Heterojunction and direct Z-scheme nanostructures are two typical representatives of an efficient photocatalyst, which is composed of two semiconductors. However, it is a great challenge to construct each of them on purpose. The photodeposition technique can be a potentially powerful tool to regulate the electron flow direction for constructing these nanostructures. In this report, CdS nanoparticles were deposited on the g-C3N4 nanosheets by photodeposition and chemical deposition methods for comparison. In the photodeposition case, PL and charge flow tracking demonstrate that a type II heterojunction is constructed because CdS is selectively deposited at the electron transfer site of g-C3N4, which leads to the photoexcited electron from g–C3N4 tending to transfer to CdS in the composites. In the latter, the CdS is randomly deposited onto the g-C3N4 nanosheets through chemical deposition. There is no preferred site for deposition or charge transfer in the composite. The results illustrate that the electron of CdS tends to recombine with the hole from g-C3N4. The direct Z-scheme is predominant for the CdS/g-C3N4 prepared by the chemical deposition route. Furthermore, the photocatalytic performance and stability also confirm the above results. On the of these, we can deduce that the photodeposition method can be used to regulating the electron transfer route. We expect this report to shed light on the rational design of heterojunction or direct Z-scheme type composites.

Photocatalytic ammonia synthesis is another important reaction to mimic natural nitrogen fixation, which has attracted more and more attention. In recent reports, sacrificial agents are often used to promote charge separation, and high-activity photocatalysts are discovered by using Nessler’s reagent method as a detection technique of ammonia production. However, there is an open question on the rationality and accuracy of the ammonia production amount in the presence of the sacrificial agent and Nessler’s reagent detection method. In this report, P25 TiO2 is employed as a model photocatalyst and alcohol as sacrificial agent, and both Nessler’s reagent and cation exchange chromatography are employed as ammonia detection methods. The different ammonia production amount was found by the different detection method. HPLC and 1H NMR results indicate that carbonyl compounds (formaldehyde, acetaldehyde, and acetone) are produced in the reaction. When the carbonyl compound was added to the ammonia standard solution, the interference effect on the detection of ammonia was found in the Nessler’s reagent method. No interference effect was found in the cation exchange chromatography. Thus, the Nessler’s reagent is not suitable for ammonia detection in the presence of alcohol as the sacrificial agent.

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Defects play a significant part in promoting photocatalytic activity for H 2 production. Various methods such as chemical reduction have been performed to metal oxide based photocatalysts. Herein, we present the NaBH 4 reduction route to introduce the defects into the graphitic carbon nitride (g-C 3 N 4 ) to enhance photocatalytic activity. A new −C≡N group is observed in the FTIR spectra of treated g-C 3 N 4 nanosheets indicating the presence of structural defects. At the same time, the B signal appears in the X-ray photoelectron spectroscopy analysis, suggesting that B is doped in the g-C 3 N 4 during the treatment. All these results manifested that multiple types of defects are introduced in the g-C 3 N 4 during the NaBH 4 treatment. The UV− vis spectra illustrate that the absorption band edge of g-C 3 N 4 is extended from 420 to 450 nm after NaBH 4 treatment. This demonstrates that the band gap of g-C 3 N 4 turns narrow owing to the introduction of defects. Photocatalytic H 2 production of defective g-C 3 N 4 is ∼5-fold better than that of pristine g-C 3 N 4 . To understand the enhanced mechanism, the apparent quantum efficiency, photoluminescent spectra, transient photocurrent and electrochemical impedance spectra are investigated. The results show that the charge separation efficiency is greatly strengthened in the defective g-C 3 N 4 . Upon these findings, the enhancement of catalytic activity can be attributed to both the broad light adsorption range and highly efficient charge separation process.

Photodeposition can effectively regulate charge transfer sites and deposit the second semiconductor at a specific site to form a direct Z-scheme.