Photothermal-enhanced catalysis in core–shell plasmonic hierarchical Cu7S4 microsphere@zeolitic imidazole framework-8

A strategy to improve reaction activity via the photothermal effect of plasmonic semiconductor nanomaterials is demonstrated in a core–shell structured catalyst.

Instruments Ltd., FLS920). The temperature of the bulk reaction solution was measured by a K/J type thermometer (Suzhou Tasi Electronic Co., Ltd, TASI-602).
The monochromatic light sources were 670, 808, 1450 nm diode laser with a fiber optic accessory (Beijing Hi-Tech Optoelectronic Co., Ltd) and the output power was calibrated by an optical power meter (Ophir Optronics Solutions Ltd, Vega). The sunlight photothermal catalysis was conducted under the simulated solar irradiation with intensity of 100 mW cm -2 obtained by a 500 W Xenon lamp with the AM 1.5G filter (Beijing Au-Light Tech Co., Ltd, CEL-S500R).
Catalyst evaluation. The cyclocondensation reaction was conducted in a 10 mL rubber-sealed flask under atmospheric pressure. The reactions (Figure 4, S11) were performed in a mixture of 1,3-cyclohexanedione (0.1 mmol), 3-methyl-2-butenal (0.1 mmol) and dichloromethane (2 mL) with the amount of catalysts adjusted to contain 10 mg ZIF-8. The measurement of the reaction extent was based on the conversion efficiency of 1,3-cyclohexanedione, whose amount was analyzed by gas chromatographs (Agilent, 7890B). All reactions conducted in our experiments performed a 99.9% selectivity with the Cu 7 S 4 @ZIF-8 catalyst, which was consistent with the previous report. 3 For the determination of the relationship between photothermal catalytic activity and the laser intensity, the initial reaction rate was calculated based on the conversion efficiency of the first hour. The recyclability test of the Cu 7 S 4 @ZIF-8 (with or without the sunlight irradiation) were performed in a mixture of 1,3-cyclohexanedione (0.6 mmol), 3-methyl-2-butenal (0.6 mmol) and dichloromethane (12 mL) with the amount of catalysts adjusted to contain 60 mg ZIF-8. The catalyst was centrifuged at 6000 rpm for 5 min after each cycle of the 6-hour catalysis. The supernatant liquid was decanted slowly and the precipitate was washed with CH 2 Cl 2 and immersed in CH 2 Cl 2 for 12 h and centrifuged at 6000 rpm for 5min.
Then the precipitate was dried in vacuum at 60 o C for at least 6 h. Then, the catalyst was reused with the injection of fresh solvent and reactants for the next catalytic cycle under the same reaction conditions. Surface temperature measurement of Cu 7 S 4 nano-heaters. The measurement of surface temperature was based on the temperature-sensitive fluorescent molecules, an Yb 3+ complex. The 410 nm laser was used to excite the fluorescent molecule and the 1450 nm laser was used as the heating source. To prepare the testing sample, the methanolic solutions of 1 mL Yb 3+ complex (1 g/L) and 1 mL Cu 7 S 4 NPs (2 g/L) were mixed and standed for 5 min to ensure the sufficient absorption of the Yb 3+ complex on the surface of the Cu 7 S 4 NPs. Then the mixture (~100 µL) was dispensed on the quartz substrate using a pipet and dried in air. The luminescence spectrum of the Yb 3+ T-sensor was collected from 900nm to 1150 nm (step length: 1.0 nm, dwell time: 0.3 s, scan number: 1). The value of the surface temperature was determined according to the standard curve.

Measurement of photothermal transduction efficiency (PTE) of core-shell
nanostructures. The photothermal transduction efficiency η of Cu 7 S 4 @ZIF-8 NPs was measured by illuminating the nanostructure dispersion to a steady-state temperature increase. 4,5 Figure 2b shows a typical thermal profile of the sample system. The energy balance of the entire system can be described as (1) where η is the photothermal transduction efficiency, A λ = 6.3 is the absorbance at the excitation wavelength of 1450 nm (measured by UV-vis-NIR spectrascopy), and I = 500 mW is the laser power. Q in,surr is the heat input due to the light absorption of the borosilicate glass cell (C glass = 0.840 J g -1 K -1 , m glass = 2.5 g) containing The heat lost term, Q out , is depicted as where T surr is the surrounding temperature, h is a heat-transfer coefficient, and A is the surface area of the glass container. At the maximum temperature (T max ), the system heat flux reaches a steady-state that the laser-induced energy input equals to the energy transfer out of the system: (4) in,np in,surr out Substituting eq 2 and 3 into eq 4, we get From the results deduced by Roper and co-workers, 4 the temperature increase as a function of time during the laser irradiation can be expressed as Fitting the data in Figure 2b, τ s was found to be 295.62 s from the heating curve. The heat transfer coefficients (hA) was calculated to be 10.79 mW/K according to eq 7 (C Cu7S4 = 76.838 J mol -1 K -1 , m Cu7S4 = 5 mg). 7        Laser power (mW)

Calculation of the Reaction Enthalpy and Apparent Activation Energy for [3+3] Cyclocondensation Reaction
We implemented our geometry optimizations with Gaussian 09 software. During calculations, the Cartesian coordinate is chosen in the input files with symmetry reserved for each molecule. The b3lyp/6-31g basis set is adopted because it is adequate to ensure an accurate results in our calculations. Each molecule is relaxed to its lowest energy state in vacuum. Afterwards, the reaction enthalpy ΔH is estimated by comparing the energy difference between products and reactants, where ΔH = ΔH p -ΔH r . Though no solvent effects are considered during our calculations, the geometry optimization results are still reasonable to mimic the real system for ΔH in fact is the total bond energy change, which is solvent irrelevant. The results were listed in the following table and the reaction enthalpy ΔH was calculated to be -370 kJ mol -1 .    The PL peak wavelength (580 nm) showed a red-shift against the onset absorption (2.35 eV). 6,7 The PL quantum yield of the hierarchical Cu 7 S 4 NPs can calculated according to a reference sample (2,4-dichloro-6-[p-(N,N-diethylamino) biphenylyl]-1,3,5-triazine, DBQ) with known quantum yield. 8 2 6 0.042 128134 17.2% 2.8 10 % 0.326 9.98 10 Such a low quantum yield indicates the poor luminescence of the Cu 7 S 4 NPs. From the perspective of the structure, the extensive grain boundaries between the nanocrystals facilitated the conversion of photo-generated electrons and holes into heat, indicating that the hierarchical Cu 7 S 4 NP was a promising photothermal semiconductor for the utilization of full solar spectrum.