Showcasing research from Prof. Pedro Gómez-Romero's group at the Catalan Institute of Nanoscience and As featured in: Nanotechnology, Bellaterra, Barcelona, Spain. Volume 14 Number 40 28 October 2022 Pages 14813-15204 Polyoxometalate intercalated MXene with enhanced electrochemical stability Nanoscale rsc.li/nanoscale Polyoxometalates (POMs) were successfully intercalated into MXene layers for the first time, with the assistance of pre- intercalated surfactant cations (cetyltrimethylammonium, CTA), which provides the needed steric and charge shielding. The work includes a systematic study with several cations, among which CTA is the only one truly leading to intercalation. Only this material features superior ISSN 2040-3372 electrochemical stability over the other materials prepared in COMMUNICATION Hilmi Volkan Demir et al. Observation of optical gain from aqueous quantum well heterostructures in water this work. The results provide a solid guide to help develop high-performance MXene/POM hybrid materials for a variety See Jun-Jie Zhu and Pedro Gomez- Romero, Nanoscale, 2022, 14, 14921. of applications. rsc.li/nanoscale Registered charity number: 207890 Nanoscale This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online PAPER View Journal | View Issue Polyoxometalate intercalated MXene with enhanced electrochemical stability† Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. Cite this: Nanoscale, 2022, 14, 14921 a Jun-Jie Zhu and Pedro Gomez-Romero *a,b MXene/polyoxometalate (POM) hybrids are useful target materials for a variety of applications. Yet, the goal of preparing simple binary hybrids by intercalation of POMs into MXene has not been achieved. We propose and demonstrate here a method to intercalate POMs ( phosphotungstate, PW12) into Ti3C2Tx MXene through the interaction between POM anions and pre-intercalated surfactant cations. A variety of quaternary ammonium cations have been used to expand Ti3C2Tx interlayer spacing. Cetyltrimethylammonium cations (CTA+) lead to an expansion of 2 nm while allowing intercalation of a considerable load (10 wt%) thanks to their tadpole-like shape and size. CTAPW12 has a layered structure compatible with Ti3C2Tx. The CTA+-delaminated Ti3C2Tx keeps the large interlayer spacing after being coupled with PW12. The PW12 clusters are dispersed and kept isolated thanks to CTA surfactant and the confinement into Ti3C2Tx layers. The redox reactions in CTA+-delaminated Ti3C2Tx/PW12 are diffusion- controlled, which proves the well-dispersed PW12 clusters are not adsorbed on the surface of Ti3C2Tx particles but within Ti3C2Tx layers. The CTA+- delaminated Ti3C2Tx/PW12 shows superior electrochemical stability (remaining redox active after 5000 cycles) over the other MXene/POM hybrids prepared in this Received 12th March 2022, work (inactive after 500 cycles). We associate this improved stability to the effective intercalation of PW12 Accepted 9th August 2022 within Ti3C2Tx layers helped by the CTA cations, as opposed to the external aggregation of PW12 clusters DOI: 10.1039/d2nr01410f into micro or nanocrystals taking place for the other cations. The results provide a solid guide to help rsc.li/nanoscale develop high-performance MXene/POM hybrid materials for a variety of applications. Introduction Polyoxometalates (POMs) are a class of metal–oxide clusters consisting of multiple metal–oxide octahedra linked together Since the discovery of Ti3C2Tx – the first MXene – in 2011 by by oxygen atoms to form nanoclusters with ordered and well- Naguib et al.,1 the MXene family is flourishing. More than 30 known structures. Their intrinsic nanosize, high surface-to- new MXenes have been synthesized.2 MXenes have a general bulk ratio and fast and reversible redox activity have launched formula Mn+1XnTx, where M is a transition metal, X is carbon them for use in applications in various fields, such as energy and/or nitrogen, n is an integer between 1 and 3, and Tx rep- storage,14,15 catalysis,16 sensors,17 etc. resents surface functional groups. Due to their two-dimen- MXenes and POMs share some target applications while sional structure, high conductivity, redox activity and hydro- featuring complementary properties and dimensionalities philicity, MXenes have shown great potential in various appli- which could lead to synergies. For example, 2D MXenes could cations, including energy storage,3 electromagnetic inter- provide an electronically conducting matrix, whereas 0D POMs ference,4 sensors,5 water remediation,6 photocatalysis,7 radio- could provide a pillaring effect opening the resulting structure frequency antennas,8 etc. and facilitating ion transport. Thus, POM-intercalated MXene Beyond the compositional richness of these MXene formu- hybrids seem interesting in their own right. And several labora- lations, their anionic charge and layered structure have tories have tried to synthesize this type of hybrid, although prompted the development of a variety of hybrids derived from intercalation has proved to be elusive. Chen et al. anchored them.9–13 POMs on the surface of Ti3C2Tx with (ionic liquid) linkers. Their hybrid material was shown to deliver high specific capacitance long term in an aqueous electrolyte.18 Chao et al. a Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, reported the in situ growth of 10–20 nm Mo and Fe-based POM Campus UAB, Bellaterra, 08193 Barcelona, Spain. E-mail: pedro.gomez@icn2.cat; nanoparticles on Ti3C2Tx nanosheets. Their hybrid material Fax: +34 936917640; Tel: +34 937373608 b showed excellent pseudocapacitive performance in lithium-ion Consejo Superior de Investigaciones Científicas (CSIC), Spain † Electronic supplementary information (ESI) available. See DOI: https://doi.org/ capacitors and sodium-ion capacitors.9 Zhou et al. embedded 10.1039/d2nr01410f phosphomolybdic acid and Ti3C2Tx in polypyrrole and built an This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14921 View Article Online Paper Nanoscale impedimetric aptamer biosensor for osteopontin with an extre- wires and 2D superlattices with excellent catalytic activity.27 It mely low detection limit, and high selectivity and stability.10 would be fascinating to design and develop this kind of self- MXene/POM hybrid materials not only show great potential in assembly reaction between POMs and cationic surfactants conventional fields, but also expand their application. For within the interlayer space of MXene. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. instance, Zong et al. developed GdW10-based polyoxometa- We report here for the first time a method to intercalate lates/Ti3C2Tx composite for contrast-enhanced computed tom- POM clusters into MXene (Ti3C2Tx) layers by utilizing the inter- ography and magnetic resonance imaging.11 Therefore, it is a actions between POM anions and organic cations pre-interca- good idea to develop MXene/POM hybrid materials both to lated into Ti3C2Tx. A variety of quaternary ammonium cations strengthen their properties and performance through syner- were used to expand the interlayer spacing and then react with gies in conventional applications as well as to explore new POMs. The cations capable of expanding the interlayer space Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. fields and extend their use to new applications. substantially enough to form nano lamellar structures were MXene/POM hybrid materials reported to date either finally able to anchor isolated POM clusters between layers. deposit crystallized POM nanoparticles on the MXene surface This POM-intercalated MXene hybrid outperforms the hybrid or anchor POM nanoclusters on the MXene surface with materials based on POM nanoparticles/MXene mentioned linkers. It might seem that POMs can only adhere onto the above in electrochemical stability owing to the superior inter- MXene surface and not get intercalated. layer hybridization. This study shows a new strategy to design The LiF/HCl-etched Ti3C2Tx has an interlayer spacing of ca. and synthesize MXene/POM hybrid materials to meet the 1.2 nm.19 Using large cations during the delamination process demands for better performance in conventional applications can lead to an expansion of this interlayer distance. For (catalysts, sensors, energy storage) as well as to enable new, instance, tetramethylammonium (TMA)-treated Ti3C2Tx has an uncharted ones. interlayer distance of around 1.5 nm.19 With extended time, even larger cations can intercalate MXene, resulting in a larger interlayer distance. To date, various cations, including tetra- ethylammonium (TEA),20 tetrabutylammonium (TBA),21 Experimental 1-ethyl-3-methylimidazolium (EMIM),22 dodecyltrimethyl- Synthesis of MXene (Ti3C2Tx, MX) ammonium (DTA),23 cetyltrimethylammonium (CTA)23 etc. Ti3C2Tx was synthesized through a LiF/HCl etching method. have been shown to intercalate MXene. The resulting interlayer Thus, 1 g Ti3AlC2 (MAX phase) was added slowly into a Teflon distance can be as large as 2.7 nm. The Keggin POMs clusters vessel containing 1 g LiF and 20 mL 9 M HCl solution, stirred are around 1 nm; thus, the interlayer space is large enough to continuously for 24 h at 35 °C. The mixture was poured into a accommodate POMs. However, this goal has not been accom- 50 mL centrifugation tube, centrifuged at 3500 rpm for plished yet due to several reasons. 5 minutes, then washed with Milli-Q water. This process was The first obstacle is the electrostatic repulsion. Since repeated until the pH of the dark-green supernatant was >6. MXene monolayers are terminated with negatively charged Finally, the mixture was centrifuged at 6000 rpm for functional groups (e.g. OH−, F−, Cl−), only cations can easily 20 minutes. The sediment was collected and used in the fol- enter the interlayers. Even with the assistance of an electric lowing dispersion. field, small anions (like Cl−, Br−) can hardly intercalate,24 not 0.1 g Ti3C2Tx sediment was dispersed in 50 mL Milli-Q to mention the much larger POMs which are all polyanions. water (shaken by hand), deaerated with Ar for 20 minutes, and On the other hand, POM salts in which the negative charge of stored in the fridge for future use. 20 mL of the Ti3C2Tx colloid these anions is balanced by a suitable amount of counter- was filtered on Celgard 3501 membrane, rinsed, dried in a cations do not suffer electrostatic repulsion but tend to crystal- vacuum oven at 60 °C and stored in an argon-filled glovebox, lize. The crystallized POM nanoparticles are too large to enter labeled as MX. MXene interlayers and can only be adhered onto the surface. Furthermore, crystallization packs POM clusters into extended arrays, preventing the direct interaction of each cluster with a Synthesis of quaternary ammonium cations-delaminated suitable substrate or electrolyte, which is the key to their MXene (TMAMX, TBAMX, DTAMX and CTAMX) success as catalysts or energy storage electrodes, respectively. 0.1 g Ti3C2Tx sediment was dispersed in each of four 50 mL On the other hand, most of the cations used to expand the 0.5 M quaternary ammonium solutions (tetramethyl- interlayer distance of MXenes can interact with POMs. And the ammonium hydroxide (TMAOH), tetrabutylammonium hydrox- fact that anionic POMs can interact with organic and inorganic ide (TBAOH), dodecyltrimethylammonium bromide (DTAB) cations can be used advantageously to control POM self-assem- and cetyltrimethylamonium bromide (CTAB)) (shaken by bly, stabilization, solubility, and function.25 For example, tetra- hand), deaerated with Ar for 20 minutes. The four mixtures ethylammonium phosphotungstate (TEAPW12) has been were shaken for four days. Four kinds of cation-exchanged shown to improve the capacitive performance in organic elec- Ti3C2Tx colloids were obtained. 20 mL of each colloid was fil- trolytes, where the activity of most inorganic POMs is less than tered on Celgard 3501 membrane, rinsed and dried with Ar optimal.26 CTA+, DTA+ and TBA+ can induce the reversible self- flow. The films can be easily peeled off from the membrane, assembly of Wells–Dawson POMs (P2W17MnO62) into 1D nano- dried in a vacuum oven at 60 °C and stored in an argon-filled 14922 | Nanoscale, 2022, 14, 14921–14934 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper glovebox. As-prepared samples were named TMAMX, TBAMX, DTAMX and CTAMX, respectively. Synthesis of MXene/POM hybrids (TMAMX-PW12, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. TBAMX-PW12, DTAMX-PW12 and CTAMX-PW12) 20 mL of each cation-exchanged Ti3C2Tx colloid was filtered on Celgard 3501 membrane, and rinsed repeatedly. Then 20 mL of 20 mM phosphotungstic acid (PW12) was added on the filter cake, filtered, rinsed, and dried with Ar flow. The films were care- fully peeled off, dried in a vacuum oven at 60 °C and stored in an Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. argon-filled glovebox. As-prepared samples were named TMAMX-PW12, TBAMX-PW12, DTAMX-PW12 and CTAMX-PW12. Material characterization Powder XRD patterns were collected on a PANalytical X’pert Pro-MRD diffractometer with Cu Kα radiation and PIXel detec- Fig. 1 Thermogravimetric curves of MX, TMAMX, TBAMX, DTAMX and tor. Scanning electron microscopy images were taken on a CTAMX in nitrogen at a heating rate of 10 °C min−1. Quanta 650 FEG microscope. High-resolution transmission electron images (HR-TEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were taken on a FEI Tecnai G2 F20 microscope. Raman as shown in Fig. 1. The pristine Ti3C2Tx (MX) suffers a slight measurements were performed on a Horiba T64000 Raman weight loss (3.2%) before 500 °C, corresponding to the spectrometer. A 532 nm laser (Cobolt Samba) was used to removal of some surface functional groups, e.g., –F, –OH and obtain the spectra. XPS measurements were performed with a –O.28,29 All the quaternary ammonium cation-exchanged Phoibos 150 analyzer (SPECS GmbH, Berlin, Germany) under MXenes undergo relatively drastic weight losses when ultra-high vacuum conditions (based pressure 10−10 mbar). heating up to 450 °C, followed by a plateau. The quaternary Monochromatic Al Kα was used as X-ray source (1486.6 eV). All ammonium cations can be completely removed before the high-resolution XPS spectra were fitted with CasaXPS 450 °C under a nitrogen atmosphere.30 Thus, the extra version 2.3.1. weight loss of quaternary ammonium cation-exchanged MXenes is associated with the removal of these organic com- Electrochemical characterization pounds.28 Among them, CTAMX presents the greatest amount of intercalated organic cation (10 wt%), and DTAMX Three-electrode characterizations were done in T-type has the smallest (3.8 wt%), as determined by comparison Swagelok cells, in which Ag/Ag+ and overloaded activated with the loss associated with pristine MXene. Since the qua- carbon served as the reference electrodes and counter electro- ternary ammonium cations will serve as the anchor sites in des, respectively. 1 M TEABF4 in acetonitrile was used as the the reactions with PW12, we normalize the weight percentage electrolyte. The three-electrode tests were carried out in an by molecular weight and calculate the concentration of qua- Argon-filled glovebox with the oxygen and water levels under ternary ammonium cation (mmol g−1) to estimate their 5 ppm. Two-electrode tests were carried out in coin cells. potential capabilities of anchoring POMs. TMAMX has the Overloaded activated carbon (5 times the mass of working elec- highest concentration (0.83 mmol g−1) of quaternary trodes) served as the positive electrode. Since the activated ammonium cation among the four samples, more than twice carbon electrodes have significantly larger specific capacitance that of TBAMX (0.35 mmol g−1) and three times that of and mass loading than MXene/POM hybrid electrodes, the CTAMX (0.27 mmol g−1). DTAMX has the lowest (0.12 mmol MXene/POM hybrid electrodes undergo the vast majority of g−1). The different concentrations will lead to different mor- electrochemical potential shift. The specific capacitance can phologies and different PW12 loading masses once they be calculated by eqn (1): interact with PW12. It We carried out powder XRD to characterize the crystal struc- C¼ ð1Þ V ture of the solid phases, as shown in Fig. 2. Table 1 presents where I is the current density (A g−1), t is the discharge time (s) the positions of (002) diffraction peaks and the interlayer dis- and V is the voltage window (V) tance. We can observe a significant expansion of the interlayer distance associated with large cations (TMA+, TBA+, DTA+ and CTA+) taking the place of Li+. Pristine Ti3C2Tx (MX) has the Results and discussion (002) diffraction peak at 7.76°, corresponding to an interlayer distance of 11.39 Å. TMA+ and TBA+ expand the interlayer dis- We carried out thermogravimetric analyses (TGA) to estimate tances to 14.53 Å and 14.95 Å, respectively. The degree of the amount of the quaternary ammonium cations in MXenes, expansion agrees well with the size of the cations: TMA+ has This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14923 View Article Online Paper Nanoscale Table 1 Positions of (002) peaks and their corresponding interlayer spacings for pristine MXene, cation-exchanged MXene and MXene/POM hybrids This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 2θ values for (002) Interlayer diffraction peaks/° distance/Å MX 7.76 11.39 TMAMX 6.08 14.53 TMAMX-PW12 7.62 11.58 TBAMX 5.90 14.95 TBAMX-PW12 6.38 13.84 DTAMX 6.10 14.48 Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. DTAMX-PW12 6.16 14.34 CTAMX 4.26 20.70 CTAMX-PW12 4.96 17.80 explain the relatively similar spacings of TBA and DTA interca- lates. On the other hand, when we compare the cases of the two anisotropic cations we find the same trend. DTA+ and CTA+ both have three methyl groups and a long aliphatic chain. The tail of CTA+ (hexadecyl) is longer than DTA+ (dodecyl), leading to a larger interlayer distance (20.70 Å). It becomes more difficult for larger spherical quaternary ammonium cations to insert into the layers.19 Our TGA results also agree with that. Increasing spherical quaternary ammonium cation size can only expand the interlayer space slightly, since fewer cations get intercalated. By contrast, tadpole-like quaternary ammonium cations can easily insert into the layers even with a long tail. Some studies have con- firmed that the interlayer space gets expanded remarkably with the increase of tail length.28,31 Indeed, the interlayer dis- tance of CTAMX is 20.70 Å, larger than DTAMX (14.48 Å). It is the largest interlayer distance among all the samples. After being coupled with PW12 clusters, the MXene/POM hybrids show a shrunken interlayer distance. The shrinkage of interlayer space must be associated with the extraction of cations during the reaction with PW12 and following the washing procedure. Among all the MXene/POM hybrids, only TMAMX-PW12 exhibits extra diffraction peaks. They match well with the XRD patterns of TMAPW12 (Fig. S1†). This con- firms that the pre-intercalated quaternary ammonium cations can fasten PW12 anions to form organic PW12 compounds. The PW12 anions have a high affinity toward the quaternary Fig. 2 XRD patterns of pristine MXene (MX), cations delaminated ammonium cations associated with a large lattice energy, MXene MXene (TMAMX, TBAMX DTAMX and CTAMX) and MXene/POM which results in the precipitation of quaternary ammonium hybrids (TMAMX-PW12, TBAMX-PW12, DTAMX-PW12 and phosphotungstate salts when these two ions are mixed in CTAMX-PW12). aqueous solution (synthesis of quaternary ammonium phos- photungstate derivatives, in ESI†). This large lattice energy acted as the driving force to fasten PW12. Even though the four methyl groups while TBA+ has four butyls. Yet, both of other MXene/POM hybrids do not show distinct extra peaks them are isotropic. from POM, this does not mean the absence of organic PW12 In the expansion of the interlayer distance we can dis- compounds. TMAMX has the highest cation concentration, tinguish two cases: isotropic and markedly anisotropic cations. anchoring more PW12 clusters and forming big enough PW12 Considering the shape of cations, TMA+ and TBA+ are crystals to present diffraction patterns. The other MXene/POM spherical, while DTA+ and CTA+ are tadpole-like. The former hybrids do not present characteristic diffraction patterns from only have one way to get intercalated whereas the latter can POMs. Several reasons can explain this: low loading mass, suffer a certain degree of conformational variation. This could nanocrystals, and intrinsically poor crystalline nature 14924 | Nanoscale, 2022, 14, 14921–14934 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper (Fig. S1†). Meanwhile, the interlayer distance of TMAMX-PW12 reacting with PW12, TMAPW12 has aggregated into micron- shrinks most significantly (from 14.53 Å to 11.58 Å), close to sized particles outside the Ti3C2Tx layers. The absence of such pristine MXene (11.39 Å), indicating that most of the TMA+ has particles in the other MXene/POM hybrids confirms that PW12 been extracted. TBAMX-PW12 and DTAMX-PW12 only show a clusters have not accumulated into extended microcrystals This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. slight shrinkage of the interlayer distance. CTAMX-PW12 (therefore diffracting). But this does not mean that PW12 clus- shows a visible decrease in the interlayer space as well as a ters are not attached to MXene. EDX analyses can verify this. lower crystallinity (indicated by broader peaks), but its inter- Table 2 lists the titanium and tungsten concentration of the layer distance (17.80 Å) is still larger than that of the others. MXene/POM hybrids derived from EDX analysis. Electron microscopy and affiliated energy-dispersive X-ray TMAMX-PW12 has the largest and DTAMX-PW12 has the smal- spectroscopy can provide evidence to prove the existence as lest amount of W among the MXene/POM hybrids. This agrees Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. well as distribution of PW12 clusters. Fig. 3 presents the SEM well with our expectation: the concentration of the intercalated images of the MXene/POM hybrids. In TMAMX-PW12 (Fig. 3a), quaternary ammonium cations affects the capability to fasten we can discern many tiny particles corresponding to PW12 clusters. A considerable amount of W can be discerned TMAPW12, spread everywhere: among Ti3C2Tx flakes and on the surface. By contrast, in TBAMX-PW12 (Fig. 3b), Table 2 EDX semi-quantitative analysis results DTAMX-PW12 (Fig. 3c) and CTAMX-PW12 (Fig. 3d), we can hardly discern any such particles. The existence of small par- W wt% Ti wt% ticles and nanosheets can explain why the XRD pattern of TMAMX-PW12 is a combination of both TMAPW12 and TMAMX-PW12 27.31 28.93 TBAMX-PW12 15.93 53.50 Ti3C2Tx diffraction patterns. Due to the high concentration of DTAMX-PW12 7.540 55.77 TMA+ in TMAMX and most of TMA+ being extracted when CTAMX-PW12 19.01 41.45 Fig. 3 SEM images of (a) TMAMX-PW12, (b) TBAMX-PW12, (c) DTAMX-PW12 and (d) CTAMX-PW12. This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14925 View Article Online Paper Nanoscale in the TBAMX-PW12 and CTAMX-PW12, confirming the get in, leading to an interlayer distance close to TBAMX-PW12 effective anchoring of PW12 clusters. In principle, such a con- and DTAMX-PW12. The larger interlayer distance corresponds centration of a constituent is high enough to present diffrac- to Ti3C2Tx layers with most CTA+ remaining and PW12 clusters tion patterns if well crystallized. The absence of POM diffrac- intercalating, reacting with these CTA+. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. tion patterns in TBAMX-PW12, DTAMX-PW12 and HAADF-STEM can provide Z-contrast imaging, in which W CTAMX-PW12 is related to poor crystallinity, such as nanocrys- would be more visible. TBAMX-PW12 (Fig. 4d) show the tals and long range disorder. Ti3C2Tx nanosheets clearly, on which several small bright par- To further investigate the distribution state of PW12 clus- ticles (<50 nm) are attached. These particles are TBAPW12 ters in the MXene/POM hybrids that we cannot discern in nanocrystals. Since the particles are much larger than the SEM, we characterized them in HR-TEM and HAADF-STEM, as interlayer distance, it is impossible for them to insert within Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. presented in Fig. 4. Ti3C2Tx layers. DTAMX-PW12 (Fig. 4e) shows a similar mor- All the HR-TEM images feature a layered structure. We can phology. We can discern thin, semi-transparent Ti3C2Tx estimate the interlayer distance through the zoom-in HR-TEM nanosheets with bright particles of 20 nm spreading. images. TBAMX-PW12 shows an interlayer distance of 1.42 nm DTAPW12 must have aggregated outside of the Ti3C2Tx layers (Fig. 4a inset), and DTAMX-PW12 shows an interlayer distance too. CTAMX-PW12 (Fig. 4f ) exhibits a different morphology. of 1.41 nm (Fig. 4b inset). Both of them agree well with the Numerous tiny bright dots spread everywhere. The EDX spec- interlayer distance derived from the XRD patterns. However, in trum in such an area reveals the coexistence of Ti and W, con- CTAMX-PW12 HR-TEM images, we can discern different inter- firming it is hybridized (Fig. S2†). Meanwhile, these observed layer distances (1.47 nm and 1.97 nm in Fig. 4c inset), which areas are still semi-transparent, meaning that they are still very are either larger or smaller than the interlayer distance thin. PW12 clusters do not form a continuous, dense aggrega- (1.78 nm) from XRD. We speculate that the two interlayer dis- tion; instead, there is a thin homogeneous discontinuous tances correspond to two intercalations. distribution. The smaller interlayer distance corresponds to Ti3C2Tx Raman spectra and XPS were used to further investigate layers where a few CTA+ are extracted, and no PW12 clusters how POM crystals combine with MXene. Fig. 5(a) compares the Fig. 4 HR-TEM images of (a) TBAMX-PW12, (b) DTAMX-PW12 and (c) CTAMX-PW12 (the inset images are zoom-in areas for observing interlayer distance). HAADF-STEM images of (d) TBAMX-PW12, (e) DTAMX-PW12 and (f ) CTAMX-PW12. 14926 | Nanoscale, 2022, 14, 14921–14934 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. Fig. 5 (a) Raman spectra of MX, TMAMX-PW12, TBAMX-PW12, CTAMX-PW12 and pristine TMAPW12 (inset (i) region for analyzing MXene with higher resolution; (ii) region for analyzing PW12 with higher resolution). (b) XPS full spectra, (c) C 1s spectra and Ti 2p spectra of MX and CTAMX-PW12. Raman spectra of the pristine MXene, pristine PW12 salt peaks to high wavenumber.34 However, MXene/POM hybrid (TMAPW12) and various MXene/POM hybrids (TMAMX-PW12: materials of larger interlayer distance do not show A1g(C) of microcrystal POMs, TBAMX-PW12: nanocrystal POMs and higher wavenumbers, which must be attributed to the CTAMX-PW12: nano-dispersed POMs). The broad-range scan increased defects. Furthermore, we can discern a slight shift of (with less resolution) reveals that the Ti3C2Tx MXene has its A1g(Ti, C, O) peaks to low wavenumbers in the case of MXene/ characteristic peaks under 800 cm−1 and PW12 features an POM hybrids. This phenomenon is associated with increased intense characteristic peak at around 1000 cm−1 which corres- ratio of flake edges to the basal planes (i.e. defects) induced by ponds to WvO stretching vibration.32 However, in the high- long-term shaking. The increased defects lead A1g(C) as well as resolution scan between 800 cm−1 and 1100 cm−1, only A1g(Ti, C, O) to shift to low wavenumbers.34 TMAMX-PW12 presents such intense peaks of PW12, while the To further compare the bonding forming and element other MXene/POM hybrids present hardly discernible broad valence, we performed XPS analysis on the pristine MXene and features (i.e. CTAMX-PW12). The undetected WvO stretching the representative hybrid material CTAMX-PW12, in which vibration peaks in TBAMX-PW12 and CTAMX-PW12 can be most POM clusters seem to spread on MXene directly. The low- ascribed to their poor crystallinity,33 which agrees well with resolution survey (Fig. 5(b)) confirmed the existence of W (W XRD results. The Raman spectra of Ti3C2Tx MXene can be sep- 4d and W 4p peaks) in CTAMX-PW12. However, the W 4f arated into several regions.34 The inset high-resolution scan peaks, the most commonly used peaks to analyze the bonding between 100 cm−1 and 1000 cm−1 presents them better. First structure of W, overlap with Ti 3p peaks, which hampered the comes the flake region consisting of A1g(Ti, C, O) mode, which acquisition of high-resolution W 4f spectra for further analysis. corresponds to out-of-plane vibrations of titanium atoms in Fig. 5(c) compares the C 1s spectra. Besides the C–Ti, C–C and the outer layer as well as carbon and surface groups. Next is CvO peaks, CTAMX-PW12 presents an extra C–H peak at the region between 230 and 470 cm−1, representing in-plane 286.2 eV from CTA cations. Meanwhile, the C–C peak, nor- (Eg(Tx)) vibrations of surface groups attached to titanium mally assigned to adventitious carbon, became stronger in atoms. The last region between 580 and 730 cm−1 is related to CTAMX-PW12, thanks to C–C in CTA cations. Ti 2p spectra carbon vibrations (both Eg(C) and A1g(C)). The Ti3C2Tx MXene (Fig. 5(d)) can disclose how the two components combined. Ti with expanded interlayer distance is supposed to shift A1g(C) (II) (2p3/2 455.8 eV, 2p1/2 461.5 eV) and Ti(III) (2p3/2 457.0 eV, This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14927 View Article Online Paper Nanoscale 2p1/2 462.9 eV) peaks are associated with surface terminating dependence on the square root of the scan rate (b = 0.5), it is a groups, while Ti(IV) is associated with the over-oxidized state, diffusion-controlled process. We can determine the b value by like TiO2. CTAMX-PW12 spectrum presents weaker Ti(II) peaks linear fitting the straight line of log i vs. log v, in which the and more intensive Ti(III) peaks than MXene, indicating that slope is the b value. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. the inclusion of CTAPW12 changed the valence of Ti. Some Herein, we performed CVs from 1 mV s−1 to 50 mV s−1 on chemical bonding was formed between CTAPW12 and surface three MXene/POM hybrids: TMAMX-PW12 (MXene with micro terminating groups. This phenomenon was also reported in POMs aggregates), TBAMX-PW12 (MXene with nano POMs other direct-anchored MXene/POM hybrid materials.35 aggregates) and CTAMX-PW12 (MXene with homogeneously The PW12 clusters are around 1 nm. If we only consider the distributed POMs clusters), as shown in Fig. 6a, d and g. The size, the interlayer space of around 1.4 nm can accommodate electrochemical behaviours of the crystallized PW12 electrode Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. the PW12 clusters. However, TMAMX, TBAMX and DTAMX all are also characterized and presented in Fig. S4.† Comparing fail to do that because of electrostatic repulsion among the CVs of pristine Ti3C2Tx (Fig. S3a†), PW12 (Fig. S4a†) and anions. As a result, the organic PW12 aggregates and forms those of MXene/POM hybrids (Fig. 6), we can classify the redox nanocrystals outside the layers. MXene cannot accommodate peaks in the MXene/POM hybrids into two types: the peaks smaller anions like Cl− even under the electric field force, totally derived from PW12, corresponding to Re1–Re3 and Ox1 owing to the negatively charged surface terminating groups.24 to Ox3; and the overlapped peak contributed from PW12 and Thus, it is not only a question of size, but also of charge, Ti3C2Tx corresponding to the reduction peak at −1.8 V. The affinity and compatibility. pairing of redox peaks is accomplished through carrying out CTAMX with a larger interlayer space provides a larger CVs in various narrow potential ranges, as shown in Fig. S5.† “charge-buffered” volume, has a higher tolerance against Re1 and Ox1 are a redox pair. Ox2 is actually an overlap of two electrostatic repulsion and is able to accommodate PW12 clus- oxidation peaks, pairing with Re2 and Re3. Ox3 is associated ters. Furthermore, CTAPW12 enjoys better structural compat- with the reduction peak buried in the cation-intercalation ibility with MXene. Since CTA+ is a surfactant, it can prevent peak of Ti3C2Tx (at −1.8 V). PW12 clusters from aggregation, and rearrange POMs clusters The energy storage mechanisms of pristine Ti3C2Tx and into isolated clusters.27 Several studies have proved that CTA+ crystallized PW12 were studied (Fig. S3 and S4†). For pristine can interact with Keggin POMs by self-assembly, and even Ti3C2Tx, only the reduction peak, corresponding to the cation- electrostatic layer-by-layer self-assembly, resulting in well- intercalation, is in part diffusion-controlled (b = 0.75). The ordered lamellar phases.36–38 The SEM image (Fig. S2c†) con- remaining rectangle shape mainly corresponds to the surface firms CTAPW12 layers-stacking morphology. We cannot rule capacitive process (b = 0.92). For crystallized PW12, the redox out the possibility of the partial crystallization of CTAPW12. pairs show various diffusion-controlled percentages. The Re1 However, even though CTAPW12 has formed single-layer short- and Ox1 redox pair, which presents the sharpest peaks, is range orderly lamellas, the layers still can accommodate them mostly diffusion-controlled (b = 0.6), whereas the other peaks, thanks to their structural compatibility. especially the broad peaks, are less diffusion-controlled or The observed isolated bright dots (PW12 clusters) have two even almost surface capacitive (b = 1 for Re2). This reveals the possible locations: on the surface of Ti3C2Tx nanosheets and nature of the multi-step redox reaction of crystallized PW12 in beneath the thin Ti3C2Tx layers. We speculate that PW12 clus- the organic electrolyte: the first step is more diffusion- ters are mostly beneath the Ti3C2Tx layers, because Ti3C2Tx controlled. layers can confine the growth of PW12 clusters and avoid the We performed linear fitting on the redox peaks of the three formation of thick aggregates. Electrochemical characteriz- MXene/POM hybrids, as shown in Fig. 6(b, c), (e, f ) and (h–l). ation, capable of distinguishing whether the associated redox The figures confirm the good fit. The fitting results are listed reactions are diffusion-controlled or surface capacitive, can in Table 3. The b-values of TMAMX-PW12 are close to 0.5, indi- help prove this hypothesis and is discussed below. cating that the redox reactions are almost 100% diffusion-con- Dunn et al. have proposed a method to distinguish whether trolled. This is not surprising with such large crystallized charge storage processes are diffusion-controlled or surface POMs particles.40 The b-values of TBAMX-PW12 increase. capacitive by carrying out CVs at various scan rates.39 The Several studies have pointed out that the energy storage dependence of the current response on the scan rate can mechanism shifts from diffusion-controlled to surface capaci- provide insights into the charge-storage mechanism according tive with decreasing particle size.41 TMAMX-PW12 has to eqn (2): PW12 microcrystals while TBAMX-PW12 has PW12 nanocrys- tals. The redox reaction in TBAMX-PW12 becomes less i ¼ av b ð2Þ diffusion-controlled since PW12 crystals are smaller. CTAMX-PW12 does not present larger b-values even though where i is the maximum peak current obtained at a specific PW12 clusters are dispersed homogeneously at the nanoscale. scan rate v, and a and b are adjustable parameters. The This is quite surprising at first glance, because in other current linear dependence on scan rate (b = 1) usually means studies (such as activated carbon/POMs,26,42 reduced graphene the charge is stored by a fast response mechanism, such as oxide/POMs,43 polypyrrole hydrogel wrapped CNT/POMs,44 surface capacitive. If the current response follows a linear and metal–organic framework/POMs45–47) where the POMs 14928 | Nanoscale, 2022, 14, 14921–14934 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. Fig. 6 CVs of (a) TMAMX-PW12 (d) TBAMX-PW12 and (g) CTAMX-PW12. Linear fitting of log currents versus log scan rates plots to determine b-values of reduction peaks and oxidation peaks: (b) and (c) for TMAMX-PW12; (e) and (f ) for TBAMX-PW12; (h) and (i) for CTAMX-PW12. ( j) Schematic illustration of the fabrication of POMs-intercalated MXene (CTAMX-PW12). clusters are nano-dispersed (bright dots detected in HAADF-STEM), the b-values are close to 1, revealing that the Table 3 b-Values derived from fitting current of reduction peaks (Re) and oxidation peaks (Ox) at various scan rates redox reactions of the nano-dispersed PW12 clusters are surface capacitive. The electrochemical behaviours of Ox1 Ox2 Ox3 Re1 Re2 Re3 CTAMX-PW12 seem to be contrary to these other studies. However, this is evidence to prove that those nano-dispersed TMAMX-PW12 0.51 0.50 0.52 0.54 0.50 0.55 TBAMX-PW12 0.66 0.57 0.62 0.55 0.96 0.67 PW12 clusters have intercalated in Ti3C2Tx layers, and have not CTAMX-PW12 0.58 0.66 0.63 0.51 0.88 0.57 just spread on the Ti3C2Tx surface. The intercalation of nano- This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14929 View Article Online Paper Nanoscale dispersed PW12 clusters remarkably reduced the crystallinity that POMs are de-attached upon cycling. This, in turn, means of MXene layers, leading to a FWHM (Full Width Half that PW12 in the form of microcrystals attached to Ti3C2Tx Maximum) of its (002) diffraction peak (Fig. 2) much wider nanosheets cannot withstand cycling. than the other MXene/POM hybrids, in which the PW12 clus- The situations for TBAMX-PW12 and DTAMX-PW12 are This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. ters have crystallized outside. In the other nano-dispersed better than TMAMX-PW12, but still not good enough. PW12 hybrid materials, the PW12 clusters are anchored on the TBAMX-PW12 maintains 21% of the initial peak current of surface of the substrates. Most of the active sites on PW12 are Ox1 after 20 cycles, and DTAMX-PW12 has 35% left. By exposed to the electrolyte directly. Their redox reactions are analogy, we can conclude that also PW12 nanocrystals are lost surface capacitive. In the case of CTAMX-PW12, the PW12 clus- upon cycling in the organic electrolyte. Meanwhile, the quasi- ters are dispersed homogeneously within Ti3C2Tx layers where rectangles derived from Ti3C2Tx almost keep their initial Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. they are surrounded by large cationic surfactant molecules. shape, indicating the good stability of Ti3C2Tx in the organic The electrolyte ions have to diffuse along the slit between electrolyte. The cation-intercalation peaks (at around −1.8 V) Ti3C2Tx layers to reach the PW12 clusters, restricting the redox sharpen after cycling, which must be associated with the acti- reactions under diffusion control. Fig. 6( j) illustrates the fabri- vation process. In pristine Ti3C2Tx, this cation-intercalation cation process of the POMs intercalated MXene. peak is less intense than in TBA or DTA-intercalated MXenes. Furthermore, we studied the electrochemical stability to This suggests enhanced cation-intercalation processes associ- check whether the PW12 clusters that can be attached on ated with the larger interlayer spacing provided by pre-interca- Ti3C2Tx maintain redox active long term. In fact, the pristine lated TBA and DTA cations. Indeed, MXenes pre-intercalated crystallized PW12 electrode undergoes severe redox activity loss with organic cations have been reported to accommodate more upon cycling (Fig. S4(b)†). Some studies applying pristine POMs cations from the electrolytes, resulting in increased electrodes have reported the same phenomenon.48 Therefore, capacities.23,31,49 Unlike the other three MXene/POM hybrids, most effort has been made in developing POMs-based hybrid the CVs of CTAMX-PW12 after 20 cycles almost overlap the first materials whose nano-dispersion improves the cycling stability. one. Even after 100 cycles, the peak current of Ox1 is still 104% Fig. 7 presents CVs of MXene/POM hybrids before and after of the first cycle, revealing its excellent electrochemical stability. the indicated number of cycles. Their galvanostatic charge–dis- To further analyze the electrochemical stability as well as to charge curves are presented in Fig. S5.† Comparing the CVs evaluate their charge-storage capability, we assembled a series before and after cycling, TMAMX-PW12 shows the most of asymmetric devices with activated carbon positive electrodes remarkable difference. All the peak currents drop dramatically. and MXene/POM negative electrodes. Fig. 8(a–c) presents the For instance, only 20% of the initial peak current of Ox1 galvanostatic charge–discharge curves. The small plateau on remains after 20 cycles. In order to establish the reason for charge and discharge curves and the variable slope confirmed this decay, we ran EDX analyses of the electrode after cycling the hybrid nature. Fig. 8(d) depicts the specific capacitance at and found a very substantial decrease of W in the sample different current densities. CTAMX-PW12 delivered higher (Fig. S6† and Table S2†). Thus, the most likely conclusion is specific capacitance than TBAMX-PW12 and TMAMX-PW12 Fig. 7 CVs of (a) TMAMX-PW12, (b) TBAMX-PW12, (c) DTAMX-PW12 and (d) CTAMX-PW12 before and after a few cycles at 2 mV s−1. 14930 | Nanoscale, 2022, 14, 14921–14934 This journal is © The Royal Society of Chemistry 2022 View Article Online Nanoscale Paper This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. Fig. 8 Galvanostatic charge–discharge curves of (a) TMAMX-PW12, (b) TBAMX-PW12 and (c) CTAMX-PW12. (d) Specific capacitance of MXene/ POM hybrids at various current densities. (e) Cycling stability test from charge–discharge data at 2 A g−1: inset are the comparison of cyclic polariz- ation curves of TBAMX-PW12 and TMAMX-PW12 before and after 500 cycles. (f ) Cyclic polarization curves of CTAMX-PW12 before and after 5000 cycles. Impedance spectra of MXene/POM hybrids (g) before and (h) after cycling. over the whole range, which must be attributed to the larger effective in enhancing volumetric capacitance.26 We cycled the interlayer distance.31 However, compared with pristine MXene asymmetric cells to test their stability (Fig. 8(e)). (30 F g−1 at 10 mV s−1 in the same electrolyte35), the inclusion TMAMX-PW12 and TBAMX-PW12 suffered a dramatic loss of CTAPW12 did not enhance the capacitance remarkably. from the beginning; only 70% and 78% of the initial capaci- PW12 has a large molecular weight (>2800 g mol−1), making it tance remained after 500 cycles, respectively. Their redox less effective in enhancing specific capacitance but more waves almost disappeared (Fig. 8(e) inset). By contrast, This journal is © The Royal Society of Chemistry 2022 Nanoscale, 2022, 14, 14921–14934 | 14931 View Article Online Paper Nanoscale CTAMX-PW12 maintained 85% after 5000 cycles. Fig. 8(f ) con- intercalate into Ti3C2Tx layers easily, but their expansion of the firmed that the POMs were still anchored on MXene and redox interlayer spacing is not enough to accommodate PW12 active after 5000 cycles. anions. CTA+ delaminated Ti3C2Tx (CTAMX) works well thanks Fig. 8(g and h) compares the electrochemical impedance to its shape and interlayer spacing (2 nm). The XRD pattern This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. spectra before and after cycling. The intercepts at the very confirms that POM-intercalated MXene (CTAMX-PW12) still high-frequency region represent equivalent series resistance keeps a relatively large interlayer distance (1.7 nm). (Rs). The arc at high frequency represents charge-transfer resis- HAADF-STEM shows the PW12 clusters spread homoge- tance Rct. The following transitory parts between the semicir- neously. The electrochemical analyses prove that the redox cles and the final linear parts represent the Warburg impe- reactions of PW12 are still diffusion-controlled in dance associated with mass transfer (diffusion). We fitted the CTAMX-PW12. Combining the information provided by XRD Open Access Article. Published on 12 August 2022. Downloaded on 11/2/2022 8:34:07 AM. spectra with a modified Randles circuit (Fig. S7†), and the patterns, HR-TEM images, HAADF-STEM images, and electro- fitted values are listed in Table S3.† All the spectra feature chemical characterization, we can reach the following con- similar Rs thanks to the same setup. The Rct of the fresh elec- clusion: assisted by CTA cations, PW12 clusters can be interca- trodes follows this order: CTAMX-PW12 < TBAMX-PW12 < lated and spread homogeneously at the nanoscale within TMAMX-PW12, which agrees well with their crystal size. In Ti3C2Tx layers. Contrary to the other POMs/MXene hybrids these MXene/POM hybrids, MXene acts as the conductive tested in this work, the CTA-dispersed-POMs intercalated scaffold while POMs provide extra redox activity but are poorly MXene can be reversibly and sustainably cycled in both three- conductive. The POM clusters near MXene can accept electrons electrode cells and two-electrode cells, showing a superior per- and quickly carry out redox reactions. By contrast, the interior formance compared with the other crystallized POM-MXene clusters of large POM crystals suffer long, inefficient electron hybrids. transfer, leading to larger charge-transfer resistance. After Thus, our work has shown how to intercalate and stabilize cycling, all the spectra present larger semicircles. This can be POMs within MXene, keeping the electroactivity of both in a explained by the fact that the expansion and shrinkage of synergic combination which could be used to harness the MXene layers caused by repeated intercalation–deintercalation electrochemical activity of POMs in a variety of applications. of cations impairs the attachment of POMs on MXene. TBAMX-PW12 and TMAMX-PW12 suffered a larger increase in Rct, agreeing well with their worse cycling stability. Conflicts of interest The difference between CTAMX-PW12 and the other MXene/POM hybrids must be related to microstructure. This There are no conflicts to declare. excellent electrochemical stability is usually found in well-dis- persed and well-anchored POMs-based hybrid materials, such as activated carbon/POMs,26 graphene/POMs43 and metal– Acknowledgements organic framework/POMs.50 TMAMX-PW12, TBAMX-PW12 and DTAMX-PW12 behave like pristine crystallized PW12 electrodes Partial funding from Ministerio de Ciencia Innovacion y because the PW12 clusters have crystallized into micro or Universidades (MCIU), the Agencia Estatal de Investigacion nanoparticles outside MXene layers. The micro or nano- (AEI) and the European Regional Development Fund (FEDER) particles are attached to or surrounded by multilayer Ti3C2Tx (grants RTI2018-099826-B-I00 and PID2021-128390OB-I00) is flakes. Ti3C2Tx serves as both an active component and the gratefully acknowledged. ICN2 is funded by the CERCA pro- conductive substrate for PW12 anchoring. The conductivity gramme/Generalitat de Catalunya, and also supported by the within the PW12 particles is still poor. Severo Ochoa Centres of Excellence programme, funded by the On the contrary, in the case of CTAMX-PW12, PW12 clusters Spanish Research Agency (AEI, grant no. SEV-2017-0706). J. J Z. are nano-dispersed, rigidly intercalated within Ti3C2Tx layers. acknowledges his scholarship (no. 201806370211) under the Every PW12 cluster has direct access to the conductive Ti3C2Tx China Scholarship Council. This work has been carried out substrate. And, last but not least, an EDX analysis of the elec- within the framework of the doctoral program (PhD) of trode after 100 cycles showed only a minor loss of W in the Material Science (Department of Physics) of Universitat sample, as compared with TMAMX-PW12 (Fig. S6† and Autonoma de Barcelona (UAB). Table S2†), all of which contribute to explaining the cycling stability. 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