Novel in-situ composites of chlorine-doped CQDs and g-C3N4 with improved interfacial connectivity and accelerated charge transfer to enhance photocatalytic degradation of tetracycline and hydrogen evolution

•A novel high efficiency photocatalyst (Cl-CCN) was prepared.•Cl-CQDs form a tight interfacial connection with g-C3N4 by C-Cl covalent bonding.•C-Cl bond improved electron transfer efficiency by constituting electron channels.•The TC degradation and H2 evolution mechanism by Cl-CCN was proposed.•Cl-CCN exhibited excellent universality, stability, and recyclability.