Photo-electrocatalysis is an efficient way to convert solar energy into chemical energy. Extending the light absorption region of the photo-electrode material is a key step to obtaining high conversion efficiency. Most investigations so far have focused on titanium oxide for its high activity and excellent chemical stability for energy conversion under UV light ^{[1, 2, 3]}. The doping of N into TiO₂ can extend the light response into the visible region as the band gap is narrowed with N doping ^{[4, 5, 6]},such as in the oxide/nitride compounds of Ta₃N₅ ^[7],TaON ^[8],and N-TiO ^[9]. In contrast to these oxide/nitride semiconductors with a specific conduction band and valence band,TiN_x exhibits metallic property in a particular energy state ^{[10, 11]},which is favorable for electron transfer in TiN_x ^{[12, 13, 14]}. In a previous report,TiN_x was reported to show photo-catalytic activity for H₂ generation under visible light in water with CH₃OH,with Na₂SO₃ and Na₂S as sacrificial electron donors ^[15]. However,the non-stoichiometric nature of TiN_x inhibited its application in photo-electrocatalysis because its intrinsic defects/vacancies can act as the recombination centers to reduce the conversion efficiency.

Cerium oxide (CeO₂) has been widely studied in catalysis for its unique redox property. Recent research indicated that CeO₂ is also a potential photo-catalyst ^{[16, 17, 18, 19]}. We have reported that the redox property of CeO₂ is favorable for the consumption of photo-holes in the photo-catalytic reaction for H₂ generation in Na₂S and Na₂SO₃ aqueous solution ^[20]. In this manuscript,a composite photo-anode of TiN_x/CeO₂ was fabricated with the electrodeposited method and its photo- electrocatalytic performance was evaluated by photo-current measurement. TiN_x/CeO₂ gave four times larger photo-current than the pristine TiN_x and CeO₂,and the photo-current stabilization was significantly increased relative to TiN_x and CeO₂. The photo-electrocatalytic mechanism is discussed with consideration of the light harvest,separation efficiency of photo-electrons and photo-holes,and the interfacial electrochemical reaction rate.

TiN_0.3 was obtained by calcining a Ti sheet (Taijin Company,China) under NH₃ at 600 °C for 3 h. Before calcination,the Ti sheet of 2.5 cm × 1.2 cm size was polished and then washed by deionized water,acetone,and deionized water.

TiN_0.3/CeO₂ was obtained by electrodepositing CeO₂ on TiN_0.3 in a two electrode cell with a current of 2 mA for 10 min. The synthesis recipe was reported in Ref.^[20]. TiN_0.3 and a graphite rod were used as the working electrode and counter electrode,respectively. The CeO₂ grew on the TiN_x substrate in a solution containing Ce(NO₃)₃ (0.01 mol/L),NH₄Cl (0.1 mol/L),and KCl (0.03 mol/L) at 70 °C. A Ti/CeO₂ photo-electrode was prepared by the same method for comparison.

The crystal structure of TiN_0.3,TiN_0.3/CeO₂,and Ti/CeO₂ was characterized by a D8 Advance X-ray diffractometer (Bruker,Germany) with Cu K_α radiation source at a scanning rate of 5°/min. The morphology of the samples was observed with a Thermal FE Environment scanning electron microscope (SEM) (FEI,Quanta 400F,Holland). Diffuse reflection spectra (DSR) were recorded on a UV-Vis spectrophotometer (Shimadzu,UV-3150,Japan; equipped with an integrating sphere) to characterize the optical properties of the samples.

The photo-electrocatalytic performance of TiN_0.3,TiN_0.3/ CeO₂,and Ti/CeO₂ was evaluated with photo-current measurement without an external potential in addition to the open circuit voltage. The test was carried out in a three-electrode cell equipped with a Pt counterelectrode and saturated calomel reference electrode. The potential was generated and the current was analyzed with a potentiostat (Chenhua,756d,Shanghai) controlled by a computer. A 10 mL Na₂SO₄ aqueous solution (0.1 mol/L) was used as the electrolyte. The photo-current was measured under light-on and light-off with a 10 s interval. Light for the photo-electrochemical measurement was produced by a Xe lamp (Changtuo,PLS-SXE300/300UV,Beijing) with 130 mW/cm² intensity (Zhongjiaojinyuan,CEL-VIS400,Beijing).

Figure 1 shows the XRD patterns of the Ti sheet and TiN_0.3,TiN_0.3/CeO₂,and Ti/CeO₂ samples. The diffraction peaks at 35.0°,38.4°,40.2°,52.8°,62.7°,and 70.1° (Fig. 1(1)) were consistent with JCPDS 44-1294,and indicated the Ti crystalline structure. After calcining the Ti sheet in a NH₃ atmosphere,a series of shoulder peaks at 37.5°,39.7°,52.1°,and 69.2° appeared in Fig. 1(2),in addition to the Ti characteristic peaks. These peaks are from hexagonal TiN_0.3 according to JCPDS 41-1352 (a = b = 0.2974 nm,c = 0.4792 nm,α = β = 90°,γ = 120°). In Fig. 1(3) and (4),a new peak at 28.5° was observed for TiN_0.3/CeO₂ and Ti/CeO₂,in addition to the diffraction peaks of TiN_0.3 and Ti substrates. This peak was identified as the (111) diffraction peak of cubic CeO₂ according to JCPDS 65-2975 (a = b = c = 0.424 nm,α = β = γ = 90°),and indicated that CeO₂ was deposited on the TiN_0.3 and Ti substrates successfully.

Fig. 1. XRD patterns of (1) Ti,(2) TiN0.3,(3) TiN0.3/CeO2,and (4) Ti/CeO2.

SEM was employed to study the morphology of TiN_0.3/CeO₂ and Ti/CeO₂. The images are shown in Fig. 2. Spherical CeO₂ particles were uniformly dispersed on the surface of the TiN_0.3 and Ti substrates. The change of substrates did not influence the dispersion and particle size of CeO₂.

Fig. 2. SEM images of (a) TiN0.3/CeO2 and (b) Ti/CeO2.

The optical absorption characterization results of TiN_0.3 and TiN_0.3/CeO₂ are shown in Fig. 3. A broad absorption band centered at 680 nm was observed for pristine TiN_0.3. Compared to TiO₂ with a UV response,the visible light harvest by TiN_0.3 originated from the reduction of the energy level structure due to the atomic substitution of N for O. After electrodepositing CeO₂ on TiN_0.3,the visible absorption band was weakened and blue-shifted to 550 nm. Meanwhile,a UV absorption band starting at 400 nm was observed. The outer CeO₂ was responsible for the UV absorption of TiN_0.3/CeO₂ ^[20],and it also caused the attenuation of visible light transmission to the TiN_0.3 layer. For the shift of the visible absorption band,we proposed that it was related to an interaction between TiN_0.3 and CeO₂.

Fig. 3. UV-Vis absorption spectra of the TiN0.3 and TiN0.3/CeO2 samples.

The photo-electrocatalytic performace (i-t) of the TiN_0.3,Ti/CeO₂,and TiN_0.3/CeO₂ samples is shown in Fig. 4. A 7 μA/cm² photo-current was instantly observed for TiN_0.3 once the light was turned on. This photo-current quickly decayed within 1 s to 2.0 μA/cm²,and then slowly decayed thereafter. With the CeO₂ photo-anode,the photo-current showed a similar generation and decay as with TiN_0.3,except that it slowly decayed within 10 s to 1 μA/cm². For the TiN_0.3/CeO₂ sample,an instant photo-current of 8.4 μA/cm² was generated by irradiating,and it remained constantly in the following light-on time. However,after two light-on/off cycles,the photo-current was slightly reduced at the end of the light-on time.

Fig. 4. i-t plots of TiN0.3,TiN0.3/CeO2,and Ti/CeO2.

The photo-current measurement was carried out in Na₂SO₄ aqueous solution without an external potential. In this electrolyte,the observed anodic photo-current originated from photo-electrocatalytic H₂O oxidation. The four times larger photo-current density of TiN_0.3/CeO₂ than those of TiN_0.3 and CeO₂ indicated its higher photo-electrocatalytic activity. The photo-current generation begins with the generation of photo-carriers under light irradiation on the photo-anode. After the separation of the photo-electrons and photo-holes inside the semiconductor,photo-electrons are transferred to the Ti substrate and arrive at the Pt counterelectrode through the outer circuit to form the observed photo-current. Meanwhile,photo-holes diffuse to the interface between the electrode and electrolyte to complete the electrochemical reaction. Therefore,the light harvest and photo-carriers separation in the semiconductor directly influence the intensity of the photo-current. In contrast to the light absorption and photo-carriers separation on the nanosecond scale,the slow electrochemical reaction rate at the electrode/solution interface gave a current stabilization on the time scale of seconds.

There are many vacancies and defects in TiN_0.3 due to its non-stoichiometric nature. These defects act as recombination centers in the photo-electrocatalytic process,and lead to the fast decay of the photo-current in TiN_0.3. For the single CeO₂ photo-anode,Ce³⁺ is oxidized to Ce⁴⁺ by the photo-holes,while it can be reproduced by Ce⁴⁺ reduction by trapping the photo-electron. That is,the Ce³⁺ centers act as the recombination centers of electrons and holes,which is unfavorable for photo-current generation in the photo-electrocatalytic H₂O oxidation process.

Compared to TiN_0.3 and CeO₂,the combined TiN_0.3/CeO₂ photo-anode exhibited the highest light utilization efficiency since both UV and visible light can be harvested for the generation of free carriers. Figure 5(a) shows the straddling gap (type I) heterojunction structure of TiN_0.3 and CeO₂. This energy band structure has no effect on the transfer of carriers in the TiN_0.3 layer,but it allows the transfer of electrons and holes in the CeO₂ layer to TiN_0.3. Photo-electrons in TiN_0.3 are recombined with photo-holes at the defect sites in TiN_0.3,which is similar to the single TiN_0.3 photo-anode. In contrast to TiN_0.3,photo-electrons in CeO₂ transferred to TiN_0.3 conduction band are driven by the energy difference between CeO₂ and TiN_0.3. But the photo-hole transfer from CeO₂ to TiN_0.3 was inhibited as they were consumed by Ce³⁺ that existed in the interface of TiN_0.3/CeO₂. That is,the carrier recombination in CeO₂ was inhibited by the specific double-layer structure of TiN_0.3/CeO₂. Therefore,the photo-current in the TiN_0.3/CeO₂ photo-anode was significantly enhanced.

Fig. 5. (a) Energy band structure of CeO2 and TiN0.3; (b) Schematic of photo-current generation in TiN0.3/CeO2.

In addition to the enhancement of the photo-current in the TiN_0.3/CeO₂ combined photo-anode,the stabilization of the photo-current was also promoted. The oxidation of H₂O occurs in the interface between CeO₂ and the electrolyte. As an important catalyst,CeO₂ has drawn much attention for its special storage and release of oxygen as oxygen vacancies,i.e.,Ce^{3+ [21, 22, 23]}. The Ce³⁺ at the CeO₂ surface acts as the adsorption site for the activation of H₂O,which accelerates the H₂O oxidation reaction. As a result,the stabilization of the photo-current was improved in the TiN_0.3/CeO₂ photo-anode. A scheme of photo-current generation in TiN_0.3/CeO₂ is shown in Fig. 5(b). Concerning the slight decline of the photo-current after several cycles with TiN_0.3/CeO₂,we believe it was related to a side reaction of the electrode. More work is now underway.

A TiN_0.3/CeO₂ photo-anode was fabricated by the electro-deposition of CeO₂ spheres on the surface of TiN_0.3 supported on a Ti substrate. The double-layer structure in TiN_0.3/CeO₂ increased the light harvest efficiency since both visible light and UV light were absorbed by TiN_0.3 and CeO₂,respectively. The photo-electrochemical measurement indicated that the stabilization and magnitude of the photo-current were significantly improved by the combination of TiN_0.3 and CeO₂. The separation of electrons and holes in the CeO₂ layer was promoted due to electron transfer to TiN_0.3 driven by the heterojuntion and hole consumption by Ce³⁺ in the TiN_0.3-CeO₂ interface. The electrochemical reaction was also accelerated due to the adsorption and activation of H₂O on Ce³⁺ sites. The TiN_0.3/CeO₂ photo-anode has potential application in photo-catalysis and photo-electrocatalytic reactions.