Boosting the photocatalytic activity of CdLa2S4 for hydrogen production using Ti3C2 MXene as a co-catalyst

Journal Pre-proof Boosting the photocatalytic activity of CdLa2 S4 for hydrogen production using Ti3 C2 MXene as a co-catalyst Lin Cheng, Qian Chen, Juan Li, Hong Liu

PII:

S0926-3373(19)31125-7

DOI:

https://doi.org/10.1016/j.apcatb.2019.118379

Reference:

APCATB 118379

To appear in:

Applied Catalysis B: Environmental

Received Date:

28 August 2019

Revised Date:

10 October 2019

Accepted Date:

1 November 2019

Please cite this article as: Cheng L, Chen Q, Li J, Liu H, Boosting the photocatalytic activity of CdLa2 S4 for hydrogen production using Ti3 C2 MXene as a co-catalyst, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118379

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Boosting the photocatalytic activity of CdLa2S4 for hydrogen production using Ti3C2 MXene as a co-catalyst

Lin Cheng, Qian Chen, Juan Li and Hong Liu*

Department of Chemical Engineering, School of Environmental and Chemical

*

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Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China

Corresponding author. Tel: 86-21-66137487. Fax: 86-21-66137725.

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E-mail: [email protected].

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Graphical Abstract

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Highlights 

Novel CdLa2S4/Ti3C2 nanocomposite was constructed.



CdLa2S4 nanoparticles were well anchored on the surface of 2D Ti3C2 nanosheets.



A maximal H2-evolution rate of 11182.4 μmol·h-1·g-1 was achieved over 1

CdLa2S4/Ti3C2. 

The apparent quantum efficiency reached 15.6% at 420 nm.



Ti3C2 is an efficient co-catalyst for photocatalytic H2-production.

Abstract: Exploring efficient, stable and low-cost photocatalysts for photocatalytic hydrogen production remains a challenge in the field of energy conversion. Here,

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two-dimensional Ti3C2 nanosheets were fabricated via etching Ti3AlC2 with HF, followed by ultrasonic exfoliation. Then CdLa2S4/Ti3C2 nanocomposites were fabricated by growing CdLa2S4 nanoparticles in situ on the surface of these Ti3C2

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nanosheets. The resultant CdLa2S4/Ti3C2 nanocomposites exhibited an excellent

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photocatalytic activity for H2 production from water splitting under visible light illumination. When the content of Ti3C2 was 1.0 wt%, the CdLa2S4/Ti3C2

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nanocomposites presented maximum hydrogen production rate of 11182.4

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μmol·g−1·h−1, which was 13.4 times as high as that of pristine CdLa2S4 and even outperformed Pt-loaded CdLa2S4. The apparent quantum efficiency reached 15.6% at

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420 nm. In addition, the CdLa2S4/Ti3C2 photocatalyst still maintained high photocatalytic

activity

after

six

cycles.

The

exceptional

performance

of

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CdLa2S4/Ti3C2 originated from the superior electrical conductivity of Ti3C2 MXene, which facilitated the separation of photo-generated electron-hole pairs. This work presents the potential of earth-abundant MXene materials in the construction of high efficiency and low-cost photocatalysts toward solar energy conversion.

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Keywords: CdLa2S4; Ti3C2; Photocatalytic; Co-catalyst; H2 evolution

1. Introduction Photocatalytic H2 evolution from water splitting is an ideal way to solve the increasing energy crisis and environmental problems by using freely available solar energy [1, 2]. The key of photocatalytic H2 production technology lies in the

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development of photocatalysts with high efficiency, high stability and low cost.

Among various semiconductor photocatalysts, CdLa2S4, a ternary metal sulfide, has attracted much attention owing to its suitable band gap, strong absorption in the

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visible region and high chemical stability [3-7]. However, the hydrogen production

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performance of CdLa2S4 alone is still unsatisfactory due to the rapid recombination of photo-induced carriers. To address this pitfall, several strategies have been developed

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to boost the charge-carrier separation of CdLa2S4. The coupling of CdLa2S4 with other

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semiconductors with suitable band gaps has been widely adopted to reduce the electron-hole recombination, hence enhancing the photocatalytic activity [6, 8, 9].

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Besides, introduction of a noble metal onto CdLa2S4 as a co-catalyst can also effectively inhibit the combination of photo-induced electrons and holes in CdLa2S4,

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thus improving the photocatalytic activity of CdLa2S4 [5, 8, 10]. For example, loading Pt or Ag effectively improved the hydrogen production performance of CdLa2S4 in water splitting [5, 8, 10]. These noble metals act as “electron sinks” in the photocatalytic system and significantly promote the separation of charge-carriers. Nevertheless, these precious metals are scarce and expensive, which largely limits 3

their application. Therefore, it is significant to develop noble metal-free co-catalysts for providing cost-effective hydrogen. MXenes

is

a

new

family

of

two-dimensional

(2D)

transition

metal

carbides/nitrides/carbonitrides discovered in 2011 by Gogotsi et al [11]. Usually, MXenes are obtained from the selective etching of MAX phase, in which M refers to transition metal, A refers to A-group element (Al or Si) and X denotes C or N element

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[11-13]. At present, MXenes are widely used as excellent electrode materials in

electrochemical fields, such as lithium batteries, super-capacitors and electrocatalysts [14-19]. Meanwhile, owing to its excellent electronic conductivity and a large number

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of exposed metal sites, MXenes as co-catalysts also have great potential in the field of

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photocatalysis [20-25]. For instance, Ran et al. [20] prepared CdS/Ti3C2 composites by a one-step hydrothermal approach with significantly improved photocatalytic

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activity for hydrogen production. Cao et al. [21] designed a 2D/2D ultrathin

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Ti3C2/Bi2WO6 hybrid that displayed a superior photocatalytic performance for CO2 reduction. Wu and colleagues [22] found that the ternary Nb2O5/C/Nb2C showed

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higher H2 production rate than that of its bare counterparts under UV light illumination. Xu et al. [23] demonstrated that Ti3C2 was a Janus co-catalyst for both

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promoting the electron extraction from CdS for photocatalytic reduction of 4-nitrophenol and impeding photocorrosion of CdS via the Cd2+ confinement effect. However, as far as we are aware, it has not been reported that MXene is used as a co-catalyst to boost the photocatalytic performance of CdLa2S4. Herein, 2D Ti3C2 nanosheets were prepared from the layered Ti3AlC2 by HF 4

etching with subsequent solvent stripping. Then, a series of CdLa2S4/Ti3C2 nanocomposites with different Ti3C2 content were synthesized by a facile hydrothermal

synthetic

route

(Fig.

1).

The

as-synthesized

CdLa2S4/Ti3C2

nanocomposites exhibited significantly improved photocatalytic activity towards H2 evolution as compared with bare CdLa2S4. When the content of Ti3C2 was 1.0 wt%, a maximum photocatalytic H2 production rate of 11182.4 μmol·g−1·h−1 was achieved

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and no noticeable decline of the photocatalytic activity was observed after six

photocatalytic cycles. Moreover, the catalytic performance of CdLa2S4/Ti3C2 was even better than that of the Pt-loaded CdLa2S4. The origin of this high activity of

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CdLa2S4/Ti3C2 was studied systematically.

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2. Experimental section 2.1 Reagents

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Hydrofluoric acid (HF, 40 wt%), thiourea (CH4N2S), lanthanum nitrate

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(La(NO3)3·6H2O), cadmium chloride hydrate (CdCl2·2.5H2O), dimethy sulfoxide (DMSO) and ethanol were supplied from Sinopharm Chemical Reagent Co., Ltd.

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China. Ti3AlC2 was obtained from Forsman Scientific (Beijing) Co., Ltd. China. All chemicals were used as received.

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2.2 Sample synthesis

2.2.1 Synthesis of Ti3C2 nanosheets Ti3C2 nanosheets were prepared according to the method reported elsewhere [21] with some modifications. 1.0 g of Ti3AlC2 powders were immersed in 20 mL 40% HF aqueous solution and stirred at room temperature (RT) for 72 h to etch out Al in 5

Ti3AlC2. The suspension was filtrated, rinsed with deionized water until the pH of the supernatant is nearly to 7, and then dried at 80 °C for 12 h. To achieve the exfoliation, the obtained multilayered Ti3C2 powder was added to 20 mL DMSO and stirred for 24 h. The resulting mixture was centrifuged and rinsed with deionized water for several times to obtain the intercalated powder. Thereafter, the collected powder was re-dispersed in deionized water and delaminated by ultrasonication. After 1 h of

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ultrasonic treatment, the suspension was centrifuged at 5000 rpm for 1 h to remove the unexfoliated Ti3C2. Eventually, the supernatant of Ti3C2 nanosheets was obtained. 2.2.2 Preparation of CdLa2S4/Ti3C2

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Firstly, 2.078 g La(NO3)3·6H2O and 0.548 g CdCl2·2.5H2O were dissolved in a

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mixed solution containing 20 mL deionized water and 10 mL ethanol, and then a certain amount of Ti3C2 was added to the above solution under intense stirring. After

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stirred for 1 h, 0.761 g thiourea was added to the suspension and stirred for 2 h. The

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resultant mixture was then transferred to a Teflon-lined autoclave and crystallized at 160 °C for 72 h. After the reaction, the samples were rinsed three times with

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deionized water, centrifuged, and dried at 80 °C for 12 h. The as-prepared CdLa2S4/Ti3C2 samples with 0.5 wt%, 1.0 wt% and 1.5 wt% Ti3C2 were labeled as

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CLST0.5, CLST1.0 and CLST1.5, respectively. 2.3 Characterization An X-ray diffractometer (XRD, Rigaku D/MAX-2500V/PC) was used to identify the crystal structure of the samples using Cu-Kα radiation. Field-emission scanning electron microscopy (FE-SEM, JSM-7500F), transmission electron microscopy (TEM, 6

JEM-2010F) and high-resolution transmission electron microscopy (HRTEM) were employed to analyze the morphologies and microstructures of the samples. UV–vis absorption spectra of the samples were collected by a Hitachi U-3010 UV-vis spectrophotometer. X-ray photoelectron spectra (XPS) were obtained on an ESCALAB 250Xi electron spectrometer. Photoluminescence (PL) spectra were recorded at RT with a Hitachi RF5301 fluorescence spectrophotometer. Fourier

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transform infrared (FT-IR) spectra were recorded at RT using a FT-IR spectrometer (AVATAR370, Nicolet). Raman spectra were measured by a Raman spectrometer (inVia,

Renishaw)

with

an

excitation

of

532

nm

laser

light.

The

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Brunauer-Emmett-Teller (BET) surface area was measured on a Micromeritics

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ASAP2460 nitrogen adsorption-desorption apparatus.

The photocurrent responses of the samples were measured by an electrochemical

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station (Chenhua CHI660E) with a standard three-electrode configuration. In the

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system, the synthesized samples acted as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, Pt foil as the counter electrode, and a 0.5 M Na2SO4

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aqueous solution as the electrolyte. An 800 W xenon arc lamp with a UV-cut off filter (λ > 420 nm) was employed as the light source. The working electrode was prepared

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by the following way: 10 mg of the synthesized sample was mixed with 100 µL ethanol to obtain a slurry. Then the slurry was coated on a 2.0 cm ×2.0 cm FTO (F-doped SnO2) substrate with a coated area of about 0.5 cm2 by the doctor-blade approach. The obtained working electrode was dried at 80 °C for 0.5 h. The optical-to-chemical conversion efficiency (η) can be calculated according to equation 7

[26]:



J p (1.23 V )  I0

where I0 is the irradiance intensity (100 mW/cm2), α is the transmittance for light incident on the photoanode (0.8), Jp is the photocurrent density at the measured potential (0 V vs. Ag/AgCl), and V is the applied potential vs. reversible hydrogen electrode (RHE). The potential versus RHE was calculated with a reference to

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Ag/AgCl according to the Nernst equation:

VRHE VAg / AgCl  0.0591 pH  0.197

where VRHE is the potential vs. RHE, and VAg/AgCl is the measured potential vs.

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Ag/AgCl.

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Electrochemical impedance spectroscopy (EIS) was conducted under the frequency from 10-2–106 Hz in 0.1 M Na2SO4. Mott-Schottky plots were undertaken in the range

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of -1.0 to +1.0 V (vs. Ag/AgCl) at a frequency of 1000 Hz in 0.1 M Na2SO4.

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2.4 Photocatalytic H2 evolution

The experiment of photocatalytic H2 evolution was implemented in a 100 mL

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gas-closed quartz reactor. A 300 W Xenon lamp equipped with a high-pass filter (λ > 420 nm) was utilized as the visible light source. In the typical setup, 50 mg of

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photocatalyst was suspended in 100 mL deionized water with 0.25 M Na2SO3 and 0.35 M Na2S as the hole scavenger. Prior to experiment, the photo-reaction system was pumped to vacuum. The reaction cell was maintained at RT by the cooling water. The resulting H2 was analyzed by online gas chromatography (GC7900, TCD detector, 5Å molecular sieve column, and N2 as carrier gas). The apparent quantum efficiency 8

(AQE) was determined under the identical photocatalytic reaction, only by replacing the high-pass filter with a band-pass filter, which has a central wavelength of 420 nm. The focused intensity of irradiation was determined to be 15.0 mW cm−2 and the irradiation area was held at 28.26 cm2. The AQE was evaluated in terms of the following equation:

2  The number of evolved H 2 molecules  100% The number of incident photons

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AQE 

3. Result and discussion 3.1 Structure and morphology

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The phase structures of the as-prepared samples were determined by XRD. As can

be seen from Fig. 2a, the peak of pure Ti3AlC2 is sharp and its position is consistent

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with that reported in literature [21, 27, 28]. After etching with 40% HF, the most

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intense peak at 2θ = 39° disappears and the (002) and (004) peaks shift towards a lower angle, confirming that Ti3AlC2 is completely transformed into Ti3C2 after HF

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etching [21, 27, 28]. Additionally, compared with Ti3AlC2, the peak intensities of Ti3C2 become much weaker, which can be ascribed to the thinner layered structure of

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Ti3C2 [21, 27]. In the XRD spectra of CdLa2S4/Ti3C2 samples, only the characteristic

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diffraction peaks of CdLa2S4 can be observed. There is no characteristic diffraction peak of Ti3C2 in the XRD spectra, which is owing to the low amount of Ti3C2 in the composites.

Raman spectra of Ti3AlC2 before and after HF treatment are shown in Fig. 2b. For Ti3AlC2 sample, peaks I and II in Fig. 2b can be assigned to Al–Ti vibrations, while 9

peaks III and IV involve Ti–C vibrations [21, 25]. After HF treatment, peaks I and II nearly disappears, while peaks III and IV become broader and weaker. The disappearance of peaks I and II demonstrates the successful etching of Ti3AlC2. While the broader and weaker peaks III and IV can be attributed to the thinner layer structure of Ti3C2 caused by etching of HF [21]. In addition, peaks III and IV in Ti3C2 slightly shift to larger wavenumbers, which is due to the strong interaction between Ti

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and surface functional groups (–O or –F) [21].

The microstructure and morphology of the samples were observed by SEM, TEM and HRTEM. It can be clearly seen that the CdLa2S4 reveals a particle-like

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morphology with particle size of 30–50 nm (Fig. 3a). Fig. 3b shows a layered

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structure of Ti3C2 after HF etching with a thickness of around tens nanometers for each layer. Fig. 3c and Fig. 3d reveal that the Ti3C2 after ultrasound peeling has a 2D

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sheet-like morphology similar to that of graphene. From the TEM image (Fig. 3e) of

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the composite sample, we can see that CdLa2S4 particles are uniformly anchored on the surface of the Ti3C2 flakes. Two obvious lattice fringes with widths of 0.294 and

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0.261 nm are observed in the HRTEM image (Fig. 3f) of the CLST 1.0 sample. The former is assigned to the (300) crystal plane of CdLa2S4 and the latter corresponds to

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the (0 1 10) crystal plane of Ti3C2 [6, 24, 25, 27]. To investigate the chemical states of CdLa2S4/Ti3C2 composites, XPS was utilized

to analyze the CLST1.0 sample. It can be seen from the survey spectrum of CLST1.0 (Fig. 4a), the composite consists of six elements: Ti, C, O, Cd, S and La. No F elements are detected, indicating that the F-terminated Ti3C2 after HF etching of 10

Ti3AlC2 is successfully converted to O-terminated Ti3C2 after hydrothermal reaction [29, 30]. Fig. 4b is the Ti 2p spectrum of the CLST1.0 sample. There are obvious absorption peaks at 455.4, 458.3, 459.7 and 464.9 eV, corresponding to Ti-C (2p3/2), Ti-O (2p3/2), Ti-C (2p1/2) and Ti-O (2p1/2), respectively [20, 31]. Fig. 4c is the C 1s spectrum of the sample. The peak at 284.2 eV corresponds to adventitious carbon (C-C bond), the peak at 285.8 eV corresponds to C-O bond, and the characteristic

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peaks at 281.6 and 289.3 correspond to C-Ti and O-C=O, respectively [21, 27, 29].

Four characteristic peaks of La 3d in Fig. 4d are located at 835.5 (La 3d5/2), 838.2 eV (La 3d5/2), 852.4 eV (La 3d3/2) and 855.1 eV (La 3d3/2) eV, respectively, indicating that

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the La element exists in the form of La3+ [3, 6]. The broad peak of S 2p (Fig. 4e) can

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be deconvoluted into two peaks at 162.5 eV and 161.3 eV, which are ascribed to S 2p1/2 and S 2p3/2 of S2-, respectively [3, 6]. Fig. 4f is the Cd 3d pattern of the sample.

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The characteristic peaks at 404.7 and 411.5 eV correspond to the inner electrons of Cd

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3d5/2 and Cd 3d3/2, respectively [3, 6], indicating that the Cd element in the sample exists in the form of Cd2+.

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The optical properties of CdLa2S4, Ti3C2 and the CdLa2S4/Ti3C2 hybrids were analyzed by the UV-vis absorption spectroscopy. As illustrated in Fig. 5a, the

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absorption edge of pristine CdLa2S4 is about 560 nm. Ti3C2 has an absorption curve similar to that of reduced graphene oxide (RGO) and other black materials [32, 33], with light absorption from 300 to 800 nm. Compared with pure CdLa2S4, the absorption edge of the CdLa2S4/Ti3C2 composites exhibit a significant red shift, and the more obvious the red shift is with the increase of Ti3C2 content, suggesting the 11

better light absorption ability of CdLa2S4/Ti3C2. The band gap energy of the CdLa2S4 sample can be estimated by Kubelka-Munk method [6]. As shown in Fig. 5b, the band gap value of CdLa2S4 is determined to be 2.32 eV. Fig. 6 is the typical Mott-Schottky plot of CdLa2S4, which reveals an n-type semiconductor feature of CdLa2S4 because of the positive slope of the plot [34, 35]. The flat band potential of CdLa2S4 from Mott-Schottky plot is extrapolated to be

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about –0.59 V vs. Ag/AgCl（i.e. –0.39 V vs. NHE). It is acknowledged that the

conduction band potential (ECB) of n-type semiconductors is about 0.1 V below the

flat-band potential [36, 37]. Thus the ECB of CdLa2S4 is calculated to be about –0.49 V,

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and the corresponding valence band potential (EVB) can be estimated to be +1.83 V.

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The N2 adsorption-desorption isotherms and the corresponding pore size distribution curves of the samples are shown in Fig. 7, and the specific surface areas

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are listed in Table 1. As displayed in Fig. 7a, the pristine CdLa2S4 exhibits no

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hysteresis loop, suggesting no pores exist in this sample. The N2 adsorption isotherms observed over Ti3C2 and CdLa2S4/Ti3C2 composites attribute to IV-type sorption

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curves, indicating the presence of mesopores in these samples [6]. The surface area of the composites slightly increase compared with that of the pristine CdLa2S4, owing to

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the larger specific surface area of Ti3C2. 3.2. Photocatalytic H2 evolution activity The photocatalytic H2 evolution performance of the CdLa2S4, Ti3C2 and CdLa2S4/Ti3C2 composites were evaluated under visible light with 0.1 M Na2S and 0.5 M Na2SO3 as sacrificial reagent. As can be seen from Fig. 8 and Table 1, Ti3C2 has 12

no hydrogen production activity due to the metallic properties of Ti3C2, which is consistent with the literatures [20, 31]. The H2 production rate of pure CdLa2S4 under visible light illumination is only 832.0 μmol·g−1·h−1, and the AQE at 420 nm is only 1.3 %. The photocatalytic activity of CdLa2S4 composites is prominently improved after coupling of Ti3C2. With increasing amount of Ti3C2, the photocatalytic H2 evolution rate of the CdLa2S4/Ti3C2 composites improves accordingly. When the

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content of Ti3C2 is 1.0 wt%, the CdLa2S4/Ti3C2 composite (CLST1.0) achieves the

maximum hydrogen production rate of 11182.4 μmol·g−1·h−1 (the AQE at 420 nm is 15.6 %), which is 13.4 times as high as that of pristine CdLa2S4. Moreover, the

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hydrogen production rate of CLST1.0 is also higher than that of 1.0 wt% Pt/CdLa2S4

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(1734.7 μmol·g−1·h−1). However, with the further increase of Ti3C2, the H2 production rate of the CdLa2S4/Ti3C2 composite decreases. Such reduced activity is due to the

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excessive amount of Ti3C2 hindering the light absorption of CdLa2S4 [29, 38]. In

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addition, excessive Ti3C2 will become the recombination center of photo-induced electrons and holes (Fig. 10), consequently reducing the photocatalytic activity.

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Cyclic experiments were conducted to estimate the stability of the CdLa2S4/Ti3C2 composites. Fig. 9a shows that the H2 production rate of the CLST1.0 sample still

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reaches to 58210.2 μmol·g-1 after six recycling tests. Compared with the initial cycle (67094.4 μmol·g-1), the hydrogen production decreased slightly. In addition, the crystal structure, chemical composition and morphology of the composite do not change significantly before and after the reaction (Fig.9b-d). The above results show that the CdLa2S4/Ti3C2 composite has good stability in photocatalytic experiments. 13

3.3. Photocatalytic mechanism for enhanced activity To shed light on the fundamental reasons for the outstanding performance of CdLa2S4/Ti3C2 hybrids, the PL spectra of as-synthesized samples were measured at 325 nm. From Fig. 10a, it can be seen that CdLa2S4 has strong fluorescence peaks at 360 and 470 nm, which indicates that CdLa2S4 has rapid electron-hole recombination rate. The fluorescence intensity of the CdLa2S4/Ti3C2 composite is lower than that of

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CdLa2S4, which indicates that the addition of Ti3C2 effectively inhibits the recombination of photo-excited electrons and hole on the catalyst. The order of

fluorescence intensity is CdLa2S4 > CLST1.5 > CLST0.5 > CLST1.0, which is in line

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with the order of photocatalytic activity. Transient photocurrent responses of pure

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CdLa2S4, Ti3C2 and CdLa2S4/Ti3C2 samples were measured to further validate the enhanced carrier separation efficiency in CdLa2S4/Ti3C2. Fig. 10b shows that the

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photocurrent intensity of pure Ti3C2 and CdLa2S4 is very low. The photocurrent

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intensity of CdLa2S4/Ti3C2 is much higher than that of CdLa2S4 and Ti3C2. The order of photocurrent intensity is CLST1.0 > CLST0.5 > CLST1.5 > CdLa2S4 > Ti3C2,

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which is in accordance with the above PL results. A maximum optical-to-chemical conversion efficiency of 0.081% is obtained for the CLST1.0 sample. The EIS plots

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of as-prepared samples are displayed in Fig. 10c. It is shown that the Nyquist semicircle diameter of the CdLa2S4/Ti3C2 composites is much smaller than that of pristine CdLa2S4. In particular, the CLST1.0 sample has the smallest radius of the arc, indicating its lowest electrochemical impedance. The results of PL, photocurrent and EIS indicate that the CdLa2S4/Ti3C2 has higher transfer and separation ability of 14

photo-induced electron-hole pairs, resulting in higher photocatalytic efficiency. In the light of the above experimental results, we proposed the mechanism of enhancing activity of CdLa2S4/Ti3C2 photocatalyst (Fig. 11). Under visible light illumination, CdLa2S4 is stimulated, and the electrons of valence band jumps to the conduction band, leaving holes in the valence band. Because O-terminated Ti3C2 is highly conductive and its Fermi level (Ef, 0.71 V vs. NHE, pH=7) [21, 24] is less

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negative than the CB of CdLa2S4, electrons can quickly transfer from CdLa2S4 to

Ti3C2 surface, and react with water absorbed on the surface of Ti3C2 to form H2.

Meanwhile, the holes remaining in the valence band of CdLa2S4 are consumed by the

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sacrificial agent. This process realizes the transfer and separation of photo-excited

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electrons and holes on the CdLa2S4 photocatalyst, thus significantly enhancing the photocatalytic activity.

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4. Conclusions

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In summary, a series of CdLa2S4/Ti3C2 nanocomposites with different content of Ti3C2 were successfully fabricated by growing CdLa2S4 nanoparticles in situ on the

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2D Ti3C2 nanosheets. The CdLa2S4/Ti3C2 nanocomposites demonstrated remarkably enhanced performance toward photocatalytic H2 production as compared with bare

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CdLa2S4. The photocatalytic H2 evolution rate of the optimized CdLa2S4/Ti3C2 (CLST1.0) upon visible light irradiation was 11182.4 μmol·g−1·h−1, which was 13.4 times as high as that of pristine CdLa2S4 and even outperformed Pt-loaded CdLa2S4. The excellent photocatalytic activity of CdLa2S4/Ti3C2 hybrids was ascribed to the superior conductivity of Ti3C2 MXene, which promoted the migration and separation 15

of photo-generated charge carriers. Moreover, the CdLa2S4/Ti3C2 hybrids exhibited excellent durability. This work demonstrates that Ti3C2 can be utilized as a low-cost and efficient co-catalyst for photocatalytic hydrogen production as well as other reactions. Declarations of interest None

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Declaration of interests The authors declare that they have no known competing financial interests or personal

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relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

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This work was supported by the National Natural Science Foundation of China

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(11872235).

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Figure Captions: Fig. 1. Synthesis process of CdLa2S4/Ti3C2 nanocomposites Fig. 2. (a) XRD patterns of Ti3AlC2, Ti3C2, CdLa2S4 and CdLa2S4/Ti3C2 nanocomposites; (b) Raman spectra of Ti3AlC2 before and after HF treatment 23

Fig. 3. SEM images of (a) CdLa2S4, (b) Ti3C2 after HF etching and (c) Ti3C2 nanosheets; TEM images of (d) Ti3C2 nanosheets and (e) CLST1.0; (f) HRTEM image of CLST1.0 Fig. 4. XPS spectra of CLST1.0: (a) survey; (b) Ti 2p; (c) C 1s; (d) La 3d; (e) S 2p; (f) Cd 3d Fig. 5. (a) UV-vis absorption spectra of CdLa2S4, Ti3C2 and CdLa2S4/Ti3C2; (b) the

ro of

plot of Kubelka-Munk function for CdLa2S4 Fig. 6. Mott-Schottky plot of CdLa2S4 (pH=6.8)

CdLa2S4, Ti3C2 and CdLa2S4/Ti3C2 composites

-p

Fig. 7. (a) N2 absorption-desorption isotherms and (b) pore size distributions of

evolution rate over different catalysts

re

Fig. 8. (a) Photocatalytic H2 evolution as a function of irradiation time and (b) H2

lP

Fig. 9. (a) Cycling experiments of photocatalytic H2 production over CLST1.0 under

na

visible light illumination; (b) XRD patterns and (c) FT-IR spectra of CLST1.0 before and after reaction; (d) TEM image of CLST1.0 after reaction

ur

Fig. 10. (a) PL spectra, (b) transient photocurrent responses and EIS plots of the CdLa2S4, Ti3C2 and CdLa2S4/Ti3C2 sample

Jo

Fig. 11. Mechanism for enhancement of photocatalytic activity of CdLa2S4/Ti3C2

24

ro of

Fig. 1

(a)

-p

CLST1.0

re

CLST0.5

10

20

30

40

Ti3C2

109 110

107

104 105

101 103

202

006

004

lP

002

004

CdLa2S4

002

Intensity (a.u.)

CLST1.5

50

Ti3AlC2

60

70

2 Theta (degree)

Jo

ur

Intensity (a.u.)

na

(b)

III

IV

II

I

Ti3AlC2 Ti3C2

200

400

600

-1 Wavenmuber (cm )

Fig. 2

25

800

1000

26

ro of

-p

re

lP

na

ur

Jo Fig. 3

(a) survey

1000

800

600

400

200

Ti-O 2p3/2 458.3 Ti-C 2p3/2 455.4

Intensity (a.u.)

S 2p

La 4d

C 1s

O 1s

Ti 2p Cd 3d3/2 Cd 3d5/2

Cd 3p

Intensity (a.u.)

La3d3/2 La3d5/2

(b) Ti 2p

Ti-O 2p1/2 464.9 Ti-C 2p1/2 459.7

470

0

465

460

455

450

Binding energy (eV)

Binding energy (eV)

(d) La 3d

(c) C 1s

290

288

286

284

282

280

860

855

(e) S 2p

845

840

835

830

re

(f) Cd 3d

Intensity (a.u.)

Intensity (a.u.)

161.3

163

162

161

160

159

na

164

lP

162.5

165

850

Binding energy (eV)

Binding energy (eV)

166

838.2 835.5

-p

292

Ti-C 281.6

852.4

ro of

C-O 285.8

O-C=O 289.3

855.1

Intensity (a.u.)

Intensity (a.u.)

C-C 284.2

158

416

Binding energy (eV)

404.7

411.5

414

412

410

408

406

Binding energy (eV)

Jo

ur

Fig. 4

27

404

402

(b) 12

300

CdLa2S4

9 (hv)2

Relative intensity (a. u.)

(a)

Ti3C2

6

CLST0.5 CLST1.0 CLST1.5 CdLa2S4

3

400

500

600

700

2.32 eV

0 1.5

800

2.0 2.5 Band gap (eV)

Wavelength(nm)

3.0

25 1000Hz

-p

15 -0.59 V

10 5

lP

0 -1.0

re

10 (C-2/F-2)10

20

-0.5

ro of

Fig. 5

0.0

0.5

1.0

Potential (V vs Ag/AgCl)

(b)

ur

180

Ti3C2

150

CLST1.5 CLST1.0 CLST0.5 CdLa2S4

Jo

120

Pore volume (cm3g-1nm-1)

Volume absorbed (cm3g-1)

(a)

na

Fig. 6

90 60 30 0

0.0

0.2

0.4

0.6

0.8

0.006 Ti3C2

0.005

CLST1.5 CLST1.0 CLST0.5 CdLa2S4

0.004 0.003 0.002 0.001 0.000 1

1.0

10

Pore diameter (nm)

Ralative pressure (P/P)

Fig. 7 28

100

H2 evolution rate (mol g-1h-1)

(b) 70000

12000

CdLa2S4 Ti3C2 CLST0.5 CLST1.0 CLST1.5 Pt/CLS

H2 evolution(mol g-1)

(a)

60000 50000 40000 30000

10000 0 1

9583.0

10000

20000

0

11182.4

2

3

4

5

8601.9 8000 6000 4000 2000

1734.7 832.0 none

0

LS

6

C

Irradition time (h)

C2 1.0 T1.5 CLS 0.5 Ti 3 S ST LST Pt/ C CL CL

(a)

H2 evolution(mol g-1)

60000

Intensity (a.u.)

50000

before after

re

40000

-p

(b)

70000

30000

10000 0 0

6

12

18

lP

20000

24

30

36

na

Irradiation time (h)

(c)

ur

before after

Jo

Transimittence (a.u.)

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Fig. 8

500

1000 1500 2000 2500 3000 3500 4000 -1

Wavenmuber (cm )

Fig. 9

29

10

20

30

40

50

2 Theta (degree)

60

70

(a)

Photocurrent density (A cm-2)

(b) CdLa2S4

PL intensity (a.u.)

CLST1.0 CLST1.5 CLST0.5

330

360

390

420

450

480

Ti3C2

160

CdLa2S4

light on light off

120

CLST0.5 CLST1.0 CLST1.5

80

40

0

510

60

40

20

0

80

100

120

Time (s)

Wavelength (nm)

(c) 75 CdLa2S4

Z''/ohm

ro of

CLST1.5 CLST0.5 CLST1.0

60 45 30

0 0

20

40

60

80

100

120

re

Z'/ohm

-p

15

Jo

ur

na

lP

Fig. 10

Fig. 11

30

140

Table 1. Textural properties, H2 evolution rate and AEQ of the as-synthesized catalysts Surface area Pore volume H2 evolution (µmol·g−1·h−1) AQE (%)

Catalyst (cm3 g−1)

4.38

0.01057

832.0

1.3

CLST0.5 6.45

0.01983

9583.0

13.4

CLST1.0 8.19

0.03447

11182.4

15.6

CLST1.5 8.84

0.03901

8601.9

Ti3C2

0.04256

-

9.69

re lP na ur Jo 31

12.0 -

-p

CdLa2S4

ro of

(m2 g−1)