Graphitic　carbon　nitride　(g-C3N4)　has　attracted　considerable　attention　with　regard　to　its　use　in　photocatalytic　solar　hydrogen　production　by　the　splitting　of　water.　High　charge　carrier　recombination　critically　limits　the　photocatalytic　activity　of　g-C3N4.　Plasmonic　metal　nanoparticles　that　can　generate　localized　surface　plasmon　resonance　(LSPR)　have　been　suggested　to　enhance　the　harvesting　of　visible　light　and　to　improve　water　splitting　efficiency.　However,　direct　contact　between　metal　nanoparticles　and　g-C3N4　reduces　the　hydrogen　generation　efficiency　owing　to　energy　loss　by　Förster　resonance　energy　transfer　(FRET),　which　competes　with　plasmon　resonance　energy　transfer　(PRET).　Decorating　g-C3N4　with　Ag@SiO2　core-shell　plasmonic　nanoparticles　increases　its　photocatalytic　ability.　Tuning　the　size　of　the　SiO2　nanogap　can　optimize　the　photocatalytic　performance　of　g-C3N4/Ag@SiO2,　which　involves　a　trade-off　between　PRET　and　FRET.　X-ray　absorption　spectroscopy　(XAS)　is　utilized　to　investigate　the　electronic　structure　of　g-C3N4　and　its　modulation　with　Ag@SiO2.　In　situ　XAS　reveals　the　dynamics　of　the　charge　carriers　under　solar　illumination.　Analytic　results　suggest　charge　redistribution,　shifting　of　the　conduction　band,　modification　of　the　unoccupied　states,　and　consequent　improvement　in　photocatalytic　activity　by　solar　illumination.　This　work　sheds　light　on　the　effect　of　LSPR　on　this　photocatalyst　with　reference　to　its　electronic　structure.　©　2020　Elsevier　B.V.