Facile and Large-Area Preparation of Porous Ag3PO4 Photoanodes

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Facile and Large-Area Preparation of Porous AgPO Photoanodes for Enhanced Photoelectrochemical Water Oxidation Qi Cao, Jun Yu, Kaiping Yuan, Miao Zhong, and Jean-Jacques Delaunay ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 31 May 2017 Downloaded from http://pubs.acs.org on May 31, 2017

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ACS Applied Materials & Interfaces

Facile and Large-Area Preparation of Porous Ag3PO4 Photoanodes for Enhanced Photoelectrochemical Water Oxidation Qi Cao,† Jun Yu,† Kaiping Yuan,‡ Miao Zhong,† Jean-Jacques Delaunay*,†

†

Graduate School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656,

Japan ‡

State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, 220 Handan

Road, Shanghai 200433, People’s Republic ofChina

* Corresponding author: [email protected] (J.-J. Delaunay)

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Abstract Photoelectrochemical (PEC) water splitting is a promising approach for renewable energy, where the development of efficient photoelectrodes, especially photoanodes for water oxidation is still challenging. In this paper, we report the novel solution-processed microcrystalline Ag3PO4 photoanodes with tunable porosity depending on the reaction time. These porous Ag3PO4 films were grown on large-area (4.5 × 4.5 cm2) silver substrates via an air-exposed and room-temperature immersion reaction. Enhanced light absorption abilities were exhibited by the synthesized Ag3PO4 films with optimized porosity resulted from prolonged reaction times (≥ 20 h), owing to which appreciable water splitting performance was demonstrated when they were utilized as photoanodes. Particularly, the highly porous 20-h Ag3PO4 photoanode presented a photocurrent density of around 4.32 mA/cm2, which is nearly three times higher than that of the non-porous 1-h Ag3PO4 photoanode (1.48 mA/cm2) at 1 V vs. Ag/AgCl. Moreover, superior stability of the 20-h Ag3PO4 photoanode has also been confirmed by the 5-h successive PEC water splitting experiment. Therefore, both the scalable and facile fabrication method, and considerable photoactivity and stability of these Ag3PO4 photoanodes together suggest their great potential for efficient solar-to-fuel energy conversion and other PEC applications.

Keywords: orthophosphate; ionic conductors; porous material; water oxidation; scalable synthesis


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Photoelectrochemical (PEC) water splitting represents a promising strategy for low-cost and environmentally friendly production of clean and renewable hydrogen energy from solar light and water.1 A key issue in this area is to fabricate nanostructured photoelectrodes with desirable properties such as wide band absorption, high carrier mobility, long carrier lifetime, good structural stability, and environmental benignity.2 The water splitting process involves two half-cell reactions, the hydrogen evolution reaction (HER) on photocathodes and oxygen evolution reaction (OER) on photoanodes. The latter is usually more complex and sluggish since it requires the removal of four electrons and two protons from two H2O molecules which are twice than those for HER. Silver phosphate (Ag3PO4), naturally crystallizing in a P4̅3n body-centered cubic structure, is an n-type semiconductor with a direct bandgap of 2.43 eV and an indirect bandgap of 2.36 eV, ensuring that Ag3PO4 absorbs solar energy of the wavelength shorter than 530 nm.3 This material has received considerable attention because of its versatile photooxidation applications such as OER catalysis and oxidation of organic pollutants,4−7 which can be traced back to as early as 1988 when Tennakone and co-workers reported the first photocleavage of water using Ag3PO4.8 Since the valence band edge of Ag3PO4 intrinsically locates at about 2.85 V vs. reversible hydrogen electrode (RHE),4,9 which is much higher than the potential of the O2/H2O redox couple (1.23 V vs. RHE), the photogenerated holes in the valence band of Ag3PO4 could have sufficient energy to oxidize water. In recent years, Ye and co-workers demonstrated that Ag3PO4 as a photoanode material, with the presence of AgNO3 as a sacrificial reagent for photogenerated electrons, exhibited even much higher photoactivity toward OER than WO3 and BiVO4, which are both well-recognized OER photocatalysts with high efficiencies.10–12 However, still not much work has been reported on using Ag3PO4 as a photoanode material for PEC water splitting mainly due to their shortcomings such as relatively high electron-hole recombination rate and poor structural stability.13 Microcrystalline films usually exhibit better stability than nanostructures-based layers when serving as photoelectrodes.14 Nevertheless, the increase in particle/crystal size of the photocatalysts would undesirably lead to the decrease in active surface areas and therefore would reduce the photocurrent densities of the photoelectrodes. In this regard, introducing porous structures into microcrystalline semiconductor films seems to be an effective strategy to achieve both improved photoactivity and -3ACS Paragon Plus Environment


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stability. Recently, porous structures have been demonstrated to be able to improve the performance of photocathode materials such as NiO15 and Cu2ZnSnS416, and photoanode materials such as α-Fe2O3,17 WO318 and fluorine-doped rutile TiO219. In this work, we prepared large-area (4.5 × 4.5 cm2) and solution-processed Ag3PO4 microcrystalline films with tunable porosity controlled by the reaction time as novel photoanodes for PEC water splitting. Enhanced water oxidation activity and stability were demonstrated by the Ag3PO4 porous films with the optimized porosity obtained for extended reaction times (≥ 20 h), suggesting great potential of these porous Ag3PO4 photoanodes to support efficient solar-to-fuel energy conversion and PEC applications. The Ag3PO4 microcrystalline films were grown on silver substrates via a room-temperature air-exposed solution process without any post-treatment (e.g., annealing) process. Figure 1a displays a schematic illustration of the origin of the porosity of the Ag3PO4 films — induced by the release of H2 bubbles. Briefly in the reaction, pieces of 4.5 × 4.5 cm2 Ag substrates were immersed into a mixture solution containing NaH2PO4 as the PO43+ source and H2O2 as the oxidation agent. Ag3PO4 was then generated on the substrate from the chemical reaction described as follows: 2 NaH2PO4 + 2 H2O2 + 6 Ag → 2 Ag3PO4 + 2 NaOH + 2 H2O + H2↑ Figure 1c−e and supplementary videos 1−3 together show the gas release phenomenon at the time points of 5 min, 5 h as well as 24 h during the reaction. Overall, it can clearly be observed that as the reaction proceeded, a large amount of H2 bubbles were generated from the reaction and released from the surface of the substrates. Specifically, at the initial stage of the reaction (i.e. 5 min), the size of the gas bubbles turned out to be quite large, suggesting that the reaction was rather violent at this time due to the strong oxidation activity of hydrogen peroxide. As the time reached 5 h, the size of the bubbles decreased obviously, making it look more like mist rather than bubbles, indicating that the reaction became milder at this stage owing to the decreased concentration of reactants. Finally, the amount of the mist-like bubbles released from the surface of the substrate decreased when the reaction time reached 24 h as can be understood from the gradual exhaustion of reactants. The digital micrographs in Figure 1b reveal that after the reaction, the color of as-obtained Ag3PO4 films were close to dark yellow, revealing their


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possibly good photoactivity when serving as photoelectrodes. The digital micrographs of all samples are shown in Figure S2 of the supporting information (SI), demonstrating the color change more completely. To determine the component and phase purity of the obtained porous films on the silver substrates, the XRD pattern and XPS spectra of the samples were recorded, as exhibited and discussed in Figure S3 of the SI. The morphologies of the obtained Ag3PO4 films on silver substrates are revealed by the SEM images of Figure 2 and Figure S5. Typically, the appearance of the Ag3PO4 film obtained after 1 h of the reaction (Figure 2a) looks similar to other previously reported solution-processed semiconductor microcrystalline photoelectrodes like α-Fe2O320 and Cu2O21 with hardly no porous structures on the surface being identified. Nevertheless as the reaction proceeded till 2 h long, the surface of Ag3PO4 became much rougher. This roughness was ascribed to the etching effect of the H2 gas bubbles released (Figure 2b). Further, when the reaction time was extended to 5 h, 9 h, 20 h and finally 24 h, distinct interconnected porous structures could clearly be distinguished from the SEM images of the samples (Figure 2c−f). The porous structures might be resulted from the continuous formation and release of H2 bubbles during the chemical reaction generating Ag3PO4 from the Ag substrate and PO43+ in the solution. Cross-sectional SEM images displayed in Figure S5 of the nearly non-porous 1-h Ag3PO4 film and highly porous 20-h sample further help to confirm the porosity degree of the films. Besides, from the cross-sectional SEM images, it is also found that the thickness of the Ag3PO4 film grown on silver foils was almost independent with the reaction time when it was longer than 1 h, as the thicknesses of both the 1-h and 20-h Ag3PO4 films were just similar to each other. The small nanoparticles (i.e. brighter white dots) which could be observed in Figure 2a−b might be the precipitation of silver from Ag3PO4 taking place during the violent chemical reaction at this stage. More detailed reasons for such silver precipitation in short-reaction-time Ag3PO4 samples are discussed in the SI. As the reaction became milder for longer reaction times, the amount of such white dots on the surface seems to decrease, as seen in Figure 2c−f. In summary, the highly porous structures of the Ag3PO4 films obtained with relatively longer reaction times (9 h, 20 h and 24 h) should have the potential to increase the active surface areas and, hence, enhance the light absorption and subsequent PEC water oxidation activity.



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Raman spectra of the samples, as displayed and discussed in Figure S6 of the SI, confirm the existence of [PO4] units and O−P−O bonds in the sample materials. The observation of multiple resonant Raman peaks indicates that the synthesized Ag3PO4 films possessed favorable optical quality.22 In addition, the slightly increased intensities of the characteristic Raman peaks of the samples obtained with extended reaction times suggest a gradual improvement of the crystallinity of Ag3PO4 as the reaction proceeded. DRS spectra of the obtained Ag3PO4 films, as shown in Figure 3, indicate that as the extended reaction time brought about promoted porosity, the Ag3PO4 films grown on silver substrates demonstrated decreased reflectance and hence increased absorption over the whole ultraviolet–visible light band. This might be attributed to the enhanced light confinement within the interconnected pores of the obtained Ag3PO4 films,23 and consequently lead to improved light capture capability for PEC water splitting. The absorption edges of all Ag3PO4 samples turn out to be close to each other at around 530 nm, which is consistent with previous related reports.3–4,9 Besides, a small peak could be identified in the wavelength range between 400 and 450 nm, as indicated in blue in Figure 3. This small contribution may come from the scattering of precipitated silver nanoparticles. It seems that as the reaction time increases from 1 h to 24 h, the intensity of this peak decreases slightly. This phenomenon has also been confirmed by the optical scattering microscopy of the Ag3PO4 films (Figure S7) since the number of the blue–green scattering dots originating from silver nanoparticles decreases in the samples obtained with longer reaction times. These results imply that when longer reaction times were used, less silver nanoparticles would have precipitated on the surface of Ag3PO4 during the reaction, which is consistent with the SEM observation results. The enhanced light absorption of the obtained porous Ag3PO4 films together with their intrinsically high photoactivity suggest the potential of the porous films as highly efficient photoanodes for PEC water oxidation. Figure 4a displays the applied bias-dependent photocurrent densities of all fabricated Ag3PO4 photoanodes together with the dark current densities. It is clear that as the reaction time increased from 1 h to 20 h, the photocurrent densities of the Ag3PO4 photoanodes increased. The 20-h Ag3PO4 photoanodes presented a photocurrent density of about 4.32 mA/cm2 at 1 V vs. Ag/AgCl, which is almost three times higher than that of the 1-h Ag3PO4 photoanode (1.48 mA/cm2). This enhancement -6ACS Paragon Plus Environment

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could be attributed to the boosted active surface areas and also light capture ability of the highly porous samples than non-porous samples. The higher incident photon-to-electron conversion efficiency (IPCE) profiles of the 20-h Ag3PO4 photoanode than 1-h Ag3PO4 photoanode, as shown in Figure S8, suggest the higher external quantum efficiencies (EQEs) of the porous photoanodes than non-porous ones, which have made contributions to the higher photocurrent densities in Figure 4a. As has been discussed before, the decrease in the amount of Ag nanoparticles on the surface of Ag3PO4 as the reaction time increased may also have a positive effect on the photocurrent density. The Ag nanoparticles at Ag3PO4/electrolyte interfaces could easily attract photoelectrons due to their superior electron affinity and serve as the recombination centers for the photogenerated electron/hole pairs and, therefore, lead to the loss of photoholes entering water oxidation reaction. It could be observed from Figure 4a that the dark current densities of Ag3PO4 photoanodes also increased gradually together with the photocurrent densities. This phenomenon here is similar with previous reports,4,13 suggesting that Ag3PO4 also have electrocatalytic activity. Therefore, the dependences of Jlight – Jdark values (i.e. photocurrent density minus dark current density) on the applied bias for all the Ag3PO4 photoanodes have been calculated to verify the contribution of photocatalytic activity in the as-recorded photocurrent densities. Figure S9 shows that the Jlight – Jdark values of different Ag3PO4 photoanodes give the same trend as that of Figure 4a, thus confirming that photocatalytic effect has indeed played a dominant role in generating the photocurrent. The slight decrease in the photocurrent density from the 20-h to 24-h Ag3PO4 photoanodes might be originated from the increased interfacial charge transfer impedance between Ag3PO4 surfaces and the electrolyte solution. This effect has been evidenced by the electrochemical impedance spectroscopy (EIS) Nyquist plots exhibited in Figure S10. On one hand, the relatively low interfacial charge transfer resistance (RCT) and thus high conductivity of all these Ag3PO4 photoanodes could be regarded as one reason for the high current densities observed in Figure 4a. On the other hand, the increased interfacial RCT derived from the increased porosity induced an increase in the density of non-conductive voids in the Ag3PO4 films obtained with longer reaction times may have resulted into the decreased current density from the 20-h to 24-h porous Ag3PO4 photoanodes.



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Besides the high photocurrent density, it is notable that the porous Ag3PO4 photoanodes obtained with prolonged reaction times (≥ 20 h) also demonstrated better stability than non-porous ones, which could be known clearly from a 1-h successive PEC water splitting test carried out at 1 V vs. Ag/AgCl (Figure 4b). The promotion in stability of the porous Ag3PO4 photoanodes may be caused by the pores inside the Ag3PO4 layer because the porous structures could help to release the stress at the near-surface region so that could help to keep them more stable than non-porous Ag3PO4 according to previous studies.24–26 SEM images of the 1-h and 20-h Ag3PO4 photoanodes after experiencing the 1-h PEC water splitting tests, shown in Figure S11, clearly confirm that the continuous water oxidation reaction on the surface of the 1-h photoanode has caused a strong surface erosion of the originally non-porous Ag3PO4 film (Figure S11a–b), leading to a non-negligible loss of active materials and decrease of the photocurrent density. However, the highly porous morphology of the 20-h Ag3PO4 photoanode (Figure S11c–d), on the contrary, remained essentially the same after the 1-h stability test, ensuring the maintenance of the high photocurrent density. The 20-h porous Ag3PO4 photoanode, moreover, displayed considerable stability even when the stability test was further extended to 5 h, with its surface morphology, phase composition, and chemical surface state maintaining essentially unchanged (Figure S12). The Faradaic efficiencies of the 5-h successive water splitting reaction using the 20-h porous Ag3PO4 photoanode are all calculated to be near 100% based on the H2 evolution amount detected utilizing a gas chromatography (GC) setup (Figure S13), which further confirm the stability of the 20-h porous Ag3PO4 photoanode in relatively long-time water splitting reactions. Overall, the high current density and improved stability manifested by the 20-h Ag3PO4 photoanodes suggest great potential for highly efficient PEC energy conversion. In summary, we present large-area (4.5 × 4.5 cm2) and solution-processed Ag3PO4 microcrystalline films with tunable porosity controlled by the reaction time as novel photoanodes for PEC water splitting. Enhanced light absorption abilities were demonstrated by the obtained Ag3PO4 films with optimized porosity originated from prolonged reaction times (≥ 20 h), which have further improved the water oxidation performance of the photoanodes made of the porous Ag3PO4 films. Specifically, the highly porous 20-h Ag3PO4 photoanode presented a photocurrent density of about 4.32 mA/cm2, which is -8ACS Paragon Plus Environment

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nearly three times higher than that of the non-porous 1-h Ag3PO4 photoanode (1.48 mA/cm2) at 1 V vs. Ag/AgCl. Furthermore, superior stability of the 20-h porous Ag3PO4 photoanode was also revealed by a 5-h successive PEC water splitting test. Overall, the scalable and facile fabrication method, as well as considerable photoactivity and stability of the porous Ag3PO4 microcrystalline photoanodes, together suggest many exciting opportunities of the Ag3PO4 porous films for use in efficient solar-to-fuel energy conversion and other PEC applications.

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Acknowledgements

This work was supported by Core-to-Core Program (Advanced Research Networks Type A) of Japan Society for the Promotion of Science (JSPS), and Japan (JSPS)–Korea (NRF) Bilateral Program. A part of this work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by "Nanotechnology Platform Project" (No. 12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. Besides, Q. Cao acknowledges the support of China Scholarship Council (No. 201506100018).

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Supporting information

The Supporting Information is available free of charge on the ACS Publications website at DOI: ..., including the supplementary videos 1–3, experimental details, supplementary Figures S1–S13, as well as additional discussions.

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Figure 1. (a) Schematic illustration for the gas bubbles release-induced etching into porous structures during the preparation of microcrystalline Ag3PO4 films; (b) Digital photography image of the original silver foil substrate (4.5 × 4.5 cm2) and the finally obtained Ag3PO4 film grown on silver after 24 h of the reaction; (c–e) Digital photography images of the chemical reaction after immersing the silver foil into the mixture solution for 5 min (c), 5 h (d) and 24 h (e), respectively. Corresponding videos of the reaction stage in panels (c–e) can be found in the supporting information.


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Figure 2. SEM images of the obtained porous Ag3PO4 photoanodes after (a) 1 h, (b) 2 h, (c) 5 h, (d) 9 h, (e) 20 h and (f) 24 h of the reaction. Inserts in each panel are SEM images recorded at a higher magnification for each sample.



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Figure 3. UV–visible DRS spectra of the synthesized Ag3PO4 microcrystalline photoanodes grown on silver substrates with different reaction times.


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Figure 4. PEC water oxidation performance of the obtained porous Ag3PO4 microcrystalline photoanodes: (a) linear sweep voltammetry (LSV) curves of all the photoanodes fabricated using different reaction times. The solid lines and dashed lines stand for the photocurrent and dark current densities, respectively; (b) Current density−time (J−t) curves of the porous Ag3PO4 photoanodes fabricated using 1 h, 20 h and 24 h reaction times recorded during 1-h continuous PEC water splitting tests at an applied potential of 1 V vs. Ag/AgCl.



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Graphical abstract


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Facile and Large-Area Preparation of Porous Ag3PO4 Photoanodes

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