Ti3C2 MXene co-catalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production

Scalable and sustainable solar hydrogen production through photocatalytic water splitting requires highly active and stable earth-abundant co-catalysts to replace expensive and rare platinum. Here we employ density functional theory calculations to direct atomic-level exploration, design and fabrication of a MXene material, Ti3C2 nanoparticles, as a highly efficient co-catalyst. Ti3C2 nanoparticles are rationally integrated with cadmium sulfide via a hydrothermal strategy to induce a super high visible-light photocatalytic hydrogen production activity of 14,342 μmol h−1 g−1 and an apparent quantum efficiency of 40.1% at 420 nm. This high performance arises from the favourable Fermi level position, electrical conductivity and hydrogen evolution capacity of Ti3C2 nanoparticles. Furthermore, Ti3C2 nanoparticles also serve as an efficient co-catalyst on ZnS or ZnxCd1−xS. This work demonstrates the potential of earth-abundant MXene family materials to construct numerous high performance and low-cost photocatalysts/photoelectrodes.


Supplementary Tables
Supplementary Table 1

NPs
As shown in Supplementary Fig. 7, the XRD patterns of Ti3AlC2, Ti3C2-E and Ti3C2 NPs are consistent with the literature 11,12 . After ultra-sonication treatment, Ti3C2 NPs show a decrease in the intensities of (002) and (004) peaks, in agreement with the dimension change from 3D Ti3C2-E to 0D Ti3C2 NPs.

Pt-CdS
The In general, the change in GH* is continuous from one coverage to the next.  Indeed, Pt-CdS-1 exhibits an obviously enhanced photocatalytic activity of 8234 mol h -1 g -1 ( Supplementary Fig. 26), compared to that of CT0 (105 mol h -1 g -1 ). This is attributed to the presence of ultra-small Pt NPs, which not only extract the photo-induced electrons from CdS, but also promote the H2 evolution, as reported in many previous references. [13][14][15] Nevertheless, Pt-CdS-1 still exhibits lower photocatalytic activity than CT2.5 (14342 mol h -1 g -1 ). Given that these two samples show similar morphologies, the superior activity of CT2.5 should mainly arise from the stronger interaction between Ti3C2 NP and CdS SMS formed in the hydrothermal reaction,

ZnS and ZnS/Ti3C2
The cubic sphalerite-structured ZnS (JCPDS No. 05-0566) is observed for both ZnS and ZnS/Ti3C2 as shown in the XRD patterns ( Supplementary Fig. 24b). Moreover, ZnS/Ti3C2 shows almost the same XRD pattern as that of ZnS, since the mechanical mixing of 1 wt.% Ti3C2 NPs with ZnS does not change its crystal structure. However, an obvious enhanced absorption in the 370-800 nm region is observed for the UV-Vis diffuse reflectance spectrum of ZnS/Ti3C2, in comparison to that of ZnS ( Supplementary Fig. 24c). Also, the color of ZnS/Ti3C2 turned grey due to the loading of black-colored Ti3C2 NPs on the surface of white ZnS.

Supplementary Note 9. Discussion of the effect of F/O atomic ratio on photocatalytic activity
The surface F signal in CT0.05, CT0.1, CT5, CT7.5, CT2.5-E or CT2.5-5000 is negligible as examined by the XPS technique ( Supplementary Fig. 28 Supplementary Fig. 29b), the most stable termination of Ti3C2 is a mix of OH* and O*, which proves that our model for free energy calculation is reasonable.

Ti3C2 at different H coverages
As displayed in Supplementary Fig. 30a

Experimental Section
Synthesis of Ti3AlC2. Elemental Ti (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 40 m), Al (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 40 m), and graphite (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 48 m) powders were mixed with a molar ratio of 3 : 1.5 : 2. The mixture was ball-milled for 48 hours and cold pressed into cylindrical discs (15 mm in diameter and 10 mm in height) under 1 GPa pressure. The discs were put into a tube furnace under flowing Ar gas and heated to 1673 K for 2 hours at a ramp rate of 20 K min -1 .
After cooling to room temperature, the discs were ball-milled for 2 hours to acquire fine powders for further investigation.

Computation Section
Active sites and H adsorption properties.  (3): where S H 2 0 is the entropy of H2 gas at standard conditions. Therefore, Eq (1) can be rewritten as Eq (4): where ∆E H is the differential H adsorption energy, which is defined by Eq (5): OH * → O * + H + + e - Under standard conditions, the free energy of H + + eis equal to ½ H2. Therefore, Eq (7) and Eq (8) can be rewritten into Eq (9) and Eq (10) The Gibbs free-energies of Eq (9) (∆G OH * 0 ) and Eq (10) (∆G O * 0 ) are obtained by Eq (11): from the value table I of reference 18 .
Both Eq (7) and Eq (8) are dependent on the pH and potential U through the chemical potential of H + and e -, respectively, while Eq (9) and Eq (10) are not. To include the effects of pH and potential U, the Eq(11) are rewritten into Eq (12) and Eq (13): Based on Eq (12) and Eq (13) (14): ∆G mix = ∆G mix 0 -(x + 2y)U SHE -(x + 2y)k b Tln10×pH (14) Therefore, we can obtain the free energy of Ti3C2 under different OH* and O* coverages. The most stable state of the surface under relevant conditions is used to construct the surface Pourbaix diagrams.