Multi-functionalized carbon nanotubes towards green fabrication of heterogeneous catalyst platforms with enhanced catalytic properties under NIR light irradiation

Metal/carbon nanotubes (CNTs) have been attractive hybrid systems due to their high specific surface area and exceptional catalytic activity, but their challenging synthesis and dispersion impede their extensive applications. Herein, we report a facile and green approach towards the fabrication of metal/CNT composites, which utilizes a versatile glycopeptide (GP) both as a stabilizer for CNTs in water and as a reducing agent for noble metal ions. The abundant hydrogen bonds in GP endow the formed GP-CNTs with excellent plasticity, enabling the availability of polymorphic CNT species from dispersion to viscous paste, gel, and even to dough by increasing their concentration. The GP molecules can reduce metal precursors at room temperature without additional reducing agents, enabling the in situ immobilization of metal nanoparticles (e.g. Au, Ag, Pt, and Pd) on the CNT surface. The combination of the excellent catalytic properties of Pd particles with photothermal conversion capability of CNTs makes the Pd/CNT composite a promising catalyst for the fast degradation of organic pollutants, as demonstrated by a model catalytic reaction using 4-nitrophenol (4-NP). The conversion of 4-NP using the Pd/CNT composite as the catalyst has increased by 1.6-fold under near infrared light illumination, benefiting from the strong light-to-heat conversion effect of CNTs. Our proposed strategy opens a new avenue for the synthesis of CNT composites as a sustainable and versatile catalyst platform.

When adding an excess amount of NaBH4, the reduction kinetics of 4-NP mainly follow a pseudofirst-order law.Apparent rate constant (kapp) of 4-NP can be given by: where c and c0 are the concentration and I and I0 are the absorption intensity at 400 nm of 4-NP at the given time t and the very beginning of the reaction, respectively. 2St is the total surface of catalytic particle and k1 is the rate constant normalized to St. kapp can be obtained from the linear correlation between ln(I/I0) or ln(c/c0) and t.As a result, GP-CNTs-Pd displayed a fast catalytic conversion of 4-NP, giving a kapp value of 0.016 s -1 , while a kapp value of 0.0021 s -1 has been measured for GP-CNTs-Au (Fig. S16).
Note that the reduction mainly proceeds on the surface of the metal NPs.If we assume both Pd and Au particles are ideally spherical, St can be thus calculated by: where m0 S0, and n are the mass and surface of a single catalyst particle and the total number of metal particles, respectively; m is the total mass of metal catalyst that can be estimated from TGA measurement (13.2 wt.% for GP-CNTs-Pd and 11.5 wt.% for GP-CNTs-Au); ρ is the density of the catalytic particle, assuming it is the same as the density of their bulk material (12.0 g/cm 3 for Pd and 19.3 g/cm 3 for Au); d is the diameter of the particle, which can be obtained from the HRTEM analysis (1.8 nm for Pd and 5.6 nm for Au). 3 V is the volume (2.5 mL here).
According to Table S2, the St was thereby fixed to be 0.26 m 2 /L.The value of GP-CNTs-Pd was calculated to be 0.062 s -1 m -2 L, which is higher than that of GP-CNTs-Au (0.0081 s -1 m -2 L), demonstrating the superior catalytic activity of GP-CNTs-Pd.The photothermal conversion efficiency (η) can be calculated based on the heating-cooling profiles and given by: where h, s, QDis, are the heat transfer coefficient, surface area of the container, heat dissipated from the light absorbed by the quartz sample cell containing with pure water, respectively.I and A808 are the laser intensity and absorption of dispersion at the wavelength of 808 nm, respectively.
where Tsur, Tmax are surrounding temperature and maximum temperature of the GP-CNTs dispersion, respectively. 4,5 calculate hs, a dimensionless parameter (θ) needs to be introduced and determined by: where mD and CD are the mass of water (1 g) and heat capacity (4.2 J/g• °C), respectively.The time constant (τs) can be determined from the slop of cooling curve (Fig. S16b) according to the equations below: where t and T are the time and cooling temperature of the dispersion during cooling stage.Hence, τs of the pristine CNTs cluster and GP-CNTs dispersion were determined to be 328.1 and 370.4 s, respectively.Correspondingly, the value η of CNTs cluster and the GP-CNTs dispersion were determined to be 4.2 % and 22.3 %, respectively.

Fig. S6 .
Fig. S6.Photos of the GP-CNTs dispersions at 0.2 mg/mL and the pH of (a) 10 and (b) 5 overnight.

Fig. S7 .
Fig. S7.SEM images of the GP-CNTs film after drying at various magnifications.

Fig. S9 .
Fig. S9.TEM images of the GP-CNTs-Pd sample after storage for 1 month.

Fig. S10 .
Fig. S10.Histograms of particle size distribution (based on over 50 particles) of (a) Pd NPs in GP-CNTs-Pd, (b) Au NPs in GP-CNTs-Au, and (c) Ag NPs in GP-CNTs-Ag determined from TEM analysis based on more than 50 particles.

Fig. S15 .
Fig. S15.TEM images of (a) Pristine CNTs cluster mixed with HAuCl4 in the absence of GP molecules.(b) Au NPs generated in the mixture of HAuCl4 (0.01 mM) and GP (0.1 mg/mL) solution.The inset in (b) is the corresponding photo of the obtained Au NPs dispersion.

Fig. S18 .
Fig. S18.(a) TEM image of the GP-CNTs-Pd dispersion after catalytic cycles.(b) Reduction reaction of 4-NP with either GP-CNTs-Pd or pristine GP molecules at pH of ~10 without NaBH4 after 15 min.

Table S1 .
Comparison of the catalytic activity of the Pd-based catalysts.Ccat, CNaBH4, C4-NP, t, T, and km are the catalyst concentration, NaBH4 concentration, 4-NP concentration, reaction time, reaction temperature, and kapp normalized with the catalyst mass, respectively.

Table S2 .
Summary for the calculation of St.