Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 695-700

#### The article information

Xiao-nong Wang, Jun Ma, Yang-guang Hu, Ran Long, Yu-jie Xiong

Ag-Cu Nanoparticles Supported on N-Doped TiO$_2$ Nanowire Arrays for Efficient Photocatalytic CO$_2$ Reduction
Ag-Cu纳米颗粒修饰的N掺杂TiO$_2$纳米棒阵列及其高效光催化CO$_2$还原应用
Chinese Journal of Chemical Physics, 2018, 31(5): 695-700

http://dx.doi.org/10.1063/1674-0068/31/cjcp1804062

### Article history

Accepted on: April 21, 2018
Ag-Cu Nanoparticles Supported on N-Doped TiO$_2$ Nanowire Arrays for Efficient Photocatalytic CO$_2$ Reduction
Xiao-nong Wang, Jun Ma, Yang-guang Hu, Ran Long, Yu-jie Xiong
Dated: Received on April 10, 2018; Accepted on April 21, 2018
Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of Chemistry and Materials Science, and National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Ran Long, E-mail: longran@ustc.edu.cn; Yu-jie Xiong, E-mail: yjxiong@ustc.edu.cn
Abstract: Photocatalytic reduction of CO$_2$ into various types of fuels has attracted great interest, and serves as a potential solution to addressing current global warming and energy challenges. In this work, Ag-Cu nanoparticles are densely supported on N-doped TiO$_2$ nanowire through a straightforward nanofabrication approach. The range of light absorption by N-doped TiO$_2$ can be tuned to match the plasmonic band of Ag nanoparticles, which allows synergizing a resonant energy transfer process with the Schottky junction. Meanwhile, Cu nanoparticles can provide active sites for the reduction of CO$_2$ molecules. Remarkably, the performance of photocatalytic CO$_2$ reduction is improved to produce CH$_4$ at a rate of 720 $\mu$mol$\cdot$g$^{-1}$$\cdoth^{-1} under full-spectrum irradiation. Key words: Photocatalytic CO_2 reduction Schottky junction Energy transfer TiO_2 Nanoparticles Ⅰ. INTRODUCTION The energy and environmental issues associated with the consumption of fossil fuel have influenced our daily life. Carbon dioxide (CO_2) largely contributes to the greenhouse gas among various emitted products. To solve both the environmental and energy issues, the conversion of CO_2 to valuable fuels such as methane (CH_4) and methanol through photocatalysis has attracted wide interests [1-6]. According to the fundamental principle, three steps are mainly involved in a process of photocatalytic CO_2 reduction: (ⅰ) absorption of incident photons by semiconductor to generate photoexcited electrons and holes, (ⅱ) separation of photoexcited electrons and holes and their migration to the surface of photocatalyst, and (ⅲ) CO_2 reduction by the electrons and oxidation reaction by the holes. For this reason, light absorption should be first engineered by selecting and modifying semiconductor in efforts to achieve high activity in photocatalytic CO_2 reduction. Since the first report on photocatalytic CO_2 reduction in 1979 [7], a variety of semiconductors have been investigated towards this application [5, 8-11]. Among various semiconductors, TiO_2 is a very promising candidate for photocatalytic reactions owing to its high stability and photocatalytic performance. However, the bandgap of TiO_2 at 3.2 eV makes it only absorb ultraviolet (UV) light, which accounts for 5% photons in the solar spectrum. In order to absorb visible light by TiO_2, great efforts have been made to modify TiO_2 [12-15]. In 2001, Asahi and co-workers reported that N-doped TiO_2 exhibits photocatalytic activity under visible-light illumination [12]. The hybridization of N 2p orbitals in doped TiO_2 could narrow the bandgap, resulting in visible-light absorption. Since then, the N-doped TiO_2 has been widely investigated for visible-light photocatalysis [16-18]. To further improve photocatalytic performance, the Schottky junction between metal and semiconductor has also been investigated to improve charge separation and transfer [19, 20]. By combining metal nanoparticles with semiconductor, photoexcited electrons can be transferred and trapped on the metal with suitable work function, which spatially separates the electrons from holes. In addition, the surface plasmon of noble metal nanoparticles (e.g., Ag nanoparticles) may promote the creation and/or separation of electron-hole pairs through two different mechanisms [21-24]: (ⅰ) local electromagnetic field enhancement, and (ⅱ) resonant energy transfer (RET) when the light absorption of semiconductor and the plasmonic band of metal nanoparticles sufficiently overlap. Thus it should be a promising approach to the enhancement of photocatalytic CO_2 reduction by integrating N-doped TiO_2 with plasmonic metal nanoparticles. Upon the accumulation of sufficient photoexcited electrons on surface, the overall photocatalytic performance is still limited by active sites. It has been reported that the integration of cocatalysts (e.g., PdCu [5], AuCu [25], and PtCu [26]) with TiO_2 can provide active sites to enhance the photocatalytic conversion of CO_2 to valuable hydrocarbons. In this article, we report a facile nanofabrication approach to combining dense Ag-Cu nanoparticles with N-doped TiO_2 nanowire arrays without the need of using surfactants. As compared with wet-chemical methods, this approach does not involve surfactants so as to make an intimate contact between metal and semiconductor, which would dramatically enhance the efficiency of electron transfer during photocatalytic CO_2 conversion. In this model, Ag nanoparticles offer a plasmonic band which sufficiently overlaps with the light absorption of N-doped TiO_2, enabling RET to improve carrier creation and separation. Meanwhile, Cu nanoparticles provide active sites for CO_2 conversion [28-30]. We specifically choose TiO_2 nanowires that have been widely investigated for solar energy harvesting and conversion process [16, 31, 32] as our material model. As compared with the disorderly dispersed nanowires, TiO_2 nanowire arrays exhibit superb performance in light trapping owing to their high aspect ratios [33, 34]. When light is introduced into the vertical arrays, multiple scattering would occur within the arrays, which effectively increases optical length and thus enhances light absorption. This design thus perfectly offers the improvement on light absorption by doping and light trapping, charge separation by Schottky junction and RET, and surface reactions by active sites, which all can enhance the performance of photocatalytic CO_2 conversion under full-spectrum irradiation. This nanofabrication technique should also provide a flexible approach to designing various hybrid structures by altering evaporation metals. Ⅱ. EXPERIMENTS A. Chemical Tetrabutyl titanate, hydrochloric acid, ethanol, and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. The water used in the experiments was deionized. All chemicals were used as received without further purification. B. Synthesis of TiO_2 nanowire arrays and N-doped TiO_2 nanowire arrays The TiO_2 nanowire arrays were prepared by a hydrothermal method. In a typical synthesis, the mixture of 12-mL deionized water and 12-mL hydrochloric acid was stirred for 5 min. Subsequently, 0.4-mL tetrabutyl titanate was added into the mixture. After stirring for 15 min, the mixture solution was transferred to a 50-mL Teflon-lined stainless steel autoclave. A piece of cleaned FTO glass was then placed at an angle against the wall of the Teflon-liner with the conducting side facing down. The hydrothermal synthesis was conducted at 150 ℃ for 3 h. After the reaction, the autoclave was cooled to room temperature naturally. The rutile TiO_2 nanowire arrays were prepared by simply annealing the sample in air at 450 ℃ for 2 h, with a heating rate of 5 ℃/min. The N-doped TiO_2 nanowire arrays were prepared by annealing the sample in a tube-type furnace in ammonia at 450 ℃ for 2 h, with a heating rate of 1 ℃/min. C. Integration of AgCu nanoparticles with TiO_2 nanowire arrays or N-doped TiO_2 nanowire arrays An ultrahigh vacuum (UHV) electron-beam evaporation system (Shenyang Scientific Instruments, China, DZS-500) was used to deposit a layer of 5 nm Ag film and 5 nm Cu film (Ag_5Cu_5) on TiO_2 nanowire arrays or N-doped TiO_2 nanowire arrays. The evaporation rate was maintained at 0.03 nm/s under the pressure of about 1\times10^{-4} mbar. D. Sample characterization Scanning electron microscopy (SEM) images were taken on a FEI Sirion 200 field-emission scanning electron microscope operated at 5 kV. X-ray powder diffraction (XRD) patterns were recorded on a Philips X'Pert Pro Super diffractometer with Cu K\alpha radiation (\lambda=1.54178 Å). UV-Vis diffuse reflectance data were recorded in the spectral region of 300-800 nm with a Shimadzu SolidSpec-3700 spectrophotometer. X-ray photoelectron spectra (XPS) were collected on an ESCALab 250 X-ray photoelectron spectrometer, using nonmonochromatized Al-K\alpha X-ray as the excitation source. E. Photoelectrochemical measurements The measurements were carried out on a CHI 660D electrochemical station (Shanghai Chenhua, China) in ambient condition under irradiation of a 300 W Xe lamp (Solaredge 700, China). The power density of full spectrum was set to be 100 mW/cm^2, the ultraviolet (UV) light was measured to be 2.7 mW/cm^2. Standard three-electrode setup was used with the fabricated samples as photoelectrode, with a Pt foil as counter electrode, and the Ag/AgCl electrode as reference electrode. The three electrodes were inserted in a quartz cell filled with 0.5-mol/L Na_2SO_4 electrolyte. The Na_2SO_4 electrolyte was purged with Ar for 30 min prior to the measurements. The photocurrent responses of the prepared photoelectrodes (i.e., I-V) were operated by measuring the photocurrent densities under chopped light irradiation with the light on/off cycles for each 10 s. F. Photocatalytic CO_2 reduction In a typical experiment, 3-cm^2 photocatalysts including 0.03-mg Au and Cu were immersed into 30-mL deionized water with 5-mL triethanolamine as a sacrificial agent in a home-made quartz bottle, followed by saturation with high-purity CO_2 for 30 min. Subsequently, light irradiation was performed using a 300-W Xe lamp with full-spectrum light, UV light or visible light as the illumination source, respectively. The light source and power intensity were consistent for all the photocurrent measurements. The photocatalytic reaction was typically performed for 4 h. The amount of CH_4, CO, and H_2 evolved was measured by gas chromatography (GC, 7890A, Ar carrier, Agilent). H_2 was detected using a thermal conductivity detector (TCD), and CH_4 was measured by a flame ionization detector (FID). CO was converted to CH_4 by a methanation reactor, and then analyzed with FID. Three replicates were collected for each sample with relative error < 10%. Ⅲ. RESULTS AND DISCUSSION A. Sample characterization Ag-Cu nanoparticles can make intimate contact with TiO_2 nanowire arrays during electron beam evaporation, which establishes the Schottky junction between the components. SEM images with different magnifications (FIG. 1 (a) and (b)) show that the Ag-Cu nanoparticles have an average size of 20 nm and tightly contact the TiO_2 nanowire arrays. These nanoparticles are constructed as an immiscible Ag-Cu binary phase diagram instead of AgCu alloy [35]. X-ray photoelectron spectroscopy (XPS, FIG. 1(c)) reveals the existence of N, Ag, and Cu elements in the sample, indicating the successful N doping. Moreover, X-ray diffraction (XRD, FIG. 1(d)) shows that all the peaks can be assigned to rutile TiO_2 (JCPDS No.21-1276), face-centered cubic (fcc) Ag, (JCPDS No.65-2871) and Cu (JCPDS No.04-0836). This verifies the formation of immiscible Ag-Cu binary phase (namely, Ag_5Cu_5). We thus name the sample with Ag-Cu nanoparticles supported on N-doped TiO_2 nanowire arrays as "Ag_5Cu_5/N-TiO_2".  FIG. 1 SEM images of Ag_5Cu_5 nanoparticles combined with N-doped TiO_2 nanowire arrays (Ag_5Cu_5/N-TiO_2) at (a) low and (b) high magnification. (c) XPS spectra of Ag_5Cu_5/N-TiO_2. (d) XRD pattern of Ag_5Cu_5/N-TiO_2. In the sample preparation process, N element is doped into the lattice of rutile TiO_2 by annealing the TiO_2 nanowire arrays in NH_3 atmosphere. As shown in FIG. 2, such N doping (N-TiO_2) results in an extension of light absorption to the visible spectrum. By doping N into the TiO_2, the light absorption range can be extended to 500 nm. This extended light absorption can thus sufficiently overlap with the plasmonic band of Ag_5Cu_5 nanoparticles. The plasmonic band can be well resolved in the absorption spectrum of Ag_5Cu_5 nanoparticles supported on the undoped TiO_2 nanowire arrays (Ag_5Cu_5/TiO_2). As compared with bare TiO_2, the light absorption in the visible spectrum should result from the plasmonic effect of Ag_5Cu_5 nanoparticles. Thus we anticipate that the N-doped TiO_2 and Ag_5Cu_5 nanoparticles can offer an overlapping light absorption in the sample of Ag_5Cu_5/N-TiO_2. Since the RET process requires sufficient overlap between semiconductor and metal nanoparticles, such a match in the spectral range can help enhance carrier creation/separation and thus photocatalytic performance. The apparent enhancement around 400-550 nm for the Ag_5Cu_5/N-doped TiO_2 sample results from the match between the Ag_5Cu_5 and N-doped TiO_2.  FIG. 2 UV-Vis diffuse reflectance spectra of TiO_2 nanowire arrays, N-doped TiO_2 nanowire arrays, Ag_5Cu_5/TiO_2 nanowire arrays and Ag_5Cu_5/N-doped TiO_2 nanowire arrays. B. Photoelectrochemical (PEC) performance Upon the completion of sample synthesis and fabrication, we investigate the PEC performance of our samples with the illumination source of UV light, visible light, and full-spectrum light, respectively. As shown in FIG. 3(a), TiO_2 can barely generate photocurrents under visible-light illumination. This situation can be improved by doping TiO_2 with nitrogen. The doping of N atoms leads to the orbital hybridization of N 2p and TiO_2 valence band, and thus narrows the bandgap of TiO_2. The resulted visible light absorption gives a relatively apparent photocurrent response under visible-light illumination.  FIG. 3 Photocurrent-potential curve of (a) TiO_2 nanowire arrays and N-doped TiO_2 nanowire arrays under visible-light irradiation, (b) N-doped TiO_2 nanowire arrays and Ag_5Cu_5/N-doped TiO_2 nanowire arrays under UV-light irradiation, (c) N-doped TiO_2 nanowire arrays and Ag_5Cu_5/N-doped TiO_2 nanowire arrays under visible-light irradiation, and (d) N-doped TiO_2 nanowire arrays and Ag_5Cu_5/N-doped TiO_2 nanowire arrays under full-spectrum light irradiation. We further evaluate the effects of Ag_5Cu_5 nanoparticles on the photocurrent response of N-doped TiO_2 nanowire arrays. Ag_5Cu_5 nanoparticles potentially can play dual roles in photocurrent enhancement-Schottky junction and plasmonic RET. To appreciate the promotion by Schottky junction, we collect photocurrents under UV-light illumination. As shown in FIG. 3(b), the photocurrents of N-doped TiO_2 nanowire arrays can be enhanced 5 times by the addition of Ag_5Cu_5 nanoparticles. Given that plasmonic effect is excluded under UV irradiation, this enhancement should result from the function of Schottky junction. The Schottky junction traps electrons on Ag_5Cu_5 nanoparticles, and thus reduces the recombination of the electron-hole pairs in the N-doped TiO_2. Furthermore, the RET process is assessed under visible-light illumination, in which the plasmonic property of Ag_5Cu_5 nanoparticles can be activated. As shown in FIG. 3(c), the Ag_5Cu_5/N-doped TiO_2 nanowire arrays give a 3.8 times stronger photocurrent response than the N-doped TiO_2. This enhancement results from the surface plasmon of metal nanoparticles whose band matches with the light absorption of N-doped TiO_2. The RET process enhances the carrier creation and separation, which can boost the PEC performance under visible-light illumination. Taken together, the integration of N-doped TiO_2 with Ag_5Cu_5 nanoparticles can significantly enhance the full-spectrum performance as shown in FIG. 3(d). As compared with the N-doped TiO_2, the photocurrent of Ag_5Cu_5/N-doped TiO_2 is enhanced about 6.4 times, owing to the synergetic effect of Schottky junction and RET process. C. Photocatalytic CO_2 reduction Upon acquiring the giant enhancement on photocurrent response, we are in the position to evaluate the performance of our samples in the photocatalytic CO_2 reduction with triethanolamine as a sacrificial agent in H_2O. FIG. 4(a-c) shows the average rates of photocatalytic CH_4, CO, and H_2 production by TiO_2 nanowire arrays, N-doped TiO_2 nanowire arrays and Ag_5Cu_5-supported N-doped TiO_2 nanowire arrays under different light illumination, respectively. The N-doped TiO_2 nanowire arrays integrated with Ag_5Cu_5 nanoparticles exhibit significantly higher photocatalytic activity than bare TiO_2 and N-doped TiO_2 in the production of total products, demonstrating the importance of synergetic Schottky junction and RET effects to photocatalysis. Specifically, the production rate of CH_4 by the Ag_5Cu_5/N-doped TiO_2 nanowire arrays is 720 \mu mol\cdotg^{-1}$$\cdot$h$^{-1}$, which is about 6 times that of the TiO$_2$ nanowire arrays (FIG. 4(a)). The production rates in full spectrum are higher than the sum of those under UV and visible illumination for the Ag$_5$Cu$_5$/N-doped TiO$_2$ nanowire arrays, indicating the synergetic effects.

 FIG. 4 The average rates of photocatalytic (a) CH$_4$, (b) CO, and (c) H$_2$ production for TiO$_2$ nanowire arrays, N-doped TiO$_2$ nanowire arrays, and Ag$_5$Cu$_5$/N-doped TiO$_2$ nanowire arrays under UV-light, visible-light, and full-spectrum irradiation, respectively. (d) Schematic illustrating the major process in the photocatalytic reactions. Performance of Ag$_5$Cu$_5$/N-doped TiO$_2$ nanowire arrays for (e) CH$_4$ and CO evolution and (f) H$_2$ evolution in 3 successive 4 h cycles under full-spectrum irradiation.

Based on the experiment results, we can summarize the major processes in the photocatalytic CO$_2$ reactions as shown in FIG. 4(d). The UV light and partial visible light can photoexcite electrons and holes in N-doped TiO$_2$. The electrons are then separated from holes and become trapped by the metal nanoparticles through the Schottky junction. Moreover, the visible light can induce the surface plasmon of Ag$_5$Cu$_5$ nanoparticles whose band has a spectral overlap with the light absorption of N-doped TiO$_2$, which enhances the creation and separation of electron-hole pairs through the RET process. Under the full-spectrum illumination, the Schottky junction can be synergized with the RET process, thereby further enhancing the photocatalytic performance in CO$_2$ reduction. We have performed the photocatalytic reaction for 3 cycles using the same Ag$_5$Cu$_5$/N-doped TiO$_2$ sample, each of which lasts 4 h under full-spectrum irradiation. As shown in FIG. 4 (e) and (f), the sample shows excellent performance stability in the recycling test.

Ⅳ. CONCLUSION

In conclusion, we have prepared the N-doped TiO$_2$ nanowire array through a hydrothermal method, followed by the deposition of Ag$_5$Cu$_5$ nanoparticles through electron beam evaporation. The doped N can expand the absorption range of TiO$_2$ to visible light, matching the surface plasmonic resonance of Ag$_5$Cu$_5$ nanoparticles. The Schottky junction between semiconductor and metal can be synergized with the RET process in this work, enhancing the photocatalytic performance in CO$_2$ conversion. Another contribution from Ag$_5$Cu$_5$ nanoparticles is the role of Cu as active sites for CO$_2$ reduction. This work provides a method for large-scale photocatalyst fabrication towards future practical applications.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Key R&D Program of China (2017YFA0207301), National Natural Science Foundation of China (No.21725102, No.21471141, No.21601173), CAS Key Research Program of Frontier Sciences (QYZDB-SSW-SLH018), CAS Interdisciplinary Innovation Team, Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (No.2016FXCX003), Anhui Provincial Natural Science Foundation (No.1608085QB24), and Chinese Universities Scientific Fund (WK2310000067).

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Ag-Cu纳米颗粒修饰的N掺杂TiO$_2$纳米棒阵列及其高效光催化CO$_2$还原应用