Cécile Le Guern
email: c.leguern@brgm.fr

Liliane Jean-Soro

Béatrice Béchet

Thierry Lebeau

Dorine Bouquet

MANAGEMENT INITIATIVES IN SUPPORT OF THE SOIL QUALITY OF URBAN ALLOTMENT GARDENS: EXAMPLES FROM NANTES (FRANCE)

Urban allotment gardens (UAG) are important for: the provision of foodstuffs, social cohesion, residents' well-being, and prevention of the formation of local heat islands during summer. The soils of these gardens however may be adversely affected by pollution threats and thus create health risks. In such cases, appropriate management becomes necessary.

For several gardens exhibiting soil contamination (e.g. Pb at 100-400 mg kg -1 ) in the city of Nantes, local actors collaborated, including: scientists, the municipality's Parks and Open Space Department, elected officials, sanitary administration, and each site's gardeners' association. The soil characterization step was performed along with a sanitary risk evaluation and discussion of management options, based on both the pollution characteristics and local context.

The most frequent option consisted of replacing the polluted soils with clean soils. Managing the excavated polluted soils on-site (e.g. for ornamental purposes) limited the economic and environmental impacts associated with this solution. Alternative solutions, including a combined system of non-accumulative cropping vegetables at the time of phytoextraction, were also employed to maintain gardening uses. In some cases, land use (gardening) was changed into, for example, an orchard, open space or ornamental space. A combination of solutions was introduced in several gardens.

The various options available for managing polluted soils, as implemented in Nantes' UAGs and based mainly on NBS, can be applied more generally in order to improve soil quality. In addition to enhancing the quality of both residents' lives and biodiversity, several solutions

Introduction

Sustainable and attractive green cities include urban allotment gardens (UAG) in their development programs. Such UAG have become widespread across the world, especially in industrialized countries [START_REF] Draper | Review and analysis of the benefits, purposes, and motivations associated with community gardening in the United States[END_REF][START_REF] Guitart | Past results and future directions in urban community gardens research[END_REF]. These gardens provide many benefits to the community, namely: assets for social cohesion, health and gardeners' well-being [START_REF] Genter | The contribution of allotment gardening to health and wellbeing: a systematic review of the literature[END_REF]; foodstuffs, especially important in times of economic crisis; contribution to water regulation and prevention of local heat islands through evapotranspiration; sources of biodiversity development [START_REF] Haase | A quantitative review of urban ecosystem service assessments: concepts, models, and implementation[END_REF][START_REF] Breuste | Allotment gardens contribute to urban ecosystem service: case study Salzburg, Austria[END_REF]Bell et al., 2016). The soils in these gardens however may be adversely affected by pollution due to their location on former urban industrial sites or near roads with intense traffic [START_REF] Hough | Assessing potential risk of heavy metal exposure from consumption of home--produced vegetables by urban populations[END_REF][START_REF] Hursthouse | Environmental pressures on and the status of urban allotments[END_REF]. Individual gardening behavior can also introduce contaminants. These soils might thus be contaminated by anthropogenic sources, but also they could sometimes be affected by geogenic anomalies and present health risks [START_REF] Jean-Soro | Origin of trace elements in an urban garden in Nantes, France[END_REF]. In such cases, finding appropriate management solutions is crucial for sanitary and economic reasons.

Several feasible options exist to manage soil pollution: i) Excavating the contaminated soil and replacing it by uncontaminated (top)soil (called ex situ management). Urban developers and city councils most often rely on such an option in order to reopen the gardens as quickly as possible. Removing the source of danger solves the problem immediately and definitively, thus avoiding any liability issue in the event of insufficient soil cleanup (precautionary principle). Nonetheless, when taking soil sustainability into consideration, landfilling low contaminated soils as the main management option is a questionable practice. Once excavated and removed from the site, soil indeed becomes a waste and cannot be reused except for very specific uses (e.g. road base course). The availability and quality of the marketed (top)soil used to replace the polluted soil also raises questions;

ii) Excavating the contaminated soil and keeping it on site (called "on-site management").

In this case, the excavated soil retains its status as soil. Depending on the pollutants involved as well as on the deadline for land use conversion and available on-site space, the soil may be put under cover or treated on-site. After a (successful) depollution process, this soil could be reused on the same site for various purposes, as long as the specific reuse is deemed compatible with soil quality;

iii) Managing the contaminants in situ (i.e. "in situ management"). More common for biodegradable or volatile organic pollutants, with as an example the (bio)venting method, such an in situ strategy is not widely used for inorganic pollutants (trace elements). Moreover, only phytoextraction, i.e. the use of plants capable of extracting trace metals from the soil, followed by transfer from roots to aerial parts can be applied in such cases.

Yet this process is time-consuming and slows the urban development process since it often results in a large discrepancy between the time needed for soil cleanup (several years to decades) and the time constraints imposed by city planning processes. However, in terms of soil sustainability, this option is undoubtedly the most appropriate, when considering the increased global need for soils as a resource to feed the world's population.

Nature-Based Solutions (or NBS) are defined as actions to protect, sustainably manage and restore natural or modified ecosystems in a way that effectively and adaptively addresses societal challenges, while simultaneously providing for human well-being and biodiversity benefits (Cohen-Shacham et al., 2016). Most researchers agree that the NBS concept is a rational strategy for promoting the ecological restoration [START_REF] Maes | Nature-Based Solutions for Europe's Sustainable Development[END_REF][START_REF] Kabisch | Nature-based solutions to climate change mitigation and adaptation in urban areas: perspectives on indicators, knowledge gaps, barriers, and opportunities for action[END_REF]. In their chapter, [START_REF] Bouzouidja | Metal contamination in urban soils: which managements using nature-based solutions for the development of a safe urban agriculture?[END_REF] reviewed the used of NBS for developing safe urban cropping for the case of metal contamination. The objectives of this paper are to discuss the range of potential solutions, with a particular emphasis on NBS, to ensure the compatibility of land use as regards soil quality in the context of low to moderate soil pollution, based on the example of Nantes' UAGs. A description of the options for three representative UAGs will be complemented by a discussion of the options selected for all contaminated UAGs.

Materials and methods

Study area

Nantes is located 40 km east of the Atlantic coast and enjoys an oceanic climate (820 mm rainfall/year); this large city in northwestern France has a high population density (4,100 inhabitants/km 2 ). The city possesses over 1,000 plots in more than 25 gardens located throughout the city (Fig. 1), with soil contamination being reported for 8 of them (Fig. 1).

Among these 8 UAGs, Crapaudine, Oblates and Eglantiers appear to be representative since they display various contamination origins and applied management solutions. The characteristics and past land use of the Crapaudine and Eglantiers UAGs are detailed in [START_REF] Béchet | Spatial variability of trace elements in allotment gardens of four European cities: assessments at city, garden, and plot scale[END_REF]. The Eglantiers garden (95 plots) is located north of Nantes in an urbanrural transition zone near the ring-road. Eglantiers was formerly a crop farm that included a small vineyard. The Crapaudine garden is a public green space containing 90 plots. Until the 1970's, the area supported market gardening. The Oblates park is situated near industrial sites.

Until recently, Oblates belonged to a religious congregation that used it as a private park, with some zones being used for cropping or an orchard. The city council bought this parcel for transformation into a public park, in the aim of setting up a 30-plot UAG and a collective cropping garden.

The geological and geochemical contexts could be determined thanks to: a local geological map (Béchennec, 2007, Fig. 1), borehole descriptions available in the national geological database, and a departmental soil geochemical study [START_REF] Le Guern | METOTRASS: méthodologie optimisée pour l'évaluation des teneurs en éléments trace (As, Pb, Cu[END_REF]. Crapaudine and Eglantiers lie on micaschist formations known to present locally mineralized veins that may contain arsenic and/or lead (in association or not) [START_REF] Le Guern | METOTRASS: méthodologie optimisée pour l'évaluation des teneurs en éléments trace (As, Pb, Cu[END_REF][START_REF] Jean-Soro | Origin of trace elements in an urban garden in Nantes, France[END_REF]. Oblates lies on granite that might also present natural anomalies in arsenic [START_REF] Le Guern | METOTRASS: méthodologie optimisée pour l'évaluation des teneurs en éléments trace (As, Pb, Cu[END_REF], but to a lesser extent than the micaschist.

From soil characterization to site management

A multi-actor cooperative approach has been carried out for this study. The overall methodology is summarized in Figure 2 and described hereafter, providing more details on the tasks carried out by scientists. The first step consisted of a preliminary soil characterization commissioned by the city. Should soil contamination be discovered, scientists conducted a more in-depth characterization. This scientific approach included: a synthesis of land use history, determination of a sampling strategy, the sampling itself with a core, and the precise georeferencing of sampling locations using a Differential Global Positioning System (DGPS). On-site measurement using a field portable X-Ray fluorescence analyzer (PXRF; NITON XL3t GOLDD), after sample preparation, was completed by additional ICP-MS analyses in the laboratory for calibration purposes. PXRF allows carrying out a screening of several trace elements on site, leading to adapting the on-site sampling strategy by taking the geochemical and lithological heterogeneity observed in situ into account. Within this framework, the number of sampling points was proportional to the level of heterogeneity. These steps are described in [START_REF] Béchet | Spatial variability of trace elements in allotment gardens of four European cities: assessments at city, garden, and plot scale[END_REF] and [START_REF] Jean-Soro | Origin of trace elements in an urban garden in Nantes, France[END_REF]. Not all plots could be sampled in each UAG (Crapaudine and Eglantiers). Some sampling was also carried out in uncropped zones. The sampling in Oblates was performed over a large area of the park before renewal.

Contaminant mapping relied on Mapinfo 8.5. All results were interpreted in light of historical land-use changes.

The second step focused on the verification of health issues, as required in the case of a questionable geochemical anomaly; soil ingestion, dust inhalation and vegetable ingestion were considered as scenarios of human exposure to contaminants. Direct contact was not considered given that medium-to-low soil contamination was typically observed in the case studies. As scientists, we sampled and analyzed the typical vegetables cropped in the contaminated plots according to the French guide on vegetation sampling [START_REF] Marot | Guide d'échantillonnage des plantes potagères dans le cadre des diagnostics environnementaux, 2ème Edition, coord[END_REF]. Such sampling was impossible in Oblates UAG because the plots were not implemented yet. Trace element concentrations in the edible parts of vegetables were measured by ICP-MS (Varian 820-MS) after careful washing, with preparation similar to cooking practice and aqua regia digestion. The contents obtained were compared with the EC regulation that defines the maximum authorized concentration of 4 trace elements (Pb, Cd, Hg, Sn) in vegetables intended for human consumption (Commission Regulation (EC) No. 1881[START_REF]Setting maximum levels for certain contaminants in foodstuffs[END_REF], European Commission, 2006). For Pb, the threshold value ranges from 0.1 to 0.3 mg kg -1 of fresh matter according to vegetables. The health risk calculation for ingestion scenarios involved a consulting firm or the local health administration, which was also assigned to validate the calculations carried out by consulting firms. For As and Pb, the local health administration established the threshold values to be used in the case of lead or arsenic anomalies (i.e. 100 mg kg -1 and 50 mg kg -1 for Pb and As, respectively), by taking into account national recommendations for lead and the local background for arsenic [START_REF] Le Guern | METOTRASS: méthodologie optimisée pour l'évaluation des teneurs en éléments trace (As, Pb, Cu[END_REF].

In the event of health risks, a third step was launched, consisting of the joint development of a management plan with the various actors (city, administration, residents/gardeners, and scientists) through technical, public and field meetings. During these rounds of meetings, various scenarios were discussed and a cost-benefit assessment was performed by the city.

Depending on the specificity of each site (i.e. spatial distribution of soil contaminants, heterogeneities, origin, transfer to vegetables) as well as the level of sensitivity expressed by the gardeners, city and administration, either a single option or a set of management options was implemented for each garden. These options included soil replacement, preservation or restoration using conventional or alternative techniques.

Results and discussion

Site specificities

Trace elements (As, Cu, Pb, and Zn ) are the main contaminants observed in Nantes' eight UAGs, with Pb being the principal contaminant. Tables 1 and2 show the concentrations for As and Pb in the topsoil of the Eglantiers and Crapaudine gardens and in the Oblates park. In most cases, Pb (for the three sites) and/or As (only for Eglantiers) contents were above acceptable sanitary levels or above the local baseline level (i.e. 100 mg kg -1 and 50 mg kg -1 for Pb and As, respectively). A comparison of the percentiles shows that Pb concentrations vary significantly: from 28 to 371 mg kg -1 . The spatial distribution (Figs. 3a and3b) confirms this strong heterogeneity of Pb concentrations in the topsoil of the Eglantiers and Crapaudine gardens. For Oblates, lead shows moderate to high levels of anomaly over the entire park (Fig. 3c), as compared to usual contents in French agricultural soils (maximum at 25 mg kg -1 for As and 60 mg kg -1 for Pb, [START_REF] Baize | Pb, Se, Zn) dans les sols agricoles en France[END_REF].

For Eglantiers (Fig. 3a), the geological context better explained the anomalies than land use history. Indeed, the central plots exhibit high Pb concentrations (100-500 mg kg -1 ) in topsoil, increasing with depth (until 1,000 to 5,780 mg kg -1 at 60 to 100 cm of depth), associated with high arsenic concentrations (up to 384 mg kg -1 ); these are of a geogenic origin due to a mineralized vein in the micaschist substratum [START_REF] Jean-Soro | Origin of trace elements in an urban garden in Nantes, France[END_REF].

As for Crapaudine (Fig. 3b), high concentrations (100-200 mg kg -1 ) are observed in the southeastern part of the garden, on topsoils only. The study of past land uses of this garden zone, from historical data and aerial photographs, suggests that these high concentrations are due to the use of plant protection products or materials in relation to more intensive market gardening activities. For the Oblates site, the presence of lead at the surface (Fig. 3c) is attributed to the impact of a former lead smelter located close to the park under prevailing winds.

Soil pollution management options to maintain (or allow) gardening

Presented herein are the options selected to maintain (or allow) gardening activity.

The primary conventional option introduced was soil input. Such was the case at the Eglantiers garden: given that Pb content was increasing with depth, fresh soil was imported onto the central part of this site (on a thickness of 80 cm). This new soil originated from a nearby site, where soil excavation was necessary for development purposes. Prior to application, the soil quality (geochemistry) was verified for the absence of geogenic or anthropogenic elements/contaminants (e.g. Pb < 60 mg kg -1 and As < 25 mg kg -1 ). Marker tape was placed between the original and new soil. Even though the municipal Parks and Open Space Department carried out the work itself, this solution proved to be quite costly (approx. €200,000 to manage roughly 5,000 m², i.e. €40/m²).

Soil input was also chosen for both the Crapaudine and Oblates sites. Only a small number of plots (about 10) in the Crapaudine garden exhibited lead concentrations above 100 mg kg -1 MS. Since a stockpile of excavated soil was available on-site, the local authority decided to replace the top layer of the cultivated soil in the plots by this stockpiled soil. Before application, the added soil was tested for its trace elements content (low level, e.g. 59 ± 6 mg kg -1 of Pb); moreover, its origin was known and assessed to be suitable as a fertile soil. This soil management solution was implemented before the growing season without causing any break in the cultivation calendar. The topsoil layer was stripped at a 40-cm thickness covering the entire plot surface. Marker tape was placed over the surface, and the new soil was spread out. The weakly contaminated soil was stored close to the garden prior to a risk-free future use. The work was completed in 4 days by 2 employees with the City's Open Space Department. This soil management solution therefore proved to be a "low-cost" option (less than 10 €/m²) since uncontaminated soil was available on-site. At Oblates' park, soil was only replaced in the future gardening zone. The excavated topsoil (30 cm), featuring a medium Pb content (103-125 mg kg -1 MS), was stored in a single location for flower growing. Another option pursued in other gardens not detailed here consisted of transitioning to raised bed culture.

At the Eglantiers garden, a secured cropping system with vegetables unable to accumulate Pb and associated with Indian Mustard (Brassica juncea) (Bouquet et al., submitted), previously used in studies devoted to phytoextraction, was tested as an alternative and innovative solution in order to avoid plot closure and soil replacement, in light of the high agronomic quality. Carried out on 4 plots, this particular solution also involved the gardeners. This in situ and participatory 2-year experiment was launched in July 2015. Vegetables and Indian mustard (Brassica juncea cv. Vitamine), used for phytoextraction, were cultivated in intercropping or as crop co-cultures and then compared to a control (without phytoextraction).

Most vegetables showed that Pb concentrations in the edible parts of tomatoes, winter cabbage, leeks and potatoes were below the EC regulatory threshold, which was set at 0.1 and 0.3 mg kg -1 (for cabbage and leeks) of fresh matter, respectively (Fig. 4). Pb concentrations in green bean pods were also below the threshold (0.2 mg kg -1 ), except for the two sub-plots 6JL and 3CD with the lowest biomass production (7.25 times lower compared to the others, Fig. 4). Detailed results of these experiments are presented in Bouquet et al. (submitted).

Regarding phytoextraction efficiency, Pb concentrations in shoots of B. juncea are very low (approx. 2 mg kg -1 dw in above-ground parts). Geogenic Pb availability (as evaluated by BCR sequential extraction), which is about half of that measured in a similar soil with the same Pb concentration yet of anthropogenic origin, partially explains this low phytoextraction efficiency [START_REF] Bouquet | Brassica juncea tested on urban soils moderately contaminated by lead: Origin of contamination and effect of chelates[END_REF]. These same authors showed that by enhancing Pb mobility Since EDTA used to enhance the phytoextraction rate is toxic for plants at millimolar concentrations and even reduces growth at micromolar concentrations (Shahid et al., 2012), bioaugmentation-assisted phytoextraction might be a relevant alternative [START_REF] Lebeau | Performance of bioaugmentation-assisted phytoextraction applied to metal contaminated soils: a review[END_REF].

Options leading to land use evolution

The management solutions presented in Section 3.2 are aimed at maintaining the land use (gardening) of the various sites. In several cases however, gardening was abandoned within specific parts (most heavily contaminated). Within this framework, NBS were proposed as much as possible to replace previous gardening activities. For example, for some plots, the land use (gardening) was modified to an orchard, open space or ornamental space. Figure 5 summarizes the various options identified and tested in the 8 contaminated gardens indicated in Figure 1. In several gardens, a number of management options were actually implemented, as depicted in Figure 6 for the Eglantiers and Oblates gardens. Although the number of experiments remains low, this case study has provided an overview of several possible options.

Role of NBS in managing low-to-moderate soil contamination in UAGs

Table 3 summarizes the various management options applied at Nantes' 8 contaminated UAGs plus an additional UAG within the urban zone (not presented in this paper). This table compares the duration, impacts of solutions (economic, social and environmental), the potential for integrating the principles of circular economy, and the city's or owner's responsibilities.

The decision to frequently change soil may be explained by the fact that this choice limits the site owner's responsibility in terms of health impacts, in addition to allowing for fast implementation. However, two issues emerge: i) the availability and quality of the marketed topsoil used for soil replacement; and ii) the low-contaminated soils that, once excavated and removed from the site, become waste and cannot be reused as soil except in very specific uses (e.g. road base course). In addition, environmental impacts are pronounced (e.g. CO 2 emissions due to soil transportation, soil consumption), and such solutions appear to be costly. This cost may nevertheless be reduced by applying the principles of a circular economy (soil reuse). NBS appear as options to be considered in the case of low-to-moderate contamination. In particular, phytoextraction is the only method available for the in situ remediation of soils contaminated by trace metals. Considered as an environmentally-friendly solution, this approach unfortunately requires many years to achieve a state of soil cleanup. For this reason, phytoextraction has yet to gain widespread popularity. When taking this constraint into consideration, the association of non-accumulating vegetables and metal-accumulating crops appears to offer a promising solution as a means of securing cropping on contaminated soils.

An alternative option consists of soil excavation followed by building up temporary vegetated stockpiles that contribute to urban landscape quality (Todd et al., 2016;Wilschut et al., 2013). Once cleaned up, the soil could be reused in various ways. Phytostabilization can also be practiced in order to avoid the dissemination of contamination, but in this case the contaminants are not removed. Raised beds with imported soil constitutes another technique for urban gardens to avoid or reduce exposure to contaminants (ingestion of soil or after plant uptake). The raised beds solution requires less capital investment than remediating or importing soil [START_REF] Clark | Urban gardens: Lead exposure, recontamination mechanisms, and implications for remediation design[END_REF]Witzling et al., 2011;[START_REF] Pfeiffer | Innovation in urban agricultural practices: Responding to diverse production environments[END_REF][START_REF] Kessler | Urban Gardening: Managing the Risks of Contaminated Soil[END_REF].

Green space and ornamental space are other options, although they lead to a change in use.

Urban orchards are defined as tree-based food production systems that can be owned and run either privately or by the community to provide wildlife habitat [START_REF] Lin | The future of urban agriculture and biodiversityecosystem services: Challenges and next steps[END_REF]. They can also increase vegetative cover, which is helpful in combating soil erosion (phytostabilization solution). Ornamental plants expand in UAG [START_REF] Lin | The future of urban agriculture and biodiversityecosystem services: Challenges and next steps[END_REF] depending on the cultural habits of gardeners (Bell et al., 2016). This new habit can then be practiced to manage polluted plots. Well located and well equipped (with urban furnitures such as seating, play areas), green common areas in UAGs exert a strong impact on the relations between gardeners (Bell et al., 2016). Ornamental, open space and shipyards also appear to be feasible solutions; they prove to be quite attractive from a quality-of-life perspective. While they do not alter soil functions, they still require the implementation of precautions by the city/owner in order to limit their liability as regards sanitary risks (e.g. exposure to dust from soils).

These solutions may also be applied in other contexts than UAGs. For instance, phytoextraction may be used to restore geochemical quality and increase the multifunctionality of soils; it could contribute to managing low-level contaminated excavated soils through ornamental spaces in the public or private space. More specifically, it could be introduced when creating new districts or within the scope of redeveloping existing districts, such as in a polluted area of Amsterdam's harbor (Wilschut et al., 2013) and in the case of polluted vacant lands in Canadian municipalities (Todd et al., 2016). Developers and urban planners would thus need to anticipate such ex situ treatment in the planning and development process, and regulations would need to evolve to allow for such an option.

It is also interesting to note that the city now systematically inspects the environmental quality of the soil of planned UAGs early enough in the planning process to adapt the initial plan according to the soil geochemical quality.

NBS thus appears to be attractive in managing low-to-moderate levels of soil contamination.

However, UAGs or land owners / the city needs to be aware of their legal responsibilities.

They must ensure limiting the exposure to soil (in particular ingestion of soil or dust, [START_REF] En Denys | Bioaccessibility of lead in high carbonate soils[END_REF][START_REF] Pascaud | Environmental and health risk assessment of Pb, Zn, As and Sb in soccer field soils and sediments from mine tailings: solid speciation and bioaccessibility[END_REF][START_REF] Kessler | Urban Gardening: Managing the Risks of Contaminated Soil[END_REF] as a means of mitigating health risks.

Grassing is one solution to avoid soil sealing, and phytostabilization is another appealing option to minimize soil erosion and trace element transfer. In addition to controlling health risks, communication outreach to the public constitutes another challenge. Along these lines, a socio-scientific approach is useful in helping managers face such challenges (Mombo et al., 2016). Success story examples point to the political transparency of the stakeholders and partnerships closely involving the city of Nantes, scientists, gardeners and other actors.

Conclusion

Environmental investigation within UAGs may reveal soil contamination, Pb being the primary contaminant detected in French [START_REF] Béchet | Spatial variability of trace elements in allotment gardens of four European cities: assessments at city, garden, and plot scale[END_REF] and in Nantes' UAGs. For three representative gardens in Nantes, detailed management scenarios have been proposed. The various options available for managing polluted soils, including several NBS (as carried out in 8 of Nantes' UAGs), can be applied more generally to cope with soils containing low-tomedium contaminant levels. Within this framework, a key challenge consists of controlling health risks. In addition to improving the quality of residents' life and biodiversity, several solutions make it possible to preserve or even restore soil functions.

Moreover, anticipating soil quality allows adapting the planning process to ensure compatibility with land uses while limiting soil management expenses.

  in the soil with EDTA, the Pb concentration increased in B. juncea shoots by up to 26 times compared to the control sample. Phytoextraction efficiency by B. juncea remains low on moderately Pb-contaminated soils and might not be the best candidate in such a context. With a maximum of 45 mg Pb kg -1 dw in shoots, 604 years are needed to remove the non-residual Pb, but much less if just the bioavailable fraction of Pb is considered (DTPA extraction).

Fig. 1 :

 1 Fig. 1: Location of the studied gardens within the city of Nantes, with emphasis on geological and historical contexts -allotment gardens requiring specifications on contamination issues are indicated along with historical land use (adapted from Béchet et al., 2016) (geological map 1/50 000,[START_REF] Béchennec | Carte géologique harmonisée du département de Loire-Atlantique[END_REF]

Fig. 5 :

 5 Fig. 5: Summary of the options adopted in 8 contaminated UAGs managed by the city of Nantes (France) in the objective of maintaining gardening as much as possible. A partial change of use was sometimes necessary: green space includes ornamental or open space.

Fig. 6 :

 6 Fig. 6: Example of multiple contaminated soil management options, including NBS for Eglantiers (a) and Oblates (b) UAGs

Table 1 :

 1 As concentrations (mg kg -1 ) in topsoil samples collected at Eglantiers garden (PXRF measurement) (Nantes, France)

Table 2 :

 2 Pb concentrations (mg kg -1 ) in topsoil samples collected at a selection of representative sites (PXRF measurement) (Nantes, France)

Table 3 :

 3 Various management options applied at Nantes' 8 contaminated UAGs plus an additional UAG within the urban zone

Table 1 :

 1 As concentrations (mg kg -1 ) in topsoil samples collected at Eglantiers garden (PXRF measurement) (Nantes, France)

		Number of
	Site	measurement	Min	25%	Median	75%	Max	Mean
		points
	Eglantiers	95	19	25	29	39	92	36

Table 2 :

 2 Pb concentrations (mg kg -1 ) in topsoil samples collected at a selection of representative sites (PXRF measurement) (Nantes, France)

		Number of
	Site	measurement	Min	25%	Median	75%	Max	Mean
		points
	Eglantiers	95	28	50	84	126	371	98
	Crapaudine	82	34	44	53	69	134	61
	Oblates	31	71	87	117	127	272	119

Acknowledgments

The authors would like to thank both the French National Research Agency (JASSUR Project -ANR-12-VBDU-0011), the Pays de la Loire Region (POLLUSOLS project) and the City of Nantes (Marie-France Ringeard) for their support. Our gratitude extends to all colleagues involved in this work, especially Laurent Lebouc and Alice Biczysko (IFSTTAR technical staff).

** Soil excavation and its local use in redevelopment projects as an alternative, with the prospect of reusing the soil for cropping in allotment gardens.


Issam Mjejri

Romain Grocassan

Aline Rougier
email: aline.rougier@icmcb.cnrs.fr

Enhanced Coloration for Hybrid Niobium based Electrochromic Devices

Keywords: Polyol synthesis, Nanoparticles, Niobium oxide, Dip Coating, Electrochromic performances, Hybrid Device

Niobium pentoxide, Nb2O5, exists in many polymorphs with diverse properties and morphologies that are dependent on the synthesis route. In this work, we design a new approach to fabricate and stabilize the metastable polymorph hexagonal nanoniobium pentoxide using soft chemistry and in particular the polyol synthesis. The electrochromic properties of homogeneous niobium pentoxide thin films deposited by Dip Coating, from as-synthesized hexagonal powder, are studied in lithium trifluoromethanesulfonimide (LiTFSI) in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) ionic liquid electrolyte versus Pt as counter electrode. The structure and morphology of the Nb2O5 thin films before and after cycling are characterized by ex-situ XRD and SEM while their optical and electrochemical properties are investigated by in-situ transmittance and cyclic voltammetry (CV), respectively. Thin films made from hexagonal Nb2O5 with spheres architecture exhibit good cycling stability, high transmittance modulations (ΔT≈33% at 550 nm), fast switching speeds (2 to 3 s for coloration and bleaching at 550 nm), and high coloration efficiency (CE≈26 cm 2 .C -1 ) with a good quality factor G(λ) = 10.4 cm 2 .C -1 .s -1 . Finally, novel electrochromic devices based on hybrid thin films combining PEDOT:PSS and oxides are assembled showing enhanced coloration and simultaneous color change due to their double sided configuration.

I.

Introduction

Currently, more than 40% of the energy consumption is in buildings either for heating or cooling, but this phenomenon is always accompanied by a rejection of carbon dioxide, CO2. To improve the energy efficiency and reduce the emission of CO2 into the atmosphere, smart windows, often based on electrochromic materials, offer a promising solution. Electrochromic (EC) materials are characterized by the ability to change their optical properties, when a voltage is applied. [START_REF] Yang | Lithium-ion Intercalation and Electrochromism in Ordered V2O5 Nanoporous Layers[END_REF][START_REF] Qu | One-pot Preparation of Crystalline-Amorphous Double-Layer Structured WO3 Films and their Electrochromic Properties[END_REF][START_REF] Mjejri | Nano-Vanadium Pentoxide Films for Electrochromic Displays[END_REF] Electrochromism occurs in a number of organic [START_REF] Granqvist | Handbook of Inorganic Electrochromic Materials[END_REF] , inorganic [START_REF] Beaujuge | The Donor-Acceptor Approach Allows a Black-to-Transmissive Switching Polymeric Electrochrome[END_REF] and hybrid materials [START_REF] Ding | Conductivity Trends of PEDOT-PSS Impregnated Fabric and the Effect of Conductivity on Electrochromic Textile[END_REF] , allowing to address multiple applications either in a transmittive mode (smart windows…) or reflective one (dis-plays….). In comparison to others, the family of inorganic oxides is commonly known for its good stability. Another advantageous specificity lies in the facility to improve the electrochromic properties by single or double doping or by addition of other materials and formation of composites . [START_REF] Arvizu | Electrochromic W1-x-yTixMoyO3 Thin Films Made by Sputter Deposition: Large Optical Modulation, Good Cycling Durability, and Approximate Color Neutrality[END_REF][START_REF] Gesheva | Optoelectronic Properties of CVD MoO3 and MoO3-WO3 Filmsand Their Applications in Electrochromic cells[END_REF] Since the discovery of electrochromism in Nb2O5 in 1980, by Reichman and Bard 9 , niobium oxide has been studied extensively and has come up as a promising electrochromic material in respect of chemical stability in many electrolytes, nice reversible colors switching and long-term cycling stability. [START_REF] Zhang | Synthesis and Electrochemical Eroperties of Niobium Pentoxide Deposited on Layered Carbide-Derived Carbon[END_REF][START_REF] Pawlicka | Synthesis of Multicolor Nb2O5 Coatings for Electrochromic Devices[END_REF] . Niobium element characterized by several valence states from +2 to +4 to +5 for NbO, NbO2 and Nb2O5, respectively [START_REF] Dash | A Simple Growth Method for Nb2O5 Films and their Optical Properties[END_REF] , exists in various oxides polymorphic forms including monoclinic (H-Nb2O5), tetragonal (M-Nb2O5), orthorhombic (T-Nb2O5), and pseudo hexagonal (TT-Nb2O5). The different crystal phases of Nb2O5 depend on the synthesis temperature. For example a low temperature (≈200 °C) leads to an amorphous phase further crystallizes at 500 °C into hexagonal or orthorhombic phases, while at medium-temperature (≈ 800 °C) niobium oxide transforms into the tetragonal phase and forms the monoclinic phase above 1000 °C [START_REF] Viet | Nanostructured Nb2O5 Polymorphs by Electrospinning for Rechargeable Lithium Batteries[END_REF][START_REF] Rani | Thin Films and Nanostructures of Niobium Pentoxide: Fundamental Properties, Synthesis Methods and Applications[END_REF] . Herein, we present a novel synthesis route to stabilize the metastable phase (TT-Nb2O5) with ultrafine powders and we demonstrate the electrochromic performance of this metastable allotropic form of niobium pentoxide.

In the literature, a variety of approaches have been employed to fabricate Nb2O5 nanostructures such as hydro/solvothermal [START_REF] He | Hydrothermal Growth and Optical Properties of Nb2O5 Nanorod Arrays[END_REF] and sol-gel, but these methods have the inconvenience of limited reproducibility and scaling up. To solve this issue, new synthesis methods able to produce a large quantity of nanometric powders with high reactivity are highly needed. As one of the solution methods, the polyol synthesis has been initially developed for metals [START_REF] Dong | Polyol Synthesis of Nanoparticles: Status and Options Regarding Metals, Oxides, Chalcogenides, and Non-Metal Elements[END_REF]17 and more recently applied to the synthesis of single-crystalline oxide nanostructures 18 . This low reaction temperature method, in which the polyol serves as both the solvent and the reducing agent, offers several advantages as compared to other chemical methods. Among them a non-exhaustive list contains (i) water-comparable solubility of simple metal-salt precursors; (ii) high boiling points (up to 320 °C); (iii) reducing properties for the instantaneous synthesis of metals; (iv) coordinating properties for surface functionalization and colloidal stabilization of nanoparticles; (v) wide adaptability of the polyols ranging from low-weight ethylene glycol (EG) to high-weight polyethylene glycols (PEGs) (vi) high-quality doped powders with specific morphologies (vii) applicable to large scale manufacturing processes. In summary, this process is ideal for the processing of very fine powders having high purity, high crystallinity, excellent reproducibility, narrow particle size distribution, uniformity, and high reactivity. 19 The electrochromic performance of niobium pentoxide depends on several parameters including the morphology and the porosity, the film thickness, the surface roughness and the deposition method. Various deposition techniques including spray pyrolysis [START_REF] Romero | Electrochromic Behaviour of Nb2O5 Thin Films with Different Morphologies Obtained by Spray Pyrolysis[END_REF] radio-frequency magnetron sputtering [START_REF] Maruyama | Electrochromic Properties of Niobium Oxide Thin Films Prepared by Radio-Frequency Magnetron Sputtering Method[END_REF] , pulsed laser deposition 22 , chemical vapour deposition 23 and Dip-coating techniques [START_REF] Dash | A Simple Growth Method for Nb2O5 Films and their Optical Properties[END_REF][START_REF] Rosario | Optimisation of the Electrochromic Properties of Nb2O5 Thin Films Produced by Sol-Gel Route Using Factorial Design[END_REF] have been used to prepare Nb2O5 thin films. Among them, Dip Coating processes present the advantage of high versatility, low cost, reasonable deposition rates, and simplicity in experimental parameter modulation. In this study, niobium pentoxide (Nb2O5) thin films were dip coated from nanopowders synthesized from a single polyol route. Herein, their electrochromic activity in lithium-based electrolyte is discussed in terms of optical contrast, cyclability, and stability. Their nice EC properties allow further integration of this oxide in complete devices optimized in respect of novel architecture and enhanced coloration.

During the recent decade, escaping from the common 5-layers battery type devices, remarkable progress have been achieved in exploring new concepts to enlarge the field of applications for electrochromic devices. Such evolution includes the development of new EC materials as well as novel architectures. For instance, in 2017 our group [START_REF] Mjejri | Double Sided Electrochromic Device based on Metal-Organic Frameworks[END_REF] reported for the first time a double sided electrochromic device based on Metal-Organic Frameworks (MOFs). Mixing the advantages of inorganic and organic materials, hybrids systems offer great potential of exploration. Indeed, while transition metal oxide (TMOs) have generally a good redox activity with high optical properties and good memory effect, they suffer, among other things from a too low electrical conductivity. On the contrary, polymer-based electrochromic films are characterized by rapid kinetics and multicolor electrochromism, while they typically show poor adhesion to the substrate. To counter balance the pros and cons of both types of materials, we suggest mixtures of metal oxide and polymer as good candidates for electrochromic application [START_REF] Roy | Polypyrrole-Vanadium Oxide NaNocomposite: Polymer Dominates Crystallanity and Oxide Dominates Conductivity[END_REF][START_REF] Mishra | Organic Nanostructures on Inorganic Ones: An Efficient Electrochromic Display by Design[END_REF] . The benefit of such approach has been recently demonstrated through various examples. Among those hybrid composite materials, mixtures of PEDOT:PSS and WO3 [START_REF] Ling | Layerby-Layer Assembly of PEDOT:PSS and O3 Nanoparticles: Enhanced Electrochromic Coloration Efficiency and Mechanism Studies by Scanning Electrochemical Microscopy[END_REF] or with TiO2 29 are characterized by good properties. Furthermore, the combination of PEDOT:PSS and V2O5 in electrochromic device shows better stability as compared to a pristine PEDOT:PSS-based device [START_REF] Lee | Organic Photovoltaic With PEDOT:PSS and V2O5 Mixture as Hole Transport Layer[END_REF][START_REF] Alhummiany | XPS Analysis of the Improved Operational Stability of Organic Solar Cells Using a V2O5 and PEDOT:PSS Composite Layer: Effect of Varied Atmospheric Conditions[END_REF] . Recently, Mishra and al., have shown that TiO2-Co3O4 coreshell has a good electrochromic behavior and superior supercapacitive poroperties. [START_REF] Mishra | TiO2-Co3O4 Core-Shell Nanorods: Bifunctional Role in Better Energy Storage and Electrochromism[END_REF] In the same context, Zhang and al., reported that the organic-inorganic nanocomposites (ATO/TiO2@PANI) [START_REF] Zhang | Enhanced Electrochromic Performances of Polyaniline-Coated Rod-Like ATO/TiO2 Nanocomposites[END_REF] present better electrochromic performance compared with pure PANI film. According to the authors the improved electrochromic properties are mainly attributed to the core/shell structure, formation of the donor-acceptor system. Based on this finding, we successfully combine two hybrids films (namely Nb2O5 -PEDOT:PSS and V2O5-PEDOT:PSS ) to build for the first time a hybrid electrochromic device with dual face.

II. Experimental details

II.1. Polyol synthesis of Nb2O5 powder and characterization

All of the chemical reagents were purchased from Acros Organics and utilized without further purification. Ammonium niobate (V) oxalate hydrate was used as niobium source and diethylene glycol (DEG) as template. In a typical synthesis, C4H4NNbO9. H2O (7.574 g) was added to 250 mL of C4H10O3. The resulting mixture was heated to 110 °C with continuous stirring to give a white solution that was refluxed at 160 °C for 1 h. At the end of the reaction, a white precipitate was obtained. The precipitate was centrifuged and washed several times with ethanol to remove the organic product and dried in an oven at 80 °C. To prepare the final Nb2O5 powders, the precipitates were calcined for 1 h at 500 °C. The powder structure was characterized by X-ray diffraction analysis (Philips PW 1820, PANalyticalX'Pert instrument, 2θ range from 15 to 60° and λCuKα1 =0.154056 nm). The unit cell parameters were refined by structural pattern matching using the Fullprof program package. Transmission electron microscopy (TEM) images were recorded with a SEI instrument (operating at 5 kV) microscope.

II.2. Film preparation and characterization

Nb2O5 films were deposited by the dip coating method from homemade powder, synthesized by the polyol process. Firstly, 2.5 g of powder was dispersed into 5 ml ethanol. The resulting dilute was stirred for 1 h at room temperature and after aliquots of the as-prepared colloidal Nb2O5 solution were deposited on the ITO (In2O3:Sn) coated glass (ccommercialized by SOLEM with a resistance of 30 Ω/ ). The thickness of the Nb2O5 films, measured using a Dektak mechanical profilometer, was of about 800 nm ± 50 nm. For the identification of the crystal phases of the thin films before and after cycling, X-ray diffraction (XRD, Philips PW 1820, PANalyticalX'Pert instrument, 2θ range from 15 to 60° and λCuKα1 =0.154056 nm) was used.

The morphology of the layers before and after cycling was investigated using a JEOL JSM-840 (operating at 15 kV) microscope.

II.3. Electrochromic measurements

Electrochemical measurements of Nb2O5 thin films on ITO/glass were carried out in a three electrodes cell configuration using a BioLogic SP50 potentiostat/galvanostat apparatus. The counter-electrode and reference electrode consisted of a platinum foil and Saturated Calomel Electrode, SCE (ESCE = 0.234 V/ENH), respectively. The operating voltage was controlled between -1.1 and -0.1 V at a scan rate of 20 mV/s, in lithium based electrolyte, namely, lithium bis-trifluoromethanesulfonimide (LiTFSI, Solvionic, purityN99.99%) in 1etyl-3-methylimidazoliumbis-(trifluoromethanesulfonyl)-imide (EMITFSI), purity N 99.99%). All the electrochemical measurements were performed at room temperature. The optical transmittance of Nb2O5 thin films was measured in-situ using a Varian Cary 5000 UV-Vis-NIR spectrophotometer.

III. Results and discussion

III.1. Polyol synthesis of Nb2O5 powder

The crystallinity of the powder synthesized by the polyol process pre-and post-calcined under air was analyzed by XRD (Figure 1). Prior to calcination, the XRD pattern (Figure 1a), with no visible peaks, corresponds to an amorphous phase, in agreement with R.A. Rani et al. [START_REF] Rani | Thin Films and Nanostructures of Niobium Pentoxide: Fundamental Properties, Synthesis Methods and Applications[END_REF] , reporting amorphous Nb2O5 at low temperature (≈200 °C). As the temperature of the calcination increases, niobium oxide becomes crystallized. At 400 °C, (Figure 1b), the XRD pattern reveals the presence of a predominantly amorphous phase with the appearance of a single peak characteristic of the hexagonal phase of Nb2O5. An increase in temperature to 500 °C (Figure 1c) leads to a well crystallized phase with narrow X-ray peaks. The d-spacing values of all diffraction peaks are identical to those of the hexagonal crystalline phase Nb2O5 with lattice constants a = b = 3.6070 Å, and c = 3.9250 Å (JCPDS # 28-0317). The absence of any additional peaks from other phase or impurity indicates that Nb2O5 with high purity can be synthesized via the polyol synthesis at 160 °C for 1 h. The hexagonal phase remains predominant after a post-thermal treatment at 600 °C (Figure 1d) with the appearance of some small peaks characteristic of orthorhombic Nb2O5.

From the temperature study, a treatment of the powders obtained by polyol process at 500 °C for 2 h under air was established as optimal conditions to synthesize the metastable Nb2O5 hexagonal phase with high crystallinity chosen for further investigation. The average crystallite size was calculated using the Scherrer's formula:

L = 0.89λ=β cosθ
Where L is the average crystallite size in nm, λ=0.154056 nm, β is the full width at the half maximum, and θ is the diffraction angle. The average crystallite size value, calculated from three of the main peaks of the XRD pattern, is about 9 nm. To complete the structural studies, refinement using an isotropic peak profile function (Caglioti function) was carried out on the hexagonal Nb2O5 compound (Figure S1). Unit-cells parameters and the reliability factors (Rp, Rwp, and Rexp) are summarized in Table S1. The isotropy of the structure well agrees with the isotropy of the morphology.

III.2. Electrochromic properties of Nb2O5 thin films

III.2.1. Cyclic voltammetry (CV) measurements

An important requirement of electrochromic materials is their stability upon cycling while keeping their structural integrity. The repetitive cycling of Nb2O5 thin film in LiTFSI-EMITFSI ionic liquid at a scan rate of 20 mV/s between -1.1V as -0.1V shows good reversibility and cyclability (Figure 3a and inset Figure 3a). No significant degradation corresponding to only 1 % decrease of the capacity after 100 cycles (inset Figure 3a) confirms this good stability. Upon cathodic and anodic sweeping, the transparent films show one-step electrochromism from transparent to blue (Figure 3b). The color change is accompanied by the appearance of a well-defined oxidation peak located at -0.63V. During the cathodic scan a simultaneous insertion of electrons and Li + ions into the Nb2O5 film causes the reduction of Nb 5+ to lower valence state, Nb 4+ and hence the film coloration. In its reduced state, the film appears blue. During the anodic scan, the deinsertion of Li + ions causes the bleaching of the film that becomes transparent. As reported in the literature [START_REF] Rosario | Optimisation of the Electrochromic Properties of Nb2O5 Thin Films Produced by Sol-Gel Route Using Factorial Design[END_REF][START_REF] Bueno | Electrochromic Properties of Undoped and Lithium Doped Nb2O5 Films Prepared by the Sol-Gel Method[END_REF] , the redox chemical reaction associated with coloration and bleaching in Nb2O5 films can be schematized as follows:

Nb2O5+ xe -+xLi + LixNb2O5
The number of exchanged Li + (x) involved during each coloration/bleaching process was estimated using the following equation: Q = F*n(Nb2O5)*x, where Q is the electrochemical capacity, F =96500 C, and n is number of moles. Assuming a density close to 100 %, x value of about 2 was calculated suggesting the participation of all Nb ions in the Nb 5+ to Nb 4+ redox process.

After 100 cycles, negligible shift in the position of the oxidation peaks was observed. The origin of such deviation remains unclear but might be ascribed to a modification of the film morphology upon cycling.

III.2.2. Optical properties

CE= ∆OD/Q

Where, Q is the amount of charge transferred per unit area. From calculated optical density (∆OD= 0.205) and the amount of charge (Q= 0.008 C/cm 2 ), a coloration efficiency 26 cm 2 /C is obtained. Such values nicely compared with values reported in the literature [START_REF] Bueno | Electrochromic Properties of Undoped and Lithium Doped Nb2O5 Films Prepared by the Sol-Gel Method[END_REF][START_REF] Mujawar | Electrochromic Properties of Spray-Deposited Niobium Oxide Thin Films[END_REF] (Table 1).

The switching optical response in between alternating potentials is a key characteristic of electrochromic materials. The switching kinetics of Nb2O5 thin film was deduced from monitoring the transmittance at a wavelength of 550 nm under the application of potentials window of -1.1 V and -0.1 V at 10 mV/s. The response times, slightly higher in the bleaching process (3 s) than in the colored one (2 s), due probably to the transition from the semiconductor (Nb2O5) to conductor (LixNb2O5) while the process is faster from a conductive to a semi-conductive behavior, remain of few seconds. To appraise the overall EC performances of the Nb2O5 films, we calculated a quality factor G(λ) using the relation: [START_REF] Liu | Highly Stable and Flexible ITO-Free Electrochromic Films with Bi-Functional Stacked MoO3/Ag/MoO3 Structures[END_REF] G(λ)= CE(λ)/𝛕𝛕 , 𝛕𝛕=( 𝛕𝛕c+ 𝛕𝛕b)/2

The value of G(550 nm) for a Nb2O5 thin films is 10.4 cm 2 .C -1 .s -1 . This value, not reported in the literature to our knowledge for Nb2O5, is 2.54 times as high as the one of typical electrochromic oxide WO3 [START_REF] Liu | Highly Stable and Flexible ITO-Free Electrochromic Films with Bi-Functional Stacked MoO3/Ag/MoO3 Structures[END_REF] . In summary, the comparison of the electrochromic characteristic of Nb2O5 thin films with the ones reported in the literature confirm the benefit of the polyol synthesis which leads to very fine and very reactive powder showing higher performances (Table 1).

Ref. ΔT (%) CE (cm 2 .C -1 ) tc (s) tb (s)

Nb2O5 [START_REF] Coskun | The Effects of Heat Treatment on Optical, Structural, Electrochromic and Bonding Properties of Nb2O5 Thin Films[END_REF] 25.4 4.63 ------Nb2O5 [START_REF] Mujawar | Electrochromic Properties of Spray-Deposited Niobium Oxide Thin Films[END_REF] ----13 3.7 4.7 Nb2O5 [START_REF] Rosario | Optimisation of the Electrochromic Properties of Nb2O5 Thin Films Produced by Sol-Gel Route Using Factorial Design[END_REF] ----22 ------Nb2O5 [START_REF] Maruyama | Electrochromic Properties of Niobium Oxide Thin Films Prepared by Radio-Frequency Magnetron Sputtering Method[END_REF] ----

III.3. Structure and morphology of the Nb2O5 films before and after cycling

The XRD pattern of the Nb2O5 film deposited by Dip-Coating is shown in Figure 5a. The d-spacing values of all diffraction peaks match the ones of the hexagonal crystalline phase (JCPDS# 007-0061) and of the ITO substrate (JCPDS # 44-1087). No major change in XRD peak position compared to Nb2O5 powders and no peaks of any other phase or impurity are detected after deposition on ITO substrate, demonstrating a good stability of Nb2O5 nanoparticles synthesized by the polyol method using these deposition conditions. Figure 5b and5c shows the XRD of the Nb2O5 film in reduced and oxidized state, after 50 cycles. All peaks are indexed to the crystal phase of hexagonal Nb2O5 indicating a remarkable stability upon repeated lithium insertion and deinsertion. A close examination of the XRD patterns upon cycling reveals an increase in the width at half height with cycling while in the reduced state the slight shift of the peaks towards low theta values well agrees with an insertion of lithium and the formation of LixNb2O5 (Figure S2). The crystallite size, calculated using the Scherrer formula, for the Nb2O5 film as deposited and reduced/oxidized Nb2O5 film after 50 cycles is about 30, 20 and 15 nm, respectively. The SEM analysis of the Nb2O5 film before cycling (Figure 5d) shows that the Dip Coating deposition leads to homogeneous films of uniform particles with smooth surfaces being relatively densely covered and containing many small cracks. Such morphology, exhibiting a large surface exchange with the electrolytes, appears well suitable for the alkaline elements insertion. SEM imaging of the Nb2O5 film after 50 cycles (Figure 5e) shows a significant change in the morphology. After cycling, the morphology of Nb2O5 film consists of an assortment of small particles that agglomerated giving places to large particles of indefinite forms. Besides the presence of visible voids indicates that upon cycling the electrolyte penetration may be progressively facilitate, promoting the insertion/deinsertion of lithium ions in the Nb2O5 matrix.

III.4. Hybrid Electrochromic Devices

III.4.1. Fabrication of hybrid electrochromic Device

Using the common 5-layer type battery device, an electrochromic device consisting of two hybrid electrochromic films separated by an electrolyte was assembled. V2O5/PEDOT:PSS and Nb2O5/PEDOT:PSS have been employed as the two electrodes and the 60% of (LiTFSI-BMTFSI blended with PMMA) and 40% of TiO2 (Sigma Aldrich) is used as the electrolyte membrane. The same protocol was utilized to prepared both (Oxide/PEDOT:PSS) electrodes. 70% of oxide nanopowders (Nb2O5 or V2O5) synthesized by polyol process were mixed with 30% PEDOT:PSS (Sigma Aldrich) in ethanol as solvent (this mass ratio between oxides and PEDOT:PSS was optimized to get the highest contrast in the visible) and then the mixture was stirred for 3 h. After the preparation of the suspension, the dip coating method was used to deposit the Nb2O5 (or V2O5)/PEDOT:PSS layer on ITO. The two hybrids films were dried in air at room temperature. The ionic electrolyte membrane was prepared by mixing 60% of (LiTFSI-BMTFSI blended with PMMA) and 40% of TiO2 and then the mixture was stirred for 12 h (Figure S3). The hybrid composite [Nb2O5+PEDOT:PSS] film was cycled in the LiTFSI-EMITFSI electrolyte for five cycles and assembled in the blue reduced state to the precycled orange hybrid composite film [V2O5+PEDOT:PSS].

III.4.2. Electrochromic performance of the hybrid Device

Figure 6a shows the superposition of the CVs for the two types of devices: (i) inorganic device LixNb2O5/electrolyte/V2O5 and (ii) hybrid device Lix[Nb2O5+PEDOT:PSS/electrolyte/V2O5+PEDOT:PSS] cycled at room temperature in ionic liquid at 10 mV/s. The combination of both cyclic voltammetry as well as in-situ reflectance shows higher electroactivity for the hybrid device associated with higher optical contrast. Indeed, the modulation in reflectance of the hybrid device is quite double the one of the inorganic ECD (∆R = 21% for inorganic ECD << 42% for hybrid ECD) (Figure 6b). In the following, we focus our attention on the hybrid device. taken between -0.7 V and 0.8 V, at a cycling rate of 10 mV s -1 (Figure 7a) illustrates a well reversible behavior with good cyclability associated with nice color switches which differ on the two faces. Nb2O5+PEDOT:PSS reversibly changed from bluewhite to dark-blue and V2O5+PEDOT:PSS from orange to blue/green (Figure 7b). The switching kinetics of the hybrid device was investigated by chronoamperometric (CA) measurements. Figure 8 shows current transient (j-t) for the ITO/Nb2O5+PEDOT:PSS/LiTFSI-EMITFSI+TiO2 in PMMA /V2O5+PEDOT:PSS/ITO for potential steps of (+1 V/30 s) and (-1 V/30 s). Both response times are in the order of a few seconds; 3 s for the oxidation and 2 s for the reduction. The optical contrasts of the two faces were evaluated using the L*a*b* color system. In fact, in CIE colorimetric space, the color is represented by three parameters: a luminance axis (L*) and two hue axes Figure 9 shows the in-situ optical reflectance spectra for the bleached and colored states from the double sided hybrid EC device at wavelengths ranging from 300 to 800 nm. The reduced blue-white state (-1V/30 s) and the oxidized dark blue state (+1 V/30 s) (Figure 9a) are associated with reflectance values of about RR ≈ 16% and RO ≈ 60% at 550 nm, respectively, corresponding to a reflectance modulation ΔR of 44%. The in-situ evolution of the reflectance spectra for the second hybrid face (V2O5+PEDOT:PSS) is depicted in (Figure 9b). The reducedgreen state (-1 V/ 30 s) and the oxidized-orange state at (+1/30 s) are associated with reflectance values of about RR ≈ 3% and RO ≈ 26% at 550 nm, respectively, with a total reflectance modulation of ΔR ≈ 23%.

IV. Conclusion

In summary, hexagonal nano-Nb2O5 was synthesized by facile and low-cost method, namely the polyol process.

FIGURE 1 .

 1 FIGURE 1. XRD patterns of the niobium oxide powders synthesized by polyol process (a) pre-calcination and post-calcination at (b) 400 °C, (c) 500 °C and (d) 600 °C under air for 2 h. ( ) correspond to orthorhombic Nb2O5.

Figure 2

 2 Figure2shows the corresponding TEM images of the powders elaborated by the polyol process. The hexagonal Nb2O5 phase is homogenous with particles regularly sized and which display 2D nanospheres morphology, with a radius ranging from 10 nm to 35 nm.

FIGURE 2 .

 2 FIGURE 2. TEM images of hexagonal Nb2O5 powder after thermal treatment at 500 °C for 2 h under air.

FIGURE 3 .

 3 FIGURE 3. The 1 st and the 100 th cyclic voltammograms for the Nb2O5/LiTFSI in EMITFSI/Pt/SCE recorded at 20 mV/s scan rate (a), Corresponding variation of the anodic capacity upon cycling (inset Fig. 3a) and Visual appearance of Nb2O5 films in the bleached and colored states (b).

  The in-situ transmittance upon cycling of Nb2O5/LiTFSI in EMITFSI/Pt/SCE is displayed in Figure 4. The colored-blue state (-1.1 V) and the bleached-transparent state at (-0.1V) are associated with transmittance values of about Tc ≈ 55 % and Tb ≈ 88 % at 550 nm, respectively, corresponding to a total optical transmittance modulation of ∆T ≈ 33%. The electrochromic performance is commonly characterized by the optical density (OD) represented by the logarithmic ratio of the transmittance in the bleached state (Tb) to the transmittance in colored state (Tc) and the coloration efficiency (CE) defined as the change in optical density per unit-inserted charge. The ∆OD and coloration efficiency (CE) are calculated using the two relations: ∆OD=log(Tb/Tc) Where, Tb and Tc are the transmittance of the Nb2O5 film in the bleached and colored states, respectively.

FIGURE 4 .

 4 FIGURE 4. Transmittance curves of the Nb2O5/LiTFSI in EMITFSI/Pt/SCE recorded in situ at 550 nm with voltammetry cyclic range -1.1 V to -0.1V.

FIGURE 5 .

 5 FIGURE 5. XRD patterns of (a) Nb2O5 film as-deposited on ITO/glass substrate (b) ex-situ XRD patterns of the reduced LixNb2O5 film and (c) ex-situ XRD patterns of the oxidized Nb2O5 film after the 50 th cycle. SEM images of (d) Nb2O5 film as-deposited on ITO/glass substrate and (e) ex-situ SEM images of the oxidized Nb2O5 film after the 50 th cycle.

FIGURE 6 .

 6 FIGURE 6. Comparaison of (a) cyclic voltammograms of the inorganic device (LixNb2O5/electrolyte/V2O5) and hybrid device (Lix[Nb2O5+PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS]), (b) the in-situ modulation reflectance of the inorganic device (LixNb2O5/electrolyte/V2O5) and hybrid device (Lix[Nb2O5+PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS]). The cyclic voltammogramms (CVs) of the (Lix[Nb2O5+PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS])taken between -0.7 V and 0.8 V, at a cycling rate of 10 mV s -1 (Figure7a) illustrates a well reversible behavior with good cyclability associated with nice color switches which differ on the two faces. Nb2O5+PEDOT:PSS reversibly changed from bluewhite to dark-blue and V2O5+PEDOT:PSS from orange to blue/green (Figure7b).

FIGURE 7 .

 7 FIGURE 7. Cyclic voltammograms of the ITO/Lix[Nb2O5+ PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS]/ITO hybrid device between -0.7 and 0.8 V with a 10 mV/s scan rate for 100 cycles (a). Schematic of ITO/Lix[Nb2O5 +PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS]/ITO electrochromic device combining Lix[Nb2O5+PEDOT:PSS] and V2O5+PEDOT:PSS films on ITO substrates separated by an electrolyte membrane consisting of LiTFSI-BMITFSI mixture with TiO2 and blended with PMMA (b).

FIGURE 8 .

 8 FIGURE 8. Chronoamperogramms of Lix[Nb2O5 +PEDOT: PSS]/electrolyte/[V2O5+PEDOT: PSS] device at -1 V for 30 s and 1 V for 30 s.

FIGURE 9 .

 9 FIGURE 9. In-situ diffuse reflectance spectra of Lix[Nb2O5 +PEDOT:PSS]/electrolyte/[V2O5+PEDOT:PSS] device at -1 V for 30 s and 1 V for 30 s (a) face based niobium and (b) face based vanadium.

  Nb2O5 electrochromic thin films were successfully deposited on ITO substrate at room temperature by Dip-Coating. Homogenous Nb2O5 films show nice electrochromic properties switching reversibly from white to blue color when cycled in lithium based ionic liquid electrolyte. Improvement of the electrochromic properties of Nb2O5 was achieved by mixing with PEDOT-PSS prior to the fabrication of a novel double-sided device built from Lix[Nb2O5+PEDOT:PSS]/ electrolyte/[V2O5+PEDOT:PSS] assembly. This double-sided electrochromic device exhibited nice optical contrast and fast color switching opening applications in the decorative field for instance.

TABLE 1 .

 1 Comparison of the electrochromic performance of Nb2O5 thin films with literature.

ACKNOWLEDGMENTS

The authors thank Marion Gayot (CNRS, Université de Bordeaux, PLACAMAT UMS 3626) for the TEM images.

ABBREVIATIONS

EC, electrochromic ECD, electrochromic device CE, coloration efficiency ΔE*, optical contrast (LiTFSI), lithium trifluoromethanesulfonimide. (EMITFSI), 1-ethyl-3-methylimidazolium bis (trifluoromethanesulfonyl) imide. PEDOT:PSS, poly(ethylene-3,4-dioxythiophene): poly (styrenesulfonicacid)).

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed description of the Full-pattern matching using isotropic peak profile function of the niobium pentoxide powders synthesized by polyol process and the unit-cells parameters and R reliability factors extracted from the pattern matching refinement of Nb2O5 powders annealed at 500 °C under air for 2 h. A Detailed description of all the experimental procedures to prepare the two hybrids films prior to their assembly in a double-sided device.

AUTHOR INFORMATION

Corresponding Author E-mail: aline.rougier@icmcb.cnrs.fr. ORCID Issam Mjejri: 0000-0001-5313-3198 Aline Rougier: 0000-0002-1340-734X

AUTHOR CONTRIBUTIONS

The manuscript was written through contributions of all authors. The authors declare no competing financial interest.