Shirley Carter-Thomas

Introduction

My choice of this topic is based on a conviction, following several years of teaching English to French students, that many of the difficulties encountered by language learners in constructing native-like discourse are linked to information structuring, the so-called third level referred to by the Praguian linguists (as opposed to the grammatical & semantic layers), and to issues related to the textual metafunction in systemic linguistics. Although French and English are typologically close (both languages have an obligatory subject and are generally considered to have an SVO structure1 ), considerable differences in word order remain and the specialised syntactic structures available for manipulating information structure are neither identical nor necessarily used in the same circumstances. I will illustrate this with various examples of it-and c'est-clefts The theoretical approach I adopt in this paper, whilst drawing on many considerations related to the textual metafunction in systemic linguistics, will also take into account some of the earlier theories of theme developed by the functional linguists of Prague [START_REF] Frantisek | Functional sentence perspective and the organisation of the text[END_REF][START_REF] Firbas | On defining the theme in functional sentence analysis[END_REF]). I will not however be adhering to the 'combining' theory still adopted by many of those who continue the the Praguian tradition. Separation of the systems of theme/rheme and Given/New would indeed seem to be essential to an understanding of how clefts and other specialised structures work. Some of the literature I have read in preparing this paper is extremely confusing from this viewpoint. The conflating approach often leads French grammar manuals for example, when speaking of clefts, to refer to a reversal of the usual thème/propos order (where propos is usually considered as the informative part of the clause providing information about the theme). Equally it would seem misleading, even with written text, to ignore the Given/New dimension and just talk about clefts simply as thematising structures [START_REF] Thompson | Introducing Functional Grammar[END_REF]. If the focalised constituent in a cleft is the Subject it would usually also be considered as theme in the canonical non-clefted structure, and only speaking about a thematising structure in this case would therefore seem to be insufficient. In my view an adequate account of specialised structures -structures that play an important role in this third informational layer of language organisation -must take into account the two separate strands. As Martin Davies has underlined the Given/New structure must be visible in writing) otherwise we would not understand what we read, (1994). A successful piece of writing is therefore one "which gives the reader the 2 necessary clue for this reconstruction" [START_REF] Banks | Decoding the information structure of journalistic clefts[END_REF]). I will, when speaking of the Given/New component, also refer to the work of linguists of a more cognitive persuasion such as Wallace [START_REF] Wallace | Cognitive constraints on information flow[END_REF] and in particular Knud Lambrecht (1994;2001), whose approach to the question of information structure is illuminating for studies of both written and oral discourse I have chosen to analyse in particular the functioning of clefts as they are one of the specialised syntactic structures (or marked word order patterns) that seem to be used with a very different frequency in French and English. Structures such as: C'est Claire qui aime le chocolat It's Claire that/who likes chocolate French makes far greater use of the 'c'est' cleft than English does of the it-cleft. [START_REF] Péry-Woodley | Textual designs: Signalling Coherence in First and Second Language Academic Writing[END_REF] in a study of students' essays in French and English found c'est-clefts for example to be used twice as frequently by the French students as the it-cleft in the English essays. A preliminary study I made of front page news stories in a British and French newspaper also revealed a similar trend. Why should this be the case? Why does French seem to need cleft structures more than English does? Which alternative information packaging strategies are used in English? As the status of clefts as a cross-linguistic class is in itself fairly controversial I have chosen to examine a fairly fixed range of data with a view to simplifying the comparison of isomorphic structures in the two languages. I will, for example, only examine the traditional it & c'est clefts as illustrated above and will not be considering pseudo-clefts, or any variants of the c'est-cleft such as the French ya or j'ai clefts. The latter in any case would seem to more of a feature of spontaneous speech ("J'ai mon pied qui me fait mal"). It and c'est-clefts though are quite frequent in both written and speech genres. As the English and French 'it' cleft systems examined appear to be syntactically identical (except for possible relative pronoun effacement in English), it thus seems to be a reasonable hypothesis that distributional differences will reflect the functional properties rather than the formal characteristics of the two languages.

Corpus and methodology

In order, however, to really compare the functioning of clefts as an information packaging strategy across the 2 languages, it is important to base the analysis on a corpus which is as close as possible not only in terms of genre and purpose but also in terms of propositional content. For this reason I decided to direct my research towards a corpus of translations. The corpus I assembled consists of a series of articles from Le Monde (a leading French quality newspaper) which were later translated into English for ' The Guardian Weekly' . The original articles and their translations were then basically identical, with regard to content, barring the odd passage which does not appear in the English version, presumably because it was judged inaccessible or uninteresting for the English reader. In a first stage I noted all the c'est qui/que cleft structures in the thirty original French articles (medium length articles of 400-700 words). This provided me with a total of twenty-six clefts. Eight of the articles examined contained no clefts but in the remaining twenty-two there were then one or two clefts per article. I then analysed the English translations of these articles to see how frequently the cleft structure was preserved in the translations. Of the twenty-six French clefts only twelve (roughly 45%) were also translated by a cleft construction in English. In the other cases non-cleft constructions were preferred.

22 articles contain at least 1 cleft 12 clefts in English 30 French articles 8 articles with no clefts 26 French cleft constructions 14 non-cleft constructions One could argue that in a corpus of translations like this the question of language interference may play a role -the translator could be influenced by the source text and in this case there might be more clefts in the English translation than in English original articles. However, as over 50% of the original French clefts were not rendered by an English cleft construction, it would seem that the translators were not unduly influenced by the original form in this respect1 . ..

General functions of clefts

The literature on clefts, and on English cleft structures in particular, is extensive. Many commentators seem to consider that questions of 'newness' (or freshness) and in particular contrastive newness are essential to an understanding of how they function. This was indeed the view taken by Halliday in the 1994 edition of IFG. Halliday explains that cleft sentences (or predicated theme) enable the receivers' attention to be explicitly directed towards the 'news' value of a particular information unit, with the tonic accent falling on a constituent after the copula. As a result this constituent will often tend to receive a contrastive emphasis -"in order to make it clear that this, and nothing else, is the news value". In writing, as he goes on to explain, where one does not have the possibility of voice accentuation, clefts are particularly useful, as they explicitly direct the reader to interpret or decode the information structure in a particular way. Halliday illustrates this with the following example:

John's father wanted him to give up the violin.

His teacher wanted him to continue.

In the 2 nd sentence here the reader would, without any other clues, normally interpret the final constituent 'continue' as having the principle news value. It was his teacher who persuaded him to continue Theme Rheme Theme Rheme Theme (& 'New') Rheme (& Given)

However, using the marked word order (cleft) structure, a tonic accent would now fall on 'teacher'. The reader would realise that the fact that John continued is Given and that not only the theme but also the principle news value is conveyed by the noun 'teacher'. It was the teacher and not the father or anyone else who persuaded him to continue. Many French linguists also highlight the contrastive and news value attached to clefts. However, due to the fact that in mainstream French linguistics theme and rheme are seen from an essentially Praguian perspective, the approach and terminology adopted are different. Experiential theme is not considered as a 'staging post or starting point (1993: 221). Although the descriptive terms may be different, the fact remains that the clefted constituent here is also considered as 'new' and receives a special emphasis. The distribution of Given and New in the cleft construction is not always, however, so straightforward as the examples quoted above; often the clefted constituent is an anaphoric and cannot by definition be considered as referentially new or fresh. Many researchers, often from the cognitive linguistic tradition, have proposed a scale of giveness and newness (for example, Prince 1978) with several sub-divisions, in order to determine the precise informational status of particular referents. What is at issue when discussing clefts is not so much the question of degrees of newness of particular referents as that of the new distribution of newsworthiness/focus brought about by the reorganisation of the clause. Using a cleft enables the enunciator to signal a particular slant or orientation, different from that of the non-clefted canonical clause. The clefted item receives additional prominence (although this does not mean it is necessarily the only item to receive this prominence) which in turn affects our interpretation of the rest of the clause. We can illustrate the above points with an example from the French newspaper corpusan example that is also translated with a cleft construction in the corresponding English article.

E1

And it was at that point that an explosion destroyed Russia's political and social system

The context is that of that the relationship between Jews and Russians. The point in time the author is referring to has been defined in the preceding sentences (the eve of the Bolshevik revolution) and is thus easily 'recoverable' for the reader. However by placing the time adverbial within the cleft, the item is automatically signalled as being particularly 'newsworthy'. It was at this moment in time and no other (ironically, considering the fact that anti-Semitism was not an issue) that the revolution occurred. As Halliday puts it: "The meaning is attend to this: this is news". Referential identifiability or recoverability is not necessarily what is at issue. For this reason I prefer to follow Peter Fries in referring to the concept of 'newsworthy' to describe the meaning of 'New' in information structure.

It is also important to point out that the real focus or newsworthiness in the above example is not just created by this item itself but by the relation it creates with the information conveyed by the relative clause, a point forcefully made by Knud Lambrecht on several occasions: The new information …. is strictly speaking not expressed in the focus denotatum itself but in the relation established between this denotatum and the rest of the clause (2001: 477). In other words the real news value or newsworthiness of the clefted constituent can only be evaluated in relation to the information presented in the rest of the clause. In the above example the newsworthiness of the temporal adverbial cleft is only really brought home when juxtaposed with the rest of the sentence. In the strict Hallidayan tradition, however, the identification of Newsworthiness is dependent on the prosodic structure of oral discourse. Transferring this approach wholesale to written discourse can therefore be somewhat problematic. It is true -as P. Fries (1994) has pointed out -that writers will generally speaking sequence their texts so that New or focal information occurs where the tonic accent would be in the spoken sentence. The researcher can of course asks subjects to read the texts aloud (cf. [START_REF] Banks | Decoding the information structure of journalistic clefts[END_REF]. However this approach seems rather artificial, and will always involve a degree of guesswork (due to both inter-and intra-speaker variability). For this reason the approach advocated by [START_REF] Knud | Information structure and sentence form[END_REF], 2001), which involves not only taking into account prosody but also including grammatical markers (such as the position and ordering of constituents, complex grammatical constructions and certain choices between related lexical items), seems more insightful. This approach is notably centred on the comparison of semantically equivalent but formally and pragmatically divergent alternatives. In the case of written clefts, it would therefore seem essential to study the various agnate and non-cleft alternatives. In a non-cleft canonical version of the above example, that is to say a clause without the copula, the initial adverbial expressions would normally be treated as Given backgrounded information. ?E1b And at that point an explosion destroyed Russia's political and social system While it is true that an attentive reader could perhaps attribute a contrastive focal reading to the initially placed adjunct in the manipulated English version, in the French version this thematic adverbial expression merely seems to refer to the background against which the revolution or explosion happened. Without the cleft the cause and effect relationship between the initially occurring temporal adjunct and the rest of the clause is not apparent. Et une déflagration mit en pièces le système politique et social de la Russie à ce moment-là ?E1c And an explosion destroyed Russia's political and social system at that point In the second manipulated example here, the adverbial expression occurs postverbally within the rheme and could conceivably be thought of as carrying contrastive focus, but the long composite nature of the rheme makes it difficult for the reader to establish a hierarchy out of the different elements that appear. The manipulated sentence is difficult to read and process in both the English and French versions. The original cleft construction would seem to be the only possibility for providing clear processing instructions to the reader and the distribution of newsworthiness in the sentence.

Results

French clefts also translated by an English cleft

As underlined above of the twenty-six French cleft constructions in my corpus only twelve were also translated with a cleft construction in the English version. Of these clefted elements, ninewere adjuncts or dependent clauses, and three were subjects or complements of their respective canonical sentences. I do not have the space in this article to discuss any more of these examples, numbered 1-12 in the appendix below, but it is perhaps significant that the great majority are adjuncts of some type (particularly temporal adverbials). Both French and English would seem to need a cleft in these cases. I would now like to concentrate on the examples of French clefts that were not translated by a cleft in English.

3.2 French c'est clefts not translated by an it English cleft 3.2.1 Rigidity of French focus structure Of the fourteen French clefts that were not translated with a cleft construction in English, a notable proportion (seven out of fourteen) would seem to be linked to the fact that English is far more flexible than French as regards focus structure and the distribution of newsworthiness in the clause (cf. [START_REF] Van Valin | A typology of the Interaction of Focus structure and Syntax[END_REF]. Whereas in English the preceding context is often sufficient to indicate the possibly newsworthy nature of an item appearing in preverbal position, this is not the case in French where grammatical subjects in theme position, in particular, cannot easily receive a newsworthy reading. According to Knud Lambrecht there is, in fact, a powerful constraint "against the co-mapping of the pragmatic relation focus and the grammatical relation subject", which means in turn that French often resorts to cleft constructions "to avoid focus-initial SV structures." (1994, p.22). Although Lambrecht's comment was made about spoken French, I would suggest that such considerations can also be a factor in written French and can go a good way towards explaining why there are more incidences of the c'est-cleft than the it-cleft in my corpus. This can be illustrated by the following example (F13) taken from an article dealing with the subject of arranged marriages -that is to say young immigrant girls who are victims of arranged marriages by their families.

E13

In Dr. Piet's view, almost all Turkish, Senegalese and a large number of North African girls are at risk

In example F13 the clefted item, the subject of the canonical clause, concerns the precise number of young girls who are likely to be victims of arranged marriages. (We notice in passing that the plural form ce sont instead of c'est is employed, although this agreement is tending to disappear, particularly in informal genres). The fact that there is a real risk has already been commented upon, but what is particularly newsworthy is the precise numbers affected: the 'quasi-totalité' of young Turkish, Senegalese girls and un grand nombre of North African girls. The cleft structure is necessary in order to direct the readers' attention towards the news value of these elements. Without the cleft structure, the reader would be inclined to attribute newsworthiness to the predicate sont menacées, in the usual rheme portion of the sentence (F13b). Cleft structures thus not only serve to signal the newsworthiness of a constituent that would normally be nonfocal but also to mark the nonfocal character of a predicate that might otherwise be construed as focal (Lambrecht, 2001, p.489) In the English translation (13E) of this part of the text, however, a cleft construction is not used. This is because it is unnecessary. E14 "There's been an increase in crimes against the community. That's worrying."

The article is dealing with the wave of anti-Semitic violence in France at the beginning of the year. The portion of the text under investigation concerns an interview with a spokesman for the Jewish community in Sarcelles, a Parisian suburb. In the original French article, the deictic pronoun is clefted. Without the cleft it would indeed be difficult to attribute focus to the element in theme position. Using a cleft thus enables the enunciator to cope with co-mapping constraint evoked earlier and to signal unambiguously the newsworthy nature of the deictic.

In the English translation, on the other hand, the cleft structure is unnecessary, as English accepts a focal element in preverbal position far more easily than French does. On reading the article the reader would automatically recognise the thematic 'That' as carrying a focal stress rather than the predicate 'worrying'. Note that in the English translation a reverse pseudo-cleft (RWH) or marked thematic equative in systemic parlance would also have been a pragmatically acceptable alternative: 'That's what's worrying' (further underlying the exclusiveness of the relation between the thematic and rhematic components). Such a structure, however, is impossible in French (as in Spanish) as once again a specifically focalised element cannot be used as a preverbal subject and clausal theme. This difference in the relative flexibility of the focus structure in the 2 languages can go a good way towards explaining the higher frequency of clefts in French than in English. Seven of the fourteen French clefts that were not translated with a cleft in English (N° 13-19) would seem to be directly linked to this question of preverbal thematic focus. Of these examples six would have been subjects of their respective canonical sentences in French, further reinforcing Lambrecht's point about the particular difficulty in French relative to the co-mapping of theme, grammatical subject and focus. English canonical rheme/focus ordering is sufficient On several other occasions in the corpus (N° 20-23), the item clefted in the c'est French cleft simply occupies the usual rhematic position towards the end of the clause in the English translation. Once again, however, the absence of the it-cleft does not seem to affect the accuracy or clarity of the translations: F20 En réaction à la funeste 'présidence à vie' instaurée au bénéfice d'un président Bourguiba devenu sénile, l'actuel chef de l'Etat avait eu l'intelligence de limiter à trois le nombre de mandats présidentiels. C'est ce verrou que le même président Ben Ali se prépare à faire sauter dans le cadre d'une réforme de la Constitution E20 The same Ben Ali is now preparing to scrap that restriction under constitutional reform.

In the English translation the element highlighted through the French cleft simply occurs in the canonical rheme position. The translation seems perfectly clear, we know exactly which restriction is being referred to and drawing further attention to it in a cleft would seem unnecessary.

?E20b It is that restriction that the same Ben Ali is now preparing to scrap under constitutional reform.

Likewise in examples 21-23, the information highlighted in the cleft of the French original falls in the usual rhematic position in the English translation. The translator could have used a cleft in these instances too. The fact that that he chose not to do so is presumably because he considered the same communicative effect to be achievable by using a structure with the highlighted element in its canonical position. Why should this be case? One hypothesis is simply that the English it-cleft structure is considered more 'marked' than the French c'est cleft, if we understand markedness here as related to the degree of the potential domain of application and hence the frequency of use. French, as we have seen, has more frequent recourse to c'est clefts, due in great part to its more rigid focus structure and consequently the structure can be considered less marked than in English. If the English translator had also clefted the element highlighted in the French originals, making them not only the focus of the newsworthiness but also the theme, they would perhaps have received more prominence than intended in the original French text.

Translation 'noise'

As often though with a linguistic analysis based on a corpus, some examples do not fit neatly into the pattern. There are a few examples of French clefts in my corpus translated with a non-cleft structure in English where the information structure does not appear to have been respected (N° 24-26). This may be partly due to a certain amount of interference or noise linked to the translations themselves. The journalistic translator, working to a tight deadline, is not a producing a lasting work of art but journalistic prose.

It is possible that the absence of certain it-clefts may simply be sins of omission, nuances of the original being elided or even unnoticed.

In the following extract, for example, the absence of the cleft in the English translation makes the sentence far less clear.

F25

Ironie de l'histoire, c'est le 11 septembre que les pays membres de l'OEA, dont les Etats-Unis, ont signé une nouvelle charte démocratique prévoyant des mécanismes régionaux de défense des régimes constitutionales.

E25

By a curious twist of history OAS member countries, including the US, signed a new Inter-American Democratic Charter on September 11 2001, which provided for regional mechanisms to defend constitutional governments.

In the original French text the adverbial expression, le 11 septembre is presented as both the thematic starting point and the focus of the clause. In the English translation, however, the nominal groups are presented as thematic and this adjunct simply occupies the usual unmarked rhematic and newsworthy slot at the end of the clause. Its positioning here, however, at a distance from 'a curious twist of history' is not only awkward but conceals the very irony that the writer was trying to convey.

E25b By a curious twist of history, it was on September 11 2001 that OAS member countries, including the US, signed a new Inter-American Democratic Charter, which provided for regional mechanisms to defend constitutional governments.

A translation using an it cleft would seem more appropriate:

Some tentative conclusions

As explained in the introduction one of my main motivations in embarking on this comparison of cleft structures in English and French was to try to pinpoint more closely their role as information packaging strategies in the two languages, from the didactic viewpoint. Many of my French students' difficulties in creating native-like discourse lie, I believe, on the informational level. Although having a sound grasp of English (and making few serious grammatical errors) they tend to reproduce certain French syntactic patterns, rather than using syntactic arrangements more suited to the communicative context in terms of theme and information structure in the target language. I recently reread a rather dense philosophical paper, written in English by a French university colleague. In his 20-page article there were no less than thirteen it-clefts, five of which no longer existed after a thorough reworking of the text. It is features like these I believe that often contribute to the rather awkward and non-native like feel of advanced English learners' discourse.

The fact that such a small corpus has been analysed discourages me from making any sweeping statements, but nevertheless a few general tendencies would seem to emerge. The it-cleft structure in English appears by no means as vital as the c'est cleft is in French. Although, French and English both have recourse to a cleft structure, particularly for the clefting of adverbial adjuncts, there are many other contexts in French where the c'est cleft is required, but where the it-cleft is optional and at times even best avoided.

The main reason for this is, as we have seen, that English focus structure is more flexible than that of French and can, given the right contextual clues, place a newsworthy reading on an element in theme position. In French, however, due to constraints against the comapping of subject and focus, a c'est cleft is vital in order to attribute a newsworthy reading to such initially occurring clausal themes1 . On other occasions in our corpus we have seen that the clefted item in the c'est French cleft simply occupies the canonical rheme position in a monoclausal version proposed by the English translator. The usual information packaging arrangements, with Given at clause outset and the climax of the newsworthiness occurring towards the right-hand side of the clause, are deemed adequate.

The use of the English it-cleft, unlike the c'est cleft, sometimes appears to be motivated more by stylistic reasons than pragmatic ones. It can sound rather formal and as other researchers have noted, is often used as a literary device, which may account for the fact that it would seem to occur with a greater frequency in certain types of 'sophisticated' writing and elevated speech rather than in spontaneous exchanges (Katz, Collins). There is no straightforward correspondence between the French c'est cleft and the English it cleft. In either language, if one employs the cleft in an inappropriate context or does not use it when required, the resulting statement, although comprehensible, will sound strange to a native speaker. This is something that learners should be made aware of. The informational level must be taken into account in language teaching. Specialised syntactic structures for example should be taught within this framework, and not simply as alternatives or clever grammatical manipulations. I also think there is a strong argument, particularly with advanced learners from the same linguistic background, for the (re)introduction of certain translation-based tasks.

Although the research on English clefts is extensive, full-length studies of French clefts seem rare [START_REF] Stacey | A functional approach to the teaching of the French c'est-cleft[END_REF] and very little research seems to have been done from a contrastive viewpoint. Contrastive research though is perhaps the only way to really understand when and why clefts and non-clefts are truly in free variation. E10 Indeed, it is thanks to the money they send home that the Moldavian economy is still functioning.

F11 Selon Eric Debarbieux, elle (la répétition) est au coeur de la logique délinquante. C'est par la réitération de ces petits faits, petites agressions ou micro-violence, que le jeune, seul ou en groupe, assoit sa domination sur les autres.

E11 He argues that is through the repetition of petty offences and acts of aggression that youngsters, either alone or in groups, achieve dominance over others E14 "There's been an increase in crimes against the community. That's worrying. But the attacks are not always anti-Semitic. I think the youngsters attack people they regard as the bourgeois of Sarcelles (…)"

F15 "Notre objectif est de dire que ces mariages concernent l'école car ils vont à l'encontre de ses valeurs d'épanouissement et de liberté", explique Alain Seksig, consiller technique de Jack Lang. C'est en effet auprès de l'infirmière ou de l'assistante sociale de leur lycée ou de leur collège que ces jeunes filles osent confier leur drame.

E15 "Our aim", said Alain Seksig, (…)"is to make schools feel concerned about these marriages(…)." The school sickroom nurse or the school social worker is often the person to whom the girls dare to talk about the ordeals they are subjected to.

F16 C'est elle qui fait remarquer la similitude de forme entre 'Huit femmes' et l'un de ses grands succès, 'Marie Octobre' de Julien Duvivier, autre huis clos à énigme.

E16 Darrieux herself points out the formal similarity between 8 femmes and one of her great successes, Julien Duvivier's Marie Octobre, another claustrophobic whodunnit.

F17 Incongrue avant le 11 septembre, la question de la raison d'être de l'Alliance atlantique fait désormais partie de la réflexion stratégique, aux Etats-Unis comme en Europe. (…/) C'est le 11 septembre qui a servi de révélateur à la crise d'identité de l'Alliance Atlantique, l'Amérique ayant fait comprendre à ses alliés que pour organiser la riposte elle jugeait plus efficace d'agir seul.

E17 The September 11 attacks revealed NATO's identity crisis, when America made it clear to their allies that they could organise a more effective response if they acted alone;

F18 (…) le tremblement de terre politique qu'est le premier tour de l'élection présidentielle a une seule cause: la dispersion, absurde, de la gauche. Celle-ci, qu'on la considère dans ses limites modérées ou en incluant l'extrême gauche, ne recule pas, d'un scrutin présidentiel à l'autre.

Ce sont ainsi 44% des Français qui ne seront pas représentés au second tour.

E18 (… ) the absurd degree to which the left was fragmented. Whether one considers only its more moderate elements or includes the far left, the left-wing vote has not in fact decreased from one presidential election to the next. This means that 44% of the French electorate will not be represented at the second round. F25 Ironie de l'histoire, c'est le 11 septembre que les pays membres de l'OEA, dont les Etats- Unis, ont signé une nouvelle charte démocratique prévoyant des mécanismes régionaux de défense des régimes constitutionnels.

E25 By a curious twist of history OAS member countries -including the US -signed a new Inter-Amercan Democratic Charter on September 11 2001, which provided for regional mecansims to defend consitutional governments.

F26 "J'avais des liens très forts avec lui (Aristide). Aujourd'hui, je me confesse en public car il faut reconnaître le mal pour l'extirper. Davantage que l'homme, c'est le vide de sa pensée qui me déçoit."

E26 "I used to be very close to Aristide. Now I publicly admit my mistake (…). I'm disappointed less with the man than with the emptiness of his thinking. "

F1 A la veille

  de la révolution bolchevique, cette vraie Russie et son intelligentsia libérale rejetaient l'antisémitisme ; elles ne voulaient même pas entendre parler d'une 'question juive. (…) Et c'est à ce moment-là qu'une déflagration mit en pièces le système politique et social de la Russie 14 th Euro-International Functional Systemic Workshop, Lisbon University, July 2002.

?

  F1b A la veille de la révolution bolchevique, cette vraie Russie et son intelligentsia libérale rejetaient l'antisémitisme ; elles ne voulaient même pas entendre parler d'une 'question juive. (…) Et à ce moment-là une déflagration mit en pièces le système politique et social de la Russie

?

  F1c A la veille de la révolution bolchevique, cette vraie Russie et son intelligentsia libérale rejetaient l'antisémitisme ; elles ne voulaient même pas entendre parler d'une 'question juive. (…)

F13

  La mesure du phénomène reste délicate. Il n'existe aucune statistique officielle…. Pour Emmanuelle Piet, ce sont la quasi-totalité des jeunes Turques, des Africaines du fleuve Sénégal et un grand nombre de jeunes Maghrébines qui sont ménacées. #F13b Pour Emmanuelle Piet, la quasi-totalité des jeunes Turques, des Africaines du fleuve Sénégal et un grand nombre de jeunes Maghrébines sont ménacées.

  The English reader would, I believe, naturally attribute focal status to the expressions detailing the population at risk. With a cleft structure the translator could have achieved the same result on the pragmatic level, but the additional presence of the it cleft would seem unnecessarily cumbersome.

	?E13b In Dr. Piet's view, it's almost all Turkish, Senegalese and a large number of
		North African girls who/that are at risk
	English would seem therefore much more flexible with regard to the distribution of focus or newsworthiness than French and would not seem to require the extra grammatical clue to indicate the location of the newsworthy parts of the clause.
	An even more striking example, illustrating this relative flexibility of English focus structure in relation to French, is the following:
	F14	"Il y a une recrudescence d'actes de délinquence qui visent la communauté. C'est
	CA qui est angoissant."
	?F14b "CELA est angoissant"

  E6 But it was in Milan that Christopher Raeburn, Decca's legendary recording producer, discovered her talent and quickly got her to sign a contract. It's an episode from the Apocalypse that we're experiencing here in Moldova.F10 C'est d'ailleurs avec l'argent qu'ils envoient chez eux que le pays tourne encore.

	F7	C'est souvent lorsqu'ils découvrent leur fille avec une cigarette, ou la soupçonnent d'avoir un petit copain, qu'ils se décident à la marier.
	E7 It is often when the parents discover their daughter with a cigarette, or suspect she has a boyfriend, that they decide to marry her off.
	F8	C'est principalement l'impossibilité de rembourser qui avait poussé à l'exil beaucoup de paysans.
	E8 It was chiefly because they could not pay back what they owed that many farmers left Afghanistan.
	F9	C'est une page de l'apocalypse que nous vivons ici.

E9

  Il y a une recrudescence d'actes de délinquance qui visent la communauté. C'est ça qui est angoissant. Mais ce ne sont pas toujours des actes antisémites. J'ai l'impression que ces jeunes agressent ceux qu'ils voient comme les bourgeois de Sarcelles(…)"

	F14 "
	F12 La consommation de boeuf fut longtemps faible dans l'archipel. (…) C'est au lendemain de l'ouverture du pays ( milieu du XIXe siècle) que les Japonais ont commencé à consommer du boeuf sous l'influence des étrangers résidants les ports de Kobe et Yokohama.
	E12 For a long time, very little beef was eaten in Japan. (…) It was after the opening up of the country in the mid-nineteenth century that the Japanese, influenced by foreigners living in the ports of Kobe and Yokohama, began to eat meat.
	French c'est clefts translated by a non-cleft structure in English
	F13 La mesure du phénomène reste délicate. Il n'existe aucune statistique officielle(…). Pour Emmanuelle Piet, ce sont la quasi-totalité des jeunes Turques, des africaines du fleuve Sénégal et un grand nombre de jeunes Maghrébines qui sont menacées.
	E13 In Dr. Piet's view, almost all Turkish, Senegalese and a large number of North African girls are at risk
	14 th Euro-International Functional Systemic Workshop, Lisbon University, July 2002.	14

  F19 Le maire (PS), Roger Madec ne nie pas les faits: " Il ne faut pas faire d'angélisme. Nous avons dans l'arrondissement une bande de jeunes d'origine musulmane qui commettent des actes d'incivilité contre les juifs. Pour eux, c'est la communauté juive qui est opulente et qui a tout. C'est faux, et il faut le dire. " E19 The mayor, Roger Madec says: "It's no good pretending this doesn't go on. We have a gang of young Muslims who commit antisocial acts against Jews. They regard the Jewish community as rich. That's not true, and we must say so. " Clefted French item occurs in canonical rheme position in English F20 En réaction à la funeste 'présidence à vie' instaurée au bénéfice d'un président Bourguiba devenu sénile, l'actuel chef de l'Etat avait eu l'intelligence de limiter à trois le nombre de mandats présidentiels. C'est ce verrou que le même président Ben Ali se prépare à faire sauter dans le cadre d'une réforme de la Constitution E20 The same Ben Ali is now preparing to scrap that restriction under constitutional reform. F21 (…) José Mantero a tout du charme massif d'un Russell Crowe, l'acteur australien rendu célèbre par son maniement du glaive dans Gladiator. Pourtant c'est dans un combat autrement plus dangereux que s'est lancé ce prêtre catholique de 39 ans, en choisissant de descendre dans l'arène médiatique, à travers la publication de ses confessions de prêtre gay. E21 (…) José Mantero seems to have all the hunky charm of Russell Crowe, the Australian actor famous for his swordplay in Gladiator. But Mantero, 39 Spanish priest, embarked on a much more perilous combat when he decided to enter the media arena and confess to (…). F22 Après cinquante sept mois d'une cohabitation.Lionel Jospin devait immédiatement démarquer … Pour cela, il a soigneusement choisi le moment, la manière et le message. (…/) La manière s'ensuit. C'est par une 'courte lettre aux Françaises et aux français', transmises à l'Agence France presse, que Lionel Jospin a fait connaître sa décision.E22 As for the medium, Jospin broke the news with a 'brief letter to Frenchwomen and Frenchmen' which he sent to the news agency, Agence France Presse.F23 Cette notoriété aiguillonne les chercheurs. Dans le monde anglo-saxon, d'abord. C'est aux Etats-Unis que la redécouverte de Le Gray a eu lieu.E23 His rise to fame has stimulated a great deal of research, initially in English-speaking countries. Le Gray was first rediscovered in the United States.Le ton monte entre les deux rives de l'atlantique…. 14 th Euro-International Functional Systemic Workshop, Lisbon University, July 2002. 16 C'est le discours de Président Bush sur l'état de l'Union et la dénonciation de l'axe du mal qui a déclenché des salves d'épithètes peu amènes. Hubert Védrine, le ministre français a tiré le premier E24 Tempers have flared up on either side of the Atlantic. President Bush's state of the Union speech and his denunciation of 'an axis of evil' has triggered a volley of hostile reaction. The French minister, Hubert Védrine, was the first to unleash his barbs.

	Translation noise
	F24 Irritations transatlantiques

Knud Lambrecht (1987) notes that this SV(O) structure tends now to be used less in oral French: the preferred-clause construction being a verb initial structure, clitic + verb (X).

th Euro-International Functional Systemic Workshop, Lisbon University, July 2002.

It would be interesting to compare these results with a corpus of English/French translations to see if the phenomena observed are mirrored. One would logically expect to find a greater number of clefts in the French translations than in the original English articles

An argument placed in preclausal position can have a focal reading in French(Lambrecht, 1994, p31). For other examples of topicalisations (or absolute theme) with a focal reading, see[START_REF] Caffarel | Interpreting French theme as a bi-layered structure: Discourse implications[END_REF].

E1 Just before the Bolshevik revolution, the real Russia and its liberal intelligentsia rejected anti-semitism. They would not even hear of such a thing as the 'Jewish question. The tsarist regime, according to Solzhenitsyn, was on the brink of lifting all the civic, social and educational restrictions that still affected the Jewish population. «And it was at that point that an explosion destroyed Russia's political and social system.» F2 Parmi les sept collègues de Danièle Darrieux, il y avait Catherine Deneuve "C'est la quatrième fois que je suis sa mère à l'écran. D'abord dans l'un de ses premiers films " (…).

E2 One of Darrieux's seven colleagues was Catherine Deneuve: "It was the fourth time I had played her mother on the screen. I did so first in one of her earliest movies " (…).

F3

Ces dix dernières années, c'est elle qui insista, à juste titre, sur les dangers de la baisse constante de l'aide publique au développement (APD).

E3 Over the past ten years it has been France that has rightly drawn attention to the dangers of a steady fall in development aid.

F4 C'est en 1220 après l'hégire que le vaste djihad a était déclenché ici.

E4 It was in the year 1220 after the hegira that a great jihad was started.


Thomas Arnoult

Colin Leclercq

Cécile Ghouila-Houri

Aurélien Mazzamurro

Manon Benedito

Romain Viard

Quentin Gallas

Eric Garnier

Denis Sipp

Abdelkrim Talbi

P Pernod

Wind tunnel integration of Micro-Magneto-Mechanical Systems (MMMS) microvalves for flow control experiments

Keywords: Micro-Magneto-Mechanical Systems (MMMS), cavity, flow control, quasi-steady jets, pulsed jets

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

I. INTRODUCTION

The open cavity is a common geometry. The flow developing over such a geometry, illustrated in Fig. 1, has been extensively studied since Roshko [START_REF] Roshko | Some measurements in a rectangular cutout[END_REF] and complex dynamic phenomena develop in it.

Figure 1: Features of the flow developing over a cavity and indications of actuators and sensors locations

The shear layer destabilizes over the cavity and impacts the downstream corner. The acoustic waves generated from this location propagate upstream and excite the shear layer instabilities at the upstream corner. This results in selfsustained oscillations in the flow, which can induce structural vibrations and noise radiation [START_REF] Rossiter | Wind tunnel experiments on the flow over rectangular cavities at subsonic and transonic speeds[END_REF]. Therefore damping these oscillations is of interest.

II. MMMS MICROVALVES

We propose a Micro-Magneto-Mechanical Systems (MMMS) microvalve design [START_REF] Pernod | MEMS magneto-mechanical microvalves (MMMS) for aerodynamic active flow control[END_REF] in order to implement an open-loop flow control strategy on a cavity. The MMMS design, presented in Fig. 2, consists in a pressurized air inlet, a channel surmounted by a flexible resonator made in elastomer material (Silastic) and a submillimetric outlet. A NdFeB permanent magnet is fixed on the flexible resonator enabling the channel height modulation and hence modulating the outlet velocity, when an electrical current runs in the coil surrounding the magnet. The microvalve can deliver a high-speed jet with an outlet velocity up to 150 m/s (for a submillimetric outlet), corresponding to a mass flow of 4 L/min, pulsed at frequencies Wind tunnel integration of Micro-Magneto-Mechanical Systems (MMMS) microvalves for flow control experiments up to 500Hz [START_REF] Viard | Magnetostatic microvalve for high momentum rate pulsed jet generation[END_REF]. The actuators outlet design was adapted to the control of the cavity flow. Therefore, the submillimetric outlet was modified in order to obtain an outlet velocity spread over a slot, as depicted in Fig. 3. The slot exit fabricated with fast prototyping techniques such as stereolithography induce an outlet angle. Therefore, the outlet jet is located in a (XY) plane rotated by 45° around the Z axis. Performances of these micro-actuators were assessed with hot wire measurements once assembled as a linear array. A DANTEC 55P11 hot wire probe was used and measurements were done at different points over a duration of 1s with a sampling frequency of 20 kHz. A quasi-steady jet mapping is presented in Fig. 4 for a driving pressure of 150 mbar. It can be observed that the jet maximum velocity for this supply pressure is about 25 m/s and that the jet spreads over 3 mm for Z = 3 mm.

Figure 2 :

 2 Figure 2: Schematic description of a MMMS micro-valve.

Figure 3 :

 3 Figure 3: Assembled microvalve with its jet adapter. Packing dimensions are indicated with arrows

Figure 4 :

 4 Figure 4: Quasi-steady jet characterization in a (XY) planeCavity flow control physics require mass flow rates of the order of 1 L/min up to 2 L/min, which make MEMS technologies suitable for these applications.III. OPEN-CAVITY FLOW CONTROLFor the first time, a linear array of 15 MMMS microvalves has been integrated to the upstream wall of an open-cavity placed in a wind tunnel, as presented in Fig.5and Fig.6. For a chosen flow regime, effects of quasi-steady and pulsed jets on the cavity oscillations were assessed through unsteady pressure measurements. Depending on the microvalves performances and the flow regime, quasi-steady jets enabled a decrease of 20 dB in the cavity fundamental frequency sound pressure level. The cavity spectrum without control and with the effect of the quasi-steady jets for a driving pressure of 290 mbar is presented in Fig.7. Regarding pulsed jets, depending on the forcing induced by the actuators a locking phenomenon was observed. Therefore, the actuators authority proved to be

Figure 5 :

 5 Figure 5: Picture of the S19 wind tunnel with the cavity dimensions

supported by the regional platform CONTRAERO in the framework of the CPER ELSAT 2020 Project. The Innovation Defense Agency (AID) has also financially sustained this work. The authors also thank RENATECH, the French national nanofabrication network, and FEDER.


Sara Ibrahim

Liana Minasyan

Pierre Bonnet

Guillaume Monier

Eric Tomasella

Thierry Sauvage

Mireille Richard-Plouet

Maryline Le Granvalet

Audrey Bonduelle

Celine Pagis

Angelique Bousquet

Photocatalytic Properties of Bismuth Oxyfluoride Thin Films Deposited by Reactive Magnetron Sputtering in Ar/O2/CF4 Atmosphere

Keywords: Bismuth oxyfluoride thin films, plasmonic Bi o /BiO0.5F2 heterojunction structure, reactive magnetron sputtering, methyl orange photodegradation, CO2 photoconversion

Introduction

Heterogeneous photocatalysis has been considered as a hot research topic to solve environmental problems and energy losses since the first report by Fujishima and Honda [START_REF] Fujishima | Electrochemical photolysis of water at a semiconductor electrode[END_REF].

Photocatalysis uses the UV or visible light irradiation for environmental remediation (pollutant degradation) and clean energy production (hydrogen production and carbon dioxide (CO2) reduction) [START_REF] Jin | Bismuth-rich Bismuth oxyhalides for environmental and energy photocatalysis[END_REF]. Among the different classes of materials, titanium dioxide (TiO2) [START_REF] Yoshitake | Preparation of Wormhole-like Mesoporous TiO2 with an Extremely Large Surface Area and Stabilization of Its Surface by Chemical Vapor Deposition[END_REF][START_REF] Jia | Microwave-assisted synthesis of anatase TiO2 nanorods with mesopores[END_REF][START_REF] Lee | The synthesis and coating process of TiO2 nanoparticles using CVD process[END_REF][START_REF] Li | In situ spectroscopic ellipsometry study of TiO2 films deposited by plasma enhanced chemical vapour deposition[END_REF][START_REF] Buraso | Synthesis of TiO2 nanoparticles via a simple precipitation method and photocatalytic performance[END_REF], zinc oxide (ZnO) [START_REF] Yang | Effect of annealing temperature on the structure and optical properties of ZnO nanoparticles[END_REF][START_REF] Jie | Role of oxygen atoms in the growth of magnetron sputterdeposited ZnO films[END_REF][START_REF] Chen | Preparation of ZnO photocatalyst for the efficient and rapid photocatalytic degradation of azo dyes[END_REF], graphite-like carbon nitride (g-C3N4) [START_REF] Li | A facile approach to synthesize novel oxygen-doped g-C3N4 with superior visible-light photoreactivity[END_REF][START_REF] Ge | Synthesis and Efficient Visible Light Photocatalytic Hydrogen Evolution of Polymeric g-C3N4 Coupled with CdS Quantum Dots[END_REF][START_REF] Wen | A review on g-C3N4-based photocatalysts, 2nd[END_REF], and bismuth-based materials [START_REF] Jin | Bismuth-rich Bismuth oxyhalides for environmental and energy photocatalysis[END_REF][START_REF] Iyyapushpam | Synthesis of room temperature bismuth oxide and its photocatalytic activity[END_REF][START_REF] Ratova | Reactive magnetron sputtering deposition of bismuth tungstate onto titania nanoparticles for enhancing visible light photocatalytic activity[END_REF][START_REF] Meng | Bismuth-based photocatalytic semiconductors: introduction, challenges and possible approaches[END_REF][START_REF] Cheng | Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications[END_REF] have been widely employed in a variety of environmental and energy photocatalytic reactions. Bismuth oxyhalides, BiOX (X: Cl, Br, I), have lately emerged as a new family of promising photocatalysts. Indeed, BiOX exhibits a unique layered structure characterized by fluorite-like [Bi2O2] 2+ slabs interleaved by double slabs of halogenide anions. The internal electric fields (IEF) between [Bi2O2] slabs and the anionic halogen layers induce the efficient separation of the photoexcited charge carriers and reveal the potential photocatalytic performance of BiOX [START_REF] Cheng | Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications[END_REF][START_REF] Zhang | First-principles studies on facet-dependent photocatalytic properties of bismuth oxyhalides (BiOXs)[END_REF]. Unlike the other bismuth oxyhalides, bismuth oxyfluoride (BiOF) material exhibits a direct band gap of 4.2 eV in the UV light range. Thus, for a long time, fluorinated bismuth derivative compounds were not considered as interesting materials for photocatalytic applications due to their high band gap, which explains why only a few studies have been done compared to the other bismuth-based halide compounds [START_REF] Cheng | Preparation and Photocatalytic Properties of a Hierarchical BiOCl/BiOF Composite Photocatalyst[END_REF][START_REF] Yang | PVA-assisted hydrothermal preparation of BiOF with remarkably enhanced photocatalytic performance[END_REF].

Recently, bismuth oxyfluoride materials have attracted significant attention since fluorine has distinctive chemistry properties from chlorine, bromine, and iodine. Due to its high electronegativity, fluorine can trap electrons firmly leading to an uneven electron distribution and enhancing the photocatalytic efficiency [START_REF] Zou | Controllable synthesis of uniform BiOF nanosheets and their improved photocatalytic activity by an exposed high-energy (002) facet and internal electric field[END_REF]. Yet, the photocatalytic activity of BiOF could be further improved by different approaches. For instance, the construction of heterojunction structures such as Ag2O/BiOF [START_REF] Yang | Enhanced photocatalytic performance of Ag2O/BiOF composite photocatalysts originating from efficient interfacial charge separation[END_REF], BiOBr/BiOF [START_REF] Jiang | BiOBr/BiOF composites for efficient degradation of rhodamine B and nitrobenzene under visible light irradiation[END_REF] and BiOCl/BiOF [START_REF] Cheng | Preparation and Photocatalytic Properties of a Hierarchical BiOCl/BiOF Composite Photocatalyst[END_REF] strengthens the visible light response, promotes the separation of the photogenerated charge carriers, and thus enhances the photocatalytic activity. Moreover, BixOyFz compounds with different atomic ratios have been reported as efficient photocatalysts. The different O/F ratios in BiO0.51F1.98 (O/F = 0.25), BiO0.9F2. [START_REF] He | IR and raman spectra properties of Bi2O3-ZnO-B2O3-BaO quaternary glass system[END_REF] (O/F = 0.38), and BiO0.67F1.66 (O/F = 0.4) could be responsible for the variation in the intensities of the internal electric fields. As the O/F ratio decreases, the amount of oxygen defects decreases, and the photocatalytic activity of BixOyFz increases. Therefore, BixOyFz materials with different O/F molar ratios exhibit better photocatalytic activity than the stoichiometric BiOF [START_REF] Gu | Effect of F/O atomic ratio on photocatalytic activity of BixOyFz[END_REF][START_REF] Ren | Influence of O/F ratio on oxygen defect and photochemical properties of BixOyFz[END_REF]. In this work, we investigate the deposition of oxyfluoride thin films by reactive magnetron sputtering of a pure Bismuth target in Ar/O2/CF4 atmosphere and teste them for gas CO2 photoconversion and for pollutant photodegradation in water. For the latter application, thin films are more favorable for their reusability and recovery without the need for filtration or any other sophisticated separation technique [START_REF] Medina | Sputtered bismuth oxide thin films as a potential photocatalytic material[END_REF]. Moreover, to our knowledge, the synthesis of bismuth oxyfluoride films by sputtering has not been reported in the literature so far. Thus, the originality of this work arises from the deposition of novel photocatalytic bismuth oxyfluoride thin films by this technique.

Materials and methods

Thin films deposition

Bismuth oxyfluoride films were grown by reactive sputtering of a Bismuth target (99.99% purity and a size of 76 x 3 mm) in Ar/O2/CF4 atmosphere onto different substrates including silicon, quartz, and vitreous carbon. Initially, the substrates were ultrasonically cleaned in water, ethanol, and acetone for 30 min each and then dried using compressed air.

Subsequently, they were placed in the vacuum chamber on a substrate holder that was rotated at five rotations per minute (rpm) without heating nor biasing. The substrate was positioned above the target with a separation distance of 13.5 cm. The system was kept under vacuum, reaching a base pressure of about 10 -6 mbar. Before any sputtering experiment, the target was sputter-cleaned in argon plasma. A first set of films was deposited at room temperature with a fixed deposition time of 3 min, a radiofrequency (13.56 MHz) power of 50 W, a working pressure of 2.2 x 10 -2 mbar. The total flow rate (ΦAr + ΦO2 + ΦCF4), where ΦAr, ΦO2 and ΦCF4 are flow rate for Ar, O2 and CF4 respectively, is kept constant at 30 standard cubic centimeters per minute (sccm); while the reactive gas ratio (Rf = ΦO2/(ΦO2 + ΦCF4)) is varied from 0 to 0.6.

A second set of films were obtained varying the rf power (50-100 W), the O2 and CF4 flow rates, and the deposition time (3-30 min).

Thin films characterization

The elemental analysis of the films deposited on vitreous carbon substrate was investigated by Rutherford Backscattering Spectrometry (RBS) at CEMHTI-CNRS (Orléans, France), using 2 MeV alpha particles. The experimental spectra were simulated with SIMNRA program to determine the concentrations of Bi, O, F, and C elements.

However, the quantification of carbon in the films that may come from the dissociation of CF4

was not so accurate. Therefore, the carbon content in the films was estimated by X-ray photoelectron spectroscopy (XPS). XPS uses the Omicron brand EA 125 model with a DAR 400 X source. It utilizes the MgKα anode (E= 1253.6 eV) and a hemispheric analyzer working at constant energy of 20 eV for the acquisition of high-resolution spectra. A high-resolution measurement was made for Bi4f, O1s, F1s, and C1s peaks. After the acquisition, the exploitation of the experimental data was carried out by the XPSPEAK41 software. First, a Shirley baseline was used to subtract the background level on the spectra. The experimental spectra were then calibrated in energy by setting the adventitious C 1s (C-C) at 285.12 eV in agreement with reference [START_REF] Dolgov | Characterization of carbon contamination under ion and hot atom bombardment in a tinplasma extreme ultraviolet light source[END_REF].

The crystal structure of the films deposited on Si (100) was investigated by X-ray diffraction, PANalytical XPert'PRO diffractometer model (CuKα1 The Raman spectroscopy measurements were done at room temperature using a Jobin-Yvon T64000 spectrometer on thin films deposited on vitreous carbon. A laser emits light at 514.5 nm with a minimum power of 50 mW, such that the power reaching the sample is ~ 200 µW, which doesn't result in local annealing. Data acquisition was made with Labspec software.

The apparatus used for Fourier Transform Infra-Red spectroscopy (FTIR) analysis is the Nicolet 5700 Fourier spectrometer in the frequency range 4000-400 cm -1 with a scan number of 80 and a resolution of 4 cm -1 . Data acquisition was made with omnic software such that a measurement of the double-side polished intrinsic Si (100) substrate alone was necessary to subtract its contribution. The measured intensities were then normalized by the thickness of the films to eliminate the influence of the material's quantity.

In order to get a local characterization of the films, high resolution transmission electron microscopy (HR-TEM) was carried out using a S/TEM Themis Z Thermo Fisher Scientific, 300kV. HR imaging was performed to observe the nanoscale nature of the crystallized particles and phase identification was conducted using selected area electron diffraction on chosen crystallized particles. Comparison with the expected diffraction planes confirmed the nature of bismuth oxy-fluoride. The observation of metallic bismuth was not possible due to the sublimation of the metal under the electron beam.

Thin films optical properties

The thickness and the optical indexes of the films were investigated by spectroscopic ellipsometry thanks to an ex-situ Jobin-Yvon Uvisel ellipsometer that utilizes a 125 W Xenon arc lamp, an IHR 320 monochromator, UV-Vis and infra-red photomultipliers allowing measurement in the range of 0.59 to 4.96 eV. Information was deduced from the simulation of the measured ellipsometric angles (Ψ and ∆) using the DeltaPsi2 software.

The transmittance (T) of the films deposited on a transparent quartz substrate was measured by a UV-Visible spectrophotometer UV-2600 Shimadzu model over a range of wavelengths between 200 and 800 nm using the UVProbe software. The absorption coefficient (α) was

n is an index that characterizes the optical absorption process, it equals to ½ or 2 for direct or indirect allowed transitions, respectively; hν is the photon energy (eV), and B is a proportionality constant known as the band tailing parameter (B = 1).

Thin films' photocatalytic properties

First, the photocatalytic activity of bismuth oxyfluoride films was investigated by measuring the evolution of the absorbance spectrum of an aqueous solution of 5.2 x 10 -5 mol L -1 of methyl orange dye (MO) as a function of the irradiation time. The deposited film was placed vertically at the bottom of the cell containing 3 mL of MO solution and stirred under darkness for 60 minutes to establish equilibrium between the dye and the photocatalyst. Then, the cell containing the film was placed at a distance of about 20 cm from a 1000 W Xenon (Xe) lamp that emits light from 200 to 900 nm. Continuous stirring was maintained to have homogeneity between the dye and the catalyst. The UV-Vis measurements were carried out every 20 min up to 120 min and then every 60 min up to 240 min. The pseudo-first-order reaction rate of the samples (k) was obtained using the Langmuir-Hinshelwood model by plotting ln (Ao /A) as a function of the irradiation time. Hence, the slope of such plot corresponds to k according to Equation 2.

(

) 2
The values of k were then normalized by the surface area of films to eliminate the geometrical surface's effect between samples that could slightly differ from size.

Secondly, the ability of bismuth oxyfluoride material in reducing CO2 was explored at IFPEN (Solaize, France). The photoconversion experiments were carried out continuously in a stainless-steel fixed bed reactor gas flow passing through the sample, operating at atmospheric pressure and controlled temperature of 30 °C. The CO2/H2O molar ratio (obtained by CO2 bubbling in a thermalized water tank) was fixed at 24, and the CO2 gas flow rate was set at 0.3 cubic centimeters per minute (cc.min -1 ). A Xe lamp Max303 Asahi provided the UV-Visible irradiation from 280 to 620 nm. The irradiance was controlled with a radiometer equipped with CCD captor (Delta Ohm) and adjusted to 80 W.m -² for 315-400 nm wavelength range.

Moreover, the reactor has an optical quartz window through which the light flux reached the material. The catalyst was supported on a filter inside the Teflon ring, where the film was crushed to obtain powder where the gas could go through. Reagents and products were then analyzed on-line by a gas chromatograph TCD sensor microchromatograph Lan3000 (SRA Instruments, µGC-TCD). Gas-phase analyses were performed every 10 minutes during 20 hours. According to Equation 3, the simplified unbalanced reaction with all the reactants and products in the gas phase, a couple of products could be obtained.

(

) 3
Then, to evaluate the photocatalytic performance of the materials for CO2 reduction, the calculated parameters must consider all the products. Two parameters are then defined: the average electron consumption rate Equation (4) and the average selectivity (Equation ( 5)), calculated after 20 hours of Time On Stream (T.O.S.).

(

) (5) 4
Where, is the stoichiometric coefficient of the electrons consumed by the product i (for example, 2 ē are needed to form H2 or CO, whereas CH4 needs 8 ē), is the concentration of product i (or j) over time (in ppmVol), Qtot is CO2 and H2O input flow rates (3.09 x 10 -4 L.min -1 ), and Vm is the molar volume of a gas at 25°C (24.5 L.mol -1 ). Average electron consumptions are given with a standard deviation of approximately ± 10%.

Results and discussion

Influence of the reactive gas ratio (Rf)

In order to investigate the synthesis of bismuth oxyfluoride thin films by reactive sputtering, the reactive gas ratio, Rf, corresponding to the O2 flow rate to the total reactive gas (O2 + CF4) flow rate ratio is varied from 0 to 0.6. Other process parameters are kept constant: 29 sccm for the argon flow rate, 1 sccm for the total reactive gas flow rate, a rf power of 50W on the pure Bi target and a deposition time of 3 min. Figure 1a shows the XRD patterns of the films (311) planes of BiO0.5F2 [START_REF] Bervas | Soft-chemistry synthesis and characterization of bismuth oxyfluorides and ammonium bismuth fluorides[END_REF]; while the peak at 27.2 o is attributed to the (012) plane of Bi [START_REF] Ibrahim | Tailoring the structural and optical properties of bismuth oxide films deposited by reactive magnetron sputtering for photocatalytic application[END_REF]. The presence of metallic Bi indicates that the amount of the injected reactive gases (O2 and CF4) is not high enough to react with all the sputtered bismuth atoms, leaving some metallic parts in the oxyfluoride matrix. We may notice that BiO0.5F2 is not the most common oxyfluoride phase. Figure 2a presents the intensity of the main peaks for BiO0.5F2 and Bi phases, normalized by film thickness, as a function of Rf. We observe a global decrease of these phases in the films until Rf = 0.6, where they are no more observed. For Rf = 0.6, the peak appearing at 2θ = 26.2 o on Figure 1 is attributed to the (111) plane of BiF3 phase. Of course, depositing a pure fluoride thin film is difficult to explain with an O2-containing gas mixture; that is why we assume the presence of an amorphous oxide compound in this film, not detectable by XRD. With this gas mixture, metallic Bi is no more present indicating that all the Bi atoms sputtered from the target are oxidized or fluorinated by the reactive gases. In parallel to this new BiF3 phase, Figure 2b shows a drop in the thickness, deduced from ellipsometry, from 80-90 nm for Rf ≤ 0.5 to around 40 nm for Rf = 0.6, suggesting a shift of the process from a metallic to a compound sputtering mode. Figure 2b shows the composition of the films deduced from RBS analysis. For all films, the percentage of bismuth is approximately constant (~ 30 %); whereas F content slightly decreases and O content slightly increases with Rf. The relatively high content of oxygen, while XRD shown the presence of oxygen-poor phases (BiO0.5F2, Bi and BiF3), confirms that amorphous bismuth oxide phases should be present into the samples. We may also remark the very low percentage of carbon in the films; whereas carbon atoms are provided from injected CF4 molecules. This can be explained by the etching of carbon in the growing films by oxygen radicals from plasma, already observed in literature [START_REF] Granier | Mechanisms Involved in the Conversion of ppHMDSO Films into SiO2-Like by Oxygen Plasma Treatment[END_REF]. However, it is difficult to quantify precisely by RBS the proportion of carbon in the films, since they are deposited on a vitreous carbon substrate. Accordingly, this aspect should be confirmed latter by XPS. To better quantitatively evaluate the presence of amorphous compounds, especially bismuth oxide, we investigate the deposited films by Raman and IR spectroscopies. The FTIR and Raman spectra are shown in Figure S1 in supplementary information. For both techniques, the measured spectra are as expected. The IR bands detected in the 700 -400 cm -1 range, especially the main one at 612 cm -1 , correspond to the Bi-O stretching vibrations in bismuth oxide and/or oxyfluoride films [START_REF] Fan | Infrared, Raman and XPS spectroscopic studies of Bi2O3-B2O3-Ga2O3 glasses[END_REF]. For Rf = 0 (ΦCF4 = 1 sccm), the intensity of these peaks is very low and a peak at 453 cm -1 emerges. The latter could be attributed to the stretching vibrations of Bi-F bonds [START_REF] Bervas | Soft-chemistry synthesis and characterization of bismuth oxyfluorides and ammonium bismuth fluorides[END_REF]. The peak at 840 cm -1 refers to Bi-O-Bi symmetric stretching vibrations in Bi2O3 but also in BiO0.5F2 [START_REF] Yang | Facile Fabrication of Dumbbell-Like β-Bi2O3/Graphene Nanocomposites and Their Highly Efficient Photocatalytic Activity[END_REF]. The presence of fluorine is also confirmed by the small C-F vibration band observed at 1122 cm -1 [START_REF] Li | The effect of Nethyl-N-hydroxyethyl perfluorooctanoamide on wettability alteration of shale reservoir[END_REF]. For Raman spectra, the bands observed at 126 cm -1 could be assigned to Bi-O and Bi-F stretching vibrations, while the stretching and bending vibrations of Bi-O-Bi and Bi-F-Bi appear at around 314 cm -1 , but they are not easily distinguished [START_REF] Hu | Preparation, nonlinear optical properties, and theoretical analysis of the non-centrosymmetric bismuth oxyfluoride, Bi7F11O5[END_REF]. The band at 624 cm -1 could be attributed to Bi-O - [START_REF] He | IR and raman spectra properties of Bi2O3-ZnO-B2O3-BaO quaternary glass system[END_REF]. Moreover, the presence of metallic bismuth is observed by the Bi-Bi vibration band at 95 cm -1 but its intensity could not be precisely determined due to some apparatus limitation in this region [START_REF] Cheng | Preparation and photocatalytic properties of a hierarchical BiOCl/ BiOF composite photocatalyst[END_REF]. Finally, the normalized contributions of Bi-O bonds at 612 cm -1 from IR were added to Figure 2a. This band can be attributed to both BiO0.5F2 and Bi2O3 compounds. However, its intensity is very low for Rf = 0 while diffraction peaks indicate the presence of BiO0.5F2 phase in this film. Hence, the main contribution to this Bi-O band seems to be the Bi2O3 compound.

         BiO 0.5 F 2  Bi  BiF 3  Si substrate Bi BiO 0.5 F 2 R f = 0.6 R f = 0.5 R f = 0.4 R f = 0.3 R f = 0.2 R f = 0.1

Intensity (a.u.) 2 (°)

R f = 0   (a)
When Rf increases, we observe a progressive increase of Bi-O bonds content, which is linked to the increasing part of amorphous Bi2O3. Hence, for Rf values between 0 to 0.5, the deposited films are composed of a decreasing part of crystallized BiO0.5F2 and Bi phases in mixture with slightly increasing part of amorphous Bi2O3. For Rf = 0.6, the previous BiO0.5F2

and Bi phases disappear, the Bi-O band intensity drastically increases, indicating higher part of amorphous Bi2O3 in parallel to a BiF3 phase emergence. For this Rf, a phase segregation of the oxyfluoride compound is then observed in a mixture of crystallized BiF3 phase and amorphous Bi2O3.

We then explored the optical properties of the films thanks to UV-visible spectroscopy and ellipsometry. The variation of the films transmittance measured by UV-visible spectroscopy is displayed in Figure 3a. are lower than the one reported for BiO0.5F2, which can be explained by the presence of metallic Bi. Indeed, a slight band gap decrease was already reported due to metallic bismuth addition in the Bi2WO6/Bi system [START_REF] Huang | Facile synthesis of Bi/Bi2WO6 nanocomposite with enhanced photocatalytic activity under visible light[END_REF]. We might also remark, that the band gap is minimum for Rf = 0.3, which could be favorable for photocatalysis. Figure 4a presents the degradation of MO in water without any photocatalysts and in the presence of the films deposited at Rf = 0.2, 0.3, and 0.6. First of all, interestingly, we observed that all the deposited films present a photocatalytic activity. We also compare film performance to pure oxyfluoride phase, by studying the degradation of MO with 1.28 mg of BiO0.5F2 powder. This mass was chosen to be twice the estimated mass of our films (⁓ 0.7 mg). Results show that even if the powder amount is twice higher, our films composed of a Bi0.5F2/Bi mixture, for 0 ≤ Rf ≤ 0.5, exhibits better photocatalytic performance. In Figure 4b, we plotted the reaction rate, k, divided by the geometrical surface area of the film, S.A., in order to eliminate the effect of sample size variation. For 0 ≤ Rf ≤ 0.5, the film deposited at Rf = 0.3 seems to exhibit the highest performance, in agreement with its lower band gap. We also observed a good reaction rate, k/SA, for the film at Rf = 0.6. However, it is difficult to compare this latter result to the other one since the present phases are different. Interestingly, the best film (Rf = 0.3) is not the one with the highest BiO0.5F2 crystallinity (Rf = 0.0). Similar results were observed with the Bi2O3/Bi system, for which an enhanced photocatalytic activity for MO photodegradation was demonstrated by the addition of a controlled Bi content [START_REF] Ibrahim | Tailoring the structural and optical properties of bismuth oxide films deposited by reactive magnetron sputtering for photocatalytic application[END_REF].

This has been interpreted as a heterojunction effect to increase the separation of the photogenerated carrier and / or an improved light absorption due to plasmonic effect of metallic nanodomains. Of course, a too high amount of metallic Bi will also lead to a decrease of light absorbed by the photocatalyst, which will reduce its activity. Hence, a compromise between both phases needs to be found. Here, the optimum ratio seems to occur at Rf = 0.3.

However, photocatalytic performance may be increased by enlarging the investigated process parameters around this promising condition.

Influence of discharge power and flow rates parameters

In a second set of measurements, the previous condition at Rf = 0.3 is taken as a reference and some process parameters are modified. First, in order to explore the effect of a higher content of metallic Bi, we increase the rf power applied to the target from 50W (reference sample) to 75 and 100W. The XRD patterns obtained for these films are plotted on Figure 5a and shows an increase of metallic Bi peak intensity detrimentally to the BiO0.5F2 ones. In these conditions, when the rf power is increased, the flux of Bi atoms reaching the substrate is increased; while the constant amount of reactive gases is not high enough to react with all the sputtered atoms, leading to incorporate more metallic Bi. However, this higher Bi content is accompanied by a decrease of the photocatalytic response: k/SA drops to values lower than 2.10 -3 min -1 .cm -2 . At the opposite, at a fixed 50 W power, increasing the flow rate of O2 (to 0.4 sccm) or CF4 (to 0.4 sccm) flow rates allows us to decrease the metallic content and drastically increase the crystallization of oxyfluoride phase in the films, as shown in Figure 5b. However, whatever the improvement of BiO0.5F2 crystallinity, the reduction of Bi content seems to limit the film photocatalytic performance (k/S.A. < 3.10 -3 min -1 .cm -2 ). Finally, even if these various process parameters allow one to finely controlled the content of both phases, the film previously obtained at Rf = 0.3 seems to be the best compromise for optimal photocatalytic properties.

Influence of the deposition time

Since the ratio between both phases seems to be the best for Rf = 0.3, we might try to increase the crystallinity part without varying their ratio. Previous thin films present a limited crystallization that could be due to their limited thickness (< 100 nm). We then tried to favor phases crystallization by increasing the deposition time. Figure 6a presents the thickness variation as a function of the deposition time. The first exciting remark is that the variation is not linear. Initially, the deposition rate is around 30 nm/min for deposition time of 3 min. It then decreases to 22-25 nm/min for longer time. This could be a sign of film densification due to its crystallization. phase is observed, even if we cannot exclude the presence of amorphous compound. The film surface roughness was also evaluated. As expected, the thickness and crystallization enhancement with deposition time leads to higher roughness. However, this increase stays moderate: from Rq = 2 nm for 3 min deposition to 10 nm for 30 min.

To investigate the film microstructure more in detail, we performed some SEM and HR-TEM analysis. Images for the film deposited at Rf = 0.3 and td = 20 min are shown in Figure 7.

First, SEM images from top and trench on Figure 7a show a porous structure with a rough surface, in agreement with the AFM measurements. When this film is scratched on TEM grid, we remark that it is composed of small grains, where their size is found to be ⁓ 10 nm, and that oxyfluoride phase is the BiO0.5F2 one. Unfortunately, Bi grains cannot be studied by TEM because of their low melting point. Indeed, this lead to the grain melting under the electron beam, which modifies their shape and size before observation. To quantify the potential presence of amorphous bismuth oxide and to check the low content of carbon into our films, XPS analysis was carried out on the same film. Figure 8 presents the XPS high resolution spectra for C 1s, Bi 4f, O 1s and F 1s binding energies. Since the binding energies for various oxyfluoride are relatively close, XPS spectra measured with pure BiO0.5F2 powder were added to Figure 8. The binding energies of 159.7 and 165.0 eV correspond to Bi 4f7/2 and Bi 4f5/2, respectively, consistent with the Bi 3+ oxidation state in BiO0.5F2 [START_REF] Gmitter | Subsurface diffusion of oxide electrolyte decomposition products in metal fluoride nanocomposite electrodes[END_REF]. The small second doublet appearing at lower binding energies in thin film is assigned to the metallic Bi 0 chemical state, in agreement with the XRD results. The highresolution XPS spectrum of O1s is decomposed into three peaks, where the peaks at 530.32, 531.73, and 532.77 eV correspond to Bi-O, C=O, and C-O bonds, respectively [START_REF] Fan | Infrared, Raman and XPS spectroscopic studies of Bi2O3-B2O3-Ga2O3 glasses[END_REF]. The peak at 684.03 can be attributed to the F species in BiO0.5F2 [START_REF] Jiang | Enhanced Visible Light Photocatalysis of Bi2O3 upon Fluorination[END_REF]. Hence, the XPS peaks detected at 159.7, 530.32, and 684.03, corresponding to Bi 4f7/2, O 1s, and F 1s, respectively, could be referred to BiO0.5F2 material as reported in literature [START_REF] Ren | Synergetic effect of Bi2WO6 micro-spheres and activated carbon mm-spheres for enhancing photoreduction activity of CO2 to CO[END_REF]. We remark, that these peaks are also in agreement with those measured on BiO0.5F2 powder. In Figure 8a, a weak C 1s peak is observed for the film. The three peaks emerging at 285.12, 286.62, and 288.62 eV are ascribed to C-C, C-O, and O-C=O bonds, respectively [START_REF] Dolgov | Characterization of carbon contamination under ion and hot atom bombardment in a tinplasma extreme ultraviolet light source[END_REF]. No peak at energy higher than 289.5 eV, corresponding to C-Fx environments, is seen. A first elemental quantification, reported on Table 1, estimates the carbon content to around 11%.

However, in our case, it is difficult to specify if this carbon originates from dusts on the surface or from carbon from CF4 reactive gas incorporated into the film during its growth. To answer this question, we clean the film surface thanks to low energy Ar + ion bombardment (E = 1 keV, I = 10 A, for 2 min). The analysis made after this bombardment is also presented in Figure 8. After bombardment, compared to the original film, the same peaks are observed; but the carbon peak is drastically reduced from 11 to 2% of elemental composition (see Table 1); whereas other peaks are rather unchanged. Indeed, most of the initially detected carbon originates from adventitious carbon, as classically observed on sample surface analyzed by XPS, and only few percent are from carbon incorporated into the thin film. This low carbon content, in agreement with RBS results (%C = 4 at.% for RF = 0.3), might be explained by an etching effect by O radicals from the plasma in Ar/O2/CF4 mixture. On the Bi 4f spectrum, the Bi2O3 phase, whose peak Bi 4f7/2 is reported at 158.7 eV, cannot be clearly seen, indicating its amount should be low. However, we may take into account presence of this phase into Bi 4f peak decomposition, shown in Figure 9. From peak areas, Bi environment is evaluated to be at 28 % of Bi, 68 % of BiO0.5F2 and 4-5 % of Bi2O3 (reported in Table 1). Details of peak position, width and area can be found in supplementary information. However, since the Bi2O3 peak is very close to the BiO0.5F2 one, its contribution part, obtained from data fitting, can be imprecise. To confirm the latter results, we calculate the contributions of the three phases from the global elemental composition, which is not influenced by peak fitting. We considered that all F atoms are from BiO0.5F2. After, the remaining O atoms are attributed to Bi2O3, and then the left Bi atoms are considered in metallic environment. Details of this calculation are explained in supplementary information.

The calculated parts of the three phases are reported in Table 1 for the film, before and after ionic bombardment, and are nicely in agreement with the Bi 4f peak decomposition. Hence, we might consider that in this film, Bi atoms are around 60-67 % in BiO0.5F2, 28-35% in pure metallic and 4-5 % in Bi2O3 environments. Hence, the latter phase seems to be a minor one compared to BiO0.5F2 and Bi.

Photocatalytic properties of thicker films

The degradation of MO in water with films deposited at Rf = 0.3 for 3 and 20 min is illustrated in Figure 10a. The film obtained after 20 min degrades 90 % of MO after only 2 hours of irradiation (compared to only 50% for film deposited during 3 min). Indeed, the crystallization improvement, thanks to higher deposition time, leads to an enhanced photocatalytic activity with a k/SA increase from 3.0 to 9.5 10 -3 min -1 .cm -2 . This good results

for thicker film may also be explained by its higher roughness and/or a more adapted thickness, reducing the light loss by reflection. To go further, we then check the stability of this best film by cycling experiments (Figure 10b). Its high performance is maintained after 3 cycles (= 9 hours), without major modification observed in its XRD pattern (Figure S2 in supporting information). Hence, this photocatalyst is stable and does not degrade upon irradiation and time.

We might also compare this results to our previous one where we deposited Bi2O3/Bi heterojunction [START_REF] Ibrahim | Tailoring the structural and optical properties of bismuth oxide films deposited by reactive magnetron sputtering for photocatalytic application[END_REF]. For the best film, with optimized Bi content, we reached comparable k/SA (= 8.0 10 -3 min -1 .cm -2 ), which is surprising considering the lower reported Bi2O3 band gap around 2.6-3.0 eV. From one hand, this can be explained by the fact that the lower bandgap of Bismuth oxide is indirect and so presents a lower absorbance than the direct bandgap of BiO0.5F2. On the other hand, the fluorination is already reported to create electric local field that reinforce the photogenerated carriers [START_REF] Jiang | Enhanced Visible Light Photocatalysis of Bi2O3 upon Fluorination[END_REF] and then limit their recombination.

Furthermore, the chemical stability of the oxyfluorinde leads to a higher stability of the photocatalyst after 3 cycles than the Bismuth oxide film [START_REF] Ibrahim | Tailoring the structural and optical properties of bismuth oxide films deposited by reactive magnetron sputtering for photocatalytic application[END_REF]. After pollutant photodegradation activity, we investigated the performance of Bismuth oxyfluoride films for CO2 photoreduction. Figure 11a 11a also shows the selectivity of these materials for the different products detected: CO, CH4 and H2. It confirms the already observed selectivity of Bi-based photocatalyst for CO production. [START_REF] Ren | Synergetic effect of Bi2WO6 micro-spheres and activated carbon mm-spheres for enhancing photoreduction activity of CO2 to CO[END_REF] This selectivity is obtained for thin films but also for powders and seems to be reinforced by fluorine presence.

It is important to recall that our films present a very low content of carbon. Hence, the detected CO comes from CO2 reduction and doesn't arise from the oxidation of carbon into the film. The Figure 11b present the production rate (rav pdt), averaged on the 20 h of the test duration and normalized by the photocatalyst mass, for the detected reduction products (CO, CH4 and H2). Compared to powder, CO gas is identified in 1000 times higher proportion using the thin films. Moreover, taking into account its higher electron consumption rate and its better selectivity for CO, the heterojunction of BiO0.5F2/Bi developed in this paper is more efficient (≈ 25 mol/h/g) than the best film Bi2O3/Bi heterojunction (≈ 10 mol/h/g). Again, this could be explained by a better carrier separation thanks to local electric field into BiO0.5F2 and its direct bandgap. These production rates are comparable to the current state of art [START_REF] Sorcar | A review of recent progress in gas phase CO2 reduction and suggestions on future advancement[END_REF]. Finally, for both photocatalytic applications, metallic Bi addition seems to enhance the photocatalytic properties. This can be attributed to different phenomena discussed in literature. First, Bi acts as electron traps, reducing the recombination of the generated e -/h + pairs on the bulk defects of BiO0.5F2 [START_REF] Zhang | Plasmonic photocatalysis[END_REF], thus enhancing the photocatalytic degradation of MO and photoconversion of CO2. Secondly, metal Bi was shown to act as noble metal nanoparticles (Au and Ag), improving the visible light absorption and activating the photocatalyst [START_REF] Wang | Plasmonic photocatalysts: harvesting visible light with noble metal nanoparticles[END_REF]. Finally, the surface plasmon polarizes the non-polar reactant molecules and improves the adsorption to the metal surface [START_REF] Zhang | Plasmonic photocatalysis[END_REF]. Therefore, this heterojunction structure favors the production of OH . radicals at the valence band, which degrade the MO solution, and the photoconversion of CO2 at the surface of BiO0.5F2 [START_REF] Li | Specific analyses of the active species on Zn0.28Cd0.72S and TiO2 photocatalysts in the degradation of methyl orange[END_REF].

Conclusions

In this paper, we succeed for the first time to deposit bismuth oxyfluoride thin films by sputtering a Bismuth target in reactive Ar/O2/CF4 atmospheres. Moreover, we demonstrate, that the reactive sputtering technique is very versatile, since it allows us to synthesize one-pot heterojunctions of BiO0.5F2/Bi with a controlled content of both phases. We also showed that, an increase in the deposition time promotes the part of crystallized phases, leading to improved photocatalytic properties. Finally, the film obtained with Rf = 0.3 and td = 20 min exhibits interesting photocatalytic activity in the degradation of MO pollutant in water, stable for at least 9h. It can also be used to selectively convert CO2 gas to CO (with a product rate around 25 mmol/h/g and a selectivity of 70%). Yet, this BiO0.5F2/Bi heterojunction will be more investigated in the future to understand the exact role of metallic Bismuth in the enhancement of the photocatalytic properties. Moreover, the nanostructuration of such system will be studied to further improve its performance.

  calculated from the transmittance by means of Lambert-Beer's law and from film thickness deduced from ellipsometry. The UV-visible spectrum was then used to determine the band gap (Eg) of semiconductors through Tauc plot, which relates the band gap and the absorption coefficient as shown in Equation 1,

  deposited depending on Rf to identify the synthesized phases. The Figure1bpresents a zoom on the main peak of the films (around 26-27°) compared to the patterns of numerous bismuth compounds: Bi, Bi2O3, BiOF, BiO0.5F2, Bi7O5F11, BiF3. Results reveal that the films deposited at 0 ≤ Rf ≤ 0.5 consist of two phases: BiO0.5F2 and metallic bismuth. Indeed, the peaks appearing at 2θ = 26.4 o , 30.4 o , 43.6 o , and 51.8 o correspond to the (111), (200), (220), and

Figure 1 :

 1 Figure 1: XRD patterns of bismuth oxyfluoride films deposited with Rf = 0-0.6.

Figure 2 :

 2 Figure 2: (a) Normalized intensities of (111) BiO0.5F2 and (012) Bi XRD peaks and of Bi-O bonds IR absorption at 612 cm -1 , and (b) elemental composition of bismuth oxyfluoride films and their thickness for a 3 min deposition time, as a function of the reactive gas ratio, Rf.

  Films deposited at Rf ≤ 0.5 exhibit low transmittance in the UV-visible range due to their metallic bismuth content, whereas the film deposited at Rf = 0.6 is more transparent in visible range. These results are confirmed by the simulation of the extinction coefficient, k, from ellipsometric measurements, plotted in Figure3b. For Rf = 0.6, a simple Tauc-Lorentz model is used to fit the measurements. Hence, the extinction coefficient is null for photon energy lower than the band gap and drastically increases at higher energies. This is in agreement with the insulator behavior of BiF3 or Bi2O3 compounds. For lower Rf, a Lorentz oscillator should be added to the previous model, considering light absorption at low photon energy due to the presence of metallic Bi. The obtained k is now composed of two parts: an absorption at energy higher than 3.5 eV, due to band gap of BiO0.5F2 or Bi2O3 compounds, and a new and large absorption around 1 eV, due to the presence of metallic Bi. From both techniques, UV-visible spectroscopy and ellipsometry, the band gap values were resolved and plotted in Figure3c. The films exhibit a direct band gap[START_REF] Gu | Effect of F/O atomic ratio on photocatalytic activity of BixOyFz[END_REF][START_REF] Huang | Electronic structures of relaxed BiOX (X=F, Cl, Br, I) photocatalysts[END_REF], with values ranging from 3.89 to 4.33 eV from ellipsometry results. Values obtained from UV-visible are close even if they are less precise because of the high contribution of Bi to the absorbance. These values

Figure 3 :

 3 Photon energy (eV)

Figure 4 :

 4 Figure 4: (a) Degradation of MO (Ct/Co) as a function of the irradiation time in the presence of films (Rf = 0.2, 0.3, and 0.6) and BiO0.5F2 powder, and (b) the reaction rate to surface area, k/S.A., as a function of Rf.

2 PFigure 5 :

 25 Figure 5: XRD patterns for thin films deposited (a) at fixed gas mixture of 0.3 sccm of O2 and 0.7 sccm of CF4(also called RF = 0.3) and various rf power and (b) at fixed rf power of 50W and various gas mixture.

Figure 6 :

 6 Figure 6: (a) The thickness and deposition rate of bismuth oxyfluoride films (Rf = 0.3) as a function of the deposition time, and (b) the XRD patterns of these films.

Figure 7 :

 7 Figure 7: Microscopy images of the film deposited at Rf = 0.3 and td = 20 min. (a) SEM pictures of top and side view, (b) TEM images. Small grains of ⁓ 10 nm size are observed. (c) SAED pattern of nanoscale particles located in the circle. The distance of 3.39 Å corresponds to the (111) plane of BiO0.5F2.

Figure 8 :

 8 Figure 8: XPS spectra obtained for (a) C 1s, (b) Bi 4f, (c) O 1s and (d) F 1s for film deposited at Rf = 0.3 for 20 min, before and after Ar + bombardment and for BiO0.5F2 powder.

Figure 9 :

 9 Figure 9: Decomposition of Bi 4f peak for the film deposited at Rf = 0.3 for 20 min after Ar + bombardment.

Figure 10 :

 10 Figure 10: (a) Degradation of MO (Ct/Co) as a function of the irradiation time in the presence of films obtained at Rf = 0.3 for 3 and 20 min of deposition times and (b) cycling curves of photocatalytic degradation for the one deposited during 20 min.

3 Figure 11 :

 311 Figure 11: The photocatalytic activity represented by (a) the average electron consumption rate and the selectivity of H2, CO, and CH4 products and (b) the average production rate of BiO0.5F2/Bi and Bi2O3/Bi film (obtained respectively at Rf = 0.3 and (O2) = 1 sccm [29]), compared to powders.

  1.5406 Å, CuKα2 1.5444 A o , 40 kV, 30 mA, and an angle step of 0.0167 o ).

Table 1 : The elemental composition, formula and phases content calculation for the bismuth oxyfluoride film deposited at Rf = 0.3 for 20 min, before and after bombardment with Ar + ions.

 1

			Elemental composition		Phases for Bi environment
		Bi	O	F	C	Formula	BiO0.5F2	Bi	Bi2O3
	Pristine	31.7	19.0	38.6	10.6	BiO0.59F1.20	60.2	35.4	4.4
	After	38.3	14.7	44.6	2.4	BiO0.38F1.16	58.4	38.3	3.2
	etching			From Bi 4f peak decomposition	67.6	27.8	4.6

  presents the electron consumption rate average in photocatalyst mass and in time (on the 20 hours of the test duration) for the BiO0.5F2/Bi films (deposited at Rf = 0.3 for 20 min of deposition time). For comparison, we also plotted results for Bi2O3/Bi best film from our previous work and for stoichiometric Bibased compounds in powder. These latter present relatively low rates compared to films, especially considering that reported values are multiplied by a factor 500. The Bi2O3/Bi heterojunction have interesting rate around 35 mol/h/g; while BiO0.5F2/Bi system is twice better with a rate around 70 mol/h/g. The Figure

Acknowledgements

The authors acknowledge the support received from the Agence Nationale de la Recherche of the French government through the program "Investissements d'Avenir" (Grant No. 16-IDEX-0001 CAP 20-25).

Supporting Information