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Article

Constructing Active Sites on Self-Supporting Ti3C2Tx (T = OH) Nanosheets for Enhanced Photocatalytic CO2 Reduction into Alcohols

by
Shuqu Zhang
,
Man Zhang
,
Wuwan Xiong
,
Jianfei Long
,
Yong Xu
,
Lixia Yang
and
Weili Dai
*
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1594; https://doi.org/10.3390/catal12121594
Submission received: 8 November 2022 / Revised: 2 December 2022 / Accepted: 4 December 2022 / Published: 6 December 2022
(This article belongs to the Special Issue Industrial Applications of Advanced Oxidation Technologies: Past and Future)
Browse Figures
Review Reports Versions Notes

Abstract

:
Ti3C2Tx (T = OH) was first prepared from Ti3AlC2 by HF etching and applied into a photocatalytic CO2 reduction. Then, the Ti3C2Tx nanosheets present interbedded a self-supporting structure and extended interlayer spacing. Meanwhile, the Ti3C2Tx nanosheets are decorated with abundant oxygen-containing functional groups in the process of etching, which not only serve as active sites but also show efficient charge migration and separation. Among Ti3C2Tx materials prepared by etching for different times, Ti3C2Tx-36 (Etching time: 36 h) showed the best performance for photoreduction of CO2 into alcohols (methanol and ethanol), giving total yield of 61 μmol g catal.−1, which is 2.8 times than that of Ti3AlC2. Moreover, excellent cycling stability for CO2 reduction is beneficial from the stable morphology and crystalline structure. This work provided novel sights into constructing surface active sites controllably.

Graphical Abstract

1. Introduction

The continuous CO2 emissions due to the depletion of fossil fuels have caused emerging problems in the environment and energy sectors [1]. Solar-driven CO2 reduction that can produce various carbon fuels is considered a desirable strategy to resolve these problems [2]. Nevertheless, the perfect photocatalytic reduction of CO2 process needs to meet the enhanced and broaden light absorption, abundant active sites and efficient charges separation [3]. At present, the researchers devote themselves improving the efficiency for photocatalytic CO2 reduction towards the abovementioned objectives.
Two-dimensional semiconductors are valuable materials for photocatalytic applications because of their larger surface area and excellent electron mobility [4,5,6,7]. As a surface catalytic reaction, the performance of photocatalytic CO2 reduction is also seriously determined by the reactive sites on the surface of photocatalysts [8]. Therefore, it is still urgent for constructing active sites on the surface of two-dimensional semiconductor photocatalysts to further enhance photocatalytic performance [9]. MXenes is formulated as Mn+1XnTx, in which M is a transition metal such as Ti, X is C or N, and T is a surface termination group such as -O or -OH [10]. They can be obtained by removing element A (mostly Al) from a ternary parent MAX phase through liquid exfoliation [11,12]. It was known that MXenes acted as a cocatalyst in photocatalytic CO2 reduction due to its huge surface and excellent electronic conductivity [13]. Its huge surface provides the anchored sites for CO2, and its excellent electronic conductivity is beneficial for migration of photogenerated electrons. However, there are still no reports about MXenes as separate photocatalyst in CO2 reduction. As reported, the terminal oxygen-containing functional group on the MXenes surface could be redox-active [14]. Therefore, it is necessary to prepare MXenes nanosheets with a large surface area and explore the role of terminal functional groups on the performance of photocatalytic CO2 reduction.
In this work, Ti3C2TX nanosheets were prepared by controllable etching, and firstly applicated into a photocatalytic CO2 reduction. The Ti3C2TX nanosheets were decorated with different types of the oxygen-containing functional group. The interbedded self-supporting structure of layered Ti3C2TX not only exposed more active sites and preserved the stability of morphology and crystalline structure, but also benefitted for charge migration and separation. Eventually, Ti3C2TX nanosheets delivered excellent performance and stability for photocatalytic CO2 reduction.

2. Results and Discussion

The XRD pattern was shown to investigate the stacking property and layered structure (Figure 1a). Diffraction peaks of Ti3C2Tx correspond to JCPDS No. 52-0875. Stacking peak {002} shifts to a lower angle compared with Ti3AlC2, indicating extended interlayer spacing in Figure 1b [15]. Ti3C2Tx-36 shows the largest specific surface area among all the photocatalysts samples, which means that the Ti3C2Tx-36 holds the largest interlayer spacing, showing the lowest {002} peak intensity. Raman spectra of different samples was shown in Figure S1. The enhanced peak intensity at 203 cm−1 suggests powerful Ti-C vibration in Ti3C2Tx [16]. The peak at 273 cm−1 belonging to Ti-Al vibration in Ti3AlC2 disappeared after etching. The enhanced Ti-C vibration and disappeared Ti-Al vibration suggest the removal of the Al layer. “Eg vibration” corresponds to out-of-plane vibration of Raman scattering for two-dimensional nanosheets. Eg vibration presents enhanced Raman scattering at 628 cm−1 for decoration of -OH groups on the terminated C atom of Ti3C2 [17]. All indicate successful formation of Ti3C2Tx and decoration of oxygen-containing functional groups on it.
Figure 2 showed SEM images for Ti3C2Tx samples with different etching time (24 h, 30 h, 36 h, 42 h and 48 h). Ti3C2Tx samples show the obvious morphological features of layered structure with broadened interlayer spacing. It can be observed that Ti3C2Tx-36 shows uniform layers and a smooth surface. However, the Ti3C2Tx-42 and Ti3C2Tx-48 tended to aggregate and stack again with the increasing etching time. It is well accepted that the catalysis generally occurs on the active sites, while the active sites mostly exist in the edges, unsaturated steps, terraces, kinks, and/or corner atoms for layered structures [3,18].
The catalysts’ surface holds a spot of active sites. The stacking, layered structure may cause the less active sites’ exposure. Ti3C2Tx-36 shows a uniform layered structure (Figure S2a). Elemental mapping spectra presented Ti, C and O elements in Figure S2b,d. Oxygen-containing functional groups are decorated on the surface. The atomic structure of Ti3C2Tx nanosheets is shown in Figure S2e. The side view for Ti3C2Tx nanosheets shows a broadening layered structure (Figure S2f). TG analysis was conducted to inspect thermostability in Figure S3. Ti3C2Tx nanosheets decorated with oxygen-containing functional groups shows the interbedded self-supporting structure, which also preserves morphological stability. Specifically, Ti3C2Tx-36 showed the best thermostability among all Ti3C2Tx samples. The specific area of Ti3AlC2, Ti3C2Tx-24, Ti3C2Tx-30, Ti3C2Tx-36, Ti3C2Tx-42 and Ti3C2Tx-48 nanosheets are 0.56, 2.49, 3.27, 3.52, 2.97 and 1.41 m2/g, respectively. Ti3C2Tx shows a larger specific surface area compared with Ti3AlC2 form Nitrogen adsorption–desorption isotherms (Figure S4). Ti3C2Tx-36 holds the highest specific surface area and pore volume. The extended interlayer spacing means more surface is exposed and the stacking structure becomes open architecture. It is reported that the open architecture is beneficial for migration and diffusion of photogenerated carriers [19]. Ti3C2Tx-42 and Ti3C2Tx-48, with prolonged etching times, present the smaller specific surface area due to the stacking layers, which is in accord with the morphological features from Figure 1.
It is necessary to investigate the photoelectric property and identify performance of carrier separation. Ti3C2Tx shows excellent UV-vis absorption ability (Figure S5a). The bandgap structure is not changed even though Ti3C2Tx nanosheets are decorated with different oxygen-containing functional groups. The bandgap of Ti3C2Tx-24, Ti3C2Tx-30, Ti3C2Tx-36, Ti3C2Tx-42 and Ti3C2Tx-48 samples is 2.21, 2.14, 2.22, 2.38, 2.26 V, respectively, from the Kubelka–Munk function (Ahv)2 vs. light energy (hv) in Figure S5b. The flat band potential (FB) of Ti3C2Tx is −0.53 V vs. SCE by Mott-Schottky spectra in Figure S5c. The conduction band (CB) can be calculated as −0.39 V vs. NHE by the following equation:
Evs. NHE= EFB + E0 + 0.059pH.
The valence band (VB) of Ti3C2Tx is 1.82, 1.75, 1.83, 1.99, and 1.87 V vs. NHE (pH = 7) by the following equation:
(EVB = ECB + Eg)
It is reported that the oxidation potential is 0.82 V (vs. NHE, pH = 7) [20,21,22]. Ti3C2Tx holds the ability to oxidize H2O to provide H protons for a CO2 reduction rection.
Ti3C2Tx-36 shows highest photocurrent, indicating efficient separation and transportation of photoinduced charge carriers (Figure 3a). In addition, Ti3C2Tx-36 shows a much smaller radius from electrochemical impedance spectroscopy (EIS) spectra (Figure 3b), demonstrating fast interfacial charge transfer. The efficient separation of photogenerated carriers and longer fluorescence (PL) lifetime imply that Ti3C2Tx-36 showed the best carrier generation and separation capability (Figure 3c,d). The enhanced photoelectric property is due to extended interlayer spacing and more introduced active sites.
The Ti3C2Tx showed the better photoelectric property, therefore, it is needed to inspect the surface property of Ti3C2Tx. FTIR spectra was conducted to analyze the functional groups in Figure 4a. A length of 775–1237 cm−1 corresponds to various Ti-O vibrational modes [23]. A length of 1607 and 1631 cm−1 absorbs O vibration. A length of 2345 and 2372 cm−1 belongs to -OH groups vibration. A length of 3396 cm−1 corresponds to absorbed H2O. The Ti3C2Tx nanosheets were prepared by HF etching, therefore, there are few -F function groups linked with the C atom after Al removal (Figure S6a,b). The abundant oxygen-containing functional groups (i.e., -O, -OH) are decorated on the terminus of Ti3C2 after etching exfoliation from XPS measurement (Figure S6c). The binding energy at 527.15, 528.45 and 530.35 eV absorb O, -OH/Ox and H2O, respectively [24]. The high-resolution XPS spectra of O 1s for the samples are analyzed to figure out the crucial oxygen-containing functional group in Figure 4b. The analysis results are listed in Figure 4c. The atomic O contents for Ti3C2Tx-24, Ti3C2Tx-30, Ti3C2Tx-36, Ti3C2Tx-42 and Ti3C2Tx-48 samples are 15.96%, 16.27%, 16.98%, 15.46% and 15.92%, respectively. Ti3C2Tx-36 shows the highest atomic O content because more oxygen-containing groups are decorated on a larger surface area. The stacking layers for Ti3C2Tx-42 and Ti3C2Tx-48 lead to the smaller O content. In Ti3C2Tx-36, the -OH/Ox and H2O showed the highest content compared with other photocatalyst samples from the integral area of the corresponding peak, which means these two oxygen-containing groups play a crucial role for better photoelectric properties. As result, Ti3C2Tx-36 shows better dispersion from the morphological features of the layered structure in Figure S2g, which is due to the wider interlayer spacing for the decoration of -OH and -F functional groups. The selected area electron diffraction (SAED) pattern of Ti3C2Tx-36 suggests preservation of hexagonal basal structure derived from Ti3AlC2 (Figure S2h) [25]. It was reported that this oxygen-containing functional group on the Ti3C2Tx terminal could be redox-active, serving as adsorption active sites for CO2 [26].
The photocatalytic CO2 reduction was proceeded to evaluate photocatalytic performance for photocatalysts. The products of photocatalytic CO2 reduction are methanol and ethanol, and Ti3C2Tx samples show enhanced photocatalytic performance with increasing irradiation time (Figure 5a,b). The produced rate for methanol and ethanol over Ti3AlC2 is 12.9 μmol g catal.−1 and 8.7 μmol g catal.−1. Among all samples, Ti3C2Tx-36 gives best methanol and ethanol yields, 38.1 μmol g catal.−1 and 22.9 μmol g catal.−1 after 4 h irradiation, respectively. The total yield for Ti3C2Tx-36 is 2.8 times than that of Ti3AlC2. The methanol and ethanol yields after 4 h irradiation of Ti3C2Tx-24, Ti3C2Tx-30, Ti3C2Tx-42 and Ti3C2Tx-48 are 19.34 and 17.7, 30.99 and 17.05, 28.85 and 18.11 and 19.1 and 14.34 μmol g catal.−1, respectively. It is noted that Ti3C2Tx-42 and Ti3C2Tx-48 with less O contents show poorer photocatalytic performance of CO2 reduction due to the restacking layers. The oxygen-containing content shows a positive correlation with production yields (Figure 4c and Figure 5a,b). Table S1 showed the comparison of photocatalytic activity for CO2 reduction (products: methanol and ethanol) by some photocatalyst systems. The enhanced performance for CO2 reduction is due to more active sites constructed on Ti3C2Tx surface and efficient carrier separation. 13CO2 was employed to replace 12CO2 to confirm carbon source of the produced methanol and ethanol with the corresponding MS spectra shown in Figure 5c,d. The intense signals of m/z = 33 and m/z = 48 are assigned to 13CH3OH and 13C2H5OH, respectively. The nearby peaks belong to the fragment peaks. It verifies that CO2 acts as the only carbon source of value-added alcohols over the Ti3C2Tx photocatalyst. To further prove the water oxidation, O2 amounts were detected during photocatalytic CO2 reduction over Ti3C2Tx-36. The calibration curves of the relationships between peak area and O2 volume was shown in Figure S7. The O2 yield with 2, 5, 8, 13 μmol g catal.−1 is increased during CO2 reduction over Ti3C2Tx-36 in 4 h in Figure S8. It is true that self-supporting Ti3C2Tx nanosheets with constructed active sites could act as an efficient photocatalyst for CO2 reduction and H2O oxidation. The cycling stability of CO2 reduction over Ti3C2Tx-36 was inspected in Figure 5e. The methanol and ethanol performance over Ti3C2Tx-36 in five cycles are 38.06 and 22.85, 32.1 and 28.64, 32.61 and 27.96, 31.5 and 27.52 and 30.49 and 27.42 μmol g catal.−1, respectively. The performance over Ti3C2Tx-36 represents little decrease after five cycling runs, but crystal structure does not change (Figure 5f). Interbedded self-supporting structures are responsible for excellent photocatalytic activity and stable morphology structure.

3. Materials and Methods

3.1. Preparation Methods

Ti3AlC2 powder (1 g) was dispersed in HF solution (10 mL) and vigorously stirred for different times at room temperature. The obtained powder was washed with deionized water until pH = 6, collected by centrifugation at 8000 rpm for 5 min and dried in the vacuum oven at 60 °C for 12 h. A series of Ti3C2Tx were labeled as Ti3C2Tx-y (y = 24 h, 30 h, 36 h, 42 h and 48 h).

3.2. Materials Characterization

Crystal structure was analyzed by X-ray diffractometer with Cu Kα radiation (Bruker AXS-D8, Karlsruhe, Germany). Raman spectra of the samples were measured by Raman spectrophotometer (Horiba JY LabRAM HR800, Paris, France). Scanning electron microscope (SEM) images were obtained by Nova NanoSEM 450, Hillsboro, IL, USA, and transmission electron microscope (TEM) analyses were conducted with JEOL, JEM−2100F (HR, Tokyo, Japan). Specific surface area and pore property were collected by TriStar II 3020, Atlanta, GA, USA. Thermogravimetric (TG) analysis was obtained from SDT Q600 (TA Instruments, New Castle, DE, USA). Fourier transform infrared spectroscopy (FTIR) were conducted by Bruker VERTEX 70 (Bruker, Karlsruhe, Germany). X-ray photoelectron spectroscopy (XPS) was performed VG Escalab 250, Waltham, MA, USA spectrometer equipped with an Al anode. UV-vis diffuse reflectance spectra (DRS) were proceeded by Shimadzu UV-2450 (Tokyo, Japan) spectrophotometer. The photoluminescence (PL) spectra were measured to inspect charge recombination (F7000, Hitachi, Tokyo, Japan). A time-resolved fluorescence spectrofluorometer was used from Edinburgh, FS5.

3.3. Photoelectrochemical Measurements

Transient photocurrent response, electrochemical impedance spectroscopy and Mott–Schottky curves were carried out on the electrochemical workstation (CHI760E, Shanghai, China) in a standard three-electrode system with the Pt mesh as the counter electrode, the Saturated Calomel Electrode as the reference electrode, and the sample loaded electrodes as the working electrode in 0.1 M Na2SO4 aqueous solution (electrolyte solution) at room temperature. The distance between the counter electrode and the working electrode is 2 cm. Indium tin oxide (ITO) with a 1.0 cm × 1.0 cm area photocatalyst was used as the working electrode. The photocurrent measurement of the photocatalyst is measured by several switching cycles of light irradiated by a 300 W xenon lamp (using a 420 nm cut off filter).

3.4. Photocatalytic Reaction

The assessment for photocatalytic performance of CO2 reduction as follows: 30 mg photocatalyst was dispersed in 30 mL deionized water and put into the reactor. The reactor was vacuumized. The saturated solution was obtained after admission with CO2 (50 mL/min, 0.5 h). The reaction temperature was controlled at 4 °C. With the increasing irradiation time (light source: 300 W xenon lamp with a 420 nm cut offfilter, PerfectLight, Beijing, China), the liquid reduction products were analyzed by gas chromatograph (GC7920-TF2A) equipped with a flame ionized detector (FID) and SE-54 capillary column. The isotope-labeled photocatalytic CO2 reduction tests were performed by replacing 12CO2 with 13CO2 gas, while keeping the other reaction conditions unaltered. The obtained mixed gas was analyzed by gas chromatography-mass spectrometry (GC Model 6890 N/MS Model 5973, Agilent Technologies, Palo Alto, CA, USA).

4. Conclusions

In this work, Ti3C2TX with abundant oxygen-containing functional groups was successfully prepared and applicated into photocatalytic CO2 reduction under visible light. The controllable content of oxygen-containing functional groups was achieved by tuning etching times as shown by the TG and XPS analysis. The exfoliation by extending the interlayer spacing exposed more active sites for generating more photo-induced carriers. The decorated oxygen-containing functional groups was beneficial for the charge migration and separation. The result was that the Ti3C2TX-36 showed the best performance for photocatalytic CO2 reduction (alcohols over production rate: 61 μmol g catal.−1), which is 2.8 time than that of Ti3AlC2. The interbedded self-supporting structure of layered Ti3C2TX after successful exfoliation showed excellent stability of morphological structure, resulting in cycling stability for CO2 reduction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121594/s1, Figure S1. Raman spectra of Ti3AlC2, Ti3C2Tx-y nanosheets (y= 24, 30, 36, 42 and 48); Figure S2. SEM image of (a) Ti3C2Tx-36, (b-d) elemental mappings of (a). (e) Atomic structure of Ti3C2Tx, (f) STEM image, (g) TEM image and (h) selected area electron diffraction (SAED) pattern of Ti3C2Tx-36; Figure S3. TG analysis of (a) Ti3AlC2, (b) Ti3C2TX-24, (c) Ti3C2TX-30, (d) Ti3C2TX-36, (e) Ti3C2TX-42 and (f) Ti3C2TX-48; Figure S4. (a) Nitrogen adsorption−desorption isotherms, (b) corresponding pore size distribution curves, and (c) information contrast of BET surface area, pore size and pore volume of Ti3C2TX with different etching times; Figure S5. (a) UV-vis absorption spectra, (b) Plots of transformed Kubelka–Munk function (Ahv)2 vs light energy (hv) and (c) Mott-Schottky spectra of Ti3C2TX-24, Ti3C2TX-30, Ti3C2TX-36, Ti3C2TX-42, Ti3C2TX-48; Figure S6. High-resolution XPS spectra of (a) Ti 2p, (b) C 1s and (c) O 1s over Ti3C2TX-36; Figure S7. Calibration curves of the relationships between peak area and O2 volume; Figure S8. O2 evolution during photocatalytic CO2 reduction over Ti3C2Tx-36; Table S1. The comparison of photocatalytic activity for CO2 reduction (products: methanol and ethanol) by some photocatalyst systems [27,28,29,30,31,32,33,34,35,36].

Author Contributions

Conceptualization, S.Z.; Methodology, M.Z.; Methodology, W.X.; Formal analysis, J.L.; Investigation, Y.X.; Supervision, L.Y.; Writing—review and editing, W.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grants 51908269, 52262037, 22006064, 52072165).

Data Availability Statement

In this manuscript, our characterizations were SEM, XRD, TEM, BET, and UV-vis. All data have been reported as the images.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01594 g001 550
Figure 1. (a) XRD pattern and (b) (002) peak of Ti3AlC2, Ti3C2Tx-y nanosheets (y = 24, 30, 36, 42 and 48).
Figure 1. (a) XRD pattern and (b) (002) peak of Ti3AlC2, Ti3C2Tx-y nanosheets (y = 24, 30, 36, 42 and 48).
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Catalysts 12 01594 g002 550
Figure 2. SEM images of (a) Ti3AlC2, (b) Ti3C2TX-24, (c) Ti3C2TX-30, (d) Ti3C2TX-36, (e) Ti3C2TX-42 and (f) Ti3C2TX-48.
Figure 2. SEM images of (a) Ti3AlC2, (b) Ti3C2TX-24, (c) Ti3C2TX-30, (d) Ti3C2TX-36, (e) Ti3C2TX-42 and (f) Ti3C2TX-48.
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Figure 3. (a) Transient photocurrent responses, (b) EIS Nyquist plots, (c) PL spectra and (d) TRPL decay spectra of prepared photocatalysts.
Figure 3. (a) Transient photocurrent responses, (b) EIS Nyquist plots, (c) PL spectra and (d) TRPL decay spectra of prepared photocatalysts.
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Catalysts 12 01594 g004 550
Figure 4. (a) FTIR spectra, (b) High-resolution XPS spectra of O 1s, (c) Table of different O group comparison of Ti3C2TX-24, Ti3C2TX-30, Ti3C2TX-36, Ti3C2TX-42 and Ti3C2TX-48.
Figure 4. (a) FTIR spectra, (b) High-resolution XPS spectra of O 1s, (c) Table of different O group comparison of Ti3C2TX-24, Ti3C2TX-30, Ti3C2TX-36, Ti3C2TX-42 and Ti3C2TX-48.
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Figure 5. Yields of (a) methanol and (b) ethanol from photocatalytic CO2 reduction under visible light. GC-MS spectra of (c) 13CH3OH and (d) 13C2H5OH using 13CO2 and 12CO2 as the source of CO2. (e) Cycling runs for CO2 reduction over Ti3C2Tx-36. (f) XRD patterns of fresh and repeated Ti3C2Tx-36.
Figure 5. Yields of (a) methanol and (b) ethanol from photocatalytic CO2 reduction under visible light. GC-MS spectra of (c) 13CH3OH and (d) 13C2H5OH using 13CO2 and 12CO2 as the source of CO2. (e) Cycling runs for CO2 reduction over Ti3C2Tx-36. (f) XRD patterns of fresh and repeated Ti3C2Tx-36.
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Zhang, S.; Zhang, M.; Xiong, W.; Long, J.; Xu, Y.; Yang, L.; Dai, W. Constructing Active Sites on Self-Supporting Ti3C2Tx (T = OH) Nanosheets for Enhanced Photocatalytic CO2 Reduction into Alcohols. Catalysts 2022, 12, 1594. https://doi.org/10.3390/catal12121594

AMA Style

Zhang S, Zhang M, Xiong W, Long J, Xu Y, Yang L, Dai W. Constructing Active Sites on Self-Supporting Ti3C2Tx (T = OH) Nanosheets for Enhanced Photocatalytic CO2 Reduction into Alcohols. Catalysts. 2022; 12(12):1594. https://doi.org/10.3390/catal12121594

Chicago/Turabian Style

Zhang, Shuqu, Man Zhang, Wuwan Xiong, Jianfei Long, Yong Xu, Lixia Yang, and Weili Dai. 2022. "Constructing Active Sites on Self-Supporting Ti3C2Tx (T = OH) Nanosheets for Enhanced Photocatalytic CO2 Reduction into Alcohols" Catalysts 12, no. 12: 1594. https://doi.org/10.3390/catal12121594

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