Producing a metal photocathode with a low work function (WF), low emissivity, and high quantum efficiency is a matter of importance in the design of the next generation of free-electron laser facilities. General rules for the design of appropriate materials are currently unclear and difficult to elucidate from observations of structure-composition relationships of known photocathodes. In this work, high-quality density-functional-theory electronic structure calculations and a simple physical model are employed to develop design rules for photocathodes based on metallic alloys. A theoretical study of metal alloys for photocathode applications is presented, in which high WF, stable copper is paired with low WF, unstable barium in two alloys,
Cu
13
Ba
and
Cu
Ba
. Surfaces terminating in a plane of
Ba
atoms have a lower computed surface energy than those terminating in
Cu
atoms due to surface segregation of the larger
Ba
atoms. This results in a significant surface dipole due to the interatomic charge transfer from the differences in electronegativity of the species. The details of the surface structure determine the direction of the dipole and thus have a strong influence on the computed WF. The computed WF of the
Cu
13
Ba

Ba
-terminated (100) surface is even lower than that of pure
Ba
, at 1.95 eV. The computed quantum efficiency (QE) of the best-performing pure
Cu
surface is
5.86
×
10
−
6
, whereas the best-performing
Cu
13
Ba
surface terminates in a plane of
Ba
atoms and has a significantly increased QE of
5.09
×
10
−
3
. A surface terminating in two planes of
Ba
atoms, the (001) surface of
Cu
Ba
, has an even higher computed QE of
1.38
×
10
−
2
.