Versatile Cutting Method for Producing Fluorescent Ultrasmall MXene

Nov 7, 2017 - Rheological Characteristics of 2D Titanium Carbide (MXene) Dispersions: A Guide for Processing MXenes. Bilen Akuzum , Kathleen Maleski ,...
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Versatile Cutting Method for Producing Fluorescent Ultrasmall MXene Sheets Zhiqiang Wang,† Jinnan Xuan,† Zhigang Zhao,‡ Qingwen Li,‡ and Fengxia Geng*,† †

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou 215123, China Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Sciences, 398 Ruoshui Road, Suzhou Industry Park, Suzhou 215123, China

‡

S Supporting Information *

ABSTRACT: As a recently created inorganic nanosheet material, MXene has received growing attention and has become a hotspot of intensive research. The efficient morphology control of this class of material could bring enormous possibilities for creating marvelous properties and functions; however, this type of research is very scarce. In this work, we demonstrate a general and mild approach for creating ultrasmall MXenes by simultaneous intralayer cutting and interlayer delamination. Taking the most commonly studied Ti3C2 as an illustrative example, the resulting product possessed monolayer thickness with a lateral dimension of 2−8 nm and exhibited bright and tunable fluorescence. Further, the method could also be employed to synthesize ultrasmall sheets of other MXene phases, for example, Nb2C or Ti2C. Importantly, although the strong covalent M−C bond was to some extent broken, all of the characterizations suggested that the chemical structure was composed of well-maintained host layers without observation of any serious damages, demonstrating the superior reaction efficiencies and safeties of our methods. This work may provide a facile and general approach to modulate various nanoscale materials and could further stimulate the vast applications of MXene materials in many optical-related fields. KEYWORDS: MXene, two-dimensional materials, intercalation, delamination, ultrasmall sheets, fluorescence

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he morphology control of nanoscale structures, especially dimensionality modulation, provides tremendous and exciting opportunities for creating materials with marvelous properties and functions. These materials can be utilized as important building components for the assembly of a wide variety of practical devices.1−3 Two main directions for reducing the dimensionality of inorganic lattices are available: (1) lowering the thickness down to the molecular or atomic level and (2) decreasing the lateral dimensions, even down to the quantum confinement range in some extreme cases. For the former, a number of unilamellar sheets have been produced by delamination from layered host compounds and have attracted considerable interest in the materials research community. The sheets possess ultimate two-dimensional anisotropy with a lateral size on the micrometer scale, whereas the thickness is on the nanometer level, endowing the material with a wealth of interesting physical phenomena and applications.4−6 For the latter, nanoparticles that reduce at least one effective dimension from three-dimensional bulk materials to a tiny size of several nanometers have been synthesized by a variety of methods.7−9 The quantum confinement effects in these systems with reduced space dimensions would modify electronic and optical properties of these structures, which is important for advancement in both fundamental science and industrial technology fields.10,11 © 2017 American Chemical Society

Although it is recognized that efficient cleavage of inorganic lamellar compounds into individual layers and simultaneously reducing the lateral size of monolayered sheets would offer unexpected opportunities,12,13 the related studies are rather limited because the bonds within host layers are usually very strong covalent bonds. An intensive reaction may yield breakage of unit layers into smaller parts, but in most cases, damage to the intrinsic chemical structure of the materials would also unavoidably happen to some extent.14,15 In the rich pool of two-dimensional materials, the so-called MXene’s family is one of the youngest, and these materials are receiving an explosion of interest due to their diverse and intriguing structural, mechanical, physical, and chemical properties.16−22 The multilayered MXenes are produced by selectively etching out A from a parent MAX phase with the general formula of Mn+1AXn, where M denotes an early transition metal, A represents a main group 3 or main group 4 element, and X is either C or N. All known MAX structures are crystallized in a layered hexagonal structure with P63/mmc symmetry with the A element interleaving between regularly Received: September 12, 2017 Accepted: November 6, 2017 Published: November 7, 2017 11559

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Figure 1. Schematic illustration of the synthetic process for preparing ultrasmall Ti3C2 sheets. After the reaction with TMAOH, the host layers fragmented into smaller parts, and at the same time, TMA ions were intercalated into the gallery, in which layer cutting and delamination simultaneously occurred upon ultrasonication, producing the targeted ultrasmall sheets.

hand. It was interesting to find that, in some occasions, defects were present on the sheet surfaces (Supporting Information, S1), which may suggest that there was a certain reactivity between TMAOH and surface Ti atoms in the structure. To enhance the reaction efficiency, in this study, we adopted multilayered MXene, that is, the phase with interlayer Al extracted with concentrated HF, as the starting crystals. TMA ions were intercalated into the gallery space, and in the meantime, the host layers were cut into smaller fragments, after which an ultrasonication treatment led to in-plane fragmentation and perpendicular cleavage of the layers, thus producing the targeted ultrasmall sheets. The crystal structure, morphology, and chemical composition of the phases were systematically investigated by X-ray powder diffraction (XRD) measurements, scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) characterizations. The starting MXene crystals were produced according to previously reported procedures, using HF to selectively extract the Al layer located between the regular stacking of Ti3C2 layers, and its characterizations can be found in the Supporting Information, S2 and S3. With the removal of the Al layer, the stacking was to some extent loosened due to the loss of metallic Ti−Al bond binding the neighboring layers, in which the basal spacing shifted from the original value of 9.9 Å at approximately 9.51° to a slightly larger value of 10.8 Å at 8.17°. EDS analysis confirmed the reactions, which showed a significant decrease in the Al content. The occurrence of O and F signals was also detected, which should be explained by their high tendency to terminate the exposed Ti surfaces, in good agreement with previously documented reports.25 The subsequent intercalation of TMA ions when reacting in aqueous TMAOH was evidenced by observation of a further basal lattice expansion to 15.2 Å in the XRD characterizations and detection of an N signal in the EDS analysis, which would facilitate the delamination into monolayers under external shearing forces. Importantly, while most of the nonbasal reflections disappeared due to the loss of layer-to-layer registry, the characteristic in-plane diffraction, that is, (110) peak, was found to be preserved, suggesting that the structure of the host layer was well-maintained with no appreciable chemical degradation or oxidation. Still, a broadening of the peak was discerned, which indicated shrinkage of the crystalline sizes. Based on the Scherrer equation, the average crystallite sizes were 27.0 and 15.8 nm for starting Ti3AlC2 and HF-etched multilayered MXene, respectively, but decreased to 12.5 nm after reaction in aqueous TMAOH. The lateral size reduction was also verified by SEM morphology observations. All of the results are very suggestive hints that the layers may have fragmented in the treatment in aqueous TMAOH. Combining

stacked Mn+1Xn layers, where the M layers are nearly close packed and X atoms fill the octahedral sites. This structure is very strongly held by metallic M−A and M−X bonds in a mixed covalent/metallic/ionic character. Therefore, the exfoliation of the structure to selectively draw the A layer or further delamination into individual Mn+1Xn layers was not realized until very recently by taking advantage of the relatively weak strength of the M−A bond compared with that of M−X. The principle involves selectively etching out the Al layer by a strong corrosive acid, such as hydrofluoric acid (HF), to weaken the interlamellar binding forces, which facilitates subsequent bulky guest intercalations and delaminations.13,23 Alternatively, we recently discovered that the Al layer can also be extracted by reacting Al in a base environment, for example, tetramethylammonium hydroxide (TMAOH), in which case TMA ions could be simultaneously intercalated, resulting in delaminations in one single step.19,24 However, the further breakage of the primary strong M−X bond to produce ultrasmall sheets of this family remains a critical challenge, which needs to be urgently addressed to further broaden our understanding and widen the applications of this important family. Here, we demonstrate an in-depth study for the fabrication of their ultrasmall sheets by simultaneous layer cutting and stacking cleavage through a mild reaction in aqueous TMAOH. Taking the most widely studied member, Ti3C2, as a representative case, the obtained ultrathin tiny structure was 2−5 nm in lateral width and 0.5−1.5 nm in thickness. The soobtained ultrasmall Ti3C2 sheets showed strong optical absorption and excitation-dependent emission behavior, endowing the materials with great potential in a variety of important applications. Furthermore, the synthetic method can also be extended to produce ultrasmall sheets of other members in this family, for example, Ti2C and Nb2C, therefore establishing its general applicability.

RESULTS AND DISCUSSION The fabrication process of ultrasmall Ti3C2 monolayers involved a simple and mild reaction in aqueous TMAOH followed by an ultrasonic treatment, as schematically demonstrated in Figure 1. The reaction with TMAOH induced both in-plane fragmentation of MXene host layers and gallery intercalation of bulky guests, which facilitated the delamination into monolayers and facile cutting into small pieces after applying ultrasonication, yielding ultrasmall monolayers. We recently reported that TMAOH could hydrolyze interlayer Al of Ti3AlC2 crystals into Al(OH)4− by destroying the strong Ti− Al bonds and simultaneously intercalating TMA+ into the gallery, resulting in delamination into unilamellar nanosheets with the aid of a gentle shear force, for example, shaking by 11560

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Figure 2. (a) Representative low-magnification, (b) high-resolution TEM images, and (c) tapping-mode AFM image for the as-prepared ultrasmall Ti3C2 sheets. Inset in (a): photograph of the colloid clearly displaying the Tyndall scattering effect, which confirmed the abundant presence of small sheets. Inset in (b): corresponding SAED pattern showing individual spots in a hexagonal arrangement, as expected from the intrinsic symmetry of Ti3C2 layers. Inset in (c): height profile of the ultrasmall Ti3C2 sheets. Statistical distribution of (d) lateral width and (e) thickness of the obtained ultrasmall Ti3C2 sheets. (f) ζ-Potential spectrum of the colloid.

characterizations (Figure 2d,e and Supporting Information, S5), giving mean values of approximately 4.0 and 1.0 nm, respectively. Lateral sizes of the sheets could to some extent be adjusted with synthesis parameters, for example, HF concentration and corrosion period, liquid cascade centrifugation speed, etc. If the starting Ti3AlC2 crystals were only contacted with HF of low concentrations or of limited period, by which the Al layer sandwiching between Ti3C2 sheets still remained, TMAOH would mainly work to extract Al. In this case, TMAOH could only intercalate into the gallery with almost no fragmentation, and sheets of relatively large sizes would be obtained. In contrast, when the Al layer was completely extracted with concentrated HF or extended reaction, TMAOH would fragment the host layer along with intercalation, and thus sheets of relatively smaller sizes would be obtained. Meanwhile, with the liquid cascade centrifugation technique normally applied to colloidal dispersions, sheet sizes could be possibly selected. The detailed data will be reported in a future work. The XRD pattern of ultrasmall sheets collected by highspeed centrifugation manifested no obvious peak except for a broad halo at the low-angle area (Supporting Information, S6), further confirming their highly delaminated nature. The ζpotential measurement on aqueous colloids of the ultrasmall sheets, shown in Figure 2f, gave a negative value reaching −60 mV, which suggested that the sheet surface was terminated with negative moieties, probably inherited from the parent MXene crystals, for example, OH− and F−. Due to the surface charges and electrostatic repulsions among the nanosized particles, these ultrasmall sheets could be dispersed in a variety of solvents, including ethanol, methanol, dimethylsulfoxide, N,Ndimethylformamide, formamide, N-methyl-2-pyrrolidone, and some others, all exhibiting excellent colloidal stability (Supporting Information, S7). The surface compositions and chemical structure of the asobtained ultrasmall Ti3C2 sheets were examined by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). The survey spectrum revealed the existence of F and O in addition to Ti and C (Supporting Information, S8). In the high-resolution Ti 2p spectrum

the aforementioned interlayer expansion with TMAOH intercalation, the structure would be readily broken into monolayers of smaller pieces. As expected, an ultrasonic treatment of the fragmented, TMA-intercalated phase in H2O produced a steady aqueous colloidal emulsion, which showed no noticeable precipitation even after being kept still for weeks at room temperature. The resulting colloid displayed a clear Tyndall scattering effect when irradiated with a laser beam, as indicated in the inset of Figure 2a, suggesting the presence of abundant nanosized crystallites dispersed in the aqueous medium. Information regarding the average hydrodynamic diameter of the ultrasmall sheets was retrieved from dynamic light scattering measurements, which observed a homogeneous Gaussian distribution of the particles sizes centering around 200 nm (Supporting Information, S4). A representative TEM image of the freshly obtained colloid is presented in Figure 2a, which unambiguously illustrates the formation of fairly uniform ultrasmall pieces with widths ranging from 2 to 8 nm. The high-resolution TEM (HRTEM) image in Figure 2b manifested clear fringes in each nanocrystal, suggesting the single-crystalline feature of the obtained ultrasmall sheets. The distance between adjacent lattice fringes was elucidated to be 1.5 Å, matching with the (11) lattice spacing of an individual Ti3C2 layer. The single crystallinity was also supported by the selected area electron diffraction (SAED) pattern acquired from one of the ultrasmall sheets (inset in Figure 2b), which exhibited individual spots instead of diffraction rings. In addition, the spots were in a hexagonal arrangement as expected for the in-plane symmetry intrinsic to Ti3C2 layers. Atomic force microscopy (AFM) images of the product, collected by dip coating a polyethylenimine-pretreated Si wafer in a fresh emulsion, and the corresponding height profile in Figure 2c revealed highly dispersed sheets on the Si substrate with a typical topographic height of ca. 1.0 nm, providing a direct and reliable evidence for the monolayer or bilayer characteristics. The average lateral width and thickness were studied with more detail in several areas and evaluated from a systematic statistical analysis performed on 200 tiny sheets from five different spots in the TEM and AFM 11561

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Figure 3. High-resolution XPS spectra of (a) Ti 2p, (b) O 1s, and (c) FT-IR spectrum for the obtained ultrasmall Ti3C2 sheets, displaying that the intrinsic chemical structure of Ti3C2 was well maintained.

in the diffusion rate-limiting process of thermal transformation from Ti0.87O2 to TiO2.30 It is well-known that MAX phases have a quite large family of members with the general composition of Mn+1AXn; we then attempted to cut and delaminate some other Al-containing phases to demonstrate the general applicability of this method. Nb2AlC and Ti2AlC were chosen as examples. Possibly due to the different stability or reactivity of their in-plane bonds, the synthesis parameters, including concentration of HF or TMAOH, reaction period, etc., have to be carefully adjusted before success. The reaction intensity should be tuned to be stronger and weaker for Nb2AlC and Ti2AlC, respectively. Figure 4a,b shows the representative TEM images for the finally

displayed in Figure 3a, no obvious trace of Ti−Al at lower energies of approximately 452 eV was observed, evidencing the removal of the Al layer while showing evident signals corresponding to Ti−C and Ti−O/F.26 The former should originate from the intrinsic nature of Ti3C2 layers, and the latter confirmed the surface termination with −O(H) and/or F. Correspondingly, the O 1s spectrum in Figure 3b could be fitted by components corresponding to O−Ti, OH−Ti, and H2O−Ti at 530.0, 531.5, and 532.8 eV, respectively,27 additionally evidencing the surface terminations of −O(H) along with strongly absorbed H2O molecules. In good consistencies, the FT-IR analysis also detected signals corresponding to −OH and Ti−O, along with Ti−C (Figure 3c).28 The conspicuous feature of Ti−C in all of the characterizations verified that the internal atomic structure of the Ti3C2 layers was almost perfectly retained from the pristine multilayered MXene crystals, and these ultrasmall sheets did not experience any chemical change or oxidation during the mild delamination and fragmentation process. It should be noticed that when the MXenes were hydrothermally treated to induce the preparation of quantum dots, it was reported that oxidation was unavoidable, and the oxidizing degree was more severe with increasing temperature.14,15 Therefore, the present approach of fragmenting host layers and weakening the interlamellar forces with TMAOH provided a mild and facile synthetic route to produce ultrasmall sheets. Stability of the obtained ultrasmall sheets against oxidizing media, for example, ambient air, H2O2, etc., was studied and compared with large sheets including those prepared with milder etchants HCl + LiF25 and HF-dimethylsulfoxide,23 from which it was surprising to see that the ultrasmall sheets manifested the highest stability (Supporting Information, S9). A recent report studied in-depth the oxidation behavior of titanium carbide sheets obtained via directly sonicating multilayered MXene,29 which found that the oxidation process was basically edge-driven and smaller sheets would be less stable. The seemingly different results may be explained by their different sheet thicknesses. The ultrasmall sheets in the present work may contain higher percentage of unilamellar materials because the synthesis process included an additional step of TMA intercalation to further weaken the interlamellar forces. Considering the oxidation reaction from Ti3C2O2 to TiO2 involves a structural recrystallization, atomic diffusion in thinner sheets is supposed to be more difficult, which may account for the unusual high stability of the ultrasmall sheets. This was also proved by the final oxidized phases, anatase/rutile TiO2 for the sheets from milder etchants, HCl + LiF and HFdimethylsulfoxide, but stacking aggregates for the ultrasmall unilamellar sheets. Similar phenomenon was also reported for titania sheets, single layer sheets exhibiting the highest stability

Figure 4. TEM images for the product of (a) Nb2C and (b) Ti2C ultrasmall sheets, demonstrating the general applicability of this synthesis procedure.

obtained Nb2C and Ti2C. The in-plane dimensions were found to be cut into sizes of 60−75 and 2−5 nm for Nb2C and Ti2C, respectively. The Nb2C materials possessed relatively larger lateral sizes. Additionally reminiscent of the fact that normally more severe conditions, higher HF concentrations and extended reaction times, were required to exfoliate Nb2AlC, this should be related with the higher chemical stability for Nb−C bonds. The broad applicability of the present approach to synthesize ultrasmall sheets of diverse materials underlined the excellent general applicability and advantages of this synthesis protocol for cutting MAX phases. The optical properties of the colloid containing ultrasmall Ti3C2 sheets were studied by ultraviolet−visible (UV−vis) absorption and photoluminescence (PL) spectroscopy. The recorded UV−vis absorption spectrum for the colloid is provided in Figure 5a, exhibiting an abrupt increasing absorption below 320 nm, which almost reached the maximum at approximately 250 nm. This spectral feature remarkably differed from that of parent MXenes or TMA-intercalated phase, in which the diffuse reflectance spectrum revealed an almost constant and featureless absorption over a wide wavelength range of 200−800 nm. Although their different detection modes made a simple and quantitative comparison 11562

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Figure 5. (a) UV−vis spectra for colloid of ultrasmall Ti3C2 sheets at various concentrations of 0.111, 0.056, 0.037, 0.028, and 0.022 mg mL−1. The diffuse scattering spectra for MXene and TMA-reacted phase are also shown for comparison. (b) Absorbance at 245 nm plotted against concentration, showing a nearly linear manner. (c) UV−vis absorption, excitation, and emission spectra of as-prepared ultrasmall Ti3C2 sheets. (d) Emission spectra under different excitation wavelengths from 340 to 500 nm. Inset in (d) shows the corresponding normalized emission spectra.

phenomenon has been attributed to the optical selection of differently sized or surface defects within the sample ensemble. With the increased excitation wavelength from 340 to 500 nm, the fluorescence peaks correspondingly red-shifted from 460 to 580 nm, as shown in Figure 5d (the inset displays the normalized emission spectra). While the origin of luminescence and excitation-dependent PL behaviors still has controversy and is awaiting discovery,35,36 these results indicated the distinctiveness of the as-prepared ultrasmall Ti3C2 sheets, and the multiple PL colors under different excitation wavelengths are important for certain practical applications. The emission quantum yield of the ultrasmall Ti3C2 sheets was determined according to the comparative method, yielding a high value of 8.9%. Furthermore, the photoluminescence under different pH values was examined, which showed no significant change in either wavelength or intensity with increasing pH close to neutral and alkaline values beyond 6, whereas the intensity decreased at low pH values (pH 2−5) (Supporting Information, S13). The possible reason for this phenomenon may be the passivation of surface terminating groups (OH− or F−) in strong acid media, providing additional evidence for the surface capping of the ultrasmall sheets.

difficult, the significant spectral change before and after delamination was most likely associated with the cleavage of three-dimensional stacked crystals into individual layers with a sub-nanometer thickness. Similar optical behaviors were also reported for MnO2 sheets delaminating from bulk layered manganese oxide, H0.13MnO2·H2O.31,32 The position of the absorption edge may be closely related to the lateral dimensions of the sheets due to the ubiquitous size effect, which still needs more elaborate experiments. Compared with normal size sheets, obviously the absorption edge exhibited a slight blue shift, which has been commonly observed in many dimensionally reduced materials due to size effect (Supporting Information, S10). To exclude the possibility of the presence of carbon dots that would strongly interfere with fluorescence behavior of samples, we characterized the sample with Raman technique, which is a powerful tool to detect carbon traces. In the Raman spectrum of the ultrasmall sheets, no G or D band characteristic of carbonaceous materials was detected, excluding the possibility of carbon production and interference with fluorescence behaviors (Supporting Information, S11). Figure 5b depicts the dependence of absorbance at 250 nm on sheet concentrations, showing a nearly linearly function, which suggested that no possible association among the ultrasmall sheets occurred in this concentration range. Noticeably, the characteristic features in various solvents were almost the same (Supporting Information, S12), suggesting that the optical behavior originated from the ultrasmall sheet structure rather than any solvation effects. The excitation spectrum of the colloid showed a maximal luminescence at ca. 405 nm, and on excitation near 405 nm, the PL spectrum manifested a broad and symmetric fluorescence peak at 509 nm, as depicted in Figure 5c. Interestingly, similar to the hydrothermally synthesized carbon or graphene quantum dots,33,34 the colloid of the so-obtained ultrasmall Ti3C2 sheets exhibited an excitation-dependent emission behavior. The

CONCLUSION In summary, a generic and mild approach for producing ultrasmall MXene sheets was developed by simultaneous intralayer cutting and interlayer delamination. Owing to the efficient but mild reactions, the intrinsic chemical structure of parent phases was well maintained in the ultrasmall sheets. For the mostly studied Ti3C2, the product was 4 nm in lateral dimensions and 1 nm in thickness on average. The attempts to extend the method to MXenes of other compositions, Nb2C and Ti2C, were also successful, although the lateral dimensions of final products seemed to be highly related with the chemical stability of MAX phases. Importantly and interestingly, the 11563

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ASSOCIATED CONTENT

ultrasmall sheets manifested optical behavior quite similar to carbon dots, displaying strong photoluminescence and excitation-dependent behavior. We believe that this work can provide a versatile approach for creating diverse nanoscale materials and additionally broaden the applications of MXene materials in optical and display fields.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06476. TEM image of Ti3C2 sheets in aqueous TMAOH, XRD, SEM, and EDS data for as-received Ti3AlC2 crystals, starting MXene crystals, and TMA-reacted phase, XRD, DLS distribution, survey XPS spectrum, oxidation stability for the ultrasmall sheets, more TEM and AFM images, digital photos showing dispersity in other solvents and optical absorption in the solvents, optical absorption comparison with large sheets, and photoluminescence at various pH (PDF)

EXPERIMENTAL SECTION Materials and Chemicals. High-purity powders of Ti3AlC2 (99.8%, 400 mesh) were purchased from Forsman Scientific (Beijing) Co., Ltd., China. HF (48−51.0%) and TMAOH (25% solution) were purchased from J&K Scientific Co., Ltd., China. All chemical reagents in this study were of analytical grade and were used as received without further purification. All solutions were prepared using distilled water with resistivity not less than 18.2 MΩ·cm (Direct-Q UV, Millipore). Producing Ultrasmall Sheets. The starting crystals of multilayered MXenes were prepared following a modified previously reported procedure by treating Ti3AlC2 powder with particle size less than 38 μm in 50% aqueous HF solution for 24 h at room temperature. To remove the excess HF, the resulting suspension was thoroughly washed with deionized water until the pH of the liquid reached 5−6. The wet sediment was then reacted in 25% aqueous TMAOH for another 24 h to induce guest intercalation and host layer fragmentation, which was separated by centrifugation at 3500 rpm for 15 min. The obtained powder was dispersed in deaerated water with a sample/water weight ratio of 1:300 and then sonicated in a water bath under flowing Ar for approximately 10 h. To remove the presence of any big particles, the system was centrifuged at 5000 rpm for 30 min, and the supernatant colloid was obtained for characterization and further uses. Morphology and Structural Characterization. The crystallographic phases of the samples were characterized by powder X-ray diffraction technique, and the patterns were recorded on a Rigaku Ultima IV diffractometer with Cu Kα radiation (λ = 0.15405 nm). The sample morphologies and dimensions were studied with a FEI/S-4700 scanning electron microscope at an accelerating voltage of 10 kV equipped with an energy-dispersive X-ray spectroscopy module for elemental analyses. TEM investigations were conducted on an FEI G20 microscope with a field emission gun operated at 200 keV. The samples were prepared by dropping a diluted suspension of the sheet colloid in water on a carbon-coated copper grid. AFM images were collected on an atomic force microscope (Bruker instrument Dimension Icon) in tapping mode. The ζ-potential surface measurements of the obtained ultrasmall sheets were carried out on a ZetaPlus ζ-potential analyzer. XPS spectra were obtained from Axis Ultra DLD equipped with an Al anode X-ray tube and helium lamp. Infrared spectra of the samples were recorded on a Varian 3100 FT-IR spectrometer in the 4000−400 cm−1 range with a nominal resolution of 0.4 cm−1 using powdered samples diluted in KBr pellets. Raman spectra were measured using an excitation wavelength of 514 nm provided by an Ar laser on an XploRA laser Raman spectrometer. UV−vis absorption measurement was carried out on a Shimadzu UV3600 UV−vis−NIR spectrophotometer, and PL emission spectra were recorded on a FluoroLog-3 fluorescence spectrophotometer. The confocal imaging was conducted on a LeciaSP5 laser scanning confocal microscope. Calculation of Quantum Yield. The quantum yield (Φ) of the ultrasmall MXene sheets was calculated by the comparative method using rhodamine 6G dissolved in water as a reference based on the following equation:

Φs = Φr ×

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Fengxia Geng: 0000-0001-5557-4165 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (51402204 and 51772201), Thousand Young Talents Program, Jiangsu Specially-Appointed Professor Program, and a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. REFERENCES (1) Xiao, F. X.; Pagliaro, M.; Xu, Y. J.; Liu, B. Layer-by-layer Assembly of Versatile Nanoarchitectures with Diverse Dimensionality: A New Perspective for Rational Construction of Multilayer Assemblies. Chem. Soc. Rev. 2016, 45, 3088−3121. (2) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and Quantum Dots. Science 1996, 271, 933−937. (3) Liu, Y. H.; Zhao, J. B.; Li, Z. K.; Mu, C.; Ma, W.; Hu, H. W.; Jiang, K.; Lin, H. R.; Ade, H.; Yan, H. Aggregation and Morphology Control Enables Multiple Cases of High-efficiency Polymer Solar Cells. Nat. Commun. 2014, 5, 5293. (4) Collman, J. P. Patterns of Organometallic Reactions Related to Homogeneous Catalysis. Acc. Chem. Res. 1968, 1, 136−143. (5) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Liquid Exfoliation of Layered Materials. Science 2013, 340, 1226419. (6) Samorì, P.; Palermo, V.; Feng, X. L. Chemical Approaches to 2D Materials. Adv. Mater. 2016, 28, 6027−6029. (7) Lim, S. Y.; Shen, W.; Gao, Z. Q. Carbon Quantum Dots and Their Applications. Chem. Soc. Rev. 2015, 44, 362−381. (8) Li, X. M.; Rui, M. C.; Song, J. Z.; Shen, Z. H.; Zeng, H. B. Carbon and Graphene Quantum Dots for Optoelectronic and Energy Devices: A Review. Adv. Funct. Mater. 2015, 25, 4929−4947. (9) Carey, G. H.; Abdelhady, A. L.; Ning, Z. J.; Thon, S. M.; Bakr, O. M.; Sargent, E. H. Colloidal Quantum Dot Solar Cells. Chem. Rev. 2015, 115, 12732−12763. (10) Cullis, A. G.; Canham, L. T. Visible Light Emission Due to Quantum Size Effects in Highly Porous Crystalline Silicon. Nature 1991, 353, 335−338. (11) Cong, S.; Tian, Y. Y.; Li, Q. W.; Zhao, Z. G.; Geng, F. X. SingleCrystalline Tungsten Oxide Quantum Dots for Fast Pseudocapacitor and Electrochromic Applications. Adv. Mater. 2014, 26, 4260−4267. (12) Nakamura, K.; Oaki, Y.; Imai, H. Monolayered Nanodots of Transition Metal Oxides. J. Am. Chem. Soc. 2013, 135, 4501−4508.

η2 Is A × s2 × r Ir As ηr

where I, η, and A represent the measured integrated emission intensity, refractive index of solvent, and absorption of the reference (r) and ultrasmall MXene sheet sample (s), respectively. 11564

DOI: 10.1021/acsnano.7b06476 ACS Nano 2017, 11, 11559−11565

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DOI: 10.1021/acsnano.7b06476 ACS Nano 2017, 11, 11559−11565