BandGapof3dTMOxides

Band gap of 3d transition metal oxides

	Oxides	Xystal color	band gap (eV)	Oxides	Xystal color	band gap (eV)	Oxides	Xystal color	band gap (eV)
		2+			3+			4+
3d0				Sc2O3	white	6	TiO2	white	3.2
3d1				Ti2O3	violet	4.08	VO2	blue-black	0.7
3d2	TiO	dark	0	V2O3	black	0.2	CrO2	black	0
3d3	VO	black, grey, or green	0	Cr2O3	green	3	MnO2	gray to black	3.4
3d4
3d5	MnO	green	3.6-4.3	Fe2O3	red-brown	Xystal 2.0/film 2.7
3d6	FeO	black
3d7	CoO	grey or olive green	2.4
3d8	NiO	green	3.0-4.3

 

As we can see that the color does not correlate with band gap, except for the case that the material is always black when the gap is lower than 1.8 eV. This is because the optical gap we normally refer is actually charge transfer gap which bare huge absorption strength which color properties can be determined by the excitation that has much less absorption strength, e.g. on-site forbidden excitation. See this post.