Ternary Ti–Mo–Fe Nanotubes as Efficient Photoanodes for Solar-Assisted Water Splitting

Abstract

Designing efficient and stable water splitting photocatalysts is an intriguing challenge for energy conversion systems. We report on the optimal fabrication of perfectly aligned nanotubes on trimetallic Ti–Mo–Fe alloy with different compositions prepared via the combination of metallurgical control and facile electrochemical anodization in organic media. The X-ray diffraction (XRD) patterns revealed the presence of composite oxides of anatase TiO2 and magnetite Fe3O4 with better stability and crystallinity. With the optimal alloy composition Ti–(5.0 atom %) Mo–(5.0 atom %) Fe anodized for 16 h, enhanced conductivity, improved photocatalytic performance, and remarkable stability were achieved in comparison with Ti–(3.0 atom %) Mo–(1.0 atom %) Fe samples. Such optimized nanotube films attained an enhanced photocatalytic activity of ∼0.272 mA/cm2 at 0.9 VSCE, which is approximately 4 times compared to the bare TiO2 nanotubes fabricated under the same conditions (∼0.041 mA/cm2 at 0.9 VSCE). That was mainly correlated with the emergence of Mo and Fe impurities within the lattice, providing excess charge carriers. Meanwhile, the nanotubes showed outstanding stability with a longer electron lifetime. Moreover, carrier density variations, lower charge transfer resistance, and charge carriers dynamics features were demonstrated via the Mott–Schottky and electrochemical impedance analyses.

Stabilized Brookite Nanotube Array Films Grown on Transparent Substrates for Photoelectrochemical Water Splitting

“…These Sn signals can be ascribed to the triggered Sn 4+ ions to diffuse from the FTO substrate to the surface during a one-step annealing process (which is performed at one certain temperature for a definite time interval), not via a two-step annealing process (which is performed at two certain temperatures and each has its definite time interval), in agreement with previous studies. , A schematic representation of the suggested mechanism of Ti–Ni–O formed via one-step, and two-step thermal annealing is shown in Figure i. Further, for the high-resolution O 1s spectrum shown in Figure d, three prominent peaks can be distinguished at 530.1, 531.8, and 533.3 eV, distinctive of crystal lattice oxygen of metal oxides, carbonyl groups associated with ethylene glycol residues during anodization, and adsorbed H 2 O, respectively . The estimated atomic percentage of Ni and Sn atoms at 300 and 500 °C is listed in Table S2.…”

“…Alongside, R T denotes the net series resistances that contain: (i) R C , (ii) R G , (ii) R FTO , and (iii) R BE , i.e., R T = R C + R G + R FTO + R BE . Q is the constant phase element (CPE) and was used as detailed in our previous work, which could be calculated using eq Z CPE = 1 j ω C n where j is the imaginary unit, ω is the angular frequency, C is the ideal capacitance, and n is an empirical constant (0 < n <1) . On the one hand, R SCL and Q SCL could be considered as the resistance and capacitance of the space charge layer, respectively.…”

“…72 Notably, the spin−orbit splitting coupling of the Ni 2p 1/2 peak is broader and smaller in area than that of the Ni 2p 3/2 counterpart. 5,73 As a result, the degeneracy (g J ) of both Ni 2p 1/2 (the lower intensity) and Ni 2p 3/2 (the higher intensity) can be only two and four states, respectively, according to the empirical relationship given by eq 8 g J 1…”

“…All treated samples showed a positive slope (Figure 9a), revealing the typical n-type behavior, in agreement with (J−V) findings. 5,99,101 Among all treated samples, TN8-500 exhibited the most negative flat band potential (V FB ) of about −1.04 V SCE accompanied by the smallest slope, thus yielding excess charge carriers' concentration according to eq 12. Additionally, the TN8-500 plot was not completely linear, which might demonstrate the presence of defect states.…”

The synergetic effect of cobalt content on enhancing the photoelectrochemical hydrogen production performance of in-situ-doped TiO2 photocatalysts

“…% of Co), providing the change in the lattice parameters of TiO2 local structure and the successful doping of cobalt. As the Co atom has a different size than Ti, substituting Co at Ti sites either by low or high percent will create Ti interstitials or vacancies, oxygen vacancies, impurities, and defects at interfaces to balance the charge neutrality [38]. The local structure change in the Co-doped TiO2 NPs was elucidated via Raman spectroscopy, see Fig.…”