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Multilayered approach for TiO2 hollow-shell protected SnO2 nanorod arrays for superior lithium storage Christian G. Carvajal, Sangeeta Rout, Rajeh Mundle, and Aswini K Pradhan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02801 • Publication Date (Web): 19 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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FE-SEM image of SnO2 nanorods surrounded with TiO2 nanotube. Absolute capacity Vs. cycle number for Liion battery. 241x133mm (150 x 150 DPI)

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Multilayered approach for TiO2 hollow-shell protected SnO2 nanorod arrays for superior lithium storage Christian G. Carvajal, Sangeeta Rout, Rajeh Mundle and Aswini K. Pradhan Center for Materials Research, Norfolk State University, 700 Park Avenue Norfolk, VA 23504, USA Abstract: The ability to control the growth of materials with nano-sized precision as well as complex hollow morphology provides rationale for the study of systems comprising of both characteristics. This study explores the design of TiO2 hollow nanotube shells deposited by Atomic Layer Deposition (ALD) on vertically aligned SnO2 nanorods grown by the Vapor-Liquid-Solid technique. The sacrificial template approach in combination with highly conformal coating advantages of the ALD resulted in a highly reproducible method to create high surface area covered by TiO2 protected SnO2 nanorods which are about 60 to 100 nm in diameter and approximately 1 µm in length. ZnO was used as a sacrificial layer to create 30nm gap in between SnO2 nanorods and 10nm of TiO2 shells. Chemical etching of the sacrificial layer was used to create the desired hollow nanocomposite. A coin half-cell battery has been assembled using TiO2 protected SnO2 nanorods as anode electrode and lithium foil as counter-electrode and tested for lithium storage during 70 cycles of charge/discharge in a range of 0.5V to 2.5V. TiO2 hollow shell demonstrated to be a good and robust enhancer for both absolute capacity and current rate capabilities of vertically aligned SnO2 nanorods, also an improvement on cyclic stability is observed. This advanced selfstanding hollow configuration provides several unique advantages for energy storage device applications including enhanced lithiation for superior energy storage performance.

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Corresponding author’s E-mail address: [email protected]


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Introduction: In recent years, hollow nanomaterials have attracted significant attention because of their unique versatile structural properties with different shapes for many applications. Hollow nanostructures have shown convenient applications in drug delivery, catalysis, micro and nano-fluidics, Li-ion batteries, supercapacitors, water splitting etc.[1-6] Until now several synthetic methods have been used to fabricate inorganic nanotubes and other hollow structures; these methods can be classified in two main categories; self-organization methods and sacrificial template methods [7,8]. Self-organization method requires thermodynamic preferred conditions to achieve the formation of hollow nanostructures by the Kirkendall route being the first category. Regarding the second category, the sacrificial template method consists on the deposition of the desired structural material onto a sacrificial template and the removal of the template afterwards, which has been considered more controllable in comparison to Kirkendall route [9, 10]. In this matter, the sacrificial template approach requires a coating technique or deposition method to create hollow structures. One of the most promising deposition methods for high aspect ratio to achieve all-solid-state three-dimensional coatings is the Atomic Layer Deposition (ALD) technique; based on self-limiting organometallic reactions, this technique alternate gasto-solid interaction between the precursors and the depositing film. The reactants are injected into the deposition chamber that allows for surface coating reactions while the by-products and excess reactants are purged by the carrier gas [11, 12]. Extremely conformal films and sacrificial layers can be deposited using ALD on complex nanostructures with precise thickness control that represents a top advantage for executing hollow microstructure and nanostructure design. Several alternative approaches of sacrificial template methods have been also reported in literature such as the use of porous anodized alumina membranes on which the desired material can be deposited followed by the selective etching of the template membrane, inorganic and polymeric nanotubes have been reported in literature by this method [13-15]. This anodized process can be used directly on metallic foil as well to create a porous etching in combination with annealing procedures to promote oxidation of the in-pore nanotubes [16, 17]. Another technique that has been reported is the use of electro-spun polymeric fibers as templates on which the active material can be deposited followed by the complete removal of the sacrificial fiber creating long hollow nanostructures [18, 19]. These techniques have been explored regarding lithium storage properties and they serve as a point of reference for the work described in this report. Semiconductors such as tin dioxide (SnO2) and similar metal-oxide materials like titanium dioxide (TiO2) have gained significant attention for technological innovations because of their lithium storage properties and their non-toxic non-reactive behavior [20, 21]. SnO2 is well known for its potential applications in Li-ion batteries, photo detectors, gas-sensors [22-24]. On the other hand TiO2 possess highly active catalytic surface, which serves as a photo catalyst in solar cells, and it is used in corrosion-protection and Li-ion batteries as well [9, 25]. Other than TiO2; nanocarbon materials such as graphene, carbon nanotubes, and graphene-oxide have been coupled with SnO2 as a capacity enhancer for Li-ion storage with promising results but also with a major disadvantage being the high nanocarbon toxicity and safety [26-28]. TiO2 anodes have shown reversible capacity, high power density and nontoxic behavior, which make this material


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a very safe candidate for application in batteries with the only disadvantage of TiO2 being its poor energy density [29-31]. SnO2 promotes considerably higher lithium storage capacity than graphite which makes it a promising material for lithium storage devices; however, recent attempts in developing high performance SnO2 based Li-ion batteries are encountering disadvantages such as poor cycling stability. One of the big challenges for SnO2 anodes used in batteries is the severe volume expansion and structural pulverization that cause a reduction of the cycling performance in Li-ion batteries [21, 32]. Different approaches involving SnO2 and TiO2 hollow structures have been reported with several techniques to overcome the volume expansion and capacity loss during Li-ion storage like that of the use of anodized alumina membranes for hollow SnO2 nanotubes with TiO2 encapsulation which have been reported with Li storage capacities of ≈ 300 mAh/g [13, 33]. Electro-spun polymeric fiber templates have been used to make SnO2-TiO2 double hollow layers showing capacities of ≈ 600 mAh/g and other non electro-spun double layer nanotubes with capacities ≤ 500 mAh/g [18, 30, 34]. Hollow SnO2-TiO2 core shell nano-spheres show capacities in the range of ≈500-700 mA h/g [35, 36] and hollow shell covered SnO2 nanowires showing a performance of ≈ 600-700 mAh/g in capacity [21]. In the present study, we grow vertically aligned SnO2 nanorods and deposit a ZnO sacrificial layer before the addition of TiO2 as the protecting material. The sacrificial layer is selectively removed leaving a gap in between the starting material and the hollow shell creating a vertical self-standing protected configuration. The novelty of this work belongs to the research line of hybrid TiO2-SnO2 composite materials for battery electrodes and the vertical hollow-shell architecture described here demonstrated the improvement of the absolute capacity and current rate capabilities of the battery compared to other hollow TiO2-SnO2 nanostructures mentioned above, also an improvement on cyclic stability was observed in comparison to the non-protected electrode. Although TiO2 hollow shell provides a contribution to the capacity by acting as a Li storage material the role of TiO2 in this study is mainly related to the hollow gap design, which allows the SnO2 nanorods to expand naturally. The overall improved performance demonstrated a good coupling of both materials in the architecture morphology using the hybrid approach. Furthermore, the capacity contribution of TiO2 was found to be moderate by performing Cyclic Voltametry where no significant contribution from TiO2 was observed on oxidation/reduction peaks related to lithium interaction. The results reported in this research include the mass of TiO2 into the corresponding calculations and the detailed performance analysis is described. Experimental: A piece of silicon of approximately 1cm2 area was used as a substrate; it was cleaned by sonication in acetone and methanol for 10 minutes each. The silicon substrate was dried under nitrogen and this process was repeated three times to ensure appropriate cleaning. Then the growth of SnO2 nanorods was performed using ZnCl2 powder and SnCl2 anhydrous powder both of 99.9% purity (4:1 weight ratio respectively). The powders were ground, mixed, and placed together at the bottom of an alumina ceramic boat with the Si substrate placed on the top of the


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boat. This setup was inserted into a Thermolyne 4800 muffle furnace in the presence of oxygen; the temperature was set gradually to increase to reach 510°C under 10°C/min rate. When the temperature reached 510°C the setup was allowed gradually to cool down to room temperature. During the vapor reaction ZnCl2 acts as inter-space separator between vertical growths of SnO2 nanorods [37]. This process generates ZnO sub products, which are then removed by a chemical bath in hydrochloric acid (HCl) of 0.1M concentration for 10 minutes. 30nm of sacrificial ZnO layer was directly deposited onto the top of SnO2 nanorods by using the Cambridge NanoTech Savannah 100 Atomic Layer Deposition System where diethyl zinc (DEZ or Zn(C2H5)2), Tetrakis (diethylamide)-titanium (TDMA-Ti or Ti(N(CH3)2)4) and water are used as precursors. The deposition of both layers was carried out at 250°C using the exposure-mode procedure, which is known to increase the coverage and growth rate [38, 39]. At first, the evacuation valve is closed and water is pulsed, after 3s the evacuation valve is opened to purge excess water, then the same process is repeated using diethyl zinc (DEZ) instead of water and this process is repeated for 335 cycles to create a 30nm layer of ZnO (0.8 Å/cycle growth rate). The protective TiO2 layer is grown right after by closing the evacuation valve and pulsing water, after 3s the evacuation valve is re-opened to purge the chamber. Then the same process is repeated using Tetrakis(diethylamide)-titanium (TDMA-Ti) instead of water for 110 cycles to create a 10nm layer (0.9 Å/cycle growthrate). Nitrogen gas was used as the carrier gas for precursors and for purging the reaction chamber. In order to create a hollow TiO2 shell, ZnO sacrificial layer was removed. In order to achieve this a small portion of TiO2 nanotube tips was etched by using a 5:1 volume ratio solution of hydrogen peroxide (H2O2) and ammonia solution 25% (NH3 + H2O), respectively. A spin coating condition of 2000rpm was used during the drop-cast etching to ensure even surface modification. One drop was placed on the spinning substrate every 20 seconds during 1 minute and washed thoroughly in DI water. Finally, ZnO sacrificial layer was etched completely by using hydrochloric acid (HCl) 0.1M concentration under spin coating during 5 minutes placing one drop every 5 seconds. The sample was kept in water for 3 hours to ensure full neutralization of any remnant acid residue and annealed under Ar at 400°C for 2 hours before further characterization. In order to evaluate the protecting properties of the hollow shell, the in-tube nanorods were grown on a stainless steel spacer disc substrate following the same procedure described above and used as anode electrode in a coin half-cell battery assembled inside an argon filled glove box. Lithium foil was used as the counter-electrode and tested for lithium storage during 70 cycles of charge/discharge from 0.5V to 2.5V. The current density applied was kept at 348mA/g. Results and Discussion: Fig 1 shows the Field Emission Scanning Electronic Microscopy (FESEM) images of each fabrication steps during hollow protective TiO2 shell deposition on SnO2 nanorods. The characteristic tetragonal phase with a spear-looking shape was found for SnO2 nanorods, the average size observed is about 50-100 nm in diameter and approximately 1 µm in length. A circular ring-like shape with a space gap of about 30 nm can be observed around the SnO2 nanorods and a very thin shell of about 10nm of a glassy looking transparent TiO2 was found, actual measurements of the dimensions of as prepared nanorods are provided in the supporting


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information (Supporting Information Fig. S1). The surface holes on the protecting TiO2 were created during the gentle etching process under spin-coating condition. It was observed that the etching by soaking into a chemical bath actually damaged several areas on the shell faster than others and destroyed the coating in random places. On the other hand, gentle etching by dropcast under spin coating condition described above resulted efficient and non-aggressive formation of spear-like structure surrounded by a circular ring. The TiO2 tips of the outer shell were opened by a mixture of hydrogen peroxide and ammonia solution (25%) in a 5:1 volume ratio, respectively under spin coating condition as described before. The opening size at the nanotube can be optimized directly from the variation of the etching time. The nano-holes on TiO2 shell allow for the ZnO sacrificial layer to be etched away leaving a hollow shell-like nanotube shape. Fig 2 shows the Energy Dispersive X-ray (EDAX) data that confirmed the presence of SnO2, TiO2, Si (substrate), Al (SEM sample holder) and very small amount of ZnO residue that remained in the back and edge of the sample after the etching process. This residue acts as a support for the hollow shell from the sample sides as illustrated in the last step of Fig 4. A comparison of the atomic percentage ratios is provided in the supporting information document (Table S1).

Figure 1. FESEM images of SnO2 nanorods (a), ZnO (30nm) and TiO2 (10nm) deposited on SnO2 nanorods by ALD (b), In-tube TiO2 protected SnO2 nanorods (c), SnO2 nanorods tilted angle by 30° (d), ZnO and TiO2 deposited on SnO2 nanorods tilted angle by 30° (e), In-tube TiO2 protected SnO2 nanorods tilted angle by 30° (f).


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Figure 2. (2a). EDAX qualitative analysis from SnO2 (a), ZnO and TiO2 deposited on SnO2 nanorods by ALD (b), and in-tube TiO2 protected SnO2 nanorods (c). Rutile and anatase are the most common phases of titanium dioxide. In our case a 10nm thick TiO2 layer deposited by ALD on SnO2 nanorods grows aligned in the same direction as the SnO2 nanorods. In order to explore the structure we performed X-ray diffraction characterization. Rutile crystal structure was found for the as grown SnO2 nanorods and TiO2 nanotube shells after thermal treatment on stainless steel substrates which are used for battery assembly purposes. The x-ray scan was carried out from 15 to 85 degrees on the 2θ axis. Fig 3 shows the diffraction pattern and the peak correlation to each crystal plane assigned according to the powder


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diffraction reference files PDF #41-1445 for SnO2 and PDF #21-1276 for TiO2. The diffraction pattern was recorded before and after the protective TiO2 hollow shell deposition and the results were compared.

Figure 3. X-Ray diffraction profile peaks and their corresponding crystal planes obtained from SnO2 nanorods before hollow TiO2 shell (red), and after hollow TiO2 shell deposition (blue).

The thin structure of TiO2 (10nm) has weak visible rutile diffraction signatures after thermal treatment by annealing at 400°C, consistent in one dimensional TiO2 nanostructures reported in literature such as rods and tubes [17, 40, 41]. Regarding the growth process of sacrificial layer and hollow-shell using ALD, Fig. 4 describes the mechanism of the atomic layer deposition of each layer at 250°C. The deposition of ZnO sacrificial layer growth involves the reaction of diethyl zinc (DEZ) and water. First H2O, at such temperature, generates hydroxyl groups bound to the nanorods surface (Nanorod(―OH)); when DEZ is pulsed, it reacts with the hydroxyl groups; the reactions occurs as follows: Nanorod + 2H2O + ∆ 250°C → Nanorod(―OH)2 + H2↑ Nanorod(―OH) + Zn(C2H5)2 → Nanorod(―O)Zn(C2H5) + C2H6↑ Nanorod (―O)Zn(C2H5) + H2O → Nanorod―ZnO(―OH) + C2H6↑ The sub-product C2H6 is washed away by nitrogen carrier gas and the same process is repeated for 335 cycles. In same manner the deposition of TiO2 protective shell involves the reaction of Tetrakis(diethylamide)-titanium (TDMA-Ti) and water; ZnO layer is covered by hydroxyl groups when the above reaction is terminated, then TDMA-Ti is pulsed and reacts with hydroxyl groups on ZnO surface as follows:


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ZnO(―OH)2 + Ti(N(CH3)2)4 → ZnO(―O)2Ti(N(CH3)2)2 + 2HN(CH3)2↑ ZnO(―O)2Ti(N(CH3)2)2 + 2H2O → ZnO―TiO2(―OH)2 + 2HN(CH3)2↑ The sub-product HN(CH3)2 is washed away by nitrogen carrier gas and the same process is repeated for 110 cycles. In order to remove ZnO layer; small perforations were etched on TiO2 by using a 5:1 volume ratio solution of hydrogen peroxide (H2O2) and ammonia solution 25% (NH3 + H2O) respectively. TiO2 etching does not follow any visible direction on the film, instead, it begins at the nanorod tip expanding itself to the rest of the nanorods body depending on etching time. This process may be attributed to the hydrophobic behavior of TiO2 due to the presence of Ti+4 ions on the surface of the film, this behavior has been reported previously in literature [42, 43].

Figure 4. Atomic layer deposition (ALD) schematic procedure utilized to achieve ZnO and TiO2 core-shell deposition on SnO2 nanorods followed by the sacrificial layer removal (ZnO) under chemical etching for in-tube TiO2 protected SnO2 nanorods formation. 1mMol concentrated HCl was used to remove ZnO sacrificial layer creating a hollow gap in between SnO2 and TiO2 as described above. Annealing at 400°C promotes TiO2 reaccommodation of atomic sites bringing structural stability to cracks or any isolated damage that may have been caused during the etching process. Fig 5 shows the progressive opening of TiO2 tube shell and its relation with etching time under spin coating condition.


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500 nm

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Figure 5. FESEM images of SnO2 nanorods covered by ZnO and TiO2 shell before chemical etching (a). 60 sec TiO2 etching by drop casting (one drop every 20 seconds under spin coating) plus complete ZnO etching (b). 90 sec TiO2 etching (one drop every 20 seconds under spin coating) plus complete ZnO etching (c). 120 sec TiO2 etching (one drop every 20 seconds under spin coating) plus complete ZnO etching (d).


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Figure 6. Charge and discharge curves (absolute capacity vs voltage) shown by SnO2 nanorods after 70 cycles of lithium storage (a), capacity vs voltage shown by TiO2 protected SnO2 nanorods (b), capacity vs cycle number for non protected SnO2 (c), capacity vs cycle number for protected SnO2 (d), FESEM image of the SnO2 nanorods electrode (open battery) after 70 cycles of lithium storage (e), FESEM image of the protected SnO2 nanorods electrode (open battery) after 70 cycles of lithium storage (f). The sample as shown in fig 5b (1 minute etching time) was used as anode electrode in a coin half-cell battery (large area image is provided in figure S3 from supporting information document). Lithium foil was used as the counter-electrode and tested for lithium storage during 70 cycles of charge/discharge from 0.5V to 2.5V. Fig 6a and 6b shows the comparison of the absolute capacity (the amount of electric charge that can be delivered at the rated voltage) by charging/discharging SnO2 nanorods with and without TiO2 hollow protective shell. A mass loading of 0.55mg was found for the non-protected SnO2 nanorods sample excluding the mass of


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stainless steel substrate, and the hollow-shell protected SnO2 nanorods displayed a loading mass of 0.59mg (including TiO2 shell and excluding stainless steel substrate mass). A reversible absolute capacity of ≈ 908 mAh/g was kept for more than 50 cycles with very little variation (+/- 6mAh/g) by the electrode containing in-tube protected nanorods fig 6b, from this point onwards the battery slowly decreased its capacity to 790 mAh/g after 20 cycles more (70 cycles total). The absolute capacity vs cycle number plot for this behavior is presented in Fig 6d demonstrating a better cycling stability during 54 charge/discharge cycles followed by a slight decrease in absolute capacity mainly caused by loss of active material discussed further in this report. In contrast, it was observed for the non-protected electrode as shown in Fig. 6a a starting capacity of 813mAh/g which gradually decreased down to 485mAh/g after 70 charge-discharge cycles. The absolute capacity vs cycle number plot for this behavior is presented in Fig. 6c. The current density applied to both batteries was kept at 348mA/g during the 70 cycles. For our TiO2 protected nanorods (fig 6d) it’s possible to observe a lower charge capacity that stabilizes itself after 20 cycles as compared to the discharge capacity; this behavior is mainly caused by the protecting TiO2 shell. The shell opening by chemical etching is time dependent as explained in fig 5, depending on how big is the shell opening it will take several cycles to stabilize Li ions through the hollow gap during the initial charge cycles (specially for hollow shells with very small opening). The same behavior has been observed in similar hollow protected SnO2 nanostructures [21].


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Figure 7. (a) Cycle number vs absolute capacity for 70 charge-discharge cycles under current density rate of C/4 (174mA/g), C/2 (348mA/g), 2C (870mA/g), 10C (1740mA/g), and back to C/2 (348mA/g) for SnO2 nanorods and TiO2 protected SnO2 nanorods. (b) Corresponding charge-discharge plots for TiO2 protected electrode at different C-rates. (c) Columbic efficiencies obtained during the 70 cycles at different C-rates (d) corresponding charge-discharge plots for non-protected SnO2 electrode at different C-rates. Fig 6e and 6f shows the SnO2 nanorods anode and TiO2 protected SnO2 anode after 70 charge/discharge cycles, both batteries where opened and characterized using FESEM. It can be observed that SnO2 nanorods generated sphere-like morphologies due to constant pulverization and expansion after lithiation/delithiation process. On the other hand, TiO2 protected SnO2 nanorods generated a composite of semi expanded SnO2 nanorods with TiO2 flakes around them. TiO2 storage of Li ions occurs through an intercalation process across the crystal structure that doesn’t cause significant volume changes for this material as observed in Fig. 6f, the volume expansion of TiO2 during this process has been reported in literature to be less than 4% [27]. The structural nature of TiO2 provides with an enhanced stability to Li insertion in comparison to SnO2 in which lithium storage occurs by reacting with Li+ in an alloy/de-alloy electrochemical mechanism (LixSn) generating a significant volume expansion after repetitive cycling [26, 44]. Fig. 6f shows the TiO2-SnO2 flake-nanorod composite generated as a result of combining both materials after 70 charge-discharge cycles of lithium storage. These observations indicate that TiO2 holds SnO2 nanorods shape and eventually breaks due to structural damage after continuous Li ion storage by intercalation mechanism in addition to the internal SnO2 volume expansion. The hollow TiO2 shell on SnO2 rod composite provides an improved performance allowing higher current densities during the charge/discharge cycles. Fig 7 shows the absolute capacity at a current density of C/4 (174mA/g), C/2 (348mA/g), 2C (870mA/g), 10C (1740mA/g), and back to C/2 (348mA/g) respectively during 70 cycles. However, it’s possible to observe that the cycling stability displayed a decay during the last cycles mainly caused by poor adhesion to stainless steel substrate, FESEM pictures of the active material peeling is reported on the supporting information document Fig S4. From the results obtained in fig 6 and fig 7 it is easy to observe that TiO2 hollow protected battery shows higher capacity, higher current rate performance, and higher cycling stability during 70 cycles in comparison with the battery without protecting TiO2 shell. The corresponding columbic efficiencies for the very first cycle are 50.1% for the non-protected SnO2 electrode and 74.9% for the TiO2 protected electrode followed by an electrochemical stability of near 100% reported in Fig. 7c. It is well known that the lithiation and delitiathion on SnO2 creates a solid electrolyte interface (SEI) and a complex LixSn/Li2O matrix that causes a higher irreversible capacity at the very first discharge cycle which is the main reason for the observed poor columbic efficiency during the first cycle on SnO2 electrodes [26, 44]. However the higher columbic efficiency shown by the protected nanorods during the first cycle could be related to the TiO2 shell that prevents the formation of the SEI and Li2O directly on SnO2 nanorods surface; to further explore the oxidation/reduction of the electrodes cyclic voltammetry data was collected.


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Figure 8 Cyclic voltammetry (CV) curves of SnO2 nanorods anode (a) and TiO2 hollow-shell protected SnO2 anode (b) for 3 cycles using a scanning rate of 0.2 mV/s. Cyclic voltammetry (CV) curves are illustrated in Fig 8. CV measurements were performed from 0V to 3V. The cycles obtained show a reduction peak at 0.8V which is attributed to the Li+ ions being inserted onto SnO2 nanorods creating Sn and Li2O in the process, then Li reacts to form LixSn clearly displayed by the reduction peak at 0.2V. During the charging process three oxidation peaks can be observed at 0.55V, 1,5V, and 1.9V, corresponding to the dealloying reaction of LixSn when Li+ is extracted and the oxidation to metallic Sn0 which reacts afterwards with Li2O to form SnO/SnO2. No visible contribution from TiO2 was observed on oxidation/reduction peaks. This could be attributed to the low capacity contribution of TiO2 and highly reversible lithium insertion/disinsertion process that takes place in TiO2 surface, which is consistent with the literature for TiO2-SnO2 composites. [21, 30, 35].

Conclusion We have introduced a multilayer approach for effective lithiation. Vertically aligned SnO2 nanorods grown by the vapor-liquid-solid route were protected with a hollow TiO2 shell using a ZnO sacrificial template created by ALD technique. A 10nm thick transparent shell with a hollow gap of about 30nm was confirmed by FESEM characterization and rutile crystal structure was consistent for both materials SnO2 and TiO2. When tested as anode for lithium storage the TiO2-protected SnO2 nanorods demonstrated higher rate capacities and improved cycling stability during 50 charge-discharge cycles of ≈ 900 mAh/g and a slow decay to 792 mAh/g at the last 20 cycles making a total of 70 cycles. SnO2 nanorods without protecting shells showed decreasing capacity gradually from 813 to 485 mAh/g after 70 cycles. The hollow gap allows for the SnO2 nanorods to expand naturally until the hollow space gets filled up; thus maintaining a highly reversible capacity for more than 50 cycles which do not happen in the sample without TiO2 shell. The performance of the hollow-shell protected nanorods battery demonstrated a superior performance in terms of capacity retention and improved rate capabilities in comparison with other reported TiO2-SnO2 composite structures such as the capacity reported for double wall


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hollow tubes grown directly in alumina anodized membranes [13, 33]; also a better capacity than hollow core shell nanospheres [35, 36]. Similar, capacity retention was reported for double shell hollow nanofiber configurations and hollow shell covered SnO2 nanowires [21, 30, 34] however our TiO2 hollow-shell protected SnO2 battery still displays higher starting performance in capacity retention and current rate density, also an improved cycling stability compared to the non-protected nanorods electrode demonstrating a good coupling of both materials in the architecture morphology. As a disadvantage, the cycling stability dropped consistently after 54 cycles showing a shorter life-spam of our TiO2 hollow-shell protected SnO2 battery in comparison to the hollow nanostructures from the mentioned literature, these results suggest low adhesion of SnO2 active material to stainless steel substrate after cycling. The design and results obtained provide evidence of significant improvement of the battery performance related to the self-standing vertical hollow-shell architecture protecting the SnO2 nanorods. The proposed multilayer design is open to be tested in different substrates with the prediction of higher cycling stability as observed in fig 6d vs 6c. This multilayered-based hollow shell fabrication can be also employed to supercapacitors, drug delivery, and nano-fluidics.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (A.K.P.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the CREST-CREAM HRD-1547771. We thank Tristan Skinner, Sangram Pradhan and Messaoud Bahoura for useful discussion.


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