Figures 3(c) and 3(d) depict the typical time response spectra of CdS:Ag NRs and undoped CdS NRs, respectively. We note that both the doped and undoped CdS NRs show good photo-response characteristics with excellent stability and reproducibility. The response time of the CdS:Ag NRs is less than 0.5 s, which is comparable to that of the undoped CdS NRs, implying that the CdS:Ag NRs have high crystalline quality with little carrier trapping centers. It is known that the trapping centers induced by the defects play an important role in determining the response speed of the photodetectors [
21]. On the other hand, we note that the photocurrent of the CdS:Ag NRs has significantly increased compared to the undoped CdS NRs under the same light intensity, which is 3 μA for CA but only 0.6 nA for C0. Accordingly, the responsivity (
R), which represents the current output of a photodetector under light illumination and can be defined as
R (A·W
-1) =
Ip/
Popt, where
Ip is the photocurrent,
Popt the incident light power, has increased to 2.7×10
4 A·W
-1 for CA, in contrast to the small value of 5.4 A·W
-1 for C0. The enhanced photoconductivity of the Ag doped CdS NRs can be attributed to the increase of the photo-carrier lifetime in the Cd:Ag NRs [
22]. In intrinsic CdS NRs, recombination will happen soon after the generation of electron-hole pairs, thus leading to the low photocurrent. In contrast, the holes tend to be trapped in the n-type CdS:Ag NRs, prolonging the lifetime of electrons by preventing the electron-hole recombination. Consequently, the photocurrent is increased since more carriers can pass through the electrodes in a certain time.