Articleshttps://doi.org/10.1038/s41893-019-0373-4

1State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, China. 2College of Chemistry, New Campus, Fuzhou University, Fuzhou, China. 3School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan, China. *e-mail: yjxu@fzu.edu.cn

Heavy metal ions (HMIs) are a major global water concern because of their toxicity, high mobility and non-biodegrad-ability1. Although various adsorbents have been investigated

for HMI removal2,3, most adsorbents applied for HMI removal suf-fer from low adsorption capacity and require tedious and sophisti-cated separation processes, such as precipitation and centrifugation. Moreover, adsorbent removal processes are generally highly selective towards capture of a specific HMI. In recent years, increasing inter-est has been shown in the use of two-dimensional materials (2DMs) for HMI removal due to their well-known advantages of high surface area and abundant surface functional groups4,5. In this field, the pro-duction of 2DM films as integrated functional materials represents an emerging paradigm for water purification. It should be noted that 2DM-based films are typically applied as pressure-driven filtration membranes for water treatment, requiring a pressure difference to drive water molecules through the membrane’s nanochannels while selectively rejecting other substances6–10. In contrast, pressure-free water purification over 2DM-based films via adsorption constitutes an appealing strategy, featuring simple water treatment without the requirement for external facilities to generate pressure. In compari-son to traditional powder adsorbents, 2DM-based films contain several advantages regarding water purification. First, assembly into films improves the environmental stability of 2DMs compared to their counterparts (dispersed nanosheets) by decreasing their exposure to the oxidation environment, which prolongs their life-time for water treatment11. In addition, it is time- and cost-effec-tive to recycle 2DM-based films after water purification. However, it remains challenging to construct efficient 2DM-based films for pressure-free water purification due to the thermodynamic sink of 2DMs present during the reassembly process. Specifically, 2DMs are thermodynamically inclined to aggregate by face-to-face restacking to form compact structures of high packing density. This results in limited exposed surfaces and restricts water penetration through 2DM-based films12,13. Consequently, 2DM-based films exhibit limited solid–liquid interactions for effective HMI removal under

pressure-free conditions. The efficient application of 2DM-based films is the ‘holy grail’ for pressure-free HMI removal.

The complexity of HMIs in waste water presents another major obstacle to the use of 2DM-based films in pressure-free water puri-fication. Along with positively charged (cationic) HMIs, such as Ag+, there are several negatively charged (anionic) HMIs such as HCrO4

−, AuCl4− and PdCl4

2−. Many HMIs pose public safety con-cerns. For example, HCrO4

− is carcinogenic and mutagenic to liv-ing organisms. Multiple HMIs originate from electronic wastes, and excessive amounts in the human body cause health concerns2,14. Consequently, it is imperative to remove these HMIs from waste water to meet discharge standards. The underlying chemistries for the removal of HMIs by 2DMs are predominantly governed by coordination chemistries or electrostatic attractions4,5,15–17, which tend intrinsically to determine that 2DMs can selectively adsorb only specific HMIs. It remains challenging to remove multifarious HMIs by a single type of 2DM, due to limited functional accessibil-ity. In this scenario, to realize the efficient pressure-free removal of multifarious HMIs over 2DM-based films, availability of the follow-ing should be systematically considered: (1) surfaces that are highly accessible to HMIs; (2) high wettability to facilitate water flow; and (3) favourable surface interactions between 2DMs and HMIs to drive the spontaneous removal of multiple HMIs.

MXenes are a family of two-dimensional (2D) transition metal carbides and nitrides. As the most studied MXene, hydrophilic Ti3C2Tx with functional groups T (for example, hydroxyl, oxygen or fluorine) is chemically versatile18–22. Herein, we report the appli-cation of Ti3C2Tx-based films, normally reported in connection with pressure-driven water filtration23,24, for pressure-free reduc-tion removal from water of multifarious HMIs (HCrO4

−, AuCl4−,

PdCl42− and Ag+). Reduced graphene oxide (RGO) is introduced as

the spacer to mitigate the restacking of Ti3C2Tx nanosheets, thereby improving their spatial accessibility. Surface hydroxylation of Ti3C2Tx is further conducted, which favours HMI removal by improving the wettability of Ti3C2Tx-based films and enhancing HMI adsorption

Microstructure and surface control of MXene films for water purificationXiuqiang Xie1,2, Chi Chen3, Nan Zhang1,2, Zi-Rong Tang2, Jianjun Jiang3 and Yi-Jun Xu   1,2*

Heavy metal ions (HMIs), such as those containing chromate and arsenic, are toxic and need to be removed from drinking water to protect public health. Films based on two-dimensional materials are promising regarding the removal of HMIs from water, but they typically use pressure-driven filtration. This study reports the application of two-dimensional titanium carbide (Ti3C2Tx MXene)-based films for pressure-free removal of multiple negatively and positively charged HMIs from water. The Ti3C2Tx MXene-based film’s microstructure was optimized by insertion of reduced graphene oxide between the layers, and the film’s surface was progressively hydroxylated to increase the accessibility of Ti3C2Tx, improve the film’s wettability and enhance the adsorption and reduction of HMIs. These steps synergistically improved the film’s HMI removal efficiency. This study provides a straightforward paradigm to manipulate the pivotal solid–liquid interactions for water purification under pres-sure-free conditions using two-dimensional materials-based films. Moreover, it could open a new vista of rationally designed, versatile, Ti3C2Tx-based films for target applications.

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and reduction. This report on 2DM-based films for effective, pres-sure-free removal of various cationic and anionic HMIs is expected to open a new avenue of rational use of 2DM-based films for water purification.

ResultsA facile synthetic strategy, developed to prepare Ti3C2Tx-based films for HMI removal, is potentially available for practical applications.

Preparation and characterizations of Ti3C2Tx films. As schemati-cally illustrated in Fig. 1a, the introduction of RGO into Ti3C2Tx films was realized using a Coulombic assembly approach followed by vacuum-assisted filtration. Because of the presence of surface functional groups such as -O, -OH and -F, Ti3C2Tx nanosheets are negatively charged with a zeta potential of −26 mV (Fig. 1b). To ensure the Coulombic assembly, positively charged RGO has been prepared, which has a zeta potential of +44 mV (Fig. 1b). After liquid-phase mixing of RGO and Ti3C2Tx, the zeta potential was −17.9 mV, a positive shift compared to that of Ti3C2Tx (Fig. 1b). This is ascribed to Coulombic interactions between Ti3C2Tx and the posi-tively charged RGO, which screens the surface charges of Ti3C2Tx. After vacuum-assisted filtration through a poly-(vinylidene fluo-ride) (PVDF) membrane, RGO-intercalated Ti3C2Tx film (denoted as RM) was obtained. A low RGO addition ratio of 5 wt% was used in the present study, generating the flexible and freestand-ing RM film (Fig. 1c). Further discussion regarding the fabrication of the composite film can be found in Supplementary Fig. 1 and Supplementary Notes. This 2D–2D integration based on Coulombic assembly is able to maximize the interaction between RGO and Ti3C2Tx, thereby effectively preventing restacking and thus maxi-mizing the spatial accessibility of Ti3C2Tx nanosheets (Fig. 1d). Supplementary Fig. 2 shows the Raman spectrum of the RM film, in which the characteristic Raman peaks of both Ti3C2Tx and RGO are found, illustrating successful hybridization between Ti3C2Tx and RGO in RM. For comparison, pure Ti3C2Tx MXene films (denoted as PM) were prepared in a similar way but without the addition of RGO. Supplementary Fig. 3 shows the X-ray diffraction (XRD) pat-terns of PM and RM, from which it can be seen that the diffrac-tion peak appears at around 6.5°, which is attributed to the (0002) interlayer spacing of Ti3C2Tx. Accordingly, the calculated interlayer

spacings for PM and RM are similar. Note that the peak intensity of RM decreases compared to that of PM, indicating a decrease in the degree of stacking order of Ti3C2Tx layers due to hybridization with RGO.

In comparison to randomly dispersed Ti3C2Tx nanosheets in solution, the vacuum-assisted filtration process led to the con-tinuous deposition of Ti3C2Tx MXene nanosheets on the substrate in a sheet-by-sheet fashion, forming oriented layered structures (Supplementary Fig. 4). The microstructure of Ti3C2Tx-based films was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure 2a shows a cross-sectional SEM image of the PM film. The highly oriented layered structure can be observed, suggesting that Ti3C2Tx nanosheets read-ily restack into a compact structure during the filtration process due to their intersheet interactions. The SEM image of the RM film shown in Fig. 2b displays a similar layered structure. Compared to PM, it is obvious that the intersheet restacking of Ti3C2Tx has been effectively mitigated by the introduction of RGO. The assembly between Ti3C2Tx and RGO at the nanoscale is evidenced by the TEM image (Fig. 2c), in which intimate contact between RGO and Ti3C2Tx is observed. Figure 2d shows an enlarged TEM image of the white rectangular area in Fig. 2c, and an interlayer spacing of 1.15 nm is clearly seen, which is characteristic of Ti3C2Tx, while the characteristic interlayer spacing of RGO is ~0.3 nm (ref. 25). Figure 2e shows the N2 adsorption–desorption isotherms of PM and RM. The specific surface area of the RM film is 125.5 m2 g−1, which is much higher than that of PM (19.6 m2 g−1). Based on these results, the microstructure optimization resulting from the introduction of RGO increases the accessible surface area of Ti3C2Tx, thereby facili-tating solid–liquid interactions and affording effective diffusion pathways for mass transfer.

Next, RM films were subjected to further treatment in HCl, pro-ducing surface-modified films (denoted as HRM). The zeta potential of HRM was measured at −20.4 mV (Supplementary Fig. 5). Changes in surface properties following HCl treatment were investigated by X-ray photo-electron spectroscopy (XPS). High-resolution XPS spectra of O 1 s core levels of RM and HRM are shown in Fig. 2f,g, respectively. The O 1 s is deconvoluted to four components centred at 529.9, 531.2, 532.0 and 533.0 eV, which are assigned to Ti(IV)-O, C-Ti-Ox, C-Ti-(OH)x and C-Ti-OH-(H2Oads)x, respectively26,27.

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a

c db

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Fig. 1 | Fabrication of Ti3C2Tx-based films. a, Schematic preparation of RM film. b, Zeta potentials of Ti3C2Tx, RGO and Ti3C2Tx/RGO mixture. a.u., arbitrary units. c, Digital photograph of the as-prepared RM. d, Schematic illustration of the mitigated restacking of Ti3C2Tx layers by RGO.

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Among the oxygen-containing surface functional groups of Ti3C2Tx—that is, C-Ti-(OH)x and C-Ti-Ox—the percentage of hydroxyl groups is 51.9% for RM films, which increased to 56.8% for HRM films. This suggests that around 10.2% of C-Ti-Ox on the surface of Ti3C2Tx was transformed to C-Ti-(OH)x by HCl treatment through hydrogenation of C-Ti-Ox. To test the wettability of the as-prepared films, water contact angle (WCA) measurements were conducted. As shown in Fig. 2h, the WCA of RM is close to that of PM, which indicates that the introduction of RGO had a negligible effect on the wettability of Ti3C2Tx-based films. Note that HRM has a much lower WCA of 64.7° than PM and RM films, signifying that surface hydroxylation of Ti3C2Tx is beneficial for water purification by facilitating water flux.

HMI removal performance of Ti3C2Tx films. Taking Cr(VI), a typical heavy metal ion pollutant, as an example, the performance of Ti3C2Tx-based films in pressure-free water purification was evaluated by their direct immersion in solutions containing Cr(VI) under ambient conditions (Cr(VI) exists as cationic HCrO4

− in the present system according to the speciation diagram)2. From Fig. 3a it can be seen that only 44% was removed after 150 min over PM, which is primarily attributed to its compact structure limiting the mass transfer of HCrO4

−. In comparison, the efficiency of HCrO4−

removal was significantly improved over RM and 91.3% of HCrO4−

was removed after 150 min. Supplementary Fig. 6 shows that RGO film removed 52.8% of HCrO4

− under the same conditions (note that RGO mass in RGO film is 20-fold that in RM). Considering the limited HCrO4

− removal capability of RGO and the low ratio of RGO in RM, the enhanced HCrO4

− removal efficiency of RM compared to PM is primarily ascribed to microstructure optimi-zation of Ti3C2Tx-based films through the introduction of RGO. Notably, HRM film exhibits further enhanced efficiency regarding

the removal of HCrO4− compared to RM (Supplementary Table 1),

demonstrating the superiority of surface hydroxylation in improv-ing the HCrO4

− removal efficiency of Ti3C2Tx-based films. The maximum Cr(VI) removal capacity for PM, RM and HRM was 68.2, 69.6 and 84 mg g−1, respectively (Supplementary Figs. 7–9). Supplementary Table 1 compares Cr(VI) removal performance of some typical 2DM-based materials, from which it can be seen that the Cr(VI) removal capacity of HRM outperformed that of two typi-cal 2DMs—RGO films (26.4 mg g−1) in the present study and MoS2-based powders (76.3 mg g−1)16. It should be mentioned here that HRM maintained mechanical integrity after HCrO4

− removal tests and was readily separated by directly removal from purified water (Supplementary Fig. 10). This endows the as-developed HRM with step-economy for water purification as compared to multi-layered Ti3C2Tx powders regarding the removal of Cr(VI) from water, which requires further tedious separation processes after water purifica-tion, including centrifugation and filtration28.

Kinetic analysis was conducted to gain deeper insight into the contribution of microstructure and surface modification to the enhancement of HCrO4

− removal efficiency in Ti3C2Tx-based films. The fitting results obtained from different models are summarized in Supplementary Table 2. With the highest correlation coefficient (R2), the intraparticle diffusion kinetic model provides the best correlation for the removal of HCrO4

− with Ti3C2Tx-based films, signifying that diffusion of HCrO4

− within these films is the rate-limiting step. Based on the intraparticle diffusion kinetic model, the removed Cr(VI) at time t (Qt) scales as a linear function of the square root of time (t1/2), with the slope being the diffusion rate constant29. As shown in Fig. 3b, the kinetics of HCrO4

− removal increases with increase in the initial concentration of HCrO4

−, which can be ascribed to the fact that a higher initial concentration mitigates the mass transfer resistance for HCrO4

− transport. This

1 µm

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b RM

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Ti3C2Tx MXene

Restacked Ti3C2Tx

PM RM HRM

87.2° 83.7° 64.7°

536 534 532 530 528

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C-Ti-(H2Oads)x C-Ti-(OH)x C-Ti-(OH)x

C-Ti-Ox Ti(IV)-O C-Ti-Ox Ti(IV)-O

RM

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Fig. 2 | Morphology and surface properties of Ti3C2Tx-based films. a,b, Cross-sectional SEM images of PM (a) and RM (b). c, TEM image showing the assembly of RGO and Ti3C2Tx in RM. d, Enlarged TEM image of the white rectangular area in c, showing the layered structure of Ti3C2Tx MXene. e, N2 adsorption–desorption isotherms of PM and RM. f,g, High-resolution O 1 s XPS spectra of RM (f) and HRM (g). a.u., arbitrary units. h, Water contact angle tests for PM, RM and HRM.

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can be interpreted as evidence that liquid–solid diffusion of HCrO4−

predominantly affects HCrO4− removal efficiency for Ti3C2Tx-based

films. As indicated in Fig. 3c and Supplementary Table 1, the dif-fusion rate constant is 2.84 mg g−1 min−1/2 for RM, which is much higher than that for PM (1.54 mg g−1 min−1/2), suggesting that the diffusion kinetics of HCrO4

− is improved by the introduction of RGO. For HRM, the diffusion rate constant further increases to 5.76 mg g−1 min−1/2, which can primarily be ascribed to the improved wettability by surface hydroxylation that favours water flux.

Compared to PM and RM, a similarly enhanced removal per-formance for HRM was found for other heavy ions, including PdCl4

2− and AuCl4− (Fig. 3d,e). Remarkably, maximum removal

capacities of 890 and 1,241 mg g−1 for Pd(II) and Au(III), respec-tively, were achieved for HRM. Despite these anionic coordination-saturated species, HRM also showed enhanced removal efficiency for cationic Ag+ compared to PM and RM (Fig. 3f). The maximal removal capacity of HRM for Ag(I) peaked at 1,172 mg g−1. The effi-cient removal of anionic coordination-saturated HMIs (HCrO4

−, PdCl4

2− and AuCl4−) and cationic Ag+ demonstrates that Ti3C2Tx-

based films overcome the general limitation of selective removal of specific HMIs over other reported 2DMs, illustrating the potential of these films in pressure-free removal of multifarious cationic and anionic HMIs in the field of water purification.

Mechanistic study of HMI removal for Ti3C2Tx films. For HCrO4

− removal, XPS results suggest that the Cr/Ti ratio in HRM increases correspondingly with increased Cr(VI) removal capac-ity (Fig. 4a), indicating the accommodation of Cr species in HRM. Concomitantly, as shown by the XRD patterns (Fig. 4b), the (0002) peak of Ti3C2Tx broadens and shifts towards lower angles during the HCrO4

− removal process, indicating the expansion and disorder-ing of the interlayer spacing of Ti3C2Tx nanosheets. Supplementary Fig. 11a compares the XRD patterns of PM, RM and HRM after Cr(VI) removal tests. Based on XRD results, the interlayer spacing changes for PM, RM and HRM before and after Cr(VI) removal

tests are shown in Supplementary Fig. 11b. As can be seen, inter-layer spacing increased for these three films after Cr(VI) removal tests, suggesting the intercalation of Cr species. The interlayer spacing change follows the order HRM > RM > PM, which cor-relates with Cr(VI) removal efficiency. The XPS and XRD results reveal that Cr species efficiently diffuse and intercalate in HRM under pressure-free conditions. It is interesting to note that the coexistence of Cr(VI) and Cr(III) was identified in HRM (Fig. 4c). Among these Cr species, the percentage of Cr(III) was calculated as 82%, suggesting that the removal of HCrO4

− is initially dominated by the reduction in Cr(VI) to Cr(III) for HRM. This reduction was accompanied by valency state changes of Ti in Ti3C2Tx. As shown in Fig. 4d, the Ti 2p core level can be fitted with four doublets (Ti 2p3/2–Ti 2p1/2), three of which belong to C-Tiδ+-Tx (δ = 1, 2 and 3) while the fourth is assigned to Ti(VI)-O26,27. After HCrO4

− removal tests, a shift of 0.1–0.2 eV towards higher binding energy was observed for the C-Tiδ+-Tx moieties. This suggests the depletion of electrons in Ti3C2Tx during the reduction removal of HCrO4

−. Consequently, it was speculated that HCrO4

− removal over Ti3C2Tx proceeds through an indirect mechanism of electron transfer from Ti3C2Tx to HCrO4

−, producing non-toxic Cr(III) that can be accommodated in Ti3C2Tx-based films. Similar reduction processes were also observed for the removal of PdCl4

2−, AuCl4− and Ag+, which were reduced

to their metallic phases (Pd, Au and Ag) according to XRD results (Supplementary Figs. 12–14). As can clearly be seen from the cor-responding digital images in Supplementary Fig. 15, the surface of HRM films was covered by a layer of metals after the removal tests due to the high removal capacity for PdCl4

2−, AuCl4− and Ag+, which

led to the absence of the characteristic XRD peak of Ti3C2Tx due to the shielding effect and the weak diffraction intensity of Ti3C2Tx. As seen from the SEM images (Supplementary Fig. 16), metal-lic reduction products are found throughout HRM, further dem-onstrating the spontaneous mass transfer and reduction of HMIs over the microstructure- and surface-modulated Ti3C2Tx film under pressure-free conditions.

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Fig. 3 | Performance of Ti3C2Tx-based films in pressure-free removal of HMis. a, Time-online profiles of HCrO4− removal for PM, RM and HRM under

pressure-free conditions. b, Effect of initial concentration on the kinetics of HCrO4− removal for HRM. c, Kinetic analysis of HCrO4

− removal for PM, RM and HRM based on the intraparticle diffusion kinetic model. d–f, Time-online profiles for removal of AuCl4

− (d), PdCl42− (e) and Ag+ (f) for PM, RM and

HRM films under pressure-free conditions. Error bars represent systematic errors in measurements.

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The effect of Ti3C2Tx surface terminations on the interaction between HMIs and Ti3C2Tx was investigated by density functional theory (DFT) calculation. Taking HCrO4

− as an example of the anionic ions studied, different adsorption configurations of HCrO4

− on O- and OH-terminated Ti3C2 are shown in Supplementary Fig. 17 and the corresponding binding energies are summarized in Fig. 4e. The binding energy of HCrO4

− on O-terminated Ti3C2 was approximately 0.72 to –1.53 eV. In comparison, the binding energy of HCrO4

− on OH-terminated Ti3C2 was significantly dif-ferent, at approximately –6.32 to –6.42 eV, indicating enhanced adsorption of HCrO4

− on OH-terminated Ti3C2 compared to O-terminated. To further understand the reductive removal of HCrO4

− for Ti3C2Tx, charge transfer in the most stable configura-tions was investigated. According to Bader analysis30, 0.51 e− could transfer from O-terminated Ti3C2 to HCrO4

− (Fig. 4f). In the case of OH-terminated Ti3C2, there was obviously enhanced electron redis-tribution between Ti3C2 and HCrO4

− (Fig. 4g). It was calculated that 1.48 e− were obtained by HCrO4

− from Ti3C2 with OH-terminations. This suggests the favourable reduction detoxification of HCrO4

− on OH-terminated Ti3C2 compared to that on O-terminated Ti3C2. Based on DFT calculations, it was found that surface hydroxylation of Ti3C2Tx facilitates HCrO4

− removal by enhancement of HCrO4−

adsorption and reduction, which contributes to the enhanced HCrO4

− detoxification performance of HRM compared to RM. In addition, for the cationic ion of Ag+, a similar contribution of sur-face hydroxylation of Ti3C2Tx to enhanced adsorption and reduc-tion was also found (Supplementary Fig. 18).

The present progressive strategy, using microstructure optimiza-tion and surface modulation, proved to be effective in enhancing the performance of Ti3C2Tx-based films in the reduction removal of HMIs from water, and can be ascribed to the following four fea-tures (Fig. 5): (1) microstructure optimization by the introduction of RGO between Ti3C2Tx layers effectively mitigates the restacking

of Ti3C2Tx, which significantly enhances the surface accessibility of Ti3C2Tx to HMIs and shortens the mass transfer. (2) Surface hydrox-ylation of Ti3C2Tx improves the wettability of Ti3C2Tx-based films. (3) Ti3C2Tx-based film removes multifarious HMIs by redox reac-tion from the oxidation state (Mox) to the reduction state (Mre), sup-pressing the limitation of other reported 2DMs using the principles of electrostatic attraction or coordination chemistry that can only selectively remove HMIs. (4) Surface hydroxylation also favours the reductive removal of HMIs by enhancing their adsorption and reduction processes. With these combined advantages, microstruc-ture- and surface-modulated Ti3C2Tx-based films efficiently remove multifarious HMIs under pressure-free conditions, including HCrO4

−, PdCl42− AuCl4

− and Ag+.

Recyclability of HRM. Previous investigations show that Ti3C2Tx nanosheets dispersed in water could potentially be irreversibly oxi-dized to TiO2, while stability was significantly improved when the nanosheets were reassembled into films11. In the case of reductive

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– removal

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δ+-T

x2p

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2p1/2

594 591 588 585 582 579 576 573 570

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Cr(VI) 2p3/2

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Cr(III) 2p3/2

After HCrO4– removal

Before HCrO4– removal

g

Fig. 4 | Characterizations and computational investigation of HCrO4− removal for HRM. a,b, Cr/Ti ratios (a) and XRD patterns (b) for HRM at different

stages of HCrO4− removal. a.u., arbitrary units. 2 Theta, the angle between the incident X-ray beam and reflected beam. c, High-resolution Cr 2p. d, Ti 2p

XPS spectra for HRM before and after HCrO4− removal tests. e, Binding energy of HCrO4

− on O- or OH-terminated Ti3C2 with different configurations. f,g, Differences in charge density of HCrO4

− on O- (f) and OH-terminated Ti3C2 (g). Turquoise and yellow regions indicate depletion and accumulation of electrons, respectively. Error bars represent systematic errors in measurements.

Mox

Mre

Favourable redox reaction Rapid diffusionTi3C2Tx MXene

Ti C Tx: surface functionalities

RGO

+–

R-NH2C

Fig. 5 | Schematic illustration of pressure-free water purification by HMR. HMIs were removed from water by microstructure- and surface dual-optimized Ti3C2Tx MXene films under pressure-free conditions.

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removal of HCrO4−, the Raman spectra of Ti3C2Tx-based film

before and after HCrO4− removal tests were identical and no TiO2

peak was found (Supplementary Fig. 19). This suggests that elec-tron transfer from Ti3C2Tx to HCrO4

− is reversible and that the regeneration of Ti3C2Tx-based films after HCrO4

− removal tests can be expected. Supplementary Fig. 20 shows the ultraviolet-visible (UV-vis) absorption spectrum for NaOH solution after soaking of the recycled HRM used for Cr(VI) removal tests, from which the absorption peak corresponding to Cr(VI) can be seen. This suggests the dissolution of Cr species from recycled HRM into NaOH solution, which is essentially critical to replenish-ment of surface active sites of recycled HRM for further Cr(VI) removal. Thus we developed a two-step method to regenerate used MXene-based films, which involves sequential treatment in NaOH solution and HCl, respectively. However, it was found that HRM film peeled off from the substrate during the regen-eration process (Supplementary Fig. 21a), which is ascribed to penetration by water and salt31. Notably, no apparent dissolution of Ti3C2Tx was observed, indicating that intersheet attraction can maintain the integrity of the film while weakening of adhesion forces between Ti3C2Tx and the PVDF substrate, due to swelling, became predominant. It has been demonstrated that Ti3C2Tx has a strong interaction with amine functional groups32. Accordingly, we used (3-aminopropyl) triethoxysilane (APTES) with amine terminations to modify the PVDF substrate, which would be expected to enhance adhesion forces between HRM and the sub-strate. Encouragingly, it was found that HRM is stable and no peel-ing was observed during the regeneration process (Supplementary Fig. 21b), which paves the way to addressing the stability issue regarding the swelling behaviour of MXene-based films used for practical applications.

The successive regeneration treatment and Cr(VI) removal test results shown in Supplementary Fig. 22 clearly illustrate the success-ful refurbishment of recycled HRM films by our two-step regenera-tion method, demonstrating the repeatable Cr(VI) removal activity recovery of Ti3C2Tx-based films. Therefore, it is believed that as-pre-pared MXene-based films with microstructure- and surface dual-modulation hold promise for water treatment due to their efficient water purification performance regarding the removal of multifari-ous heavy metal ions, excellent mechanical stability, regenerability and operational convenience. It is also worth noting that Ti3C2Tx shows higher antibacterial efficiency compared to the widely reported antibacterial agent graphene oxide, which is highly advan-tageous for improving the overall water purification performance in practical applications33–35.

DiscussionIn summary, microstructure optimization resulting from the intro-duction of RGO efficiently mitigates the restacking of Ti3C2Tx nanosheets, thereby facilitating interaction between Ti3C2Tx and HMIs in water. Surface modification by HCl increases the content of hydroxyl groups on Ti3C2Tx, which not only improves the wet-tability of Ti3C2Tx-based films but also favours reduction in HMIs through enhancement of adsorption and charge transfer from Ti3C2Tx to HMIs. The combination of microstructure optimiza-tion and control of surface properties of Ti3C2Tx-based films leads to the enhanced removal of multifarious HMIs from water under pressure-free conditions, and removal capacities of 84, 890, 1,241 and 1,172 mg g−1 for Cr(VI), Pd(II), Au(III) and Ag(I), respectively, were achieved. Regeneration of recycled Ti3C2Tx-based films after Cr(VI) removal tests was achieved by sequential treatment with NaOH solution and HCl. This microstructure- and surface dual-modulation strategy provides a promising method for improvement of solid–liquid interactions over Ti3C2Tx-based functional films, which is also of fundamental importance in regard to other liquid-phase applications of Ti3C2Tx MXene for sustainable energy storage

technologies, including super-capacitors and lithium- and sodium-ion capacitors.

MethodsExperiment details on the fabrication of Ti3C2Tx-based films, characterization techniques and performance measurement procedures are presented below.

Preparation of Ti3C2Tx-based films. The colloid of Ti3C2Tx nanosheets was prepared as described previously20, and was then diluted to obtain a concentration of 0.5 mg ml−1. Positively charged RGO was obtained by refluxing graphene oxide with branched polyethylenimine (molecular weight = 25,000, Aldrich) followed by washing with ethanol and deionized water to remove excess branched polyethylenimine36. Preparation of RGO-intercalated Ti3C2Tx films was performed by a Coulombic assembly process followed by vacuum-assisted filtration. Typically, 1.2 ml of RGO suspension (0.5 mg ml−1) was added dropwise to 22.8 m of Ti3C2Tx solution and stirred at room temperature for 1 h. The mixture was then filtrated through a PVDF membrane (diameter 47 mm, pore size 0.2 μm, Tianjin Jinteng). After drying, RGO-intercalated Ti3C2Tx film was obtained. For comparison, pure Ti3C2Tx MXene and RGO films were prepared in a similar way but without the addition of RGO or Ti3C2Tx. The mass of each as-prepared film was 12 mg, with a RGO ratio of 5 wt%. Hydrochloric acid treatment of RM was performed by immersion of RM in 1 M HCl aqueous solution for 24 h. To remove additional HCl, the HRM film was then immersed in deionized water for 24 h followed by washing with deionized water to pH ~6.0. After drying, HCl-treated RM (HRM) was obtained. For modification of PVDF substrates, 0.15 ml APTES was dissolved in 30 ml ethanol; PVDF substrates were then immersed in APTES solution for 1 h at room temperature. APTES-treated PVDF substrates were then washed thoroughly with ethanol to remove excess APTES and dried at 60 °C for 2 h for further use.

Characterization. Zeta potentials (ξ) of Ti3C2Tx and RGO were determined by dynamic light-scattering analysis (Zeta sizer 3000HSA) at room temperature. XRD measurements were conducted on a Bruker D8 Advance X-ray diffractometer (40 kV, 40 mA) using Ni-filtered Cu Kα radiation at a scan rate of 0.02° s−1. Field-emission scanning electron microscopy was used to determine the morphology of films on an FEI Nova NANOSEM 230 spectrophotometer. TEM images were obtained using a JEOL JEM 2010 EX instrument at an acceleration voltage of 200 kV. XPS measurements were carried out on a Thermo Scientific ESCA Lab 250 spectrometer, which consists of monochromatic Al Kα as the X-ray source, a hemispherical analyser and a sample stage with multi-axial adjustability. Micromeritics ASAP 3020 equipment was used to determine the N2 adsorption–desorption isotherms and Brunauer–Emmett–Teller (BET) surface areas at 77 K. The samples were de-gassed at 160 °C for 3 h and then analysed at 77 K. The relative pressure (P/P0) range used for calculation of BET surface area was 0.05–0.35. Wettability of the as-prepared films was determined by digitized images of a 4-μl sessile water drop with the OCA20 Contact angle system equipped with a charge-coupled device camera. The water contact angle was measured directly from the image of the deionized water droplet.

HMI removal tests. Removal of HMIs from water for the as-prepared Ti3C2Tx-based films was performed under ambient conditions. To test for the removal of Cr(VI), Ti3C2Tx-based films were added to 30 ml of a 20-mg l−1 aqueous solution of Cr(VI) (pH = 5.14) in a beaker. Three millilitres of Cr(VI) solution was sampled at specific time intervals and Cr(VI) concentration was analysed on a Varian UV-vis spectrophotometer (Cary-50, Varian Co.). Because PdCl2 is slightly soluble in water, HCl was used to dissolve it, resulting in a solution of H2PdCl4 with a concentration of 28.2 mM. The H2PdCl4 solution was then diluted to obtain 200 ml of Pd(II) solution with a concentration of 200 mg l−1. For Ag(I) and Au(III), AgNO3 and chloroauric acid tetrahydrate (AuCl3·HCl·4H2O) were dissolved separately in 200 ml deionzed water. The concentration of Ag(I) and Au(III) was 200 mg l−1. The Ti3C2Tx-based films were then added to the solutions of Pd(II), Ag(I) and Au(III) and the concentration change was quantified by an inductively coupled plasma emission spectroscopy instrument (PerkinElmer, Optima 2000DV). Removal capacities of Ti3C2Tx-based films were calculated based on the HMIs (Cr(VI), Pd(II), Au(III) and Ag(I)) removed per mass of film.

Regeneration of recycled HRM. The recycled HRM produced by the Cr(VI) removal tests was soaked in 30 ml NaOH solution (0.1 mol l–1) for 5 h. After washing with deionized water, recycled HRM was immersed in HCl (1 mol l–1, 30 ml) for 12 h. The regenerated HRM was then rinsed and dried for further tests.

Computational details. Our first-principle calculations were performed using VASP code37, based on DFT38,39. The 4 × 4 super-cells of Ti3C2Tx were chosen as adsorbents for HCrO4. The c axes were set at 30 Å to ensure sufficient vacuum to avoid interactions between HCrO4 molecules in adjacent super cells. Exchange–correlation energy was calculated using general gradient approximation with the Perdue–Burke–Ernzerhof exchange–correlation functional40. The effect of van der Waals interactions was estimated and implemented in the optimized exchange van der Waals functional B86b of the Becke (optB86b vdW) functional41,42. The

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plane wave cut-off energy was 580 eV, and k-point meshes of 2 × 2 × 1 and 3 × 3 × 1 in the Monkhorst Pack43 sampling scheme were used for geometry optimization and electronic self-consistent computation, respectively. The convergence condition for the energy was 10−4 eV, and the structures were relaxed until the force on each atom was <0.03 eV Å–1. Spin polarization was considered in all calculations. Structure drawing and charge density visualization were conducted in VESTA44.

Reporting Summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Data availabilityThe data that support the findings of this study are available from the corresponding author upon request.

Received: 2 February 2018; Accepted: 1 August 2019; Published online: 9 September 2019

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acknowledgementsSupport from the National Natural Science Foundation of China (Nos. U1463204, 51802040, 21872029, 21802020 and 21173045), the Award Program for Minjiang Scholar Professorship, the Natural Science Foundation of Fujian Province for Distinguished Young Investigator Rolling Grant (No. 2017J07002), the Independent Research Project of the State Key Laboratory of Photocatalysis on Energy and Environment (No. 2014A05) and the First Program of Fujian Province for Top Creative Young Talents is gratefully acknowledged. Computational resources were provided by Intelligent Electronics Institute, Huazhong University of Science and Technology, China.

author contributionsY.-J.X., Z.-R.T. and X.X. conceived and designed this work. X.X. conducted the experiments. C.C. and J.J. performed the computational investigations. N.Z. conducted SEM and N2 adsorption–desorption analysis. X.X. and Y.-J.X. wrote and revised the manuscript. All authors participated in discussion and reviewed the manuscript before submission.

Competing interestsThe authors declare no competing interests.

additional informationSupplementary information is available for this paper at https://doi.org/10.1038/s41893-019-0373-4.

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