Adsorption and Photoactivity of Tetra(4-carboxyphenyl)porphyrin (TCPP) on Nanoparticulate TiO₂

Abstract

Tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbs strongly onto nanoparticulate TiO2 and serves as an efficient photosensitizer for solar-energy conversion by TCPP-sensitized TiO2 electrodes. Nanoparticulate TiO2 electrodes were prepared from Degussa P25 TiO2 powder in the standard manner for a Grätzel cell. Adsorption studies of TCPP onto these sintered TiO2 electrodes gave a saturation surface coverage of 47 μmol/g. Adsorption studies of TCPP onto colloidal dispersions of Degussa P25 in ethanol gave a saturation surface coverage of 77 μmol/g. The difference between the saturation coverages is attributed to the reduction of the available surface area in the TiO2 films after sintering, from 55 m2/g as a free colloid to about 34 m2/g as a sintered electrode. The nature of the binding of TCPP onto the TiO2 electrodes was investigated using X-ray photoelectron spectroscopy (XPS) and Resonance Raman Spectroscopy (RRS). In the XPS spectra of TiO2 with adsorbed TCPP, the O (1s) and Ti (2p3/2) peaks of TiO2 were shifted to a higher binding energy value, by about 0.3 eV, and the O (1s) and N (1s) peaks of TCPP were shifted to a higher binding energy, by about 0.7 eV. Upon adsorption of TCPP, one of the Ti (2p3/2) peaks of TiO2 disappeared, suggesting complexation and removal of surface states. The RRS results indicated that for cases in which TCPP was adsorbed onto TiO2 films from ethanolic solutions of about 1 μM concentration, the porphyrin spectrum showed distinctive interactions with the surface, but for cases in which it was adsorbed from higher concentrations, the RRS spectra were similar to spectra of TCPP powder, indicating the dominance of porphyrin−porphyrin interactions. We conclude that lateral interactions between adsorbed TCPP are significant upon adsorption from all but the lowest (micromolar) initial concentrations. Photovoltaic cells with TCPP-sensitized TiO2 electrodes gave good solar-energy conversion efficiencies. At light simulating one sun (AM 1.5), a cell sensitized by TCPP gives a short-circuit photocurrent of about 6 mA/cm2 and an open-circuit photopotential of 485 mV. The incident photon-to-current conversion efficiency was 55% at the Soret peak and 25−45% at the Q-band peaks; the cells have a fill factor of 60−70% and an overall energy conversion efficiency of about 3%.

Durable Unsymmetrical Zinc Phthalocyanine for Near IR Sensitization of Nanocrystalline TiO₂ Films with Non-Volatile Redox Electrolytes

“…The energy of the π* level of the phthalocyanine is decreased by this delocalization, which explains the observed shift of the peaks. Similar types of interactions were observed in case of porphyrins adsorbed on TiO 2 surface [7]. …”

Photosensitized Degradation of Dyes in Polyoxometalate Solutions Versus TiO2 Dispersions under Visible‐Light Irradiation: Mechanistic Implications

“…At given time intervals, aliquots (3 mL) were sampled, buffered to pH 9 by the addition of a Na 2 CO 3 / NaHCO 3 solution (1 mL), and subsequently analyzed by HPLC or UVvisible spectroscopy. The concentration of H 2 O 2 formed during the irradiation was determined by the spectrophotometric DPD method [37] immediately after irradiation. The consecutive reaction mode [Eq.…”

Improvement in the methylene blue adsorption capacity and photocatalytic activity of H2-reduced rutile-TiO2 caused by Ni(II)porphyrin preadsorption

“…Thus, the MB-photodegradation capacity observed for rutileTiO 2 xerogel is possibly due to a combined effect of: (i) the sensitivity of rutile to visible light irradiation due to its low band gap and (ii) electron transfer from photoexcited MB adsorbed onto the TiO 2 xerogel surface, similar to the effect observed on the TiO 2 -P25 surface sensitized with phthalocyanines [15][16][17] or Ru(II) complexes [18][19][20]. Thermal treatment is also used to activate the TiO 2 surface for the chemical adsorption of polypyridil or porphyrin complexes [21][22][23][24][25]. Specifically, metallic- for the adsorption process [26][27][28].…”