The photosensing properties of vertically aligned ZnIn2S4 nanopetal films grown hydrothermally on FTO coated glass are examined without and with surface dispensed agarose gel. For the ZnIn2S4 nanopetals photodetector (without gel) there is no photoresponse for zero bias. Most interestingly, with surface dispensation of agarose gel, the hybrid electronic-iontronic interface system shows a strong photoresponse even under zero bias, a highly efficient self-powered UV-visible light photodetector. Indeed, the zero bias ZnIn2S4/gel hybrid photoresponse is a factor of 100 stronger as compared to the response of the only ZnIn2S4 device (at -1.5 V bias) and that too without any significant degradation in response time. The possible operating mechanisms are proposed.