Remarkable differences in the voltammetric response towards hydrogen peroxide, oxygen and Ru(NH3)63+ of electrode interfaces modified with HF or LiF-HCl etched Ti3C2Tx MXene

An electrochemical study was performed on the behavior of Ti3C2Tx MXenes prepared by using either HF (MXene1) or LiF/HCl as etchants (MXene2). The use of two redox probes indicates the presence of a higher negative charge density on MXene2 in comparison to MXene1. The characterization of two nanomaterials shows that titanium and fluoride are present higher by one order of magnitude at the interface of MXene2, compared to MXene1. The high Ti and F content is accompanied by a 82-fold larger (249 μA·cm−2 vs. 5.64 μA·cm−2) anodic peak at the peak potential near 0.4 V (vs. Ag/AgCl). Similarly, the peak current on MXene2 is 317-fold higher for the oxygen reduction at pH 7.0 (at a voltage of −0.84 V) and 215-fold higher for the reduction of H2O2 at −0.89 V, when compared to MXene1. Graphical abstract Difference in electrochemical behavior of MXene prepared by HF (MXene1) and LiF/HCl (MXene2) as etchants. Difference in electrochemical behavior of MXene prepared by HF (MXene1) and LiF/HCl (MXene2) as etchants.

Etching of MAX phases by HF has been initially widely used to produce Ti 3 C 2 T x MXene [17,18]. The process however requires handling noxious HF, what is not a green and sustainable way for producing MXene. Alternative etching ways were seeking such as (NH 4 HF 2 ) salt-based approach [19] and subsequently a combination of LiF and HCl (making HF in-situ) was introduced [20]. Presence of Li + ions in the LiF-HCl etchant has a significant effect on the properties of Veronika Gajdosova and Lenka Lorencova contributed equally to this work.
MXene. While, a lattice c parameter of HF-produced Ti 3 C 2 T x was 20 Å, in case of LiF-HCl protocol, it increased to a value of ≈40 Å. Thus, Li + ions and water molecules were intercalated and thus no other intercalants are needed to make stable MXene sheets. As a result, the obtained Ti 3 C 2 T x MXene flakes are larger with much lower density of defects and at the same time with higher yield of dispersed MXene flakes [21]. Thus, LiF-HCl etchant is recommended to be applied when high electrical conductivity, larger flake sizes, and high mechanical stability are desired, and HF would be more suitable for preparing Ti 3 C 2 T x with smaller or more defective flakes [21,22]. Several techniques were applied to study differences in the composition of Ti 3 C 2 T x MXene produced either by HF or LiF-HCl method including 1 H, 13 C and 19 F NMR, SEM, XRD, EDS techniques [23] from an application for Liion batteries point of view. Similar techniques have been applied also to study differences in the properties of Ti 3 C 2 T x MXene prepared using an original "clay" method [20] and a new MILD method based on higher concentration of LiF and HCl not requiring sonication [24]. The first study describing differences in the electrochemical behavior of Ti 3 C 2 T x MXene prepared either by a "clay" or a MILD method was recently published [25].
MXene-modified interfaces besides energy storage applications [26] as batteries or supercapacitors can be effectively applied as electrochemical sensors and biosensors [14]. The differences in the electrochemical behavior of Ti 3 C 2 T x MXene prepared using HF or LiF/HCl as etchants are described here for the first time. Besides electrochemical examination of two types of Ti 3 C 2 T x MXene materials, a battery of techniques was applied, as well, with a conclusion that the amount of Ti and F present on the surface of Ti 3 C 2 T x MXene most likely plays a cardinal role in the electrochemical properties of this nanomaterial.

Materials and methods
This section is part of the ESM file.

Atomic force microscopy of MXene1 and MXene2
The size of MXene flakes was visualised by AFM. The results indicate that MXene2 contains larger particles compared to MXene1 (Fig. S8). The results obtained by AFM are in a good agreement with the size of MXene flakes observed by DLS imaging as shown in Fig. S5. The intensity distribution reveals a Z-average of 132 nm and a polydispersity index of 0.366, with two peaks located at 121 nm (92.7%), and 28.8 nm (7.3%) for MXene1. In contrast, for MXene2 the following parameters were obtained: a Z-average of 178 nm, a polydispersity index of 0.379 with two peaks located at 189 nm (93.6%), and 2.6 μm (6.4%).

Electrochemical characterization of MXene1 and MXene2
Redox behavior in the plain electrolyte under N 2 atmosphere We investigated electrochemical performance of MXene1 and MXene2 in aqueous solutions by CV. Quite stable electrochemical behavior of both types of Ti 3 C 2 T X MXenes in the cathodic potential window for GCE/MXene1 (Fig. 1a) and for GCE/MXene2 is observed (Fig. 1c).
In case of MXene1, a drop of a current density from 111 μA ·cm −2 to 79 μA·cm −2 at −0.75 V i.e. drop to 71% of the original value in 10 scans in the cathodic potential window 0 V → -1 V was observed (Fig. 1a). In the anodic potential window (0 V → 1 V) two peaks were observed at a potential of 0.39 V and 0.60 V. The peak current at 0.39 V sharply decreased and after 3 scans it was stable (4.7 μA·cm −2 → 1.6 μA·cm −2 , i.e. drop to 34% of the original value) (Fig. 1b).
Electrochemical investigation of GCE/MXene2 in a plain electrolyte proved presence of high anodic current of 16.5 μA (249 μA·cm −2 ) at +0.39 V in the 1st CV scan (Fig. 1d), while under the same conditions an anodic current of 0.30 μA (5.64 μA·cm −2 ) ( Fig. 1b) was observed for GCE/MXene1. After background subtraction, there is 82-fold increase in the current density on GCE/MXene2 in comparison to GCE/ MXene1.
In case of MXene2, a drop of a current density from 196 μA·cm −2 to 142 μA·cm −2 at −0.8 V i.e. drop to 72% of the original value in 10 scans was observed, while scanning the potential in the cathodic potential window 0 V → -1 V (Fig. 1c). In the anodic potential window (0 V → 1 V) only one dominant peak at a potential of 0.38 V was seen. The peak current at 0.38 V sharply decreased and after 3 scans it was stable (253 μA·cm −2 → 1.8 μA·cm −2 , i.e. drop to 0.7% of the original value) (Fig. 1d). There is also difference between redox behavior of MXene2 and MXene1, since at GCE/ MXene2 an anodic peak at a potential of~−0.95 V is present (Fig. 1c).
Thus, it can be concluded that the electrochemical stability for both types of MXene is very similar in the cathodic potential window with a striking difference of the electrochemical stability and behavior in the anodic potential window. The anodic oxidation of both MXene types is an irreversible process, and the oxidation peak disappears after 3 CV cycles.

Electrochemical behavior using electrochemical redox probes
The Ti 3 C 2 T X MXene is not stable in an anodic potential window required to study redox behavior of an inner-sphere electrochemical redox probe -ferricyanide/ferricyanide [25,27]. Therefore, the redox behavior of an outer-sphere electrochemical redox probe Ru(NH 3 ) 6 Cl 3 was studied. This redox probe was also used for calculation of an electrochemically active surface area.
From the slope of a linear plot of i p vs. v 1/2 for the cathodic part of the CV for GCE/MXene1 and GCE/MXene2 in 5 mM Ru(NH 3 ) 6 Cl 3 (inset figures in Fig. S6), the electroactive surface area was calculated as follows: (5.27 ± 0.07) mm 2 for GCE, (5.32 ± 0.08) mm 2 for GCE/MXene1 and (6.62 ± 0.08) mm 2 for GCE/MXene2. This really suggests that MXene2 is much better delaminated compared to MXene1 with a higher electrochemical/interfacial surface area of the nanomaterial exposed to the electrolyte.
There is a striking difference in the electrochemical behavior of Ru(NH 3 ) 6 Cl 3 between GCE/MXene1 and GCE/ MXene2 (Table S1 and S2; Fig. 2). An anodic peak potential for Ru(NH 3 ) 6 Cl 3 at both GCE/MXene1 and GCE/MXene2  (Table S1 and S2, Fig. 2b). A cathodic peak potential remains stable only for GCE/MXene1 with a value of −213 mV (Table S1, Fig. 2a), while for GCE/ MXene2 it increases from −215 mV to −225 mV with an increasing scan rate from 0.1 V·s −1 to 0.8 V·s −1 (Table S2, Fig. 2a). Moreover, there is a change of the E pc vs. v is linear with a slope of −0.0188 V / (V·s −1 ) for GCE/MXene2 within the interval of scan rates from 0.2 V·s −1 to 0.7 V·s −1 (Fig. 2a). At the same time there is also a linear dependence of ΔE vs. v with a slope of 0.0181 V / (V·s −1 ) for GCE/MXene2 within the interval of scan rates from 0.2 V·s −1 to 0.7 V·s −1 (Fig. 2c).
Besides differences in the electrochemical behavior of Ru(NH 3 ) 6 Cl 3 between GCE/MXene1 and GCE/MXene2, when a peak potential is taken into account, there are also significant differences, when peak current is considered. For GCE/MXene1, the value of i pc / i pa decreased from 1.26 (0.1 V·s −1 ) to a value of 1.08 (0.8 V·s −1 ) (Fig. 2d). For GCE/MXene2, after an initial drop from a value of 1.24 to 1.20 (0.1 V·s −1 → 0.2 V·s −1 ), there is an increase in the value of i pc / i pa from a value of 1.20 to a value of 1.32 (0.2 V·s −1 → 0.8 V·s −1 ) (Fig. 2d). Interestingly, GCE modified by both nanomaterials exhibits the same initial i pc / i pa of 1.24-1.25 at a low potential scan rate of 0.1 V·s −1 (Fig. 2d). Possibly, the irreversibility of the redox behavior on GCE/ MXene1 and GCE/MXene2 is caused by different affinity of Ru 2+ and Ru 3+ species towards both MXene nanomaterials (initial i pc / i pa far from an ideal value of 1.00 i.e. 1.24-1.25).
We can conclude from the electrochemical investigations using two redox probes that most likely GCE/MXene2 has a higher density of a negative charge on the surface compared to GCE/MXene1, as judged by the use of a ferricyanide/ ferrocyanide as a redox couple. A higher density of a negative interfacial charge on GCE/MXene2 compared to GCE/ MXene1 is consistent with a redox behavior of a redox probe Ru(NH 3 ) 6 Cl 3 , since a high density of negative charge on GCE/MXene2 will have a high affinity towards Ru 3+ redox species over Ru 2+ redox species. Thus, GCE/MXene2 will pre-concentrate Ru 3+ over Ru 2+ , a feature, which is more obvious at high scan rates, not allowing Ru 3+ species to be diluted in the bulk of the electrolyte.

Oxygen reduction reaction (ORR)
Naturally abundant 2D materials are investigated for their exceptional electrocatalytic properties as in energy-related reactions including hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) or promising energy conversion process, carbon dioxide reduction reaction (CO 2 RR). ORR presents a pertinent reaction in the majority of energy conversion and storage devices; for example, in fuel cells and rechargeable metal-air batteries. ORR progresses via a direct four-electron pathway or a two-step two-electron process that produces a H 2 O 2 intermediate [28].
The electrocatalytic activity of MXene1 and MXene2 towards ORR in alkaline, neutral and acidic media was also studied and can be seen in Fig. 3.
In the alkaline electrolyte (0.1 M NaOH) ORR on GCE, GCE/MXene1 and GCE/MXene2 is very similar (a potential at which ORR starts and a current density) (Fig. 3a-c), when subtracted CVs are considered (Fig. 3c). This really indicates that very high pH does not keep Ti 3 C 2 T X MXene in an active state and that at alkaline pH Ti 3 C 2 T X MXene is not electrochemically stable. The same conclusion was recently made by Nayak et al. [25].
At neutral pH, the situation is completely different for GCE/MXene1 and for GCE/MXene2. ORR starts at a potential of −131 mVon GCE (Fig. 3d), at a potential of −190 mVat GCE/MXene1 (Fig. 3d) and at a potential of −100 mV at GCE/MXene2 (Fig. 3e). ORR exhibits a maximal current density of 147 μA·cm −2 at a potential of −698 mV on GCE, a maximal current density of 157 μA·cm −2 at a potential of −674 mV on GCE/MXene1 and a maximal current density of 1270 μA·cm −2 at a potential of −840 mV on GCE/ MXene2. This really means that GCE/MXene1 exhibits ORR parameters very similar to GCE (Fig. 3d, f) and that MXene prepared by HF etching does not exhibit significant intrinsic ORR capabilities. At neutral pH, there is only a mild intrinsic ORR on GCE/MXene1 (J GCE/MXene1 -J GCE = 3.6 μA·cm −2 ) read at −840 mV, compared to intrinsic ORR on GCE/MXene2 (J GCE/MXene2 -J GCE = 1270 μA·cm −2 ). Thus, it is concluded that there is 317-fold higher intrinsic ORR on GCE/MXene2 in comparison to intrinsic ORR on GCE/MXene1 at pH 7 (Fig. 3f).
At an acidic pH, ORR starts at a potential of −198 mV on GCE (Fig. 3g), at a potential of −105 mV at GCE/MXene1 (Fig. 3g) and at a potential of −46 mV at GCE/MXene2 (Fig.  3h). ORR exhibits a peak current density of 91 μA·cm −2 at a potential of −574 mV on GCE, a peak current density of 196 μA·cm −2 at a potential of −640 mV on GCE/MXene1 and a peak current density of 2270 μA·cm −2 at a potential of −671 mV on GCE/MXene2. At an acidic pH, there is only a mild intrinsic ORR on GCE/MXene1 (J GCE/MXene1 -J GCE = 81 μA·cm −2 ) read at −671 mV, compared to intrinsic ORR on GCE/MXene2 (J GCE/MXene2 -J GCE = 2160 μA·cm −2 ). Thus, there is 27-fold higher intrinsic ORR on GCE/MXene2 in comparison to intrinsic ORR on GCE/MXene1 at an acidic pH (0.1 M H 2 SO 4 ) (Fig. 3i).

Electrochemical reduction of H 2 O 2
The electrochemical determination is H 2 O 2 is widely applied using various types of nanomaterials [29][30][31]. H 2 O 2 reduction starts at a potential of −224 mV at GCE, at a potential of −190 mV at GCE/MXene1 and at a potential of −140 mV at GCE/MXene2 (Fig. 4). H 2 O 2 reduction exhibits a peak current density of 87 μA cm −2 at a potential of −818 mVon GCE, a peak current density of 107 μA·cm −2 at a potential of −686 mV on GCE/MXene1 and a peak current density of 2100 μA·cm −2 at a potential of −894 mV on GCE/MXene2. When H 2 O 2 reduction is considered, there is only a mild intrinsic activity towards H 2 O 2 reduction at a potential of −894 mV on GCE/MXene1 (i.e. J GCE/MXene1 -J GCE ) of 9.4 μA·cm −2 , while on GCE/MXene2 the activity towards H 2 O 2 reduction is 215-fold higher (J GCE/MXene2 -J GCE = 2020 μA·cm −2 ).

TOF-SIMS analysis
The electrochemical behavior of MXene1 and MXene2 cannot be explained by any typical characterization techniques applied for that purpose (i.e. XPS, Raman spectroscopy, contact angle measurements) all provided in the ESM file. This is why for that purpose we tried to use TOF-SIMS method, which can determine interfacial composition of nanomaterial at the nanoscale.
Secondary Ion Mass Spectrometry (SIMS) employing Time-of-Flight (TOF) analyzer is an analytical tool for examination of elemental/molecular composition of interfacial layers with high mass resolution on the order of 10,000 m/z, high lateral resolution of 100 nm, a depth resolution of 1 nm and with sensitivity down to ppm-ppb level [32]. The technique allows to acquire 2D map of the interface with a spatial distribution of selected ions [32]. SIMS spectra analysis confirmed that the fragmentation of MXene1 and MXene2 produced the following ions: C + , CH 3 + , C 2 H 5 + , Ti + , C 4 H 7 + , TiO + in a positive polarity and fragments O − , OH − , F − in a negative polarity. The main differences were in the intensity of particular fragments (Table 1), when the ratio MXene2/ MXene1 was calculated (Table 1, last line). The highest differences between these two types of MXene were observed for content of Ti + (12.5-fold) and F − (39fold) ions. This really means that although the average composition of both MXenes as determined by XPS (Table S1) is slightly different, interfacial composition of MXene1 is dramatically different from MXene2 as determined by TOF SIMS. Hence, it is concluded that differences in the interfacial density of various chemical species (Ti and F content) between these two types of MXenes are the reason for the dramatic differences in the electrochemical behavior of both MXenes. Moreover, 2D SIMS analysis also revealed that distribution of both ions Ti + and F − is more homogeneous on MXene2 compared to MXene1 (Fig. S9). Further details regarding TOF SIMS analysis are provided in ESM (Figs. S10-S17).

Conclusions
The study revealed a striking difference in the electrochemical behavior of MXenes prepared either using HF (MXene1) or LiF/HCl (MXene2) etching routes. Electrochemical investigation of GCE/MXene2 in a plain electrolyte proved presence of a peak at potential of +0.39 V with 82-fold higher density on GCE/MXene2 in comparison to GCE/MXene1. Additional electrochemical experiments using two redox probes -outer one (Ru 3+ complex for calculation of electrochemical surface area) and inner one (ferricyanide/ferrocyanide redox couple to investigate R ct ) indicate higher negative charge on the surface MXene2 compared to MXene1. Finally, ORR at neutral pH was 317-fold more effective at GCE/MXene2 compared to MXene1 and significantly higher (215-fold) H 2 O 2 reduction was observed on GCE/MXene2 in comparison to GCE/ MXene1, as well.
Although traditional characterization techniques cannot explain such staggering difference in the electrochemical behavior for GCE/MXene2 in comparison to GCE/MXene1, TOF SIMS indicate significantly higher amount of Ti + (12.5-fold) and F − (39-fold) ions on MXene2 compared to MXene1. This is why additional studies are needed to prove if there is a direct relationship between interfacial content of F − ions and electrochemical activity of MXenes.
Better understanding of electrochemical performance of MXenes is needed for further development of highly robust electrochemical catalytic and affinity-based biosensors.  Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.