO 2 and Gd 3+ -doped CeO 2 (10 atom % Gd) at 5 K. As we all know, the stoichiometric CeO 2 should be nonmagnetic. However, the obvious magnetic property was observed for the prepared porous CeO 2 and Gd-doped CeO 2 . The coercivity field (H c ) and the remanent magnetization (M r ) of Gd-doped CeO 2 were larger than those of porous CeO 2 . For porous Gd 3+ -doped CeO 2 , the H c is about 6838 Oe, and M r is about 1.02 × 10 -3 emu at 5 K. What leads to the stronger magnetic property of the porous Gd 3+ -doped CeO 2 ? The mechanism of the origin of magnetism in doped CeO 2 is still under investigation. It has been proved that the oxygen defects and Ce 3+ ions existed in porous CeO 2 deposits by XRD and XPS, and the Ce 3+ ions in deposits can lead to the magnetic properties of CeO 2 . 13b,27 Therefore, the stronger magnetic properties of Gd-doped CeO 2 may be caused by the higher concentration of Gd 3+ ions in deposits.

CeO 2 is an important three-way catalyst, and we studied the catalytic activity of porous Gd-doped CeO 2 (10 at% Gd), porous CeO 2 , and CeO 2 nanoparticles. Catalytic activities of CeO 2 samples as a function of reaction temperature are shown in Figure 12a. The CO conversion increases with increasing reaction temperature for all samples. It can be clearly observed that porous Gd-doped CeO 2 shows much higher activity than porous CeO 2 and CeO 2 nanoparticles. For porous Gd-doped CeO 2 , a 47% CO conversion is achieved at 180 °C, while only 23% and 8% CO conversions are obtained at the same temperature for porous CeO 2 and CeO 2 nanoparticles, respectively. For the same CO conversion (20%), the reaction temperature for porous Gd-doped CeO 2 can be 30 and 76 °C lower than those for porous CeO 2 and CeO 2 nanoparticles, respectively. The large difference in catalytic activity can be partly due to the variation in surface area. It is well-known that the catalytic process is mainly related to the adsorption and desorption of gas molecules on the surface of catalyst. 28 Therefore, the porous structures with high surface area enable better contact with gas molecules, and accordingly show better catalytic performance compared with CeO 2 nanoparticles. In addition to the surface area, the oxygen vacancies in CeO 2 are also an important factor for better performance as they can act as promoting sites for CO conversion. 1d On the basis of the above results, the oxygen vacancy concentration in porous Gddoped CeO 2 is much larger than those of porous CeO 2 and CeO 2 nanoparticles, which further promotes the catalytic performance of porous Gd-doped CeO 2 . Figure 12b shows CO conversion as a function of doping concentration for the as-deposited porous Gd-doped CeO 2 at 180 °C, indicating oxidation ability increases with doping concentration.

Conclusions

In summary, a facile electrochemical deposition route was developed for the direct growth of porous Gd-doped CeO 2 . The porous Gd 3+ -doped CeO 2 (10 atom % Gd) displays a typical type I adsorption isotherm and yields a specific surface area of 135 m 2 /g. The pore diameters of porous Gd 3+ -doped CeO 2 (10 atom % Gd) are distributed with peaks centered at around 4 nm. The XPS results demonstrated the obtained Gd 3+ -doped CeO 2 deposits have a small quantity of Ce 3+ oxidation state. The optical band gap values of porous Gd 3+ -doped CeO 2 (10 atom % Gd) nanocrystals can be determined as 2.48 eV, which is much smaller than 3.10 eV of porous CeO 2 . In addition, when Gd 3+ ions were doped into CeO 2 lattice, the luminescence of Gd 3+ -doped CeO 2 deposits was remarkably enhanced, which can be attributed to the rapid oxygen vacancy increase. The magnetic properties of porous CeO 2 may be caused by the Ce 3+ ions in deposits, and the stronger magnetic properties of Gddoped CeO 2 were caused by more RE 3+ ions in deposits. Furthermore, better catalytic actives of porous Gd 3+ -doped CeO 2 nanocrystals were obtained compared with those of porous CeO 2 and CeO 2 nanoparticles.

Figure 1 .

 1 Figure 1. SEM images of porous CeO2 prepared in a solution of 0.02 mol/L Ce(NO3)3 + 0.1 mol/L NH4NO3 with current density of 1.0 mA/cm 2 with different magnifications: (a) ×5 00; (b) ×30 000.

Figure 2 .

 2 Figure 2. SEM images of porous Gd-doped CeO2 prepared in a solution of 0.01 M Ce(NO3)3-0.01 M Gd(NO3)3-0.1 M NH4NO3 with current density of 1.0 mA/cm 2 : (a) ×15 000; (b) ×50 000.

Figure 3 .

 3 Figure 3. EDS pattern of the porous Gd-doped CeO2 composite foam structures.

Figure 4 .

 4 Figure 4. XPS spectra of Gd-doped CeO2: (a) Ce 3d in Ce0.9Gd0.1O2-δ (1); Ce 3d in Ce0.88Gd0.12O2-δ (2); Ce 3d of porous CeO2 (inset); (b) Gd 3d; and (c) O1s spectra in Ce 0.9Gd0.1O2-δ.

Figure 5 .

 5 Figure 5. XRD patterns of (a) Gd-doped CeO2 (10 atom % Gd) and (b) CeO 2 porous structures.

Figure 6 .

 6 Figure 6. Raman spectra of (a) Gd-doped CeO2 (10 atom % Gd) and (b) CeO 2 porous structures taken at room temperature.

Figure 7 .

 7 Figure 7. N2 adsorption-desorption isotherms of (a) Gd-doped CeO2 (10 atom % Gd) and (b) CeO 2 porous structures.

Figure 8 .

 8 Figure 8. DFT pore size distributions of (a) Gd-doped CeO2 (10 atom % Gd) and (b) CeO 2 porous structures.

Figure 9 .

 9 Figure 9. UV-vis absorption spectra of porous Gd-doped CeO2 (10 atom % Gd) (red line), porous CeO 2 (blue line), and bulk CeO2 (black line).

Figure 10 .

 10 Figure 10. Excitation (a) and emission (b) spectra of porous Gd-doped CeO2 (10 atom % Gd) (red line), porous CeO2 (black line), and bulk CeO 2 (blue line) in hexane solution.

Figure 11 .

 11 Figure 11. The magnetization hysteresis loops of porous Gd-doped CeO2 (10 atom % Gd) (red line), porous CeO2 (black line), and bulk CeO 2 (blue line) at 5 K.

Acknowledgment. This work was supported by the Natural Science Foundation of China (Grant No. 20603048 and 20873184), the Guangdong Province Science & Technology Development Program (2008B010600040), and the Foundation of Potentially Important Natural Science Research of Sun Yat-Sen University.