Defect engineering
Metal oxide cavities and tubular nanostructures of functional
compounds including Ti, Ta, Fe, Mo, W or Nb oxides were
engineered in terms of morphology (namely anodic self-organized
nanotubes with controlled diameter, length, wall thickness),
crystallographic features, and electronic and optical
properties.
Particularly impressive were the results in engineering
semiconductor electronic properties. We identified defined
lattice defects, such as Ti3+ states and specific oxygen
vacancies to provide or mediate strongly the efficiency of
photocatalysis. Remarkable is particularly the work on noble
metal free photocatalysis.[1-10] This defect engineering was
achieved by strategies such as thermal hydrogenation,[1,6,10-12]
ion implantation[2,4,7] or suboxide or hydride oxidation.[3,5]
We demonstrated the manipulation of such defects (nature,
density and energetics) to be as well an effective path for
tuning the oxide electronic transport and co-catalytic
properties. Opposite defect engineering (eliminating native
defects) led to anatase TiO2 with nanotwinned grain structures
in the nanotube walls - these tubes show a high conductivity
(charge carrier mobility), highly useful for fabricating
photo-anodes for dye sensitized solar cells. Most importantly,
we demonstrated that an adequate reduction of TiO2 (to a grey
form) can provide co-catalyst free photocatalytic H2 evolution.
We established that such activity originates from specific
lattice defect (suitable oxygen vacancies and Ti3+ states) that
are intrinsic of mildly reduced, grey TiO2, while
black titania requires co-catalyst activation. We explored
the formation of grey forms for various TiO2 polymorphs,
e.g. anatase, rutile and brookite, and for different
morphologies, including nanotubes, nanopowders and single
crystals.
Some literature citations:
[1] N. Liu, C. Schneider, D. Freitag, U. Venkatesan, V. R. R.
Marthala, M. Hartmann, B. Winter, E. Spiecker, A. Osvet, E. M.
Zolnhofer, K. Meyer, T. Nakajima, X. Zhou, P. Schmuki, Angew.
Chemie Int. Ed. 2014, 53, 14201.
[2] N. Liu, V. Haeublein, X. Zhou, U. Venkatesan, M. Hartmann,
M. MačKović, T. Nakajima, E. Spiecker, A. Osvet, L. Frey, P.
Schmuki, Nano Lett. 2015, 15, 6815.
[3] X. Zhou, E. M. Zolnhofer, N. T. Nguyen, N. Liu, K. Meyer, P.
Schmuki, Angew. Chemie - Int. Ed. 2015, 54, 13385.
[4] X. Zhou, V. Haeublein, N. Liu, N. T. Nguyen, E. M. Zolnhofer,
H. Tsuchiya, M. S. Killian, K. Meyer, L. Frey, P. Schmuki, Angew.
Chemie Int. Ed. 2016, 55, 3763.
[5] X. Zhou, N. Liu, J. Schmidt, A. Kahnt, A. Osvet, S. Romeis,
E. M. Zolnhofer, V. R. R. Marthala, D. M. Guldi, W. Peukert, M.
Hartmann, K. Meyer, P. Schmuki, Adv. Mater. 2017, 29.
[6] N. Liu, X. Zhou, N. T. Nguyen, K. Peters, F. Zoller, I.
Hwang, C. Schneider, M. E. Miehlich, D. Freitag, K. Meyer, D.
Fattakhova-Rohlfing, P. Schmuki, ChemSusChem 2017, 10, 62.
[7] N. Liu, H. G. Steinruck, A. Osvet, Y. Yang, P. Schmuki,
Appl. Phys. Lett. 2017, 110.
[8] X. Zhou, N. Liu, P. Schmuki, ACS Catal. 2017, 7, 3210.
[9] A. Naldoni, M. Altomare, G. Zoppellaro, N. Liu, S. Kment, R.
Zbořil, P. Schmuki, ACS Catal. 2019, 9, 345.
[10] X. Zhou, E. Wierzbicka, N. Liu, P. Schmuki, Chem. Commun.
2019, 55, 533.
[11] E. Wierzbicka, X. Zhou, N. Denisov, J. E. Yoo, D. Fehn, N.
Liu, K. Meyer, P. Schmuki, ChemSusChem 2019, 12, 1900.
[12] S. Mohajernia, P. Andryskova, G. Zoppellaro, S. Hejazi, S.
Kment, R. Zboril, J. Schmidt, P. Schmuki, J. Mater. Chem. A
2020, 8, 1432.