Accelerated pyro-catalytic hydrogen production enabled by plasmonic local heating of Au on pyroelectric BaTiO3 nanoparticles

The greatest challenge that limits the application of pyro-catalytic materials is the lack of highly frequent thermal cycling due to the enormous heat capacity of ambient environment, resulting in low pyro-catalytic efficiency. Here, we introduce localized plasmonic heat sources to rapidly yet efficiently heat up pyro-catalytic material itself without wasting energy to raise the surrounding temperature, triggering a significantly expedited pyro-catalytic reaction and enabling multiple pyro-catalytic cycling per unit time. In our work, plasmonic metal/pyro-catalyst composite is fabricated by in situ grown gold nanoparticles on three-dimensional structured coral-like BaTiO3 nanoparticles, which achieves a high hydrogen production rate of 133.1 ± 4.4 μmol·g−1·h−1 under pulsed laser irradiation. We also use theoretical analysis to study the effect of plasmonic local heating on pyro-catalysis. The synergy between plasmonic local heating and pyro-catalysis will bring new opportunities in pyro-catalysis for pollutant treatment, clean energy production, and biological applications.


Results and Discussion
The d-spacing (d(hkl)) of material can be estimated by Bragg's Laws, 2d(hkl)sinθ(hkl)=nλ (1) where n, (hkl), θ and λ are diffraction order (n=1 is adopted), crystallographic orientation (miller indices), Bragg angle (in radius) and wavelength of X-ray, respectively. For tetragonal unit cell, the relation between d-spacing and lattice parameters is 1 : By using XRD data and Equations (1) and (2), the calculated lattice parameters are shown in Table S1. Thus, the tetragonality (defined as the ratio of lattice parameters, c/a) of the material can be obtained by c/a=1.003 where F(R) is the so-called remission or Kubelka-Munk function, R is reflectance, h is the Planck's constant, ν is the photon frequency, A is a constant, Eg is the energy bandgap, and n is selected as 2 since BaTiO3 is an indirect bandgap material 4,5,6 .   As shown in Fig. S4, the average particle size of the Au nanospheres is found to be abound 18 nm.

Energy conversion efficiency calculation:
The light-to-chemical energy conversion efficiency is calculated as follows.
The value of the output chemical energy due to pyro-catalysis can be simply calculated as,
According to the above equation, the chemical energy output due to pyro-catalysis per second in the present case is: The light-to-chemical energy conversion efficiency can be calculated as, ɳ=Echem/Einput=0.023 mW/500 mW=0.0046% (6) This value is quite low since the catalyst used is dispersed in the liquid and the majority of illuminated part is actually water, not the catalyst.    As shown in the Fig.S7 and S8, after pyro-catalysis, there is nearly no damage on Au/BaTiO3 NPs.   As shown in Fig. S11, temperature change of water during plasmon induced pyrocatalysis is negligible. The overall temperature of water was monitored by a thermometer. Fig.S12 schematically illustrates the pyro-catalysis mechanism. Initially, both free electrons and screen charges will accumulate at the surface of nanoparticles to compensate the polarization-induced surface bound changes (Fig.S12a). When the temperature is raised from T1 to T2, the spontaneous polarization is reduced. Hence the positively polarized surface will release electrons to have the hydrogen evolution reaction (HER) and the negatively polarized surface will accept electrons from water molecules to have the oxygen evolution reaction (OER) (Fig.S12b). Fig.S12c depicts the equilibrium state at the higher temperature of T2. When the temperature is decreased from T2 to T1, the spontaneous polarization is increased. Hence the whole process is reversed, that is, the positively polarized surface will accept electrons to have OER and the negatively polarized surface will release electrons to have HER (Fig.S12d). with two different cut planes, i.e., P t and P b , used to calculate the temperature difference in z-direction. c, Surface-averaged temperature of P c (blue curve, marked as "center") and P e (red curve, marked as "extremity"). d, Surface-averaged temperature of P t (blue curve, marked as "top") and P b (red curve, marked as "bottom"). Source data are provided as a Source Data file.
To calculate the thermoelectrically induced charges, we set two circular cut planes, marked as P c and P e in Fig. S13a. The temperature difference between the two planes (P c and P e ) will induce current flows due to thermoelectric effect in the axial direction of the cylindrical particle, as marked by red arrows. The length of red arrow schematically shows the magnitude of current density (not drawn to scale, Fig. S13a left panel). Similarly, we also consider the current flows in a direction perpendicular to the axial direction, as marked by blue arrows in Fig. S13b left panel. Two cut planes parallel to the xy-plane are created within the cylindrical BaTiO3 particle. By drawing tangent lines from the center of the Au nanosphere to the BaTiO3 surface (Fig. S13b, right panel), the plane P t is formed by all the points of tangency. Simple calculation shows that P t is 7.98 nm from the top of the cylindrical BaTiO3 particle. For symmetry consideration, the other cut plane P b is parallel to P t but at an equal distance of 7.98 nm above the bottom of BaTiO3 particle, serving as a reference plane for calculating the temperature difference. These two planes are reasonably chosen to evaluate the thermoelectric effect in z-direction since the BaTiO3 surface above P t (highlighted by red colored zone in Fig. S13b) is regarded as being directly heated by the Au nanosphere and thus be treated as the heat source in the thermal process. The surface-averaged temperature in the P c and P e cut planes are given in Fig. S13c. And the surface-averaged temperatures in the P t and P b cut planes are given in Fig. S13d. It can be seen that, the temperature difference in axial direction is much higher as P c and P e cut planes are far apart. On the contrary, the surface-averaged temperature difference in P t and P b cut planes is much smaller due to their smaller separation distance (84.04 nm).
The quantity of electrons induced by thermoelectric effect can be derived as follows 17 : Where I, σ, E, V, l, A, t, S, and ΔT are the current, the electrical conductivity 18 , electric field, thermoelectric voltage, the distance between two cut planes, surface area, time, Seebeck coefficient and the temperature difference between two end planes, respectively. It should be noted that although the surface-averaged temperature difference in P t -P b cut planes is much smaller than that in P c -P e cut planes ( Fig.S13c and Fig.S13d), the larger surface area of P t -P b (542000 nm 2 vs. 7584 nm 2 ) and their smaller separation distance (84.04 nm vs. 500 nm) finally result in a larger value in electrons induced by thermoelectricity (Fig.S14). The total pyroelectric charges over the surface of BaTiO3 NPs within the heating process during one pulse irradiation on a 9-nm Au NP can be calculated by: where p is the pyroelectric coefficient with a value between 20 and 30 nC·cm -2 ·K -1 , T is the temperature rise in heating (or drop during cooling), ̂ is the unit vector along the surface normal direction, and ̂ is the unit vector along spontaneous polarization direction.
Without losing generality, we compare the cases for polarizations along the axial direction and z-direction. The integral of ∆ (̂•̂) over the upper surface (corresponding polarization along z-axis) of the cylindrical particle can be up to 1.7110 13 m 2 K (Fig. S15), which is much larger than the integral over the P c cut plane (corresponding polarization along axial direction) (location of P c can be found from Fig.   S13a). Moreover, the pyroelectric charges generated on the P c cut plane are difficult to diffuse to the surface for catalytic reaction due to the low conductivity of BaTiO3.
Therefore, we consider only the polarization along z-axis for the estimation of the upper limit of H2 production.
Taking the pyroelectric coefficient (p) of 30 nC·cm -2 ·K -1 as an example, we can easily calculate that the pyroelectric charges are 5.1210 -17 C, which are much larger than those due to thermoelectric effect (Fig. S14). So the thermoelectric effect induced catalysis can be neglected in the present case.
The above results show that the maximum amount of pyroelectric charges produced over the surface of one BaTiO3 NP by one single Au NP during the heating process by one-pulse illumination is about 5.12×10 -17 C, which is equivalent to 320 electrons. The pyroelectric charges produced over the lower half surface of the cylinder during the cooling process may also contribute to the catalytic H2 production. They are estimated to be around 60 % of that produced from the upper half surface during the heating process since the average temperature change of the lower half surface is around 60% of that of the upper half surface (Fig. S13d). So the total available pyroelectric charges for pyro-catalysis during one complete heating/cooling cycle is 512 electrons.
In the present case, the repetition rate of the laser is 10 Hz. Hence, the maximum number of electrons produced over the surface of one BaTiO3 NP after one-hour nanosecond laser irradiation on one 9-nm Au NP for pyro-catalysis (None particle) can be calculated as, The total number of BaTiO3 NPs in 1 gram is, where ρ is density of BaTiO3 (= 6060 kg·m -3 ).
Thus, after one-hour nanosecond laser irradiation, for 1 g of BaTiO3 NPs, the total number available electrons for pyro-catalysis is n=None particle × N=3.85×10 20 .
Terephthalic acid can react with •OH to produce a highly fluorescent product, 2hydroxyterephthalic acid, which emits a unique fluorescence signal with its peak wavelength at 425 nm 19 . The PL intensity of 2-hydroxyterephtalic acid relies on the amount of •OH generated in water 19 . This sensitive and specific method has been widely used for the •OH detection 20,21,22 . In the current work, around 0.5 mg Au/BaTiO3 NPs were dispersed in 4 mL terephthalic acid aqueous solution (0.5 mM) with a concentration of 2 mM NaOH in a quartz reactor. After experiencing various irradiation time by a 532 nm nanosecond laser, the PL spectra of the generated 2hydroxyterephthalic acid was measured via a spectrofluorometer under an excitation wavelength of 321 nm. In Fig. 16, with increasing reaction time, the 425 nm fluorescence intensity gradually increases, which indicates the increasing amount of •OH radicals synthesized under pyro-catalysis.