A Facile Chemical Solution Transportation for Direct Recycling Iron

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A Facile Chemical Solution Transportation for Direct Recycling Iron Oxides Rust Waste to Hematite Films Jiaji Wang, Lei Li, Yan Lei, Yange Zhang, Pinjiang Li, Congxu Zhu, Ke Wang, Zhi Zheng, and Xiaogang Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02581 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 21, 2018

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ACS Sustainable Chemistry & Engineering

A Facile Chemical Solution Transportation for Direct Recycling Iron Oxides Rust Waste to Hematite Films Jiaji Wang†, Lei Li†‡, Yan Lei†§, Yange Zhang†§, Pinjiang Li†§, Congxu Zhu†§, Ke Wang†, Zhi Zheng†§ and Xiaogang Yang*†‡§ †. Key Laboratory for Micro-Nano Energy Storage and Conversion Materials of Henan Province, College of Advanced Materials and Energy, Institute of Surface Micro and Nanomaterials, Xuchang University, 88 Bayi Road, Xuchang, Henan 461000, China ‡. Henan Key Material Laboratory, North China University of Water Resources and Electric Power, 36 North 3rd Ring Road, Zhengzhou, Henan, 450045, China §. Henan Joint International Research Laboratory of Nanomaterials for Energy and Catalysis, Xuchang University, 88 Bayi Road, Xuchang, Henan 461000, China * Corresponding author: Xiaogang Yang, Email: [email protected]

ACS Paragon Plus Environment

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Abstract: Convenient and economic recycling of the environmental solid waste is one of the most essential and challengeable issues for the sustainable development of modern society. In this work, for the first time, a chemical solution transportation method has been developed to convert the iron oxides and related rust into (photo)electroactive hematite films on conductive Fdoped SnO2 substrate in a hydrothermal system. We found that the zeta potential of the substrate and the precursor particles could be modulated by controlling the NaNO3 electrolyte concentration. The decrease of the positive surface charge densities caused a less adsorption of Fe(C2O4)33- intermediates on the iron oxide precursors, while the intermediates adsorption at the F-SnO2 substrate increased relatively. As a result, the small hematite nanocrystals were deposited on the substrate through an island nucleation mechanism, which transferred into a continuous dense hematite film with transportation time increase. These hematite films exhibited high photoelectrochemical and electrochemical activities in water oxidation for hydrogen generation. This direct chemical transportation method could be a promising solution for sustainable recycling of metal rust waste.

Keywords: Iron rust, hematite film, chemical solution transportation, nucleation, water splitting


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Introduction Current modern human civilization is undoubtedly based on huge consuming of energy and materials, for instance the fossil fuels in terawatt scale and iron in giga tons scale.1-2 Due to the wide applications of the iron and steel components, large amount of hydrated iron oxide rust wastes are generated from the iron corrosion reactions with the environmental oxygen and water.3 On the other hand, iron oxides, such as FeO, Fe3O4 (magnetite), α-Fe2O3 (hematite), γFe2O3 (maghemite), α-FeOOH (goethite) and other oxides, are one group of the important functional materials widely used in pigments,4 magnetics,5-6 catalysts,7-11 photocatalysts,12 food additives,13 lithium ion batteries14 and heavy metal absorbents.15-16 Currently, the world steel industries already applied high utilization of the steel waste as raw materials, the high temperature recycling process is energy intensive.17 Alternatively, Zhu et al. developed a twostep hydrothermal process to prepare hematite nanospheres for batteries, where the iron rust was initially dissolved into nitric acid to generate a nitrate solution.18 Recent studies suggested that the hematite film-like materials can be applied for photoelectrochemically water splitting to hydrogen fuel.19 These hematite films can be prepared through various methods, such as e-beam evaporated deposition, chemical vapor deposition, atomic layer deposition, sol-gel deposition, spray pyrolysis, electrophoretic deposition and hydrothermal methods.20 Most of these methods require high purity chemical reagents as precursor. Therefore, the study in economically recycling solid waste into novel functional materials at low temperature is of great significance for the sustainable development.21-23 Both the novel recycling reaction and mechanism for hematite film deposition are required. Inspired by the nature geological mineralization process, the nucleation and growth of the aragonite coral was facilitated by the surface proteins and ions.24 By adjusting the surface energy


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and/or nucleation energy,25 new surfaces were formed to replace the substrate through three typical mechanisms.26 For instance, strong interaction of biphenyl dicarboxylic acid with the step edges on Cu caused an edge passivation and impeded the mass diffusion and film growth.27 Through tuning the surface energies of glass substrate, the nucleation and crystallization of small pharmaceutical molecules or protein could be improved.28 In addition, the biomineralization of CaCO3 films required a polymer substrate for enhanced ion absorption and nucleation.29 These examples suggested that the surface ion adsorption played a very important role for the nucleation and film growth. Because the previous hydrothermal methods displayed many advantages (e.g. lower temperature, environmental benign solvent, good solubility for many metal salts and minimizing green-house gas) in environmental waste treatments.30-33 In this work, we applied the hydrothermal transportation to convert iron oxides and rusts waste into hematite films, which can be used as anode materials for (photo)electrochemical hydrogen generation. There are two important challenges to be solved: the first one is the iron species transporting to the reaction sites on the fluorine-doped tin oxide (FTO) substrate, which can be achieved by an oxalate complex;34 the second one is the surface adsorption and nucleation, which can be modulated by adjusting the surface charge densities with NaNO3 electrolyte. Our previous work34 focused on the epitaxial hematite deposition on the various substrates from single soluble precursor of ferrioxalate. For the direct solution deposition from iron oxide solid waste precursors, few effective methods have been reported. We simply varied the nitrate concentration to tune the zeta potential of the precursor particles and FTO substrates, the surface anion adsorption, nucleation and growth can be conveniently controlled for the hematite films. As many researches are focusing on the water splitting reaction catalysts35-37 or oxygen evolution reaction catalysts38 for


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solar fuels or metal-air batteries, the prepared hematite films are evaluated as promising photoanode and electric anode materials, respectively. Our hydrothermal based chemical transportation method can directly convert iron oxide rust into useful hematite photoanode materials or electrocatalytic anode materials under relative lower temperature without producing large amount of green-house gas, which will benefit our sustainable society. Experimental sections All the chemical reagents, such as Fe2O3, FeOOH, Fe3O4 powders, oxalic acid (H2C2O4), sodium nitrate (NaNO3), ammonia and hydrogen peroxide are analytical grade and commercially purchased from Sinopharm Chemical Co. (Shanghai). The iron oxide rust was collected from iron coil (Q195) and stainless-steel wires (316) used in the campus of Xuchang University. Typically, the FTO substrates (Huanan Xiangcheng Sci. Co, Shenzhen) were ultrasonically washed by NH3-H2O2-H2O, and deionized water, before dried in ambient air. Then, the mixed solution of 20 mL containing of 0.5~3 mol/L NaNO3 solution, 0.05 mol/L H2C2O4 and 3 mmol hematite powders were added in a PTFE-lined inner. After the hematite particles settled down, a clean FTO substrate was carefully immersed into the solution leaning on the inner wall with the conductive side facing down. Then the inner was sealed in a stainless-steel autoclave without agitation, and heated in oven at a temperature of 150-180 °C for 12 h. After the autoclave cooling down, a brown-color film on the FTO side was obtained. For the iron rust precursor, Fe3O4 or FeOOH powders, similar process was employed by substituting the Fe2O3 powder, while other reaction parameters were kept as constants. The nucleation, morphology and size of the obtained hematite films and the iron rust were characterized on a field-emission scanning electron microscopy (FE-SEM: FEI Nova, NanoSEM 450) coupled with an EDS analyzer (Oxford Instrument). The high-resolution TEM image of the


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hematite films were obtained on a transmission electron microscope (FEI, Tecnai G20 S-Twin) with an accelerating voltage at 200 kV. The Raman scatter spectra were collected on a Renishaw inVia Raman microscope, excited with green laser at 532 nm. The element binding energies of the sample surface were investigated on an X-ray photoelectron spectroscopy (XPS: Thermo Scientific Escalab 250Xi using Al Kα as excitation). The surface potentials of the Fe2O3 colloids and SnO2 colloids in NaNO3 were analyzed on a Zetasizer system (Malvern, Nano ZS) in acidic solution (pH=3, adjusted by dilute HNO3). The optoelectronic responses of the films are also recorded on a home-made transient surface photovoltage (TSPV) analyzer (Brilliant Eazy Quantel Nd:YAG nanosecond laser at 355 nm coupled with Tektronix TDS 3054C oscilloscope) at the ambient conditions.39 The films for photoelectrochemical measurement were annealed in air at 700°C for 25 min and a quick cooling treatment. Additionally, the films used for electrocatalytic water oxidation were annealed in air at 300-500 °C. The photoelectrochemical and electrochemical analysis were carried out in 1 M NaOH (pH=13.5) with three-electrode configuration (Pt wires as counter electrode, and Hg/HgO as reference electrode), where the current and potential were controlled with a potentiostat (660E, CH Instrument, Shanghai). The potential

versus

reversible

hydrogen

electrode

(RHE)

was

obtained

from

Vvs.RHE=Vvs.Hg/HgO+13.5×0.059+0.098. The simulated solar simulator (AM 1.5G, Model 94023, Newport) was utilized as the light source (100 mW/cm2). Results and discussion Due to the acidic atmosphere, rain exposure and periodic sunlight illumination, even a stainless-steel can be oxidized and form the flaky iron oxide, hydroxide and hydrate through a corrosion process over years.40-41 The product is commonly named as iron rust. In Figure 1, we demonstrated that the iron rust raw materials could be successfully converted to hematite films


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on FTO substrate. The loosen brownish rust on the iron wires can be observed in Figure 1(a), which was then applied as the iron oxide precursors in our chemical transporting deposition. In the EDS spectrum (Figure 1b), the Fe, O, Mo and Al elements can be conspicuously identified (molar ratio of Mo/Fe/O equals to 0.04/1.00/1.57), which was ascribed to the stainless-steel 316 or Q195 steel. In Figure 1(c), the rust exhibited flake-like morphologies with the size of several hundred nanometers in plane and tens nanometers in thick. When the rust was chosen as the precursor in the solution transportation process, the produced brown-reddish films in Figure 1(d) exhibited uniform smooth surface on the substrate. From the EDS spectrum of the film in Figure 1e, the Fe and O are ascribed to the hematite films, while the Sn, Na and Ca are mainly attributed to the F-doped SnO2 and soda-lime glass (containing SiO2, Na2O, Na2CO3, Al2O3 and CaO) substrates. For the peak of Al, it can be ascribed either to the solid waste precursor or to the glass substrate (will be confirmed by the XPS later). For the peak of Au in Figure 1b and 1e, it is due to the conductive layer sputtered for SEM observation. In Figure 1(f), the prepared film showed a continuous surface composed of many spherical nanoparticles (~200-300 nm) and a few small nanoparticles (~100 nm). This confirmed the rust could be successfully transported and deposited on the FTO substrate. The inset in Figure 1(f) and the Figure S1 showed the HR-TEM image of the hematite film, where the distance between the two consecutive lattice planes was ca. 2.68 Å. This value corresponded to the (104) plane of α-Fe2O3 (2.70 Å, in JCPDS card, No. 33-0664). Moreover, the different domain sizes indicated that the surface nucleation may not be formed in the same stage. We also noticed that the Mo impurities in the rust did not transport to the iron oxide films.


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Figure 1. Iron oxide films deposited from the rust precursor through the chemical solution transportation: (a) optical image of iron rusts, (b) EDS spectrum and (c) SEM image of the rusts; (d) optical image, (e) EDS spectrum and (f) SEM image of the as-deposited iron oxide films. Inset in (f) is the HR-TEM image of the hematite film peeled off from FTO substrate. The phase purity and surface compositions of the films are revealed by the Raman and XPS spectra. When we used other analytical grade commercial chemical reagents (Fe3O4, FeOOH or Fe2O3) as precursors, all the products are brownish smooth films without significant deposition rate difference. In Figure 2(a), the prepared iron oxide films and Fe2O3 colloids are characterized by Raman spectra. The Raman peaks at 226 cm-1, 246 cm-1, 295 cm-1, 411 cm-1, 500 cm-1 and 612 cm-1 corresponded to the typical active modes (A1g and Eg) of hematite.34, 39 We did not observe any other iron oxides. The broad peak at about 660 cm-1 is owing to the defects in the asprepared films. For the films prepared from the iron rust, this defect related Raman peak is relatively stronger than other pure chemical precursors. Conspicuously, all the deposited films exhibited very similar Raman scatter peaks with very small position shifts, which confirmed the obtained films were hematite phase (Corundum structure). Although the chemical state of the Fe


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element for Fe3O4 is not same as Fe2O3 or FeOOH, this has less little influence on the crystalline phase of the final produced films. This is possibly due to the same Fe intermediates that contributes to the film deposition.

Figure 2. Raman spectra (a) and XPS survey spectra (b) of the hematite films on FTO substrates prepared from various precursors. Moreover, we applied the XPS technique to investigate the surface elemental composition on the iron oxide films deposited from rust, Fe2O3 and FeOOH precursors. In the XPS survey spectra (Figure 2b), all the C1s peak was set at 284.8 eV as reference. The peaks of the Fe3p, Fe3s, Fe2p3/2, Fe2p1/2 band and Auger peak of Fe were clearly indexed in the Figure 2(b), which were attributed to the hematite film. While, the O2s, O1s peaks and Auger peak of oxygen O(A) could be assigned to the hematite. No peak from Al, Ca or Na from the obtained films could be detected. We also noticed that there were small Sn3d or Si2s signals in the film


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prepared from iron rust, which might be due to the edge parts (FTO), or glass substrate uncovered by the hematite film. The chemical transportation of Sn or Si species during the hematite deposition is less likely to happen, since none of these peaks has been detected on the films from analytical grade FeOOH or Fe2O3 precursors (Figure 2b). Moreover, because no Sn or Si impurities were observed in iron oxide rust from Figure 1b, the interpretation that the Sn or Si signal originated in the FTO substrate sounds more reliable. From all the three spectra, we have not detected any Mo signals, indicating that the Mo oxides cannot be transported or deposited under this condition. This is consistent with the EDS results in Figure 1(e). The peaks at 710.7711.1 eV and 535.3-535.5 eV are assigned to the Fe2p3/2 and O1s bands, respectively, revealing the Fe3+ and O2- states of the deposited films. Therefore, we could conclude the films are composed of α-Fe2O3. Table 1. Hematite film deposition in various solutions at 155 °C for 12 h. NaNO3

H2C2O4

α-Fe2O3

mol/L

mmol/L

mmol

1

3

0

0.5

7.48→5.75

No film

2

0

50

0.5

1.44→3.00

No film

3

0.01

50

0.5

1.40→2.88

No film

4

0.15

50

0.5

1.15→4.57

Very thin film

5

3

50

0.5

0.69→6.04

Continuous dense film

6

10

50

0.5

1.25→5.45

Less & discrete film

Entry

pH

Film deposition

We then investigated the influence of the hydrothermal conditions on the hematite film deposition. The compositions of the transportation solution were presented in Table 1. No films could be obtained from the α-Fe2O3 precursor, when either NaNO3 or H2C2O4 is absent in the


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Entry 1 and 2. The low concentration of oxalic acid at 50 mmol/L in 20 mL solution was fixed for the film deposition, and we studied the film deposition by increasing the concentration of the sodium nitrate from 0.01 mol/L to 10 mol/L. Interestingly, we found that the film deposition rate increased at a concentration of 0.15 mol/L for a uniform hematite film. The optimized concentration of sodium nitrate was about 3 mol/L. When the [NaNO3] increased to 8 mol/L and 10 mol/L, the deposition became very difficult and slow, where the films were not continuous but covered with large discrete island particles. Other nitrate, such as ammonium nitrate, potassium nitrate or lithium nitrate showed similar influence, indicating the nitrate anion rather than cations significantly contributed to the surface nucleation and film growth. The pH of the solution slightly increased after the hydrothermal deposition. Higher concentration of H2C2O4 (e.g., 100-300 mol/L) can also be applied for a good hematite film deposition. In this work, the molar ratio of the H2C2O4/Fe2O3 was set 2/1 and lower than the theoretical ratio 3/1 of the Fe(C2O4)33-. This suggested that the H2C2O4 is insufficient for Fe2O3 precursors to generate a pure solution. The reaction may not be regarded as a completely serial-conversion from Fe2O3 to Fe(C2O4)33-, and to Fe2O3 at the substrate in a single soluble precursor process. To explore the nucleation and films growth mechanism, we scrutinized the typical film deposition (entry 5 in Table 1) at 155 °C by quenching the deposition after a specific interval. From the images of the films deposited for 1 h (in Figure 3a), there were many small nanoparticles with sizes in tens of nanometers appearing at the terrace or kinks of FTO substrate. It was worth noting that all the films were bare without any conductive coating layer (e.g., Au). Inset (a1) at the top-right was the bare FTO substrate without any nucleation. And inset (a2) at the bottom-right displayed the initial nucleation crystals (marked in brown color). This clearly demonstrated an island nucleation mechanism. When the deposition time was prolonged to 2 h,


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the small nucleation crystals would continuously grow into short rod-like structures in Figure 3(b), further confirming the island-growth model. We also found that some smaller nucleation crystals were formed besides these larger crystals, indicating the surface nucleation occurred during the whole film deposition. When the deposition time increased to 3 h, the FTO substrate was almost continuously covered by the hematite film (particle size ~120 nm) in Figure 3(c), no discrete Fe2O3 particles or sharp morphology of the FTO surface (a1 inset in Figure 3a) could be observed. This indicated an island plus layer nucleation in this stage. When the deposition time increased to 6 h, the FTO surface are covered by the conformal film with spherical particle size at ~200 nm in Figure 3(d).

Figure 3. Nucleation and film growth of the hematite on FTO substrate after various hydrothermal intervals: (a) 1 h, a1 inset in (a) is the bare FTO substrate without any nucleation; a2 inset with brownish color in (a) demonstrates the small Fe2O3 nucleation on FTO substrate;


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(b) 2 h, inset with brownish color is the larger hematite particles; (c) 3 h and (d) 6 h, covered by hematite nanoparticles film. The images are taken without any Au conductive coatings. As we have proved the pH of the solution and the surface isoelectric point of the substrate significantly influenced on the deposition behavior by single molecule precursor in other work,34 the surface charges of the substrate drastically affected the surface anions absorption during the deposition. Thus, we speculated that the electrolyte salts effect may influenced on the surface charges or zeta potentials, which was analogous to those surface modifications by the charged polymers. Different from those previous works, there are precursor particles with high surface area and strong surface charge, which can cause competitive adsorption of the anions from the solution. For the surface charge adsorption analysis, the zeta potential of the hematite and SnO2 colloids are measured, due to the difficulty in the quantitively evaluation of the corresponding films. When the concentration of sodium nitrate increased from 0.001 mol/L to 0.2 mol/L, the zeta potential of the hematite colloids quickly decreased from 60 mV to 10 mV in Figure 4(a); while the zeta potential of the SnO2 colloids decreased from 17 mV to 5 mV in Figure 4(b). To our knowledge, the isoelectric point (7.2-8.8) of the hematite is higher than 4-5.5 of the SnO2.34, 42

When the pH value of the solution is at about 3, both the hematite and SnO2 particles

displayed a positive charge at surface (e.g., Surf-OH2+). The decrease of the zeta potential on the hematite and SnO2 particles with increased NaNO3 concentration is consistent with the Derjaguin-Landau and Verwey-Overbeek (DLVO) theory.43 Under this condition, the interaction between the two particles is influenced both the repulsive electrostatic forces and the attractive Van der Waals forces.44 The increased electrolyte concentration significantly compacts the thickness of the electric double layer, while the zeta potential decreases.45 With the concentration of NaNO3 increment, the stability of the hematite particles drastically decreased and formed


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some precipitation in short while, revealing the reduced electrostatic repulsion forces. Due to the difficulty in the zeta potential measurement on a less stable colloid dispersion in the region of high NaNO3 concentration, the theoretical zeta potentials on hematite and SnO2 particles are expected to decrease to nearly zero (> 1 mol/L). This predicted trend, where the zeta potential decreased close zero, is in good agreement with the recent literature reports on the hematite particles at higher concentration of NaNO3.46 Obviously, the low zeta potential region (red dashed circles in Figure 4) is favorable for the chemical transportation deposition of hematite films, which is different from the single molecule precursor.34 This also suggested that some new adsorption controlled mechanism must be proposed for better understanding this transportation process.

Figure 4. The relationship between the NaNO3 concentration and zeta potentials of the Fe2O3 (a) and SnO2 (b) colloids in pH=3 solutions (adjusted by dilute NHO3). Dashed lines are simulated according to the DLVO method.43 Red dashed ovals showed the good film deposition range. The film deposition is sensitive to the surface absorption of the intermediate molecule on the substrate. There was no hematite deposition on the glass side, no matter facing up or down in the solution. This is possibly due to the lower isoelectric point (IEP:1.8-2.2 for SiO2)47 and


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neutralized surface, which are coincided with the previous deposition results by using a single molecule precursor.34 Like the charged polymer modified surfaces for better intermediate adsorption, we used sol-gel method to deposit a thin SnO2 layer on the glass slide, the electrostatic attraction of anions by the surface positive charges of the new substrate can be drastically increased compared with the electrostatic repulsion by the negative (or zero) charged glass surface. As a result, a brownish hematite layer could be deposited on new SnO2 layer. This confirmed the surface absorption inherently contributed to the intermediate molecules adsorption and nucleation. We also checked the effect of other anions, for instance the NaCl and Na2SO4, on the nucleation and film deposition. Surprisingly, no films could be deposited on the FTO substrate in very wide concentration range. This indicated the NaNO3 did not simply influence on the ion strength but on the surface adsorption in Stern layer. We checked the zeta potential of the hematite colloid in other electrolytes, and found that the NaCl (3 mmol/L) has almost no different from NaNO3, while the Na2SO4 (3 mmol/L) resulted in a very low zeta potential (4.2 mV) due to its strong divalent absorption effect.48 At the pH=3, the surface of hematite may form Surf-OH2+-SO4H-,49 which resulted in an uncontrollable neutralization of surface positive charge. Thus, the SO42- adsorption on hematite or SnO2 leads to a very low adsorption of other anions. The colloid dispersion quickly generated a precipitation, which further confirmed this wary. Obviously, the optimal NaNO3 causes a moderate decrease of surface positive charges, which keep partial surface adsorption of intermediate anions. Based on these results, a possible chemical solution transportation process can be proposed. Firstly, the iron oxide precursors (e.g., Fe2O3) will adsorb the H+, Na+, NO3- and C2O42- ions in the electric double layer34, 44, 50 and display positive charges in the acidic solution. First, the hydrothermal treatment causes a dissolution of the iron oxide solid to soluble Fe(C2O4)33-


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anions50-51 as intermediate (Equation 1). Next, as the Fe2O3 precursor particles and the F-SnO2 substrate all are positively charged, an adsorption of Fe(C2O4)33- anion forms at the surface SurfOH2+ sites (Equation 2). After that, the surface adsorbed Fe(C2O4)33- anions will decompose into Fe2O3 in situ at the active site (Equation 3) at the decomposition temperature.34 Fe 2 O 3 +6C 2 O 42 − + 6H +  → 2Fe(C 2 O 4 )33− +3H 2 O

(1)

Surf-OH +2 +Fe(C 2 O 4 )33−  → Surf-OH 2+ -Fe(C 2 O 4 )33-

(2)

Surf-OH +2 -Fe(C 2 O 4 )33- +1.5H 2 O  → Surf-0.5(Fe 2 O 3 )-OH 2+ +3(C 2 O 4 ) 2- +3H +

(3)

When the FeOOH was used as precursor, similar dissolution of iron species would generate ferrioxalate anions as Equation 4: 2FeOOH+6C 2 O 42 − + 6H +  → 2Fe(C 2 O 4 )33− +4H 2 O

(4)

Overall, the transportation-deposition process (using Fe2O3 as example) can be formulated as Equation 5: dissolution  → 2Fe(C 2 O 4 )33− +3H 2 O Fe 2 O 3 +6C 2 O 24 − + 6H + ←  deposition

(5)

where the dissolution process mainly take place at the iron oxide solid precursor, the deposition process occurs both at the solid precursors or the substrate, depending on the zeta potentials of their surfaces. When the concentration of the NaNO3 is relative low (1.8 V vs. RHE. This current could be furtherly enhanced when the films were annealed at the temperature of 400 °C for 25 min. Its onset potential negatively shifted to 1.7 V vs. RHE, indicating a overpotential of 0.47 V. Although this value is much lower than the state-of-art oxygen evolution catalysts,57-58 such as the IrOx, a-NiFeOx, a-CoPi, Co3O4,


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the performance can be significantly enhanced when the rust compositions, product morphologies, crystallinity are optimized in the future.

Figure 5. Photoelectric response under transient light of hematite films: (a) TSPV of the sample prepared from Fe2O3 powders; (b) TSPV of the sample prepared from iron rust; (c) photocurrent of the hematite films prepared from Fe2O3 powders after annealing at 700 °C; (d) electrocatalytic water oxidation of the hematite films prepared from iron rust under dark condition, the samples were annealed at 300 °C, 400 °C and 500 °C, respectively. The electrolyte is 1 M NaOH.

Conclusions In this work, we developed a chemical solution transportation method to convert iron oxide rust to hematite films for the first time, using sodium nitrate and oxalic acid as the electrolyte and transporting reagent, respectively. By controlling the hydrothermal parameters such as


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concentration, temperature and pH, we found that the transportation mechanism was highly sensitive to the concentration of NaNO3 electrolyte. The film thickness could be controlled by the deposition time as well. The hematite films prepared from pure iron oxide/hydroxide chemicals demonstrated high photoelectrochemical activity for the water splitting. While for the hematite prepared based on the iron oxide rust, the PEC activity for water oxidation is poor due to some impurities. However, this film displayed promising water oxidation activity in the dark condition as an oxygen evolution catalyst. The methods and materials developed in this work may show potential environmental and sustainable applications in solid waste recycling and in other fields.

ASSOCIATED CONTENT Supporting Information. The HR-TEM image of the hematite film, I-t curve of the hematite photoanode and the X-ray photoelectron spectra (XPS) of Mn2p of the hematite films can be found in the supporting information. AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Funding Sources National Natural Science Foundation of China. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (Project No. U1604121, 61504117 and 21673200). REFERENCES 1.

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Table of Content (TOC)

SYNOPSIS Direct recycling iron based solid waste into applicable hematite films can be a sustainable process.


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Figure 1. 109x52mm (300 x 300 DPI)



Figure 2. 101x135mm (300 x 300 DPI)


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Figure 3. 102x68mm (300 x 300 DPI)



Figure 4. 50x16mm (300 x 300 DPI)


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Scheme 1. 114x64mm (300 x 300 DPI)



Figure 5. 101x67mm (300 x 300 DPI)


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A Facile Chemical Solution Transportation for Direct Recycling Iron

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