  Oxides containing metals or metalloids from the {\it p}-block of the periodic
table ({\it e.g.}, In, Sn, Sb, Pb, Bi) are of technological interest as
transparent conductors and light absorbers for solar energy conversion due to
the tunability of their electronic conductivity and optical absorption.
Comparatively, these oxides have found limited applications in hydrogen
photoelectrolysis primarily due to their high electronegativity, which impedes
electron transfer for reducing protons into hydrogen. We have shown recently
that inserting {\it s}-block cations into {\it p}-block metal oxides is
effective at lowering electronegativities while affording further control of
band gaps. Here, we explain the origins of this dual tunability by
demonstrating the mediator role of {\it s}-block cations in modulating orbital
hybridization while not contributing to frontier electronic states. From this
result, we carry out a comprehensive computational study of 109 ternary oxides
of {\it s}- and {\it p}-block metal elements as candidate photocatalysts for
solar hydrogen generation. We downselect the most desirable materials using
band gaps and band edges obtained from Hubbard-corrected density-functional
theory with Hubbard parameters computed entirely from first principles,
evaluate the stability of these oxides in aqueous conditions, and characterize
experimentally four of the remaining materials, synthesized with high phase
uniformity, to validate and further develop the computational models. We thus
propose nine oxide semiconductors, including CsIn$_3$O$_5$, Sr$_2$In$_2$O$_5$,
and KSbO$_2$ which, to the extent of our literature review, have not been
previously considered as water-splitting photocatalysts.
