Visible-Light-Driven Photocatalytic H2 Production Using Composites of Co–Al Layered Double Hydroxides and Graphene Derivatives

The direct conversion of solar energy into chemical energy represents an enormous challenge for current science. One of the commonly proposed photocatalytic systems is composed of a photosensitizer (PS) and a catalyst, together with a sacrificial electron donor (ED) when only the reduction of protons to H2 is addressed. Layered double hydroxides (LDH) have emerged as effective catalysts. Herein, two Co–Al LDH and their composites with graphene oxide (GO) or graphene quantum dots (GQD) have been prepared by coprecipitation and urea hydrolysis, which determined their structure and so their catalytic performance, giving H2 productions between 1409 and 8643 μmol g–1 using a ruthenium complex as PS and triethanolamine as ED at 450 nm. The influence of different factors, including the integration of both components, on their catalytic behavior, has been studied. The proper arrangement between the particles of both components seems to be the determining factor for achieving a synergistic interaction between LDH and GO or GQD. The novel Co–Al LDH composite with intercalated GQD achieved an outstanding catalytic efficiency (8643 μmol H2 g–1) and exhibited excellent reusability after 3 reaction cycles, thus representing an optimal integration between graphene materials and Co–Al LDH for visible light driven H2 photocatalytic production.


FTIR-ATR assignments:
FTIR-ATR spectra corroborated the formation of composites, according to the bands observed at ca. 3500 cm −1 , attributed to OH stretching, at 2187 cm −1 , which corresponded to cyanate groups (CNO − ), present in intermediate decomposition products from the hydrolysis of urea 3 , at approximately 1620 cm −1 , assigned to adsorbed water, and at 1550-1585 cm −1 , ascribed to C=C stretching vibrations.Those bands at 1352-1377 cm −1 were associated to the stretching mode of both NO3 − and CO3 2− present in LDH layers 4 .Bands below 800 cm −1 were assigned to metaloxygen (M-O) stretching and bending vibrations in the brucite-like lattice 5 .GO presented an absorption band at 236 nm attributed to the π → π* transition of aromatic rings and a shoulder at 310 nm corresponding to the n → π* transition of C=O bonds 6 .Bands at 207 nm and 329 nm attributed to π → π* and n → π* transitions, respectively, were present in GQD 7 .The band gap for GO and GQD was 3.3 eV and 3.5 eV, respectively.) C e (g L -1 ) q m = 62 mg g -1 = 109 mmol g -1 R 2 = 0.98 a

Figure S7 .
Figure S7.N2 adsorption-desorption isotherm of GO.GO presented a type IV isotherm with hysteresis loop at a relative pressure between 0.45 and 0.96, specific surface area of 59 m² g −1 and pore volume of 0.066 cm 3 g −1 .

Figure
Figure S12.a) Graphene oxide with two insets at higher magnification.Fast Fourier Transform (FFT) analysis provides typical interplanar distances for GO.b) Graphene Quantum Dots (GQD).One of them is magnified at the upper left inset whose corresponding FFT is displaying (100) planes with a spacing of 0.21 nm.

Figure
Figure S14.a) TEM micrograph of pristine LDH synthetized by urea homogeneous precipitation (LDHu).b) HAADF and Al and Co elemental maps are shown.Again, a 3:1 (Co:Al) ratio was obtained.

Figure S15 .
Figure S15.Particle size distribution of GQD by TEM.

Table S1 .
Composition of LDH and composites.
a Based on XRF measurements; b Determined by elemental analysis; c Determined by ATG analyses.