This is the submitted version of the article: Navas D., Donoso J.P., Magon C., Sotomayor-Torres C.M., Moreno M., Lozano H., Benavente E., González G.. Ammonium hexadeca-oxo-heptavanadate microsquares. A new member in the family of the V7O16 mixed-valence nanostructures. New Journal of Chemistry, (2019). 43. : 17548 - . 10.1039/c9nj02188d. Available at: https://dx.doi.org/10.1039/c9nj02188d NJC View Article Online PAPER View Journal Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. Ammonium hexadeca-oxo-heptavanadate microsquares. A new member in the family of Cite this: DOI: 10.1039/c9nj02188d the V7O16 mixed-valence nanostructures† Daniel Navas, a José Pedro Donoso, b Claudio Magon, b Clivia M. Sotomayor-Torres, cd Mabel Moreno, ef Harold Lozano, b Eglantina Benavente *ag and Guillermo González *h This paper presents a new mixed valence heptavanadate (NH4)2V7O16, obtained from a two-stage treatment in a single container of ammonium metavanadate with hexadecylamine in an acetic medium. The hydrolysis of the precursor under normal conditions leads to an intermediate, layered hybrid nanocomposite, V2O5/alkylamine, which after an in situ hydrothermal treatment is transformed almost quantitatively into an amine-free microcrystalline phase constituted by perfect square microparticles of (NH4)2V7O16. The analysis of composition, structure and morphology of the product points to a mixed valence vanadium oxide with a high content of V(IV) (approximately 73%). The microsquares have a tetragonal structure similar to that of BaV7O16, as well as to those proposed for the VOx/amine hybrid nanocomposite series: nanotubes, nano urchins and the compound (en)V7O16. The results suggest that all these phases belong to the V7O16 family, but that they differ in the amine content, the degree of reduction and the curvature of the network. The feasibility of obtaining flat nanostructures based on V7O16 without Received 29th April 2019, templates, beyond demonstrating the robustness of the structural unit V7O16 in networks with different Accepted 19th June 2019 degrees of reduction and curvatures, clarifies the role of alkylamines in this type of anti-entropic DOI: 10.1039/c9nj02188d supramolecular process. First, the amine provides a stable platform that allows for an orderly reduction of the network under hydrothermal conditions and, when the medium favours the stability of the V–amine rsc.li/njc bond, the alkylamine contributes to the driving force that leads to the curving of the V–O network. Introduction properties and ensuing potential applications.1 The often colourful appearance of vanadium species—the origin of the Vanadium oxide-based compounds have received a great deal element name (Vanadis, Scandinavian Goddess of Beauty)— of attention because of their special physical and chemical denotes a challenging rich redox and coordination chemistry,2 useful for developing materials sensitive to low-energy stimuli which are important for applications, for instance, in chemical a Universidad Tecnologica Metropolitana, Departamento de Quı́mica, and biochemical catalysis, energy storage or sensor devices. P. O. Box 9845, Santiago, Chile. E-mail: ebenaven@utem.cl b Vanadium is a widely distributed element in the biosphere, IFSC, Universidade de São Paulo, P. O. Box 369, 13560-970 São Carlos, SP, Brazil c Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, which as a heavy metal is in general considered a harmful Campus UAB Bellaterra, 08193 Barcelona, Spain pollutant, particularly when there is vanadium(V).3 However, at d ICREA, Pg. Lluı́s Companys 23, 08010 Barcelona, Spain trace levels, vanadium has proven to be essential for some e Max Planck Institute of Microstructure Physics, Weinberg 2, 06120, Halle, living organisms.4 The development of knowledge about Germany f oxidation states, redox chemistry and coordination of vana- Universidad SEK, Instituto de Investigación Interdisciplinar en Ciencias Biomédicas SEK (I3CBSEK), Facultad Ciencias de la Salud, Fernando Manterola 0789, dium species associated with their biological role in enzymes Providencia, Santiago, Chile such as some peroxidases or nitrogenases ones constitutes an g Programa Institucional de Fomento a la I + D + i, Universidad Tecnológica important topic of modern bio-inorganic chemistry. Due to the Metropolitana, Ignacio Valdivieso 2409, P. O. Box 8940577, San Joaquı́n, recognised therapeutic effects of some vanadium-based formu- Santiago, Chile h lations that mimic the action of insulin in mammals and their Universidad de Chile, Departamento de Quı́mica, P. O. Box 653, Santiago, Chile. E-mail: ggonzale@uchile.cl possible effect to treat some types of cancer,5 the interest in this † Electronic supplementary information (ESI) available. See DOI: 10.1039/ chemistry has increased markedly. Vanadium multivalence c9nj02188d is also very useful in homogeneous chemical catalysis. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem. View Article Online Paper NJC For example, in the oxidation of organic compounds with structural unit in these nanostructures appears to be the same, peroxides, H2O2 or oxygen assisted by vanadium compounds.6 which is also very similar to that determined by Wang et al.23 by Interestingly, this same type of reaction could be used to the XRD single crystal analysis of the compound BaV7O16, remediate contaminated soils by reducing the relative concen- prepared in a small amount from metallic vanadium by tration of vanadium(V) performing a Felton-like heterogeneous electrochemical-hydrothermal synthesis. The BaV7O16 has a catalytic process in situ.7 Recently, laminar hybrid nanocompo- laminar structure where each sheet is a flat polyanionic species sites based on vanadium oxide have also been shown to be constructed by condensation of the V7O162 units. In bulk, Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. effective as co-catalysts in heterojunctions with ZnO/fatty-acid these sheets are stacked in a direction normal to the molecular hybrids for the photodegradation of dyes in aqueous systems plane with Ba2+ counterions located in the interlaminar space. with visible light.8 Each molecular unit V7O162 is formed in turn by two layers, The vanadium oxide materials have demonstrated promis- each of three distorted VO6 octahedrons oriented in the ing performance as materials for lithium-,9 sodium-10 or opposite direction, which are joined together by a tetrahedral potassium-ion batteries.11 However, the efficiency of these VO4 unit. systems depends on the stability of the host vanadium oxide Although the morphological analysis of the VOxNT and the network. A robust network capable of accommodating lithium NU clearly shows that the tubes are generated by a rolling ions without severely altering its structure prevents the segre- process,24 in our opinion there are still open questions about gation of phases that would affect the transport of mass and the driving force and the mechanism of the process. Consider- charge on the electrode.12 Vanadium oxide based layered ing the rich diversity of coordination and the versatile redox nanostructures with various vanadium oxidation states are also chemistry typical of vanadium oxides, it is difficult to under- studied as gas sensor materials.13,14 stand the generation of morphologically pure mixed valence Among the vanadium derivatives of mixed valence are the phases such as VOxNTs and NUs under hydrothermal condi- hybrid tubular nanostructures of vanadium oxides with alkyl- tions. Undoubtedly, as has been repeatedly argued in the amines, particularly the nanotubes (VOxNTs) and nano-urchins literature,25,26 an important factor is the change in the oxida- (NUs), in which the average oxidation state of vanadium is tion state of vanadium produced by reaction with electron around +4.42 and +4.38, respectively.15 The VOx-NTs synthe- donors such as water or amines at hydrothermal temperatures. sised for the first time by the Nesper group are open tubes with However, the role of amphiphiles like long chain amines in the walls in the range of 5–10 nm thick, formed by approximately process has been little discussed. In this sense, the reports by 5–6 concentric layers of oxide interspersed with long-chain both Rappolo and Worle are interesting.15,20 These authors monoalkylamines.16 The NUs synthesised for the first time reported that the same VOxNT synthesis strategy but using by our laboratory are three-dimensional micrometric particles ethylenediamine instead of long carbon chain amines as a (9–12 mm in diameter) formed by highly dense VOxNT spherical template, leads to a laminar product structurally similar to matrices.17 The preparation strategies of both VOxNTs and NUs VOxNTs and NUs, but with an almost flat open morphology, are similar; essentially through the hydrothermal treatment of with just a small curvature. The elementary cell of the product a flat laminar precursor which can be described as the inter- is qualitatively similar to those of VOxNTs and NUs, differing calation of the corresponding amine in the vanadium(v) oxide significantly only on the c axis. hydrogel (V2O5). In fact, this intercalation is carried out by a In this paper, we report the preparation of a new carbonless supramolecular process based on the sol–gel condensation of member of the VOxNT and NU families, the ammonium the oxide on a double layer of surfactant (amine) that acts as a hexadeca-oxo-heptavanadate (NH4)2V7O16, where ammonium two-dimensional (2D) template. Virtually any precursor of V2O5 ions replace the long carbon chain amines. The tetragonal hydrogel can be used as a source of vanadium.18 The hydro- symmetry of the crystalline structure of the product, very thermal treatment (6–8 days at 180 1C) of the lamellar solid similar to that of its homologue BaV7O16, is reflected in its leads, almost quantitatively, to practically pure tubular almost perfect square-planar morphology. The preparation products, provided that the carbon chain of amine Cn is in strategy and properties of (NH4)2V7O16 corroborate the the range (CnH2n+1NH2 with 4 r n r 22). VOxNTs may also be structural stability of this family of vanadium oxides in a wide produced using diamines (H2N[CH2]nNH2 with 4 r n r 22).19 range of oxidation states. In addition, the results help to At the same time, vanadium(v) is partially reduced to understand the role of amine amphiphiles in both the coiling vanadium(IV). However, according to Vera-Robles et al.,18 the mechanism and the supramolecular nature of the redox generation of VOxNTs can also be achieved with excellent yields process underlying the formation of this class of mixed valence by chemical oxidation of a vanadium precursor in a lower vanadium oxides. oxidation state but provided an amphiphilic amine is present. Vanadium oxide nanotubes with mixed valence V5+/V4+ are structurally very stable species, capable of maintaining their Experimental section tubular structure even after exchanging their original organic component with other low molecular weight amines, including Materials and methods ammonia, or with a variety of inorganic cations.20–22 Regardless All chemical reagents used in the experiments were obtained of the oxidation state of vanadium in VOxNTs or NUs, the from commercial sources as guaranteed grade reagents New J. Chem. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 View Article Online NJC Paper purchased from Aldrich. All reagents were of analytical grade and were used without further purification. Synthesis NH4VO3 puriss Z99% (0.236 g, 3 mmol) and 1-hexadecylamine (HDA) puriss Z90% (0.268 g, 1 mmol) are mixed with 10 mL of absolute ethanol in a 100 mL flask. After 2 hours of vigorous Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. stirring, 5 mL of glacial acetic acid is added. The yellow suspension is stirred for 30 minutes and 10 mL of double distilled water is added. An orange composite is formed and is stirred for 48 hours. This suspension is aged for 5 days. The Scheme 1 Synthesis of (NH4)2V7O16. hydrothermal treatment is performed in a Teflon-lined Parr Bomb at 180 1C, at differing durations of 24 hours, 3 days, 7 days and 10 days. The resulting black solid is washed several The first stage corresponds to the hydrolysis at room tem- times with ethanol and dried under active vacuum at 80 1C. The perature of the vanadium precursor NH4VO3 in aqueous amine amount of resulting black solid was 0.150 g, yield above 80%. solution, acidified after approximately two hours with acetic The chemical composition obtained from elemental analysis acid, which leads to a suspension of an orange solid. The (%) for the products is found to be, for the intercalation interruption of the reaction at this point allows the orange compound, experimental (calculated for V2O5(HDA)1.02.3H2O intermediate to be denoted as a vanadium(V)/amine laminar with 3.07% VO2): C 40.03 (39.99), N 2.99 (2.99), H 8.46 (8.46); and nanocomposite similar to those previously detected in the for the microsquares, experimental (calculated for (NH4)2V7O16 preparation of VOxNTs and NUs.17 The second reaction stage with 1.38% V2O5/HDA and 6.48% VO2): C 0.65 (0.63), N 4.02 (4.02), consists of the hydrothermal treatment of the previous orange H 1.25 (1.25). suspension, which—on acquiring a black colour—points to the The content of vanadium(IV) in the products determined by reduction of V(V) to (IV). In addition, no nanotube formation colorimetric titration of the samples with KMnO4 15 is found to was detected, and the main product was identified as the be, for the intercalation compound, 4.2  0.3% and for the amine-free ammonium vanadate (NH4)2V7O16 (Scheme 1). This microsquares, 73.3  0.2%. Experimental details of the titra- new product has a lamellar nature, but instead of the typical tion are presented in the ESI.† appearance of nanocomposites of vanadium oxide, it exhibits a disordered superposition of extended leaves, consisting of Characterisation methods a phase constituted by crystalline particles that are almost perfectly square and flat. Structural studies. Information was obtained from Powder The chemical analysis of the orange solid obtained in the X-Ray Diffraction (XRD), transmission electron microscopy first reaction stage shows a high content of carbon and hydro- (TEM) and scanning electron microscopy (SEM). Room temperature gen (40.0 and 8.5%, respectively) and a small amount of XRD data were collected using a Siemens D5000 diffractometer vanadium(IV). Analysis fits the stoichiometry of V2O5(HDA)1.0 (Cu Ka. l = 1.5418 Å, Operation voltage 40 kV). Data were collected 2.3H2O with an impurity equivalent to about 3.1% VO2, which in a continuous manner with a speed of 0.501 min 1 over the range approximately agrees with the content of V(IV) determined 1.71 o 2y o 801. The morphological characterisation of the experimentally (ca. 4.2%). The presence of a small amount of products was performed by field emission scanning electron micro- V(IV) in the product is consistent with the ESR studies of xerogel scopy (FESEM) using a JEOL JSM-6700F field-emission scanning V2O5 reported in the literature.27,28 The diffraction pattern of electron micrograph with a suitable resolution to study the external the product (Fig. 1a) dominated by the reflections of Bragg 00l morphology. The scanning electron microscopy (SEM) images were (l = 1 to 3) corresponds to a laminar nanocomposite with an obtained by using an EVO MA 10 ZEISS microscope. The samples interlaminar space of approximately 2.68 nm. It is assumed were prepared by the drop casting suspension technique and placed on a holey carbon support. TEM and electron diffraction (ED) were conducted using a JEOL-2000FX operating at 200 kV. The chemical composition of the samples was determined by elemental chemical analysis using a SISONS model EA-1108 analyser. The FT-IR spectra were recorded using the KBr technique in a PerkinElmer series 2000 apparatus in the 4000–450 cm 1 region. Results The preparation of (NH4)2V7O16 microsquares is a one-pot process, which, however, involves two well-defined consecutive Fig. 1 Laminar nanocomposite V2O5/HDA (a). X-ray diffraction (XRD) and stages (Scheme 1). structural model (insert); scanning electron microscopy (SEM) image (b). This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem. View Article Online Paper NJC Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. Fig. 3 X-ray photoelectron spectrum (XPS) of (NH4)2V7O16: (a) long-scan spectrum; (b) high-resolution XPS of the V2p and O1s state region. Fig. 2 (NH4)2V7O16 microsquares morphology: (a) and (b) SEM images; (Voigt functions, CASA software) taking particularly into account (c)–(e) transition electron microscopy (TEM) images. the element photoionisation cross sections and the BE difference as well as the intensity ratio of the peaks V2p3/2 and V2p1/2 of 7.4 eV and 2 : 1, respectively. Results are shown in Fig. 3b and that the amine is located in the interlamellar space with its Table S1 (ESI†). From fitted components of the peak V2p3/2 polar heads oriented towards the vanadium–oxygen sheets corresponding to both the V4+ and V5+ ions, centred at with their hydrocarbon chains self-assembled in an inter- 515.88 eV and 517.33 eV, respectively, the average surface formal digitated manner with an inclination of 371 to optimise the oxidation state of vanadium in (NH4)2V7O16 was found to be 4.25. van der Waals interactions between the carbon chains29 (model Within experimental errors, this value agrees reasonably with inserted in Fig. 1a). In Fig. 1b, the image SEM shows the layered both values, that measured by redox titration (4.27) and that nature with multi-layer plates. calculated from the stoichiometry of the compound (4.29) Fig. 2(a) and (b) illustrate SEM micrographs of a typical (Table S1, ESI†). sample of black microcrystalline solids obtained from a hydro- To further investigate the chemical nature of (NH4)2V7O16, thermal reaction (stage 2). The obtained phase is practically we analysed the molecular structure of microsquares using constituted only by well-formed square microcrystals with Fourier transform infrared spectroscopy (FTIR). In Fig. 4 we lateral sizes ranging from 1.8 to 4.4 mm and widths between compared the FTIR spectra of (NH4)2V7O16 with that of the 200 and 500 nm. The images of the surface of the crystals reveal laminar intermediate V2O5–amine detected in its preparation. characteristics that indicate that these squares are of a laminar In the nanocomposite (Fig. 4a), the vibrational signatures of nature and that they correspond to the stacking of nanometric hexadecylamine and vanadium oxide are clearly detected. We sheets, practically all with 2D aspect ratios close to one. observe the asymmetric stretching and bending vibration The TEM images reproduced in Fig. 2(c) and (d) corroborate modes of the NH2 group at 3203 and 1514 cm 1, respectively. the almost completely symmetrical shape of the crystals. Meanwhile, the vibrations ns(CH2) and nas(CH2) appear at The presence of the ultrathin films points to an exfoliation of 2848 and 2920 cm 1, respectively. On the other hand, we the crystals produced by the preparation of the samples. The composition of the product calculated from the elemental analysis and the redox titration corresponds to the compound (NH4)2V7O16, contaminated by a small amount of unreacted precursor nanocomposite (approximately 1.3% w/w) and an excess of V(IV) equivalent to ca. 6.5% of VO2. To corroborate the mixed valence state of the (NH4)2V7O16 microsquares, we performed the X-ray Photoelectron Spectroscopy (XPS) study illustrated in Fig. 3. We used the oxygen vanadyl peak O1s energy level, at 530.0 eV, as reference bonding energy (BE) as well as to compensate charge and to determine the Shirley back- ground underneath the V2p region. The long scan spectrum (Fig. 3a) shows the composition of the sample where the absence of the important contribution of possible impurities is apparent. Fig. 3b illustrates the high resolution XPS spectrum in the V2p states region where we observe the typical two peaks structure of V2p, due to spin–orbit splitting, and the single O1s peak from the oxygen atoms bonded to the vanadium atoms.30,31 To estimate the valence state composition of the sample, we analysed the Fig. 4 Fourier transform infrared spectroscopy (FTIR) spectra of the V2O5/ relative peak area ratio of V2p to O1s. Consequently, we simulated HDA nanocomposite (a), (NH4)2V7O16 microsquares (b), and (NH4)2V7O16 the spectrum with mixed Lorentzian–Gaussian functions N–H stretching vibration modes (c). New J. Chem. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 View Article Online NJC Paper observe the absorptions associated with the V2O5 network in narrow equidistant Bragg peaks that point to the presence of the region 1000–480 cm 1: the VOV stretch mode (bridged layered microcrystals with preferential orientation. The inter- oxygen) at 721 cm 1; the mode of stretching of the group with laminar space d determined from the complete set of diffracto- tri-coordinated oxygen, at 644 cm 1; and the combination grams is 0.89 nm. This basal separation is similar to that of stretching and bending modes of groups with a triple- observed in other intercalated ammonium ions: for example, coordinated oxygen at 511 cm 1. In the spectrum of the micro 0.90 nm in the NH4+/amine VOxNTs,35 0.94 nm in (NH4)0.5V2O5 squares, there is only one absorption band assignable to the mH2O,36 or NH4+ in interleaf xerogels,37 as well as the separa- Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. mode of stretching vanadyl bonds, a sharp peak at a relatively tion in the intercalation of metal ions, such as Na+ and Ca2+ in low frequency, 941 cm 1.32,33 This is interesting because it is a VOxNTs with the interlaminar distances of 0.90 nm38 and frequency that corresponds to a high content of V(IV) in the approximately 0.90–1.10 nm,22 respectively. The indexing of compound and also because this band shape would reflect that the XRD pattern of the nanosquares consigned in Fig. 5(a) and there is only one type of atom of V. The atoms V(IV) and V(IV) are 10 in Fig. 5(b), including the planes of the Bragg reflections indistinguishable under conditions of measurement, pointing series (00l) and (hk0), corresponds to a two-dimensional tetra- to a charge delocalization, characteristic of mixed valence gonal network with a basal distance, a, of 6.1 (Fig. S1, ESI†). The structures. A further description of the IR spectrum of the reflection peaks (hk0) have an approximately Lorentzian shape; products and their comparison with the VOxNT spectrum,33 however, they generally show an asymmetrical sawtooth profile in the characteristic frequency range of the oxovanadates, is that suggests a turbostratic disorder between the layers.39 shown as additional information (Table S3, ESI†). The spec- In fact, no (khl) reflection is detected. The DRX pattern of trum of (NH4)2V7O16 (Fig. 4b) differs from that of the nano- (NH4)2V7O16 XRD qualitatively coincides with those observed in composite, particularly in the spectral region around 3500 and VOxNTs19 and NUs17 as well as in (en)2V7O16,20 where the reflec- 1350 cm 1. The characteristic absorptions of hexadecylamine tions (hk0) seem to be insensitive to the type of intercalated host. are practically absent, but instead, there are features assignable The typical selected area electron diffraction (SAED) pattern to the N–H vibration of the ammonium ion, both free and with of the microsquares illustrated in Fig. 6 suggests a tetragonal hydrogen bonds to the vanadium oxide matrix. We suggest that network with a = 0.615 nm agreeing with the XRD results. the band centred at 3426 cm 1 comprises the asymmetrical and Consequently, the crystal structure we propose for (NH4)2V7O16 symmetric stretching vibrations of the free N–H bonds, while is similar to that of BaV7O16—a tetragonal network with those centred at 3196 and 2922 cm 1 belong to the asymmetric a B 0.617 nm—23 and only slightly different from those and symmetric stretching modes of the NH bonds coupled reported for the VOxNTs: lattice networks with a = 0.61,22 to the matrix, respectively (Table S2, ESI†). It is also 0.615,40 or 0.62 nm,18 as well as that of the triclinic network worth mentioning the reduced symmetry of the NH4+ ion in (en)V7O16 with a = 0.616 and b = 0.617.15,20 (NH4)2V7O16 (from td to C2v), which allows the detection of its In the structure of the anion V7O162 , the VO6 units are four fundamental modes of vibration by infrared solid spectro- distorted octahedra with the axial V–O bond opposite to the scopy (KBr). In addition, the coexistence of free N–H bonds and vanadyl group VQO significantly weaker than the equatorial H bonds observed at frequencies similar to those registered in ones (Fig. 7a) Therefore, the structure of V7O162 can be seen, the gas phase systems reflects the nature of the interlaminar as shown in Fig. 7b. This is like a complex polyanionic flat sheet phase in this compound.34 neutralized by NH4+ ions distributed on the surface. In turn, Fig. 5 shows the typical XRD pattern of the microsquares each sheet is formed by two parallel layers of square-pyramidal (NH4)2V7O16 characterised mainly by a sequence of relatively polyhedrons, oriented in the opposite direction with respect to the VQO bond, joined together by tetrahedral VO4 units separated periodically. Therefore, bulk square microcrystals would be stacked (V7O6)n2n sheets interspersed with NH4+ ions (Fig. 7b). This description agrees with the intensities pattern of the harmonics (00l)—odd–even—observed in the diffractogram (Fig. 5a). This phenomenon, where the intensities of the even-order reflections—especially the (002) ones—appear Fig. 5 XRD pattern of the (NH4)2V7O16 microsquares (a); part of the same Fig. 6 Selected area electron diffraction (SAED) pattern of a microcrystal amplified by 10 (b). of (NH4)2V7O16. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 New J. Chem. View Article Online Paper NJC vanadium oxide(V) or by oxidation of vanadium(IV), is only achieved when an alkylamine is used as a template. Discussion The formation of nanotubes has been explained mainly in Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. terms of the flexibility of the system associated with the partial reduction of vanadium. However, the results of this work encourage a somewhat more detailed analysis of the mechanistic Fig. 7 Structural model of (NH4)2V7O16, adapted from the structure of aspects involved in these anti-entropic processes. In general, the BaV7O16 reported by Wang et al.22 (a) Model of the anionic cluster formation of the different V7O16-based tubular nanostructures may (V7O16)2 ; (b) model of stacked bilayers for the bulk product. be seen as the conjugation of two important processes, the partial reduction of vanadium and the curvature of the network. The conjugation of structural changes and redox processes promoted markedly depressed, is similar, for example, to the stacking of by hydrothermal conditions is not new in the chemistry of the double layers in ribbons and sheets of V2O5nH2O gels oxovanadates. An example of this is the influence of temperature modeled by Oka et al.,41 or as observed for the partially reduced and counterion on the synthesis and decomposition of sodium V2O5, (NH4)0.5(V2O5).42 As far as we know, such an X-ray inter- and tetramethylammonium decavanadate (TBA) salts as described, ference pattern has never been observed in the nanostructures for example, by J. Livage’s group.45 Both salts can be precipitated V7O16, VOxNTs or NUs, probably due to poor coherence between from V2O5 solutions in the presence of NaOH or TBAOH at both the plates because of their curvature. alkaline and neutral pH. However, the thermal decomposition of The crystal structure of (NH4)2V7O16 appears to be thermally these two solids leads to different products. NaV10O28 is decom- quite stable under hydrothermal conditions. Indeed, as observed posed in a mixture of sodium polyoxovanadate NaV3O8 and in Fig. S2 (ESI†), the products obtained from a series of experi- NaVO3, while that of TBA is partially reduced, transforming into ments performed using thermal treatments of different durations, laminar polyoxovanadate TMA(V4O10). These same polyoxo- from 0.5 to 7 days, show practically the same DRX pattern. metalates can also be obtained directly from V2O5 by hydro- However, after thermal treatment for longer than seven days, they thermal treatment. In addition, the authors showed that these begin to decompose. Fig. 8 shows a typical SEM image of the polymeric solids may also be precipitated from the respective product obtained after 10 days. The formation of a new rather decavanadate solutions previously pretreated at 200 1C (hydro- homogenous vanadium oxide phase, mostly constituted by cross- thermal conditions). This is despite the fact that an 51V NMR shaped particles is evident. Although we have not yet studied this study showed that both solutions contained only vanadium(V). phase in detail, the preliminary characterisation showed that the This suggests that the reduction stage induced by the presence micro crosses do not contain nitrogen and that in them vanadium of the organic cation would occur together with the precipita- oxidation is near to that of VO2. DRX analysis (Fig. S3, ESI†) shows tion of the species in solution. Apparently, a template effect a pattern that agrees with that of the phase V0.87O2 which is similar to that leading to the transformation of V2O5 into mixed similar to our six-times rotationally symmetrical vanadium oxide- valence species V7O16 would occur. based nanostructures with a cog-like architecture43 as well as to The first stage in the synthesis of V3O17 species (Scheme 1) the hexangular starfruit-like VO2, reported more recently.44 always corresponds to the formation of a laminar nanocompo- Our results agree with that reported in the literature site, stabilised by the two dimensional self-assembly of the concerning the importance of long-chain alkylamines in the surfactant and interaction of its head with the vanadium oxide. hydrothermal synthesis of morphologically pure phases of The vanadium structure in these nanocomposites is like that of mixed valence vanadium oxides.25 The formation of micro- their hydrogel,46 where water can easily be replaced by amine in squares, as well as VOx nanotubes, obtainable by reduction of the space of the intermediate layer due to the relatively high Lewis-base activity of the latter. In contrast to the ligand exchange nature of the previous step, the second is character- ized by the partial reduction of vanadium in the compound. The vanadium oxide reduction process promoted by the hydrothermal conditions leading to these compounds appears to be an orderly process that is only possible if it occurs on the surface of a template stable enough to allow concomitantly both vanadium reduction and bond rearrangements involved in the transformation of V2O5 into V7O16 units. An example of processes in which the template determines the structure of the Fig. 8 Vanadium oxide micro crosses. SEM images of the product products is the synthesis of perfectly ordered mesoporous obtained after 10 days of hydrothermal treatment. silicates and aluminosilicates, known as MCM-41 based New J. Chem. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2019 View Article Online NJC Paper materials. The use of long-chain tetraalkylammonium salts as the reaction medium in the case of microsquares. Although the templates, similar to the amines we use, makes it possible to increase in acidity of the medium does not prevent the formation generate hybrid sheets of two-dimensional hybrid silicates and of the intermediate nanocomposite under normal reaction con- convert them into tubular species by modulating the reaction ditions, it is enough to induce the segregation and replacement of conditions.47 the amine under hydrothermal conditions. This is possibly mainly The long-chain monoalkylamines or diamines seem to be due to the destabilisation of the vanadium–nitrogen interaction ideal for producing both processes. As far as we know, amines by the protonation of the amine. Published on 20 June 2019. Downloaded by Universitat Autonoma de Barcelona on 10/6/2019 5:05:26 PM. are the only amphiphiles able to produce this type of structure With respect to the reduction procedures, our experiments, in such a successful way. Indeed, other types of surfactants or many others previously described in the literature cited such as alkyl thiols or polyethers are equally suitable for above, have not allowed it to be determined whether the forming laminar species but fail to stabilise tubular hybrids vanadium-reducing electron donor agent is the amine, the or planar mixed-valence species as they do the amines. In these water, or both. However, we now believe that the reduction of cases, the hydrothermal treatment produces a more enhanced vanadium(V) to vanadium(IV) must occur while the oxide reduction leading to template-free compounds with a mean structure is supported by the amine template. In the case of vanadium oxidation state very close to 4+ that corresponds to nanotubes where the amine remains attached to the inorganic Magnelli phases.48 Interestingly, these products show charac- structure, the reduction and rolling up processes are probably teristic morphologies with a hexagonal symmetry like, for concomitant. Therefore, it is expected that the amount of example, the nanofruits42 or our nanocogs (Fig. S4, ESI†),43 charge transferred to the vanadium oxide structure is exactly obtained using polyethers (Pluronics) or alkyl thiols as that necessary to achieve the minimum energy determined by templates, respectively. the curvature of the network. On the other hand, in the case of Although other surfactants such as alkyl thiols can form microsquares, both oxide reduction and amine/ammonium bilayers as strong as amines, in the latter their self-assembly exchange must also occur concomitantly, until the minimum capability is combined with their peculiar coordinating ability energy of the V7O16 flat network is reached. derived from the electronic structure of the nitrogen atom, a Considering that a flat V7O16 structure is as or more stable typical monofunctional s-donor. This contrasts with surfac- than that of the nanotubes, it is necessary to explain why both tants with donor atoms such as oxygen or sulphur which, types of compounds can be obtained independently as because they also are p-donors, generate a smaller ligand field. morphologically pure phases. The volume change of the VO5 Thus, it is expected that in the intercalation compounds the polyhedra associated with the reduction of vanadium(V) of amine nitrogen atoms are relatively strongly coordinated to certain atoms of the structure can lead to a curvature of the vanadium, especially at relatively high pH values. system.49 However, it seems to us that that variable alone is not The role of the coordination of alkylamine in the promotion enough to justify such homogenous systems. We believe that of the partial reduction of V2O5 seems to arise from the the presence of the amine plays an important role in the modification of the electronic structure of the oxide. The formation of nanotubes. structure of V2O5 in the hydrogel can be roughly described as As already mentioned above, at relatively high pH—like that a condensation of the VO5 units in which the vanadium used to obtain the nanotubes—the V–N bonds and the alkyl acquires a distorted octahedral configuration by coordinating chain self-assembly do resist hydrothermal conditions. How- a water molecule. In this context, the frontier orbitals in V2O5 ever, at high pressure and temperature, together with the and oxovanadates, HOMO and LUMO, would have mainly the change in water pK (Kw B 10 12 at 180 1C and a self-generated character of oxygen-p and vanadium-t2g, respectively. Due to pressure of approximately 10 bar), the diffusion of water into the the interaction with the oxygen-filled p-orbitals, the t2g orbitals interlaminar space is favoured which increases the degree of are antibonding. Therefore, by substituting the water molecules ionization and solvation of the amine group leading to a greater by purely Sigma ligands, such as amines, the t2g orbitals will effective volume of the head of the surfactant. As schematically become partially nonbonded, the LUMO energy will be lower illustrated in Fig. S5 (ESI†), the characteristic cylindrical shape of and the vanadium reduction will be energetically more suitable. the surfactant is transformed into a cone; in other words, there is Comparing the average oxidation state of vanadium in the a decrease in the so-called critical packaging parameter,50 favour- microsquares described here with those in the VOxNTs and ing the curvature of the template. In this way, this typical process NUs seems to indicate that the degree of reduction of vanadium of the supramolecular chemistry of the amphiphilic species would is related to the morphology of the products. Given the fact that contribute to the necessary driving force for the rolling of the the greater the planarity of the structure, the greater the degree V7O16 sheets. of reduction, it is quite reasonable if we consider that planarity Although our experiments contribute to the postulation of favours the delocalisation of electrons in the system. plausible mechanisms to explain the morphological richness The fact that the same type of intermediary, the nano- and chemical versatility of the V7O16 family, a deeper knowl- composite V2O5/alkylamine, generates in one case organic– edge of these systems as well as the development of new inorganic hybrid nanotubes and in the other one, purely applications, particularly in the field of catalysis, energy con- inorganic microsquares is due only to a subtle difference in version and molecular magnetism, is still required. 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