Fig. authors.40 In the dark (before UV-vis illumination) oxygen

Fig.

11 Shows the time resolved rise and decay curve of photocurrent for the samples of ZnO, ZnO/P, ZnO/S, and ZnO/S/P with fixed bias voltage 20 V in the air as well as in vacuum under UV-Visible illumination with fixed photo-flux (7300 lux).               3.6.1 Rise and Decay in Air:                              Fig.

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11. (a) and Fig. 11. (d) Shows that after dark current stabilization When the light was switched on, the photocurrent in ZnO and ZnO/S/P nanostructures were initially found to increase rapidly followed by the slow rate of increase with time. The initial fast rise in photocurrent may be due to fast process of photo generation of electron hole pairs. The slow rise of photocurrent may be due to the slow process of desorption of oxygen molecules on the surface of ZnO and ZnO/S/P nanostructures.   Fig. 11.

(b) and Fig. 11. (c) Shows that after dark current stabilization when the light was switched on, the photocurrent in ZnO/P and ZnO/S nanostructures were initially found to increase rapidly followed by a fast rate of decrease with time which may be due to traps/defects.

Therefore, it was indicated that the seed and polymer (peg) created the defects/impurities. Then again, it was slowly increasing the photocurrent may be due to the slow process of desorption of oxygen molecules on the surface of ZnO/P and ZnO/S nanostructures. When the UV-Vis light was switched off, initially the current decreased very fast which could be attributed to recombination of free electrons and holes. Later the current decreased slowly, which could be attributed to the slow process of surface re-adsorption of oxygen molecules due to the presence of free electrons at the surface of ZnO/P. The decay curve was going below the dark current, so it had negative photoconductivity due to the presence of traps/defects.

39 Similar anomalous photoconducting behaviour had been reported in ZnO, ZnS, CdS and other materials by several authors.40In the dark (before UV-vis illumination) oxygen molecules tend to be adsorbed on the ZnO surface by capturing free electrons and producing adsorbed oxygen ions as follows:  (adsorbed species) Where O2 is the oxygen molecule, e- is the free electron and is the adsorbed oxygen ion on the surface of ZnO nanoparticles. The adsorbed oxygen ions create a barrier near the surface, which generates a low current. The UV-Vis photoresponse in ZnO was generally governed by two processes: (1) a slow surface related process (ii) a fast bulk related process.

The surface related photoconductivity was primarily related to the adsorption and desorption of the chemisorbed oxygen at the surface of the ZnO. During the UV-Vis illumination, photogenerated electron-hole pairs are generated at the surfaces as. where h  is the photon energy of UV-vis light, h+ is the photogenerated hole in the valence band, and is the photogenerated electron in the conduction band. This fast process of generation of photocarriers results in an initial fast rise in photocurrent under UV-Vis illumination. The photogenerated holes recombine with the adsorbed oxygen ions on the surface, producing oxygen molecules; this reaction also eliminates the barrier near the surface of the nanostructure.

This process was described by the following equation: At the same time, the desorption of adsorbed oxygen ions on the surface of nanoparticles leaves captured electrons in the conduction band, thereby, increasing the sample conductivity and contributing to the photocurrent. When the UV-Vis light was switched off, initially the current decreased very fast, which could be attributed to recombination of free electrons and holes. Later, the current decreased slowly, which could be attributed to the slow process of surface re-adsorption of oxygen molecules due to the presence of free electrons at the surface of ZnO.

                The Rise and Decay of ZnO (pure) and ZnO/S/P have same patterns, but different kinds of dark-current, photocurrent, sensitivity, and efficiency it was shown in table 2. The dark-current of ZnO/S/P sample was higher than ZnO (pure) sample, so it was saying that ZnO/S/P had more free electrons than ZnO (pure) sample.  The photocurrent of ZnO/S/P was also higher than the photocurrent of ZnO (pure). So it was saying that ZnO/S/P created more photo-generation of electron hole pairs than ZnO(pure).The dark-current of ZnO/S sample was higher than ZnO/P sample, so it was saying that ZnO/S has more free electrons than ZnO/P sample and remaining samples. The photocurrent of ZnO/S was also higher than the photocurrent of ZnO/P and remaining samples. So it was saying that ZnO/S sample created more photo-generation of electron hole pairs than remaining samples.

The sensitivity of the ZnO(pure) sample was higher than remaining samples. The sensitivity of the ZnO/P was lower than remaining samples. So it was showing that the sensitivity of ZnO(pure) was high but the photocurrent and dark current was lower than others. The dark current of ZnO/S was higher than ZnO/P but the ZnO/P dark current was higher than the remaining samples.

So it was saying that the ZnO/S and ZnO/P had more free electrons than remaining samples.