Noncovalent nanohybrids between meso-(phydroxyphenyl)porphyrin (TPPH) and graphene oxide (GO) sheets were studied as a function of pH. The overall charge of the TPPH molecule changes between negative (−4), neutral, and positive (+2) depending on the pH of the solution. Results of Fourier transform infrared spectroscopy, thermogravimetric analysis, and elemental analysis confirm successful noncovalent functionalization of GO sheets with TPPH. We applied a number of methods to probe the ground-state as well as the excited-state interaction between the components of the new material. The experimental results were additionally supported by theoretical calculations that included optimizations of the ground-state structures of TPPH and TPPH 2+ and their complexes with a molecular model of GO. It was demonstrated that both TPPH and TPPH 2+ molecules can be assembled onto the surface of GO, but it was clearly shown that the stronger interaction with GO occurs for TPPH 2+ . The stronger interaction in the acidic environment can be rationalized by the electrostatic attraction between positively charged TPPH 2+ and negatively charged GO, whereas the interaction between TPPH 4− and GO at basic pH was largely suppressed. Our comprehensive analysis of the emission quenching led to the conclusion that it was solely attributed to static quenching of the porphyrin by GO. Surprisingly, fluorescence was not detected for the nanohybrid, which indicates that a very fast deactivation process must take place. Ultrafast time-resolved transient absorption spectroscopy demonstrated that although the singlet excited-state lifetime of TPPH 2+ adsorbed on the GO sheets was decreased in the presence of GO from 1.4 ns to 12 ps, no electron-transfer products were detected. It is highly plausible that electron transfer takes place and is followed by fast back electron transfer.

We have demonstrated that the photocatalytic system, containing Eosin Y (EY) as a sensitizer, triethanolamine (TEOA) as a sacrificial electron donor, CoSO 4 as a catalyst, and graphene oxide (GO), exhibited a 9-fold increase in hydrogen production rate compared to the analogous system in the absence of graphene oxide. Interaction of Eosin Y (EY) with graphene oxide (GO) in the ground state and excited state was probed by steady-state and time-resolved absorption and emission measurements. Analysis of the emission quenching of EY by GO revealed that the measured decrease of the fluorescence in the presence of GO was solely attributed to inner filter effects I and II and an absorbance change of EY itself at the excitation wavelength in the presence of GO. Femtosecond and nanosecond transient absorption spectroscopy experiments pointed to a lack of electron transfer from the excited states of EY, neither singlet nor triplet excited states, to GO sheets. It was demonstrated that the electron donor triethanolamine (TEOA) participates in the primary photochemical reaction, and the electron transfer to GO occurs from the EY radical anion and not directly from the excited state of EY. GO sheets were photochemically reduced using EY and TEOA under ambient conditions. After illumination of GO in the presence of TEOA and EY, the formation of reduced graphene oxide (rGO) was confirmed through optical absorption, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and X-ray photoelectron spectroscopies (XPS). This indicates that the electron transfer to GO is not followed by back electron transfer and thus can be further transferred to hydrogen evolution catalysts. The existence of a stable charge-separation state explains the role of graphene in the improvement of photocatalytic efficiency in the Eosin Y-based systems.

Fast electron transfer from the photoexcited ZnTPPH singlet state to GO sheets was detected by ultrafast time-resolved transient absorption spectroscopy.

Graphene-based nanohybrids are good candidates for various applications. However, graphene exhibits some unwanted features such as low solubility in an aqueous solution or tendency to aggregate, limiting its potential applications. On the contrary, its derivatives, such as graphene oxide (GO) and reduced graphene oxide (RGO), have excellent properties and can be easily produced in large quantities. GO/RGO nanohybrids with porphyrins were shown to possess great potential in the field of photocatalytic hydrogen production, pollutant photodegradation, optical sensing, or drug delivery. Despite the rapid progress in experimental research on the porphyrin-graphene hybrids some fundamental questions about the structures and the interaction between components in these systems still remain open. In this work, we combine detailed experimental and theoretical studies to investigate the nature of the interaction between the GO/RGO and two metal-free porphyrins 5,10,15,20-tetrakis(4-aminophenyl) porphyrin (TAPP) and 5,10,15,20-tetrakis(4-hydroxyphenyl) porphyrin (TPPH)]. The two porphyrins form stable nanohybrids with GO/RGO support, although both porphyrins exhibited a slightly higher affinity to RGO. We validated finite, Lerf–Klinowski-type (Lerf et al. in J Phys Chem B 102:4477, 1998) structural models of GO ($$\hbox {C}_{59}\hbox {O}_{26}\hbox {H}_{26}$$ C 59 O 26 H 26 ) and RGO ($$\hbox {C}_{59}\hbox {O}_{17}\hbox {H}_{26}$$ C 59 O 17 H 26 ) and successfully used them in ab initio absorption spectra simulations to track back the origin of experimentally observed spectral features. We also investigated the nature of low-lying excited states with high-level wavefunction-based methods and shown that states’ density becomes denser upon nanohybrid formation. The studied nanohybrids are non-emissive, and our study suggests that this is due to excited states that gain significant charge-transfer character. The presented efficient simulation protocol may ease the properties screening of new GO/RGO-nanohybrids.

The present study explored the correlation between the photocatalytic activity toward hydrogen production of the graphene-based materials and graphene oxide (GO) morphology. In this work we applied the technique based on the combination of time-dependent sonication and iterative centrifugation cascades, which were designed to achieve nanosheets size and the number of layers selection. First such obtained GO dispersions were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical spectroscopy. Those combined measurements showed that the intensity of the π-π peak at 230 nm seems to be very sensitive to the number of layers of nanosheets. Next, GO dispersions were used to establish influence of the size and the number of layers of GO flakes on the photocatalytic hydrogen production in the photocatalytic system, containing eosin Y as a sensitizer, triethanolamine as a sacrificial electron donor, and CoSO4 as precatalyst. The H2 production efficiency varied by a factor of 3.7 for GO dispersions sonicated for various amount of time. Interestingly it was found that too long ultrasound treatment had negative impact on the GO enhancement of hydrogen production which was related to the fragmentation of GO flakes. The photocatalytic system produced the highest amount of H2 when graphene oxide occurs as monolayers and efficiency becomes lower with the decrease of GO sheets size. Our results demonstrate the importance of optimizing the size and the number of layers of the GO flakes prior to preparation of GO-based materials.