Three dimensional metal/N-doped nanoplate carbon catalysts for oxygen reduction, the reason for using a layered nanoreactor

A layered nanoreactor (zinc hydroxide gallate/nitrate nanohybrid) has been designed as a nano-vessel to confine the gallate/nitrate reaction inside zinc hydroxide layers for production of metal/nitrogen-doped carbon catalysts. Metals (Fe2+, Co2+ and Ni2+) doped and bare zinc hydroxide nitrates (ZHN) were synthesized as the α-phase hydroxide hosts. By an incomplete ion-exchange process, nitrate anions between the layers of the hosts were then partially replaced by the gallate anions to produce the layered nanoreactors. Under heat-treatment, the reaction between the remaining un-exchanged nitrate anions and the organic moiety inside the basal spacing of each nanohybrid plate resulted in obtaining highly porous 3D metal/nitrogen-doped carbon nanosheets. These catalysts were then used as extremely efficient electrocatalysts for catalyzing oxygen reduction reaction (ORR). This study is intended to show the way to get maximum electrocatalytic activity of the metal/N-doped carbon catalysts toward the ORR. This exceptionally high ORR performance originates from the increased available surface, the best pore size range and the uniform distribution of the active sites in the produced catalysts, all provided by the use of new idea of the layered nanoreactor.


Thermal and phase stability behavior of the FZHN precursor and its FZHNG nanohybrid
shows TG/DTG curves for the FZHN, organic compound (gallic acid), the FZHG gallate nanohybrid and the FZHNG nitrate/gallate nanohybrid. As shown in the figure, FZHN decomposes in two major steps and a minor step. The initial major step is owed to the removal of the intercalated water molecules with respective weight loss of 5.5% at temperature maximum of 88 ˚C as following, Zn 5 (OH) 8 (NO 3 ) 2 .2H 2 O→ Zn 5 (OH) 8 (NO 3 ) 2 + 2 H 2 O (1) with theoretical weight loss of 5.8 %. The minor step (it can be seen as a shoulder in the DTG curve) at about 150 ˚C is attributed to the converting of anhydrous ZHN to another type of ZHN (Zn 3 (OH) 4 (NO 3 ) 2 , card No: 52-0627 with 0.69 nm basal spacing) 1 and a small amount of zinc oxide with theoretical weight loss of 6.1 % as following, Zn 5 (OH) 8 (NO 3 ) 2 → Zn 3 (OH) 4 (NO 3 ) 2 + 2 ZnO For our sample, this weight loss is 7.0 %. The second major step is due to the collapsing the layered material and production of zinc oxide with respective weight loss of 28 % at temperature maximum of 173 ˚C. The equation for this step is as following, Zn 3 (OH) 4 (NO 3 ) 2 → 3 ZnO + 2 NO 2 + ½ O 2 + 2 H 2 O with theoretical weight loss of 25.6 %.
Therefore, the sum of the theoretical weight losses is 37.5 % and for our sample is 40 %. It shows a good agreement between theatrical and practical data for thermal behavior of the FZHN.
Meanwhile, it should be noted that some difference between theoretical and practical values is due to the removal of the surface adsorbed water molecules for the FZHN. Overall, this TG curve in consistent with XRD data confirm that the layered hydroxide cannot keep its structure stable at the temperatures higher than 200 ˚C. However, by FTIR analysis, it was observed that the nitrate anions can still be intercalated between zinc oxide layers mostly in the stable state at temperature as high as 400 ˚C and only at the temperatures higher than 400 ˚C, nitrate anions are decomposed and released as NO 2 gas molecules 2 . This is evidenced by observation of the continued weight losses in the FZHN sample at temperatures higher than 200 ˚C to 800 ˚C.
Gallic acid decomposes by heat-treatment under inert gas in two steps with the weight losses of 44.0 and 25.0 % at temperatures maxima of 264 and 319 ˚C, respectively (Fig. S2b). The release of CO 2 by decomposition of acidic group of gallic acid, and, also the decomposition of phenolic groups along with production of water is assumed to be the reasons to observe the first weight loss step of gallic acid 2,3 with theoretical weight loss of 44.1 % according to following equation in good agreement to our obtained result.
The second weight loss can be due to decomposition of benzene ring to carbon with theoretical weight loss of 23.5 %.
C 12 H 6 O 3 → 4C + 3CO + C 5 H 6 (5) As observed, gallic acid cannot be stable at temperatures higher than 320 ˚C and the obtained residue carbon is 13 % at 800 ˚C (2C is remained from 7C due to gallic acid). Figure S2c shows the TG/DTG curves for the FZHG. Three major steps of the weight losses of 5.5, 27.0 and 27 % occurring at temperatures maxima of 80, 590 and 795 ˚C are observed. Except the first weight loss due to the removal of the surface adsorbed water molecules at about 80 ˚C, any major weight loss cannot be seen up to around 600 ˚C in the TG curve. Given that the surface adsorbed water weight loss is 5.5 % and the intercalated water weight loss is theoretically considered to be 4.2 % (a total of ≈10 %), 10 % weight loss is equal to 300 ˚C in the TG curve and, therefore, the next step weight loss occurred from around 300 ˚C. In accordance with XRD patterns for the heat-treated products of the FZHG (Fig. S3a), the weight loss at 300 ˚C and at higher temperatures is due to the dehydroxylation process. In fact, the dehydroxylation started gradually around 300 ˚C and completed at around 600 ˚C. As shown in Fig. S3a, XRD pattern confirms that the layered structure can still be observed for the FZHG heat-treated at 600 ˚C. Moreover, FTIR spectra for the heat-treated products of the FZHG (Fig. S4a) show that gallate anion peaks are similar to those of the unheated nanohybrid, but with less intensities and some red shifts. This confirms that gallate anions positioned between the inorganic layers are not completely decomposed even at 600 ˚C. However, the dehydroxylation weight loss (Zn 5 (OH) 8 (C 7 H 5 O 5 ) 2 to Zn 5 O 5 (C 7 H 5 O 5 ) 2 ) should be 10.6 % (theoretically) and the obtained weight loss at 600 ˚C is 27.0 %. It means that the second weight loss for the FZHG is due to both dehydroxylation and a follow up of the first step decomposition of gallic acid. Accordingly, the presence of gallate anion between zinc hydroxide layers could profoundly enhance thermal resistance not only for organic compound, but also for the layered hydroxide structure. Complete decomposition of gallate anions and producing carbon material is obtained at the temperatures higher than 600 ˚C (see Figs. S3a and S4a). Alternatively, a further major weight loss is considered at around 790 ˚C in the TG/DTG curve of the FZHG. If we suppose that the heat-treatment of the gallate anions inside the FZHG layers would produce carbon material in the same manner as that of free gallic acid according to following formula, all the residual material should be 57 %. It is while; we can see that the residual product is about 40 % for the FZHG after heat-treatment at 850 ˚C. According to TG curve ( Fig. S2c) and XRD data for the FZHG (Fig. S3b), one can deduce that a new event happens at temperatures higher than 700 ˚C. This is the reduction of ZnO to metallic zinc and its evaporation on the surface of the produced carbon 4-6 . Nevertheless, the evaporation does not occur for the dopant metals of Ni, Co and Fe and, they could be only reduced to metallic form as evidenced by the XRD patterns of the metal-doped nanohybrids heat-treated at 800 ˚C (Fig. S3b) 6 . According to data shown in Fig.   S3b, it is worthwhile to mention that the presence of the metal dopant agents could catalyze the reduction and evaporation of ZnO. Accordingly, ZnO peaks are completely being eliminated in the XRD patterns and FTIR spectra (Fig. S4a) for the heat-treated products of metal doped layered nanohybrids of NZHG, CZHG and FZHG at 800 ˚C, but not for the undoped ZHG nanohybrid. Additionally, the EDS analysis was performed on the CZHG and FZHG samples heat-treated at 800 ˚C to confirm the elemental compositions of the produced metals as observed in their XRD patterns. The skewer-like metallic cobalt (obtained via fusion of the melted cobalt nanoparticles) and carbon nano plates are seen in the FESEM image of the heat-treated CZHG sample (Fig. S5a). Also, EDS data shows some dissolved oxygen, carbon and metallic zinc for the obtained metallic cobalt and iron (Figs. S5b-f ).
For the iron-doped zinc hydroxide nitrate/gallate nanohybrid (FZHNG), we used a STA to get better insight into the reaction between nitrate and gallate anions inside the inorganic layers.
As shown in Fig. S2d, there are two minor weight losses of 2.9 and 8.6 % at temperatures maxima of 96.6 and 167.7 ˚C. The first one is due to the removal of the adsorbed water and the second one is owed to the dehydroxylation process for the zinc hydroxide nitrate moiety in each nanohybrid plate (see Fig. S6a and Fig. S1). This small amount of the dehydroxylation weight loss (8.6 %) demonstrates that only dehydroxylation process occurs at this temperature, but not the decomposition of nitrate anions along with the release of NO 2 . It means that the nitrate anions are present mostly between zinc oxide layers encapsulated by the zinc hydroxide gallate layers. DTA analysis confirms the two mentioned endothermic processes. At around 400 ˚C, there is an exothermic process followed by an intense endothermic process accompanied to the main weight loss of 32 % occurring at temperature maximum of 432 ˚C. The exothermic process can be due to the reaction between nitrate and gallate anions, resulting in the burning some organic anions and also starting the formation of nitrogen doped carbon material. In fact, during the heat-treatment process, the basal spacing of the nanohybrids acts as a layered nanoreactor for the reaction between gallate and nitrate moieties to produce N-doped carbon catalysts 7,8 . The inorganic layers can shield the content of the basal spacing (gallate and nitrate) from direct heat and thus prevent the evaporation and burning out the anions. As seen in Fig. S2d, unlike to free gallic acid and FZHG nanohybrid, gallate anions inside the FZHNG layers are not decomposed in two steps, but in one step due to the gallate-nitrate reaction. An endothermic reaction can also be observed after 700 ˚C due to the reaction between the produced carbon and zinc oxide materials as following, 5 ZnO (s) + C (s) → Zn (g) + CO (g) Due to phase stability of the materials during the heat-treatment process and confirming those events observed in the thermal analysis for the FZHN and its FZHNG nanohybrid, an insitu heating XRD instrument was used to monitor the structural degradation of the samples in the temperature range from 25 to 800 ˚C at a constant heating rate of 5 ˚C min −1 and an Ar flow rate of 50 mL min −1 . Figure S6a shows